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Home , Allopurinol, Cytidine, Folinic acid, Megaloblastic anemia, Mercaptopurine

The new england journal of medicine

Review Article

Jeffrey M. Drazen, M.D., Editor Drug-Induced Megaloblastic

Charles S. Hesdorffer, M.D., and Dan L. Longo, M.D.​​

ore than 50 years ago, Victor Herbert first described the From George Washington University and concept that defective nucleoprotein synthesis, attributable to various the Washington DC Veterans Affairs Med- 1 ical Center, Washington, DC (C.S.H.). causes, results in the development of . Megaloblas- Address reprint requests to Dr. Hesdorffer M at the Department of Medicine, Wash- tic anemia is characterized by the presence of a hypercellular marrow with large, abnormal hematopoietic progenitor cells with a characteristic finely stippled, lacy ington DC Veterans Affairs Medical Cen- ter, 50 Irving St. NW, Washington, DC nuclear chromatin pattern. These abnormal progenitor cells, or megaloblasts, were 20422 or at ­charles​.­hesdorffer@​­va​.­gov. first described by Paul Ehrlich in 1880. Leukopenia and thrombocytopenia are N Engl J Med 2015;373:1649-58. frequently present. Although the marrow is hypercellular, many of the cells die DOI: 10.1056/NEJMra1508861 within it in a process called ineffective erythropoiesis. Megaloblastosis usually Copyright © 2015 Massachusetts Medical Society. results from a deficiency of B12 (cobalamin) or folic acid, or a deficiency in their metabolism; however, any interference with the synthesis of , pyrimi- dines, or protein may result in megaloblastosis.2 Megaloblastic maturation is the morphologic result of any biochemical defect that causes a slowing of DNA synthesis. The hallmark of this megaloblastosis is nuclear-cytoplasmic dissociation; the nucleus remains immature in appearance while the cytoplasm matures more normally. This dissociation, which is the result of DNA synthesis that is retarded relative to normal RNA and protein synthesis, is manifested in the marrow and other proliferating tissues in the body by large cells containing a large nucleus with a diffuse and immature-appearing chromatin content, surrounded by normal-appearing cytoplasm.3 However, a high mean cor- puscular volume does not necessarily imply a diagnosis of megaloblastic anemia. A high is noted also in cases of abuse, hypothy- roidism, , myelodysplasia, and any condition in which the reticu- locyte count is considerably elevated (such as in ); it may also be a congenital finding. Since it was first described in 1849 by Thomas Addison,4 megaloblastic anemia has been attributed to both congenital (uncommon) and acquired (common) prob- lems. It is most frequently related to deficiency due to defective absorp- tion, folic acid deficiency due to malnutrition, or both. However, because of the correction of most of the dietary causes of vitamin B12 and deficiency, drug- induced megaloblastic anemia has become a more prominent cause of megaloblas- tic anemia. The drugs that may cause this condition are commonly used in clinical practice, and their effects on DNA synthesis pathways may be underappreciated (Table 1). A number of biochemical processes in DNA synthesis are vulnerable to inhibi- tion by drugs, but among the most important of these is the new synthesis of . Figure 1 shows the structure of and their associated termi- nology. Thymidine is a component of DNA but not RNA, and it is present in cells in rate-limiting amounts. The other nucleotides tend to be present in excess. Thymidine can be salvaged from the turnover of DNA, but the main source is the addition of a methyl group to the 5-position of the ring to convert

