Methotrexate actions

Mechanism of Action An antidote to folic acid antagonists that may limit methotrexate action on normal cells by competing with methotrexate for the same transport processes into the cells Therapeutic Effect Reverses toxic effects of folic acid antagonists. Reverses folic acid deficiency. 

Uga, H., Knramoii, C., Ohta, A., et al. (2006). A new mechanism of methotrexate action revealed by target screening with affinity beads. Mol Pharmacol, 70, 1832-1839. 

Different biochemical mechanisms of acquired resistance to methotrexate can affect each known step in methotrexate action, including (I) impaired transport of methotrexate into cells ... 

The classification of these dmgs as antimetaboHtes stems from the mode of action as antagonists to the natural metaboHc processes leading to either DNA, RNA, or proteiu synthesis (13) (see Nucleic acids Proteins). They either inhibit function of a key en2yme involved in protein synthesis or are recmited into the cell division process as DNA synthesis terrninators. For example, methotrexate (8) is a foHc acid [59-30-3], antagonist and... 

In view of the well-documented inhibition of dihydrofolate reductase by aminopterin (325), methotrexate (326) and related compounds it is generally accepted that this inhibitory effect constitutes the primary metabolic action of folate analogues and results in a block in the conversion of folate and dihydrofolate (DHF) to THF and its derivatives. As a consequence of this block, tissues become deficient in the THF derivatives, and this deficiency has many consequences similar to those resulting from nutritional folate deficiency. The crucial effect, however, is a depression of thymidylate synthesis with a consequent failure in DNA synthesis and arrest of cell division that has lethal results in rapidly proliferating tissues such as intestinal mucosa and bone marrow (B-69MI21604, B-69MI21605). 

When a sulfonamide is administered with an oral anticoagulant, the action of the anticoagulant may be enhanced. The risk of bone marrow suppression may be increased when a sulfonamide is administered with methotrexate When a sulfonamide is administered with a hydantoin, the serum hydantoin level may be increased. 

The answer is c. (Katzung, pp 608-609, 932-9.13.) Methotrexate is classified as an anti metabolite with therapeutic uses in cancer chemotherapy and as an immunosuppressive agent indicated in the treatment of severe active classical rheumatoid arthritis. Leucovorin is related to methotrexate in that it is an antagonist of its actions. It can supply a source of reduced folate for the methylation reactions that are prevented by methotrexate. 

Chan, E.S., and Cronstein, B.N. (2002) Molecular action of methotrexate in inflammatory diseases. Arthritis Research. 4, 266-273. 

Let s start with three examples of antitumor drugs that work by the inhibition of DNA synthesis methotrexate (MTX, Methopterin), 5-fluorouracil (Fluril), and 6-mercaptopurine (Purinethol). These are all old cancer drags, marketed under numerous trade names, including those indicated above. Although the mechanism of action of each one is different, at the end of the day they all inhibit DNA synthesis. 

These are pyrimidine derivatives and are effective because of differences in susceptibility between the enzymes in humans and in the infective organism. Anticancer agents based on folic acid, e.g. methotrexate, inhibit dihydrofolate reductase, but they are less selective than the antimicrobial agents and rely on a stronger binding to the enzyme than the natural substrate has. They also block pyrimidine biosynthesis. Methotrexate treatment is potentially lethal to the patient, and is usually followed by rescue with folinic acid (A -formyl-tetrahydrofolic acid) to counteract the folate-antagonist action. The rationale is that folinic acid rescues normal cells more effectively than it does tumour cells. 

Change in site of action e.g., increased synthesis of dihydrofolate reductase may occur as a compensatory response to methotrexate. 

Pharmacology The mechanism of action in RA is unknown it may affect immune function. Methotrexate inhibits dihydrofolic acid reductase and interferes with DNA synthesis, repair, and cellular replication. 

The answer is E. Methotrexate is an analog of folic acid that binds with very high affinity to the substrate-binding site of dihydrofolate reductase, the enzyme that catalyzes conversion of DHF to THE, which is used in various forms by enzymes of both the purine and pyrimidine de novo synthetic pathways. Thus, synthesis of dTMP from dUMP catalyzed by thymidylate synthetase and several steps in purine synthesis catalyzed by formyltransferase are indirectly blocked by the action of methotrexate because both those enzymes require THE coenzymes. 

In Fig. 1 various targets of some important cytostatic agents are depicted. Their main mechanisms of action can be briefly summarized as follows. Pentostatin blocks purine nucleotides by inhibiting adenosine deaminase. 6-Mercaptopurine and 6-thioguanine inhibit purine ring biosynthesis and they inhibit nucleotide interconversions. Methotrexate by inhibiting dihydrofolate reduction blocks thymidine monophosphate and purine synthesis. 5-Fluorouracil also blocks thymidine monophosphate synthesis. Dactinomycin, daunorubicin, doxorubicin and mitoxantrone intercalate with DNA and inhibit RNA synthesis. L-asparaginase deaminates... 

Methotrexate is a folic acid analogue. Its mechanism of action is based on the inhibition of dihydrofolate reductase. Inhibition of dihydrofolate reductase leads to depletion of the tetrahydrofolate cofactors that are required for the synthesis of purines and thymidylate (see Fig. 2). Enzymes that are required for purine and thymidylate synthesis are also directly inhibited by the polyglutamates of methotrexate which accumulate with dihydrofolate reductase inhibition. The mechanisms that can cause resistance include decreased transport of methotrexate into the tumor cells, a decreased affinity of the antifolate for dihydrofolate reductase, increased concentrations of intracellular dihydrofolate reductase and decreased thymidylate synthetase activity. 

Its mechanism of action is based on intercalation in the minor groove of double stranded DNA, interference with RNA polymerase and with topoiso-merase II. Its primary indications are rhabdomyosarcoma and Wilms tumor in children. In combination with methotrexate, it is used in the treatment of choriocarcinoma. 

Anakinra (Kineret) is the first antirheumatic agent that acts by blocking the action of IL-1. This drug was recently approved for the treatment of moderately to severely active rheumatoid arthritis in adults who have not responded to therapy with one or more DMARDs. Anakinra may be used alone or in combination with DMARDs other than the TNF antagonists. Clinical trials have shown anakinra to be more effective than placebo, either alone or in conjunction with methotrexate. 

Salicylates, probenecid, and sulfonamides inhibit the renal tubular secretion of methotrexate and may displace it from plasma proteins. Asparaginase inhibits protein synthesis and may protect cells from methotrexate cytotoxicity by delaying progression from Gj-phase to S-phase. Methotrexate may either enhance or inhibit the action of fluorouracil, depending on its sequence of administration. 

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... 

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