The purpose of these experiments was to assess the synergistic activity of silibinin with chemotherapy agents in clinical use against prostate cancer. Silybin-phytosome, a commercially available formulation containing silibinin, has recently been studied in a phase I clinical trial. The silibinin doses used in the present study are clinically achievable based on the preliminary phase I data. DU145, PC-3 and LNCaP prostate cancer cells were seeded in 96-well plates in triplicate. Twenty-four hours later, silibinin (10, 20 and 40 microM) and either mitoxantrone or docetaxel were added to the designated wells. Seventy-two hours post-treatment, cell viability was determined with a tetrazolium-based assay. The combination index (CI) for determination of a synergistic effect was calculated, with values of <0.9 indicating synergy and values >1.1 antagonism. Apoptosis was also assessed using a luminescent assay after 72 hr of treatment with media alone, silibinin, mitoxantrone, or silibinin plus mitoxantrone. Silibinin showed a synergistic effect with mitoxantrone, as measured by reduction in cell viability. The CI values ranged from 0.413 to 2.650 for the combination of silibinin and mitoxantrone; in contrast, treatment with docetaxel and silibinin showed little or no synergy, with CI values of 0.898-4.469. In concordance with these findings, the addition of silibinin increased the level of apoptosis compared to mitoxantrone alone, particularly in the PC-3 cells. The combination of silibinin and mitoxantrone exhibits a pattern of synergy in reducing cell viability with increased apoptosis. These data are important in the planning of future clinical applications of silibinin.
(c) 2007 Wiley-Liss, Inc.