MXPA00006270A - Kappa receptor opioid peptides - Google Patents

Abstract

Peptides which exhibit high selectivity for the kappa opioid receptor (KOR) and long duration of peripheral action without significant entry into the brain are created which are sequences of four D-isomer amino acid residues having a C-terminus which is a mono- or di-substituted amide. Representative compounds, which have an affinity for the KOR at least 1,000 times their affinity for the mu opioid receptor and an ED50 of not greater than about 0.5 mg/kg, include H-D-Phe-D-Phe-D-Nle-D-Arg-NHEt, H-D-Phe-D-Phe-D-Nle-D-Arg-morpholinyl, H-D-Phe-D-Phe- D-Nle-D-Arg-NH-4-picolyl, H-D-Phe-D-Phe- D-Nle-D-Arg-NHPr, H-D-Phe-D-Phe-D-Nle-D-Arg-thiomorpholinyl, H-D-Phe-D-Phe- D-Nle-D-Arg-NEt2, H-D-Phe-D-Phe- D-Nle-D-Arg-NHMe, H-D-Phe-D-Phe-D-Leu-D-Orn-morpholinyl, H-D-4Fpa-D-Phe-D-Nle-D-Arg-NH- 4-picolyl, H-D-Phe-D-Phe-D- Nle-D-Arg-NH-cyclopropyl, H-D-Ala- (2Thi)-D-3, 4Cpa-D-Leu-D-Arg-morpholinyl, H-D-Phe-D-Phe-D-Nle-D-Gmf-morpholinyl, H-D-Phe-D-Phe- D-Leu-D-Orn-NH(Aeb), H-D-Phe-D-Phe- D-Leu-D-Lys-morpholinyl, H-D-Phe-D- Phe-D-Nle-D-Arg-piperazinyl, and H-D-Phe-D- Phe-D-Nle-D-Arg-NH(Hoh).

Description

OPIOID PEPTIDES OF THE KAPPA RECEIVER Description The present invention relates generally to synthetic opioid peptides, particularly to opioid peptides that are highly selective kappa receptor agonists and, more particularly, to such agonists that (a) do not penetrate the brain and (b) exhibit an anti-nociceptive activity. long-term in vivo. BACKGROUND OF THE INVENTION Kappa opioid receptors (KORs) are present in the brain, the spinal column, and in the central and peripheral terminals and cell bodies of the primary sensory afferents (somatic and visceral), as well as in immune cells. The molecules that activate the KORs are commonly referred to as kappa agonists. The activation of the KORs that are located in the brain has been shown to produce an analgesic affection. This discovery led to attempts to develop kappa agonists that penetrate the brain, not peptides, for use as original analgesics that would be devoid of the undesirable side effects (constipation, respiratory depression, dependence and addiction) of the morphine analogues that act on opioid receptors. (MORs). The analgesic activity, as well as the lack of "mu opioid-like" side effects, of this class of compounds has been established in both animals and humans. However, the development of systemic kappa agonists was discontinued because it was shown that they also induced specific side effects such as diuresis, sedation and dysphoria, mediated through kappa receptors located in the brain. In addition to supra-spinal KORs, KORs located either in the periphery or the spine can also produce analgesia. However, neither the peripheral KORs nor the spinal ones were associated with any of the side effects of the systemic kappa agonists. Therefore, as long as it is possible to create opioid agonists of kappa receptors that do not enter the brain (after peripheral or spinal administration), it would be possible to obtain safe and original analgesics. Kappa agonists produce peripheral anti-nociception in models of both intestinal and colonic hyperalgesia induced by local and mild inflammation, and irritable bowel syndrome (IBS), which includes exaggerated visceral pain due to visceral hyper-sensitivity possibly linked to inflammation. local, is also a target for a peripheral kappa agonist. In addition to the gastro-intestinal tract, other viscera that show a pathological condition involving activation and / or sensitization (ie, local inflammation) of the primary sensory afferents are also considered to represent appropriate targets for such an opioid of the kappa receptors. Kappa agonists also block neurogenic inflammation in somatic tissues by inhibiting the release of substance P from primary sensory afferents and they are also known to act on the immune system and mainly have an inhibitory role on immune cells. Peptides that do not enter the brain, which exhibit high affinity for KOR versus MOR, which have high potency and efficacy, and which exhibit a long duration of action in vivo are particularly desired. U.S. Patent No. 5,610,271 discloses tetra-peptides containing four amino acid residues of isomer D that bind to KORs, but such peptides do not exhibit all of the desirable characteristics noted above. SUMMARY OF THE INVENTION A genus of peptides has been discovered which exhibit a high selectivity for KOR and long duration of action in vivo and which do not exhibit any significant penetration to the brain. These peptides comprise a four amino acid sequence of D isomer having a C-terminus which is a mono amide or a disubstituted amide. These compounds have the following general formula: X-Xaa1-Xaa2-Xaa3-Xaa4-substituted atpide, where Xaax is (A) D-Phe, (C? Me) D-Phe, D-Tyr, D-Tic or D- Ala (cyclopentyl or thienyl), A being H, N02, F, Cl or CH3; Xaa2 isnd.

