WO2005105093A2

WO2005105093A2 - METHOD OF ARRESTING THE SIDE EFFECTS OF µ-OPIOID RECEPTOR AGONISTS DURING THEIR ADMINISTRATION

Info

Publication number: WO2005105093A2
Application number: PCT/US2004/027669
Authority: WO
Inventors: Laura M. Bohn; Julia K. L. Walker; Robert J. Lefkowitz; Marc G. Caron
Original assignee: Duke University
Priority date: 2003-08-26
Filing date: 2004-08-25
Publication date: 2005-11-10
Also published as: WO2005105093A3

Abstract

The present invention provides methods for decreasing the side effects of µ opioid receptor agonists and methods for screening compounds for activity in reducing the side effects of µ opioid receptor agonists.

Description

METHOD OF ARRESTING THE SIDE EFFECTS OF μ-OPIOID RECEPTOR AGONISTS DURING THEIR ADMINISTRATION

This invention was made with Government support under Grant Nos. DA14600, NS19576, and HL16037 awarded by the National Institutes of Health. The Government has certain rights in the invention.

FIELD OF THE INVENTION The invention relates to methods for decreasing the side effects of μ opioid receptor agonists and to methods for screening compounds for activity in reducing the side effects of μ opioid receptor agonists.

BACKGROUND OF THE INVENTION G protein coupled receptors (GPCRs) have important roles in mediating fundamental physiological processes such as vision, olfaction, cardiovascular function, and pain perception. Cellular communication through GPCRs requires the coordination of processes governing receptor activation, desensitization, and resensitization. However, the relative contribution of desensitization mechanisms to the overall homeostatic process still remains largely unexplored in vivo. GPCR kinases (GRKs) act to phosphorylate activated receptors and promote their interaction with jSarrestins. This, in turn, prevents further coupling with G proteins and disrupts normal activation of the second messenger-signaling cascade. By this mechanism, GRKs and arrestins can act to reduce μ opioid receptor sensitivity to ligand activation and GPCR signaling, thereby leading to desensitization of the receptor (S. Ferguson, et al., Annu RevBiochem 67, 653 (1998)). At least seven GRKs (GRK 1-7) and four arrestins (visual and cone arrestin, jSarrestin-l and -2) have been discovered; however, the functional significance of such redundancy is largely unclear. Over expression or inactivation of certain GRKs leads to modulation of receptor responsiveness (W. Koch, et al, Science 268, 1350 (1995); H. Rockman et al, Proc NatlAcad Sci USA 93, 9954 (1996); D. Choi et al. JBiol Chem 272, 17223 (1997); G. Iaccarino et al., Am JPhysiol 275, H1298 (1998); K. Peppel, et al, JBiol Chem 272, 25425 (1997); H. Rockman, et al., JBiol Chem 273, 18180 (1998). J. Walker et al., Am JPhysiol 276, R1214 (1999); Gainetdinov et al. 1999 Neuron 24:1029-1036; and Gainetdinov et al. 2003 Neuron 38:291-303). In addition, mice that are deficient in /3arrestin-l display increased cardiac contractility in response to B-adrenergic receptor agonists (D. Conner et al., Circ Res 81, 1021 (1997)). In the absence of /5arrestin-2, the μ opioid receptor demonstrates higher levels of agonist- induced activation resulting in enhanced and prolonged analgesia to morphine in mice. (See US 6,528,271). Morphine and the other opioid analgesics are extremely effective. They bind to a series of opioid receptors including the μ opioid receptor, a GPCR. They also exert unwanted side effects such as constipation and more gravely, respiratory suppression. Clinicians often hesitate to use the drug because of the complications presented by these side effects. A universal regulatory mechanism desensitizes G-protein coupled receptors such as the μ opioid receptor after agonist stimulation. First, the activated receptors are phosphorylated by a GPCR kinase (GRK) and