1Alpha,25dihydroxyvitamin D3 and platinum drugs act synergistically to inhibit the growth of prostate cancer cell lines.

The majority of men who die from prostate cancer (PC) have hormone-refractory disease. To date, chemotherapeutic agents have had little or no impact on the survival of such patients. To explore a new approach for the treatment of hormone-refractory PC, we examined the combination effects of cis- or carboplatin with vitamin D. 1alpha,25-Dihydroxyvitamin D3 (1alpha,25(OH)2D3) and its synthetic analogue, Ro 25-6760, have an antiproliferative effect on some prostate cancer cell lines. Consequently, the growth-inhibitory effects of the drugs were measured, both singularly and in combination with cis- or carboplatin, on PC cells. Our results show that although each of the drugs alone displayed antiproliferative activity, the growth inhibition of PC cells was further enhanced by the combination of 1alpha,25(OH)2D3 or Ro 25-6760 and either platinum agent. The greatest enhancement of inhibition occurred using smaller concentrations of the platinum compound in combination with higher concentrations of 1alpha,25(OH)2D3. Isobologram analysis revealed that 1alpha,25(OH)2D3 and platinum acted in a synergistic manner to inhibit the growth of PC cells. Our findings suggest that there is potential clinical value in combining 1alpha,25(OH)2D3 with platinum compounds for the treatment of advanced-stage human PC.