MODULE

Agents Used in Anemias & Hematopoietic Growth Factors

A. Learning Objective

(please refer to Competency Standard from Indonesian Medical Council / Konsil Kedokteran Indonesia and Level of competency of Hematology, appendix 1 and 2 ).

Level of Competency of Hematology ( see, appendix 1 and 2 )

HematologyAplastic/hypoplastic anemia 2 Iron deficiency anemia 4Macrocytic anemia 3A Hemolytic anemia 3A Hemoglobinopathy 2 Anemia associated with chronic diseases 3APolycytemia 2 rombocytopenia 2 Thrombocytosis 2 Hemophilia 2 Von willebrand's disease 1 DIC 2

You should be able to:

1. Describe the normal mechanism of regulation of iron absoption and storage in the body2. List the anemias for wich iron supplementation is indicated and those for which it is

contraindicated3. Describe the acute and chronic toxity of iron4. Vitamin B12 and Folic acid cycle5. Describe the clinical application of B12 and Folic acid6. Describe the side effect of folic acid as sole therapy for megaloblastic anemia7. Name of major growth factors and their clinical uses

IRON AND IRON SALTS

Introduction

Iron deficiency is the most common nutritional cause of anemia in humans. It can result from inadequate iron intake, malabsorption, blood loss, or an increased requirement, as with pregnancy. When severe, it results in a characteristic microcytic, hypochromic anemia. The impact of iron deficiency is not limited to the erythron (Dallman, 1982). Iron also is an essential component of myoglobin; heme enzymes such as the cytochromes, catalase, and peroxidase; and the metalloflavoprotein enzymes, including xanthine oxidase and the mitochondrial enzyme -glycerophosphate oxidase. Iron deficiency can affect metabolism in muscle independently of the effect of anemia on oxygen delivery. This may reflect a reduction in the activity of iron-dependent mitochondrial enzymes. Iron deficiency also has been associated with behavioral and learning problems in children, abnormalities in catecholamine metabolism, and possibly, impaired heat production. Awareness of the ubiquitous role of iron has stimulated considerable interest in the early and accurate detection of iron deficiency and in its prevention.

Iron and the Environment.

Iron exists in the environment largely as ferric oxide or hydroxide or as polymers. In this state, its biological availability is limited unless solubilized by acid or chelating agents. For example, bacteria and some plants produce high-affinity chelating agents that extract iron from the surrounding environment. Most mammals have little difficulty in acquiring iron; this is explained by an ample iron intake and perhaps also by a greater efficiency in absorbing iron. Humans, however, appear to be an exception. Although total dietary intake of elemental iron in human beings usually exceeds requirements, the bioavailability of the iron in the diet is limited.

Metabolism of Iron.

The body store of iron is divided between essential iron-containing compounds and excess iron, which is held in storage. Quantitatively, hemoglobin dominates the essential fraction (Table 53-2). This protein, with a molecular weight of 64,500, contains four atoms of iron per molecule, amounting to 1.1 mg of iron per milliliter of red blood cells (20 mmol). Other forms of essential iron include myoglobin and a variety of heme and nonheme iron-dependent enzymes. Ferritin is a protein-iron storage complex that exists as individual molecules or as aggregates. Apoferritin has a molecular weight of about 450,000 and is composed of 24 polypeptide subunits that form an outer shell, within which resides a storage cavity for polynuclear hydrous ferric oxide phosphate. More than 30% of the weight of ferritin may be iron (4000 atoms of iron per ferritin molecule). Ferritin aggregates, referred to as hemosiderin and visible by light microscopy, constitute about one-third of normal stores, a fraction that increases as stores enlarge. The two predominant sites of iron storage are the reticuloendothelial system and the hepatocytes, although some storage also occurs in muscle.

Internal exchange of iron is accomplished by the plasma protein transferrin. This 76-kd 1-glycoprotein has two binding sites for ferric iron. Iron is delivered from transferrin to intracellular sites by means of specific transferrin receptors in the plasma membrane. The iron-transferrin complex binds to the receptor, and the ternary complex is taken up by receptor-mediated endocytosis. Iron subsequently dissociates in the acidic, intracellular vesicular compartment (the endosomes), and the receptor returns the apotransferrin to the cell surface, where it is released into the extracellular environment (Klausner et al. , 1983).

Cells regulate their expression of transferrin receptors and intracellular ferritin in response to the iron supply. When iron is plentiful, the synthesis of transferrin receptors is reduced and ferritin production is increased (Rouault, 2002). Conversely, with iron deficiency, cells express a greater number of transferrin receptors and reduce ferritin concentrations to maximize uptake and prevent diversion of iron to storage forms. Apoferritin synthesis is regulated posttranscriptionally by two cytoplasmic binding proteins (IRP-1 and IRP-2) and an iron-regulating element on mRNA (IRE). When iron is in short supply, IRP binds to mRNA IRE and inhibits apoferritin translation. Conversely, when iron is abundant, binding is blocked and apoferritin synthesis increases (Klausner et al. , 1993 ).