then the phosphorylated receptor binds a regulatory molecule arrestin. In the absence of 3arrestin-2, μ opioid receptor activation persists and reaches higher levels of agonist-induced activation and results in enhanced and prolonged analgesia in response to morphine in mice. Furthermore, following chronic administration of morphine, mice lacking /3arrestin-2 experience dramatically attenuated morphine tolerance. The mice do, however become physically dependent upon the morphine as indicated by the presence of naltrexone-precipitated withdrawal symptoms in such mice. Therefore, it appears that the major arrestin associated with desensitization of the μ opioid receptor is jSarrestin-2. It has not previously been clear, however, if the side effects of these analgesics are attenuated or potentiated by blocking desensitization of the μ opioid receptor. Since jSarrestin-2 knockout mice experience enhanced and prolonged antinociception with opioid analgesic use, the logical prediction was that effects such as constipation and respiratory suppression would also be enhanced. SUMMARY OF THE INVENTION A first aspect of the present invention is a method of reducing the side effects of a μ opioid receptor agonist analgesic in a patient being treated with the agonist comprising administering to said patient a composition that interferes with or reduces the desensitization of the μ opioid receptor. A second aspect of the invention is a method of treating a patient being treated with analgesic μ opioid receptor agonist comprising administering to said patient a composition which blocks the translocation of 3arrestin-2 to the receptor sufficiently to reduce the side effects of the μ opioid receptor agonist analgesic. A third aspect of the invention is a method of treating a patient being treated with an analgesic μ opioid receptor agonist comprising administering to said patient a composition which blocks the GRK phosphorylation of the μ opioid receptor sufficiently to reduce the side effects of the μ opioid receptor agonist analgesic. A particular aspect of the present invention is a method of screening a compound for activity in controlling the side affects of opioid analgesics. The method comprises determining whether or not the compound inhibits desensitization of the μ opioid receptor. The inhibition of such desensitization by the compound indicates the compound may be active in controlling opioid side effects. Any degree of inhibition may be examined, with greater inhibition of desensitization indicating potentially greater activity of the compound being tested. Further aspects of the present invention include compounds produced or identified by the methods described hereinabove and pharmaceutical formulations of the same, along with the use of such compounds for the preparation of a medicament for the treatment of the side effects of μ opioid receptor agonist analgesics such as morphine, and/or for the control of pain, in a subject in need thereof. The foregoing and other aspects of the present invention are explained in detail in the drawings herein and the specification set forth below.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 A represents a comparison of breathing frequency in wild type versus jδarrestin-2 knock out mice before and after administration of morphine. Figure IB represents a comparison of percent respiratory suppression in wild type versus /3arrestin-2 knock out mice after administration of various concentrations ofmo hine. Figure 2A represents a comparison of accumulated fecal boli in wild type mice and /3arrestin-2 knock out mice following treatment with saline or morphine. Figure 2B represents accumulated fecal boli in wild type and /3arrestin-2 knock out mice following treatment with differing morphine concentrations or following treatment with saline.

