Mu opioid receptor agonist analogs of the endomorphins

Abstract

The invention relates to cyclic peptide agonists that bind to the mu (morphine) opioid receptor and their use in the treatment of acute and/or chronic pain. Embodiments of the invention are directed to cyclic pentapeptide and hexapeptide analogs of endomorphin that have (i) a carboxy-terminal extension with an amidated hydrophilic amino acid and (ii) a substitution in amino acid position 2. These peptide analogs exhibit decreased tolerance relative to morphine, increased solubility compared to similar tetrapeptide analogs, while maintaining favorable or improved therapeutic ratios of analgesia to side effects.

1 . A cyclic peptide of Formula I:
   H-Tyr-cyclo[X 1 -X 2 -X 3 -X 4 ]-X 5   (I),
   
       wherein
 X 1  and X 4  each independently is an acidic amino acid or a basic amino acid; 
 X 2  and X 3  each independently is an aromatic amino acid; 
 X 5  is Ala-NHR, Arg-NHR, Asn-NHR, Asp-NHR, Cys-NHR, Glu-NHR, Gln-NHR, Gly-NHR, His-NHR, Ile-NHR, Leu-NHR, Met-NHR, Orn-NHR, Phe-NHR, Pro-NHR, Ser-NHR, Thr-NHR, Trp-NHR, Tyr-NHR, or Val-NHR, wherein R is H or an alkyl group; and there is an amide bond between an amino group and a carboxylic acid group on side chains of amino acids X 1  and X 4 , 
 with the proviso that when X 1  is an acidic amino acid, then X 4  is a basic amino acid; and when X 1  is a basic amino acid, then X 4  is an acidic amino acid. 
 
     
     
         2 . The peptide of  claim 1  wherein:
 (i) X 1  is selected from the group consisting of D-Lys, D-Orn, Lys, and Orn; and X 4  is selected from the group consisting of D-Asp, D-Glu, Asp, and Glu; or 
 (ii) X 1  is selected from the group consisting of D-Asp, D-Glu, Asp, and Glu; and X 4  is selected from the group consisting of D-Lys, D-Orn, Lys, and Orn. 
 
     
     
         3 . The peptide of  claim 1 , wherein:
 X 2  is selected from the group consisting of Trp, Phe, and N-alkyl-Phe, wherein the alkyl group of N-alkyl-Phe comprises 1 to about 6 carbon atoms; and   X 3  is selected from the group consisting of Phe, D-Phe, and p-Y-Phe, wherein Y is NO 2 , F, Cl, or Br.   
     
     
         4 . The peptide of  claim 3 , wherein X 2  is N-methyl-Phe. 
     
     
         5 . The peptide of  claim 3 , wherein X 3  is p-Cl-Phe. 
     
     
         6 . (canceled) 
     
     
         7 . The peptide of  claim 1 , wherein R is H and X 5  is Ala-NH 2 , Arg-NH 2 , Asn-NH 2 , Asp-NH 2 , Cys-NH 2 , Glu-NH 2 , Gln-NH 2 , Gly-NH 2 , His-NH 2 , Ile-NH 2 , Leu-NH 2 , Met-NH 2 , Orn-NH 2 , Phe-NH 2 , Pro-NH 2 , Ser-NH 2 , Thr-NH 2 , Trp-NH 2 , Tyr-NH 2 , or Val-NH 2 . 
     
     
         8 . The peptide of  claim 1 , wherein the alkyl group is a methyl, ethyl, propyl, isopropyl, butyl, isobutyl, pentyl, isopentyl, hexyl, isohexyl, heptyl, or isoheptyl group. 
     
     
         9 . The peptide of  claim 1 , selected from the group consisting of:
 Tyr-cyclo[D-Lys-Trp-Phe-Glu]-Gly-NH 2  (SEQ ID NO:3), Tyr-cyclo[D-Glu-Phe-Phe-Lys]-Gly-NH 2  (SEQ ID NO:4),   and   Tyr-cyclo[D-Orn-Phe-p-Cl-Phe-Asp]-Val-NH 2  (SEQ ID NO:7).   
     
