folic acid safety and toxicity: a brief review14

Folic acid safety and toxicity: a brief review14

A,nJC/in Nuir 1989:50:353-8. Printed in USA. © 1989 American Society forClinical Nutrition 353

CE Butterworth, Jr, and Tsunenobu Tamura

ABSTRACT Oral folic acid (pteroylglutamic acid) is generally regarded as not toxic fornormal humans but it may cause neurological injury when given to patients with undiagnosedpernicious anemia. The vitamin should be given with caution to drug-treated epileptic patients

because seizure control may be affected. Some studies suggest that folic acid supplements inter-fere with intestinal zinc absorption in humans and animals but others do not confirm such aneffect. The weight ofcurrent evidence favors the view that daily supplements of 5-1 5 mg folicacid do not have significant adverse effects on Zn nutriture in healthy nonpregnant subjects.Because antifolate medications are now being used to treat a wide range of malignant and

nonmalignant disorders, further investigation is needed concerning folate metabolism and thesafety ofsupplements in patients with these disorders. Am JC/in Nutr b989;50:353-8.

Introduction

KEY WORDS Folic acid, safety, toxicity,

interaction, antifolate

When synthetic folic acid (oxidized pteroylglutamicacid) was first introduced into clinical medicine 43 y ago,there was only meager information regarding human re-quirements. Lacking firm knowledge about its concen-tration in foods, bioavaibability, tissue content, and turn-over, early investigators empirically chose dose levelsnow known to be far in excess of nutritional require-ments. Fortunately, good effects outweighed the bad (ifany). In the treatment of tropical sprue, Suarez et al (1)employed oral doses of 100 mg daily for 2 wk or singleoral doses ofSOO mg which are, respectively, � 200-1000times present-day estimates of the nutritional require-ment, yet no toxicity was observed. In a well-respectedtextbook of pharmacology (2) the following statementappears: “Oral fobic acid is not toxic for man. Even withdoses as high as 1 5 mg per day, there have been no sub-

stantiated reports ofside effects.”Hunter et ab (3) in 1970 described mental changes,

sleep disturbances, and gastrointestinal symptoms overa b-mo period in 14 normal volunteers who ingested 15mg fobic acid/d. Several others reported no toxic side

effects with fobic acid supplementation in normal sub-jects (4-6). Sheehy (4) described a man who took 60 mgfolic acid/d for 3 y with no apparent toxic effect. Tayloret al (7) described renal hypertrophy and epithebial cellhyperplasia in rats after large doses offolic acid but therehave been no reports of renal toxic effects in humans.In a brief review of the toxicity of fobic acid and othervitamins, DiPalma and Ritchie (8) concluded that most

pernicious anemia, epilepsy, zinc and folate

water-soluble vitamins are nontoxic in ordinary dosesbut they noted that “even folic acid may be toxic in ex-ceptional circumstances.” The following review was un-

dertaken as a result of several recent reports suggestingpossible undesirable side effects of fobic acid supplemen-tation. The discussion will be limited to studies of syn-thetic folic acid because there is essentially no informa-tion available regarding consumption of excessiveamounts ofdietary fobate.

Neurologic disease in pernicious anemia

Undoubtedly the most dreaded toxic effect of fobic

acid is its ability to mask the anemic manifestations ofpernicious anemia while allowing the neurological dis-ease (posterolateral spinal cord degeneration [PCDJ) toprogress. Fortunately this devastating complication isunlikely to occur with the amounts of folate in ordinary

diets and in over-the-counter vitamin supplements.Moreover, the diagnosis ofpemnicious anemia can be ex-

I From the Department of Nutrition Sciences, University of Ala-

bama at Birmingham, UAB Station, Birmingham, AL 35294.2 Presented in part at the AIN/ASCN Public Information Sympo-

sium on Nutrient Safety and Toxicity, May 4, 1988. Las Vegas, NV.3 Supported by NIH contract N0l-HD52927 and grants 4 P01-

CA28103 and 5 ROl-DK33703.

