The cytotoxic activity of ecteinascidin 743 (ET-743), a natural product derived from the marine tunicate Ecteinascidia turbinata that exhibits potent anti-tumor activity in pre-clinical systems and promising activity in phase I and II clinical trials, was investigated in a number of cell systems with well-defined deficiencies in DNA-repair mechanisms. ET-743 binds to N2 of guanine in the minor groove, but its activity does not appear to be related to DNA-topoisomerase I poisoning as the drug is equally active in wild-type yeast and in yeast with a deletion in the DNA-topoisomerase I gene. Defects in the mismatch repair pathway, usually associated with increased resistance to methylating agents and cisplatin, did not affect the cytotoxic activity of ET-743. However, ET-743 did show decreased activity (from 2-to 8-fold) in nucleotide excision repair ( Ecteinascidin 743 (ET-743) is a natural product derived from the marine tunicate Ecteinascidia turbinata, selected for its significant anti-tumor activity in different in vitro and in vivo models. [1][2][3] Currently, ET-743 is undergoing phase II clinical trials after having shown activity in phase I clinical studies with responses observed in different human tumors, including soft tissue sarcoma, osteosarcoma, melanoma and breast cancer. 4,5 The mechanism of action of ET-743 has yet to be fully defined, but DNA appears to be the primary target. Indeed, it has been shown to bind in the minor groove of DNA and to alkylate the N2 position of guanine with some degree of specificity. 6,7 Such ET-743 minor-groove alkylation induces a bend in the DNA helix toward the major groove, a finding not common to the other minor groove-alkylating agents, which bend DNA into the minor groove. 8 No single-strand breaks, double-strand breaks or DNAprotein cross-links could be found by alkaline elution in cells exposed to IC 50 concentrations of ET-743. 9 However, Takebayashi et al. 10 showed that ET-743 was able to induce DNA-topoisomerase I cross-linking but that high concentrations were required to produce the effect.To provide some insight into the mechanism of action of ET-743, we have evaluated its cytotoxic activity using a panel of different cellular systems characterized by well-defined deficiencies in different mammalian repair pathways. Our data indicate that ET-743 has a unique mechanism of interaction with DNA. MATERIAL AND METHODS Cells and drugsYeast strains and the isogenic derivatives CY2823 (⌬rad52) and CY2278 (⌬topI) (kindly provided by Dr. M. Lopes, Instituto F.I.R.C. di Oncologia Molecolare, Milan, Italy) were grown in YPD plates (1% yeast extract, 2% bacto-peptone, 2% glucose) with or without the drugs.The Chinese hamster ovary (CHO) parental cell line CHO-AA8 and the UV-sensitive DNA repair-deficient mutant cell lines CHO-UV23, CHO-UV61 and CHO-UV96 (hereafter named UV23, UV61 and UV96) 11 were kindly provided by Dr. M. Stefanini (CNR IGBE, Pavia, Italy). The UV96 cell line was transfected by the calcium phosphate technique using 10 g of a CMV-ERCC1 plasmid enc...