WO2007012838A2 - Drug delivery system - Google Patents

Abstract

The invention relates to peptides and peptide analogues/derivatives, for use as peptide drug carrier molecules, and peptide drug carrier-drug conjugates which are transported across the wall of the gut into the blood by PepTl protein. There is provided a drug carrier comprising a peptide, or derivative or analogue thereof, the peptide comprising at least two amino acid residues linked by a peptide bond. The amino acid residues are arranged such that the peptide bond is protected from hydrolysis. The peptide comprises a functional group for attachment to a drug. The functional group to which the drug is attached is provided on the second or C-terminal residue of the peptide.

Description

DRUG DELIVERY SYSTEM

The present invention relates to drug delivery systems and particularly, although not exclusively, to drug delivery systems which exploit the PepTl pathway. More specifically, the invention relates to novel peptides and peptide analogues/derivatives, for use as peptide drug carrier molecules, and peptide drug carrier-drug conjugates which are transported across the wall of the gut into the blood by PepTl protein, and uses thereof in medicine. The invention further extends to methods of synthesising such peptide drug carriers.

A large proportion of drugs used in medicine, that are orally administered, are subject to structural modification and, in some cases, substantial degradation in the gut, and this can often lead to a decrease in the biological activity of the drug. Accordingly, the medicinal efficacy of such drugs can be limited when taken orally. Furthermore, a large number of drugs that exhibit medicinal properties cannot be administered to a patient orally because they have poor solubility, or they are unable to diffuse across the wall of the gut into the bloodstream. Therefore, unfortunately, such drugs are either totally rejected for use in medical treatment, or have to be administered to patients by intravenous injection, which is invasive and has associated problems with many patients. Accordingly, there is a need to develop mechanisms by which drugs, which are either injected intravenously or are not used at all, can be administered orally and transferred into the blood via the gut without any loss in biological activity, hi addition, there is a need to improve the transportation of drug molecules across the wall of the gut in respect of those drugs which are currently administered orally, but which show decreased or low levels of medicinal activity.

PepTl is a trans-membrane protein that is highly expressed in the jejenum region of the small intestine, and transports small peptides, such as the breakdown products of protein in food, across the wall of the gut into the bloodstream. PepTl transports di-peptides and tri-peptides across the gut wall efficiently. Substrate transportation by PepTl is driven by proton and electrochemical gradients and provides a mechanism by which peptidic drugs such as β-lactam antibiotics, and ACE inhibitors, for example, Captopril, can be orally absorbed by patients. Accordingly, drugs that do not naturally diffuse across the villi of the small intestine, or those which have poor solubility, and which are only administerable by intravenous injection may be made orally administrable by transporting them across the wall of the gut into the bloodstream via the PepTl pathway. In addition, the PepTl pathway may also be exploited to improve the transportation of drugs which are currently administered orally, but which show decreased levels of biological activity, for example, because they are modified or degraded in the gut before they are transported into the blood.

Therefore, it is an aim of embodiments of the present invention to address the above problems and problems with the prior art, and to provide a drug delivery system, which could be made available to the medical community, so that drugs, which are normally administered orally but which exhibit reduced or low levels of medicinal activity can have their performance improved. In addition, the drug delivery system could also enable drugs that are administered intravenously or which are not used at all, to be administered orally.

According to a first aspect of the present invention, there is provided a drug carrier comprising a peptide, or derivative or analogue thereof, the peptide comprising at least two amino acid residues linked by a peptide bond, the amino acid residues being arranged such that the peptide bond is protected from hydrolysis, and the peptide comprising a functional group for attachment to a drug, wherein the functional group to which the drug is attached is provided on the second or C-terminal residue of the peptide.

Preferably, the drug carrier is adapted to, or is capable of carrying or transporting a drug, preferably in vivo. The term "drug" as used herein is intended to encompass any pharmaceutically or medicinally active compound or molecule. For example, the drug may have a poor solubility, or may be too polar to cross a membrane when in use. Examples of suitable drugs, which may be used in accordance with the invention may include antivirals, antibiotics, β-blockers, neurotransmitters, hormonal, and anti-cancer drugs. Preferred examples of drugs may include adrenaline, dopamine, GABA, acyclovir, sulfonamides, enalaprilate, burimamide-based H₂ antagonists, propranolol, bestatin, or steroidal drugs.

Advantageously, and preferably, the peptide drug carrier according to the first aspect enables drugs, which are either not used at all in medicine, or which have to be administered intravenously, to be administered to a patient orally. In addition, advantageously, the drug carrier improves the performance of drugs, which may be normally administered orally, but which may exhibit reduced or low levels of medicinal activity when taken orally, such as drugs with poor solubility. Administering drugs orally, i.e. by mouth, is much simpler and less invasive than by intravenous injection, which is very off-putting for the majority of patients, and has a range of other health risks. Therefore, advantageously, use of the drug carrier according to the present invention, will greatly increase the number of drugs that can be used, and administered orally.

The inventor has found that the drug carrier according to the present invention may have a drug molecule attached to the functional group of the peptide, thereby forming a 'drug carrier-drug' combination. In addition, the inventor has found that this combination has improved transportation properties, for example, across the wall of the gut. The inventor does not wish to be bound by any hypothesis, but believe that the combination may be transportable, moved or carried from a first site to a second site by an active transport mechanism. An example of an active transport mechanism is a symporter, which may be a proton-dependant symporter. In particular, the inventor believes that such combinations may be transported via the PepTl pathway. Accordingly, the drug carrier is preferably adapted to act as a PepTl substrate.

PepTl is most strongly expressed in the jejunum of the small intestine. However, PepTl has also been isolated from the liver, brain, and from the cortex and medulla of the kidneys. Hence, it will be appreciated that the drug carrier in accordance with the invention may be transported in any of the gut, liver, brain, or in the kidneys etc, and as such, the drug carrier may be transported in any of these tissues. However, in a preferred embodiment, the drug carrier according to the invention may be transported across a biological membrane, for example, the lining of the gut. A suitable example, may be in the small intestine, and particularly, in the jejunum.

It is therefore preferred that the drug carrier according to the invention is adapted to carry or transport a drug across a biological membrane. By the term

"biological membrane", we mean at least one layer of epithelial cells, for example, the lining of the gut. A suitable example of a biological membrane may be in the small intestine, and particularly, in the jejunum.

A second isoform, PepT2, which shares approximately 50% sequence homology with PepTl, has also been found in the kidneys, where it reabsorbs peptides from the glomerular filtrate. Therefore, it will be appreciated that the drug carrier according to the invention will also have the advantageous properties of being able to exploit the PepT2 pathway, being transported thereby. It will be appreciated that the natural PepTl or PepT2 substrates have naturally occurring stereochemistry, i.e. they comprise natural L-isomer amino acids.

The drug carrier in accordance with the first aspect of the present invention shows significant surprising advantages over the prior art due to the ability of the peptide according to the first aspect to be attached to a drug via its functional group, and the peptide's ability to transport a drug in vivo. The advantages of the peptide according to the invention are that:-

(i) it allows surprisingly efficient binding of a peptide-drug combination to a transporter (as described in Example 2); (ii) it renders the combination substantially resistant to hydrolysis, unlike most peptides; and

(iii) it allows surprisingly rapid in vivo transport of the combination across the wall of the gut into the bloodstream (as described in Example 3).

Preferably, the peptide comprises at least two amino acids or derivatives or analogues thereof, or at least three amino acids or derivatives or analogues thereof, or at least four amino acids or derivatives or analogues thereof. Hence, for example the peptide may comprise a dipeptide or a tripeptide or derivatives or analogues thereof. However, preferably, the peptide of the present invention comprises a dipeptide. Advantageously, dipeptides are conveniently small molecules compared to longer peptides, and are therefore relatively simple to synthesise. Moreover, due to their small size, they also exhibit good transportation properties via the PepTl/PepT2 pathway.

The amino acids may be selected from the repertoire of twenty amino acids commonly found in proteins. The drug carrier peptide may comprise an acidic or a basic amino acid. The peptide may comprise a hydrophobic or a hydrophilic amino acid. Preferably, the peptide comprises a serine, aspartate or glutamate residue as the second or C-terminal residue.

The inventor has found that a serine, aspartate, or glutamate residue represents an advantageous means of attaching a drug to the peptide. Advantageously, the drug carrier is substantially resistant to hydrolysis, for example, by peptidases.

Preferably, the peptide is not a thiopeptide, and preferably, not a thiodipeptide. By the term "thiopeptide" used herein, we mean at least two amino acids joined together, comprising at least one thio- (sulphur) functional group. For the purposes of the present invention, thiopeptides are considered to lack a peptide bond (CONH) where the two amino acid residues are linked to each other. This is because of the presence of the thio group, which substitutes the carbonyl group (CO) on one of the two residues of the resultant thiopeptide forming a CSNH bond.

Where the peptide comprises a plurality of amino acid residues bonded together, preferably, the number of peptide bonds in the peptide is kept to a minimum. Preferably, the peptide comprises less than four peptide bonds, more preferably, less than three peptide bonds, even more preferably, less than two peptide bonds.

The functional group to which the drug is attached is provided on the second or C-terminal residue of the peptide. Hence, for example, if the peptide is a dipeptide, the functional group is provided on the C-terminal residue of the dipeptide. If the peptide is a tripeptide, in a first preferred embodiment, the functional group is provided on the central (second) residue, alternatively, in a second preferred embodiment, the functional group is provided on the (third) C-terminal residue. The second residue of a peptide is generally regarded as being a stereochemically sensitive site. Accordingly, the inventor was surprised to find that the drug carriers in accordance with the invention, in which a drug or drug model was attached to the functional site provided on the second residue, could bind to PepTl and also be transported thereby across a membrane.