(A ') D-Phe, D-lNal, D-2Nal, D-Tyr or D-Trp, A' where A or 3,4C12; Xaa3 is D-Nle, (B) D-Leu, D-Hle, D-Met, D-Val, D-Phe or D-Ala (cyclopentyl), B being H or CaMe; Xaa4 is D-Arg, D-Har, D-nArg, D-Lys, D-Ily, D-Arg (Et2), D-Har (Et2), D-Amf, D-Gmf, D-Dbu, D- Orn, or D-Ior. Preferred amides include ethylamide, morpholide, thiomorpholide, 4-picolyl amide, piperazide, propyl amide, cyclopro-pill-amide, diethylamide, and substituted benzylamide. In a particular aspect, the invention provides a synthetic opioid peptide amide or a pharmaceutically acceptable salt thereof having an affinity for the kappa opioid receptor which is at least 1,000 times its affinity for the mu opioid receptor and which exhibits a long duration of action when administered in vivo, which peptide has the formula: H-Xaa1-Xaa2-Xaa3-Xaa4-Q, where Xaa2 is (A) D-Phe, (CaMe) D-Phe, D-Tyr, D-Tic or D-Ala (cyclopentyl or thienyl), A being H, N02, F, Cl or CH3; Xaa2 is (A ') D-Phe, D-lNal, D-2Nal, D-Tyr or D-Trp, A' where A or 3,4C12; Xaa3 is D-Nle, (B) D-Leu, D-Hle, D-Met, D-Val, D-Phe or D-Ala (cyclopentyl), B being H or CaMe; Xaa4 is D-Arg, D-Har, D-nArg, D-Lys, D-Ily, D-Arg (Et2), D-Har (Et2), D-Amf, D-Gmf, D-Dbu, D- Orn or D-Ior; and Q is NR_R2, morpholinyl, thiomorpholinyl, (C) piperidinyl, piperazinyl, 4-mono- or 4,4-di-substituted pipe-razinyl, or e-lysyl, Rx being lower alkyl, substituted lower alkyl, benzyl, substituted benzyl, aminocyclohexyl, 2-thiazolyl, 2-picolyl, 3-picolyl, 4-picolyl, a? - (acyl-amino) -polymethylene or a poly (oxyethylene) group, and R2 being H or lower alkyl; and C being H, 4-hydroxy or 4-oxo. In a further aspect, the invention includes the use of these compounds in the treatment of human patients suffering from visceral pain and the like, bladder instability or the like, or IBD or autoimmune diseases, as well as in the similar treatment of mammals. non-human ~~ BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENTS The nomenclature used to define the peptides is that specified by Schroder & Lubke, The Peptides, Academic Press, 1965 where, according to conventional representation, the term N appears on the left and the term C appears on the right. Where the amino acid residue has isomeric forms, it is the form of the L isomer of the amino acid that is represented herein, unless otherwise expressly indicated. As indicated above, the invention provides peptides that are selective for KOR and not only exhibit an intense affinity for KOR but exhibit long duration of bioactivity in vivo. These selective kappa opioid peptides have at least 1,000 times greater binding affinity for KOR than for MOR, many compounds having at least 10,000 times higher affinity, and some compounds exhibiting an affinity of 20,000 or more times. However, for many indications it is important that, together with such high selectivity, the kappa agonists exhibit both a lack of significant penetration to the brain and a prolonged duration of anti-nociceptive activity in vivo. Thus, in addition to the aforementioned selectivity, the preferred compounds do not exhibit significant penetration to the brain while retaining substantial activity for at least about one hour, the most preferred compounds remaining significantly active for at least about 2 hours, and the compounds of the greatest preference remaining significantly active for at least about 2 hours, and the most preferred compounds exhibiting such significant activity for three hours or more. The abbreviations indicated hereinafter are used throughout this document. By D-Nle is meant D-norleucine and D-Hle represents D-homoleucine. D-Har represents D-homoaxginine, and D-nArg represents D-norarginine, which is a shorter carbon than D-Arg. By D-Nal is meant the D isomer of alanine, which is substituted by naphthyl in the β-carbon. Preferably, D-2Nal is used, ie the binding to naphthalene is in position 2 in the structure of the ring; however, D-lNal can also be used. D-Cpa and D-Fpa are used to represent, respectively, chloro-D-Phe and fluoro-D-Phe, with D-4Cpa, D-2Fpa, D-3Fpa and D-4Fpa being preferred. D-Npa means nitro-D-Phe, and D-Mpa is used to represent methyl-D-Phe. D-3,4Cpa means 3-dichloro-D-Phe. D-Acp represents D-Ala (cyclopentyl). D-Orn represents D-ornithine while D-Dbu represents alpha, gamma-diamino butyric acid. CML represents C "methyl Leu, and CMP represents CJVIe Phe. By D-4Amf is meant D-4 (NH2CH2) Phe, and by D-Gmf is meant D-Amf (amidino), which represents D-Phe where the position 4 is replaced with CH2NHC (NH) NH2. By D-Tic is meant D-1, 2, 3, -tetrahydro-isoquinoline-3-carboxylic acid. In Ala (Thi), Thi represents the thienyl group, which is preferably linked in its 2 position with alanine, although 3-thienyl is an equivalent. By Ily e lor is meant, respectively, isopropyl Lys and isopropyl Orn, where the amino group of the side chain is alkylated with isopropyl. By lower alkyl is meant ± C6 and includes cycloalkyl, and Cx-C4 is preferred, including cyclopropyl and cyclobutyl. Me, Et, Pr, Ipr, By, Pn and Bzl are used to represent methyl, ethyl, propyl, isopropyl, butyl, pentyl and benzyl. By Cyp it is meant cyclopropyl, and by Cyb is meant cyclobutyl. Although the linkage is preferably to one end of an alkyl chain, the linkage may be elsewhere in the chain, for example 3-pentyl, which may also be referred to as ethylpropyl. Ahx represents y-amino-hexyl, ie (CH2) b-NH2. 4aCx is used to represent 4-aminocyclohexyl, and hEt is used to represent hydroxyethyl, ie -CH2CH2OH. Substituted benzyl includes 4Nbz and 4Abz, which represent 4-nitrobenzyl and 4-aminobenzyl, and Aeb is used to represent 4- (2-amino-2-carboxyethyl) benzyl, i.e.

