Abstract
Taxol, a unique tubulin active agent, was found to demonstrate a marked schedule-dependent synergistic interaction with cisplatin (DDP) in the killing of human ovarian carcinoma 2008 cells in vitro as determined by median effect analysis. The interaction was highly synergistic when 19 h taxol exposure was followed by 1 h concurrent exposure to taxol and DDP. The combination indices (CIs) on this schedule were 0.11 +/- 0.1, 0.25 +/- 0.15 and 0.39 +/- 0.14 at 20%, 50% and 80% cell kill respectively. However, the interaction was antagonistic when 1 h exposure to DDP was followed by 20 h exposure to taxol, or when cells were exposed to DDP and taxol for 1 h concurrently. When taxol preceded DDP, synergy was also observed with the 11-fold DDP-resistant 2008/C13*5.25 subline, which yielded CI values of 0.21 +/- 0.02, 0.30 +/- 0.11 and 0.31 +/- 0.17 at 20%, 50% and 80% cell kill respectively. At an IC50 concentration, taxol had no effect on [3H]cis-dichloro(ethylenediamine) platinum uptake, on the permeability of the plasma membrane or on glutathione or metallothionein levels in 2008 or 2008/C13*5.25 cells. Mitotic arrest in these cells was observed only at taxol concentrations well above those required for synergy with DDP, suggesting that the mechanism underlying the synergistic interaction was not a taxol-induced alteration in cell cycle kinetics. Of additional interest was the fact that the 2008/C13*5.25 cells were hypersensitive to taxol, and that this was partially explained by an alteration in the biochemical pharmacology of taxol. Although cellular taxol accumulation reached steady state within 2 h in both cell lines, taxol efflux was slower and the taxol was more extensively bound in 2008/C13*5.25 cells than in 2008 cells. In addition, the 2008/C13*5.25 cells had only 55% of the parental levels of beta-tubulin content. However, in another pair of DDP-sensitive and -resistant ovarian cell lines no taxol hypersensitivity and no change in beta-tubulin content was observed, indicating that the DDP-resistant and taxol-hypersensitive phenotypes do not segregate together. We conclude that taxol interacts synergistically with DDP in a manner that is highly schedule dependent, and that the hypersensitivity of 2008/C13*5.25 cells no taxol is unrelated to the mechanism of synergy. These in vitro observations suggest that drug schedule will be an important determinant of the activity and toxicity of the DDP and taxol drug combination in clinical studies.