Effects of camptothecin conjugated to a somatostatin analog vector on growth of tumor cell lines in culture and related tumors in rodents

Two CPT-SSA conjugates, JF-10-71 and JF-10-81, containing a chemically adjustable release-rate carbamate linker, have been reported previously by us to potently inhibit growth of human neuroblastoma IMR32 cells overexpressing somatostatin receptor type II (SSTR2) but are stable under buffer incubation conditions or in rat plasma. Further experiments now reveal that the conjugates performed well against many additional cell lines, particularly somatostatin receptor containing rat pancreatic CA20948 cells that were actually more sensitive to the conjugates than free camptothecin itself. JF-10-71 and JF-10-81 also were examined for their inhibitory effects on the growth of this and several other tumors transplanted into rats (CA20948) or nude mice. CA20948 tumors, known to overexpress SSTR2 and grown in Lewis rats, were treated, respectively, with nontoxic 400 nmol/kg intraperitoneal (ip) doses of JF-10-71 or JF-10-81. Also, SSTR2-positive human SCLC NCI-H69 tumors transplanted in nude mice were treated in a similar fashion. Human prostate PC-3 tumors, which do not contain high concentrations of SSTR2, also were grown in nude mice and treated with a 400 nmol/kg ip dose of JF-10-71. Both cytotoxic conjugates significantly inhibited growth of SSTR2-specific pancreatic and SCLC tumors, but JF-10-81 did not significantly affect PC-3 tumor growth. These experimental results suggested that CPT-SSA conjugates can effectively target and kill tumor cells growing in vivo and that the effect is mediated by somatostatin receptors resulting in either release of camptothecin at the cell surface or, more likely, after receptor-mediated cellular internalization.