For 2-, 3- and 4-? Icolilo (2Pic, 3Pic and 4Pic) is meant. Methylpyridine groups with the binding via a methylene in the position ^ 2, 3 or 4. By Mor is meant morpholinyl, ie ~ - / \ -N O / \ and by Tmo it is meant thiomorpholinyl, • N c / By Pip it is meant piperidinyl (piperidyl), and by 4-Hyp and OxP it is meant 4-hydroxypiperidin-1-yl and 4-oxo-piperidin-1-yl. By Ppz it is meant piperazinyl. Ecp represents 4-ethylcarbamoyl-piperazin-1-yl; Pep represents 4-phenylalanba-moylpiperazin-1-yl. Quaternary ammonium fractions, such as 4,4-dimethyl piperazin-1-yl (Dmp) or other di-lower alkyl substitutions, may also be used. g / \ By 2Tzl is meant 2-thiazolyl, ie _r C? II II N - CH. By Ely is meant e-lysyl, where the side chain amino group of L-lysine is connected by an amide bond to the term C. As indicated above, R1 can be a group? (acylamino) polymethylene or a poly (oxyethylene) group, such as Aao, Aoo, Hoh, Ghx or Gao. Aao represents 8- (acetylamino) -3,6-dioxaoct-1-yl, ie CH2CH2-0-CH2-CH2-0-CH2CH2-NH-Ac. Aoo represents 8-amino-3,6-dioxaoct-l-yl, ie CH2CH2-0-CH2-CH2-0-CH2CH2-NH2. Hoh represents 6- (L-hydroorothylamino) -hex-1-yl, ie (CH 2) 6-NH- (L-hydroorotyl); L-hydroorotic acid is C4N2H5 (02) -COOH. Ghx represents 6- (D-gluconylamino) -hexyl, eTT say (CH2) 6-NH-CO- (CHOH) 4-CH2OH. Gao represents 6- (D-gluconylamino) -3,6-dioxaoct-l-yl, ie CH2CH2-0-CH2CH2-0-CH2CH2-NH-CO (CHOH) 4-CH2OH. D-Phe or D-Phe substituted at position 1 is preferred. The phenyl ring may be substituted at positions 2, 3 and / or 4, and common substitutions by chlorine or fluorine at positions 2 and 4 are preferred. Carbon atom can also be methylated. Other equivalent residues resembling D-Phe can also be used, and these include D-Ala (thienyl), D-Ala (cyclopentyl), D-Tyr and D-Tic. The residue in the 2-position is also preferably D-Phe or substituted D-Phe, such substitutions preferably including a substituent on the carbon of the 4-position of the phenyl ring or positions 3 and 4. Alternatively, it can be used D-alanine substituted by naphthyl, as well as D-Trp and D-Tyr. Position 3 is preferably occupied by a residue such as D-Nle, D-Leu, D-CML, D-Hle, D-Met or D-Val; however, D-Ala can also be used (cyclopentyl) or D-Phe. D-Arg (which may be substituted with diethyl) and D-Orn (which may be alkylated in its delta-ano group, as with isopropyl), are generally preferred for the 4-position; however, D-nARg and other equivalent residues can be used, such as D-Lys (which may also be alkylated at its amino-epsilon-amino group) and D-Har (which may be substituted with diethyl). Even more, D-Gmf, D-Dbu, D-4Amf and D-His can also be used. Although it could be expected that a good duration of the biological action would result from the use of a sequence of 4 amino acids of the D isomer, it was surprising to find that the duration of the action was generally quite short for the unsubstituted amide and that a long duration of action it was obtained only by the incorporation of a substituted amide in the term C. The simple substitutions may be in the form of ethyl, methyl, propyl, cyclopropyl and picolyl, as well as other equivalent residues, such as hydroxyethyl, thiazolyl, aminocyclohexyl, benzyl and substituted benzyl. Generally, lower alkyl or picolyl substituents are preferred for simple substituted amides. Instead of a simple substituted amide, a dialkyl substitution, for example diethylamino, is an alternative; however, preferably, such disubstituted C terminus is occupied by a morpholinyl, thiomorpholinyl or piperidinyl moiety, the latter being unsubstituted or substituted by 4-hydroxy or 4-oxo. A piperazinyl or 4-mono- or 4,4-piperazinyl disubstituted fraction can also be used, such as e-lysyl. It has been found that ligation is generally an attribute of the amino acid sequence of the tetrapeptide, and preferably the selective kappa receptor opioid peptides must exhibit a binding affinity to the kappa receptor, such that its Kx is equal to about 2 nM or less. The long duration of the action, which is primarily believed to be an attribute of the structure of the amide attached to the term C, can be effectively tested by means of the anti-nociceptive assay described hereinafter, and the most preferred peptides exhibit substantial biological activity by two or three hours and have no significant effect on the brain. A preferred sub-genus of the opioid peptide genus designated hereinbefore has the formula: H-Xaa1-Xaa2-Xaa3-Xaa4-Q, where Xaax is D-Phe (unsubstituted or substituted by CaMe, 2F, 4F or 4C1) or D-Ala (cyclopentyl or thienyl); Xaa2 is (A ') D-Phe, D-lNal, D-2Nal, D-Tyr or D-Trp, A' being H, 4F, 4C1, 4N02 or 3, 4C12; Xaa3 is D-Nle, D-Leu, D-CML, D-Met or D-Acp; Xaa4 is D-Arg, D-Arg (Et2), D-Lys, D-Ily, D-Har, D-Har (Et2), D-nArg, D-Orn, D-Ior, D-Dbu, D- Amf, and D-Gmf; and Q is NRXR2, Mor, Tmo, Pip, 4-Hyp, OxP, or PpZ, Rx being Me, Et, Pr, Bu, hEt, Cyp, Bzl or 4-picolyl, and R2 being H or Et. A further preferred sub-genus of the kappa opioid peptides has the formula: H-Xaa1-Xaa2-Xaa3-Xaa4-Q, where Xaa_ is D-Phe, D-4Fpa, D-2Fpa, D-4Cpa, D-Acp or D-Ala (Thi); Xaa2 is D-Phe, D-4FPa, D-4Cpa, C-3,4Cpa, D-lNal, D-2Nal, or D-Trp; Xaa3 is D-Nle, D-Met, D-CML or D-Leu; Xaa4 is D-Arg, D-Lys, D-Har, D-nArg or D-Orn; and Q is NRXR2, Mor, Tmo, Pip, 4-Hyp, or PpZ, R? being Et, Pr, Bu, Cyp, hEt, Bzl or 4-Pic, and R2 being H or Et. A further preferred sub-genus of the kappa opioid peptides has the formula: H-Xaax-Xaa2-Xaa3-Xaa4-Q, where Xaax is D-Phe, D-4Fpa, D-2Fpa, D-Acp or D-Ala ( 2Thi); Xaa2 is (A) D-Phe, D-lNal, D-2Nal, or D-Trp, A being 4F or 4C1; Xaa3 is D-Nle, D-Met or D-Leu; Xaa4 is D-Arg, D-Harm D-nArg, D-Lys, D-Orn or D-Gmf; and Q is NHRX, Mor, Tmo, Pip, or PpZ, Rx being Et, Pr, or 4Pic. Another preferred sub-genus of the kappa opioid peptides has the formula: H-Xaa1-Xaa2-Xaa3-Xaa4-Q, where Xaa_ is D-Phe, D-4Fpa, D-2Fpa, D-Acp or D-Ala (2Thi ); Xaa2 is (A) D-Phe, D-lNal, D-2Nal, or D-Trp, A being 3,4C12 or 4C1; Xaa3 is D-Nle or D-Leu; Xaa4 is D-Arg, D-Orn or D-Gmf; and Q is NHRX, Mor, Tmo, Pep, Ppz or N (Et) 2, R? being Et, Pr, Cyp, 4Pic, Aeb or Hoh. It has been found that the above genders and subgenders of the opioid peptides have a prolonged duration of anti-nociceptive activity in vivo as a result of incorporating a substituted amide at the C-terminus of the amino acid residue of the 4 position. This particular unexpected attribute makes such peptides particularly valuable as some of them remain active in vivo for periods of three hours and longer. Certain tetra-peptides having the aforementioned sequence but having a simple C-terminal amide also demonstrate high selectivity for KOR, as compared to MOR; however, they generally exhibit only a short-term duration of action. It is fully expected that such opioid peptides will exhibit a long term term when synthesized so as to have a substituted amide, such as morpholide, in the C-terminus. It has been consistently found that, when a primary amide tetra- When the peptide shows high ligation and selective to KOR, the corresponding substituted amides, such as for example ethylamide and morpholide, when synthesized, exhibit anti-nociceptive activity for a prolonged period measured in hours, that is to say at least one hour, without significant input to the brain. Although the preferred amino acid sequences are indicated in the above formulas, it should be understood by those skilled in the art of peptide chemistry that one or more of the designated amino acid residues may be substituted by a conservative