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Resistance of clear cell carcinoma (CCC) of the ovary to platinum-based chemotherapy is associated with a poor prognosis. However, the mechanism underlying the resistance of CCC to platinum has not yet been understood. We conducted the present study to clarify the mechanism of cisplatin (CDDP) resistance in CCC cells. Eleven CCC and 5 serous adenocarcinoma (SAC) cell lines were used in this study. The IC₅₀ to CDDP ranged from 1.3 to 18.0 µM for CCC cells and from 2.2 to 13.0 µM for SAC cells. There was no correlation between multidrug resistance-associated protein expression and the sensitivity to CDDP in CCC cells. In contrast, the doubling time for CCC cells was significantly longer than that for SAC cells (61.4 vs. 29.8 h). A significant reverse correlation between the S-phase fraction and the response to CDDP was observed (r = 0.647, p < 0.05). The present study suggests that the resistance of CCC to CDDP may be caused by low cell proliferation.

We conducted the present study to determine the chemoresistance mechanisms in clear cell carcinoma of the ovary (CCC). Five human CCC cell lines (HAC-2, RMG-I, RMG-II, KK, and KOC7c) were used in this study. The sensitivity of the cells to the anticancer agents was determined by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay and we assessed drug sensitivity by calculating assay area under the curve (AUC) for each agent. The expression of multi-drug resistance genes (MDR-1, MRP-1, MRP-2) was detected by reverse transcriptionpolymerase chain reaction (RT-PCR). Glutathione (GSH) concentration was measured by an enzymatic assay. Topoisomerase (topo) I activity was assayed in terms of relaxation of supercoiled plasmid substrate DNA. The IC 50 to anticancer agents ranged widely. The assay AUC indicated that 3 of 5 cell lines (RMG-I, RMG-II, and KK) were sensitive to paclitaxel (PTX), 3 (HAC-2, RMG-I, and RMG-II) were sensitive to 7-ethyl-10-hydroxycamptothecin (SN-38), which is an active metabolite of camptothecin (CPT-11), and only one (HAC-2) was sensitive to cisplatin (CDDP). All cell lines were resistant to mitomycin-C (MMC) and etoposide (VP-16). The MRP-1 gene was detected in all cell lines. Only one cell line showed both MRP-2 and MDR-1 gene expression. Except for HAC-2 cells, expression of MRP genes was related to CDDP resistance, and MDR-1 gene expression was associated with PTX resistance. GSH concentrations increased after exposure to CDDP or MMC in all cell lines. There was a significant correlation between topo-I enzymatic activity and the response to SN-38. The present study revealed several resistance mechanisms in CCC and the results suggested that PTX and CPT-11 might be effective agents to treat CCC.

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Objective: The aim of this study was to evaluate the combination effect of paclitaxel (PTX) and cisplatin (CDDP) and to determine the mechanisms of interaction between these agents. Methods and Results: We used human ovarian adenocarcinoma cell lines, namely a parent cell line (KF), a CDDP-resistant cell line (KFr) and a PTX-resistant cell line (KFTx).The combination effect of PTX and CDDP was synergistic on KF and KFTx and additive on KFr. The incidence of anaphase or telophase, evaluated by immunofluorescence microscopy, decreased with PTX and significantly decreased with PTX and CDDP in KF and KFTx. The concentration of PTX, which was measured by high-performance liquid chromatography, was higher in KF and KFTx cells treated with a combination of PTX and CDDP than those treated with PTX alone. Multidrug resistance gene mRNA appeared in KFTx and its expression decreased after exposure to PTX and CDDP. After exposure to CDDP, the expression of multidrug resistance-associated protein (MRP) and the concentration of glutathione increased in KF, but not in KFr or KFTx. MRP expression slightly increased in KF and KFTx after exposure to PTX. In contrast, its expression decreased in KFr. Conclusion: The present study suggests that CDDP enhances PTX accumulation and that the interaction of these agents is synergistic in CDDP-sensitive cells.