Isobologram analysis of the combined effects of anti-tumour platinum complexes and ionizing radiation on mammalian cells.

Chinese hamster ovary cells have been treated in vitro with the platinum coordination complexes cis-PAD or CHIP and with radiation, either alone or in combination with different doses and time intervals. The isobologram method has been used to make a graphic comparison of these combined-modality data in terms of additivity and enhancement. The data showed enhancement of the radiation effect by these platinum complexes in many combinations, and a truly synergistic effect in one case. This method of analysis points to the limited usefulness of the parameter dose-modifying effect (DMF) since the most synergistic combination did not have the highest DMF.