WO1999033558A1

WO1999033558A1 - Method for producing an aqueous colloidal dispersion of nanoparticles

Info

Publication number: WO1999033558A1
Application number: PCT/EP1997/007308
Authority: WO
Inventors: David Quintanar-Guerrero; Eric Allemann; Robert Gurny; Hatem Fessi; Eric Doelker
Original assignee: Universite De Geneve
Priority date: 1997-12-29
Filing date: 1997-12-29
Publication date: 1999-07-08
Also published as: AU5860798A

Abstract

The method for producing an aqueous colloidal dispersion of nanoparticles comprises a first step which consists to emulsify a partially water-soluble organic solvent, containing a water-insoluble polymer in a weight/volume percentage at which nanoparticles are formed in a second step, in an aqueous solution of a stabilizing agent, using a low energy source, and a second step which consists to distillate the organic solvent from the oil-in-water emulsion formed in the first step, to cause the formation of nanoparticles, in suspension in the aqueous phase.

Description

METHOD FOR PRODUCING AN AQUEOUS COLLOIDAL DISPERSION OF

NANOPARTICLES

The invention pertains to the area of dispersions in a liquid phase of water-insoluble materials, and more particularly the invention relates to a method for producing an aqueous colloidal dispersion Of nanoparticles, using the emulsification-diffusion technique.

Aqueous colloidal dispersions of nanoparticles, and in particular pseudolatexes which are aqueous colloidal dispersions containing nanoparticles of water insoluble preformed polymers, are currently offered for use as aqueous coating means or as pharmaceutical vectors.

At present, various techniques are known for preparing aqueous colloidal dispersions containing nanoparticles, and in particular pseudolatexes. These techniques include emulsification-evaporation, nanoprecipitation, salting-out and emulsification-diffusion, which have in common that they involve the use of an organic solution, containing the nanoparticle components, which functions as an internal phase during preparation, and of an aqueous solution, containing stabilizers which will constitute the dispersion medium for the nanoparticles.

The emulsification-evaporation technique is a well- established technique based on the classical procedure disclosed in US-A-4.177.177 (Vanderhoff) . In this technique, a polymer solution in a water- immiscible organic solvent such as chloroform or methylene chloride is emulsified in an aqueous phase containing emulsifiers . This crude emulsion is then submitted to a high energy mixing step using a high energy source such as ultrasounds, homogenizers, high pressure dispersers, colloid mills or microfluidizers, in order to reduce the droplet size. The polymer emulsion resulting from such a treatment is very fine and contains very small droplets (below 0.5 μm in diameter) . This emulsification is followed by the removal of the solvent, by vacuum distillation, producing a fine aqueous dispersion of nanospheres . In this emulsification- evaporation method, each emulsion droplet will form one polymer particle when the solvent is removed. Consequently, the homogenization step is the determining factor in obtaining submicronic particles.

In order to avoid the problem of the homogenization step, which requires high energy, other techniques have been developped.

The nanoprecipitation technique was first disclosed in EP-A-0.274.961. In this method, polymer, drug and, optionally, a lipophilic stabilizer (e.g., phospholipids) are dissolved in a semipolar water-miscible solvent, such as acetone or ethanol . This solution is poured or injected into an aqueous solution containing a stabilizer (e.g., poly (vinyl alcohol) (PVAL) or poloxamer 188) under magnetic stirring. Nanoparticles are formed instantaneously by the rapid diffusion of the solvent, which is then eliminated from the suspension under reduced pressure.

The usefulness of this simple technique is limited to water-miscible solvents in which the diffusion rate is sufficient to produce spontaneous emulsification . Also, this technique can be used only for drugs soluble in this type of solvents. A major drawback with this technique is the difficulty to choose a drug/polymer/solvent/non-solvent system in which non-aggregated nanoparticles would be formed and the drug efficiently entrapped.

The salting-out technique was first disclosed in the International Patent Application WO 88/08011. This technique is based on the separation of a totally water- miscible solvent, in particular acetone, from aqueous solutions via a salting-out effect. Typically, the polymer and the drug are dissolved in acetone and this solution is emulsified under vigorous mechanical stirring in an aqueous gel containing the salting-out agent and a colloidal stabilizer. This oil-in-water emulsion is diluted with a sufficient volume of water or of aqueous solution, in order to enhance the diffusion of acetone into the aqueous phase, thus inducing the formation of nanospheres .