However, it is preferred that the drug carrier comprises a dipeptide, or derivative or analogue thereof, the peptide comprising at least two amino acid residues linked by a peptide bond, wherein the amino acid residues are arranged such that the peptide bond is protected from hydrolysis, and wherein the peptide comprises a functional group for attachment to a drug, and wherein the C-terminal residue of the peptide comprises serine. It is preferred that the functional group is provided on the serine residue.

It is also preferred that the drug carrier comprises a dipeptide, or derivative or analogue thereof, the peptide comprising at least two amino acid residues linked by a peptide bond, wherein the amino acid residues are arranged such that the peptide bond is protected from hydrolysis, and wherein the peptide comprises a functional group for attachment to a drug, and wherein the C-terminal residue of the peptide comprises an acidic amino acid, for example, aspartate or glutamate. It is preferred that the functional group is provided on the aspartate or glutamate residue.

Peptides according to the invention may often be subject to degradation by a number of means. For example, such degradation may occur by proteases in biological systems, which target and digest (or hydrolyse) peptide bonds present in peptides. A consequential problem with such degradation is that it may limit the bioavailability of the peptide according to the invention, and hence the ability of the peptide to achieve its biological function.

Accordingly, the inventor investigated ways in which the peptide bond in the peptide according to the invention may be protected from hydrolysis. The inventor investigated several types of ways of modifying the peptide such that it was hydrolysis resistant. However, they found that two particular modifications (incorporation of a D-amino acid residue and peptide bond N-alkylation) were surprisingly effective for protecting the peptide bond in the peptide according to the invention from hydrolysis.

In one preferred embodiment of the invention, the peptide in the drug carrier according to the first aspect of the invention comprises at least one D-amino acid residue (D-isomer).

Hence, there is provided a drug carrier comprising a peptide, or derivative or analogue thereof, the peptide comprising at least two amino acid residues linked by a peptide bond, wherein at least one amino acid residue is a D-isomer, the peptide comprising a functional group for attachment to a drug.

It will be appreciated that naturally occurring amino acids in living organisms are in the L-optical isomeric form. By the term "D-isomer", we mean that the amino acid is the D-optical isomer of an amino acid, as opposed to the corresponding L- optical isomer. The Examples and Figure 3 describe an example of how to synthesise a D-analogue peptide according to the invention.

If the peptide is a dipeptide, then it comprises two amino acid residues linked together by a peptide bond. In vivo, naturally occurring peptidases are unable to recognise and hydrolyse peptide bonds, which link together amino acids in which at least one of those linked amino acids is a D-isomer. Hence, it is preferred that in order to suitably protect that peptide bond from the hydrolytic action of a peptidase, preferably at least one of the amino acid residues is modified such that it adopts the D-isomeric form. Hence, hydrolysis of the peptide bond is not possible, and the peptide bond is therefore protected.

The second amino acid residue (C-terminal residue) in a dipeptide may be a D- isomer. However, it is preferred that the first amino acid residue (N-terminal residue) in a dipeptide is a D-isomer. It is also envisaged that both the first and the second amino acid residues may be D-isomers.

If the peptide is a tripeptide, then it comprises three amino acid residues linked together by two peptide bonds. Hence, it is preferred that in order to suitably protect both of the peptide bonds from the hydrolytic action of a peptidase, preferably at least two of the amino acid residues are modified such that they adopt a D-isomeric form.

Hence, hydrolysis is not possible, and the peptide bonds are protected.

Hence, it will be appreciated that the preferred number of amino acid residues which are D-isomers will be determined by the total number of amino acid residues and therefore peptide bonds present in the peptide. It is preferred that at least one amino acid residue linked by each peptide bond in the peptide according to the invention is a D-isomer. Accordingly, the or each peptide bond is suitably protected from hydrolysis.

Where the peptide comprises at least one D-amino acid residue, it is preferred that the drug carrier has a Formula L-

H₂N- CR¹R²- CO -NR³ CR⁴R⁵ - COX

Formula I

wherein R¹, R², R³, R⁴, and R⁵ may be independently selected from a group consisting of a hydrogen; a linear alkyl group; a branched alkyl group; a dialkyl group; an N-alkyl group; an alkoxy group; and a side chain group of an amino acid residue; and, wherein X may be independently selected from a hydroxyl group; an amino acid residue; an amide; an amide link to a further amino acid residue; and a peptide.

Where R , R , or R comprise a side chain of the amino acid, it is preferred that at least one of the side chains has non-naturally occurring stereochemistry, i.e. is a D-isomer. It is preferred that R² may be hydrogen. It is preferred that R³ may be hydrogen. It is preferred that R⁵ may be hydrogen. It is preferred that X may be a hydroxyl group.

In a preferred embodiment of the peptide comprising at least one D-amino acid residue, the drug carrier has a Formula II:-

H₂N - CHR¹ - CO - NH - CHR⁴ - COOH

Formula II

wherein R¹, and R⁴ may be independently selected from a group consisting of a hydrogen; a linear alkyl group; a branched alkyl group; a dialkyl group; an N-alkyl group; an alkoxy group; and a side chain group of an amino acid residue. Preferably, the side chain of an amino acid has non-naturally occurring stereochemistry, i.e. it is a D-isomer.

It is more preferred that R¹, and R⁴ of Formula II may be independently selected from a group consisting of a hydrogen; a linear alkyl group; a branched alkyl group; an alkyl chain attached to other functional groups; and a side chain group of an amino acid residue. Preferably, the side chain of an amino acid has non-naturally occurring stereochemistry. Examples of suitable functional group include amine; amide; ester; acid; alcohol; ether; thiol; thioether; and aryl, or aromatic compounds. It will be appreciated that R¹ and R⁴ are functional groups on first and second amino acid residues of the peptide, respectively.

The peptide in the drug carrier of the invention exhibits enhanced resistance to hydrolysis. The preparation of peptides using D-amino acids rather than L-amino acids greatly decreases any unwanted breakdown of such an agent by normal metabolic processes, decreasing the amounts of agent which need to be administered, along with the frequency of its administration. Furthermore, since the functional group to which a drug is attached is provided on the second amino acid residue of the peptide, this provides a stable drug carrier which can bind to and be transported by PepTl/PepT2 carrier systems. As mentioned previously, the natural substrates of PepTl or PepT2 carrier systems each have naturally occurring stereochemistry, i.e. they comprise natural L- isomer amino acids. Accordingly, the affinity of the PepTl/PepT2 enzymes for their corresponding substrates is defined by the fact that they recognise L-amino acids in a certain spatial arrangement. Hence, the inventor of the invention expected that the inclusion of a D-isomer in the peptide of the drug carrier to result in a substantial lowering of affinity, both in general for proteins that bind/hydrolyse natural L- peptides, and specifically for PepTl/PepT2. As a result, the inventor was most surprised to see that the incorporation of a D-isomer amino acid in the peptide of the drug carrier, (i) resulted in little or no reduction in binding to, and transport by, PepTl, and (ii) also resulted in complete inhibition of hydrolysis. Accordingly, inclusion of a D-isomer provided two surprising advantages.

Specifically, the inventor found that replacement of the amino acid residue in a dipeptide by a D-residue in dipeptides leads to only a modest reduction in binding to PepTl, and also to almost complete inhibition of hydrolysis. Hence, the inventor believes that replacing the first amino acid residue by a D-amino acid in a dipeptide offers special and totally unexpected advantages for using such entities as drug carriers, if suitably adapted elsewhere in the structure (i.e. attachment of the drug to a suitable functional group on the second amino acid residue).

In another preferred embodiment of the invention, a nitrogen atom in the peptide bond of the peptide in the drug carrier according to the first aspect of the invention is alkylated.

Hence, there is provided a drug carrier comprising a peptide, or derivative or analogue thereof, the peptide comprising at least two amino acid residues linked by a peptide bond, wherein a nitrogen atom of the peptide bond is alkylated, and a functional group for attachment to a drug.

It is preferred that the N-alkylated drug carrier is adapted to carry or transport a drug across a biological membrane, as defined herein. The Examples and Figure 4 describe an example of how to synthesise an N-alkylated peptide according to the invention.

In vivo, naturally occurring peptidases are unable to recognise and hydrolyse peptide bonds, which are alkylated on the nitrogen atom. If the peptide is a dipeptide, then it comprises two amino acid residues linked together by a peptide bond. Hence, it is preferred that in order to suitably protect that peptide bond from the hydrolytic action of a peptidase, preferably the nitrogen atom in the peptide bond is alkylated. Hence, hydrolysis is not possible, and the peptide bond is therefore protected.

If the peptide is a tripeptide, then it comprises three amino acid residues linked together by two peptide bonds. Hence, it is preferred that in order to suitably protect both of the peptide bonds from the hydrolytic action of a peptidase, preferably each nitrogen atoms in each peptide bond is alkylated. Hence, hydrolysis is not possible, and the peptide bonds are protected.

Hence, it will be appreciated that the preferred number of nitrogen atoms, which are alkylated will be determined by the total number of amino acid residues and therefore peptide bonds present in the peptide. It is preferred that each nitrogen atom in the or each peptide bond in the peptide according to the invention is alkylated.

Accordingly, the or each peptide bond is suitably protected from hydrolysis.

