Biologic and clinical developments of cisplatin combined with radiation: concepts, utility, projections for new trials, and the emergence of carboplatin.

Controlled experiments have shown that more than one mechanism leads to the potentiation of radiation-induced cell killing by cisplatin, and that this potentiation is not uniformly expressed among different cell types. A firm investigative base for the design of clinical trials using cisplatin and radiation has not been established. Coincident with this deficiency of experimental guidance, the independent clinical investigator has developed an array of therapeutic strategies applying different doses and sequences of cisplatin and radiation to a variety of tumor types. Results of clinical studies integrating cisplatin and radiation that can be judged for perceived survival benefit are evaluated in comparison with existing radiobiologic information. Both the clinical and radiobiologic results lead to similar conclusions at this time. Cells that are relatively sensitive to the cytotoxic action of cisplatin alone would best be considered for combined treatment with radiation. Large and infrequent, rather than small and frequent, individual administrations of cisplatin are better used with radiation for enhanced therapeutic effectiveness. Administration of cisplatin close in time to radiation is best for therapeutic response, although perceived efficacy follows from rather flexible integrations of these two modalities. It is not possible to know if clinical efficacy results from radiation potentiation as opposed to some degree of additivity of the two modalities. It is nonetheless useful to anticipate strategies that might lead to radiation potentiation by cisplatin in therapeutic designs. Two general mechanisms by which cisplatin potentiates radiation-induced cell killing are identified. One mechanism of potentiation is free radical-mediated, at least in part leads to an active radiolytic species following one-electron reduction of cisplatin, and is more readily expressed with bacterial cells than with mammalian cells in tissue culture. A second mechanism of potentiation is biochemical in nature, involves an effect of cisplatin on cellular components in ways that inhibit the recovery of radiation-induced damage, and likely applies more to the potentiation of oxic mammalian cells than bacterial cells. The latter mechanism is not universally supported in the literature. However, a unifying hypothesis, and one in need of confirmation at this time, is that the biochemical mechanism of radiation potentiation by cisplatin operates in oxic mammalian cells that are inherently sensitive to the cytotoxic action of cisplatin. This hypothesis ostensibly applies to tumor cells that are responsive to chemotherapy with cisplatin.(ABSTRACT TRUNCATED AT 400 WORDS)