The utility of this technique is however generally limited to drugs soluble in water-miscible solvents, in particular acetone-soluble drugs, to salting-out agents that enable phase separation without precipitation and to soluble stabilizers which are compatible with saturated aqueous solutions and which do not coacervate or precipitate in the presence of the solvent. A major drawback is the use of a high quantity of salt which gives to the aqueous phase a fixed pH and which must be eliminated in a subsequent purification step. Another drawback is that it is necessary to remove the solvent and a considerable amount of water to obtain a high polymer concentration in the final dispersion. Moreover, the acetone is mixed in the water which renders recycling of acetone problematic.

The more recent technique for preparing nanoparticles is the emulsification-diffusion technique proposed in

Eur. J. Pharm. Biopharm, 41 14-18 (1995) . The method involves the emulsification of a partially water-soluble (partially water-miscible) solvent, previously saturated with water and containing a polymer, in an aqueous phase previously saturated with the solvent and containing a stabilizer. The subsequent addition of water to the system causes the solvent to diffuse into the external phase, resulting in the aggregation of polymer in nanoparticles.

This method is of interest from a technological standpoint, since it does not need comminuting forces as in the emulsification-evaporation technique does, it is highly efficient, reproducible and easy to scale-up.

However, as in the salting-out technique, it is necessary to remove the solvent and a considerable amount of water to obtain a high polymer concentration in the final dispersion. It is an object of the present invention to provide a method for producing an aqueous colloidal dispersion containing a high concentration of nanoparticles which is not affected by one or more of the above drawbacks of the known methods, and, in particular, which does not require comminuting forces and which does not require the removing of toxic solvent, of stabilizer, of salting-out agent and/or of a considerable amount of water.

The object of the present invention is achieved by a method for producing an aqueous colloidal dispersion of nanoparticles, characterized in that it comprises: a) the emulsification of a partially water-soluble organic solvent, containing a water-insoluble material in a weight/volume percentage at which nanoparticles are formed in step b) , in an aqueous solution of a stabilizing agent, using a low energy source; b) the distillation of the organic solvent from the oil-in-water emulsion formed in step a) to cause the formation of nanoparticles in suspension in the aqueous phase .

Thanks to the present invention, there is provided a method effective for producing aqueous colloidal dispersions containing a high concentration of nanoparticles, which is based on the emulsification-diffusion technique, which does not require the use of high energy source for homogenization, which does not require the removing of a considerable amount of water, in which pharmaceutically acceptable organic solvents may be used with a possibility of solvent reuse, in which pharmaceutically acceptable stabilizers may be used, and which is simple to implement, easy to scale up, of a low cost and reproducible.

An advantage of the aqueous colloidal dispersions having a high concentration of nanoparticles obtained by the method of the present invention is that they can be used directly for coatings or as pharmaceutical vectors without additional purification step.

The method of the present invention can be advantageously applied for producing aqueous colloidal dispersions of nanoparticles with ingredients entrapped therein, when the partially water-soluble organic solvent further contains additional ingredients.

It is to be noted that in the present description and claims, the terms "partially water-soluble solvent" or

"partially water-miscible solvent" mean a solvent at least sparingly soluble in water in the sense of the European

Pharmacopoeia .

Also, it is to be noted that in the present specification and claims, the term "nanoparticles" means particles having a mean particle size not greater than 1 μm.

We will now describe the present invention in a more detailed manner.

To obtain an aqueous colloidal dispersion of nanoparticles according to the present invention, the first steps are to prepare the organic phase, also named internal phase, and the aqueous phase, also named external phase.

The organic phase is prepared by dissolving the selected water- insoluble material, and optionally additional ingredients, in a partially water-soluble organic solvent.

The aqueous phase is prepared separately by dissolving the selected stabilizing agent in water.

In a preferred embodiment of the present invention, the partially water-soluble organic solvent is previously added with a certain amount of water extending up to saturation and/or the water is previously added with a certain amount of the partially water-soluble organic solvent extending up to saturation, and in a particularly preferred embodiment of the invention, the partially water soluble organic solvent is previously saturated with water and the water is previously saturated with the partially water-soluble organic solvent in order to ensure initial thermodynamic equilibrium in the subsequent step.

