Folic acid toxicity

Aminolevulinic acid dehydratase 3-aminotriazole toxicity to, 1, 139 Aminopterin—see Folic acid, 4-amino-Aminopyrine as antipyretic, 1, 172 biological activity, 5, 295 Aminyl, dimethyl-ESR, 7, 19 Amiphenazole... 

Folic acid, 4-amino-4-deoxy-10-methyl-, 1, 164 3, 325 as anticancer drug, 1, 263 biological activity, 3, 325 Folic acid, 4-amino-10-methyl-toxicity, 1, 141 Folic acid, 7,8-dihydro-biosynthesis, 3, 320 synthesis, 1, 161, 3, 307 Folic acid, 4-dimethylamino-hydrolysis, 3, 294 Folic acid, 5-formiminotetrahydro-biological activity, 3, 325 Folic acid, 5-formyl-5,6,7,8-tetrahydro-biological activity, 3, 325 chirality, 3, 281 occurrence, 3, 325 Folic acid, 10-forfnyltetrahydro-biological activity, 3, 325 Folic acid, 5,10-methenyl-5,6,7,8-tetrahydro-biological activity, 3, 325 chirality, 3, 281 Folic acid, 5-methyl-chirality, 3, 281 Folic acid, 9-methyl-toxicity, 1, 141... 

Overall, supplementation with folic acid is considered safe as the vitamin has low acute and chronic toxicity. 

The classic example is that of Prontosil (Figure 2.12) in which the compound is active against bacterial infection in animals though inactive against the bacteria in pure culture. The toxicity in animals is the result of reduction to the sulfanilamide (4-aminobenzenesulfonamide) that competitively blocks the incorporation of 4-aminobenzoate into the vitamin folic acid. 

Toxicities are GI (stomatitis, diarrhea, nausea, vomiting), hematologic (thrombocytopenia, leukopenia), pulmonary (fibrosis, pneumonitis), and hepatic (elevated enzymes, rare cirrhosis). Concomitant folic acid may reduce some adverse effects without loss of efficacy. Liver injury tests (aspartate aminotransferase or alanine aminotransferase) should be monitored periodically, but a liver biopsy is recommended during therapy only in patients with persistently elevated hepatic enzymes. MTX is teratogenic, and patients should use contraception and discontinue the drug if conception is planned. 

Methotrexate, an antimetabolite, is indicated for moderate to severe psoriasis. It is particularly beneficial for psoriatic arthritis. It is also indicated for patients refractory to topical or UV therapy. Methotrexate can be administered orally, subcutaneously, or intramuscularly. The starting dose is 7.5 to 15 mg per week, increased incrementally by 2.5 mg every 2 to 4 weeks until response maximal doses are approximately 25 mg/wk. Adverse effects include nausea, vomiting, mucosal ulceration, stomatitis, malaise, headache, macrocytic anemia, and hepatic and pulmonary toxicity. Nausea and macrocytic anemia can be ameliorated by giving oral folic acid 1 to 5 mg/day. Methotrexate should be avoided in patients with active infections and in those with liver disease. It is contraindicated in pregnancy because it is teratogenic. 

Recently, tritiated folic acid became available, making possible a nonmicrobiological method for studying the metabolism of folic acid. It obviates the toxic effects of folic acid antagonists on microbial assay organisms. This technique was used to follow the uptake, metabolism, and excretory products of folic acid (A4, J2, J3). 

Johnson and colleagues made a provocative observation in the course of exploratory preclinical toxicological studies of vincristine, namely, that folinic acid (Leucovorin citrovorum factor 5-formyl-5,6,7,8-tetrahy-drofolic acid) was able to protect mice from the toxicity of high doses of vincristine lb). Vincristine, at a dose of 2.5 mg/kg administered intravenously, resulted in a mortality of 90% over a period of 30 days, but treatment with folinic acid lowered the mortality to 25%. The protection against vincristine toxicity did not occur when folic acid was substituted for folinic acid. A report has appeared (45) indicating that there is no specific protective effect of folinic acid against vincristine toxicity in mice and that the protection can be observed by comparable treatment with isotonic saline solution. As discussed in Section Vll, there is not conclusive evidence that folinic acid is able to ameliorate vincristine toxicity in humans (46). 

Folic acid is vital for both humans and bacteria. Bacteria synthesize this compound, but humans are unable to synthesize it and, consequently, obtain the necessary amounts from the diet, principally from green vegetables and yeast. This allows selectivity of action. Therefore, sulfa drugs are toxic to bacteria because folic acid biosynthesis is inhibited, whereas they produce little or no ill effects in humans. The structural relationships between carboxylic acids and sulfonic acids that we have observed in rationalizing chemical reactivity are now seen to extend to some biological properties. 

In addition, most bacteria are not able to utilize folic acid of exogen origin, so they synthesize the folic acid necessary for vital functions by themselves. This is the difference between bacterial and animal cells, and it is the reason behind the selective toxicity of sulfonamides. 

All of these compounds are inhibitors of dihydrofolate reductase in bacteria, plasmodia, and humans. Fortunately, they have a significantly higher affinity to bacterial and protozoal dihydrofolate reductase. Pyrimethamine, for example, inhibits dihydrofolate reductase in parasites in concentrations that are a several hundred times lower than that required to inhibit dihydrofolate reductase in humans. This is the basis of their selective toxicity. Selective toxicity can be elevated upon the host organism s production of folic acid, which parasites are not able to use. 

