Development of a novel class of cyclic hexapeptide oxytocin antagonists based on a natural product

Abstract

A new structural class of cyclic hexapeptide oxytocin antagonists derived from Streptomyces silvensis and typified by L-365,209 (cyclo-[L-prolyl1-D-phenylalanyl2-L- isoleucyl3-D-dehydropiperazyl4-L-dehydroperazyl5-D-(N- methyl)phenylalanyl6]) was recently reported. In this paper we further delineate the structure-activity profile for this new class by systematic study of L-365,209 analogs obtained by total synthesis. The optimal combination of cyclic amino acid ring sizes at positions 1, 4, and 5 and the role of the N-alkyl substituent at position 6 was elucidated. The lipophilic amino acids at positions 2 and 3 and the unusual amino acid D-dehydropiperazic acid at position 4 were found to be the most critical residues for obtaining good oxytocin receptor affinity. Analogs containing a basic side chain at the less critical 5- and 6-positions maintained good receptor affinity and also had useful levels of water solubility for intravenous formulation. By combining potency- and solubility-enhancing substitutions, several analogs were identified that have the desired combination of properties in vitro (22, cyclo-[L-prolyl-D-tryptophanyl-L-isoleucyl-D-pipecolyl-L-pipeco lyl-D- histidyl]; 25, cyclo-[L-prolyl-D-2-naphthylalanyl-L-isoleucyl-D-pipecolyl-L -pipecolyl-D- histidyl]; 26, cyclo-[L-prolyl-D-tryptophanyl-L-isoleucyl-D-dehydropiperazyl-L-++ pipecolyl-D-histidyl]; 33, cyclo-[L-prolyl-D-tryptophanyl-L-isoleucyl-D-pipecolyl-L- piperazinylcarboxy-D-(N-methyl)phenylalanyl]; 34, cyclo-[L-prolyl-D-phenylalanyl-L-isoleucyl-D-dehydropiperazyl-L-or nithyl- D-(N-methyl)phenylalanyl]). In general, this class exhibited good selectivity for binding to the oxytocin receptor versus the arginine vasopressin V1a and V2 receptor subtypes, although increased V2 receptor affinity was observed in one case (32, cyclo[L-prolyl-D-2-naphthylalanyl-L-isoleucyl-D-pipecolyl-L- lysyl-D-(N- methyl)phenylalanyl]). Unexpectedly, compound 33 was found to stimulate contractions of the isolated rat uterus via activation of the uterine bradykinin receptor. Compounds 22, 25, 26, 33, and 34 were found to be potent antagonists of oxytocin-stimulated contraction of the rat uterus in vitro and in vivo. Compounds 22 and 25 were additionally characterized as potent antagonists of oxytocin-stimulated uterine contractions in the near-term pregnant rhesus monkey. These studies thus demonstrate the selectivity and efficacy of certain members of this novel class of antagonists and suggest their use as pharmacological tools in further defining the role of oxytocin in both term and preterm labor.

Small molecule ligands for oxytocin and vasopressin receptors

“…Specific goals were oral bioavailability and adequate aqueous solubility for intravenous formulation in a single compound. Numerous analogs from modification of the natural product 48 and compounds prepared by total synthesis 49 were studied. Each approach was successful in identifying potent, selective, and soluble antagonists, and L-366,670 and L-366,948, respectively, are optimal examples (Fig.…”

HOBt·DCHA-Mediated Synthesis of Sterically Hindered Peptides employing Fmoc-Amino Acid Chlorides in Both Solution-Phase and Solid Phase Methods

“…The acid chloride coupling mediated by an organic tertiary amine (pyridine, diisopropylethylamine (DIEA)), an essential component of such a reaction protocol, is known to result in the formation of oxazol-5(4H)-one, [5] leading to stereomutation and=or premature deblocking of Fmoc-group, due to the prolongation of the course of the reaction (usually required for the incorporation of hindered amino acids). This has been circumvented by the use of co-coupling agents such as silver cyanide [6,7] potassium salts of 1-hydroxybenzotriazole (KOBt) [8,9] and 1-hydroxy-7-aza-benzotriazole (KOAt), [10,11] t-butyldimethylsilyloxy benzotriazole (TBDMS) derivatives of HOBt (TBMS-OBt) and HOAt (TBDMS-OAt), [12,13] and zinc dust. [14,15] The use of KOBt=KOAt=TBDMS-OBt or TBDMS-OAt converts acid chloride to its benzotriazole=azabenzotriazole ester, which acts as an acylating agent, whereas in the presence of zinc dust-mediated coupling, the liberated HCl is directly abstracted.…”

Asymmetric syntheses of pipecolic acid and derivatives

“…In addition, L-pipecolic acid, as well as other amino acids showing a 6-membered ring, have been used in peptide chemistry as analogues of L-proline (Copeland et al, 1990;Williams et al, 1992). Introduction of these compounds into peptides induce a fi-turn, and this modification of the secondary structure can result in an advantageous change of the biological activity.…”