Iododeoxyuridine chemosensitization of cis-diamminedichloroplatinum(II) in human bladder cancer cells.

We investigated the effect of iododeoxyuridine (IdUrd) exposure on cis-diamminedichloroplatinum (CDDP) cytotoxicity in the human bladder cancer cell line 647V. Following a 48-h incubation with 2-20 microM IdUrd, a 1-h exposure to 0-120 microM CDDP produced a dose-dependent increase in CDDP cytotoxicity as measured by clonogenic survival. IdUrd exposure of 2, 5, 10, and 20 microM prior to CDDP resulted in dose-modifying factors at 10% survival of 1.2, 1.6, 2.0, and 3.5, respectively. The increase in CDDP cytotoxicity appears to be associated with the level of DNA thymidine replacement in DNA by IdUrd over the range of 13-36%. Atomic absorption spectrophotometric analysis of DNA extracted from 647V cells showed that IdUrd substitution did not affect the total amount of platinum bound to the DNA or the persistence of the bound platinum over a 24-h period post-CDDP exposure versus control cells. IdUrd, unlike thymidine, was found to form two monofunctional adducts with CDDP both in vitro and in vivo. IdUrd was also found to form a mixed bifunctional adduct with deoxyguanosine (dGua) and CDDP in vitro. 1H NMR analysis of purified IdUrd-Pt and IdUrd-Pt-dGua adducts confirmed the identity of these adducts. High pressure liquid chromatography analysis of [3H]IdUrd-labeled 647V DNA digests exposed to CDDP showed the presence of two monofunctional adducts. Unlike the free solution production of adducts in vitro, the predominant adduct formed was not IdUrd-Pt. Results using 125IdUrd-labeled 647V DNA suggests that this adduct is 5-Pt-deoxyuridine. We were not able, however, to detect the presence of the bifunctional adducts IdUrd-Pt-dGua or dUrd-Pt-dGua. This was most likely due to the extremely low proportion of mixed bifunctional adducts produced in vivo. Nonetheless, these results suggest that IdUrd DNA incorporation may enhance CDDP cytotoxicity through the increase of available sites for Pt adduct formation. A Phase I clinical trial of this approach is planned.