The cellular pharmacology of the tritium-labeled cisplatin analogue dichloro(ethylenediamine)platinum(II) ([3H]DEP) was compared in cisplatin-sensitive 2008 and resistant 2008/C13*5.25 human ovarian carcinoma cells. The cellular content of total [3H], ultrafiltrable [3H], and free native [3H]DEP was measured during and following incubation with 5 µm [3H]DEP. While the rate constant for [3H]DEP uptake in the resistant cells was reduced to 25% of that in the sensitive cells, DNA intrastrand adduct formation was reduced even further to 11%, indicating the presence of defects in both uptake and the ability of intracellular drug to access or react with DNA. The latter could not be accounted for by enhanced repair. Together, these defects were sufficient to account for the 11-fold level of resistance. At steady state, the intracellular to extracellular concentration ratio for native [3H]DEP was 7.7 in the sensitive cells and 11.7 in the resistant cells, suggesting the presence of a trapping or concentrative mechanism. Thus, despite the slower initial influx, the resistant cells eventually accumulated more free [3H]DEP than the sensitive cells. We conclude that the resistant phenotype in these cells is accounted for primarily by impaired uptake and decreased reaction of [3H]DEP with DNA rather than by changes in efflux or DNA repair.
Supported by Grant CA 52151 from the NIH, Grant CH-377 from the American Cancer Society, Grant 100-R40 from Bristol-Myers Squibb, and a grant from the Academy of Finland.