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

UNLABELLED: 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. IN CONCLUSION: 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.

Fluorouracil: biochemistry and pharmacology.

Fluorouracil (5FU) is still considered the most active antineoplastic agent in the treatment of advanced colorectal cancer. The drug needs to be converted to the nucleotide level in order to exert its effect. It can be incorporated into RNA leading to interference with the maturation of nuclear RNA. However, its conversion to 5-fluoro-2'deoxy-5' monophosphate (FdUMP) leading to inhibition of thymidylate synthase (TS) and subsequently of DNA synthesis, is considered to be its main mechanism of action. In the presence of a folate cofactor a covalent ternary complex is formed, the stability of which is the main determinant of the action of 5FU. Resistance against 5FU can be mainly attributed to aberrations in its metabolism or to alterations of TS, eg, gene amplification, altered kinetics in respect to nucleotides or folates. Biochemical modulation of 5FU metabolism can be applied to overcome resistance against 5FU. A variety of normal purines, pyrimidines, and other antimetabolites have been studied in this respect, but only some of them have been clinically successful. Delayed administration of uridine has recently been shown to "rescue" mice and patients from toxicity, while pretreatment with leucovorin is the most promising combination to enhance the therapeutic efficacy. 5FU is frequently administered in an intravenous (IV) injection, and shows a rapid distribution and a triphasic elimination. The nonlinearity of 5FU pharmacokinetics is related to saturation of its degradation. Continuous infusion of 5FU led to different kinetics. Regional administration, such as hepatic artery infusion, offers a way to achieve higher drug concentrations in liver metastases and is accompanied by lower systemic concentration. The current status of the biochemical and pharmacokinetic data is reviewed.