Cell-cycle arrest and p53-independent induction of apoptosis in vitro by the new anticancer drugs 5-FdUrd-P-FdCydOct and dCydPam-P-FdUrd in DU-145 human prostate cancer cells

Purpose: Current therapies have limited impact on the progression of metastatic hormone-refractory prostate cancer. Therefore, we investigated the utility of now heterodinucleoside phosphate dimers of 5-fluorodeoxyuridine (5-FdUrd) in p53-mutated and androgen-independent DU-145 human prostate tumour cells. Methods: The effects of the dimers were assessed in vitro by a cell proliferation assay for cytotoxicity, flow cytometry for cell cycle distribution, confocal laser scanning microscopy for the detection of apoptotic bodies, poly(ADP-ribose) polymerase cleavage for caspase 3 activity and by a thymidylate synthetase assay. Results: The new dimers N-4-palmitoyl-2'-deoxycytidylyl-(3'--> 5')-5-fluoro-2'-deoxyuridine (dCydPam-P-FdUrd) and 2'-deoxy-5-fluorouridylyl-(3'--> 5')-2'-deoxy-5-fluoro-N-4-octadecyclytidine (5-FdUrd-P-FdCydOct) caused marked cytotoxicity with IC50 values of 3-4 mu M. 5-FdUrd-P-FdCydOct at 200 mu M was capable of eradicating 100% of tumour cells whereas 10% of the cells were resistant to 5-FdUrd. Cytotoxicity was caused by a dramatic S-phase arrest, resulting in an increase of this cell population from 34% to 85% with 5-FdUrd-P-FdCydOct and to 81% with dCydPam-P-FdUrd. S-phase arrest was followed by apoptosis, as shown by 85% of the cells staining positive for Apo 2.7 antibody, a six- to eight-fold increased caspase 3 activity and DNA fragmentation. Thymidylate synthase activity was inhibited by 50% at 0.6-0.7 mu M dimer concentration. The dimers were hydrolysed in vitro by phosphodiesterase I and human serum to the corresponding nucleosides and nucleoside monophosphates. Conclusions: The new dimers dCydPam-P-FdUrd and 5-FdUrd-P-FdCydOct are effective prodrugs of 5-FdUrd and have potential value for the treatment of p53-mutated and hormone-independent human prostate carcinomas.