amino acid substitution, for example a basic amino acid by another, or a hydrophobic amino acid by another, by example D-Ile by D-Leu. Similarly, they can also be modified as is generally known in this matter various waste; for example, D-Phe (as indicated above) can be modified by incorporating a halogen or nitro group usually in the 3 or 4 position, or both, or the alpha-carbon can be methylated. It is considered that such modifications produce equivalent kappa receptor opioid peptides. The peptides can be synthesized by any suitable method, such as by techniques exclusively in solid phase or addition of classical solution or alternatively by partial solid phase techniques or by fragment condensation techniques. For example, solid phase exclusively peptide synthesis (SPPS) techniques are pointed out in the Stewart & Young, Solid-Phase Peptide Synthesis, 2a. Edition, Pierce Chemical Company, Rockford, Illinois, 1984, and are implemented by the disclosure of United States Patent No. 4,105,603. The fragment condensation synthesis method is exemplified in U.S. Patent No. 3,972,859, and other available syntheses are exemplified in U.S. Patent Nos. 3,842,067 and 3,862,925. The classical synthesis by addition of solution is described in detail in Bodanzsky et al., Peptide Syn thesis, 2a. edition, John Wiley & Sons, New York, 1976. Common to the chemical synthesis of coupling type peptides is the protection of any labile side chain of an amino acid that is being coupled, and usually the protection also of the alpha-amino group, so that the addition takes place in the carboxyl group of the individual amino acid or di-peptide or tripeptide that is being added. Such protecting groups are well known in the art, and tert-butyloxycarbonyl (Boc), benzyloxycarbonyl (Z) and 9-fluorenylmethoxycarbonyl (Fmoc) are often used as preferred alpha-amino protecting groups in SPPS or classical synthesis in solution, although there is a wide variety of other alpha-amino protecting groups that can be used alternatively. When SPPS is used, the C-terminal amino acid residue is coupled to a solid resin support such as support of O-CH2-polystyrene, resin support of 0-CH2-benzyl-polyamide, resin support of -NH-benzhydrylamine (BHA), or resins support of -NH-para-methylbenzhydrylamine (MBHA). The use of BHA resins or MBHA is often preferred when the unsubstituted amide is desired because the break directly gives the C-terminal amide. When an N-methylamide is desired, it can be generated from an N-methyl BHA resin. Other simple substituted amides can be synthesized by the method set forth in W. Kornreich et al., In t. J. Peptide Protein Res. , 25: 414-420, 1985, and also in U.S. Patent No. 4,701,499. Peptides having di-substituted amides in the C-terminus, such as N-morpholinyl or N-piperidinyl, are preferably prepared via classical synthesis in solution or by condensation of fragments in solution. Once synthesized, these tetra-peptides are easily purified using state-of-the-art methods for purification of short peptides, for example reverse phase high performance liquid chromatography (RP-HPLC), or other appropriate methods. Such purification is described in detail in J. Rivier et al., J. Chroma tography, 288: 303-328, 1984, and C. Miller and J. Rivier, Peptide Science, Biopslymers, 40: 265-317 (1996), and Specific examples of such purification after solid phase synthesis or the like are shown in U.S. Patent No. 5,098,995. A variety of assays may be employed to test whether the tetra-peptides exhibit high selectivity for KOR, strong anti-nociceptive bio-activity, long duration of bioactivity in vivo, and lack of penetration to the brain. Receptor assays are well known in the art, and mouse, rat, guinea pig and human KORs have recently been cloned. With the exception of gpKOR, the cloned KORs are extremely similar, all containing around 380 amino acids. The amino acid sequence of hKOR has 93.9 and 93.4% homology with rKOR and mKOR, respectively. In contrast, hKOR differs significantly from hMOR and the human delta opioid receptor (hDOR), having, respectively, only 60.2 and 59.1% amino acid sequence identity. The KORS, as well as the other opioid receptors, are receptors coupled to the G proteins, which comprise seven trans-membranes, classical (Gi). These cloned receptors allow to easily analyze a particular candidate peptide; for example, the analysis regarding both KOR and MOR can be carried out in order to determine the selectivity. Human KOR, MOR and DOR have been stably expressed in a mouse cancer cell line, derived from a hippocampal neuroblastoma (HN.9.10) and are available for use in in vi tro analysis. There are also several well-accepted in vivo tests that have generally become standards for determining the anti-nociceptive activity of an opioid compound. These tests generally employ mice and include the tail movement test, the claw pressure test, the acetic acid reaction test, the tail prick test and the tail immersion test. Vonvoigtlander, P.F. and collaborators, J. Pharm. Exper. Therapeutics, 224: 7-12 (1983) describe several such tests for opioid compounds. The binding affinity refers to the strength of the interaction between ligand and receptor. To demonstrate the binding affinity for opioid receptors, the peptides of the invention were evaluated using competition binding studies. These studies were carried out using kappa (hKOR) and mu (hMOR) opioid human receptors expressed in stable transfected cell lines (HN.9.10, derived from mouse hippocampal neuroblastoma). In these studies, test compounds (unlabelled or cold ligand) are used at concentrations that increase to displace the specific binding of a radiolabelled ligand that has a high affinity and selectivity for the receptor studied. 3H-U-69,593 and A-DAMGO were used as ligands in studies of hKOR and hMOR, respectively. Both ligands are commercially available (NEN-Dupont). DAMGO is the acronym for [D-Ala2, MePHe4, Gly-ol5] -encephaliña. The affinity of the radio-ligands is defined by the concentration of radio-ligand that results in semi-maximal specific binding (KD) in saturation studies. The KD for 3H-U-69,593 in hKOR and for 3H-DAMGO in hMOR is around 0.3 nM and 3.0 nM, respectively. The affinity of the test compound (unlabelled or cold ligand) is determined in competition-ligation studies by calculating the inhibitory constant (K_.) According to the following formula: K ± = IC50 / 1- (F / KD) where: IC50 = concentration of the cold ligand that inhibits 50% of the specific binding of the radio-ligand; F = concentration of free radio-ligand; KD = affinity of the radio-ligand determined in saturation studies. When these assays are carried out under specific conditions with relatively low receptor concentrations, the K ± value calculated for the test compound is a good approximation of its dissociation constant KD, which represents the concentration of ligand required to occupy one half ( 50%) of the ligation sites. A low K value. In the nanomolar and sub-nanomolar range, it is considered to identify a high affinity ligand in the field of opioids. Preferred analogs have a K¿ value for KOR of about 2 nanomolar (nM) or less, while more preferred analogues have a K ± value of about 1 nM or less. Because KOR receptors are widely distributed throughout the body, kappa receptor opioid peptides will have a substantial effect in modulating many peripheral actions, and if they are highly selective to KOR, they will have minimal side effects and should be good physiological drugs. .