DETAILED DESCRIPTION OF THE INVENTION Pain perception (nociception) is mediated by a cascade of events from the point of the stimulus to integrative circuits in the brain. Nociception involves signals that are mediated by several classes of receptors and signal transduction mechanisms. Important GPCRs for nociception are the substance P receptor and the opiate receptor. Antinociception has been known for more than 1000 years to be induced by the alkaloid compound, morphine, which functions as an agonist at the μ opioid receptor. The activity of agonists for signaling through GPCRs is usually limited by cellular mechanisms that dampen the signal of the agonist, a process referred to as desensitization. These mechanisms include phosphorylation of agonist-activated receptors by specific receptor kinases called GRKs followed by the interaction of the phosphorylated GPCR with any of the members of the arrestin family of proteins. It has been discovered that reducing the desensitization of the μ opioid receptor, for example, by interfering with jSarrestin (e.g., jSarrestin-2) mediated desensitization, while leading to enhanced and prolonged antinociception, also surprisingly leads to a reduction in both the symptoms of constipation and respiratory depression associated with μ opioid receptor agonists. Prior to describing this invention in further detail, however, the following terms will first be defined. The term "arrestin " as used herein has its ordinary meaning in the art and is intended to encompass all types of arrestin, including but not limited to visual arrestin (sometimes referred to as Arrestin 1), jSarrestin 1 (sometimes referred to as Arrestin 2), and jSarrestin 2 (sometimes referred to as Arrestm 3) and cone arrestin (sometimes referred to as Arrestin 4). The term "jSarrestin" as used herein is intended to encompass all types of jSarrestin, including but not limited to jSarrestin 1 and jSarrestin 2. The phrases "concurrent administration," "administration in combination," "simultaneous administration," or "administered simultaneously" as used herein, interchangeably mean that the compounds are administered at the same point in time or immediately following one another. In the latter case, the two compounds are administered at times sufficiently close that the results observed are indistinguishable from those achieved when the compounds are administered at the same point in time. The production of jSarrestin 2 knockout mice can be carried out in view techniques known to those skilled in the art, such as described in U.S. Patents Nos. 5,767,337 to Roses et al.; 5,569,827 to Kessous-Elbaz et al; and 5,569,824 to Donehower et al.; and 6,528,271 and A. Harada et al., Nature 369, 488 (1994) (the disclosures of which incorporated by reference herein in their entirety).

Assay techniques The step of determining whether or not GRK is phosphorylating the μ opioid receptor or if jSarrestin (e.g., j8arrestin2) is binding to the phosphorylated μ opioid receptor or in general if the μ opioid receptor is being desensitized or not may be carried out by any suitable technique, including in vitro assay and in cellulo assay (e.g., in a cell that contains the jSarrestin 2 and the phosphorylated μ opioid receptor). A particularly suitable technique for in vivo assay is disclosed in U.S. Patent No. 5,891,646 to Barak et al. (the disclosure of which is to be incorporated by reference herein in its entirety). In general, this technique involves providing a cell that expresses μ opioid receptor as a G-protein coupled receptor, and contains a jSarrestin (e.g., 3arrestin 2) protein conjugated to an optically detectable molecule (e.g., green fluorescent protein). The test compound is then introduced into the cell (e.g., by microinjection, by electroporation, by suspending the cell in an aqueous solution that contains the test compound, by contacting the cell to liposomes that contain the test compound, by insertion of a heterologous nucleic acid into the cell that encodes and expresses the test compound, etc.). Upon agonist addition, translocation of the molecule from the cytosol of the cell to the activated receptor at the plasma membrane is then monitored or examined, with the inhibition of such translocation indicating that the test compound inhibits the binding of jSarrestin to the phosphorylated μ opioid receptor either directly or indirectly (for example by inhibiting the kinase activity of GRK). The cell is preferably a mammalian cell, but any suitable cell can be employed, including bacterial cells, yeast cells, fungal cells, plant cells, and other animal cells, so long as they express μ opioid receptor and phosphorylate, or can be induced to phosphorylate, the same, and contain the desired jSarrestin (e.g., iSarrestin 2) protein coupled to an optically detectable molecule (e.g., either by exogenous introduction or expression of the jSarrestin conjugate therein).