Where a nitrogen atom in the peptide bond of the peptide is alkylated, it is preferred that the drug carrier has a Formula III:-

H₂N - CR¹R² - CO - NR^A - CR⁴R⁵ - COX

Formula III

wherein R¹, R², R⁴, and R⁵ may be defined as above in respect of Formula I and II, and wherein R^A may be independently selected from a group consisting of a linear alkyl group; a branched alkyl group; and a derivatised alkyl group, such as a hydroxyalkyl group. In a preferred embodiment where a nitrogen atom in the peptide bond of the peptide is alkylated, the drug carrier has a Formula IV:-

H₂N - CHR¹ - CO -NR^A - CHR⁴ - COOH

Formula IV

wherein R , and R may be independently selected from a group consisting of a hydrogen; a linear alkyl group; a branched alkyl group; a dialkyl group; an N-alkyl group; an alkoxy group; and a side chain group of an amino acid residue; and wherein

R^Λ is defined as above in respect of Formula III. Preferably, the side chain of an amino acid has naturally occurring or non-naturally occurring stereochemistry.

The compound where a nitrogen atom in the peptide bond of the peptide is alkylated may have Formula III or IV (as referred to above), wherein R^A may be independently selected from a group consisting of a linear alkyl group; a branched alkyl; and a derivatised alkyl group, such as a hydroxyalkyl group.

Preferably, R^A comprises an alkyl chain. The alkyl group (or alkyl chain) may comprise a C₁-C₂₀ chain, and preferably, a C₁-C₁₅ chain. It is envisaged that the alkyl group or the alkyl chain may comprise a C₁-Ci_O chain, and more preferably, a C₁-C₆ chain, and most preferably a C₁-C₃ chain. The chain may be straight or branched. However, preferably, the chain is straight. The alkyl group or alkyl chain may be a methyl, ethyl, propyl, butyl, or a pentyl chain. In a preferred embodiment, R^Λ is a methyl group. Accordingly, the peptide is referred to as being N-methylated. Hence, preferably, each nitrogen atom in the or each peptide bond is methylated.

The peptide according to a preferred embodiment of the present invention in which a nitrogen atom in the peptide bond of the peptide is alkylated exhibits enhanced resistance to hydrolysis. The preparation of peptides in which the or each peptide bond is alkylated greatly decreases any unwanted breakdown of the peptide by normal metabolic processes, decreasing the amounts of peptide which need to be administered, along with the frequency of its administration. Furthermore, since the functional group to which a drug is attached is provided on the second amino acid residue of the peptide, this provides a stable drug carrier which can bind to and be transported by PepTl/PepT2 carrier systems.

The inventor of the invention expected that the inclusion of N-alkylated peptide bond(s) in peptide substrates for PepTl would lead to a substantial lowering of affinity, both in general for proteins that bind/hydrolyse natural L-peptides, and specifically for PepTl/PepT2. As a result, the inventor was most surprised to see that the N-methylation of the peptide bond of the peptide of the drug carrier, (i) resulted in little or no reduction in binding to, and transport by, PepTl, and (ii) also resulted in complete inhibition of hydrolysis. Accordingly, inclusion of N-alkyl group in the peptide bond provided two surprising advantages.

Specifically, the inventor found that N-alkylating the peptide bond in dipeptides leads to only a modest reduction in binding to PepTl, and also to almost complete inhibition of hydrolysis. Hence, the inventor believes that N-alkylating the amide bond in a dipeptide offers special and totally unexpected advantages for using such entities as drug carriers, if suitably adapted elsewhere in the structure (i.e. attachment of the drug to a suitable functional group on the second amino acid residue).

It will be appreciated that in preferred embodiments, the drug carrier may comprise at least one D-isomer and in addition comprise an N-alkylated peptide bond. Hence, in a second aspect there is provided a drug carrier comprising a peptide, or derivative or analogue thereof, the peptide comprising at least two amino acid residues linked by a peptide bond, wherein a nitrogen atom of the peptide bond is alkylated, and wherein at least one amino acid residue is a D-isomer, the peptide comprising a functional group for attachment to a drug, the functional group being provided on a second amino acid residue of the peptide. For example, in the case of dipeptides, the functional group would be provided on the C-terminal amino acid residue. The inventor was surprised to find that the drug carrier according to the invention comprising a D-isomer or N-alkylation in combination with the functional group on the second residue was most effective at transporting a drug across a biological membrane. Such a drug carrier exhibited the desired hydrolysis resistance and in addition, was efficiently transported by PepTl .

The following definitions apply to each of the above-mentioned formulae I-IV.

Preferably, R⁴ in Formulae I-IV is adapted to be attached to a drug molecule. In a preferred embodiment, R⁴ comprises or contains an alcohol or a carboxylic acid group. Preferably, R⁴ comprises an alkyl chain, which alkyl chain is attached to an alcohol or a carboxylic acid group. The alkyl group or alkyl chain may comprise a C₁- C₂o chain, and preferably, a C₁-C₁₅ chain. It is envisaged that the alkyl group or the alkyl chain may comprise a C₁-C₁₀ chain, and more preferably, a C₁-C₆ chain, and most preferably a C₁-C₃ chain. The chain may be straight or branched. However, preferably, the chain is straight. The alkyl group or alkyl chain may be a methyl, ethyl, propyl, butyl, or a pentyl chain.

Preferably, R comprises a side chain group of an amino acid residue. It is especially preferred that R⁴ comprises an amino acid side chain group of a DNA encoded amino acid. The amino acid side chain group of R⁴ may be independently selected from an amino acid side chain group of an acidic, basic, hydrophobic or a hydrophilic amino acid residue.

It is preferred that R⁴ is an amino acid side chain comprising an alcohol or a carboxylic acid group. Hence, in a preferred embodiment, R⁴ is an amino acid side chain, which incorporates either an alcohol or a carboxylic group. Accordingly, it is preferred that R comprises an amino acid side chain group independently selected from a group consisting of serine; threonine; glutamic acid; aspartic acid; and tyrosine. Most preferably, R⁴ comprises an amino acid side chain group of serine; glutamic acid; or aspartic acid. R⁴ may comprise spacing means, which spacing means is adapted to distance the drag away from the peptide when bound thereto, and preferably the C-terminal amino acid thereof. This may be advantageous, for example, in cases where it is not easy to attach a particular drug to the reactive group of R , and this may depend on the particular structure and/or type of drug being attached thereto. In such a case, it would be useful to either (i) attach the drug molecule to a spacing means, and then attach the spacing means together with the drug to R⁴; or (ii) have a spacing means on R⁴, with the reactive group of R⁴ towards, or on the end of, the spacing means. The peptide- drug combinations illustrated in Figures 7 and 8 (specifically, groups F, G, H and T) each comprise suitable spacing means in accordance with the invention.

The skilled technician will appreciate the types of chains, which may be incorporated in the compound as a spacing means (or spacer). For example, the spacing means may comprise an alkyl chain, or an alkyl chain incorporating ether, amino, ester, amide and/or carbonyl groups, with appropriate functionalisation at its termini for attachment to the peptide compound, and also preferably, the drug molecule. The attachment of the spacing means to the peptide and drug may be, for example, by an ester linkage.

The spacing means may comprise at least one, preferably at least two, and more preferably, at least three atoms in a chain. It will be appreciated that the actual type of atom is less important than the number and size of the atom, which impart the distancing effect of the drug away from the peptide. Hence, by way of example, the atom(s) in the spacing means may be a carbon or oxygen atom, or combinations thereof.

The spacing means may comprise a straight or branched chain. However, preferably, the chain is straight. It is envisaged that the spacing means (or spacer) may comprise at least 5, 10, 15, 20, 25, or 30 or more atoms. It will be appreciated that the formula and length of the spacer will be determined by the type of drug molecule to be attached to the peptide. The spacing means may comprise a repeated unit, or chain. For example, the spacing means may comprise [-CH₂-J_n, wherein the value of n is an integer of at least 1. However, n is an integer, which may be greater than 1, and hence, is essentially a repeated unit of [-CH₂-]_n. Another example of a suitable spacing means comprises [- CH₂-O-CH₂-J_n, wherein n is an integer of at least one. However, n is an integer, which may be greater than 1, and hence, is essentially a repeated unit of [-CH2-O-CH₂-]_n.

Advantageously, and preferably, when a drug molecule is attached to the R⁴ group of the peptide, the peptide comprises a C-terminal COOH group, which is preferred for substrate recognition by PepTl protein, and transportation thereby. The

Examples and Figure 1 shows a structure of a substrate for PepTl, which preferably comprises a dipeptide with a C-terminal carboxylic acid group.

Preferably, R¹ comprises an alkyl chain. The alkyl group or alkyl chain may comprise a C₁-C₂₀ chain as defined herein. R¹ may comprise a side chain group of any amino acid residue. It is especially preferred that R¹ comprises an amino acid side chain group of a DNA encoded amino acid. R¹ may comprise a side chain group independently selected from an amino acid side chain group of an acidic, basic, hydrophobic or a hydrophilic amino acid residue. For example, the amino acid side chain group (column 4 of Figure 1) may be independently selected from a group consisting of (i) H (glycine); (ii) Me (alanine); (iii) CH₂Ph (phenylalanine); (iv)

CHMe₂ (valine); (v) CH₂OH (serine); (vi) CH₂SH (cysteine); (vii) CH₂CO₂H

(aspartate); (viii) CH₂CONH₂ (asparagine); and (ix) (CH₂)₄NH₂ (lysine).

In a preferred embodiment, R¹ is selected from any of the side chain groups shown in column 4 in Figure 1.

In a preferred embodiment, R¹ and R⁴ may be a side chain group of any amino acid residue. It is envisaged that R¹ and R⁴ may be the same as each other, or different from each other.