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deoxyuridylate to deoxythymidylate (Fig. 2). This Table 1. Drugs That Cause Megaloblastic Anemia. process depends crucially on folate Mechanism of Action and Agent Type of Medication or Indication and vitamin B12. Drugs cause megaloblastic anemia by impair- Modulates metabolism ing the cellular availability or use of folic acid or Immunomodulator vitamin B12. This may occur because of interfer- ence with the absorption, plasma transport, or Mycophenolate mofetil Immunomodulator delivery of folate or vitamin B12, competition for Thioguanine Antineoplastic agent reducing , end-product inhibition of Antineoplastic agent cofactor-mediated reactions, or physical destruc- tion of the (Fig. 1).2 Antineoplastic agent Antineoplastic agent Small amounts of vitamin B12 are required on a daily basis (1 to 2.5 μg). Because folate food Antineoplastic agent fortification tends to obscure the hematologic Immunomodulator, consequences of , the early antineoplastic agent effects of drugs that interfere with vitamin B 12 oxidase inhibitor may be neurologic complications rather than the development of clinically significant anemia.5 Interferes with pyrimidine synthesis These neurologic manifestations can be modest arabinoside Antineoplastic agent or more dramatic, with the development of my- Antineoplastic agent elopathy, neuropathy, optic atrophy, and neuro- Antineoplastic agent psychiatric and, rarely, autonomic disturbances such as bladder or erectile dysfunction. A discus- Hydroxyurea Antineoplastic agent sion of these neurologic problems and their Methotrexate Antineoplastic agent biochemical basis is beyond the scope of this Mercaptopurine Immunomodulator, article, but they have been reviewed in detail by antineoplastic agent other authors.5-10 Clearly, such potentially devas- Antineoplastic agent tating drug effects underscore the need for the clinician to be alert to them. Antibacterial agent Anesthetic agent Drugs That Alter Purine Gadolinium (paramagnetic metal ion) Contrast agent in magnetic Metabolism, Pyrimidine resonance imaging Metabolism, or Both Leflunomide Immunomodulator in patients with psoriasis or In both purine and pyrimidine synthesis (Fig. 2), Teriflunomide Immunomodulator in patients with the methyl group is donated by 5,10-methylene multiple sclerosis tetrahydrofolate. The consequences of inhibition Decreases absorption of folic acid of pyrimidine synthesis are more dangerous to the cell than inhibition of purine synthesis. Thy- Alcohol midylate synthase converts deoxyuridylate to Aminosalicylic acid For tuberculosis and inflammatory thymidylate by transferring the methyl group bowel disease from methylene tetrahydrofolate, and in the pro- Birth-control pills Hormones cess it yields dihydrofolate. Estrogens Hormones In order for the reaction Tetracyclines Antibiotic to continue, the folate must reacquire a methyl group to donate. The first step is to regenerate and other penicillins Antibiotic tetrahydrofolate from dihydrofolate. This is ac- Chloramphenicol Antibiotic complished through the action of dihydrofolate Nitrofurantoin Urinary antiseptic reductase. Tetrahydrofolate is then converted to 5,10-methylene tetrahydrofolate through the ac- Erythromycin Antibiotic