4 Address reprint requests to CE Butterworth, Jr. Department of Nu-trition Sciences, University ofAlabama at Birmingham, UAB Station.Birmingham, AL 35294.

ReceivedJune 13, 1988.

Accepted for publication August 23, 1988.

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354 BUTTERWORTH AND TAMURA

cluded with a reasonable degree ofcertainty ifthe serumvitamin B- 12 is normal. Because vitamin B- 12 assays arenow widely available in diagnostic laboratories, the testshould probably be performed as a precautionary mea-sure before pharmacologic doses of folic acid are pre-scribed.

Because ofa virtual absence ofdata on the prevalenceof folate-induced PCD, an informal survey was con-ducted in 197 1 (CE Butterworth Jr, unpublished obser-vations, 1988). A first-class letter was sent to the entiremembership of the American Society of Hematology

N 1200 members) with a stamped, self-addressed postcard asking, “Have you personally cared for any patientduring the last ten years whose illness was, in your opin-ion, made worse by the daily ingestion of 400 micro-grams (or less) of folic acid, in addition to amounts pres-ent in the diet? Yes . No .“ There were 860 re-sponses, representing 72% of the mailing. Of these, 98%had no knowledge of anyone made worse by a supple-ment of 400 �tg folic acid/d during the previous 10 y. Afollow-up inquiry requesting further information wasthen sent to each ofthe 20 respondents who gave a “yes”answer to the question. There were seven responses ofwhich only three qualified as cases ofPCD during the 10-y period: one with a daily intake ofO. 1 mg, one with 5.0mg, and one unknown. The 5.0-mg dose was prescribedby a physician. There were no peer-reviewed publica-tions of case reports. Quite obviously there are manyweaknesses in a study ofthis type but it suggests that ag-gravation ofpernicious anemia and PCD associated withthe use of over-the-counter folic acid supplements is ex-tremely rare (ie, one or two cases nationwide in a lO-yperiod). Sixty-five percent of respondents reported that

they had encountered subjects whose illness might havebeen improved or prevented by a 400-�tg supplement offolic acid.

In one of the early references to neurobogic injury inpernicious anemia, Heinle and Welch (9) made the fob-lowing statement, “. . . we have treated and maintained47 patients with synthetic fobic acid for periods up to one

year. Of this number only 2 others (in addition to thepatient described) have exhibited definite neurologicalrelapse.” The one case described in detail by Heinle andWelch (9) displayed dramatic improvement in the ane-mia and improved sense of well-being over a period of79 d. At that time his four remaining teeth were extractedand he was placed on a soft diet, after which his neuro-

logic condition rapidly deteriorated. Questions may beasked as to whether poor diet and/or anesthesia (perhapswith nitrous oxide) played significant contributory robesin the neurological deterioration. One is also led to won-der if some of the other 47 patients would have fulfilledpresent-day criteria for the diagnosis of pernicious ane-

mia. Because ofethicab considerations it is likely that theprecise incidence of folate-induced, subacute combineddegeneration ofthe spinal cord in pernicious anemia will

never be known.

Folate interaction with anticonvulsant drugs

Chanarin et al (10) in 1960 first described the possibil-ity that folic acid supplements in high doses may reverse

the effectiveness of anticonvulsant medication. Sincethat time published reports as to whether folic acid ther-apy affects fit frequency have been controversial (8). In astudy on the effect of intravenous folic acid on drug-treated epileptics, it was observed that some subjects ap-pear to be relatively sensitive and others more tolerant tothe infusion (1 1). Six subjects received 75 mg folic acidintravenously in a period of 30 mm without electroen-cephalogram (EEG) changes, seizure, or other ill effects.