Figures 6, 7 and 8 illustrate a selection of preferred peptides in accordance with the invention. For example, in a preferred embodiment, the peptide may comprise a dipeptide comprising alanine as the first amino acid residue, and serine as the second amino acid residue. The drug may be attached to the reactive group of the serine residue.

hi another preferred embodiment, the peptide may comprise alanine as the first amino acid residue, and aspartate as the second amino acid residue. The drug molecule maybe attached to the reactive group of the aspartate residue.

hi another preferred embodiment, the peptide may comprise alanine as the first amino acid residue, and glutamate as the second amino acid residue. The drug molecule may be attached to the reactive group of the glutamate residue.

The functional group to which a drug is to be attached may be protected by a protection group, preferably prior to attachment of the drug thereto. Suitable protection groups may include t-butyl, benzyl, methoxybenzyl, allyl or fluorenyl attached to nitrogen, oxygen or sulphur via ester, thioether, or carbamate linkages. The functional group may be deprotected before attachment of a drug to the compound by the addition of a suitable reducing agent. The reducing agent may comprise a one electron reducing agent. For example, when benzyl-type protection is to be removed, the use of a suitable reducing agent comprising sodium/liquid ammonia is found to be effective, without significant reaction with the peptide functional group. De-protection results in the freeing up of the functional group ready for attachment of the drug.

Derivatives or analogues of the peptide compound according to the first and second aspects of the invention may include derivatives or analogues that further increase the peptide's half-life in vivo. There are a number of ways by which peptide derivatives or analogues that have enhanced stability in biological contexts can be designed and produced. Such peptide derivatives may have improved bioavailability as a result of increased resistance to protease-mediated degradation. Preferably, a peptide derivative or analogue suitable for use according to the invention is more protease-resistant than the peptide from which it is derived. Protease-resistance of a peptide derivative and the peptide from which it is derived may be evaluated by means of well-known protein degradation assays. The relative values of protease resistance for the peptide and the peptide derivative or analogue may then be compared.

It is preferred that the derivative or analogue exhibits enhanced resistance to hydrolysis by, for example, peptidases. Examples of derivatives or analogues capable of increasing the half-life of the peptide according to the invention include peptoid derivatives, and peptide-peptoid hybrids.

Peptoid derivatives of the peptide of the invention may be readily designed from knowledge of the structure of the peptide. Peptoid compounds have two properties that make them suitable for use as peptide derivatives/analogues according to the invention:-

(i) hi peptoid residues, no hydrogen bond involving the NH would be possible. (ii) The peptoids are resistance to enzymatic degradation.

Commercially available software may be used to develop peptoid derivatives according to well-established protocols.

Retropeptoids, (in which all amino acids are replaced by peptoid residues in reversed order) are also able to mimic peptides. A retropeptoid is expected to bind in the opposite direction in the ligand-binding groove, as compared to a peptide or peptoid-peptide hybrid containing one peptoid residue. As a result, the side chains of the peptoid residues are able to point in the same direction as the side chains in the original peptide.

According to a third aspect of the invention, there is provided a drug conjugate comprising a drug, which drug is linked to a drug carrier according to the first or second aspect of the present invention.

Hence, the drug carrier according to the first or second aspect may be adapted to be attached to a drug molecule, thereby forming a 'drug carrier-drug' conjugate, hereinafter referred to as a 'conjugate'. Preferably, the attachment of the drug to the drug carrier is by covalent bonding. It will be appreciated by the skilled technician that covalent bonding between the drug molecule and the drag carrier according to the first or second aspect may be achieved by reacting a functional or reactive group provided on the second amino acid residue of the drug carrier, and a functional or reactive group on the drug. Preferably, the drag carrier comprises at least one functional group, which functional group is adapted to react with the drug molecule.

The skilled technician will appreciate the types of functional or reactive groups, which would react with the drag molecule. For example, the at least one functional group may be an oxygen group, a carbonyl group, a hydroxyl group or a carboxylic acid group, which is present on the second amino acid residue of the peptide, i.e. on the C-terminal residue if the drag carrier is a dipeptide.

The amino acids in the peptide in the drag carrier may already comprise a functional group for attaching a drag molecule thereto. For example, the amino acids serine, and tyrosine have a hydroxyl functional group to which a drag molecule may be attached; and the amino acids glutamic acid and aspartic acid have a carboxylic acid functional group to which a drag may be attached. However, it will be appreciated that the peptide may comprise amino acids, which do not comprise a suitable functional group for attaching a drag thereto, and so it will be preferred to modify at least one amino acid with a functional group. Such modification may be carried out by the introduction of hydroxy, carboxy, thio or amino groups, either before or after formation of the dipeptide sub-structure.

It should be appreciated that the present invention does not extend to the selection of the drag itself. The inventor did not investigate the biological activity of any drug molecule being attached to the drag carrier according to the first or second aspect. The inventor tested a number of different drag analogue molecules or 'test' molecules (and these are illustrated in Figure 10) to investigate the efficacy of binding said analogue molecules to the drag carrier according to the first or second aspect, to thereby form the conjugate. To this end, the inventor determined Ki values of each drag conjugate synthesised, as described in Example 3. The inventor also tested the transportation of the drug conjugate via the PepTl pathway using efflux experiments, as described in Example 4. These drug analogue molecules did not have any biological activity, as they were merely test molecules.

However, these drug analogue molecules or 'test' molecules were specifically chosen by the inventor to resemble biologically active drug molecules.

Therefore, it will be appreciated that it would be preferred to attach a biologically active drug molecule to the drug carrier according to the first or second aspect. Hence, by way of example only, a biologically active drug molecule which would be suitable for attachment to the peptide according to the first or second aspect, or the drug conjugate according to the third aspect, include those possessing alcohol, thiol, acid or amino groups, for which subsequent hydrolysis would release the desired active drug. The advantages of conjugation include increased oral absorption of drugs with for example low solubility, or high polarity, control of their release (e.g. longer lasting analogues, or delayed release), and selective absorption by cells that express PepTl/PepT2 proteins (e.g. in the lung).

Examples of drugs that would be suitable for attachment to the peptide drug carrier, and for which the conjugates that were synthesised and tested were chosen as models, include the following (for which "c/ conjugate B/C", indicates the corresponding class of model compounds from Figure 8 or Sty- Antibiotics: norfloxacin, ciprofloxacin, ofloxacin (cf conjugates B) Anticancer drugs: methotrexate (cf conjugates B) taxol (c/conjugates C) Antihistamines: cetirizine (c/conjugates B) fexofenadine (c/conjugates B or C) terfenadine (c/conjugates C) Antihypertensives: valsartan, captopril (c/conjugates B) losartan (c/conjugates C) Antiinflammatories: ibuprofen and related analogues (c/conjugates B) prostaglandins and thromboxanes (c/conjugates C)

Antimalarials: quinine, and analogues such as mefloquine (c/conjugates C)

Antivirals: AZT, lamivudine, acyclovir (c/conjugates C) Beta blockers: epinephrine, terbutaline, propranolol (c/conjugates C)

Bronchodilators: adrenaline, salbutamol (cf conjugates C)

Cholersterol lowering agents: nicotinic acid (B26), acipimox (cf conjugates B) compactin ((/conjugates C) CNS drugs: adrenaline, apomorphine ((/conjugates C)

Sedatives: oxazepam, lorazepam, temazepam ((/conjugates C)

Steroids: analogues of estradiol, testosterone, cortisone ((/conjugates C)

Preferably, the drug comprises at least one functional group with which the functional group of the drug carrier may react. As with above, the skilled technician will appreciate the types of functional or reactive groups, which would react with the drug carrier. For example, the functional group on the drug molecule may comprise a carboxylic acid group or a hydroxyl group. Preferably, attachment of the drug occurs at residue 1 or 2 of the drug carrier of the first or second aspect is by means of an ester linkage.

It is envisaged that attachment of the drug to the drug carrier according to the first or second aspect may be by means of an ester linkage, or an ether linkage, or an amide linkage.

Changes to any of R¹, R², R³, R⁴, R^A, and/or R⁵, in the Formulae given herein may make it possible to modify the drug carrier in accordance with the invention so that any adverse features on the drug may be minimised by the nature of the first residue of the peptide. Preferably, R¹ on the first amino acid residue (N-terminal) is suitable for such modification. For example, it may be beneficial to modify the net charge of the drug carrier according to the first or second aspect, such that the conjugate is pharmaceutically acceptable for use in medicine. For example, if the drug being attached to the drug carrier is acidic, then it may be advantageous to neutralise the net charge of the resultant drug carrier-drug conjugate according to the third aspect by using a basic drug carrier. Hence, the net charge of the drug carrier may be modulated by selection and/or modification of any of R¹, R², R³, R⁴, R^Λ, and/or R⁵ groups. Preferably, the net charge may be modulated by selection and/or modification of the R¹ group. It is also envisaged that it could be possible to selectively detach the drug molecule from the conjugate, by designing the drug carrier so that the linkage between the drug carrier and drug can be broken when the conjugate reaches its target environment or position in vivo. For example, it is possible to design the peptide drug carrier with appropriate amino acid residues such that the linkage with the drug can be broken when the conjugate is present in an acidic environment, for example, an area of wound tissue, which may have a low pH. Alternatively, some enzymes may only be expressed or be fully functional in certain tissues, and the peptide-drug bond may be digested by such enzymes, when the conjugate reaches that particular tissue(s).

Accordingly, the drug carrier according to the first or second aspect may be capable of being released or detached from the drug molecule.

The drug carrier according to the invention may be in the form of an L-isomer.

