BACKGROUND: We analyzed the cytotoxic properties of the new heterodinucleoside phosphate dimer 5-FdU-NOAC, which is composed of the cytotoxic drugs 5-FdU and N(4)-octadecyl-1-beta-D-arabinofuranosylcytosine (NOAC) against human prostate tumor cells. METHODS: 5-FdU-NOAC effects on cell proliferation, cell cycle distribution, thymidylate synthase activity, and apoptosis were investigated in vitro in the two human prostate carcinoma cell lines DU-145 and PC-3 and compared to cells treated with the corresponding single drugs 5-FdU and NOAC. RESULTS: Treatment of the cells with 5-FdU-NOAC resulted in IC(50) values of 3.9-5 microM and in a complete inhibition of cell proliferation at 200 microM after 96 hr compared to 5-FdU, where 10% of the cells remained resistant. Flow cytometric analysis revealed cell cycle perturbations in S-phase only in the DU-145 cells. 5-FdU-NOAC caused 50% inhibition of thymidylate synthase after 90 min at 0.6 microM in both cell lines. Apoptotic cell fractions in DU-145 (66%) and in PC-3 (34%) cells were found after treatment with 5-FdU-NOAC for 96 hr. DNA fragmentation further confirmed the induction of apoptosis. CONCLUSIONS: 5-FdU-NOAC inhibits thymidylate synthase and cell cycle progression causing proliferation arrest and apoptosis in DU-145 and PC-3 cells, suggesting a potential role of 5-FdU-NOAC for the treatment of prostate cancer.