Cytotoxic profiles of various antineoplastic agents against SKOV-3, OV-90 and TOV-21G cells. Panel A, cytotoxicity was measured using clonogenic assays. For the guanine O6-chloroethylating agent 90CE, cells were pretreated with 10 µM O6-benzylguanine for 2 h to abrogate activity of O6-methylguanine-DNA methyltransferase (MGMT). O6-Benzylguanine was present in the medium throughout treatment and colony propagation. Panel B, for clarification, values of LC90 (concentration giving 90% lethality) and fold resistance (degrees of resistance in OV-90 and SKOV-3 relative to TOV-21G) are tabulated.