The flow of iron through the plasma amounts to a total of 30 to 40 mg per day in the adult (about 0.46 mg/kg of body weight) (Finch and Huebers, 1982). The major internal circulation of iron involves the erythron and reticuloendothelial cells (Figure 53-3). About 80% of the iron in plasma goes to the erythroid marrow to be packaged into new erythrocytes; these normally circulate for about 120 days before being catabolized by the reticuloendothelial system. At that time, a portion of the iron is immediately returned to the plasma bound to transferrin, while another portion is incorporated into the ferritin stores of reticuloendothelial cells and returned to the circulation more gradually. Isotopic studies indicate some degree of iron wastage in this process, wherein defective cells or unused portions of their iron are transferred to the reticuloendothelial cell during maturation, bypassing the circulating blood. With abnormalities in erythrocyte maturation, the predominant portion of iron assimilated by the erythroid marrow may be rapidly localized in the reticuloendothelial cells as defective red cell precursors are broken down; this is termed ineffective erythropoiesis. The rate of iron turnover in plasma may be reduced by one-half or more with red cell aplasia, with all the iron directed to the hepatocytes for storage.

The most remarkable feature of iron metabolism is the degree to which body stores are conserved. Only 10% of the total is lost per year by normal men, i.e., about 1 mg per day. Two-thirds of this iron is excreted from the gastrointestinal tract as extravasated red cells, iron in bile, and iron in exfoliated mucosal cells. The other third is accounted for by small amounts of iron in desquamated skin and in the urine. Physiological losses of iron in men vary over a narrow range, from 0.5 mg in the iron-deficient individual to 1.5 to 2 mg per day when excessive iron is consumed. Additional losses of iron occur in women due to menstruation. While the average loss in menstruating women is about 0.5 mg per day, 10% of menstruating women lose more than 2 mg per day. Pregnancy and lactation impose an even greater requirement for iron (Table 53-3). Other causes of iron loss include blood donation, the use of antiinflammatory drugs that cause bleeding from the gastric mucosa, and gastrointestinal disease with associated bleeding. Two much rarer causes are the hemosiderinuria that follows intravascular hemolysis, and pulmonary siderosis, where iron deposited in the lungs becomes unavailable to the rest of the body.

The limited physiological losses of iron point to the primary importance of absorption in determining of the body's iron content. Unfortunately, this process is understood only in general terms (Roy and Enns, 2000; Morgan and Oates, 2002). After acidification and partial digestion of food in the stomach, iron is presented to the intestinal mucosa as either inorganic iron or heme iron. These fractions are taken up by the absorptive cells of the duodenum and upper small intestine, and the iron is transported either directly into the plasma or stored as mucosal ferritin. Absorption appears to be regulated by two transporters:

DCT1, which controls uptake from the intestinal lumen, and a second transporter that governs movement of mucosal cell iron across the basolateral membrane to bind to plasma protein. Mucosal cell iron transport and the delivery of iron to transferrin from reticuloendothelial stores both are determined by the HFE gene, a major histocompatibility complex class 1 molecule localized to chromosome 6 (Peters et al. , 1993; Beutler, 2003). Regulation is finely tuned to prevent iron overload in times of iron excess while allowing for increased absorption and mobilization of iron stores with iron deficiency (Roy and Andrews, 2001; Sheth and Brittenham, 2000). A predominant negative regulator of iron absorption in the small intestine is hepcidin, a 25-amino acid peptide made by hepatocytes (Ganz, 2003). The synthesis of hepcidin is greatly stimulated by inflammation or by iron overload. A deficient hepcidin response to iron loading can contribute to iron overload and one type of hemochromatosis. In anemia of chronic disease, hepcidin production can be increased up to one hundredfold, potentially accounting for characteristic features of this condition, namely poor gastrointestinal uptake and enhanced sequestration of iron in the reticuloendothelial system.

Normal iron absorption is about 1 mg per day in adult men and 1.4 mg per day in adult women; 3 to 4 mg of dietary iron is the most that normally can be absorbed. Increased iron absorption is seen whenever iron stores are depleted or when erythropoiesis is increased or ineffective. Patients with hereditary hemochromatosis due to HFE mutations demonstrate increased iron absorption and loss of the normal regulation of iron delivery to transferrin by reticuloendothelial cells (Beutler, 2003; Ajioka and Kushner, 2003). The resulting increased saturation of transferrin permits abnormal iron deposition in nonhematopoietic tissues.

Iron Requirements and the Availability of Dietary Iron.