The water-insoluble material may be, for example, a polymer, a lipid, a wax and the like.

In a preferred embodiment, the water-insoluble material is a polymer, and in a particularly preferred embodiment, the water-insoluble polymer is selected from biodegradable polymers such as such as poly (D, L-lactic acid) (PLA) and poly ( -caprolactone) (PCL) and non-biodegradable polymers such as Eudragit Q^y E, cellulose acetate phthalate (CAP) , cellulose acetate trimellitate (CAT) , and ethylene vinyl acetate copolymer (EVAC) .

The partially water-soluble organic solvent should be selected on the basis of its volatility and low toxicity, in particular when a pharmaceutical application for the resulting aqueous colloidal dispersion is considered.

Examples of partially water-soluble organic solvents particularly preferred are ethyl acetate (AcEt) and 2-butanone, because of their widely recognized low toxicity, good solubilizing properties and low boiling points .

The additional ingredients may be any ingredients which can be entrapped in the nanoparticles, such as for example drugs, cosmetics, products for veterinary and agricultural use, food products or additives, colorants or any other ingredients which can be useful when they are entrapped in nanoparticles.

Stabilizing agents particularly preferred for the emulsification and for stabilizing the final dispersion are poly (vinyl alcohol) (PVAL) and poloxamer 407 because of their good water solubility, suitability for ingestion and compatibility with the system.

The key restriction is that the stabilizing agent should allow the formation of stable emulsions with the partially water-soluble solvent and that it should prevent coalescence during solvent displacement.

In a particularly preferred embodiment of the present invention, the stabilizing agent is contained in the aqueous phase in an amount of not more than 5 % w/v.

The above-mentioned aqueous phase is then mixed with the organic solution of the material, using a low energy source such as, for example, a propeller, a magnetic stirrer, a shaker and the like, to produce, when the addition is finished, an emulsion of the oil-in-water type.

When using a propeller, a particularly advantageous stirring rate for the propeller is about 1500 rpm.

The emulsification step is advantageously carried out at room temperature, but other temperatures may be used.

In accordance with the invention, distillation of the organic solvent from the oil-in-water emulsion is then carried out, to cause the displacement of the partially water-soluble solvent of the internal phase into the external phase and, consequently, to cause the formation of the particles in suspension in the aqueous phase.

In a particularly preferred embodiment of the present invention, the distillation is a vacuum distillation.

In the method of the present invention, the formation of nanoparticles is highly dependent on the water-insoluble material concentration in the internal phase and a transition from nano- to microparticles is observed at high water-insoluble material concentration. For this reason, the amount of water-insoluble material in the organic phase is critical for achieving the mean particle size desired for the final particles.

Thus, in the method of the present invention, the weight/volume percentage of the material in the organic solvent should be not greater than the critical weight/volume percentage at which the particles formed during distillation have a mean particle size of 1 μm.

This critical percentage varies depending on the material/solvent/stabilizer system and on the stirring rate of the propeller, in view of the known effect of an increased stirring rate on decreasing the particle size.

The critical percentage at which the particles formed during distillation have a mean particle size of 1 μm is easily obtained by an optimization for each material/solvent/stabilizer system at a specific stirring rate.

To illustrate the effect of the concentration of the water- insoluble material in the organic solvent on particle size, in the present invention, we will now explain the present invention, referring to Figures related to an example of a non-biodegradable polymer particularly preferred of the present invention, named Eudragit (S) E-100 (obtained from Rδhm GmbH, Darmstadt, Germany), which however is not yet commercially available as a colloidal dispersion, and to an example of another non-biodegradable polymer, named cellulose acetate phthalate (CAP) .

Figure 1 represents the influence of the percentage of Eudragit E in the internal phase (ethyl acetate) on the mean particle size, when the external phase contains

5 % w/v of PVAL as stabilizer and the rate of stirring is 1500 rpm. Figure 2 represents scanning electron micrographs (at x 4500) of Eudragit E particles, prepared at different concentrations of Eudragit E in the internal phase, namely: a) 10 % w/v in chloroform (Comparative Example hereafter) ; b) 40 % w/v in ethyl acetate, c) 30 % w/v in ethyl acetate; d) 20 % w/v in ethyl acetate (Example 3 hereafter) ; e) 10 % w/v in ethyl acetate (Example 1 hereafter) .