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tion of serine hydroxymethyl transferase. If dihy- Table 1. (Continued.) drofolate is not reduced and methylated, the cell Mechanism of Action and Agent Type of Medication or Indication is starved of thymidylate, and DNA synthesis Antineoplastic agent, slows. It is this crucial role of dihydrofolate re- immunosuppressive agent ductase in thymidine biosynthesis Phenobarbital Antiseizure agent that makes it a target for antineoplastic thera- 11 Antiseizure agent py. This pathway is also targeted in antibacte- rial therapy, especially by sulfa drugs. Quinine Antimalarial agent Purine and pyrimidine antagonists or ana- Chloroquine Antimalarial agent logues are commonly used in the treatment of Primaquine Antimalarial agent , as immune antagonists, and as antiviral agents. Inhibitors of thymidylate synthase are Artemether lumefantrine Antimalarial agent called suicide substrates because they irrevers- Sulfadoxine– Antimalarial agent ibly inhibit the . of this class Glutethimide Hypnotic sedative include fluorouracil and 5-fluorodeoxyuridine. Has folate analogue activity Both are converted within cells to 5-fluorode- oxyuridylate, which then inhibits thymidylate Methotrexate Immunomodulator, 11-13 antineoplastic agent synthase. , which masquerade as a pu- Antineoplastic agent rine or a pyrimidine, inhibit DNA synthesis by Antineoplastic agent preventing these substances from becoming in- Proguanil Antineoplastic agent corporated into DNA during the S phase of the . Purine synthesis inhibitors include a Pyrimethamine Antimalarial agent number of commonly used drugs (Table 1): aza- Trimethoprim Antibacterial agent thioprine, an immunosuppressant agent used in Decreases absorption of vitamin B12 organ transplantation, autoimmune disorders, Cycloserine For tuberculosis and and inflammatory bowel disease; mycopheno- psychiatric conditions late mofetil, an immunosuppressant agent used Isoniazid For tuberculosis to prevent rejection in organ transplantation that inhibits purine synthesis by blocking inosi- Metformin For diabetes and prediabetes tol monophosphate dehydrogenase; methotrex- Aminosalicylic acid For tuberculosis and ate, a direct inhibitor of inflammatory bowel disease that indirectly inhibits purine synthesis by For , familial Mediterranean blocking the metabolism of folic acid; and allo- , and Behçet’s disease purinol, which is used to treat Antibiotic because it inhibits the enzyme xanthine oxidore-

Histamine2- antagonists (H2 blockers) ductase. Pyrimidine synthesis inhibitors are also used Proton-pump inhibitors in active moderate-to-severe rheumatoid arthri- Increases of vitamin B12 tis and psoriatic arthritis. For example, lefluno- nitroprusside mide inhibits T-cell responses and induces a shift of CD4 T cells from the type 1 helper T Destroys vitamin B12 (Th1) cell (proinflammatory) to type 2 helper T Nitric oxide (Th2) cell subpopulation. This process results in Has unknown mechanism a beneficial effect in diseases in which T cells Arsenic play a major role in the initiation and propaga- 14 tion of inflammation. Both leflunomide and its metabolite teriflunomide, which is approved for use in multiple sclerosis, inhibit dihydroorotate dehydrogenase.14,15

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– PURINES O PENTOSE NH2 O O N N 6 N 6 N P 7 5 1 7 5 1 O 8 8 O O 5’ 2 2 O P Base 9 4 3 9 4 3 O – P N N NH – O Glycosidic N N 2 O O O bond R R – 3’ 2’ HO OH = H = NH2 O O

Nucleoside H H3O H 4 N 4 N 4 N 5 3 5 3 5 3 2 2 2 Nucleotide monophosphate 6 1 6 1 6 1 N O N O N O Nucleotide diphosphate R R R Nucleotide triphosphate Cytosine Thymidine

Figure 1. . A is a base plus a sugar, and the sugar can be either a ribose or deoxyribose. If the sugar is a ribose, then the purine bases are called adenine or guanine, depending on the position of the 2-amino group, and the pyrimidines are called cytosine, uridine, and thymidine. When the sugar is a deoxyribose, the purine nucleosides are called and and the pyrimidines are called , , and deoxythymidine. When a phosphate group is added, the structures become nucleotides and the bases are called adenylate, guanylate, cytidylate, uridylate, and thymidylate, respectively. These names refer to monophosphate nu- cleotides. If two phosphate groups are attached, they are referred to as diphosphate nucleotides and are called diphosphate, diphosphate, and so forth. If three phosphates are present, they are triphosphates.