These ligation assays employing KORs and MORs are easy to carry out and can be easily carried out with peptides identified or initially synthesized to determine whether such peptides are KOR selective and have high affinity. Such ligation assays can be carried out in a variety of ways, as is well known to those skilled in the art, and a detailed example of an assay of this general type is noted in Perrin, M. et al., Endocrinology, 118 : 1171-1179, 1986. The present invention is further described by the following examples. However, such examples should not be construed as limiting in any way the spirit or scope of the present invention, which are defined by the claims at the end of this descriptive chapter. Example 1 The peptide having the formula: H-D-Phe-D-Phe-D-Nle-D-Arg-NHEt is synthesized appropriately, as is well known in the field of peptide synthesis. For example, the tri-peptide: (alpha-amino protecting group) D-Phe-D-Phe-D-Nle (carboxyl protecting group) is initially synthesized using classical chemistry in solution. For example, the tri-peptide can be prepared by dissolving H-D-Nle-OMe in DMF and adding N-ethylmorpholine (NEM) or the like to adjust the pH. This solution is then combined with a BOC-protected D-Phe-OH solution in DMF, containing NEM. To this reaction mixture is added an activating or coupling agent, such as benzotriazole-1-yl-oxy-tris- (dimethylamino) -phosphonium hexafluoro-phosphate (BOP) or a mixture of N, N'-diisopropylcarbodiimide (DIC). ) and N-hydroxybenzo-triazole (HOBt). After the reaction is complete, the medium is evaporated to dryness, and the product is then purified and re-crystallized appropriately. The Boc protecting group is then removed with trifluoroacetic acid (TFA), and the di-peptide is re-dissolved in DMF. A Boc-protected D-Phe solution, dissolved in DMF, with NEM, is added. The reaction is repeated using BOP, as described above, to create the tripeptide which, after the solution is evaporated to dryness, is purified and re-crystallized. The product obtained as a result is Boc-D-Phe-D-Phe-D-Nle-OCH3. The methyl ester is then suitably converted to the free acid, such as by dissolving in a mixture of dioxane or DMSO and water and adding sodium hydroxide. After completion of the reaction, separation, purification and re-crystallization provide the tri-peptide Boc-D-Phe-D-Phe-D-Nle-OH. The tri-peptide is dissolved in DMF containing NEM, and reacted with D-Arg (Tos) -NHEt, again using BOP as the coupling agent. Alternatively, the methyl ester of the tri-peptide can be converted to azide, if desired, by treatment with an 80% solution of hydrazine hydrate to produce the hydrazide, which is isolated and then treated with sodium nitrite and mineral acid in DMF. The azide is immediately reacted with D-Arg (Tos) -NHEt in DMF solution containing triethylamine. After the reaction is completed, the mixture is evaporated to dryness, then purified and adequately re-crystallized. The N-terminus and the side chain of D-Arg are then deprotected and re-purification and re-crystallization are carried out, yielding the ethylamide of the desired tetra-peptide (peptide No. 1). The peptide is judged to be homogeneous by reverse phase HPLC using two different mobile phases: a gradient of acetonitrile in water containing 0.1% trifluoroacetic acid and a gradient of acetonitrile in buffer of __ triethylamine phosphate, pH 7, and also by silica melted, capillary electrophoresis using a phosphate buffer, pH 2.5. The purity of the peptide by these methods is estimated to be over 98%. Mass spectrometry using electro-dew ionization and ion trap analysis showed a pseudo-molecular ion [MH] + at m / z 609.4, which is consistent with the calculated mass of m / z 609.5 for this tetra-peptide. Fragmentation analysis of the pseudo-molecular ion showed a series of ions at m / z ratios consistent with the amino acid sequence expected for the prepared structure. Ligation assays with cells expressing human KOR and MOR are carried out as mentioned hereinabove. The affinities of the test peptide for hKOR and hMOR, stably expressed in mouse hippocampal neuroblastoma cells (HN.9.10), are determined by competitive displacement of 3H-U-69,593 for hKOR or 3H-DAMG0 for hMOR, as described. Data from at least three experiments are combined, and values of the inhibitory dissociation constant (KJ (95% confidence limits) are calculated using an appropriate program, such as the Ligand program of Munson and Rodbard, Anal. Biochem, 107: 220-239, 1980. The cloned KOR is linked to peptide No. 1 with high affinity, as determined by the competitive displacement of the ligated radio-ligand, and it is determined that K is around 0.05 ± 0.02 nM. Difference in affinity is dramatic, compared to similarly stably transfected cancer cells expressing human MOR, where the Kx value is 1.890 + 990 nM.Thus, peptide No. 1 binds more strongly to hKOR than to hMOR by a factor of about 38,000 The tests of the peptide in the acetic acid reaction assay in mouse (as described below) show an ED50 value of about 0.09 mg / kg and that the peptide continues to exhibit more than 50 Anti-nocicepc% ion after 3 hours. In this way, it is considered that peptide No. 1 exhibits an extremely long duration of action. Example 2 Opioid peptides having the general formula: H-D-Phe-D-Phe-D-Nle-D-Arg-Q, as indicated in Table A, are synthesized and tested as described in Example 1 It is considered that peptides 2 to 15 exhibit a long duration of anti-nociceptive bioactivity. Example 3 Opioid peptides having the general formula: H-Xaa1-Xaa2-Xaa3-Xaa4-Q, as indicated in Table B, are synthesized and tested as described in Example 1.

