Structure-activity relationship studies optimizing the antiproliferative activity of novel cyclic somatostatin analogues containing a restrained cyclic β-amino acid

The cyclic somatostatin analogue cyclo[Pro(1)-Phe(2)-D-Trp(3)-Lys(4)-Thr(5)-Phe(6)] (L-363,301) displays high biological activity in inhibiting the release of growth hormone, insulin, and glucagon. According to the sequence Of L-363,301, we synthesized a number of cyclic hexa- and pentapeptides containing nonnatural alpha- and beta-amino acids. The N- fluorenylmethoxycarbonyl protected cyclic beta-amino acid [1S, 2S, 5R]-2-amino-3,5-dimethyl-2-cyclohex-3-enecarboxylic acid (c beta AA), for the replacement of the Phe(6)-Pro(1) moiety of L-363,301, was synthesized in two steps by an enantioselective multicomponent reaction using (-)-8-phenylmenthol as a chiral auxiliary. The resulting peptide cyclo[c beta AA(1)-Tyr(2)-D-Trp(3)-Nle(4)-Thr(Trt)(5)] (Trt = triphenylmethyl) shows high antiproliferative effects in an in vitro assay with A431 cancer cells. The same peptide without the Trt group does not reveal any biological activity, whereas L-363,301 and closely related hexapeptides show only minor activity. By comparison of the solution structure of cyclo-[c beta AA(1)-Tyr(2)-D-Trp(3)-Nle(4)-Thr(Trt)(5)] with the structure of L-363,301, a nearly perfect match of the beta II'-turn region with D-Trp in the i + 1 position was observed. The cyclic beta-amino acid c beta AA is likely needed for the bioactive conformation of the peptide.