IN VITRO CYTOTOXICITY OF CPT-11 (SN-38) ALONE AND IN COMBINATION WITH CISPLATIN ON OVARIAN CANCER CELLS

Abstract

We investigated the in vitro cytotoxic effects of CPT-11, a new derivative of camptothecin, and its metabolizable form in vivo, SN-38, on cisplatin-resistant (SHIN-3) and -sensitive (MN-1) ovarian cancer cell lines using the MTT assay. We also attempted to identify the optimal schedule of administration for cisplatin combined with low-dose continuous exposure to CPT-11 (3 μg/ml) or SN-38 (5 ng/ml). With either CPT-11 or SN-38 alone, a marked schedule-dependent inhibition of growth was obtained with exposure for over 40 hours to concentrations of those agents applicable to clinical use (CPT-11 : <7.58 μg/ml, SN-38 : <72 ng/ml), even in a cisplatin-resistant cell line. Increasing the assay AUC of these agents only by dose escalation did not enhance cytotoxity with both MN-1 (CPT-11 : 3-50 μg/ml, SN-38 : 2.16-72 ng/ml) and SHIN-3 (CPT-11 : 5-50 μg/ml, SN-38 : 21.6-720 ng/ml) cell lines up to 72 hours of exposure. Pretreatment of SHIN-3 cells with either CPT-11 (3 μg/ml) or SN-38 (5 ng/ml) for 48 hours before the administration of cisplatin (4-20 μg/ml) appeared to produce an inhibition which exceeded that produced by either a longer (96 hours) or shorter (0 hour) pretreatment with them. Flow cytometric analysis showed that treatment with CPT-11 and SN-38 in SHIN-3 cell line was associated with a peak in G₂/M phase at 24 hours and an increase in the G₀/G₁ phase fraction for up to 96 hours. It is thus suggested that the continuous administration of a low dose of CPT-11 or of SN-38 may be useful clinically. The in vitro cytotoxicity of cisplatin could be increased by pretreatment with either CPT-11 or SN-38.