Dihydrofolate reductase antimetabolites

Methotrexate belongs to the class of antimetabolites. As a derivative of folic acid it inhibits the enzyme dihydrofolate reductase resulting in a decreased production of thymidine and purine bases essential for RNA and DNA synthesis. This interruption of the cellular metabolism and mitosis leads to cell death. 

Methotrexate is an antimetabolite of folic acid and has immunosuppressant properties. It inhibits the enzyme dihydrofolate reductase that is required for the synthesis of purines and pyrimidines. It is used in malignant disease, Crohn s disease, rheumatic disease and psoriasis. Folic acid is given with methotrexate to reduce the occurrence of side-effects particularly the risk of mucositis. 

Inhibition of nucleobase synthesis (2). Tetrahydrofolic acid (THF) is required for the synthesis of both purine bases and thymidine. Formation of THF from folic acid involves dihydrofolate reductase (p. 272). The folate analogues aminopterin and methotrexate (ame-thopterin) inhibit enzyme activity as false substrates. As cellular stores of THF are depleted, synthesis of DNA and RNA building blocks ceases. The effect of these antimetabolites can be reversed Ltillmann, Color Atlas of Pharmacology 2000 Thieme All rights reserved. Usage subject to terms and conditions of iicense. 

A while later, pyrimethamine (33.1.60) was suggested as a result of intensive research of antimetabolites of folic acid. Trimethoprim (33.1.51) is the result of later research. The structural similarity of these drugs with the pteridine fragment of folic acid undoubtedly determines their affinity with binding regions of dihydrofolate reductase. 

I I 3. The answer is c. (Hardman, pp 1243-1247.) Antimetabolites of folic acid such as methotrexate, which is an important cancer chemotherapeutic agent, exert their effect by inhibiting the catalytic activity of the enzyme dihydrofolate reductase. The enzyme functions to keep folic acid in a reduced state. The first step in the reaction is the reduction of folic acid to 7,8-dihydrofolic acid (FH2), which requires the cofactor nicotinamide adenine dinucleotide phosphate (NADPH). The second step is the conversion of FH2 to 5,6,7,8-tetrahydrofolic acid (FH ). This part of the reduction reaction requires nicotinamide adenine dinucleotide (NADH) or NADPH. The reduced forms of folic acid are involved in one-carbon transfer reactions that are required during the synthesis of purines and pyrimidine thymidylate. The affinity of methotrexate for dihydrofolate reductase is much greater than for the substrates of folic acid and FH2. The action of... 

A considerable number of enzymes occupy a central and crucial role in the activity of drugs. Dihydrofolate reductase, an enzyme involved in purine and amino acid biosynthesis, is the target of antibacterial sulfanilamides, which act both as bacteriostatics and antimalarials. These drugs act on the enzyme in different ways, some being so-called antimetabolites (i.e., reversible enzyme inhibitors). Some diuretics act on carbonic... 

Mammals must obtain their tetrahydrofolate requirements from their diet, but microorganisms are able to synthesize this material. This offers scope for selective action and led to the use of sulphanilamide and other antibacterial sulpha drugs, compounds which competitively inhibit dihydropteroate synthase, the biosynthetic enzyme incorporating p-aminobenzoic acid into the structure. These sulpha drugs thus act as antimetabolites of p-aminobenzoate. Specific dihydrofolate reductase inhibitors have also become especially useful as antibacterials,... 

Antimetabolites that are used to prevent the formation of DNA may be classified as antifolates, purine antimetabolites and pyrimidine antimetabolites (Table 7.5). Antifolates are believed to inhibit dihydrofolate reductase (DHFR). This enzyme is responsible for catalysing the conversion of dihydro-folic acid (DHF or FH2) to tetrahydrofolic acid (THF or FH4), which occurs in... 

