Reduced peptide bond pseudopeptide analogues of neurotensin: binding and biological activities, and in vitro metabolic stability

A series of pseudopeptide analogues of neurotensin was produced by systematically replacing the five peptide bonds in neurotensin-(8-13) with CH2NH (psi, reduced) bonds. All these analogues were synthesized with a free amino terminus (H derivatives) and with a N-terminal tert-butyloxycarbonyl group (Boc derivatives). The compounds were screened in vitro for agonist or antagonist activity and for metabolic stability by testing (1) their ability to inhibit the binding of radiolabelled neurotensin to homogenates of newborn mouse brain; (2) their ability to contract isolated guinea-pig ileum preparations; and (3) their degradation in the presence of rat brain homogenates. All the analogues bound to the mouse brain Neurotensin Receptor and all exhibited agonist activity in the guinea-pig ileum assay. Only the H- and Boc-[psi 8,9] derivatives were at least as potent as their parent compounds neurotensin-(8-13) and Boc-neurotensin-(8-13) in the binding and biological assays. All the other pseudopeptide analogues with reduced bonds at position 9-10, 10-11, 11-12 and 12-13 showed a marked reduction in potency ranging from 2 to 4 orders of magnitude. All the derivatives that were protected at their N terminus either by the presence of a Boc group or by the presence of a reduced bond at position 8-9 and 9-10 were slowly degraded by rat brain homogenates. The other derivatives were, in contrast, quite rapidly degraded. There was a good correlation between binding and biological potencies for those analogues that were resistant to degradation. Interestingly, the degradation-resistant H-[psi 8,9] compound exhibited higher binding and biological potency then neurotensin. It is therefore expected that this analogue will produce highly potent and long-lasting neurotensin-like effects in vivo, and preliminary experiments indicate that this is indeed the case.