Received for publication March 10, 2006.
Revised June 8, 2006.
Accepted for publication June 8, 2006.

In vitro and in vivo anti-tumor effects of cytotoxic camptothecin-bombesin conjugates are mediated by specific interaction with cellular bombesin receptors

Abstract

Most human tumors overexpress or ectopically express peptide hormone/neurotransmitter receptors, which are being increasingly studied as a means to selectively deliver cytotoxic agents. Whereas 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 bombesin (CPT-Bn) analogs, in which CPT was coupled via a novel carbamate linker (L2), 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-Tyr⁶,-Ala¹¹,Phe¹³,Nle¹⁴] Bn (6-14) [CPT-L2-BA3] bound with high affinity and had high potency for all three human Bn receptor subtypes, whereas CPT-L2-[D-Tyr⁶,-Ala¹¹,DPhe¹³,Nle¹⁴] Bn (6-14) [DPhe-CPT-L2-BA3] had >1400 fold lower affinity/potency. ¹²⁵I-CPT-L2-BA3 but not ¹²⁵I-DPhe-CPT-L2-BA3 was internalized by Bn receptor subtype-containing cells. CPT-L2-BA3 displayed significantly more cytotoxicity than DPhe-CPT-L2-BA3 towards NCI-H1299 lung cancer cells in both MTT 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 CPT-L2-BA3 will be a useful agent for delivering receptor-mediated cytotoxicity to many different human tumors.

Key words: bombesin receptors, camptothecin, lung cancer, peptide metabolism, receptor-mediated cytotoxicity, tumor proliferation

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