Combined modulation by leucovorin and alpha-2a interferon of fluoropyrimidine mediated growth inhibition

One way to improve fluoropyrimidine activity is the use of leucovorin (LV). Another way is the use of alpha-2a interferon (alpha-IF). The mechanism of the alpha-IF effect on fluoropyrimidines has not yet been elucidated. Besides, only limited data area available on double modulation (LV and alpha-IF) of fluoropyrimidines. Therefore, the modulating capacity of both drugs was tested in a fluoropyrimidine resistant (COLO 320) and a sensitive (SW 948) cell line. Also, the binding capacity of thymidylate synthase (TS) to 5-fluoro-2'-deoxyuridine-5'-monophosphate (FdUMP) and TS catalytic activity were studied in both cell lines as well as the effects of 5-fluorouracil (5-FU) and alpha IF on enzyme activity. COLO 320 had, compared to SW 948, a 7.5 fold higher FdUMP binding capacity to TS. TS activity was 4.4 and 11.3 fold higher at 10 and 1 mM substrate, respectively. In COLO 320 enhancement of 5-FU, either by LV or by alpha-IF, was not possible. Since LV did enhance 5-fluoro-2' deoxyuridine (FUdR) activity, it is conceivable that 5-FU mediated growth inhibition in COLO 320 is not TS mediated. SW 948 was sensitive to both modulating agents with a 2.4 fold lower IC50 for 5-FU/LV, 6.8 fold lower IC50 for 5-FU/alpha-IF and a 11.2 fold lower IC50 for 5-FU/LV/alpha-IF. Effects of LV and alpha-IF on FUdR were comparable but less pronounced, with a 3.4 fold lower IC50 for FUdR/LV/alpha-IF compared with FUdR alone. Thymidine, which circumvents TS inhibition, neutralized the synergistic effects of alpha-IF, indicating that alpha-IF enhancement is mediated via inhibition of DNA synthesis. However, no direct effects of alpha-IF on FdUMP binding or catalytic activity could be demonstrated.

alpha-IF can increase 5-FU/LV mediated growth inhibition in fluoropyrimidine sensitive colorectal cancer cells. FdUMP binding capacity and catalytic activity of TS may predict sensitivity to (modulation of) fluoropyrimidines.