Test compounds The present invention can be used with test compounds (or "probe molecules"), or libraries (where groups of different probe molecules are employed), of any type. In general, such probe molecules are organic compounds, including but not limited to oligomers, non-oligomers, or combinations thereof. Non-oligomers include a wide variety of organic molecules, such as heterocyclics, aromatics, alicyclics, aliphatics and combinations thereof, comprising steroids, antibiotics, enzyme inhibitors, ligands, hormones, drugs, alkaloids, opioids, benzodiazepenes, terpenes, porphyrins, toxins, catalysts, as well as combinations thereof. Oligomers include peptides (that is, oligopeptides) and proteins, oligonucleotides (the term oligonucleotide also referred to simply as "nucleotide", herein) such as DNA and RNA, oligosaccharides, polylipids, polyesters, polyamides, polyurethanes, polyureas, polyethers, poly (phosphorus derivatives) such as phosphates, phosphonates, phosphoramides, phosphonamides, phosphites, phosphinamides, etc., poly (sulfur derivatives) such as sulfones, sulfonates, sulfites, sulfonamides, sulfenamides, etc., where for the phosphorous and sulfur derivatives the indicated heteroatom for the most part will be bonded to C, H, N, O or S, and combinations thereof. Numerous methods of synthesizing or applying such probe molecules on solid supports (where the probe molecule may be either covalently or non-covalently bound to the solid support) are known, and such probe molecules can be made in accordance with procedures known to those skilled in the art. See, e.g., U.S. Pat. No. 5,565,324 to Still et al, U.S. Pat. No. 5,284,514 to Ellman et al, U.S. Pat. No. 5,445,934 to Fodor et al. (the disclosures of all United States patents cited herein are to be incorporated herein by reference in their entirety).