Methotexate is an antineoplastic folic add analogue that blocks the conversion of dihydrofolate (FHj) to tetrahydro-folate (FH4) by binding to dihydrofolate reductase (DHFR) enzyme. Folate is essential for the normal synthesis of purines and pyrimidines, and therefore DNA and RNA. In order for folate to function as a cofactor, it must be reduced to FH by DHFR. Methotrexate binds to DHFR, prevents the conversion of FH2 to FH4, and, consequently, inhibits purine and pyrimidine synthesis. The antimetabolites are considered cell cycle specific, with most activity for cells in the S (synthesis) phase. With high-dose methotrexate, leucovorin rescue is often used to prevent severe toxicity to normal body tissues. Leucovorin (folinic acid) is a reduced form of folate (similar to FH ) that does not require the use of DHFR. Leucovorin is transported into healthy cells and is utilized for DNA and RNA synthesis. Tumor cells tend to have impaired transport mechanisms and usually cannot use leucovorin. Leucovorin is usually started within 24 to 36 hours of high-dose methotrexate administration and continues until methotrexate serum levels are below nontoxic levels (0.1 to 0.05 mol/L). 

Figure 7-16. The transfer of a one-carbon unit from serine to deoxyuridine monophosphate (dUMP) to form deoxythymidine monophosphate (dTMP). FH4 is oxidized to FH2 (dihydrofolate) in this reaction. FH2 is reduced to FH4 by dihydrofolate reductase. Hi indicate the steps at which the antimetabolites methotrexate and 5-fluo-rouracil (5-FU) act.

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... 

However, in view of its apparent success, this work is bound to advance further our understanding of the modes of action and structure-activity relationships of non-classical antimetabolites, and to produce new leads for the rational design of future drugs for chemotherapy. It should be remembered that it was the rational approach , based on the concepts of antimetabolites and of dihydrofolate reductase inhibitors as chemotherapeutic agents, that has... 

Methotrexate, a common antimetabolite, was introduced several decades ago for the treatment of psoriasis and remains an effective therapeutic approach. It is a synthetic analogue of folic acid that acts as a competitive inhibitor of the enzyme dihydrofolate reductase, that is responsible for the conversion of dihydrofolate to tetrahydrofolate. Tetrahydrofolate is an essential cofactor for the synthesis of thymidy-late and purine nucleotides required for DNA and RNA synthesis. Methotrexate inhibits replication and function of T and B cells and suppresses secretion of various cytokines such as IL-1, IFN-y,... 

Dihydropteroate synthase Sporozoans (eg, plasmodium, toxoplasma, and eimeria species) lack the ability to utilize exogenous folate and therefore possess enzymes for its synthesis these enzymes can be inhibited by drugs. Sulfonamides, which are antimetabolites of PABA, inhibit dihydropteroate synthase. Sequential blockade ctin be achieved with a sulfonamide and an inhibitor of dihydrofolate reductase (eg, pyrimethamine) such drug combinations are effective in malaria and toxoplasmosis. 

Resistance to End Product Repression, a) Antimetabolites. The use of toxic antimetabolites to select resistant cultures often yields mutants whose normal biosynthetic pathway enzymes are not repressed by the end product. For example, trifluoreoleucine selects mutants with derepressed levels (as much as 10-fold) of leucine biosynthetic enzymes (Calvo and Calvo, 1967). Certain canavanine-resistant mutants produce 30 times more of the arginine pathway enzymes than do their sensitive parents (Jacoby and Gorini, 1967). Mutants of Lactobacillus casei resistant to dichloroamethopterin are derepressed 80-fold in their ability to form thymidylate synthetase (Crusberg and Kisliuk, 1969). When Diplococcus pneumoniae mutants are selected on the basis of resistance to amethopterin, these cultures produce 100 times as much dihydrofolate reductase as the parent culture (Sirotnak et al, 1969). 

C. Antimetabolites. These agents interfere with DNA synthesis at various stages. For example, methotrexate binds reversibly to dihydrofolate reductase, preventing synthesis of purine and pyrimidine nucleotides. 

---