Preferably, the or each amino acid is an L-isomer. Advantageously, use of an L- isomer improves the binding between the PepTl protein and the drug carrier. However, it will be appreciated that the drug carrier may comprise at least one D- isomer amino acid residue.

It will be appreciated that the drug conjugate is adapted to bind to PepTl (or PepT2) carrier protein such that they have sufficient affinity for each other, wherein the binding therebetween is sufficiently strong such that the conjugate remains bound to the carrier protein during transportation using the PepTl pathway, i.e. while being transported across the small intestine. In addition, the binding is sufficiently weak such that the conjugate can be detached from the carrier protein when it has been transported across the small intestine.

Accordingly, the drug conjugate may be adapted to bind with PepTl protein or PepT2 protein with a Ki of between approximately 0.01-1OmM, and preferably, between approximately 0.05mM-5mM. It is preferred that the drug conjugate is adapted to bind with PepTl protein or PepT2 protein with a Ki of between approximately 0.1-3mM, more preferably, between approximately 0.2-ImM. Preferably, the drug conjugate is adapted to bind with PepTl protein or PepT2 protein with a Ki less than 5 mM, more preferably, less than 4 mM, even more preferably, less than 3 mM, and even more preferably, less than 2 mM. It is most preferred that the Ki of binding is less than 1 mM.

According to a fourth aspect of the invention, there is provided a conjugate according to the third aspect, for use as a medicament.

According to a fifth aspect there is provided use of the conjugate according to a third aspect for the preparation of an orally administrable medicament.

Preferably, the medicament or conjugate is orally administered to an individual. The medicament or conjugate may be adapted to be transported into the bloodstream via a PepTl/T2 pathway.

It will be appreciated that the medicament may be used to treat a wide variety of disease conditions, which will be determined by the nature of the drug attached to the peptide. Examples of suitable drugs which may be carried by the peptide are given above. Hence, by way of example only, the medicament may be used to treat cancer, allergic reactions, hypertension, inflammation, malaria, viral infection, bronchial infections, e.g. bronchitis.

It will be appreciated that the conjugate according to the third aspect of the present invention may be used in a monotherapy (i.e. use of the compound or derivatives thereof according to the invention alone). Alternatively, the conjugate according to the invention may be used as an adjunct, or in combination with, known therapies.

The conjugate according to the invention may be combined in compositions having a number of different forms depending, in particular on the manner in which the composition is to be used. Thus, for example, the composition may be in the form of a powder, tablet, capsule, liquid, gel, hydrogel, aerosol, spray, micelle, liposome or any other suitable form that may be administered to a person or animal. It will be appreciated that the vehicle of the composition of the invention should be one which is well tolerated by the subject to whom it is given, and preferably enables oral delivery of the compound.

Compositions comprising the conjugate according to the invention may be used in a number of ways. For instance, systemic administration is preferred, in which case the conjugate may be contained within a composition that is preferably ingested orally in the form of a tablet, capsule or liquid. Alternatively, it is possible that the composition may be administered by inhalation (e.g. intranasally). In some circumstances, the composition may be administered by injection into the blood stream. Injections may be intravenous (bolus or infusion) or subcutaneous (bolus or infusion).

The conjugate may also be incorporated within a slow or delayed release device. Such devices may, for example, be ingested and retained in the gut, and the conjugate may be released over weeks or even months. Such devices may be particularly advantageous when long-term treatment with the conjugate according to the invention is required and which would normally require frequent administration (e.g. at least daily injection).

Due to the 1:1 stoichiometry of the drug:conjugate, it will be appreciated that the amount of conjugate, and therefore drug, required in the conjugate according to the present invention, is determined by its biological activity and bioavailability, which in turn depends on the mode of administration, the physicochemical properties of the drug employed, and whether the drug is being used as a monotherapy or in a combined therapy. The frequency of administration will also be influenced by the above-mentioned factors and particularly the half-life of the conjugate within the subject being treated.

Optimal dosages to be administered may be determined by those skilled in the art, and will vary with the particular conjugate/drug in use, the strength of the preparation, the mode of administration, and the advancement of the disease condition. Additional factors depending on the particular subject being treated will result in a need to adjust dosages, including subject age, weight, gender, diet, and time of administration.

Known procedures, such as those conventionally employed by the pharmaceutical industry (e.g. in vivo experimentation, clinical trials, etc.), may be used to establish specific formulations of the compound according to the invention, and precise therapeutic regimes (such as daily doses and the frequency of administration).

Generally, a daily dose of between 0.01 μg/kg of body weight and 1.0 g/kg of body weight of the conjugate according to the invention may be used for the prevention and/or treatment of the specific medical condition. More preferably, the daily dose is between 0.01 mg/kg of body weight and 100 mg/kg of body weight. Daily doses may be given as a single administration (e.g. a single daily tablet). Alternatively, the conjugate may require administration twice or more times during a day. As an example, the conjugate according to the invention may be administered as two (or more depending upon the severity of the condition) daily doses of between 25 mgs and 5000 mgs. A patient receiving treatment may take a first dose upon waking and then a second dose in the evening (if on a two dose regime) or at 3 or 4 hourly intervals thereafter. Alternatively, a slow release device may be used to provide optimal doses to a patient without the need to administer repeated doses.

This invention provides a pharmaceutical composition comprising a therapeutically effective amount of a drug and the compound or drug carrier according to the present invention. In one embodiment, the amount of the conjugate is an amount from about 0.01 mg to about 800 mg. In another embodiment, the amount of the conjugate is an amount from about 0.01 mg to about 500 mg. In another embodiment, the amount of the conjugate is an amount from about 0.01 mg to about 250 mg. In another embodiment, the amount of the conjugate is an amount from about 0.1 mg to about 60 mg. In another embodiment, the amount of the conjugate is an amount from about 0.1 mg to about 20 mg. This invention provides a process for making a pharmaceutical composition, the process comprising combining a therapeutically effective amount of a drug, the compound or drug carrier according to the present invention, and a pharmaceutically acceptable vehicle. A "therapeutically effective amount" is any amount of a drug which, when administered to a subject provides prevention and/or treatment of a specific medical condition. A "subject" is a vertebrate, mammal, domestic animal or human being.

A "pharmaceutically acceptable vehicle" as referred to herein is any physiological vehicle known to those of ordinary skill in the art useful in formulating pharmaceutical compositions. Preferably, the pharmaceutically acceptable vehicle is adapted for oral administration. In a preferred embodiment, the pharmaceutical vehicle is a liquid and the pharmaceutical composition is in the form of a solution. In another embodiment, the pharmaceutically acceptable vehicle is a solid and the composition is in the form of a powder or tablet. In a further embodiment, the pharmaceutical vehicle is a gel and the composition is in the form of a cream or the like.

A solid vehicle can include one or more substances, which may also act as flavouring agents, lubricants, solubilisers, suspending agents, fillers, glidants, compression aids, binders or tablet-disintegrating agents; it can also be an encapsulating material, hi powders, the vehicle is a finely divided solid that is in a mixture with the finely divided active drug. In tablets, the drug is mixed with a vehicle having the necessary compression properties in suitable proportions and compacted in the shape and size desired. The powders and tablets preferably contain up to 99% of the active drug. Suitable solid vehicles include, for example, calcium phosphate, magnesium stearate, talc, sugars, lactose, dextrin, starch, gelatin, cellulose, polyvinylpyrrolidine, low melting waxes and ion exchange resins.

Liquid vehicles are used in preparing solutions, suspensions, emulsions, syrups, elixirs and pressurized compositions. The active drug can be dissolved or suspended in a pharmaceutically acceptable liquid vehicle such as water, an organic solvent, a mixture of both or pharmaceutically acceptable oils or fats. The liquid vehicle can contain other suitable pharmaceutical additives such as solubilisers, emulsifiers, buffers, preservatives, sweeteners, flavouring agents, suspending agents, thickening agents, colours, viscosity regulators, stabilizers or osmo-regulators. Suitable examples of liquid vehicles for oral and parenteral administration include water (partially containing additives as above, e.g. cellulose derivatives, preferably sodium carboxymethyl cellulose solution), alcohols (including monohydric alcohols and polyhydric alcohols, e.g. glycols) and their derivatives, and oils (e.g. fractionated coconut oil and arachis oil). For parenteral administration, the vehicle can also be an oily ester such as ethyl oleate and isopropyl myristate. Sterile liquid vehicles are useful in sterile liquid form compositions for parenteral administration. The liquid vehicle for pressurized compositions can be halogenated hydrocarbon or other pharmaceutically acceptable propellent.

In some cases, where it is desired to inject the conjugate, liquid pharmaceutical compositions which are sterile solutions or suspensions can be utilized by for example, intramuscular, intrathecal, epidural, intraperitoneal, intravenous and particularly subcutaneous, intracerebral or intracerebro ventricular injection. The drug may be prepared as a sterile solid composition that may be dissolved or suspended at the time of administration using sterile water, saline, or other appropriate sterile injectable medium. Vehicles are intended to include necessary and inert binders, suspending agents, lubricants, flavourants, sweeteners, preservatives, dyes, and coatings.

The conjugant according to the invention may be administered orally in the form of a sterile solution or suspension containing other solutes or suspending agents

(for example, enough saline or glucose to make the solution isotonic), bile salts, acacia, gelatin, sorbitan monoleate, polysorbate 80 (oleate esters of sorbitol and its anhydrides copolymerized with ethylene oxide) and the like. Preferably, the conjugate according to the invention is administered orally either in liquid or solid composition form. Compositions suitable for oral administration include solid forms, such as pills, capsules, granules, tablets, and powders, and liquid forms, such as solutions, syrups, elixirs, and suspensions. Forms useful for parenteral administration include sterile solutions, emulsions, and suspensions. The drag carrier according to the first or second aspect of the invention may be synthesised using common known chemical synthesis techniques. An example method for synthesising the drug carrier according to the invention is disclosed in Figures 3 and 4, and is discussed in the Example.

