The Role of the Merrifield Solid Phase Method in the Discovery and Exploration of a New Class of Selective Vasopressin Hypotensive Agonists

The Merrifield solid phase method (MSPM) has been of inestimable value in studies aimed at the design of selective agonists and antagonists of the V 1a (vascular), V 1b (pituitary), V 2 (renal) and OT (uterine) receptors for vasopressin (VP) and oxytocin (OT). Here we describe how, 40 years after the landmark syntheses of oxytocin and vasopressin by Vincent du Vigneaud and colleagues, it led to the discovery of a new class of vasopressin agonists which exhibit selective hypotensive activity. We also point to the role of serendipity in this discovery. We furthermore show how the MSPM has been of inestimable value in facilitating a rapid and comprehensive structure activity study of the lead hypotensive peptide: d(CH 2 ) 5 [D-Tyr(Et) 2 ,Arg 3 ,Val 4 ]AVP (A) giving rise to d(CH 2 ) 5 [D-Tyr(Pr i ) 2 ,Arg 3 ,Val 4 ,Lys 7 ,Eda 9 ]LVP (B) which exhibits a 30-fold enhancement in vasodepressor potency relative to (A). Here, we also report a structure activity study of (B) with single modifications at position 3 (Lys, Nar) and 4 (Cha, Nle, Leu, Abu, Nva, Thr, Har) and combined modifications at positions 3 and 9 (Nar and EdaG) and 7 and 9 (Arg and Eda ← retro Tyr). All modifications of (B) are well tolerated with good retention of vasodepressor potency. These findings offer promising clues to the design of more potent VP hypotensive agonists and of critically needed antagonists of the putative VP vasodilating receptor. The Merrifield Solid Phase Method will continue to play a pivotal role in these studies.