In vitro assessment of cytotoxic agent combinations for hormone-refractory prostate cancer treatment

We have investigated new drug combinations of potential clinical value for treatment of hormone-refractory prostate cancer. Combinations of paclitaxel, carboplatin and mitoxantrone, and combinations of these three drugs with compounds targeting important pathways for cancer progression, 13-cis-retinoic acid and chelerythrine, were assessed. The drugs combinations were incubated for 72 h in steroid-free conditions with two androgen-independent cell lines, DU145 and PC3. Cytotoxicity assay was performed using resazurin and Hoescht 33342. Synergism and antagonism were measured by the combination index, and calculated for each combination by the median-effect method. All six compounds exhibited cytotoxic effects when tested alone. Paclitaxel exhibited the highest and 13-cis-retinoic acid the lowest effect on both cell lines. Paclitaxel demonstrated synergism or additivity with 13-cis-retinoic acid in both cell lines, whereas antagonistic effects were observed when it was tested in combination with carboplatin. Chelerythrine showed additive effects with mitoxantrone in both cell lines and with paclitaxel in PC3 cells. Our results suggest that combination of paclitaxel and 13-cis-retinoic acid, and of chelerythrine with mitoxantrone and paclitaxel, may have clinical value for the treatment of hormone-refractory prostate cancer.