EXPLOITATION OF FOLATE AND ANTIFOLATE POLYGLUTAMYLATION TO ACHIEVE SELECTIVE ANTICANCER CHEMOTHERAPY

Synthesis of poly(gamma-glutamate) metabolites of natural folates and antifolates is a critical process. Folylpolyglutamates are essential f or cell proliferation. Polyglutamates of glutamate (Glu)-containing an tifolates are often critical for their cytotoxic action and are releva nt to antifolate resistance. However, the role of polyglutamate synthe sis in selectivity is less clear. We have undertaken a research progra m to further define the significance of polyglutamate metabolism and t o devise ways to exploit this metabolism to achieve greater therapeuti c selectivity in cancer chemotherapy. This article briefly reviews sev eral approaches tested thus far. Inhibition of folylpolyglutamate synt hesis should lead to cell death. Current ornithine (Om)-containing fol ate-based inhibitors of the enzyme responsible for their synthesis, fo lylpolyglutamate synthetase (FPGS), are poor ly transported, apparentl y because of interference by the protonated delta-amine. Replacement o f Orn with 4,4-difluoroOrn, the delta-amine of which has a much lower pK(alpha) and is thus less protonated at physiological pH, was explore d. Since it is unclear how polyglutamylation contributes to selectivit y, we explored generic means either to eliminate or to enhance polyglu tamylation. The data indicate that substitution for Glu in an antifola te by some Glu analogs in which the gamma-COOH is either altered or re placed (e.g., gamma-tetrazole-Glu) leads to loss of both FPGS substrat e activity and binding; antifolate target specificity is unchanged, wh ile uptake is actually enhanced. Substitution of 3,3-difluoroGlu for G lu leads to enhanced polyglutamylation (although probably only to the diglutamate), retention of target specificity, and at least equal upta ke. Comparative studies of the same antifolate containing different re placements for Glu, such as gamma-tetrazole-Glu (no polyglutamylation) or 3,3-difluoroGlu (enhanced polyglutamylation), will be useful in ex ploring the role and significance of polyglutamylation.