Iron requirements are determined by obligatory physiological losses and the needs imposed by growth. Thus, adult men require only 13 g/kg per day (about 1 mg), whereas menstruating women require about 21 g/kg per day (about 1.4 mg). In the last two trimesters of pregnancy, requirements increase to about 80 g/kg per day (5 to 6 mg), and infants have similar requirements due to their rapid growth. These requirements (Table 53-4) must be considered in the context of the amount of dietary iron available for absorption.

In developed countries, the normal adult diet contains about 6 mg of iron per 1000 calories, providing an average daily intake for adult men of between 12 and 20 mg and for adult women of between 8 and 15 mg. Foods high in iron (5 mg/100 g) include organ meats such as liver and heart, brewer's yeast, wheat germ, egg yolks, oysters, and certain dried beans and fruits; foods low in iron (1 mg/100 g) include milk and milk products and most nongreen vegetables. The content of iron in food is affected further by the manner of its preparation, since iron may be added from cooking in iron pots.

Although the iron content of the diet obviously is important, of greater nutritional significance is the bioavailability of iron in food. Heme iron, which constitutes only 6% of dietary iron, is far more available and is absorbed independent of the diet composition; it therefore represents 30% of iron absorbed (Conrad and Umbreit, 2002).

The nonheme fraction nonetheless represents by far the largest amount of dietary iron ingested by the economically underprivileged. In a vegetarian diet, nonheme iron is absorbed very poorly because of the inhibitory action of a variety of dietary components, particularly phosphates. Ascorbic acid and meat facilitate the absorption of nonheme iron. Ascorbate forms complexes with and/or reduces ferric to ferrous iron. Meat facilitates the absorption of iron by stimulating production of gastric acid; other effects also may be involved. Either of these substances can increase availability severalfold. Thus, assessment of available dietary iron should include both the amount of iron ingested and an estimate of its availability (Figure 53-4) (Monsen et al. , 1978 ).

A comparison of iron requirements with available dietary iron is seen in Table 53-4. Obviously, pregnancy and infancy represent periods of negative balance. Menstruating women also are at risk, whereas iron balance in adult men and nonmenstruating women is reasonably secure. The difference between dietary supply and requirements is reflected in the size of iron stores, which are low or absent when iron balance is precarious and high when iron balance is favorable (Table 53-2). Thus, in infants after the third month of life and in pregnant women after the first trimester, stores of iron are negligible. Menstruating women have approximately one-third the stored iron found in adult men, indicative of the extent to which the additional average daily loss of about 0.5 mg of iron affects iron balance.

Iron Deficiency.

Iron deficiency is the most common nutritional disorder (World Health Organization, 2003; Hoffbrand and Herbert, 1999). The prevalence of iron-deficiency anemia in the United States is on the order of 1% to 4% (Anonymous, 2002) and depends on the economic status of the population. In developing countries, up to 20% to 40% of infants and pregnant women may be affected. Better iron balance has resulted from the practice of fortifying flour, the use of iron-fortified formulas for infants, and the prescription of medicinal iron supplements during pregnancy.

Iron-deficiency anemia results from dietary intake of iron that is inadequate to meet normal requirements (nutritional iron deficiency), blood loss, or interference with iron absorption. Most nutritional iron deficiency in the United States is mild. More severe iron deficiency is usually the result of blood loss, either from the gastrointestinal tract, or in women, from the uterus. Impaired absorption of iron from food results most often from partial gastrectomy or malabsorption in the small intestine. Finally, treatment of patients with erythropoietin can result in a functional iron deficiency (Beutler, 2003).

Iron deficiency in infants and young children can lead to behavioral disturbances and can impair development, which may not be fully reversible. Iron deficiency in children also can lead to an increased risk of lead toxicity secondary to pica and an increased absorption of heavy metals. Premature and low-birth-weight infants are at greatest risk for developing iron deficiency, especially if they are not breast-fed and/or do not receive iron-fortified formula. After age 2 to 3, the requirement for iron declines until adolescence, when rapid growth combined with irregular dietary habits again increase the risk of iron deficiency. Adolescent girls are at greatest risk; the dietary iron intake of most girls ages 11 to 18 is insufficient to meet their requirements.

The recognition of iron deficiency rests on an appreciation of the sequence of events that lead to depletion of iron stores (Hillman and Finch, 1997; Beutler, 2003). A negative balance first results in a reduction of iron stores, and eventually a parallel decrease in red-cell iron and iron-related enzymes (Figure 53-5). In adults, depletion of iron stores may be recognized by a plasma ferritin of less than 12 g/L and the absence of reticuloendothelial hemosiderin in the marrow aspirate. Iron-deficient erythropoiesis is identified by a decreased saturation of transferrin to less than 16% and/or by an increase above normal in red-cell protoporphyrin. Iron-deficiency anemia is associated with a recognizable decrease in the concentration of hemoglobin in blood. However, the physiological variation in hemoglobin levels is so great that only about half the individuals with iron-deficient erythropoiesis are identified from their anemia. Moreover, "normal" hemoglobin and iron values in infancy and childhood are lower because of the more restricted supply of iron in young children (Dallman et al. , 1980 ).

