REPLICATION-DEPENDENT AND REPLICATION-INDEPENDENT CAMPTOTHECIN CYTOTOXICITY OF 7 HUMAN COLON-TUMOR CELL-LINES

Anticancer inhibitors of topoisomerase I (TOPI, EC 5.99.1.2) cause the reversible stabilization of the TOP1-DNA covalent complex (cleavable complex). The cleavable complex can be converted into a double-strand break, the presumed cytotoxic lesion, by active replication forks. Cyt otoxicity independent of DNA replication has also been demonstrated, a nd suggested to have possible clinical significance. To assess the imp ortance of the replication-independent mechanism of camptothecin (CPT) cytotoxicity we have analyzed replication-dependent and replication-i ndependent cytotoxicity following a brief CPT treatment (40 min) of se ven human colon tumor cell lines. The cell lines were exposed to CPT i n the presence or absence of aphidicolin, an inhibitor of DNA polymera ses alpha, delta, or epsilon. The seven cell lines responded similarly to CPT: treatments of less than 0.5 mu M caused cytotoxicity only whe n DNA replication was ongoing, as evidenced by a plateau in the cytoto xicity curve corresponding to the S-phase fraction and the prevention of this cytotoxicity by aphidicolin cotreatment; at higher CPT doses, the cytotoxicity exceeded the S-phase fraction and was not prevented b y aphidicolin. The CPT sensitivity among the cell lines, measured as t he concentration required to inhibit cell growth by 25%, was between 0 .17 and 0.43 mu M without aphidicolin and 2-10 mu M with aphidicolin c otreatment; with aphidicolin in cotreatment, 20-fold greater CPT conce ntrations were required, on average among the cell lines, to achieve c ytotoxicity equivalent to CPT treatment alone. The potential of the lo wer dose and longer duration treatments of camptothecins used in the c linical setting to produce cytotoxicity independent of DNA replication is discussed.