Effects of the combination of camptothecin and doxorubicin or etoposide onrat glioma cells and camptothecin-resistant variants

From the rat C6 glioma cell line in culture, we selected camptothecin-resis tant 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 range d between 15-fold (C6(CPT10)) and 30-fold (C6(CPT50) and C6(CPT100)). The C 6(CPT10) cell line presented a collateral sensitivity to etoposide (3.6-fol d), while the C6(CPT50) and C6(CPT100) cell lines were cross-resistant to e toposide (1.8-fold) The resistant lines were characterised by a two-fold re duced content and catalytic activity of topoisomerase 1, and C6(CPT50) and C6(CPT100) presented a significant increase in topoisomerase II alpha. cont ent and catalytic activity and a marked overexpression of P-glycoprotein. W e 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 an tagonistic effects on cell survival using the median effect principle. The simultaneous combination of camptothecin and doxorubicin or etoposide was a dditive 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 line s. The sequential combination of doxorubicin and camptothecin gave additivi ty in the order camptothecin --> doxorubicin and antagonism in the order do xorubicin --> 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 a nd doxorubicin. (C) 2001 Cancer Research Campaign http://www.bjcancer.com.