One subject demonstrated a transiently abnormal EEGpattern with characteristic spikes but no seizure activityor other ill effects after receiving 150 mg folic acid in 30mm. The eighth patient displayed a burst of spikes andslow waves on the EEG but no seizure after receiving 7.2mg folic acid intravenously over a period of 3 mm. Typi-cal seizures and EEG tracings followed subsequent infu-sions of 14.4 and 19.2 mg fobic acid. It may be concludedthat some individuals with drug-controlled epilepsy aresensitive to rapid intravenous infusions ofas little as 7.0mg folic acid. In contrast, others tolerated up to 20 timesthis amount without seizure activity or other ill effects.

Folic acid-zinc interactions

During the last several years interest has developed inrelationships between fobic acid and zinc (12-19). Thereis good evidence that bovine hepatic folic acid conjugaseis a Zn metalloenzyme (20) and that folic acid polygluta-mates are not well absorbed by the intestine in Zn-defi-cient humans (2 1). However, there are conflicting me-ports as to whether folic acid supplements exert debeteri-ous effects on Zn nutriture in man. Milne et ab (12) in1984 reported that a folic acid supplement of400 �ig ev-ery other day, in addition to a dietary folate intake of1 50-1 80 �g/d, was associated with increased fecal bossesof Zn. However, urinary bosses appeared to compensatefor fecal bosses so that net Zn balance was not affected.Nevertheless the authors cautioned about “the possibil-ity ofadverse effects from supplemental pteroybglutamic

acid.”Also in 1984, in a detailed and complex study of 450

pregnant women, Mukhei�jee et al (1 3) observed a highdegree of correlation between the occurrence of preg-nancy complications or fetal distress and the combina-tion of low plasma Zn and high folate concentration inthe maternal blood sample. They postulated that folicacid supplementation and iron supplements may eachinterfere with the intestinal absorption ofZn. They sug-gested further that prenatal vitamin supplements, whichnot uncommonly contain 800-1000 �g folic acid, inhibitintestinal absorption of Zn, which in turn was responsi-ble for the observed fetal distress.

Simmer et at (14) recently gave a cautiously affirma-tive answer to the question, are Fe-folate supplements


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harmful? Zn absorption was assessed by measuringplasma Zn concentration after an oral dose ofZn sulfate(25 mg Zn) rather than by metabolic balance. Compari-sons were made ofthe area under the curve and the peakconcentration at 1 , 2, 3 or 4 h, a procedure that is subjectto error because ofvariations in gastric emptying, periph-eral uptake, and renal excretion. It does not give infor-mation concerning bong-term absorption or net meta-bolic balance. The test was performed in 10 healthy preg-nant women and repeated after 2 wk of oralsupplementation with a commercial product containing350 �tg folic acid and 100 mg ferrous Fe. The averagedietary Zn intake ofthe subjects was 12. 1 mg/d (60% ofRecommended Dietary Allowances [22] based on 7-ddiet histories). A similar test was performed in a group ofeight men and two nonpregnant women with twice asmuch Zn sulfate (50 mg Zn) after a 2-wk period of oralsupplementation with 350 �zg folic acid, rather than thefolic acid and Fe combination. Treatment depressed thebioavaibability of Zn in both groups, according to thestated criteria (14). The effect was more pronounced withthe fobic acid and Fe combination than with folic acidalone. It should be noted that the Zn-absorption test wasperformed after a period ofsuppbementation; the Zn andfolic acid were not administered simultaneously. Moredata are needed concerning possible effects of folate sup-plementation on Zn balance in pregnant women.