Nitrous oxide, an anesthetic gas that has be- tion into DNA or RNA, it may also inhibit RNA come increasingly popular for use as a recre- polymerase. ational drug, may cause megaloblastic anemia by blocking the conversion of vitamin B from 12 Drugs That Interfere with the reduced to the oxidized form. In the cyto- Absorption of Folic Acid plasm, methionine synthase requires the re-

duced form of vitamin B12 () to Folic acid (pteroylglutamic acid) cannot be syn- convert homocysteine to methionine. In con- thesized in humans, so it must be obtained in trast, in the mitochondria, the oxidized form of the diet, where its major sources are green leafy

vitamin B12 (5′-deoxyadenosylcobalamin) converts vegetables, citrus fruits, liver, and whole grains. methylmalonyl–coenzyme A (CoA) to succinyl Dietary (5-methyltetrahydrofolate and CoA. Thus, in the mitochondria, nitrous oxide formyltetrahydrofolate) are readily transported

will inhibit the activity of methylmalonyl CoA across the intestinal membranes. Vitamin B12– mutase, leading to the impairment of methyla- dependent methionine synthetase converts tion reactions and DNA synthesis.16-19 5-methyltetrahydrofolate to tetrahydrofolate, the form of folate that is required for nucleotide Inhibitors of biosynthesis. Reductase Methyltetrahydrofolate is required for the con- version of methionine to S-adenosylmethionine Although they are not as ubiquitous as drugs (SAM). Thus, when folate levels are low, SAM is that interfere with DNA synthesis, cytosine ara- depleted, resulting in a reduction in the methyla- binoside, hydroxyurea, and gemcitabine inhibit tion of cytosine in DNA. The consequences of the function of ribonucleotide reductase. This this reduced DNA methylation include enhanced inhibition blocks the conversion of di- gene transcription and DNA strand breaks; these phosphate or triphosphate to its corresponding are key factors leading to many adverse effects, . Cytosine arabinoside — including the possibility of malignant transfor- once it is rapidly phosphorylated to its active mation. metabolite, 5-triphosphate cytosine arabinoside Conversely, since folate acts as a cofactor that — inhibits DNA polymerase. After its incorpora- is regenerated in a cyclic manner, any drug that

1652 n engl j med 373;17 nejm.org October 22, 2015 The New England Journal of Medicine Downloaded from nejm.org by NIDIA ZAPATA on October 22, 2015. For personal use only. No other uses without permission. Copyright © 2015 Massachusetts Medical Society. All rights reserved. Drug-Induced Megaloblastic Anemia blocks the completion of this cycle (Fig. 2) will result in the accumulation of one of the metabo- Deoxyuridylate Thymidylate Thymidylate lites of the vitamin in an unusable form, giving synthase rise to a megaloblastic anemia. In this way, vita- min B12 deficiency leads to an accumulation of 5-methyltetrahydrofolate, which leads to mega- loblastosis that is, on a peripheral-blood smear, Folinic Folic indistinguishable from that associated with folic acid acid acid deficiency. The interrelationship of folic acid and vitamin B metabolism in this cyclical 5,10-methylene 12 tetrahydrofolate Dihydrofolate pathway is informative in determining the treat- ment that is required to correct the problem of 10 either folic acid or B12 deficiency. Thus, any drug that interferes with the intra- Dihydrofolate cellular concentration of folic acid, its intracel- reductase lular conversion to its appropriate metabolites, Serine or both can lead to megaloblastic anemia. Drugs Serine hydroxymethyl may cause a perturbation in the intracellular transferase concentration by decreasing intestinal absorp- tion, decreasing transport and delivery to cells, Tetrahydrofolate decreasing transport across cell membranes, de- Methionine creasing cellular retention (which includes in- Methionine Vitamin B creased excretion), increasing destruction, and synthase 12 increasing the requirement for folic acid. Some drugs affect the conversion or use of folic acid Homocysteine 5-Methyltetrahydrofolate by interfering with the availability of vitamin B12 or interfering with the enzymes involved in the CELL MEMBRANE conversion of folic acid to its appropriate me- 2,5,20,21 tabolites. PLASMA 5-Methyltetrahydrofolate Many drugs interfere with the absorption or proper distribution of folic acid. These include Figure 2. The Synthesis of Thymidylate, the DNA Nucleotide That Is a Rate- alcohol, antiseizure agents, contraceptive drugs, Limiting Factor in the Synthesis of DNA. and antibiotics (Table 1). Both folic acid and vitamin B12 play a critical role as cofactors in the pathway Alcohol is associated with the development that leads to the synthesis of thymidylate. Folate is absorbed in the small of megaloblastic anemia because of a low-folate intestine mainly as 5-methyltetrahydrofolate. Once it enters the cell, it is demethylated to form tetrahydrofolate in a vitamin B12–dependent enzy- diet in persons with and because of matic step that generates methionine from homocysteine. Homocysteine an inhibition of intestinal absorption, metabolic levels are increased in cobalamin deficiency because of the inhibition of use, and hepatic uptake and storage of folate.22,23 this conversion. The tetrahydrofolate is then remethylated in a reaction in Alcohol is not thought to act through the dihy- which serine donates a methyl group and pyridoxine () is a co- drofolate reductase pathway. Rather, the likely factor. The product is 5,10-methylene tetrahydrofolate. That methyl group is subsequently added to the 5-carbon of uridylate to form thymidylate effect of alcohol is on the intestinal mucosa, (). As a consequence of donating the methyl group, where it can interfere with both vitamin B12 and 5,10-methylene tetrahydrofolate becomes dihydrofolate. Dihydrofolate is then reduced by dihydrofolate reductase to generate tetrahydrofolate. folate absorption. The effect on vitamin B12 ab- sorption may be due to direct toxic effects on Fluorouracil blocks thymidylate synthase. Methotrexate blocks dihydrofolate the gastric mucosa that cause interference with reductase. the production of intrinsic factor.22 also has a direct effect on the maturation of hemato- poietic progenitor cells in the marrow. This ef- mains controversial. The use of contraceptives fect may be due to the inhibition of a specific results in a partial inhibition of intestinal decon- enzyme, 10-formyl-tetrahydrofolate dehydroge- jugation of polyglutamyl forms of folic acid.25,26 nase, as shown in studies in animals.24 This may explain why folic acid levels are usu- The mechanism by which folate absorption is ally normal in women who receive contracep- affected by the use of oral contraceptives re- tives, and it implies that absorption remains