It is considered that peptides 16 to 39 exhibit long duration of anti-nociceptive bioactivity. Example 4: Opioid peptides having the general formula: H-Xaax-Xaa2-Xaa3-Xaa4-Q, as indicated in Table C, are synthesized and tested as described in Example 1.

It is considered that peptides 40 to 53 exhibit long duration of anti-nociceptive bioactivity. Example 5 Opioid peptides having the general formula: H-D-Phe-Xaa2-Xaa3-Xaa4-Q, as indicated in Table D, are synthesized and tested as described in Example 1.

It is considered that peptides 54 to 58 exhibit long duration of anti-nociceptive bio-activity. Example 6: Opioid peptides having the general formula: H-Xaaj.-Xaa2-Xaa3-D-Arg-Q, as indicated in Table E, are synthesized and tested as described in Example 1.

It is considered that peptides 59 to 65 exhibit long duration of anti-nociceptive bioactivity. Example 7 Opioid peptides having the general formula: H-Xaax-Xaa2-Xaa3-Xaa4-Q, as indicated in Table F, are synthesized and tested as described in Example 1.

It is considered that the opioid peptides of Table F show high selectivity for KOR, as compared to MOR, and that they exhibit anti-nociceptive bioactivity in vivo.

Example 8: Opioid peptides having the general formula: H-Xaa-L-Xaa2-Xaa3-Xaa4-Q, as indicated in Table G, are synthesized and tested as described in Example 1.

It is considered that peptides 88 to 108 exhibit long duration of anti-nociceptive bioactivity. Example 9 Selected peptides that are identified in Tables A-G have been specifically subjected in addition to in vivo tests for determination of the duration of action of their opioid properties, and the results are reported in Table H below. The numbers of the peptides correspond to those of the previous tables and the figures regarding the relation μ /? they are simply reproduced for reference purposes. In vivo tests are carried out using a mouse reaction test (WT) which is suitable for determining the length of the duration of anti-nociceptive biological activity. This test is described in detail in an article by G.A. Bentley et al., Br. J. Pharma c, 73: 325-332, 1981, and employs conscious, male ICR mice, which are purchased from Harian and weigh between 20 and 30 grams. Mice are fasted for 12 to 16 hours before the start of the test. The nociceptive behavior, that is, the reaction, is monitored by intra-peritoneal administration (i.p.) of diluted acetic acid. 10 ml of 0.6% aqueous acetic acid per kg of body weight are used. The reaction is scored for 15 minutes after the administration of acetic acid. In a first step, the compounds are tested in doses of 3 to 4 increments, given by intravenous route, and at a single pre-treatment time (less than 5 minutes before the acetic acid injection). This step is used to determine the potency (WT-ED50) as well as an effective sub-maximal dose (around 80-90% anti-nociception). In a second step, this sub-maximal effective dose for each specific peptide is administered at various pre-treatment times (i.e., -5 minutes, -60 minutes, -120 minutes and -180 minutes) before administration of the acid acetic in order to determine the duration of the action. Throughout the entire test, a control group of mice to which only the vehicle is administered without the candidate peptide is used. The number of reactions is counted in a period of 15 minutes, starting at the moment in which the acetic acid is injected, and the bio-activity, that is, anti-nociception, is expressed as a percentage, and is calculated as follows: 100 X (control group reactions - reactions in treated group) / control group reactions - Because each sub-maximal dose will very likely vary so that it is not directly comparable, the results are mathematically normalized, as is known in the matter, to provide comparable values that are indicated in Table H.

In Table H, the anti-nociceptive activity remaining after 1, 2 and 3 hours is expressed as a percentage of the activity found at -5 minutes. Values greater than 100% indicate greater anti-nociception than at the beginning of the experiment. It is believed that the opioid peptide should be effective to reduce the reaction by at least about 25% at a time of 1 hour to be considered to have long duration of action in vivo. In addition to using this test to determine the duration of anti-nociceptive activity, it is also used to measure the bio-potency in vivo (short term) of the peptide. This value is given in the table under the heading WT-ED50 in mg per kg of body weight. The value is a measure of the dose needed to reduce the number of reactions in the mouse that is being tested at 50% (compared to a control mouse) over a period of 15 minutes.