Figure 3 represents the influence of the percentage of cellulose acetate phthalate in the internal phase

(2-butanone) on mean particle size when the external phase contains 5 % w/v of poloxamer 407 and the stirring rate is

1500 rpm.

The particle sizes were determined using a Coulter ®

Nanosizer (Coulter Electronics, Harpenden (UK) ) .

For the scanning electron microscopy (SE) , a concentrated dispersion of nanoparticles was finely spread over a slab and dried under vacuum. The sample was shadowed in a cathodic evaporator with a gold layer (^ 20 nm thick) . The surface morphology of the nanoparticles was observed by SEM using a JSM-6400 scanning electron microscope (JEOL, Tokyo, Japan) .

It is shown in Figure 1 that, for an Eudragit ® E/ethyl acetate/PVAL system, when the PVAL content of the external phase is 5.00 % and the stirring rate of the propeller is 1500 rpm, the critical weight/volume percentage of Eudragit ® E to ethyl acetate to obtain a mean particle size of 1000 nm is about 28 % w/v. However, in slightly different experimental conditions, this percentage varies.

The decrease of the size of the particles in relation with the diminution of the concentration of Eudragit Qy E in ethyl acetate is clearly apparent from Fig. 2 b) to Fig. 2 e) . Also, it is clear from Fig. 2 e) that only nanoparticles are obtained when Eudragit £) E is contained in a partially water-soluble solvent such as ethyl acetate at 10 % w/v or less .

Conversely, in the Comparative Example when Eudragit SΕ is contained in a non water-miscible solvent such as chloroform at a 10 % w/v, only microparticles are obtained, as shown in Fig. 2 a) .

Another non-biodegradable polymer preferred for the present invention is cellulose acetate phthalate (CAP) .

It is shown in Figure 3 an example of a

CAP/2 -butanone/poloxamer system when the poloxamer 407 content of the external phase is 5.00 % w/v and the stirring rate of the propeller is about 1500 rpm. In this case, the critical weight/volume percentage of CAP to 2-butanone to obtain a mean particle size of 1000 nm is about 38 % w/v. However, in slightly different experimental conditions, this percentage varies.

As indicated above, it is pointed out that in the present invention, the water-insoluble materials are not limited to water- insoluble polymers.

The Examples below will illustrate the method of the present invention without limiting its scope in any way.

EXAMPLES Materials

Water insoluble polymers used in the examples were Eudragit ® E (obtained from Rohm (GmbH Darmstadt, Germany) ; poly(_<£- caprolactone) (PCL) ((Tone ©767) obtained from Union Carbide (Danbury, USA) ) ; ethylene vinyl acetate copolymer (EVAC, vinyl acetate content 40 %) (obtained from Fluka (Buchs, Switzerland)); poly (D, L-lactic acid) (PLA) ( (Medisorb ® obtained by Medisorb (Cincinatti, OH, USA)); cellulose acetate phthalate (CAP) (obtained from Fluka (Buchs, Switzerland)); and cellulose acetate trimellitate (CAT) (obtained from Eastman (Kingsport, USA));

Stabilizing agents used in the present examples were poly(vinyl alcohol) (PVAL) (Mw 26 000) (Mowiol ® 4-88 , Hoechst, Frankfurt -am-Main, Germany) and poloxamer 407 (Pluronic ® F-127, BASF, Ludwigshafen, Germany).

The partially water-soluble solvents used in the present examples were ethyl acetate (EtAc) CZ,²⁰ = 1.372; water solubility = 10 mg/ml) and 2-butanone ("l²⁰ = 1.378; water solubility = 275 mg/ml) , of HPLC and of analytical grade, respectively (Fluka) .

Distilled water used in the present examples was purified using a Milli-Q system (millipore, USA-Bedford, MO) .

All other chemicals were of analytical grade and used without further purification.

Particle size analysis

The average particle size and polydispersity index (scale from 0 to 9) were determined using a Coulter U^y Nanosizer (Coulter Electronics, Harpenden (UK) . The measurements were made in triplicate for all the batches prepared.