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adequate until some additional clinically impor- a carrier protein.20 may reduce the bind- tant problem with absorption or dietary defi- ing of folate to its serum protein carrier. Simi- ciency is superimposed on the use of the contra- larly, phenytoin and other anticonvulsants that ceptive. bear structural resemblances to folate may cause Phenytoin and other anticonvulsant agents a decrease in serum folate levels by reducing the have also been associated with the development transport of folate.27 Finally, and again because of megaloblastic anemia. However, a key differ- of the structural similarities between anticon- ence between women who use birth-control pills vulsants and folic acid, the therapeutic effects of and persons who receive phenytoin and other some of these drugs have been thought to be due antiseizure medications is that folate levels are partially to their folate analogue activities. Of noted to be low in people who receive phenytoin. note, phenytoin and other anticonvulsants have Phenytoin does not seem to have any effect on been noted to cause immunosuppression and the folate metabolism pathways, nor does it ap- even myelosuppression. In addition, administra- pear to affect the excretion of folate.27 However, tion of folate in persons with seizures has been most antiseizure medications increase hepatic reported to increase the incidence of seizures in microsomal enzyme activity, and it is believed those persons, whereas low folate states have that this increase in activity may result in an been associated with improved seizure control.27 increase in the use of folic acid , thus leading to a decrease in serum folate levels. Similarly, anti- Drugs That Interfere with seizure drugs may enhance hepatic detoxifica- the Metabolism of Folic Acid tion enzymes, thus causing an increased break- down of folic acid. Drugs that are commonly termed folate ana- Antiseizure medications are also associated logues lead to a break in the important cyclic with a considerable decrease in the intestinal pathway in which folic acid is critically involved absorption of folic acid. Folate uses an active in returning dihydrofolate to tetrahydrofolate transport mechanism in the intestinal mucosa, (Fig. 1). Many folate analogues have been syn- as evidenced by the fact that some forms of thesized for a variety of therapeutic purposes. folate, such as methyltetrahydrofolate, are ab- Most commonly, they are used in the treatment sorbed more readily than others. However, since of malignant diseases such as , as well the various antiseizure medications are distinct- as various solid tumors, including lung, bladder, ly different from one another, it is unlikely that breast, and head and neck cancers, mesothelio- all these drugs would have a similar direct effect ma, and sarcomas. Common to each of these on the intestinal mucosa that would result in a drugs is the ability to bind to the enzyme dihy- decrease in active absorption of folic acid. Rather, drofolate reductase, thus inhibiting the reduc- it would appear that the effect is through a sec- tion of dihydrofolate to tetrahydrofolate. In this ondary action such as “solvent drag” (movement way, a true deficiency of reduced folic acid (i.e., of folate across the cell membrane by bulk trans- a deoxidized form of folic acid caused by dihy- port following the movement of rather drofolate reductase in the metabolism of dihy- than being facilitated by ion channels or cellular drofolate to tetrahydrofolate, which involves the pumps), sodium exchange, or an effect on intes- use of NADPH as an electron donor) is pro- tinal ATPase.27 Again, the probable reason that duced. anticonvulsants are not a more common cause Clinically, this folate analogue–generated de- of megaloblastic anemia is that the gastrointes- fect can be corrected by administration of re- tinal tract has a vast excess capacity for the ab- duced folic acid in the form of folinic acid sorption of important nutrients such as folic (10-formyl-tetrahydrofolate), since the reduced acid. Thus, some added compromise to absorp- folic acid is beyond the block to the pathway tion or significant diminution in the intake of caused by the folate analogue. The use of folinic folic acid is necessary for anemia to become an acid should be considered prophylactically in overt problem in patients. The addition of more patients who receive high doses of methotrexate. folic acid to the patient’s diet will probably pre- Folinic acid therapy should be initiated 24 hours vent or correct the problem. after the administration of methotrexate in or- Folate transport in the blood is facilitated by der to offset the adverse effects of methotrexate