Opioid peptides are useful as analgesics and for other pharmacological applications to treat pathologies associated with the KOR system. They exhibit advantages over μ-agonist analgesics, for example morphine, which have undesirable effects, such as constipation, respiratory depression and itching. It is highly desirable that these opioid peptides do not significantly cross the blood / brain barrier, in order to safeguard against potential side effects that may result. The safety of these compounds in relation to brain penetration is determined by comparing their potency to stimulate peripheral effects versus their potency to stimulate central effects. Peripheral effects are measured using the mouse reaction test (WT) described previously. The central effects due to the action on kappa receptors located in the brain are means using the test of the movement of the mouse tail (TF). The mouse tail movement test is an acute somatic pain test, designed to evaluate the potency and duration of action of centrally acting analgesics. Nociception induced by immersion of the tail in hot water (52 'C) results in a rapid removal of the tail, also known as tail movement. Analgesic compounds that act centrally are expected to increase the latency of tail removal in a dose-related manner. The test is described in Vanderah, T.W. and collaborators, J. Pharm. Exper. Therapeutics, 262: 190-197, 1992. Safety is assessed by using a brain penetration index (BPI), which is defined as: BPI = TF-ED50 / WT-ED50 where the ED50 values are the doses that produce maximum mean effect in the mouse reaction test (WT-ED50) and mouse tail movement test (TF-ED50), respectively, when given iv A high BPI value indicates low penetration to the brain and indicates that it is feasible for the compound to exhibit a wide margin of safety (lack of side effects in the brain) when used for the purposes described in this application. Preferred opioid peptides have BPI values equal to or greater than 100, the most preferred opioid peptides having a BPI value greater than 300. Systemic non-peptide kappa agonists (e.g., Enadoline and U-69,593) have BPI values less than 5, which indicates that one is occurring. significant penetration to the brain, as also evidenced by the side effects (diuresis, dysphoria, and sedation) that they produce when used clinically. The BPI values for some representative opioid peptides are shown in Table I, below: Because these peptides are strongly bound to KOR, they are also useful in in vitro assays to study receptors and determine which receptors may be present in a particular tissue sample. In this way, they are useful for diagnosis in this respect and potentially also for in vivo diagnosis. Generally, these opioid peptides can be used to achieve anti-nociception when treating visceral pain and also to treat rheumatoid arthritis. They are particularly useful for treating post-abdominal surgery symptoms, such as digestive disorders and pain. They are also considered effective for treating IBS, bladder instability, incontinence, and other indications where local inflammation results in pain states in the intestine or in other viscera, for example, inflammatory bowel disease (IBD) and dysmenorrhea. The ability of the opioid peptide to reduce the immune response may be advantageous for combating IBD and other indications, such as autoimmune diseases. The administration of the peptides can be used to produce local analgesic activity with respect to both acute and chronic inflammatory conditions. They can be used to treat digestive ailments that have symptoms such as bleeding, nausea or inhibitions in intestinal transit associated with pain, for example obstruction of the large intestine possibly caused by spinal contractions. Opioid peptides are also effective in producing peripheral analgesia, and can be targeted to relieve post-operative pain, as well as chronic pain, such as that caused by inflammation of the gastro-intestinal and visceral tissues, and also to provide relief during abstinence from addiction. to drugs. The compounds of the invention can be administered in the form of non-toxic, pharmaceutically acceptable salts, such as acid addition salts, as is known in the art. Illustrative of such acid addition salts are hydrochloride, hydrobromide, sulfate, phosphate, nitrate, oxalate, fumarate, gluconate, tannate, pamoate, maleate, acetate, citrate, benzoate, succinate, alginate, malate, ascorbate, tartrate, and the like. If the active ingredient is to be administered in tablet form, the tablet may contain a non-toxic, pharmaceutically acceptable diluent that includes a binder, such as tragacanth, corn starch or gelatin. Intravenous administration in isotonic saline, phosphate buffer, mannitol or glucose solutions, may also be carried out. The pharmaceutical compositions will usually contain an effective amount of the peptide in conjunction with a pharmaceutically acceptable, conventional carrier or diluent. Generally, the composition will contain an anti-nociceptive amount, i.e., an amount that is effective in blocking pain. Typically, the dose will be from about 1 microgram to about 10 milligrams of the peptide per kilogram of host body weight when given intravenously. The compositions can be administered as needed; for example, they can be administered repeatedly at intervals of 3-6 hours. The nature of these compounds may possibly allow effective oral administration; however, oral doses may be higher. If it is desirable to deliver the opioid peptide for extended periods of time, for example, for periods of a week or more from a single administration, slow release forms, dosages per deposit or implant can be used. For example, a slow release depot formulation, suitable for injection, may contain the peptide or a salt thereof dispersed or encapsulated in a non-antigenic or non-toxic, slow-degrading polymer, such as a polylactic acid / polyglycolic acid polymer. , as described in U.S. Patent No. 3,773,919. It is also known that administration by slow release can be achieved via a silastic implant. These compounds can be administered to mammals, including humans, intravenously, subcutaneously, intra-muscularly, percutaneously, intra-nasally, intra-pulmonarly, orally, topically, intra-rectally, intra-vaginally, or by spinal dosing, to achieve anti-nociception, such as inhibition of inverse gastro-intestinal transit, induced by peritoneal irritation. They can also be used to relieve post-operative pain. Effective doses will vary with the form of administration and the particular species of mammal being treated. An example of a typical dosage form is a bacteriostatic solution in water at a pH of about 3 to 8, for example about 6, which contains the peptide, which solution is continuously administered parenterally to provide a dose in the range of about 0.3 μg to 3 mg / kg of body weight per day. It is considered that these compounds are well tolerated in vivo, and are considered particularly suitable for administration by subcutaneous injection in a bacteriostatic solution in water or the like. Although the invention has been described in relation to its preferred embodiments, it should be understood that changes and modifications that would be obvious to a person skilled in the art can be made, without departing from the scope of the invention, which are pointed out in the appended claims. to the present. For example, other substitutions known in the art that do not depart considerably from the effectiveness of the peptides can be employed in the peptides of the invention. Other substituted D-Phe residues, such as (4Br) D-Phe or (2, 4C12) D-Phe, can be used in position 2. Both D-Lys (Bu) and D-Lys (Et2) are considered as equivalents of D-Ily and D-Arg (Et2). The N-terminus of the tetra-peptide can be permethylated, as is known in the art, if desired. Diamino compounds such as linkers can be used to create dimers of two tetra-peptide amides. Linkers that have been used successfully include 1,6-diaminohexane, 1,5-diamino-3-oxapentane, and 1,8-diamino-3,6-dioxaoctane. The resulting dimers are considered as equivalents of the respective monomers.

Claims (23)

