Modulation of fluorouracil metabolism and cytotoxicity by nitrothiobenzylinosine

Biochemical pharmacology and analysis of fluoropyrimidines alone and in combination with modulators

After more than three decades since their introduction, fluoropyrimidines, especially FUra, are still a mainstay in the treatment of various solid malignancies. The antitumor effects of fluoropyrimidines are dependent upon metabolic activation. FdUMP, FUTP and FdUTP were identified as the key cytotoxic metabolites that interfere with the proper function of thymidylate synthase and nucleic acids. The relevance of these metabolites is cell-type specific. Recently, fluorouridine diphospho sugars have been detected, but the precise function of this class of metabolites is currently unknown. In mammalian systems fluoropyrimidines and their natural counterparts share the same metabolic pathways since the substrate properties in enzyme-catalyzed reactions are frequently comparable. Ongoing studies indicate that the metabolism and action of fluoropyrimidines exhibit circadian rhythms, which appear to be due to variations in the activity of metabolizing enzymes. Essential for the expanding knowledge of the pathways and effects of fluoropyrimidines has been the constant improvement of analytical methods. These include ligand binding techniques, numerous dedicated HPLC systems and 19F-NMR. Because the overall response rates achieved with fluoropyrimidines are modest, strategies based on biochemical modulation have been devised to enhance their therapeutic index. Biochemical modulators include a wide range of various compounds with different modes of action. In recently completed clinical trials, combinations of FUra with leucovorin, a precursor for 5,10-methylene tetrahydrofolate, or with levamisole, an anthelminthic with immunomodulatory activity, appeared to be superior to FUra alone. At the preclinical level combinations of fluoropyrimidines with, e.g. interferons or L-histidinol were demonstrated to be interesting candidates for further testing. The future therapeutic utility of fluoropyrimidines will depend on both the improvement of combination regimens currently used in the treatment of cancer patients and the judicious clinical implementation of promising experimental modulation strategies. Moreover, novel fluoropyrimidines with superior pharmacological properties may become important as part of or instead of modulation approaches.

Drug-induced perturbations in the in vivo distribution of oncological radiotracers--II. 5-[125I]iodo-2'-deoxyuridine influenced by nitrobenzylthioinosine-5'-phosphate (NBMPR-P) and acyclothymidine (ACT)

Nitrobenzylthioinosine (NBMPR), a potent inhibitor of facilitated nucleoside transport in vitro and in vivo, and acyclothymidine (ACT), a potent inhibitor of pyrimidine nucleoside phosphorylase in vitro, have been used in an attempt to modulate the biodistribution of 125I-labelled iododeoxyuridine ([125I]IUdR). ACT or NBMPR-P (a water-soluble prodrug of NBMPR) were injected into BDF1 mice bearing implanted Lewis lung tumors, according to protocols which would provide high and low plasma levels of the inhibitor. Compared with controls, both inhibitors induced transient, marginal increases in hepatic, renal and blood levels of [125I]IUdR, and decreased levels in tumors at short time intervals after injection. It is concluded that there is a mild tumor-sparing effect when either NBMPR or ACT are administered together with single i.v. diagnostic doses of [125I]IUdR.

Modulation of dipyridamole action by alpha 1 acid glycoprotein. Reduced potentiation of quinazoline antifolate (CB3717) cytotoxicity by dipyridamole

