To create cytotoxic hybrid analogs of
somatostatin (SST), octapeptides
RC-160 (D-Phe-Cys-Tyr-D-Trp- Lys-Val-Cys-Trp-NH2) and
RC-121 (D-Phe-Cys-Tyr-D-Trp- Lys-Val-Cys-Thr-NH2) were linked to
doxorubicin (DOX) or its superactive derivative,
2-pyrrolino-DOX (AN-201). The conjugation was performed by coupling N-9-fluorenylmethoxycarbonyl (N-Fmoc)-DOX-14-O-hemiglutarate or 2-pyrrolino-DOX-14-O-hemiglutarate to the amino terminus of [Lys(Fmoc)5]
RC-160 yielding AN-163 and AN-258, respectively, after deprotection. The respective cytotoxic conjugates of
RC-121 (AN-162 and AN-238) were prepared similarly. In vitro tests on human
cancer cell lines-MKN-45
gastric cancer, MDA-MB-231
breast cancer, PC-3
prostate cancer, and MIA PaCa-2
pancreatic cancer-demonstrated that the antiproliferative activity of the cytotoxic radicals in these conjugates was virtually retained. In H-345 human
small cell lung carcinoma cell line, conjugates of
RC-121 preserved the cytotoxic activity of their radicals, but the hybrids with
RC-160 showed approximately 10 times lower activity. The ability of the carriers and the hybrids to inhibit the binding of 125I-labeled
RC-160 to receptors for SST on rat pituitary membrane preparation was also determined. The cytotoxic conjugates inhibited 50% of the specific binding of the radioligand in the nanomolar concentration range (IC50 < 80 nM). When SST-like activities of
AN-238 and its carrier,
RC-121, were compared in the rat pituitary superfusion system, both compounds were found to suppress a stimulated
growth hormone release at nanomolar concentrations. Preliminary studies in animal models of breast and
prostate cancers showed that
AN-238 is less toxic than
AN-201 and more potent in inhibiting
tumor growth. These highly active cytotoxic analogs of SST have been designed as targeted
antitumor agents for the treatment of various
cancers expressing receptors for SST octapeptides.