     
         10 . A pharmaceutical composition comprising a pharmaceutically acceptable carrier and the peptide of  claim 1 . 
     
     
         11 . A method of treating pain comprising administering to a subject an analgesic amount of the peptide of  claim 1 . 
     
     
         12 . The method of  claim 11 , wherein the pain is due to a gastrointestinal disorder. 
     
     
         13 . The method of  claims 11 , wherein the pain is chronic pain. 
     
     
         14 . The method of  claim 11 , wherein the pain is neuropathic pain. 
     
     
         15 . The method of  claim 11 , further comprising administering an opioid drug to the subject. 
     
     
         16 . The method of  claim 15 , wherein the steps of administering the peptide and the opioid drug are performed concurrently. 
     
     
         17 . The method of  claim 15 , wherein the steps of administering the peptide and the opioid drug are performed alternately on a rotating basis. 
     
     
         18 . The method of  claim 11 , wherein the step of administering the peptide is performed by administering repeated, increasing doses of the peptide to the subject until full antinociception is achieved, and then maintaining the blood level of the peptide at the level obtained at full antinociception. 
     
     
         19 . The method of  claim 11 , wherein the peptide is administered parenterally. 
     
     
         20 . The method of  claim 11 , wherein the peptide comprises at least one peptide selected from the group consisting of:
 Tyr-cyclo[D-Lys-Trp-Phe-Glu]-Gly-NH 2  (SEQ ID NO:3),   Tyr-cyclo[D-Glu-Phe-Phe-Lys]-Gly-NH 2  (SEQ ID NO:4),   and   Tyr-cyclo[D-Orn-Phe-p-Cl-Phe-Asp]-Val-NH 2  (SEQ ID NO:7).   
     
     
         21 . A method for treating a drug dependence comprising administering to a subject a therapeutically effective amount of the peptide of  claim 1 . 
     
     
         22 . The method of  claim 21 , wherein the peptide is administered parenterally. 
     
     
         23 . The method of  claim 21 , wherein the peptide comprises at least one peptide selected from the group consisting of:
 Tyr-cyclo[D-Lys-Trp-Phe-Glu]-Gly-NH 2  (SEQ ID NO:3),   Tyr-cyclo[D-Glu-Phe-Phe-Lys]-Gly-NH 2  (SEQ ID NO:4),   and   Tyr-cyclo[D-Orn-Phe-p-Cl-Phe-Asp]-Val-NH 2  (SEQ ID NO:7).   
     
     
         24 . A method of activating a mu-opioid receptor comprising contacting the mu-opioid receptor with the peptide of  claim 1 . 
     
     
         25 . The method of  claim 24 , wherein the peptide comprises at least one peptide selected from the group consisting of:
 Tyr-cyclo[D-Lys-Trp-Phe-Glu]-Gly-NH 2  (SEQ ID NO:3),   Tyr-cyclo[D-Glu-Phe-Phe-Lys]-Gly-NH 2  (SEQ ID NO:4),   and   Tyr-cyclo[D-Orn-Phe-p-Cl-Phe-Asp]-Val-NH 2  (SEQ ID NO:7).   
     
     
         26 . A method for measuring the quantity of a mu opioid receptor in a sample, comprising:
 (i) contacting a sample suspected of containing a mu opioid receptor with the peptide of  claim 1  to form a compound-receptor complex;   (ii) detecting the complex formed in step (i); and   (iii) quantifying the amount of complex detected in step (ii).   
     
     
         27 . A competitive assay method for detecting the presence of a molecule that binds to a mu opioid receptor comprising:
 (i) contacting a sample suspected of containing a molecule that binds to a mu opioid receptor with a mu opioid receptor and the peptide of  claim 1 , wherein the peptide and receptor form a compound-receptor complex;   (ii) measuring the amount of the complex formed in step (i); and   (iii) comparing the amount of complex measured in step (ii) with the amount of a complex formed between the mu opioid receptor and the peptide in the absence of the sample.