Control of side effects and active compounds. As noted above, the present invention provides a method of controlling/reducing the side effects of μ opioid receptor agonists analgesics in a subject, comprising inhibiting jSarrestin (e.g., jSarrestin 2) binding to phosphorylated μ opioid receptor or other blocking of the desensitization of the μ opioid receptor in said subject in an amount effective to prevent the side effects of μ opioid receptor agonists in the subject. The method may be carried out with or without concurrently administering a μ opioid receptor agonist such as moφhine (or other opiate, as described below) as long as it is given sometime during the use of the agonist in the subject. i The inhibiting of jSarrestin (e.g., jSarrestin 2) binding phosphorylated μ opioid receptor can be carried out directly or indirectly by any suitable means, including but not limited to knockout of the jSarrestin (e.g., jSarrestin 2) gene, disabling or down regulating the kinase responsible for phosphorylation of the μ opioid receptor, administration of an antisense oligonucleotide that down regulates expression of the arrestin (e.g., jSarrestin 2), or the administration of an active compound that competitively inhibits binding of the jSarrestin (e.g., jSarrestin 2) to phosphorylated μ opioid receptor. Obviously, functional μ opioid receptor itself must remain in the cells (particularly nerve cells) of the subject so that the primary analgesic activity of the μ opioid receptor agonist can be exerted. Subjects that may be treated by the compounds identified by the present invention include both human subjects and animal subjects (e.g., dogs, cats, horses, cattle) for veterinary purposes. Thus, as noted above, further aspects of the present invention include active compounds produced or identified by the methods described hereinabove and pharmaceutical formulations of the same (e.g., said compound in a sterile pyrogen- free saline solution), along with the use of such compounds for the preparation of a medicament for the reduction of the side effects of μ opioid receptor agonists such as morphine, in a subject in need thereof, in combination with a μ opioid receptor agonist such as morphine. In addition to morphine, other μ opioid receptor agonists, typically opiates, that may be used in conjunction with the present invention include, but are not limited to, codeine, oxycodeine, hydromorphone, diamorphine, methadone, fentanyl, sufentanil, buprenorphine, meperidine (Demerol®), etc. The active compounds described above may be combined with a pharmaceutical carrier in accordance with known techniques to provide a pharmaceutical formulation useful carrying out the methods described above. See, e.g., Remington, The Science And Practice of Pharmacy (9h Ed, 1995). In the manufacture of a pharmaceutical formulation according to the invention, the active compound (including the physiologically acceptable salts thereof) is typically admixed with, inter alia, an acceptable carrier. The carrier must, of course, be acceptable in the sense of being compatible with any other ingredients in the formulation and must not be deleterious to the patient. The carrier may be a solid or a liquid, or both, and is preferably formulated with the compound as a unit-dose formulation, for example, a tablet, which may contain from 0.01 or 0.5% to 95% or 99% by weight of the active compound. One or more active compounds may be incorporated in the formulations of the invention, which may be prepared by any of the well-known techniques of pharmacy consisting essentially of admixing the components, optionally including one or more accessory ingredients. The formulations of the invention include those suitable for oral, rectal, buccal (e.g., sub-lingual), parenteral (e.g., subcutaneous, intramuscular, intradermal, or intravenous), and topical (i.e., both skin and mucosal surfaces) administration; the most suitable route in any given case will depend on the nature and severity of the condition being treated and on the nature of the particular active compound which is being used. Formulations of the present invention suitable for parenteral administration comprise sterile aqueous and non-aqueous injection solutions of the active compound, which preparations are preferably isotonic with the blood of the intended recipient. These preparations may contain anti-oxidants, buffers, bacteriostats and solutes that render the formulation isotonic with the blood of the intended recipient. Aqueous and non-aqueous sterile suspensions may include suspending agents and thickening agents. The formulations may be presented in uni λdose or multi-dose containers, for example sealed ampoules and vials, and may be stored in a freeze-dried (lyophilized) condition requiring only the addition of the sterile liquid carrier, for example, saline or water-for-injection immediately prior to use. Extemporaneous injection solutions and suspensions may be prepared from sterile powders, granules and tablets of the kind previously described. For example, there may be provided an injectable, stable, sterile composition comprising a compound, or a salt thereof, in a unit dosage form in a sealed container. The compound or salt is provided in the form of a lyophilizate that is capable of being reconstituted with a suitable pharmaceutically acceptable carrier to form a liquid composition suitable for injection thereof into a subject. The unit dosage form typically comprises from about 10 mg to about 10 grams of the compound or salt. When the compound or salt is substantially water-insoluble, a sufficient amount of emulsifying agent that is physiologically acceptable maybe employed in sufficient quantity to emulsify the compound or salt in an aqueous carrier. One such useful emulsifying agent is phosphatidyl choline. Formulations suitable for topical application to the skin preferably take the form of an ointment, cream, lotion, paste, gel, spray, aerosol, or oil. Carriers that may be used include petroleum jelly, lanoline, polyethylene glycols, alcohols, transdermal enhancers, and combinations of two or more thereof. Formulations suitable for transdermal administration may be presented as discrete patches adapted to remain in intimate contact with the epidermis of the recipient for a prolonged period of time. Formulations suitable for transdermal administration may also be delivered by iontophoresis (see, for example, Pharmaceutical Research 3(6):318 (1986)) and typically take the form of an optionally buffered aqueous solution of the active compound. Suitable formulations comprise citrate or bis\tris buffer (pH 6) or ethanol/water and contain from 0.1 to 0.2M active ingredient. Formulations suitable for oral administration may be presented in discrete units, such as capsules, cachets, lozenges, or tablets, each containing a predetermined amount of the active compound; as a powder or granules; as a solution or a suspension in an aqueous or non-aqueous liquid; or as an oil-in-water or water-in-oil emulsion. Such formulations may be prepared by any suitable method of pharmacy that includes the step of bringing into association the active compound and a suitable carrier (which may contain one or more accessory ingredients as noted above). In general, the formulations of the invention are prepared by uniformly and intimately admixing the active compound with a liquid or finely divided solid carrier, or both, and then, if necessary, shaping the resulting mixture. For example, a tablet may be prepared by compressing or molding a powder or granules containing the active compound, optionally with one or more accessory ingredients. Compressed tablets may be prepared by compressing, in a suitable machine, the compound in a free- flowing form, such as a powder or granules optionally mixed with a binder, lubricant, inert diluent, and/or surface active/dispersing agent(s). Molded tablets may be made by molding, in a suitable machine, the powdered compound moistened with an inert liquid binder. The present invention is explained in greater detail in the following non- limiting Examples.

