Aminopterin antimetabolite

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. 

These two groups of compounds have different mechanisms for inducing sterility. The antimetabolites have been considered as compounds wherein a metabolite essential to cell development has been changed in one or several ways and, when introduced into an animal, will elicit signs associated with a specific lack of the metabolite (10y 11). Some of the more promising are methotrexate, aminopterin (N- p- [ (2,4-diamino-6-pteridyl)methyl]amino benzoyl glutamic acid), 5-fluorouracil, and S-fluoro-orotic acid. 

Aminopterin, methotrexate, S-fluorouracil, 5-fluoro-orotic acid, tepa, metepa, and apholate all sterilized at the higher concentration, but only the antimetabolites sterilized at the lower concentration. Whenever a compound induces sterility, a second series of tests is run to determine the range of effective concentrations. Some antimetabolites sterilized at concentrations as low as 0.0025%, whereas the minimum for alkylating agents was 0.25%. In the third series of tests, the specific sex sterilized is determined. The chemosterilants are presented in the food at sterilizing concentrations to individual sexes of flies. These flies are then mated with normal flies of the opposite sex. All the chemicals mentioned above sterilized the females, but aminopterin and methotrexate failed to sterilize the male flies, and 5-fluorouracil produced male sterility in only a few tests. Tepa, 5-fluoro-orotic acid, metepa, and apholate, when included in the food, regularly sterilized both sexes. Because of the inability of aminopterin, methotrexate, and 5-fluorouracil to sterilize males, these compounds were not considered further. 

The isolation in the 1940s of the antianemic factor now known as folic acid and its subsequent synthesis resulted, after many false leads, in the first successful antivitamin antimetabolite aminopterin (Table 4-5). This compound produced sustained remissions in leukemia. The N10-methyl homolog, now designated as MTX (Table 4-5), superseded aminopterin the same year. It is still the only clinically significant antifolate carcinolytic drug. 

The unique aspects of thymine synthesis allow the design of antimetabolites that may have greater specificity in this process. Aminopterin and methotrexate have been used with some success. These are inhibitors of tetrahydrofolate formation where, at the correct dosage, the conversion of dUMP to dTMP appears to be more sensitive than many other reactions and is preferentially inhibited. This inhibits DNA formation and cell proliferation. Other... 

With aminopterin, the reductase forms a complex with a low dissociation constant. Thus, in the presence of aminopterin many of the reductase molecules are trapped in an inactive form. The coenzymes necessary for purine biosynthesis are not formed, and that pathway is blocked. In this manner aminopterin interferes with the progress of leukemia and with the proliferation of normal bone marrow. Unfortunately, the cells of individuals treated for leukemia overcome the metabolic block by building up a resistance to the antimetabolites by increasing the level of the reductase. We will now consider the mechanism of action of each of these coenzymes separately. 

AIC also accumulated when E. coli was grown in the presence of aminopterin (4-aminopteroylglutamic acid), a folic acid antagonist. Since addition of aminopterin or p-aminobenzoic acid antimetabolites e.g., sulfonamides) results in the accumulation of AIC, it is presumed that the latter act by creating a folic acid deficiency. Thus, folic acid or a related derivative, and not p-aminobenzoic acid, may be directly concerned with purine synthesis. This is in agreement with Woods hypothesis that the primary action of sulfonamides is the inhibition of folic acid formation, and that purine synthesis is a secondary event in which p-aminobenzoic acid participates only indirectly. ... 

The known role of vitamin B12 and folic acid in the formation of labile methyl groups (339) helps to explain the replaceability of folic acid (330) or vitamin B12 (331) by thymidine in certain deficient oi anisms. Also of interest is the role of folic acid compounds in the pathway leading to formation of the thymine methyl group. Low concentrations of a folic acid antimetabolite, aminopterin, blocked the utilization of deoxyiuidine for thymidine synthesis (333). The antimetabolite effects of aminopterin on the utilization of one-carbon donors have been known for some time (333, 334). The details of thymine biosynthetis will be discassed elsewhere in this volume (Chapter 24). 

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