Example 1

Ethyl acetate and water were mutually saturated for 1 min before use, in order to ensure initial thermodynamic equilibrium of both liquids. Typically, 4 g of Eudragit E were dissolved in 40 ml of water-saturated ethyl acetate and this organic solution (internal phase) was emulsified at room temperature with 80 ml of a 5 % w/v PVAL ethyl acetate saturated aqueous solution (external phase) , using a propeller stirrer (Heidolph-Elektro, KG type E-60, propeller: IKA 1381, Germany) at 1500 rpm for ten minutes. The oil-in-water emulsion formed was subjected to vacuum distillation at 35°C and 60 mmHg until complete solvent evaporation. Generally, the solvent was recovered and used to prepare new batches. The mean particle size of the particles obtained by this method as well as the polydispersity are indicated in Table 1 below.

Example 2

The method was repeated in the same manner as in Example 1, except that 1.5 % w/v of PVAL was used in the external phase, instead of 5.0 % w/v of PVAL. The mean particle size of the particles obtained by this method and the polydispersity are indicated in Table 1 below.

Example 3

The method was repeated in the same manner as in Example 1, except that 8 g of Eudragit E was used instead of 4 g of Eudragit E. The mean particle size of the particles obtained by this method and the polydispersity are indicated in Table 1 below.

Example 4 The method was repeated in the same manner as in Example 1, except that PCL was used, instead of Eudragit E. The mean particle size of the particles obtained by this method and the polydispersity are indicated in Table 1 below.

Example 5

The method was repeated in the same manner as in Example 1, except that EVAC was used, instead of Eudragit E. The mean particle size of the particles obtained by this method and the polydispersity are indicated in Table 1 below.

Example 6

The method was repeated in the same manner as in Example 1, except that PLA was used, instead of Eudragit E. The mean particle size of the particles obtained by this method and the polydispersity are indicated in Table 1 below.

Example 7

2-butanone and water were mutually saturated for 1 min before use, in order to ensure initial thermodynamic equilibrium of both liquids. Typically, 4 g of CAP were dissolved in 40 ml of water-saturated 2-butanone and this organic solution (internal phase) was emulsified at room temperature with 80 ml of a 5 % w/v poloxamer 407 2-butanone saturated aqueous solution (external phase) , using a propeller (Heidolph-Elektro, KG type E-60, propeller: IKA 1381, Germany) at 1500 rpm for ten minutes. The oil-in-water emulsion formed was subjected to vacuum distillation at 35°C and 60 mmHg until complete solvent evaporation. Generally, the solvent was recovered and used to prepare new batches. The mean particle size of the particles obtained by this method as well as the polydispersity are indicated in Table 1 below.

Example 8 The method was repeated in the same manner as in Example 7, except that 1.25 % w/v of poloxamer 407 was used, instead of 5.00 % w/v of poloxamer 407. The mean particle size of the particles obtained by this method as well as the polydispersity are indicated in Table 1 below.

Example 9

The method was repeated in the same manner as in Example 7, except that 8 g of CAP was used, instead of 4 g of CAP. The mean particle size of the particles obtained by this method as well as the polydispersity are indicated in Table 1 below.

Example 10

The method was repeated in the same manner as in Example 7, except that 12 g of CAP was used, instead of 4 g of CAP. The mean particle size of the particles obtained by this method as well as the polydispersity are indicated in Table

1 below.

Example 11 The method was repeated in the same manner as in Example 7, except that CAT was used, instead of CAP. The mean size of the particles obtained by this method as well as the polydispersity are indicated in Table 1 below. 99/33558

14

Comparative Example

The method was repeated in the same manner as in Example 1 , except that chloroform was used instead of ethyl acetate.

The mean particle size of the particles obtained was

5006 nm and the polydispersity was 5.

TABLE 1

Examples of nanoparticles obtained by the invention.

