Folate reduction inhibitors

The active form of folate is the tetrahydro-derivative that is formed through reduction by dihydrofolate reductase. This enzymatic reaction (Figure 29.5) is inhibited by trimethoprim, leading to a decrease in the folate coenzymes for purine, pyrimidine, and amino acid synthesis. Bacterial reductase has a much stronger affinity for trimethoprim than does the mammalian enzyme, which accounts for the drug s selective toxicity. [Note Examples of other folate reductase inhibitors include pyrimethamine, which is used with sulfonamides in parasitic infections (see p. 353), and methotrexate, which is used in cancer chemotherapy (see p. 378).]... 

Another way to reduce the supply of deoxynucleotides for cell replication is to target the reduction of dihydrofolate to tetrahydrofo-late. Folate antagonists are used in antimicrobial and anticancer chemotherapy. These compounds are competitive inhibitors of dihydrofolate reductase because they resemble the natural substrate. For example, methotrexate is used in antitumor therapy. 

Two diastereomers of 5,10-dideaza-5,6,7,8-tetrahydrofolic acid, DDATHF, 127, both potent inhibitors of folate metabolism114 and de novo purine synthesis115, have been synthesized116 by catalytic reduction of the unsaturated intermediate diethyl 2-acetyl-5,10-dideaza-9,10-didehydrofolate with Adams catalyst and carrier-free tritium gas in AcOH and 3H20 solution. Each of the separated (6R) and (65) diastereomers had specific activity 11.2 Ci mmol-1 and contained tritium almost exclusively at the metabolically stable positions C(5), C(6), C(7), C(9), C(10) and the phenyl ring of DDATHF. 

DIHYDROFOLATE REDUCTASE INHIBITORS have as a target the enzyme dihydrofolate reductase, and are known as folate antagonists. These include anttcancer agents ( antimetabolites ) such as methotrexate, antibacterial AGENTS such as trimethoprim, and the antiprotozoals pyrimethamine and proguanil (which are used to treat malaria). Folate is required for synthesis of purine nucleotides, which in turn are essential for DNA synthesis and cell division. In mammals it is necessary to convert body folates, through two separate enzyme-catalysed reduction... 

Dietary folates must be chemically reduced to their tetrahy-dro forms, with four hydrogens on the pteridine ring, to be active. The enzyme responsible for this reduction is dihydrofolate reductase (DHFR), a key enzyme whose actions are inhibited by methotrexate and other antifolates. The result of this inhibition is depletion of intracellular pools of reduced folates (tetrahydrofolates) essential for thymidylate and purine synthesis. Lack of either thymidine or purines prevents synthesis of DNA. The DHFR-mediated effects of antifolate drugs on normal and probably also on cancerous cells may be neutralized by supplying reduced folates exogenously. The reduced folate used clinically for rescue is leucovorin (folinic acid), which bypasses the metabolic block induced by DHFR inhibitors. ... 

Soft Dihydrofolate Reductase (DHFR) Inhibitors. A series of esters was synthesized recently as possible dihydrofolate reductase (DHFR)inhibitors that are susceptible toward hydrolytic degradation (148,206-208). DHFR is involved in the reduction of dihydrofolate into tetrahydrofolate, and reduced folates are important cofactors in the biosynthesis of nucleic acids and amino acids. Hence, DHFR inhibitors can limit cellular growth. Consequently, classical DHFR inhibitors such as methotrexate (99) (Fig. 15.24) or nonclassical DHFR inhibitors such as trimetrexate (100) have shown antineoplastic or antiprotozoal... 

A key enzyme in pyrimidine biosynthesis, thymidylate synthetase, catalyzes the reductive methylation of 2 -deoxyuridylate (dUMP) to thymidylate (dTMP) with the concomitant conversion of 5,10-methylene-H4-folate to 7,8-dihydrofolate. The cofactor serves both as a 1-carbon carrier and a reductant. There is substantive evidence based on direct and indirect studies with the inhibitor 5 -fluoro-2 -deoxyuridylate, which will not be reviewed here, that the dUMP is covalently bound to the enzyme, possibly through a thiol group [59]. The intermediate ternary complex is hypothesized to have the following structure (10), where the CH2 unit is attached through the N-5 of H4-folate. The ring opening of the 5,10-methylene-H4-folate is... 

The antimicrobial activity of the trimethoprim/sulfamethoxazole combination results from actions on two steps of the biosynthetic pathway for tetrahydrofolic acid. Trimethoprim prevents the reduction of dihydrofolate to tetrahydrofolate (Figure 43-2). Mammalian cells use preformed folates from the diet and do not synthesize the compound. Trimethoprim is a highly selective inhibitor of dihydrofolate reductase of lower organisms -100,000 times more drug is required to inhibit human reductase than the bacterial enzyme. This relative selectivity is vital because the enzyme is essential to all species. 

Thus, the formation of thymidylate involves the formation of a carbon-carbon bond and a reduction 5,10-methyIene H4-folate is both donor of the one-carbon unit and reductant. It is apparent that a catalytic amount of H4-folate will serve in the cycle. This is consistent with the observation that in S. faecalis extracts the synthesis of thymidylate in the presence of excess H4-folate was unaffected by aminopterin, a potent inhibitor of tetrahydrofolate dehydrogenase when this reaction was conducted with catalytic amounts of H4-folate and excess NADPH, thymidylate synthesis was blocked by aminopterin (5). 

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