Most human
tumors overexpress or ectopically express
peptide hormone/
neurotransmitter receptors, which are being increasingly studied as a means to selectively deliver
cytotoxic agents. Although a number of
peptide ligand-constructs demonstrate
tumor cytotoxicity, the role of specific tumoral receptor interaction in its mediation is unclear. To address this question, we synthesized
camptothecin (
CPT)
bombesin (Bn) analogs, in which
CPT was coupled via a novel
carbamate linker, L2 [N-(N-methyl-amino-ethyl)-
glycine carbamate], that were chemically similar but differed markedly in their potency/affinity for human Bn receptors. We then examined their ability to interact with Bn receptors and cause in vitro and in vivo
tumor cytotoxicity.
CPT-L2-[D-Tyr(6),beta-Ala(11),D-Phe(13),Nle(14)] Bn (6-14) (BA3) bound with high affinity and had high potency for all three human Bn receptor subtypes, whereas
CPT-L2-[D-Tyr(6),beta-Ala(11), D-Phe(13),Nle(14)] Bn (6-14) [D-Phe-
CPT-L2-BA3] had >1400-fold lower affinity/potency. (125)I-CPT-L2-BA3 but not (125)I-D-Phe-CPT-L2-BA3 was internalized by Bn receptor subtype-containing cells. CPT-L2-BA3 displayed significantly more cytotoxicity than D-Phe-CPT-L2-BA3 toward NCI-H1299
lung cancer cells in both 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-2H-tetrazolium
bromide and clonogenic assays and more potently inhibited H1299 xenograft growth in nude mice. CPT-L2-BA3 was also metabolically more stable than its parent
peptide and inhibited growth of a number of other tumor cell lines in vitro and in vivo. These results demonstrate that specific tumoral receptor interaction is important in mediating the ability of
peptide ligand-cytotoxic constructs to cause cytotoxicity. Because many
tumors overexpress Bn receptors, these results also demonstrate that CPT-L2-BA3 will be a useful agent for delivering receptor-mediated cytotoxicity to many different human
tumors.