Internal phase 40 ml External phase 80 ml Stirring rate 1500 rpm

Ex . Polymer Solvent Stabilizer Mean size PcT No . ( %w/v) (% w/v) (nm)

1 Eudr .E (10) EtAc PVAL (5.00) 573 4

2 Eudr .E (10) EtAc PVAL (1.25) 590 4

3 Eudr .E (20) EtAc PVAL (5.00) 817 4

4 PCL (10) EtAc PVAL (5.00) 543 3

5 EVAC (10) EtAc PVAL (5.00) 435 5

6 PLA (10) EtAc PVAL (5.00) 472 2

7 CAP (10) 2-butanone Poloxamer 407 (5. .00) 260 2

8 CAP (10) 2-butanone Poloxamer 407 (1. .25) 308 4

9 CAP (20) 2-butanone Poloxamer 407 (5, .00) 614 5

10 CAP (30) 2-butanone Poloxamer 407 (5, .00) 770 5

11 CAT (10) 2-butanone Poloxamer 407 (5, .00) 811 7

Comparative Example

Eudr . E ( 10 ) CHC1- PVAL ( 5 . 00 ) 5006

*Pd : Polydispersity ( index expressed from 0 to 9 )

As shown in Table 1, nanoparticles were obtained with the polymers tested, indicating that the method of the present invention also can be advantageously applied to non- biodegradable polymers commonly used in pharmaceutical coating methods and to biodegradable polymers. The method of the present invention thus makes it possible to prepare aqueous colloidal dispersions containing high concentration of nanoparticles of water-insoluble material, for example pseudolatexes, from a conventional oil-in-water emulsion with an ordinary mechanical stirring without requiring homogenization, without requiring dilution with water, by direct displacement of a partially water-soluble solvent during distillation. Further, the dispersion obtained by the method of the present invention may be used directly for coatings without additional treatments.

Claims

1. A method for producing an aqueous colloidal dispersion of nanoparticles, characterized in that it comprises : a) the emulsification of a partially water-soluble organic solvent, containing a water-insoluble material in a weight/volume percentage at which nanoparticles are formed in step b) , in an aqueous solution of a stabilizing agent, using a low energy source; b) the distillation of the organic solvent from the oil-in-water emulsion formed in step a) to cause the formation of nanoparticles, in suspension in the aqueous phase .

2. A method for producing an aqueous colloidal dispersion of nanoparticles according to claim 1, characterized in that the partially water-soluble organic solvent is previously added with a certain amount of water extending up to the saturation and the water is previously added with a certain amount of the partially water-soluble organic solvent extending up to the saturation.

3. A method for producing an aqueous colloidal dispersion of nanoparticles according to claim 1 or 2 , characterized in that the partially water soluble organic solvent is previously saturated with water and/or the water is previously saturated with the partially water-soluble organic solven .

4. A method for producing an aqueous colloidal dispersion of nanoparticles according to any one of claims 1 to 3 , characterized in that the water-insoluble material is a polymer.

5. A method for producing an aqueous colloidal dispersion of nanoparticles according to claim 4, characterized in that the polymer is selected from biodegradable polymers such as such as poly (D, L-lactic acid) (PLA) and poly ( ┬ú -caprolactone) (PCL) and non-biodegradable polymers such as Eudragit ┬« E, cellulose acetate phthalate (CAP) , cellulose acetate trimellitate (CAT) , and ethylene vinyl acetate copolymer (EVAC) .

6. A method for producing an aqueous colloidal dispersion of nanoparticles according to any one of claims 1 to 5, characterized in that the partially water-soluble solvent is selected from ethyl acetate (EtAc) or 2-butanone .

7. A method for producing an aqueous colloidal dispersion of nanoparticles according to any one of claims 1 to 6, characterized in that the stabilizing agent is selected from poly (vinyl alcohol) and poloxamer 407.

8. A method for producing an aqueous colloidal dispersion of nanoparticles according to any one of claims 1 to 7, characterized in that the stabilizing agent is contained in an amount of not more than 5.00 % w/v in the aqueous solution.

9. A method for producing an aqueous colloidal dispersion of nanoparticles according to any one of claims

1 to 8, characterized in that the emulsification is made by using a propeller stirrer rotating at 1500 rpm.

10. A method for producing an aqueous colloidal dispersion of nanoparticle according to any one of claims 1 to 9, characterized in that the distillation of the solvent is a vacuum distillation.

11. A method for producing an aqueous colloidal dispersion of nanoparticles according to any one of claims 1 to 10, characterized in that the partially water-soluble organic solvent containing the water-insoluble material further contains one or more additional ingredients.