2',2'-Difluorodeoxycytidine (dFdC; gemcitabine) is a new antineoplastic agent that is active against ovarian carcinoma, non-small-cell lung carcinoma, and head and neck squamous cell carcinoma. cis-diamminedichloroplatinum (CDDP; cisplatin) is used commonly for the treatment of these tumors. Because the two drugs have mechanisms of action that might be complementary, we investigated a possible synergism between dFdC and CDDP on growth inhibition. The combination was tested in the human ovarian carcinoma cell line A2780, its CDDP-resistant variant ADDP and its dFdC-resistant variant AG6000, the human head and neck squamous cell carcinoma cell line UMSCC-22B, and the murine colon carcinoma cell line C26-10. The cells were exposed to dFdC and CDDP as single agents and to combinations in a molar ratio of 1:500 for 1, 4, 24, and 72 h with a total culture time of 72 h. Synergy was evaluated using the multiple drug effect analysis. In A2780 and ADDP cells, simultaneous exposure to the drugs for 24 and 72 h resulted in synergism, but shorter exposure times were antagonistic. No synergism was found in the UMSCC-22B and C26-10 cell lines at prolonged simultaneous exposure. However, a preincubation with CDDP for 4 h followed by a dFdC incubation for 1, 4, 24, and 72 h was synergistic in all cell lines except C26-10 cells. A 4-h preincubation with dFdC followed by an incubation with the combination for 20 and 68 h was synergistic in all cell lines. Initial studies of the mechanism of interaction concentrated on the effect of CDDP on dFdCTP accumulation and DNA strand break formation. In all cell lines, CDDP failed to increase dFdCTP accumulation at 4- or 24-h exposure to dFdC; in two cell lines, CDDP even tended to decrease dFdCTP accumulation. Neither dFdC nor CDDP caused more than 25% double strand break formation, whereas in the combination, CDDP even tended to decrease this type of DNA damage. The synergistic interaction between the two drugs is possibly the result of dFdC incorporation into DNA and/or CDDP-DNA adduct formation, which may be affected by each other.

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