Ghishan et al (1 5) described two animal experimentsand a series of in vitro studies designed to examine rela-tionships between Zn and fobate. Perfusion of 30-cm seg-ments ofrat small bowel in situ revealed diminished mu-cosal uptake ofradioactive Zn when folate was also pres-ent in the lumen. Previous intraperitoneal injection of

fobate did not block mucosal Zn uptake. For reasons thatare not stated, the reciprocal relationship, ie, the effect ofZn on radioactive folate absorption was not studied inthe same system but rather in an in vitro preparation ofeverted ratjejunal segments. It was observed that a 1000-fold molar excess ofZn did not significantly alter muco-sal-to-serosal transport ofradioactive fobate; however, in-hibition was observed when the molar ratio of Zn to fo-late was 2500: 1 . Ghishan et al (1 5) also described in vitroexperiments concerning the effect of charcoal-bindingand pH. The findings suggest that Zn and folate form aninsoluble complex at low pH but the complexes dissolvewhen the pH is raised above 6.0.

Keating et al (16) also studied the Zn-folate interactionin Sprague-Dawley rats and humans but under some-what different circumstances. After intragastric adminis-tration of 65Zn to rats, the researchers observed no sig-nificant differences in hepatic and renal Zn retention atmolar ratios ofeither 1 :20 or 2: 1 for folic acid:Zn. A sim-ilar lack of effect on growth pattern and Zn uptake intobone was observed with diets representing these samemolar ratios of fobic acid:Zn. When six healthy youngmen took 25-mg oral doses ofZn with or without 10 mgfobic acid, similar patterns of increase in plasma Zn were

observed, with peak levels at 2 h after ingestion of thedose.

Fuller et al (17) determined the effect of folic acid inthe diet on tissue fobate and Zn bevels in rats during preg-nancy and lactation. High-folic-acid diets (100 mg fobicacid added to 1 kg basal diet) fed for over 40 d signifi-

cantly increased blood fobate bevels but did not changeZn concentrations in various tissues. The authors suggestthat if the results are applicable to humans, then somereassurance is provided that prenatal folate supplemen-

tation does not necessarily cause Zn depletion in preg-nant women.

A study currently in progress has afforded the opportu-nity to compare plasma and erythrocyte concentration

ofZn in a group ofyoung women receiving a 10-mg dailysupplement of oral folic acid with an identical group re-ceiving a placebo (1 8). Fifty subjects with cervical dyspla-sia, of whom 27 received fobic acid and 23 received pla-

cebo, were evaluated at the end of 2 mo. At the end of 4mo, 12 ofthe same subjects in the folic acid group and 9in the placebo group were available for another compari-son. In spite of a dramatic difference in plasma anderythrocyte folate concentration between the supple-

mented and placebo groups (p < 0.001 at both 2 and 4mo), there was no significant difference between the twogroups with regard to Zn. Subsequently, 49 subjects inthe folic acid-supplemented group and 58 subjects in theplacebo groups completed the 6-mo randomized, dou-ble-blind clinical intervention trial. The median valuefor Zn concentration in plasma and erythrocytes was vir-tualby identical in the two groups of subjects and re-mained virtually unchanged throughout 6 mo of folatesupplementation.

A recently published abstract by Krebs et ab (19) re-ports no impairment of absorption of a stable isotope ofZn (70Zn) in three normal adults when administered witha simultaneous dose of 30 mg fobic acid. Another groupof eight subjects aged 24 ± 12 y with the fragile-X syn-

drome who had received 16 ± 5 mg fobic acid/d for 1-4y was also studied; mean fasting Zn concentrations in

plasma were not significantly different from values fromadult control subjects. Neutrophil Zn levels and the ac-tivities of two Zn-dependent enzymes (serum alkalinephosphatase and erythrocyte #{244}-aminolevulinic acid de-hydratase) were also within normal limits in fobic acid-supplemented subjects.

