Discovery and design of novel and selective vasopressin and oxytocin agonists and antagonists: the role of bioassays

Synthetic oxytocin and vasopressin agonists and antagonists have become imp ortant tools for research and were instrumental in the identification of th e four known receptor subtypes, V-1a, V-2, V-1b (V-3) and oxytocin, of thes e peptide hormones. However, the relative lack of receptor selectivity, par ticularly of the antagonists, has limited their usefulness as experimental probes and their potential as therapeutic agents. We now present some findi ngs from our continuing studies aimed at the design of more selective oxyto cin and vasopressin agonists and antagonists and a structure-activity relat ionship update on our recently discovered novel hypotensive vasopressin pep tides. Bioassays have been, and continue to be, of critical importance in l eading to the discovery of the novel agonists, antagonists and hypotensive peptides reported here. This paper highlights three main aspects of these s tudies. (1) Replacement of the tyrosine(2) and/or phenylalanine(3) residues in the V-2 agonist deamino,[Val(4), D-Arg(8)]arginine-vasopressin (dVDAVP) by thienylalanine resulted in selective V-2 agonists with strikingly high potencies. However, the peptide solutions were unstable and lost activity o ver time. These highly potent V-2 agonists, which are devoid of vasopressor activity, are promising leads for improving drugs for treating diabetes in sipidus, enuresis and coagulation disorders. (2) Diaminopropionic acid and diaminobutyric acid substitution at position-5 in oxytocin and in V-1a anta gonists yielded, respectively, the first specific antagonist for the oxytoc in receptor, desGly-NH2,d(CH2)(5)[D-Trp(2),Thr(4),Dap(5)]OVT and the first specific antagonist for the vasopressin V-1a receptor, d(CH2)(5)[Tyr(Me)(2) ,Dab(5)]AVP. The availability of single receptor subtype-specific or select ive antagonists will enhance our ability to delineate receptor functions. U tilising these new receptor specific probes, we were able to show that the uterotonic action of vasopressin is mediated principally by oxytocin and no t by V-1a receptors. (3) Replacement of the phenylalanine(3) residue in the V-1a/V-2/oxytocin antagonist, d(CH2)(5)[D-Tyr(Et)(2),Val(4)]AVP, with argi nine(3) yielded the novel, selective, hypotensive vasopressin peptide, d(CH 2)(5)[D-Tyr(Et)(2),Arg(3),Val(4)]AVP (Peptide I). Bioassay characterisation s of Peptide I show that its vasodepressor action is independent of the per ipheral autonomic, bradykinin, nitric oxide and prostaglandin systems and i s not mediated by the known classical oxytocin and vasopressin receptors. T hese findings suggest the existence of a new vasopressin receptor subtype t hat may be relevant to the vasodilating action of vasopressin in regional v ascular beds. Iodinatable hypotensive peptides have been synthesised and co uld be developed as markers for the putative new receptor. Ongoing structur e-activity relationship studies on Peptide I have led to more potent and se lective hypotensive peptides for use as new research tools and as leads for the development of a new class of antihypertensive agents.