EXAMPLES jSarrestin 2-Knockout (KO) mice and their wild-type (WT) littermate controls were analyzed following morphine or saline treatment for respiratory suppression and accumulation of fecal boli. A. Respiratory Suppression The respiratory rates of the jSarrestin 2-Knockout (KO) mice and their wild- type (WT) littermate controls were analyzed using whole body plethysmography following morphine treatment. The effects of morphine (50 mg/kg, s.c.) over 2 hours (Fig. 1 A) and the dose response curve accumulated over the 2-hour test period (Fig. IB) reveal marked differences between the genotypes. At each dose tested, the WT mice experienced the onset of respiratory suppression; however, the jSarrestin 2-KO mice did not. While it might have been expected that the jSarrestin 2-KO mice would experience greater respiratory suppression, they, in fact, were actually protected from this side effect of morphine. Saline produced no significant differences between the genotypes (Fig. IB). WT and jSarrestin 2-KO mice were treated with saline or morphine following a 30 min habituation period. Mice were only used once. Measurements were performed with a 12-chamber Buxco whole body plethysmograph and WT and KO mice were assessed simultaneously (PLY3211V2.1; Buxco Electronics, Sharon, CT). Breathing frequency was recorded electronically by computer software (BioSystem XA software; Buxco Electronics). Figure 1 A illustrates a comparison of the measurements of breathing frequency in the wild type and /3arrestin 2-knock out mice; measurements were taken over 2 hours and are presented as the average number of breath/min (after morphine, WT vs. KO PO.001, two-way ANOVA, n=5). The dose response data of Figure IB reflect % respiratory suppression in the wild type and jSarrestin 2-knock out mice based on the average number of breaths/min measured during a 2 h period following morphine treatment, as normalized by each mouse's breathing rate in the last 15 min of the habituation period (WT vs. KO *P<0.05, **P<0.01, ***P<0.0001, Student's t test; n=5-8). B. Accumulation of fecal boli An additional side effect associated with the use of opioid analgesics is the development of persistent constipation. Opioid receptors have also been shown to be critical in mediating this effect and the μ opioid receptor knockout mice no longer experience inhibition of gastrointestinal transit following morphine treatment. It was determined whether morphine-induced acute constipation would be enhanced or prolonged in the jSarrestin 2-KO mice. Morphine's effect on gastrointestinal function was assessed by measurement of fecal boli production over time wherein the boli were collected and weighed over a 6-hour period. Saline treatment resulted in a similar profile of fecal production in both genotypes (Fig. 2A). Morphine (20 mg/kg, s.c.) induced an initial suppression of defecation in both groups of mice; however, the jSarrestin 2-KO mice recover fully after 4 hours while the WT mice continue to experience the suppression throughout the test (Fig.2A) relative to the saline treatment. The dose response curve demonstrates that jSarrestin 2-KO mice display reduced defecation suppression compared to the WT until the dose is maximized to 50 mg/kg, s.c. (Fig. 2B). Mice were provided food and water ad libitum before the test period and both genotypes consumed comparable amounts of food as measured over several 24-hour periods. No food or water was available during the test. Mice were caged in a light- controlled, ventilated, sound-attenuating cages with grid floors. Fecal boli were collected from an aluminum tray under each mouse every hour following the injection of saline or morphine. Mice were only used once. Figure 2A illustrates the amount of feces accumulated (by weight) over time following saline or morphine (20 mg/kg, s.c.) in the wild type and jSarrestin 2-knock out mice (WT vs. KO saline: P>0.05; morphine: PO.001, two-way ANOVA; n=8-9). Figure 2B illustrates the total mass of feces produced over the entire 6-hour test period for saline or morphine (10, 20, 50 mg/kg, s.c.) treatment in the wild type and jSarrestin 2-knock out mice (WT vs. KO, **P<0.01, *P<0.05, Student's t test; n=6-9). C. Conclusion Morphine's effect on analgesia, gastrointestinal transit and respiration are mediated by μ opioid receptor in peripheral as well as central neuronal sites. Opioid receptors in these respective regions may be subject to different cellular complements of regulatory proteins and may hence show different sensitivities to the loss of jSarrestin 2 or work in concert with other neurotransmitter systems. These observations suggest that while the analgesic properties of morphine are enhanced in jSarrestin-2 knockout mice, the removal of j8arrestin-2 may actually be protective against morphine-induced constipation and respiratory suppression. Therefore, it is clear that a modulator of μ opioid receptor desensitization may prove to have therapeutic value in enhancing and prolonging the analgesic effects of morphine in the absence of antinociceptive tolerance, while at the same time, preventing constipation and respiratory suppression. The foregoing is illustrative of the present invention, and is not to be construed as limiting thereof. The invention is defined by the following claims, with equivalents of the claims to be included therein.