Dipyridamole potentiates the cytotoxicity of N10-propargyl-5,8-dideazafolic acid (CB3717), an antifolate inhibitor of thymidylate synthase, by inhibiting both thymidine (TdR) salvage and deoxyuridine (UdR) efflux. Dipyridamole binds to the serum component alpha 1acid glycoprotein (alpha 1AGP) and hence the effects of alpha 1AGP on dipyridamole-induced changes in nucleoside transport and CB3717 cytotoxicity have been investigated. Using A549 lung cancer cells in vitro, alpha 1AGP reduced the inhibition of nucleoside transport by dipyridamole in a concentration-dependent manner. Between 10 and 200 times the concentration of dipyridamole was needed to inhibit TdR uptake to the same degree in medium containing 1 mg/ml alpha 1AGP (a physiological concentration) when compared to the uptake in alpha 1AGP-free medium. Although dipyridamole inhibited UdR efflux more than TdR efflux, inhibition of UdR efflux was reduced less than the inhibition of TdR efflux in the presence of 1 mg/ml alpha 1AGP. Thus, clinically achievable levels of dipyridamole (2.5-7.5 microM), even in the presence of physiological alpha 1AGP concentrations, caused significant inhibition of nucleotide uptake and efflux. The cytotoxicity of CB3717 was increased 2-3-fold by 3 and 10 microM dipyridamole in alpha 1AGP-free medium, whereas dipyridamole did not significantly (P greater than or equal to 0.05) potentiate CB3717 cytotoxicity in the presence of 1 mg/ml alpha 1AGP. Measured free dipyridamole levels indicated that the impaired inhibition of nucleoside transport and the lack of potentiation of CB3717 cytotoxicity in the presence of alpha 1AGP was due solely to the binding of dipyridamole to alpha 1AGP. It is concluded that alpha 1AGP levels will be a major determinant of the ability of dipyridamole to modulate the activity of antimetabolites in vivo.

Enhancement of 5-fluorouracil's anticancer activity by dipyridamole

Although the interaction between FUra and DP in HCT 116 cells is fairly complex, data from other investigators indicate that in cell lines in which inhibition of TS is growth limiting at relatively low concentrations of fluoropyrimidines, DP appears to augment the cytotoxicity of FUra and FdUrd by blocking the salvage of dThd (Miller et al., 1987; Schwartz et al., 1987). The previous in vitro data regarding the ability of DP to modulate the toxicity of fluoropyrimidines was obtained in exponentially growing cells. An additional observation that warrants consideration is a report that the inhibition of nucleoside incorporation by DP changed as a function of time in culture (Zhen et al., 1986). Hepatoma 3924A cells in lag and log phase were highly sensitive to DP with IC50 values for dThd incorporation of 0.2 and 0.32 microM, respectively. In contrast, stationary phase cells were insensitive to DP (IC50 = 38.9 microM). Amphotericin B, an antifungal agent which perturbs cell membranes, restored the sensitivity to DP in stationary cells. Several investigators have presented information on the effect of DP on fluoropyrimidines in normal tissues. Lee and Park (1987) examined the effect of DP on FUra and MTX toxicity in a soft-agar cloning assay against two human cancer cell lines and on pooled normal human bone marrow (CFU-C). DP (1 microM) potentiated the action of both MTX (0.1 microM) and FUra (5 microM) on Hep-2 (epidermoid carcinoma), MCF-7 (breast carcinoma) and CFU-C in medium supplemented with either non-dialyzed or dialyzed serum. Woodcock et al. (1987) incubated gallbladder mucosa, obtained from patients undergoing elective surgery for cholelithiasis, with control medium or varying concentrations of DP for 1 hr, and then exposed the mucosal cells to 2.5 microCi [3H]-FdUrd (2.5 microM). After 1 hr, the uptake of FdUrd into the tissue was inhibited to 49% and 42% of control by 0.1 microM and 1 microM, respectively.

Potentiation of quinazoline antifolate (CB3717) toxicity by dipyridamole in human lung carcinoma, A549, cells

A potent quinazoline antifolate inhibitor of thymidylate synthase, CB3717, inhibits the growth of A549 human lung carcinoma cells: ID50 2.74 +/- 0.53 microM. The toxic effects of thymidylate synthase inhibition may be prevented by salvage of exogenous thymidine. The nucleoside transport inhibitor, dipyridamole, at the non-toxic concentration of 1 microM, inhibited [3H]thymidine uptake/incorporation by more than 95% and significantly reduced the ID50 of CB3717 to 0.98 +/- 0.28 microM. Elimination of salvageable thymidine by the use of dialysed serum also enhanced CB3717 toxicity. Since dipyridamole was equally effective in the presence or absence of dialysed serum and was more effective than dialysed serum alone, inhibition of nucleoside efflux may be an important aspect of its potentiation. Efflux of [5-3H]deoxyuridine was inhibited by 89% and [3H]thymidine efflux by 61% in the presence of 1 microM dipyridamole. Inhibition of thymidylate synthase increases the deoxyuridine nucleotide/thymidine nucleotide pool ratio. Dipyridamole could exacerbate the nucleotide pool imbalance caused by CB3717, thereby potentiating its toxicity.