Divergent experimental techniques

It is difficult to reconcile the conflicting results re-ported in the literature regarding a deleterious effect offolic acid supplements on Zn nutriture. A major sourceof confusion is the noncomparability of results due towidely different experimental techniques. For example,some human studies have employed plasma Zn responsecurves after an oral dose of Zn with or without fobate.This approach is subject to error because of variations


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356 BUTFERWORTH AND TAMURA

TABLE 1

Zinc-folate interactions as described in published reports by various groups of investigators

SystemMolar ratio of

Zn to folate* Result Reference

Human: metabolic balance 135: 1 Increased fecal Zn; net balance unaffected 12Human: plasma Zn response curve 750: 1 Flat response curve after oral Zn dose 14Rat: gut perfusion 1:4.5 Zn uptake inhibited 15Rat: everted gut sac 1000:1

2500:1Folate uptake not inhibitedFolate uptake inhibited

15

Rat: growth, tissue uptake 20: 1 Growth not inhibited; bone Zn uptake normal 16Human: plasma Zn response curve I 7: 1 Normal curve ofplasma Zn response after oral dose 16Rats: tissue Zn levels 4.4:1

I3.4:1

1:2.5

1.2:1

No depression oftissue Zn 17

Human: red blood cell and plasma Zn 7: 1 No depression ofblood Zn after 2 and 4 mo 18

Human: plasma Zn, Zn metalloenzyme, 7: 1 No depression ofblood Zn and metalloenzyme 19absorption ofZn activity after 1-4 y, normal Zn absorption

* Normal value is 250: 1 based on Recommended Dietary Allowances (22)(l5 mg [230 �mol] Zn and 0.4 mg [0.91 �imolJ folic acid).

in such factors as gastric emptying, intestinal absorptionrate, and excretion by the kidney. Some have comparedincreases in blood bevels ofZn but not folic acid, and viceversa, when both had been administered orally. Otherstudies compared increases in plasma Zn after a 1-2-wkperiod ofsupplementation with folic acid (ie, both nutri-ents were not administered simultaneously). These re-sults must be regarded with skepticism until confirmedby more reliable procedures. Investigations which in oneway or another reflect long-term metabolic balance inman (12, 18, 19) or animals (16, 17) have not indicateddeleterious effects of folic acid supplements on Zn. Asummary ofsome published studies ofZn-folate interac-tions is presented in Table 1.

Antifolate medications

Virtually nothing is known about the safety or toxicityof oral folic acid supplements in humans who are beingtreated with drugs known to interfere with folate metab-olism. A wide variety of folic acid antagonists is beingused to treat such diverse illnesses as cancer (23), leuke-mia (24, 25), psoriasis (26), rheumatoid arthritis (27, 28),polymyositis (29, 30), dermatomyositis (30, 3 1), Reiterdisease (32), Wegener granubomatosis (33), sarcoidosis(34), bronchial asthma (35), sclerosing cholangitis (36),primary biliary cirrhosis (37), bacterial infection (38),malaria (39, 40), hypertension (41), Crohn disease (42),ulcerative colitis(25), gout (43), and epilepsy (10). A par-tial list ofthese diseases along with the antifolate medica-tion commonly used in their treatment is presented inTable 2. Methotrexate, pyrimethamine, trimethoprim,and triamterene are all known to be inhibitors of dihy-drofolic reductase. Recently a new dihydrofobic reduc-tase inhibitor, trimetrexate, has been used successfullyin the treatment of Pneumocvstis carinii pneumonia in

patients with AIDS (44). Species differences exist regard-ing enzyme susceptibility to inhibition but little is knownregarding differences between human individuals or in

different human tissues. The use of so-called folic acidantagonists in these disorders might seem to imply thatan overabundance offolate in the diet would be harmful.There is little or no evidence to support such a view, noris it known iffolic acid supplements negate the therapeu-tic effectiveness ofthese medications. Indeed, because fo-lic acid is itself an inhibitor of dihydrofolate reductase

(45, 46) it could quite possibly be not only safe but bene-ficial in the treatment ofthese disorders. Alternatively itmay be possible to titrate dosages of folic acid and anti-folate medication to spare certain pathways while inhib-iting others. For example, sulfasalazine is not only aninhibitor of dihydrofolic reductase but also of methyl-enetetrahydrofolate reductase, serine transhydroxy-methylase (47), and human jejunat brush border conju-gase (48). Colchicine suppresses blood folate levels inmice, dogs, and humans (43) although the mechanism is

