Solid-phase synthesis and biological properties of psi [CH2NH] pseudopeptide analogues of a highly potent somatostatin octapeptide.

A series of psi [CH2NH] pseudopeptide analogues of a potent somatostatin octapeptide analogue, H-D-Phe-Cys-Try-D-Trp-Lys-Val-Cys-Thr-NH2, were synthesized by using a newly developed solid-phase method for direct introduction of the CH2NH peptide bond isostere. Analogues were obtained in normal yields via a combination of sodium cyanoborohydride mediated reductive alkylation of resin-bound peptide amine with a tert-butoxycarbonyl amino acid aldehyde for introduction of a CH2NH bond and normal solid-phase peptide chemistry. A racemization test on a model pseudodipeptide indicated that no more than 6% racemization took place during the aldehyde preparation and alkylation steps. Analogues were examined for their ability to inhibit growth hormone release in vivo in sodium pentobarbital anesthetized rats and in vitro from cultured rat anterior pituitary cells. Analogues modified at the amide carbonyl of Cys2 and Lys5 showed the highest in vivo activity (20 and 8 times more potent than SRIF, respectively) and in vitro activity (0.17 and 0.67 times as active as SRIF). This compared to in vivo and in vitro potencies of 8000% and 100%, respectively, for the parent analogue. Modification of the other cyclic ring amide carbonyls of Tyr3, D-Trp4, and Val6 or the N- or C-terminal amide carbonyl of D-Phe1 or Cys7 gave analogues with considerably lower in vitro or in vivo potencies. The results appear to offer support for a proposed type II beta-turn solution conformation centered on the D-Trp-Lys portion of SRIF octapeptide analogues, resulting in possible hydrogen-bonding interactions between Tyr3 and Val6 and D-Phe1 and Thr8 residues in the parent peptide. These would then be disrupted by methylene replacement of the carbonyl groups with concomitant loss of biological activity as was observed.