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Summary From the rat C6 glioma cell line in culture, we selected camptothecin-resistant variants by growth in the presence of increasing amounts of this drug (C6 CPT10 , C6 CPT50 and C6 CPT100 , growing respectively with 10, 50 and 100 ng ml -1 camptothecin). The degree of resistance to camptothecin ranged between 15-fold (C6 CPT10 ) and 30-fold (C6 CPT50 and C6 CPT100 ). The C6 CPT10 cell line presented a collateral sensitivity to etoposide (3.6-fold), while the C6 CPT50 and C6 CPT100 cell lines were cross-resistant to etoposide (1.8-fold) The resistant lines were characterised by a two-fold reduced content and catalytic activity of topoisomerase I, and C6 CPT50 and C6 CPT100 presented a significant increase in topoisomerase IIα content and catalytic activity and a marked overexpression of P-glycoprotein. We explored the cytotoxicity of combinations of a topoisomerase I inhibitor (camptothecin) and a topoisomerase II inhibitor (doxorubicin or etoposide) at several molar ratios, allowing the evaluation of their synergistic or antagonistic effects on cell survival using the median effect principle. The simultaneous combination of camptothecin and doxorubicin or etoposide was additive or antagonistic in C6 cells, slightly synergistic in the C6 CPT10 line and never more than additive in the C6 CPT50 and C6 CPT100 cell lines. The sequential combination of doxorubicin and camptothecin gave additivity in the order camptothecin → doxorubicin and antagonism in the order doxorubicin → camptothecin. Clinical protocols combining a topoisomerase I and a topoisomerase II inhibitor should be considered with caution because antagonistic effects have been observed with combinations of camptothecin and doxorubicin.

Our results indicate that (1) the cytotoxicity of irinotecan was likely due to the drug itself and not to its metabolite SN-38, and (2) that irinotecan uptake was more predictive of its cytotoxicity than topoisomerase I availability and activity in these two cell lines.

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