In mild iron deficiency, identifying the underlying cause is more important than any symptoms related to the deficiency state. Because of the frequency of iron deficiency in infants and in menstruating or pregnant women, the need for exhaustive evaluation of such individuals usually is determined by the severity of the anemia. However, iron deficiency in men or postmenopausal women necessitates a search for a site of bleeding.

Although the presence of microcytic anemia is the most common indicator of iron deficiency, laboratory testssuch as quantitation of transferrin saturation, red cell protoporphyrin, and plasma ferritinare required to distinguish iron deficiency from other causes of microcytosis. Such measurements are particularly useful when circulating red cells are not yet microcytic because of the recent nature of blood loss, but iron supply nonetheless limits erythropoiesis. More difficult is the differentiation of true iron deficiency from iron-deficient erythropoiesis due to inflammation. In the latter condition, iron stores actually are increased, but the release of iron from reticuloendothelial cells is blocked; the concentration of iron in plasma is decreased, and the supply of iron to the erythroid marrow becomes inadequate. The increased stores of iron in this condition may be demonstrated directly by examination of an aspirate of marrow or may be inferred from determination of an elevated plasma concentration of ferritin.

Treatment of Iron Deficiency

General Therapeutic Principles.

The response of iron-deficiency anemia to iron therapy is influenced by several factors, including the severity of anemia, the ability of the patient to tolerate and absorb medicinal iron, and the presence of other complicating illnesses. Therapeutic effectiveness is best measured by the resulting increase in the rate of production of red cells. The magnitude of the marrow response to iron therapy is proportional to the severity of the anemia (level of erythropoietin stimulation) and the amount of iron delivered to marrow precursors.

The patient's ability to tolerate and absorb medicinal iron is a key factor in determining the rate of response to therapy. The small intestine regulates absorption, and with increasing doses of oral iron, limits the entry of iron into the bloodstream. This provides a natural ceiling on how much iron can be supplied by oral therapy. In the patient with a moderately severe iron-deficiency anemia, tolerable doses of oral iron will deliver, at most, 40 to 60 mg of iron per day to the erythroid marrow. This is an amount sufficient for production rates of two to three times normal.

Complicating illness also can interfere with the response of an iron-deficiency anemia to iron therapy. By decreasing the number of red cell precursors, intrinsic disease of the marrow can blunt the response. Inflammatory illnesses suppress the rate of red cell production, both by reducing iron absorption and reticuloendothelial release and by direct inhibition of erythropoietin and erythroid precursors. Continued blood loss can mask the response as measured by recovery of the hemoglobin or hematocrit.

Clinically, the effectiveness of iron therapy is best evaluated by tracking the reticulocyte response and the rise in the hemoglobin or the hematocrit. An increase in the reticulocyte count is not observed for at least 4 to 7 days after beginning therapy. A measurable increase in the hemoglobin level takes even longer. A decision as to the effectiveness of treatment should not be made for 3 to 4 weeks after the start of treatment. An increase of 20 g per liter or more in the concentration of hemoglobin by that time should be considered a positive response, assuming that no other change in the patient's clinical status can account for the improvement and that the patient has not been transfused.

If the response to oral iron is inadequate, the diagnosis must be reconsidered. A full laboratory evaluation should be conducted, and poor compliance by the patient or the presence of a concurrent inflammatory disease must be explored. A source of continued bleeding obviously should be sought. If no other explanation can be found, an evaluation of the patient's ability to absorb oral iron should be considered. There is no justification for merely continuing oral iron therapy beyond 3 to 4 weeks if a favorable response has not occurred.

Once a response to oral iron is demonstrated, therapy should be continued until the hemoglobin returns to normal. Treatment may be extended if it is desirable to replenish iron stores. This may require a considerable period of time, since the rate of absorption of iron by the intestine will decrease markedly as iron stores are reconstituted. The prophylactic use of oral iron should be reserved for patients at high risk, including pregnant women, women with excessive menstrual blood loss, and infants. Iron supplements also may be of value for rapidly growing infants who are consuming substandard diets and for adults with a recognized cause of chronic blood loss. Except for infants, in whom the use of supplemented formulas is routine, the use of over-the-counter mixtures of vitamins and minerals to prevent iron deficiency should be discouraged.

Therapy with Oral Iron.

Orally administered ferrous sulfate is the treatment of choice for iron deficiency. Ferrous salts are absorbed about three times as well as ferric salts, and the discrepancy becomes even greater at high dosages. Variations in the particular ferrous salt have relatively little effect on bioavailability, and the sulfate, fumarate, succinate, gluconate, and other ferrous salts are absorbed to approximately the same extent.