1. REIVIMDICACY 1. A synthetic opioid peptide amide or a pharmaceutically acceptable salt thereof having an affinity for the kappa opioid receptor which is at least 10,000 times its affinity for the mu opioid receptor and which exhibits long duration of action when administered in vivo, which peptide has the formula: H-Xaa1-Xaa2-Xaa3-Xaa4-Q, where Xaax is (A) D-Phe, (CaMe) D-Phe, D-Tyr, D-Tic or D-Ala ( cyclopentyl or thienyl), A being H, N02, F, Cl or CH3; Xaa2 is (A ') D-Phe, D-lNal, D-2Nal, D-Tyr or D-Trp, A' where A or 3,4C12; Xaa3 is D-Nle, (B) D-Leu, D-Hle, D-Met, D-Val, D-Phe or D-Ala (cyclopentyl), B being H or CaMe; Xaa4 is D-Arg, D-Har, D-nArg, D-Lys, D-Ily, D-Arg (Et2), D-Har (Et2), D-Amf, D-Gmf, D-Dbu, D- Orn or D-Ior; and Q is NRXR2, morpholinyl, thiomorpholyl, (C) piperidinyl, piperazinyl, 4-mono- or 4,4-piperazinyl di-substituted, or e-lysyl, R_ being lower alkyl, substituted lower alkyl, benzyl, substituted benzyl , aminocyclohexyloyl, 2-thiazolyl, 2-picolyl, 3-picolyl, 4-picolyl, a group? - (acylamino) polymethylene or poly (oxyethylene), and R2 being H or lower alkyl; and C being H, 4-hydroxy, or 4-oxo.
2. 2. The synthetic peptide according to claim 1, wherein Xaa2 is D-Phe, Xaa3 is D-Leu or D-Nle, and Xaa4 is D-Arg or D-Orn.
3. 3. The synthetic peptide according to claim 1 or 2, wherein Q is NHRX and Rx is ethyl, propyl, butyl, cyclopropyl or cyclobutyl.
4. 4. The synthetic peptide according to claim 1 or 2, wherein Q is morpholinyl or thiomorpholinyl.
5. 5. The synthetic peptide according to claim 1 or 2, wherein Q is NHRX and Rx is 4-picolyl.
6. 6. The synthetic peptide according to claim 1 or 2, wherein Q is N (Et) 2, NH (Aeb), Ppz or Pcp.
7. 7. The synthetic peptide according to claim 1 or 2, wherein Q is NHRX, and R ± is Aao, Aoo, Hoh, Ghx or Gao.
8. 8. The synthetic peptide according to any of claims 1-7, wherein Xaa2 is D-Phe, D-Ala (2-thienyl) or D-4FPa.
9. 9. The synthetic peptide according to claim 1, wherein Xaa4 is D-Gmf.
10. 10. The synthetic peptide according to claim 1, wherein Xaa2 is D-4Cpa or D-3,4Cpa.
11. 11. A pharmaceutical composition comprising an anti-nociceptive amount of a synthetic peptide according to any of claims 1-10 and a pharmaceutically acceptable liquid or solid carrier therefor.
12. 12. A method of treatment, comprising administering an amount of the pharmaceutical composition of claim 11, which is effective (a) to achieve anti-nociception where there is visceral pain, rheumatoid arthritis, abdominal post-surgery symptoms or acute or chronic pain, or (b) to counteract bladder instability, incontinence or digestive disorders, or (c) to combat IBD or autoimmune diseases.
13. 13. The synthetic opioid peptide according to claim 1, having a WT-ED50 value of about 0.5 mg / kg or less, which peptide has the formula: H-Xaa1-Xaa2-Xaa3-Xaa4-Q, where Xaa : is D-Phe (unsubstituted or substituted by ^ C ^ Me, 2F, 4F or 4C1) or D-Ala (cyclopentyl or thienyl); Xaa2 is (A *) D-Phe, D-lNal, D-2Nal or D-Trp, A 'being H, 4F, 4C1, 4N02 or 3,4C12; Xaa3 is D-Nle, D-Leu, D-CML, D-Met or D-Acp; Xaa4 is D-Arg, D-Arg (Et2), D-Lys, D-Ily, D-Har, D-Har (Et2), D-nArg, D-Orn, D-Ior, D-Dbu, D- Amf, and D-Gmf; and Q is NRXR2, Mor, Tmo, Pip, 4-Hyp, OxP, or Ppz, Rx being Me, Et, Pr, Bu, hEt, Cyp, Bzl, or 4-picolyl, and R2 being H or Et. The synthetic peptide according to claim 13, wherein Xaa2 is D-Phe, D-4Cpa, or D-3,4Cpa, Xaa3 is D-Leu or D-Nle and Xaa4 is D-Arg, D-Orn or D-Gmf. 15. The synthetic peptide according to claim 13 or 14, wherein Q is NHRX and Rx is Et, hEt, Pr _ or 4-picolyl. 16. The synthetic peptide according to claim 13 or 14, wherein Q is N (Et2) or NH (Aeb). 17. The synthetic peptide according to claim 13 or 14, wherein Q is morpholinyl or thiomorpholinyl. 18. The synthetic peptide according to claim 13 or 14, where Q is NHRX and Rx is ethyl or 4-picolyl. 19. The synthetic peptide according to claim 13 or 14, wherein Q is Ppz, Pep or NH (Hoh). 20. The synthetic peptide according to any one of claims 13-19, wherein Xaax is D-Phe or D-Ala (2-thienyl) or D-Fpa 21. The synthetic peptide according to claim 1, having one of the following formulas: HD-Phe-D-Phe-D-Nle-D-Arg-NHEt, HD-Phe-D-Phe-D-Nle-D-Arg-morpholinyl, HD-Phe-D-Phe- D-Nle-D-Arg-NH- -picolyl, HD-Phe-D-Phe-D-Nle-D-Arg-NHPr, HD-Phe-D-Phe-D-Nle-D-Arg-thiomorpholinyl, HD -Phe-D-Phe-D-Nle-D-Arg-NEt2, HD-Phe-D-Phe-D-Nle-D-Arg-NHMe, HD-Phe-D-Phe-D-Leu-D-Orn -morpholinyl, HD-4Fpa-D-Phe-D-Nle-D-Arg-NH-4-picolyl, HD-Phe-D-Phe-D-Nle-D-Arg-NH-cyclopropyl, HD-Ala (2Thi ) -D-3, Cpa-D-Leu-D-Arg-morpholinyl, HD-Phe-D-Phe-D-Nle-D-Gmf-morpholinyl, HD-Phe-D-Phe-D-Leu-D- Orn-NH (Aeb), HD-Phe-D-Phe-D-Leu-D-Lys-morpholinyl, HD-Phe-D-Phe-D-Nle-D-Arg-piperazinyl, and HD-Phe-D- Phe-D-Nle-D-Arg-NH (Hoh) 22. The synthetic opioid peptide according to claim 1, having an ED50 value of about 0.5 mg / kg or less, which Eptide has the formula: H-Xaa1-Xaa2-Xaa3-Xaa4-Q, where Xaax is D-Phe, D-4Fpa, D-2Fpa, D-Acp or D-Ala (2Thi); Xaa2 is (A) D-Phe, D-lNal, D-2Nal, or D-Trp, A being 4F or 4C1; Xaa3 is D-Nle, D-Met or D-Leu; Xaa4 is D-Arg, D-Har, D-nArg, D-Lys, D-Orn or D-Gmf; and Q is NHRX, Mor, Tmo, Pip, or Ppz, "R1 being Et, Pr, or 4Pic 23. The synthetic opioid peptide according to claim 1, having an ED50 value of about 0.5 mg / kg or less. , which peptide has the formula: H-Xaa1-Xaa2-Xaa3-Xaa4-Q, where Xaax is D-Phe, D-4Fpa, D-2Fpa or D-Ala (2Thi); Xaa2 is (A) D-Phe , D-lNal, D-2Nal or D-Tp, A being 3,4C12 or 4C1, Xaa3 is D-Nle or D-Leu, Xaa4 is D-Arg, D-Orn or D-Gmf, and Q is NHR_. , Mor, Tmo, Pep, Ppz or N (Et) 2, Rx being Et, Pr, Cyp, 4Pic, Aeb or Hoh.