TABLE 2Diseases treated with drugs known to inte rfere with folate metabolism

Disease Drug

Cancer, leukemia MethotrexatePsoriasis MethotrexateRheumatoid arthritis MethotrexateBronchial asthma MethotrexateBacterial infection TrimethoprimMalaria PyrimethamineHypertension Triamterene

Crohn disease SulfasalazineGout ColchicineEpilepsy PhenytoinAIDS Trimetrexate


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FOLIC ACID SAFETY AND TOXICITY 357

not established. It is not known whether the beneficialeffects of colchicine therapy in gout are due to suppres-sion of folate-mediated pathways of purine biosynthesisor to some more general antiinflammatory action. Thesafety of folate supplementation remains an open ques-

tion for patients with the diseases listed in Table 2,whether or not they are receiving antifolate medication.

Summary

Daily oral supplements of 5-10 mg folic acid appearto be well-tolerated and without toxicity in normal non-pregnant subjects; some studies suggest the desirabilityof ensuring an adequate intake of Zn if folate supple-ments are to be used during pregnancy. Neurobogic in-jury may occur when folic acid supplements are givento patients with undiagnosed pernicious anemia. This isprobably quite rare and avoidable with more widespreaduse of screening tests for serum vitamin B-b2. Folic acidsupplements should be used with caution in patientswith epilepsy because seizure activity may be induced insome (but not all) drug-controlled subjects. Althoughhigh concentrations of folate may interfere with the in-testinal absorption of Zn in experimental animals, theweight of current evidence indicates that daily oral sup-plements of 5-b 5 mg folate do not adversely affect Znbalance in normal humans over periods of 6 mo to 4 y.Further investigation is needed concerning fobate metab-olism in a wide range of clinical disorders now beingtreated with antifolate medications. 13

References

1. Suarez RM, Spies TD, Suarez RM, Jr. The use of folic acid in

sprue. Ann Intern Med l947;26:643-77.2. Hillman RS. Vitamin B12, folic acid, and the treatment of

megaloblastic anemias. In: Gilman AG, Goodman LS, Rail TW,Murad F, eds. Goodman and Gilman’s the pharmacological basisoftherapeutics. 7th ed. New York: Macmillan, 1985:1323-37.

3. Hunter R, Barnes J, Oakeley HF, Matthews DM. Toxicity of folicacid given in pharmacological doses to healthy volunteers. Lancet

1970; 1:61-3.4. Sheehy TW. Folic acid: lack oftoxicity. Lancet 1973; l:37(letter).5. Hellstrom L. Lack oftoxicity offolic acid given in pharmacological

doses to healthy volunteers. Lancet 197 1; 1:59-61.6. Richens A. Toxicity offolic acid. Lancet 1971; i:9l2(letter).7. Taylor DM, Threlfall G, Buck AT. Chemically-induced renal

hypertrophy in the rat. Biochem Pharmacol I 968; 17:1567-74.

8. DiPalma JR, Ritchie DM. Vitamin toxicity. Ann Rev PharmacolToxicol 1977; 17:133-48.

9. Heinle RW, Welch AD. Folic acid in pernicious anemia. Failureto prevent neurologic relapse. JAMA 1947; 133:739-41.

10. Chanarin I, Laidlow J, Loughridge LW, Mollin DL. Megaloblasticanaemia due to phenobarbitone. The convulsant action oftherapeutic doses offolic acid. Br Med J 1960; 1:1099-102.

I 1. Chien LT, Krumdieck CL, Scott CW, Butterworth CE Jr. Harmfuleffect of megadoses of vitamins: electroencephalogram abnor-malities and seizures induced by intravenous folate in drug-treated

epileptics. Am J Clin Nutr 1975;28:5 1-8.