It will be appreciated by the skilled technician that there are many ways that the drag carrier according to the invention could be made. It will be appreciated that small changes to any of the steps of the synthesis disclosed herein may be made while still benefiting from the invention.

According to a sixth aspect, there is provided a method of treating an individual, the method comprising administering to an individual in need of such treatment, a drag conjugate according to the third aspect.

Preferably, the drag conjugate comprises a drag molecule attached to a drag carrier according to the first or second aspect.

All of the features described herein (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined with any of the above aspects in any combination, except combinations where at least some of such features and/or steps are mutually exclusive.

For a better understanding of the invention, and to show how embodiments of the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which:-

Figure 1 represents a Table showing variations in peptide conjugates that may be tested in accordance with the invention; Figure 2 summarises how a drag carrier according to the first or second aspect of the invention relates to natural dipeptide substrates for PepTl;

Figure 3 shows a schematic diagram for the synthesis of a D-analogue peptide drag carrier according to the invention,; Figure 4 shows a schematic diagram for the synthesis of an N-alkyl peptide drug carrier according to the invention;

Figure 5 shows a schematic diagram for the synthesis of a drug conjugate in accordance with the third aspect of the present invention; Figure 6 shows further variations in peptide conjugates that may be tested in accordance with the invention;

Figure 7 shows the types of peptide conjugates that may be tested in accordance with the invention;

Figure 8 shows D-analogues of peptide conjugates that may be tested in accordance with the invention and for which the corresponding thiopeptides have been reported in the Applicant's co-pending International Patent Application No. PCT/GB2005/000151;

Figure 9 shows JV-alkyl analogues of peptide conjugates that may be tested in accordance with the invention and for which the corresponding thiopeptides have been reported in the Applicant's co-pending International Patent Application No. PCT/GB2005/000151;

Figure 10 the specific D- and iV-alkyl analogues that were synthesised, being representative examples of the types A-I presented in Figures 7-9; and

Figure 11 shows a table summarising the binding data (Ki values), and efflux transport data, for the conjugates.

Example 1 - Making peptide compounds according to the invention

The aim of the present invention was to make a drug carrier compound, which enables the preparation of drug-drug carrier conjugates, which would allow the oral administration of drugs that are not currently administrable by mouth. In addition, such drug-drug carrier conjugates could also be used to improve the efficacy of drugs which are currently administered orally, but which show decreased levels of biological activity when administered by mouth. In order to achieve this aim, the following steps were carried out:- a) A range of peptide drug carrier analogues were prepared; b) Methods for synthesising the drug carriers were developed, both to facilitate the rapid synthesis of a wide range of analogues, and also to enable the preparation of the drug carrier cheaply and on a large scale; c) Bio-assays were developed to test the efficacy of the drug carrier analogues; d) A transport bioassay was developed that works quickly, and with small amounts of the drug substrate.

1) Preparation of a range of peptide carrier molecules

The inventor based his work on a 3D substrate model of a substrate, shown as structure A, for the PepTl protein, which is shown in Figure 2. While attempting to refine the proposed PepTl substrate model shown in Figure 2, a peptide analogue drug carrier in which the first amino acid residue was a D-isomer as shown as structure B in Figure 2 was prepared. In addition, the inventor produced a peptide analogue drug carrier in which the nitrogen atom of the peptide bond was alkylated (shown in Figure 2 as being methylated) as shown as structure C in Figure 2.

The inventor was surprised to discover that Structures B and C both exhibit three main advantageous features, namely (i) they bind efficiently to PepTl (Ki

0.3mM, typical natural substrates, in vitro assay); (ii) they are rapidly transported in vivo; and (iii) they are resistant to hydrolysis, unlike most peptides. Hence, the peptides shown as structures B and C in Figure 2 were believed to be potential carriers for drugs that do not naturally diffuse across the villi of the small intestine, or that have poor solubility, provided that a method of attaching a drug to compound B or compound C, could be devised.

Structure B (the D-isomer analogue) was modified by the substitution of a CO₂Y group, on to the methyl group to form structure D. The Y-group is provided for the attachment a drug compound. Structure C (the N-alkyl analogue) was also modified by the substitution of a CO₂Y group, on to the methyl group to form structure E. The Y-group is provided for the attachment a drug compound. Structures D and E are also shown in column 1 of Figure 1.

Accordingly, serine, aspartate or glutamate analogues of compounds D and E as shown in Figure 2 were made, in which Y is hydroxy or carboxylic acid group, connected to the drug via an ester linkage, thereby forming a drug conjugate. 2) Preparation of a protected peptide D-analogue drug carrier

A method was then devised for preparing a set of D-Xaa-L-Xaa carriers (where D-Xaa denotes that the amino acid at position one of the dipeptide may be any amino acid and is a D-isomer analogue, and where L-Xaa denotes that the amino acid at position two of the dipeptide may be any amino acid and is an L-isomer analogue) that are suitably protected for the attachment of drugs that possess a carboxylic acid or alcohol reactive group. The method of synthesising a protected peptide drug carrier compound is summarized in Figure 3. The particular strategy and choice of protecting groups were designed to enable the preparation of large amounts of drug carrier molecules, to which a drug molecule could then be attached using standard methodology (as described in J.H. Jones - "The Chemical Synthesis of Peptides", 1991, Oxford University Press.) via an ester bond, prior to deprotection of the N-C termini). Suitable protection groups include t-butyl, benzyl, methoxybenzyl, allyl or fluorenyl attached to nitrogen, oxygen or sulphur via ester, ether, thioether, or carbamate linkages.

The final step before attachment of a drug to the carrier molecule, is the deprotection of the functional group on the carrier to which the drug molecule is to be attached. This has been achieved by the addition of Na/NH₃ (liquid), i.e. sodium/liquid ammonia (or similar one electron reducing agents, or H₂ZPd-C), for the removal of benzyl-type protection, thereby freeing up the functional group ready for attachment of the drag.

Following attachment of the drag to the drag carrier in accordance with the present invention, the conjugate was then globally deprotected. This method has enabled the preparation of a large range of D-analogue drag-carrier conjugates very quickly.

3) Preparation of a protected peptide N-alkyl drug carrier A method was devised for preparing a set of L-Xaa-iV-methyl-L-Xaa carriers that are suitably protected for the attachment of drags that possess either a carboxylic acid or alcohol reactive groups. L-Xaa denotes that the amino acid at position one of the dipeptide may be any amino acid and is an L-isomer analogue, L-Xaa denotes that the amino acid at position two of the dipeptide may be any amino acid and is an L- isomer analogue, and the nitrogen atom in the peptide bond joining the two amino acid residues together is alkylated with a methyl group.

The method of synthesizing a protected dipeptide drug carrier compound is summarized in Figure 4. The key steps in the synthetic route involve the formation of an Fmoc-iV-protected oxazolidinone amino acid (with suitable protection), followed by reduction to give the required N-methylated protected amino acid. The oxazolidinone was formed by treatment with 6 equivalents of paraformaldehyde and a catalytic amount of pTSA in refluxing toluene. Reduction was then achieved by the addition of 3 equivalents of triethylsilane to a solution of the oxazolidinone in TFA/ CHCl₃. The particular strategy and choice of protecting groups was designed to enable the preparation of large amounts of drug carrier molecules, to which a drug molecule could then be attached (e.g. using standard methodology via an ester bond, prior to deprotection of the Ν/C-termini). Suitable protecting groups include t-butyl, benzyl, methoxybenzyl, allyl or fluorenyl attached to nitrogen, oxygen or sulphur via ester, ether, thioether, or carbamate linkages.

As with (2) described above, the final step before attachment of a drug to the carrier molecule, is the deprotection of the functional group on the carrier to which the drug molecule is to be attached. This has been achieved by the addition of NaME₃ (liquid), i.e. sodium/liquid ammonia (or similar one electron reducing agents, or H₂/Pd-C), for the removal of benzyl-type protection, thereby freeing up the functional group ready for attachment of the drug.

Following attachment of the drug to the drug carrier/compound in accordance with the present invention, the conjugate was then globally deprotected. This method has enabled the preparation of a large range of N-alkyl drug-carrier conjugates very quickly. 4) Preparation of a drug conjugate

Referring to Figure 5, there is shown a schematic illustration showing the mechanism for a successful attachment of a range of carboxylic acids, which may act as 'test' drug molecules, to the D-analogue drug carrier peptide produced in (2) above. Although not illustrated, it will be appreciated that it is equally possible using the same mechanism shown in Figure 5 to attach a range of 'test' drug molecules to the N-alkyl drug carrier peptide produced in (3) above. Following attachment of the carboxylic acid 'test' molecule to the drug carrier, the conjugate may then be deprotected so that the efficacy of transportation could be investigated.

Referring to Figure 6, there are shown six drug carrier-test molecule compounds prepared by the inventor. The first four compounds are D-analogue dipeptide drag carriers attached to a 'test' drug compound of O-benzyl:-

Compound 1 shown in Figure 6 is a dipeptide D-AlaAsp(OBn), in which the first amino acid residue is a D-isomer of alanine, and the second amino acid residue is an L-isomer of aspartate (O-benzyl is attached to the aspartate residue);

Compound 2 shown in Figure 6 is a dipeptide D-ValAsp(OBn), in which the first amino acid residue is a D-isomer of valine, and the second amino acid residue is an L-isomer of aspartate (O-benzyl is attached to the aspartate residue);

Compound 3 shown in Figure 6 is a dipeptide D-PheAsp(OBn), in which the first amino acid residue is a D-isomer of phenylalanine, and the second amino acid residue is an L-isomer of aspartate (O-benzyl is attached to the aspartate residue); and Compound 4 shown in Figure 6 is a dipeptide D-AlaSer(OBn), in which the first amino acid residue is a D-isomer of alanine, and the second amino acid residue is an L-isomer of serine (O-benzyl is attached to the serine residue).