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The absorption of vitamin B12 from food is shown port of vitamin B12 across membranes and in the in Figure 3. Animal products (meat, fish, chicken, circulation.35 The effect of metformin on vitamin and dairy products) are the only natural source B12 absorption is reversible with calcium because of vitamin B12 (cobalamin), which is synthesized the ileal absorption of vitamin B12, as indicated in bacteria. Cobalamin is a coenzyme in the in- previously, is a calcium-dependent process.37 teractions of only two enzymes in mammals. Another factor that may play a role in the

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— Transcobalamin I

— Folic acid

— Vitamin B12

— Intrinsic factor

— Transcobalamin II

— Cubilin STOMACH

Transcobalamin I

R DE Folic acid D LA B Vitamin B12 LL A G PANCREAS

Intrinsic factor

DU OD ENUM

Absorption ILEAL Vitamin B12–Intrinsic-factor ILEUM CELL complex

Cubilin

J E JU N UM Transcobalamin II

MESENTERIC VESSEL TERMINAL ILEUM

intestinal absorption of vitamin B12 is an altera- gastric pH has been noted in persons who have

tion in the intestinal pH. Since vitamin B12 is a been receiving long-standing phenytoin therapy. weak acid, its absorption is decreased in an al- This may partially explain why antacids such as

kaline environment. Phenytoin in solution has a histamine2-receptor antagonists (H2 blockers) very high pH (approximately 12), and an elevated and proton-pump inhibitors may, on rare occa-

