Novel E-ring camptothecin keto analogues (S38809 and S39625) are stable, potent, and selective topoisomerase I inhibitors without being substrates of drug efflux transporters

• Published in: Molecular cancer therapeutics 2007-12, Vol.6 (12), p.3229-3238
• Main Authors: Takagi, Kazutaka, Dexheimer, Thomas S, Redon, Christophe, Sordet, Olivier, Agama, Keli, Lavielle, Gilbert, Pierré, Alain, Bates, Susan E, Pommier, Yves
• Format: Article
• Language: English
• Subjects: Antineoplastic Agents, Phytogenic - chemistry; Antineoplastic Agents, Phytogenic - metabolism; Antineoplastic Agents, Phytogenic - pharmacology; ATP-Binding Cassette Transporters - metabolism; Base Sequence; Biological Transport; Camptothecin - analogs & derivatives; Camptothecin - chemistry; Camptothecin - metabolism; Camptothecin - pharmacology; camptothecins; Cell Line, Tumor; DNA - metabolism; DNA Primers; drug resistance; Enzyme Inhibitors - chemistry; Enzyme Inhibitors - metabolism; Enzyme Inhibitors - pharmacology; histone H2AX; Humans; Hydrolysis; Molecular Structure; topoisomerase; Topoisomerase I Inhibitors; yeast
• ISSN: 1535-7163; 1538-8514
• DOI: 10.1158/1535-7163.MCT-07-0441

Camptothecin (CPT) analogues are powerful anticancer agents but are chemically unstable due to their α-hydroxylactone six-membered E-ring structure, which is essential for trapping topoisomerase I (Top1)-DNA cleavage complexes. To stabilize the E-ring, CPT keto analogues with a five-membered E-ring lacking the oxygen of the lactone ring (S38809 and S39625) have been synthesized. S39625 has been selected for advanced preclinical development based on its promising activity in tumor models. Here, we show that both keto analogues are active against purified Top1 and selective against Top1 in yeast and human cancer cells. The keto analogues show improved cytotoxicity toward colon, breast, and prostate cancer cells and leukemia cells compared with CPT. The drug-induced Top1-DNA cleavage complexes induced by the keto analogues show remarkable persistence both with purified Top1 and in cells following 1-h drug treatments. Moreover, we find that S39625 is not a substrate for either the ABCB1 (multidrug resistance-1/P-glycoprotein) or ABCG2 (mitoxantrone resistance/breast cancer resistance protein) drug efflux transporters, which sets S39625 apart from the clinically used CPT analogues topotecan or SN-38 (active metabolite of irinotecan). Finally, we show that nanomolar concentrations of S38809 or S39625 induce intense and persistent histone γ-H2AX. The chemical stability of the keto analogues and the ability of S39625 to produce high levels of persistent Top1-DNA cleavage complex and its potent antiproliferative activity against human cancer cell lines make S39625 a promising new anticancer drug candidate. Histone γ-H2AX could be used as a biomarker for the upcoming clinical trials of S39625. [Mol Cancer Ther 2007;6(12):3229–38]