Pharmacology Leucovorin is one of several active, chemically reduced derivatives of folic acid. It is useful as an antidote to drugs that act as folic acid antagonists. Administration of leucovorin can counteract the therapeutic and toxic effects of folic acid antagonists such as methotrexate, which act by inhibiting dihydrofolate reductase. 

Folic acid antagonist overdosage In the treatment of accidental overdosages of folic acid antagonists, administer leucovorin as promptly as possible. As the time interval between antifolate administration (eg, methotrexate) and leucovorin rescue increases, leucovorin s effectiveness in counteracting toxicity decreases. 

All patients with methanol toxicity should be given folic acid 50 milligrams intravenously every 4 hours to increase the metabolism of formic acid. In ethylene glycol ingestion, folate, thiamine and pyri-doxine should all be administered, to enhance the metabolism of the poison to non-toxic products, and minimize oxalic acid production. Calcium supplements are required for symptomatic hypocalcaemia. 

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. 

The serious toxic effect is hyperkalemia. Triamterene produces relatively few other side effects which includes nausea, vomiting, dizziness etc. Megaloblastic anaemia has been reported in patients with alcoholic cirrhosis, which is probably due to inhibition of dihydrofolate reductase in patients with reduced folic acid intake. 

Morgan SL, Baggott IE, Vaughn WH et al. The effect of folic acid supplementation on the toxicity of low-dose methotrexate in patients with rheumatoid axthiitis. Arthritis Rheum 1990 33 9-18. 

Nausea and mucosal ulcers are the most common toxicities. Progressive dose-related hepatotoxicity in the form of enzyme elevation occurs frequently, but cirrhosis is rare (< 1%). Liver toxicity is not related to serum methotrexate concentrations, and liver biopsy follow-up is only recommended every 5 years. A rare hypersensitivity-like lung reaction with acute shortness of breath is documented, as are pseudolymphomatous reactions. The incidence of gastrointestinal and liver function test abnormalities can be reduced by the use of leucovorin 24 hours after each weekly dose or by the use of daily folic acid, although this may decrease the efficacy of the methotrexate. This drug is contraindicated in pregnancy. 

Vitamins are chemically unrelated organic compounds that cannot be synthesized by humans and, therefore, must must be supplied by the diet. Nine vitamins (folic acid, cobalamin, ascorbic acid, pyridoxine, thiamine, niacin, riboflavin, biotin, and pantothenic acid) are classified as water-soluble, whereas four vitamins (vitamins A, D, K, and E) are termed fat-soluble (Figure 28.1). Vitamins are required to perform specific cellular functions, for example, many of the water-soluble vitamins are precursors of coenzymes for the enzymes of intermediary metabolism. In contrast to the water-soluble vitamins, only one fat soluble vitamin (vitamin K) has a coenzyme function. These vitamins are released, absorbed, and transported with the fat of the diet. They are not readily excreted in the urine, and significant quantities are stored in Die liver and adipose tissue. In fact, consumption of vitamins A and D in exoess of the recommended dietary allowances can lead to accumulation of toxic quantities of these compounds. 

The mode of action of the sulfonamides as antagonists of 4-aminobenzoic acid (PAB) is well documented, as is the effect of physicochemical properties of the sulfonamide molecule, e.g. pK, on potency (B-81MI10802). Sulfonamides compete with PAB in the biosynthesis of folic acid (44), a vital precursor for several coenzymes found in all living cells. Mammalian cells cannot synthesize folic acid (44), and rely on its uptake as an essential vitamin. However, bacteria depend on its synthesis from pteridine precursors, hence the selective toxicity of sulfonamides for bacterial cells. Sulfonamides may compete with PAB at an enzyme site during the assembly of folic acid (44) or they may deplete the pteridine supply of the cell by forming covalently-bonded species such as (45) or they may replace PAB as an enzyme substrate to generate coupled products such as (46) which are useless to the cell. 

When present in excess methionine is toxic and must be removed. Transamination to the corresponding 2-oxoacid (Fig. 24-16, step c) occurs in both animals and plants. Oxidative decarboxylation of this oxoacid initiates a major catabolic pathway,305 which probably involves (3 oxidation of the resulting acyl-CoA. In bacteria another catabolic reaction of methionine is y-elimination of methanethiol and deamination to 2-oxobutyrate (reaction d, Fig. 24-16 Fig. 14-7).306 Conversion to homocysteine, via the transmethylation pathway, is also a major catabolic route which is especially important because of the toxicity of excess homocysteine. A hereditary deficiency of cystathionine (3-synthase is associated with greatly elevated homocysteine concentrations in blood and urine and often disastrous early cardiovascular disease.299,307 309b About 5-7% of the general population has an increased level of homocysteine and is also at increased risk of artery disease. An adequate intake of vitamin B6 and especially of folic acid, which is needed for recycling of homocysteine to methionine, is helpful. However, if methionine is in excess it must be removed via the previously discussed transsulfuration pathway (Fig. 24-16, steps h and z ).310 The products are cysteine and 2-oxobutyrate. The latter can be oxidatively decarboxylated to propionyl-CoA and further metabolized, or it can be converted into leucine (Fig. 24-17) and cysteine may be converted to glutathione.2993... 

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