Ferrous sulfate (FEOSOL, others) is the hydrated salt, FeSO47H2O, which contains 20% iron. Dried ferrous sulfate (32% elemental iron) is also available. Ferrous fumarate (FEOSTAT, others) contains 33% iron and is moderately soluble in water, stable, and almost tasteless. Ferrous gluconate (FERGON, others) also has been successfully used in the therapy of iron-deficiency anemia. The gluconate contains 12% iron. Polysaccharide-iron complex (NIFEREX, others), a compound of ferrihydrite and carbohydrate, is another preparation with comparable absorption. The effective dose of all of these preparations is based on iron content.

Other iron compounds have utility in fortification of foods. Reduced iron (metallic iron, elemental iron) is as effective as ferrous sulfate, provided that the material employed has a small particle size. Large-particle ferrum reductum and iron phosphate salts have a much lower bioavailability, and their use for the fortification of foods is undoubtedly responsible for some of the confusion concerning effectiveness. Ferric edetate has been shown to have good bioavailability and to have advantages for maintenance of the normal appearance and taste of food.

The amount of iron, rather than the mass of the total salt in iron tablets, is important. It also is essential that the coating of the tablet dissolve rapidly in the stomach. Surprisingly, since iron usually is absorbed in the upper small intestine, certain delayed-release preparations have been reported to be effective and have been said to be even more effective than ferrous sulfate when taken with meals. However, reports of

absorption from such preparations vary. Because a number of forms of delayed-release preparations are on the market, and information on their bioavailability is limited, the effectiveness of most such preparations must be considered questionable.

A variety of substances designed to enhance the absorption of iron has been marketed, including surface-acting agents, carbohydrates, inorganic salts, amino acids, and vitamins. When present in an amount of 200 mg or more, ascorbic acid increases the absorption of medicinal iron by at least 30%. However, the increased uptake is associated with a significant increase in the incidence of side effects; therefore, the addition of ascorbic acid seems to have little advantage over increasing the amount of iron administered. It is inadvisable to use preparations that contain other compounds with therapeutic actions of their own, such as vitamin B12, folate, or cobalt, because the patient's response to the combination cannot easily be interpreted.

The average dose for the treatment of iron-deficiency anemia is about 200 mg of iron per day (2 to 3 mg/kg), given in three equal doses of 65 mg. Children weighing 15 to 30 kg can take half the average adult dose, while small children and infants can tolerate relatively large doses of ironfor example, 5 mg/kg. The dose used is a compromise between the desired therapeutic action and the toxic effects. Prophylaxis and mild nutritional iron deficiency may be managed with modest doses. When the object is the prevention of iron deficiency in pregnant women, for example, doses of 15 to 30 mg of iron per day are adequate to meet the 3- to 6-mg daily requirement of the last two trimesters. When the purpose is to treat iron-deficiency anemia, but the circumstances do not demand haste, a total dose of about 100 mg (35 mg three times daily) may be used.

The responses expected for different dosage regimens of oral iron are given in Table 53-5. These effects are modified by the severity of the iron-deficiency anemia and by the time of ingestion of iron relative to meals. Bioavailability of iron ingested with food is probably one-half or one-third of that seen in the fasting subject (Grebe et al. , 1975 ). Antacids also reduce iron absorption if given concurrently. It is always preferable to administer iron in the fasting state, even if the dose must be reduced because of gastrointestinal side effects. For patients who require maximal therapy to encourage a rapid response or to counteract continued bleeding, as much as 120 mg of iron may be administered four times a day. Sustained high rates of red cell production require an uninterrupted supply of iron, and oral doses should be spaced equally to maintain a continuous high concentration of iron in plasma.

The duration of treatment is governed by the rate of recovery of hemoglobin and the desire to create iron stores. The former depends on the severity of the anemia. With a daily rate of repair of 2 g of hemoglobin per liter of whole blood, the red cell mass usually is reconstituted within 1 to 2 months. Thus, an individual with a hemoglobin of 50 g per liter may achieve a normal complement of 150 g per liter in about 50 days, whereas an individual with a hemoglobin of 100 g per liter may take only half that time. The creation of stores of iron requires many months of oral iron administration. The rate of absorption decreases rapidly after recovery from anemia, and after 3 to 4 months of treatment, stores may increase at a rate of not much more than 100 mg per month. Much of the strategy of continued therapy depends on the estimated future iron balance. Patients with an inadequate diet may require continued therapy with low doses of iron. If the bleeding has stopped, no further therapy is required after the hemoglobin has returned to normal. With continued bleeding, long-term, high-dose therapy clearly is indicated.

