Development of high affinity camptothecin-bombesin conjugates that have targeted cytotoxicity for bombesin receptor-containing tumor cells.
Mammalian bombesin (BN) receptors are among those most frequently overexpressed by a number of common tumors including prostate, breast, lung, and colon cancers. The aim of this study was to develop a camptothecin-bombesin (CPT-BN) conjugate that interacts with all classes of BN receptors and possibly functions as a prodrug via a labile linker with site-specific cytotoxicity for cancer cells bearing these receptors. CPT was coupled to analogs of [D-Tyr6,beta-Ala11,Phe13,Nle14]BN-(6-14) (BA0) using carbamate linkers (L1 and L2) with built-in nucleophile-assisted releasing groups for intracellular cleavage of free cytotoxic agents. One conjugate, CPT-L2-BA3, bound to all three BN receptor classes with high affinity and functioned as a full agonist at each. 125I-CPT-L2-BA3 was rapidly internalized by cells expressing each BN receptor class and, using fluorescent imaging, was found to co-localize with BN receptors initially and later to be internalized in cytoplasmic compartments. HPLC analysis of internalized ligand showed that 40% was intact, 25% was metabolized by releasing free CPT, and 35% was metabolized to other breakdown products. CPT-L2-BA3 inhibited the growth of NCI-H1299 non-small cell lung cancer cells in 3-(4,5-dimethylthiazol-2-yl)-2.5-diphenyl-2H-tetrazolium bromide (MTT) and clonal growth assays. CPT-L2-BA3 was cytotoxic in an MTT assay for cells transfected with each class of BN receptor; however, it had significantly less effect in cells lacking BN receptors. These results indicate that CTP-L2-BA3 is a potent agonist that is cytotoxic for cells overexpressing any of the three BN receptor classes and functions as a prodrug for receptor-mediated cytoxicity. It therefore should be a useful prototype to explore the effectiveness of tumor-specific cytotoxicity delivery using a receptor-mediated mechanism.
Moody
Terry W
TW
Center for Cancer Research, NCI, National Institutes of Health, Bethesda, Maryland 20892, USA.
Mantey
Samuel A
SA
Pradhan
Tapas K
TK
Schumann
Michael
M
Nakagawa
Tomoo
T
Martinez
Alfredo
A
Fuselier
Joseph
J
Coy
David H
DH
Jensen
Robert T
RT
D000818:Animals; D000970:Antineoplastic Agents / Q000378:metabolism / Q000633:toxicity*; D001839:Bombesin; D002166:Camptothecin / Q000378:metabolism / Q000633:toxicity*; D016923:Cell Death / Q000187:drug effects; D016503:Drug Delivery Systems; D015195:Drug Design; D051379:Mice; D009369:Neoplasms / Q000188:drug therapy* / Q000378:metabolism / Q000473:pathology; D011485:Protein Binding; D018004:Receptors, Bombesin / Q000037:antagonists & inhibitors* / Q000378:metabolism
D000970:Antineoplastic Agents; D018004:Receptors, Bombesin; D001839:Bombesin; D002166:Camptothecin