Functional activity of new C-terminal cyclic-neurotensin fragment analogs

Neurotensin (NT, pGlu-Leu-Tyr-Glu-Asn-Lys-Pro-Arg-Arg-Pro-Tyr-Ile-Leu) is a tridecapeptide that displays a wide spectrum of biological actions. Cyclic derivatives of a hexapeptide NT [(8-13)] (N alpha MeArg-Lys-Pro-Trp-Tle-Leu, Tle = tert-leucine) were designed and prepared by a combination of solution and solid-phase peptide synthetic methodologies. As reported previously, several analogs possessed nanomolar binding affinities for NT receptors in newborn (10-day-old) mouse brain membrane preparations. In this study, we determined the functional ability of these analogs to mobilize intracellular free calcium, [Ca2+]i, in HT-29 cells (human colonic adenocarcinoma). Of greatest interest were the cyclic compounds 2, 6 and 9 that had Ki values of 0.19, 3.50 and 4.18 microM for [3H]NT labeled receptors in the HT-29 cell membrane assay, respectively. In the functional assay, compounds 2 and 6 mobilized [Ca2+] with EC50 values of 0.13 and 20 microM, respectively. In comparison, Compound 9 blocked the NT-induced mobilization of [Ca2+]i, with an IC50 of 1.70 microM. The present findings indicate that small molecule cyclic analogs, that possess functional activity, can be designed and may have therapeutic utility in the treatment of schizophrenia and possibly other neurological disorders.