Untoward Effects of Oral Preparations of Iron.

Intolerance to oral preparations of iron primarily is a function of the amount of soluble iron in the upper GI tract and of psychological factors. Side effects include heartburn, nausea, upper gastric discomfort, and diarrhea or constipation. A good policy is to initiate therapy at a small dosage, to demonstrate freedom from symptoms at that level, and then gradually to increase the dosage to that desired. With a dose of 200 mg of iron per day divided into three equal portions, symptoms occur in approximately 25% of treated individuals versus 13% among those receiving placebo; this increases to approximately 40% when the dosage of iron is doubled. Nausea and upper abdominal pain are increasingly common at high dosage. Constipation and diarrhea, perhaps related to iron-induced changes in the intestinal bacterial flora, are not more prevalent at higher dosage, nor is heartburn. If a liquid is given, one can place the iron solution on the back of the tongue with a dropper to prevent transient staining of teeth.

The normal individual apparently is able to control absorption of iron despite high intake, and it is only individuals with underlying disorders that augment the absorption of iron who run the hazard of developing iron overload (hemochromatosis). However, hemochromatosis is a relatively common genetic disorder, present in 0.5% of the population.

Iron Poisoning.

Large amounts of ferrous salts are toxic, but fatalities are rare in adults. Most deaths occur in children, particularly between the ages of 12 and 24 months. As little as 1 to 2 g of iron may cause death, but 2 to 10 g usually is ingested in fatal cases. The frequency of iron poisoning relates to its availability in the household, particularly the supply that remains after a pregnancy. The colored sugar coating of many of the commercially available tablets gives them the appearance of candy. All iron preparations should be kept in childproof bottles.

Signs and symptoms of severe poisoning may occur within 30 minutes after ingestion or may be delayed for several hours. They include abdominal pain, diarrhea, or vomiting of brown or bloody stomach contents containing pills. Of particular concern are pallor or cyanosis, lassitude, drowsiness, hyperventilation due to acidosis, and cardiovascular collapse. If death does not occur within 6 hours, there may be a transient period of apparent recovery, followed by death in 12 to 24 hours. The corrosive injury to the stomach may result in pyloric stenosis or gastric scarring. Hemorrhagic gastroenteritis and hepatic damage are prominent findings at autopsy. In the evaluation of a child thought to have ingested iron, a color test for iron in the gastric contents and an emergency determination of the concentration of iron in plasma can be performed. If the latter is less than 63 mol (3.5 mg per liter), the child is not in immediate danger. However, vomiting should be induced when there is iron in the stomach, and an x-ray should be taken to evaluate the number of pills remaining in the small bowel (iron tablets are radiopaque). Iron in the upper GI tract can be precipitated by lavage with sodium bicarbonate or phosphate solution, although the clinical benefit is questionable. When the plasma concentration of iron is greater than the total iron-binding capacity (63 mol; 3.5 mg per liter), deferoxamine should be administered; dosage and routes of administration are detailed in Chapter 65. Shock, dehydration, and acid-base abnormalities should be treated in the conventional manner. Most important is the speed of diagnosis and therapy. With early effective treatment, the mortality from iron poisoning can be reduced from as high as 45% to about 1%.

Therapy with Parenteral Iron.

When oral iron therapy fails, parenteral iron administration may be an effective alternative (Silverstein and Rodgers, 2004). The rate of response to parenteral therapy is similar to that which follows usual oral doses. Common indications are iron malabsorption (e.g., sprue, short bowel syndrome), severe oral iron intolerance, as a routine supplement to total parenteral nutrition, and in patients who are receiving erythropoietin (Eschbach et al. , 1987 ). Parenteral iron also has been given to iron-deficient patients and pregnant women to create iron stores, something that would take months to achieve by the oral route. Parenteral iron therapy should be used only when clearly indicated, since acute hypersensitivity, including anaphylactic and anaphylactoid reactions, can occur in 0.2% to 3% of patients. The belief that the response to parenteral iron, especially iron dextran, is faster than oral iron is open to debate. In otherwise healthy individuals, the rate of hemoglobin response is determined by the balance between the severity of the anemia (the level of erythropoietin stimulus) and the delivery of iron to the marrow from iron absorption and iron stores. When a large intravenous dose of iron dextran is given to a severely anemic patient, the hematologic response can exceed that seen with oral iron for 1 to 3 weeks. Subsequently, however, the response is no better than that seen with oral iron.