The last two compounds shown in Figure 6 are N-alkyl dipeptide drag carriers attached to a 'test' drag compound of O-benzyl:- Compound 5 shown in Figure 6 is a dipeptide Ala(N-Me)Asp(OBn), in which the first amino acid residue is an L-isomer of alanine, and the second amino acid residue is an L-isomer of aspartate, and the nitrogen atom of the peptide bond attaching the two residues together has been alkylated (O-benzyl is attached to the aspartate residue); and

Compound 6 shown in Figure 6 is a dipeptide Ala(N-Me)Ser(OBn) in which the first amino acid residue is an L-isomer of alanine, and the second amino acid residue is an L-isomer of serine, and the nitrogen atom of the peptide bond attaching the two residues together has been alkylated (O-benzyl is attached to the serine residue).

Drug conjugates 1-6 shown in Figure 6 have been demonstrated to all bind well to PepTl (see Example 3 below). In addition, actual transportation of some analogues by the PepTl pathway has also been shown (see Example 3 below).

The inventor went on to synthesise a large number of peptide-drug analogue conjugates, which were tested for binding to PepTl, and also transportation by the PepTl pathway (efflux transport). With reference to Figure 7, structure 1 is the normal dipeptide substrate for PepTl. Structure 2 is a thiodipeptide in which the carbonyl group is substituted for a thiocarbonyl group (to confer hydrolysis resistance to the 'peptide' bond) and the amino acid side chain is designated 'R', potential spacers 'S', and the drug 'D'. Compounds of structure 2 form the basis of the Applicant's co-pending International Patent Application No. PCT/GB2005/000151. Structure 3 is an example of a conjugate in accordance with an embodiment of the present invention in which the peptide bond is protected from hydrolysis by alkylation of the nitrogen atom of the peptide bond and 'R', 'S' and 'D' are as in structure 2. Structure 4 is another embodiment of the present invention in which the peptide bond is protected from hydrolysis by replacement of the natural L-amino acid with a D- amino acid, and 'R', 'S' and 'D' are as in structure 2. The dipeptide carriers of the present invention are separated for convenience only in to nine classes (A-I) as shown in Figure 7 and as described more fully below.

The conjugates shown in Figures 7 to 10 are divided in to nine groups, denoted by classes A-I:- Group A (1 A-7A) represents a dipeptide (either a D-isomer analogue as shown in Figure 8, or an N-alkylated analogue as shown in Figure 9) comprising a phenylmethyl group attached to an oxygen atom on the second amino acid residue of a dipeptide carrier, in which the R¹ group on the first amino acid residue is varied as shown by numbers 1-7 in Figures 8 and 9;

Group B (8B-30B) represents a dipeptide (either a D-isomer analogue as shown in Figure 8, or an N-alkylated analogue as shown in Figure 9) comprising alanine as the first amino acid residue, and serine as the second amino acid residue, to which various R² groups (i.e. the drug analogues) are bound; Group C (31C-37C) represents a dipeptide (either a D-isomer analogue as shown in Figure 8, or an N-alkylated analogue as shown in Figure 9) comprising alanine as the first amino acid residue, and aspartate (rather than serine) as the second

¹X amino acid residue, to which various R groups (i.e. the drug analogues) are bound;

Group D (38D-40D) represents a dipeptide (either a D-isomer analogue as shown in Figure 8, or an N-alkylated analogue as shown in Figure 9) comprising alanine as the first amino acid residue, and serine as the second amino acid residue, in which the R group is varied as shown;

Group E (41E) represents a dipeptide (either a D-isomer analogue as shown in Figure 8, or an N-alkylated analogue as shown in Figure 9) comprising alanine as the first amino acid residue, and glutamate (rather than the shorter aspartate) as the second amino acid residue, bound to the drug analogue as shown;

Group F (42F & 43F) represents a dipeptide (either a D-isomer analogue as shown in Figure 8, or an N-alkylated analogue as shown in Figure 9) comprising alanine as the first amino acid residue, and serine as the second amino acid residue, to which various R⁵ groups (i.e. the drug analogues) are bound (via effectively a succinyl spacer);

Group G (45G-47G) represents a dipeptide (either a D-isomer analogue as shown in Figure 8, or an N-alkylated analogue as shown in Figure 9) comprising alanine as the first amino acid residue, and aspartate as the second amino acid residue, in which the length of the polyethylene glycol spacer chain [-CH₂-O-CH₂-]_n is varied from n = 0, 1, or 2 as shown, and are linked to a drug analogue;

Group H (48H & 49H) represents a dipeptide (either a D-isomer analogue as shown in Figure 8, or an N-alkylated analogue as shown in Figure 9) comprising alanine as the first amino acid residue, and serine as the second amino acid residue, in which the length of the alkylene spacer chain [-CH₂-] is varied from n = 9 or 15, and are linked to a drug analogue; and

Group I (501-521) represents a dipeptide (either a D-isomer analogue as shown in Figure 9, or an N-alkylated analogue as shown in Figure 9) comprising alanine as the first amino acid residue, and serine as the second amino acid residue, in which the length of the alkylene spacer chain [-CH₂-]_n is varied from n = 8, 12, or 14, and are linked to a drug analogue.

Referring to Figure 11, there is shown a table summarising the data for each conjugate shown in Figure 10. Hence, the table shows Ki binding data for each conjugate synthesised, and also efflux transport data for a large proportion of the conjugates synthesised.

Example 2 - Binding of peptide-drug conjugates to PepTl protein

Uptake of the radiolabeled dipeptide [³H]-D-Phe-L-Gln (17.4 Ci/mmole, custom synthesised, Cambridge Research Biochemicals, Stockton-on-Tees, UK) in competition with the compounds to be tested, was performed as previously described (Meredith et a!. J. Physiol, 1998, 512, 629-634). Briefly, 5 oocytes were incubated at room temperature in lOOμL of uptake medium (95mM NaCl, 2mM KCl, ImM CaCl₂, 0.42mM MgCl₂, 1OmM Tris/Mes pH 5.5) with tracer (0.4μM) [³H]-D-Phe-L-Gln with or without a test compound at the appropriate concentration. After incubation for one hour, the oocytes were washed sequentially five times in ImL of ice-cold uptake medium, then lysed individually with lOOμL 2% (w/v) SDS in vials, and liquid scintillation counted.

Quantitative binding data has been produced for a large number of dipeptide carriers with attached drug surrogates using the methodology described in Meredith et al., 1998, discussed supra, and is shown in Figure 11. Each of the peptide carrier molecules shown is in accordance with the present invention, with compounds 1, 2, 3, & 5 shown in Figure 6 comprising an aspartate amino acid side chain group in the second amino acid residue, and compounds 4 & 6 comprising a serine amino acid side chain group on the second amino acid residue. As shown in Figure 11, the Ki binding values for these test carrier drug conjugates range from between 0.12 mM to 1.37 mM. Accordingly, the binding data for each conjugate according to the invention illustrates that they are very good substrates for PepTl carrier protein.

Example 3 - Transport of peptides using an Efflux Assay

Efflux experiments were performed as described in Temple et al., J. Biol. Chem., 1998, 273, 20-22. Briefly, PepTl -expressing oocytes were micro-injected with 4.6nl of [³H]-D-Phe-L-Gln (17.4 Ci/mmole) and allowed to recover for 15 minutes. 5 oocytes were incubated in lOOμL uptake solution (pH 5.5) containing a potential substrate (i.e. a dipeptide-drug analogue conjugate) at 1OmM, with a negative control (i.e. uptake solution pH 5.5) and a positive control (i.e. 2OmM dipeptide GIy-Gm in pH 5.5 uptake solution). After 90 minutes incubation at room termperature, 50μL of the medium was removed and placed in a scintillation vial for counting. The oocytes were washed sequentially five times in ImL of ice-cold uptake medium, then lysed individually with lOOμL 2% (w/v) SDS in vials, and liquid scintillation counted.

Efflux of the radiolabeled dipeptide is a positive indication of active transport via PepTl. However, it should be appreciated that negative efflux results do not necessarily means that substrates are not transported via PepTl, as it is dependent on the Ki value of a specific conjugate to PepTl protein.

In the assays carried out, the dipeptide Gly-Gln (which is known to be transported by PepTl) caused about 70 % of labelled D-Phe-L-Gln to efflux from oocytes. At the same concentration, the test substrates (the conjugates being tested) were assessed, and their effective transport expressed as a percentage of the efflux caused by Gly-Gln.

A summary of the efflux transport data carried out is shown in the table in Figure 11. It can be seen that the following conjugates were transported as well as the control peptide Gly-Gln: entry 4, ie. the dipeptide D-AlaSer(OBn); and entry 6, ie. dipeptide Ala(N-Me)Ser(OBn). In both cases, only 43% of labelled control remained. In addition, the following conjugates were transported (although less rapidly than the control peptide under experimental conditions): entry 2, i.e. the dipeptide D- ValAsp(OBn); and entry 3, i.e. the dipeptide D-PheAsρ(OBn). For entry 2, only 57% of the labelled control remained, and for entry 3 only 65% of the labelled control remained. Accordingly, these conjugates are still considered to be transported well. Furthermore, it will be appreciated that even though transportation for these conjugates is less than the control, transportation using the PepTl pathway has still been demonstrated, and that the peptides still exhibit transport efficacy.

