Irradiation enhances cellular uptake of carboplatin.

Because radiation is known to damage cellular membranes, the purpose of this study was to determine whether irradiation of cultured cells might modify the cellular uptake of the chemotherapy agent carboplatin. Total intracellular platinum was measured using atomic absorption spectrometry in cultured V79 cells and in four Chinese hamster ovary (CHO) cell lines. Intracellular carboplatin concentrations increased linearly with radiation dose (10-50 Gy) under both hypoxic and oxic irradiation conditions. Similar doses of radiation did not significantly increase the uptake of a nontoxic platinum compound [Pt(NH3)4Cl2.H2O] (p > 0.5). Compared to unirradiated controls, there was no increase in intracellular carboplatin concentrations when carboplatin was irradiated prior to administration to the cell cultures (p > 0.5). Within the 32.5 min or less required to deliver the radiation, a dose of 50 Gy produced approximately a 50% increase in intracellular platinum in V79 cells and approximately an increase of a factor of 1.3-1.6 in the CHO cell lines. Although the increase in drug uptake would be expected to be less than 10% for most cell lines at the doses of radiation used to investigate radiosensitization by carboplatin, this level of increase may play a significant role in the radioenhancement observed in UV41 cells because these excision-repair--deficient cells are much more sensitive to carboplatin as measured by cytotoxicity. These results suggest that some of the enhanced cell killing that results when cells are exposed to carboplatin in combination with radiation may be attributed to an increased cellular uptake. One mechanism of radiopotentiation may be an enhanced chemotoxicity resulting from a radiation-induced increase in carboplatin uptake.