12. Milne DB, Canfield WK, Mahalko JR. Sandstead HH. Effect of

oral folic acid supplements on zinc, copper, and iron absorption

and excretion. Am J Clin Nutr 1984; 39:535-9.

13. Mukherjee MD, Sandstead HH, Ratnaparkhi MV, Johnson LK,

Milne DB, Stelling HP. Maternal zinc, iron, folic acid, and proteinnutriture and outcome of human pregnancy. Am J Clin Nutr

1984;40:496-507.

14. Simmer K, Iles CA, James C, Thompson RPH. Are iron-folate

supplements harmful? Am J Clin Nutr l987;45:I22-5.

15. Ghishan FK, Said HM, Wilson PC, Murrell JE, Greene HL.

Intestinal transport of zinc and folic acid: a mutual inhibitory

effect. Am J Clin Nutr 1986;43:258-62.

16. Keating JN, Wada L, Stokstad ELR, King JC. Folic acid: effect ofzinc absorption in humans and in the rat. Am J Clin Nutr 1987;46:

835-9.

17. Fuller NJ, Evans PH, Howlett M, Bates CJ. The effects of dietaryfolate and zinc on the outcome of pregnancy and early growth inrats. BrJ Nutr 1988; 59:251-9.

18. Butterworth CE Jr, Hatch K, Cole P. et al. Zinc concentration inplasma and erythrocytes of subjects receiving folic acid

supplementation. Am J Clin Nutr 1988;47:484-6.

19. Krebs NF, Hambidge KM. Hagerman RJ, et al. The effects of

pharmacologic doses of folate on zinc absorption and zinc status.Am J Clin Nutr 1988;47:783(abstr).

20. Silink M, Reddel R, Bethel M, Rowe PB. �y-Glutamyl hydrolase(conjugase). Purification and properties of the bovine hepatic

enzyme. J Biol Chem 1975;250:5982-94.

21. Tamura T, Shane B, Baer MT, King JC, Margen 5, Stokstad ELR.

Absorption of mono- and polyglutamyl folates in zinc-depleted

man. Am J Clin Nutr 1978; 31:1984-7.

22. Committee on Dietary Allowances, Food and Nutrition Board,National Research Council. Recommended dietary allowances.

9th ed. Washington, DC: National Academy ofSciences, 1980.

23. Berlin NI, Rall D, Mead JAR, et al. Folic acid antagonists. Effects

on the cell and the patient. Combined clinical staff conference at

the National Institute of Health. Ann Intern Med 1963; 59:93 1-

56.

24. Farber 5, Diamond LK, Mercer RD. Sylvester RF, Wolff JA.

Temporary remissions in acute leukemia in children produced byfolic acid antagonist, 4-aminopteroyl-glutamic acid (aminopterin).

N Engl J Med l948;238:787-93.

25. Stebbins R, Scott J, Herbert V. Drug-induced megaloblastic

anemias. Semin Hematol 1973; 10:235-51.

26. Hanno R, Gruber GO, Owen LG, Callen JP. Methotrexate in

psoriasis. A brief review of indications, usage, and complications

ofmethotrexate therapy. J Am Acad Dermatol l980;2: 17 1-4.

27. Williams HJ, Willkens RF, Samuelson CO Jr, et al. Comparison

of low-dose oral pulse methotrexate and placebo in the treatmentofrheumatoid arthritis. A controlled clinical trial. Arthritis Rheum

1985;28:72 1-30.

28. Morgan SL, Baggott JE, Altz-Smith M. Folate status of rheumatoidarthritis patients receiving long-term, low-dose methotrexatetherapy. Arthritis Rheum 1987; 30:1348-56.

29. Arnett FC, Whelton JC, Zizic TM, Stevens MB. Methotrexate

therapy in polymyositis. Ann Rheum Dis 1973;32:536-46.

30. Metzger AL, Bohan A, Goldberg LS, Bluestone R, Pearson CM.

Polymyositis and dermatomyositis: combined methotrexate and

corticosteroid therapy. Ann Intern Med 1974; 81:182-9.