Three preparations of iron are FDA-approved for parenteral therapy, sodium ferric gluconate complex in sucrose (FERRLECIT), iron sucrose (SACCHARATE), and iron dextran (INFED, DEXFERRUM). Unlike iron dextran, which requires processing by macrophages that may require several weeks, approximately 80% of sodium ferric gluconate is delivered to transferrin with 24 hours. Sodium ferric gluconate also has a much lower risk of inducing serious anaphylactic reactions than iron dextran (Michael et al. , 2002 ). No deaths were reported with 25 million infusions of sodium ferric gluconate, whereas there were 31 infusion-related deaths reported from approximately half the number of patients treated with iron dextran (Faich and Strobos, 1999). Thus, sodium ferric gluconate is the preferred agent for parenteral iron therapy. Currently, iron dextran is reserved for noncompliant patients or for those who are seriously inconvenienced by the multiple infusions that may be required for treatment with sodium ferric gluconate (Beutler, 2003). Sodium ferric gluconate complex in sucrose (FERRLECIT) was approved by the FDA for the treatment of iron deficiency in patients undergoing chronic hemodialysis who are receiving supplemental erythropoietin therapy (Eichbaum et al. , 2003 ).

Iron sucrose (SACCHARATE) also is approved for use in the United States, providing yet another form of parenteral iron for treatment of patients with iron deficiency intractable to therapy with oral iron. Like sodium ferric gluconate, iron sucrose appears to be better tolerated than iron dextran (Fishbane and Kowalski, 2000; Michaud, et al. , 2002 ). However, the experience with iron sucrose is much more limited than that with either iron dextran or sodium ferric gluconate.

Iron dextran injection (INFED, DEXFERRUM) is a colloidal solution of ferric oxyhydroxide complexed with polymerized dextran (molecular weight approximately 180,000) that contains 50 mg/ml of elemental iron. It can be administered by either intravenous (preferred) or intramuscular injection. When given by

deep intramuscular injection, it is gradually mobilized via the lymphatics and transported to reticuloendothelial cells; the iron then is released from the dextran complex. Intravenous administration gives a more reliable response. Given intravenously in a dose of less than 500 mg, the iron dextran complex is cleared exponentially with a plasma half-life of 6 hours. When 1 g or more is administered intravenously as total dose therapy, reticuloendothelial cell clearance is constant at 10 to 20 mg/hour. This slow rate of clearance results in a brownish discoloration of the plasma for several days and an elevation of the serum iron for 1 to 2 weeks.

Once the iron is released from the dextran within the reticuloendothelial cells, it is either incorporated into stores or transported via transferrin to the erythroid marrow. While a portion of the processed iron is rapidly made available to the marrow, a significant fraction is only gradually converted to usable iron stores. All of the iron eventually is released, although many months are required before the process is complete. During this time, the iron dextran stores in reticuloendothelial cells can confuse the clinician who attempts to evaluate the iron status of the patient.

Intramuscular injection of iron dextran should only be initiated after a test dose of 0.5 ml (25 mg of iron). If no adverse reactions are observed, the injections can proceed. The daily dose ordinarily should not exceed 0.5 ml (25 mg of iron) for infants weighing less than 4.5 kg (10 lb), 1 ml (50 mg of iron) for children weighing less than 9 kg (20 lb), and 2 ml (100 mg of iron) for other patients. Iron dextran should be injected only into the muscle mass of the upper outer quadrant of the buttock using a z-track technique (displacement of the skin laterally before injection). However, local reactions and the concern about malignant change at the site of injection (Weinbren et al. , 1978 ) make intramuscular administration inappropriate except when the intravenous route is inaccessible.

A test injection of 0.5 ml of undiluted iron dextran or an equivalent amount (25 mg of iron) diluted in saline also should precede intravenous administration of a therapeutic dose of iron dextran. The patient should be observed for signs of immediate anaphylaxis, and for an hour after injection for any signs of vascular instability or hypersensitivity, including respiratory distress, hypotension, tachycardia, or back or chest pain. When widely spaced, total-dose infusion therapy is given, a test dose injection should be given before each infusion because hypersensitivity can appear at any time. Furthermore, the patient should be monitored closely throughout the infusion for signs of cardiovascular instability. Delayed hypersensitivity reactions also are observed, especially in patients with rheumatoid arthritis or a history of allergies. Fever, malaise, lymphadenopathy, arthralgias, and urticaria can develop days or weeks following injection and last for prolonged periods of time. Therefore, iron dextran should be used with extreme caution in patients with rheumatoid arthritis or other connective tissue diseases, and during the acute phase of an inflammatory illness. Once hypersensitivity is documented, iron dextran therapy must be abandoned.

Before initiating iron dextran therapy, the total dose of iron required to repair the patient's iron-deficient state should be calculated. Relevant factors are the hemoglobin deficit, the need to reconstitute iron stores, and continued excess losses of iron, as seen with hemodialysis and chronic GI bleeding. Iron dextran solution (50 mg/ml of elemental iron) can be administered undiluted in daily doses of 2 ml until the total dose is reached or given as a single total-dose infusion. In the latter case, the iron dextran should be diluted in 250 to 1000 ml of 0.9% saline and infused over an hour or more.

