Will cyclopentenyl cytosine (CPEC) ever have a future in the clinic?

Novel developments in the use of antimetabolites

Antimetabolites are the most widely used and most efficacious group of anticancer drugs. Antimetabolites are also the oldest rationally designed anticancer drugs, targeted against RNA and DNA, and can, therefore, be considered as the first generation of targeted drugs. Unfortunately, resistance often develops, leading to the design of new antimetabolites, which either have a novel mechanism of action, bypass resistance or in combination enhance the effect of other drugs, such as another antimetabolite, other DNA, or protein kinase targeted anticancer drugs. Several novel antimetabolites are in clinical development. The cytidine-analog fluorocyclopentenylcytosine (RX-3117) is active in gemcitabine-resistant tumors and is activated by uridine-cytidine-kinase, can be incorporated into RNA and DNA and can downregulate DNA-methyltransferase-1. TAS-114 is a new generation dUTPase inhibitor. dUTPase normally prevents incorporation of dUTP and of the 5FU-nucleotide FdUTP into DNA. However, inhibition of dUTPase will enhance their incorporation, thereby increasing thymine-less cell-death. The formulation TAS-102 (trifluorothymidine and thymidine-phosphorylase-inhibitor) acts by incorporation into DNA and has shown efficacy in tumors progressing on 5FU therapy. Gemcitabine and cytarabine prodrugs were tested in model systems and have entered clinical evaluation. The elaidic-acid prodrugs of gemcitabine (CP-4126, CO101) and cytarabine (elacytarabine) failed in randomized Phase III studies. Two other gemcitabine prodrugs LY2334737 (gemcitabine with a valproic acid at the 5'-position) and NUC1031 (a 5'-arylphosphoamidate prodrug, with a side-chain at the 5'-phosphate) are in early clinical development. In summary, several novel antimetabolites show promise in clinical development, either because of a novel mechanism of action, or clever combination or by innovative prodrug design.

Metabolism, mechanism of action and sensitivity profile of fluorocyclopentenylcytosine (RX-3117; TV-1360)

A novel cytidine analog fluorocyclopentenylcytosine (RX-3117; TV-1360) was characterized for its cytotoxicity in a 59-cell line panel and further characterized for cytotoxicity, metabolism and mechanism of action in 15 additional cancer cell lines, including gemcitabine-resistant variants. In both panels sensitivity varied 75-fold (IC50: 0.4- > 30 μM RX-3117). RX-3117 showed a different sensitivity profile compared to cyclopentenyl-cytosine (CPEC) and azacytidine, substrates for uridine-cytidine-kinase (UCK). Dipyridamole, an inhibitor of the equilibrative-nucleoside-transporter protected against RX-3117. Uridine and cytidine protected against RX-3117, but deoxycytidine (substrate for deoxycytidine-kinase [dCK]) not, although it protected against gemcitabine, demonstrating that RX-3117 is a substrate for UCK and not for dCK. UCK activity was abundant in all cell lines, including the gemcitabine-resistant variants. RX-3117 was a very poor substrate for cytidine deaminase (66,000-fold less than gemcitabine). RX-3117 was rapidly metabolised to its nucleotides predominantly the triphosphate, which was highest in the most sensitive cells (U937, A2780) and lowest in the least sensitive (CCRF-CEM). RX-3117 did not significantly affect cytidine and uridine nucleotide pools. Incorporation of RX-3117 into RNA and DNA was higher in sensitive A2780 and low in insensitive SW1573 cells. In sensitive U937 cells 1 μM RX-3117 resulted in 90% inhibition of RNA synthesis but 100 μM RX-3117 was required in A2780 and CCRF-CEM cells. RX-3117 at IC50 values did not affect the integrity of RNA. DNA synthesis was completely inhibited in sensitive U937 cells at 1 μM, but in other cells even higher concentrations only resulted in a partial inhibition. At IC50 values RX-3117 downregulated the expression of DNA methyltransferase. In conclusion, RX-3117 showed a completely different sensitivity profile compared to gemcitabine and CPEC, its uptake is transporter dependent and is activated by UCK. RX-3117 is incorporated into RNA and DNA, did not affect RNA integrity, depleted DNA methyltransferase and inhibited RNA and DNA synthesis. Nucleotide formation is related with sensitivity.