D. Meredith and C.A.R. (Cell MoI. Life Sci., 2000; 57:754-78) suggests that that there is extensive correlation between binding of the drug conjugate in oocytes with transport of the drug conjugate in oocytes in vitro, and furthermore, that there is extensive correlation with transport in vitro with transport in the gut of rats in vivo. Hence, the inventor is convinced that the in vitro binding and transport data illustrated in Figure 9 is a good model for illustrating binding and transport in vivo.

In addition to these aforementioned peptide analogues, additional analogues were prepared as shown in Figures 1 and 6. It should be noted that two of the modifications (structures D and E in column 1, and all of column 4 of Figure 1) relate to the drug carrier compound itself, and therefore necessitate changes early in the synthesis. These latter two modifications are important from a drug delivery standpoint, because changes to the side-chain of the first amino acid residue in the peptide allows the 'fine-tuning' of the drug delivery system so that any adverse features on the drug are off-set by the nature of amino acid residue 1; for example, if there were preferred overall neutrality of the drug-drug carrier conjugate at pH 5.5. Changing the second amino acid residue to aspartate enables the attachment of drugs possessing a hydroxyl function, which leads to a massive increase in the potential number of drugs that might be suitable for attachment to the carrier.

Conclusion

In conclusion, the experiments and data described herein illustrate the efficacy of the peptide drug carrier in accordance with the invention, and how drug-carrier conjugates may be transported across membranes in vivo using the PepTl protein. In particular, the inventor believes that modifying the peptide such that it is hydrolysis resistant (either by use of a D-isomer amino acid residue or by N-alkylation) is particularly advantageous for use in carrying drugs via the PepTl/PepT2 pathway. The drag analogues used, as shown in Figures 7 to 10, had strong similarities with the structure of biologically active drag molecules. Hence, the inventor designed and synthesised a large number of peptide-drug analogue conjugates. The Ki values of with binding to PepTl protein were then determined, as shown in Figure 11. The results indicate that they would act as good substrates to PepTl/PepT2. In addition, the inventor has tested the transportation of a large number of the conjugates using efflux studies, as shown in Figure 11. These studies show that the conjugates are successfully transported by the PepTl pathway.

Claims

1. A drug carrier comprising a peptide, or derivative or analogue thereof, the peptide comprising at least two amino acid residues linked by a peptide bond, the amino acid residues being arranged such that the peptide bond is protected from hydrolysis, and the peptide comprising a functional group for attachment to a drug, wherein the functional group to which the drug is attached is provided on the second or C-terminal residue of the peptide.

2. A drug carrier according to claim 1, wherein a nitrogen atom in the peptide bond of the peptide is alkylated.

3. A drug carrier according to claim 2, wherein each nitrogen atom in the or each peptide bond in the peptide is alkylated.

4. A drug carrier according to claim 2 or 3, wherein the drug carrier has a Formula III:-

H₂N - CR¹R² - CO - NR^A - CR⁴R⁵ - COX

Formula III

wherein R¹, R², R⁴, and R⁵ are independently selected from a group consisting of a hydrogen; a linear alkyl group; a branched alkyl group; a dialkyl group; an N- alkyl group; an alkoxy group; and a side chain group of an amino acid residue and R^A is independently selected from a group consisting of a linear alkyl group; a branched alkyl group; and a derivatised alkyl group.

5. A drug carrier according to claim 2 or 3, wherein the drug carrier has a Formula IV:-

H₂N - CHR¹ - CO - NR^A - CHR⁴ - COOH Formula IV

wherein R¹, and R⁴ are independently selected from a group consisting of a hydrogen; a linear alkyl group; a branched alkyl group; a dialkyl group; an N-alkyl group; an alkoxy group; and a side chain group of an amino acid residue, and R is independently selected from a group consisting of a linear alkyl group; a branched alkyl group; and a derivatised alkyl group.

6. A drug carrier according to claim 4 or 5, wherein R^A comprises an alkyl chain.

7. A drug carrier according to claim 6, wherein the alkyl group comprises a C₁- C₂₀ chain.

8. A drug carrier according to claim 6 or 7, wherein the alkyl group comprises a methyl, ethyl, propyl, butyl, or a pentyl chain.

9. A drug carrier according to claim 4 or 5, wherein R^A is a methyl group.

10. A drug carrier according to claim 1, wherein the peptide comprises at least one D-amino acid residue (D-isomer).

11. A drug carrier according to claim 10, wherein the D-amino acid residue is the first or N-terminal residue.

12. A drug carrier according to claim 10 or 11, wherein at least one amino acid residue linked by each peptide bond in the peptide is a D-isomer.

13. A drug carrier according to claim 10, 11 or 12, wherein the drug carrier has a Formula I:-

H₂N - CR¹R² - CO - NR³ - CR⁴R⁵ - COX

Formula I wherein R¹, R², R³, R⁴, and R⁵ are independently selected from a group consisting of a hydrogen; a linear alkyl group; a branched alkyl group; a dialkyl group; an N-alkyl group; an alkoxy group; and a side chain group of an amino acid residue; and, X is independently selected from a group consisting of hydroxyl group; an amino acid residue; an amide; an amide link to a further amino acid residue; and a peptide.

14. A drug carrier according to claim 10, 11 or 12, wherein the drug carrier has a Formula IL-

H₂N - CHR¹ - CO - NH - CHR⁴ - COOH

Formula II

wherein R¹, and R⁴ are independently selected from a group consisting of a hydrogen; a linear alkyl group; a branched alkyl group; a dialkyl group; an N-alkyl group; an alkoxy group; and a side chain group of an amino acid residue.

15. A drug carrier according to any one of claims 2 to 14, wherein the drug carrier is adapted to, or is capable of being transported by a PepTl or a PepT2 protein.

16. A drug carrier according to claim 1, wherein the drug carrier is adapted to, or is capable of being transported by a PepTl or a PepT2 protein.

17. A drug carrier according to claim 1 or 16, wherein the peptide comprises a serine, aspartate or glutamate residue as the second or C-terminal residue.

18. A drug carrier according to claim 1, 16 or 17, wherein the functional group for attachment to the drug comprises a hydrolysable ester, ether or amide group.

19. A drug carrier comprising a peptide, or derivative or analogue thereof, the peptide comprising at least two amino acid residues linked by a peptide bond, wherein a nitrogen atom of the peptide bond is alkylated, and wherein at least one amino acid residue is a D-isomer, the peptide comprising a functional group for attachment to a drug, the functional group being provided on a second amino acid residue of the peptide.

20. A drug carrier according to claim 19, wherein the drug carrier is adapted to, or is capable of being transported by a PepTl or a PepT2 protein.

21. A drug carrier according to claim 19 or 20, wherein the peptide comprises a serine, aspartate or glutamate residue as the second or C-terminal residue.

22. A drug carrier according to claim 19, 20 or 21, wherein the functional group for attachment to the drug comprises a hydrolysable ester, ether or amide group.

23. A drug carrier according to any one of claims 4 to 14, wherein R⁴ comprises an alcohol or a carboxylic acid group.

24. A drug carrier according to any one of claims 4 to 14 and 23, wherein R⁴ comprises a side chain group of an amino acid residue.

25. A drug carrier according to claim 24, wherein the amino acid side chain group is independently selected from a group consisting of serine; threonine; glutamic acid; aspartic acid; and tyrosine.

26. A drug carrier according to any one of claims 4 to 14 and 23 to 25, wherein R⁴ comprises spacing means, which spacing means is adapted to distance the drug away from the peptide when bound thereto.

27. A drug carrier according to claim 26, wherein the spacing means comprises an alkyl chain, or an alkyl chain incorporating ether, amino, ester, amide and/or carbonyl groups, with appropriate functionalisation at its termini for attachment to the peptide compound, and the drug molecule.

28. A drug carrier according to claim 26 or 27, wherein the spacing means comprises [-CH₂-J_n, wherein the value of n is an integer of at least 1.

29. A drug carrier according to claim 26 or 27, wherein the spacing means comprises [-CH₂-O-CH₂-]_n, wherein n is an integer of at least 1.

30. A drug carrier according to any one of claims 4 to 14 and 23 to 29, wherein R¹ comprises a side chain group of any amino acid residue.

31. A drug carrier according to claim 30, wherein the amino acid side chain group of R¹ is independently selected from a group consisting of (i) H (glycine); (ii) Me (alanine); (iii) CH₂Ph (phenylalanine); (iv) CHMe₂ (valine); (v) CH₂OH (serine); (vi) CH₂SH (cysteine); (vii) CH₂CO₂H (aspartate); (viii) CH₂COMI₂ (asparagine); and (ix) (CH₂)₄NH₂ (lysine).

32. A drug conjugate comprising a drug, which drug is linked to a drug carrier according to any one of the preceding claims.

33. A drug conjugate according to claim 32, wherein the drug carrier is capable of being released or detached from the drug molecule.

34. A drug conjugate according to claim 32 or 33, for use as a medicament.

35. Use of the drug conjugate according to any one of claims 32 to 34 for the preparation of an orally administrable medicament.

36. A method of treating an individual, the method comprising administering to an individual in need of such treatment a conjugate according to any of claims 32 to 35.

37. A method of treating an individual according to claim 36, wherein the drug conjugate comprises a drug molecule attached to a drug carrier according to any one of claims 1 to 31.