Dynamics of cell cycle phase perturbations by trabectedin (ET-743) in nucleotide excision repair (NER)-deficient and NER-proficient cells, unravelled by a novel mathematical simulation approach

Objectives : Trabectedin (ET- 743, Yondelis (R)) is a natural marine product, with antitumour activity, currently in phase II/ III clinical trials. Previous studies have shown that cells hypersensitive to ultraviolet (UV)- rays because of nucleotide excision repair (NER) deficiency, were resistant to trabectedin. The purpose of this study was to investigate whether this resistance was associated with different drug- induced cell cycle perturbations. Materials and Methods : An isogenic NER- proficient cellular system (CHO- AA8) and a NER- deficient one (CHO- UV- 96), lacking functional ERCC- 1, were studied. Flow cytometric assays showed progressive accumulation of cells in G(2) + M phase in NER-proficient but not in NER- deficient cells. Applying a computer simulation method, we realized that the dynamics of the cell cycle perturbations in all phases were complex. Results : Cells exposed to trabectedin during G(1) and G(2) + M first experienced a G(1) block, while those exposed in S phase were delayed in S and G(2) + M phases but eventually divided. In the presence of functional NER, exit from the G(1) block was faster; then, cells progressed slowly through S phase and were subsequently blocked in G(2) + M phase. This G(2) + M processing of trabectedin- induced damage in NER- proficient cells was unable to restore cell cycling, suggesting a difficulty in repairing the damage. Conclusions : This might be due either to important damage left unrepaired by previous G(1) repair, or that NER activity itself caused DNA damage, or both. We speculate that in UV- 96 cells repair mechanisms other than NER are activated both in G(1) and G(2) + M phases.