3 1 . Malaviya AN, Many A, Schwartz RS. Treatment of dermato-myositis with methotrexate. Lancet 1968:2:485-8.

32. Owen ET, Cohen ML. Methotrexate in Reiter’s disease. Ann

Rheum Dis 1979; 38:48-50.


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358 BUTFERWORTH AND TAMURA

33. Capizzi RL, Bertino JR. Methotrexate therapy of Wegener’sgranulomatosis. Ann Intern Med 197 l;74:74-9.

34. Lacher MJ. Spontaneous remission or response to methotrexate insarcoidosis. Ann Intern Med 1968;69:l247-8.

35. Mullarkey MF, Blumenstein BA, Andrade WP, Bailey GA, OlasonI, Wetzel CE. Methotrexate in the treatment of corticosteroid-dependent asthma. A double-blind crossover study. N Engl J Med1988;3l8:603-7.

36. Kaplan MM, Arora 5, Pincus SH. Primary sclerosing cholangitis

and low-dose oral pulse methotrexate therapy. Clinical andhistologic response. Ann Intern Med 1987; 106:231-5.

37. Kaplan MM, Knox TA, Arora S. Primary biliary cirrhosis treatedwith low-dose oral pulse methotrexate. Ann Intern Med 1988; 109:

429-31.38. Mandell GL, Sande MA. Antimicrobial agents. Sulfonamides,

trimethoprim-sulfamethoxazole, and agents for urinary tractinfections. In: Gilman AG, Goodman LS, Rall TW, Murad F, eds.

Goodman and Gilman’s the pharmacological basis of therapeutics.7th ed. New York: Macmillan, 1985:1095-1 14.

39. Webster LT Jr. Drugs used in the chemotherapy of protozoalinfections. Malaria. In: Gilman AG, Goodman LS, Rall RW,

Murad F, eds. Goodman and Gilman’s the pharmacological basisoftherapeutics. 7th ed. New York: Macmillan, 1985:1029-48.

40. Falco EA, Goodwin LG, Hitchings GH, Rollo IM, Russell PB. 2:4-Diaminopyrimidines-a new series of antimalarials. Br J

Pharmacol 195 1;6: I 85-200.

41. Baer JE, Beyer KH. Subcellular phamacology of natriuretic and

potassium-sparing drugs. In: Edwards KDG, ed. Drugs affectingkidney function and metabolism. Progress in biochemical

pharmacology. Vol 7. Basel: S Karger, 1972:59-93.

42. Franklin JL, Rosenberg IH. Impaired folic acid absorption in

inflammatory bowel disease: effects of salicylazosulfapyridine

(Azulfidine). Gastroenterology 1973;64:5 17-25.43. Luketic GC, Santini R Jr. Butterworth CE Jr. Depression of whole

blood folate activity by colchicine. Proc Soc Exp Biol Med1965; 120:13-6.

44. Allegra Ci, Chabner BA, Tuazon CU, et al. Trimetrexate for the

treatment ofPneumocystis carinii pneumonia in patients with theacquired immunodeficiency syndrome. N Engl J Med l987;3l7:

978-85.

45. Bertino JR, Perkins JP, Johns DO. Purification and properties ofdihydrofolate reductase from Ehrlich ascites carcinoma cells.Biochemistry l965;4:839-46.

46. Morales DR. Greenberg DM. Purification and properties ofdihydrofolate reductase of sheep liver. Biochim Biophys Acta

1964;85:360-76.

47. Selhub J, Dhar GJ, Rosenberg IH. Inhibition offolate enzymes bysulfasalazine. J Clin Invest 1978;6l:221-4.

48. Reisenauer AM, Halsied CH. Human jejunal brush border folateconjugase; characteristics and inhibition by salicylazosulfapyri-dine. Biochim Biophys Acts 198 l;659:62-9.


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