Claims

WHAT IS CLAIMED IS:

1. A method of screening a compound for activity in reducing side effects of a μ opioid receptor agonist analgesic, comprising: determining whether or not said compound inhibits jSarrestin binding to a phosphorylated μ opioid receptor; the inhibition of such binding by said compound indicating said compound may be active in reducing side effects of μ opioid receptor agonist analgesics.

2. The method of claim 1, wherein the jSarrestin is jSarrestin 2.

3. A method of screening a compound for activity in reducing side effects of a μ opioid receptor agonist analgesic, comprising: (a) providing a cell that expresses a μ opioid receptor and a G protein coupled receptor kinase, and that contains a conjugate comprising a jSarrestin and an optically detectable molecule; (b) exposing the cell to the compound; (c) exposing the cell to an agonist for the μ opioid receptor; and (d) monitoring for inhibition of jSarrestin binding to phosphorylated μ opioid receptor, the inhibition of such binding by said compound indicating said compound may be active in reducing side effects of μ opioid receptor agonist analgesics.

4. The method of claim 3, wherein the /3arrestin is jSarrestin 2.

5. The method of claim 3, wherein the μ opioid receptor agonist is selected from the group consisting of morphine, codeine, oxycodeine, hydromorphone, diamorphine, methadone, fentanyl, sufentanil, buprenorphine and meperidine.

6. The method of claim 3, wherein the μ opioid receptor agonist is morphine.

7. The method of claim 3, wherein the monitoring for inhibition of jSarrestin binding to phosphorylated μ opioid receptor comprises: (i) monitoring for translocation of the /3arrestin to the phosphorylated μ opioid receptor; (ii) monitoring for binding of the jSarrestin to the phosphorylated μ opioid receptor; or (iii) monitoring for inhibition of GRK phosphorylation of the μ opioid receptor.

8. A method of reducing the side effects of a μ opioid receptor agonist analgesic in a patient being treated with the agonist comprising administering to said patient a composition that interferes with or reduces the desensitization of the μ opioid receptor.

9. The method of claim 8, wherein the μ opioid receptor agonist is an opioid analgesic.

10. The method of claim 9, wherein the opioid analgesic is selected from the group consisting of morphine, codeine, oxycodeine, hydromorphone, diamorphine, methadone, fentanyl, sufentanil, buprenorphine and meperidine.

11. A method of treating a patient being treated with μ opioid receptor agonist analgesic comprising administering to said patient a composition which blocks the translocation of /3arrestin 2 in said patient sufficiently to reduce the side effects of the μ opioid receptor agonist.

12. The method of claim 11, wherein the μ opioid receptor agonist is an opioid analgesic.

13. The method of claim 12, wherein the opioid analgesic is selected from the group consisting of morphine, codeine, oxycodeine, hydromorphone, diamorphine, methadone, fentanyl, sufentanil, buprenorphine and meperidine.

14. A method of treating a patient being treated with an μ opioid receptor agonist analgesic comprising administering to said patient a composition which blocks GRK phosphorylation of μ opioid receptor in said patient sufficiently to reduce the side effects of the μ opioid receptor agonist.

15. The method of claim 14, wherein the μ opioid receptor agonist is an opioid analgesic.

16. The method of claim 15, wherein the opioid analgesic is selected from the group consisting of morphine, codeine, oxycodeine, hydromorphone, diamorphine, methadone, fentanyl, sufentanil, buprenorphine and meperidine.