ANALOGS OF GROWTH HORMONE-RELEASING FACTOR (1-29) AMIDE CONTAINING THE REDUCED PEPTIDE-BOND ISOSTERE IN THE N-TERMINAL REGION

Previous peptide structure-activity investigations employing the ɸ[CH2NH] peptide bond isostere have produced antagonists when inserted into various sequences. These include bombesin, in which the incorporation of Leu13fy[CH2NH]Leu14 produced a potent antagonist, and tetragastrin, with which Boc-Trp-Leu^[CH2NH]Asp-Phe-NH2 is an antagonist. In this study, we chose to investigate the effect of this isostere on growth hormone-releasing factor (1–29) amide. Analogues were prepared by solid-phase synthesis and the isosteres incorporated by racemization-free reductive alkylation with a preformed protected amino acid aldehyde in the presence of NaBH3CN. The aldehydes were prepared by the reduction of the protected N,O-dimethyl hydroxamates with LiAlH4 at 0 °C. The purified analogues were assayed in a 4-day primary culture of male rat anterior pituitary cells for growth hormone (GH) release. Potential antagonists were retested in the presence of GRF(1-29)NH2. The following results were obtained: At position 5–6, a very weak agonist was produced with «0.01% activity. Incorporation of the isostere in positions 1–2, 2–3, and 6–7 gave weak agonists with ~0.1% activity. Agonists with 0.39% and 1.6% activity were produced by incorporation at 10–11 and 34, respectively. The analogue [Ser9ɸ[CH2NH]Tyr10]GRF(l-29)NH2was found to be an antagonist in the 10 µM range vs 1 nM GRF and had no agonist activity at doses as high as 0.1 mM. © 1990, American Chemical Society. All rights reserved.