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Figure 3 (facing page). Absorption of Folic Acid potential drug effect. Agents that are more po- and Vitamin B12 (Cobalamin). tent, such as purine or pyrimidine analogues or After ingestion, vitamin B12 is bound in the mouth to folate antagonists, are likely to result in anemia haptocorrin (transcobalamin I), from which it becomes that may occur rapidly, whereas with the use of disassociated in the stomach because of the presence less potent inhibitors, megaloblastic anemia may of gastric enzymes and acid. The haptocorrin is replaced develop more slowly. by intrinsic factor, which is secreted by the parietal cells If the mechanism of action of the offending of the stomach. The vitamin B12–intrinsic-factor com- plex attaches to the receptor cubilin, which is present on agent is not related to a deficiency in folic acid

the surface of the epithelial cells of the terminal ileum or vitamin B12 absorption, the anemia will not be and facilitates the absorption of the vitamin B12–intrinsic- corrected by vitamin supplementation. Thus, be- factor complex. Intrinsic factor is degraded within the cause of the significant downstream effects, it ileal cells, and vitamin B12 is absorbed into the blood- stream, where it becomes bound to transcobalamin II, can be corrected only by the removal of the agent which transports it to the various organs for DNA syn- from the therapeutic regimen or, when possible, thesis. Contrary to this more complex process of vita- bypassing the antagonism with folinic acid. min B12 absorption, folic acid is readily absorbed in the Perhaps a more consequential complication jejunum by means of a passive process. of vitamin B12 and is the resul- tant hyperhomocysteinemia.37 Homocysteine is a neurotoxic amino acid that is not found in pro- sions, be associated with the development of teins. An elevated homocysteine level has been megaloblastic anemia, most usually in patients implicated in many pathologic conditions, includ- who have continued to receive these agents for 2 ing cardiovascular diseases, fetal neural-tube de- years or longer.38-40 fects, and, perhaps more questionably, in several Another mechanism may be the inhibition of neurodegenerative disorders such as stroke, Par- intrinsic-factor production, given that proton- kinson’s disease, and Alzheimer’s disease.38-41 pump inhibitors do not act only on acid produc- Elimination of homocysteine is regulated by tion by parietal cells but may also act on intrin- both the transmethylation and transsulfuration sic-factor production. Thus, the continued use of pathways and is thus affected by folate, whereas such agents may be more likely to cause anemia the conversion of homocysteine to methionine is than intermittent use, which is the recommend- mediated by methionine synthase, the activity of ed approach to the use of these acid-lowering which is regulated through vitamin B12. Homo- agents.38 also seems to play an important role in the regulation of neurogenesis and apoptosis.41-46 Other effects of folate and vitamin B defi- Management 12 ciency are related to the fact that they play key The key to managing megaloblastic anemia is roles as methyl donors in one-carbon metabo- determining the cause of the megaloblastosis, lism. The results of methyl-donor deficiency have deciding whether the causative agent is expend- been noted in studies of intrauterine growth able in the patient’s treatment, and discontinu- retardation. In addition, reduced Stat3 signaling ing the agent or switching to an alternative targeted by miR-124 has been associated with regimen, if possible. If the causative drug is es- long-term postnatal brain defects.47 Even depres- sential to the patient’s treatment and there are sion may be affected by folate deficiency owing no acceptable alternatives, one should ensure to effects on the synthesis of neurotransmitters 48 that folate and vitamin B12 intake are adequate. such as monoamine metabolites. Both vitamins can be supplemented orally. Thus, the consequences of drug-induced

Regardless of the drug or the specific mecha- changes in folate and vitamin B12 physiology can nism by which it may cause megaloblastic ane- be substantial. Early recognition is critical for mia, understanding the consequences of block- the prevention of irreversible consequences. ing the specific process is critical. The most important issue is recognizing the problem in Disclosure forms provided by the authors are available with the first place and relating it to the use of a the full text of this article at NEJM.org. We thank Dr. Jerry Spivak of Johns Hopkins School of Medi- drug. Physicians who administer any agent that cine and Dr. Franklin Bunn of Harvard Medical School for their blocks DNA synthesis should be aware of the review of an earlier version of the manuscript.

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