When hemodialysis patients are started on erythropoietin, oral iron therapy alone generally is insufficient to guarantee an optimal hemoglobin response. It therefore is recommended that sufficient parenteral iron be given to maintain a plasma ferritin level between 100 and 800 g/L and a transferrin saturation of between 20% and 50% (Goodnough et al. , 2000 ). One approach is to administer an initial intravenous dose of 200 to 500 mg, followed by weekly or every-other-week injections of 25 to 100 mg of iron dextran to replace ongoing blood loss (Besarab et al. , 1999 ). With repeated doses of iron dextranespecially multiple, total-dose infusions such as those sometimes used in the treatment of chronic GI blood lossaccumulations of slowly metabolized iron dextran stores in reticuloendothelial cells can be impressive. The plasma ferritin level also can rise to levels associated with iron overload. While disease-related hemochromatosis has been associated with an increased risk of infections and cardiovascular disease, this has not been shown to be true in hemodialysis patients treated with iron dextran (Owen, 1999). It seems prudent, however, to withhold the drug whenever the plasma ferritin rises above 800 g/L.

Reactions to intravenous iron include headache, malaise, fever, generalized lymphadenopathy, arthralgias, urticaria, and in some patients with rheumatoid arthritis, exacerbation of the disease. Phlebitis may occur with prolonged infusions of a concentrated solution or when an intramuscular preparation containing 0.5% phenol is used in error. Of greatest concern is the rare anaphylactic reaction, which may be fatal despite treatment (Faich and Strobos, 1999).

Table 53-2. The Body Content of Iron  MG/KG OF

BODY WEIGHT

  Male FemaleEssential iron    Hemoglobin 31 28Myoglobin and enzymes 6 5Storage iron 13 4Total 50 37     

Table 53-3. Iron Requirements for Pregnancy  AVERAGE, mg RANGE, mgExternal iron loss 170 150-200Expansion of red cell mass 450 200-600Fetal iron 270 200-370Iron in placenta and cord 90 30-170Blood loss at delivery 150 90-310Total requirement* 980 580-1340Cost of pregnancya 680 440-1050*Blood loss at delivery not included. aIron lost by the mother; expansion of red cell mass not included. SOURCE: Council on Foods and Nutrition. Iron deficiency in the United States. JAMA 1968, 203:407-412. Used with permission 

Table 53-4. Daily Iron Intake and AbsorptionSUBJECT IRON

REQUIREMENT, mg/kg

AVAILABLE IRON IN POOR DIET-GOOD DIET, mg/kg

SAFETY FACTOR, AVAILABLE IRON/REQUIREMENT

Infant 67 33-66 0.5-1Child 22 48-96 2-4Adolescent (male) 21 30-60 1.5-3Adolescent (female) 20 30-60 1.5-3Adult (male) 13 26-52 2-4Adult (female) 21 18-36 1-2Mid-to-late pregnancy 80 18-36 0.22-0.45

Table 53-5. Average Response to Oral Iron  ESTIMATED

ABSORPTION 

TOTAL DOSE, mg of iron per day

% mg INCREASE IN HEMOGLOBIN, g/liter of blood per day

35 40 14 0.7105 24 25 1.4195 18 35 1.9390 12 45 2.2       

Figure 33-2. Enzymatic reactions that use folates. Section 1 shows the vitamin B12-dependent reaction that allows most dietary folates to enter the tetrahydrofolate cofactor pool and becomes the "folate trap" in vitamin E{12 deficiency. Section 2 shows the dTMP cycle. Section 3 shows the pathway by which folate enters the tetrahydrofolate cofactor pool. Double arrows indicate pathways with more than one intermediate step. (Reproduced, with permission, from Katzung BG [editor]: Basic & Clinical Pharmacology, 8th ed. McGraw-Hill, 2001 .)

VITAMIN B12, FOLIC ACID, AND THE TREATMENT OF MEGALOBLASTIC ANEMIAS

Introduction

Vitamin B12 and folic acid are dietary essentials. A deficiency of either vitamin impairs DNA synthesis in any cell in which chromosomal replication and division are taking place. Since tissues with the greatest rate of cell turnover show the most dramatic changes, the hematopoietic system is especially sensitive to deficiencies of these vitamins. An early sign of deficiency is megaloblastic anemia. Abnormal macrocytic red blood cells are produced, and the patient becomes severely anemic. Recognized in the 19th century, this pattern of abnormal hematopoiesis, termed pernicious anemia, spurred investigations that ultimately led to the discovery of vitamin B12 and folic acid. Even today, the characteristic abnormality in red blood cell morphology is important for diagnosis and as a therapeutic guide following administration of the vitamins.

Relationships between Vitamin B12 and Folic Acid.

The major roles of vitamin B12 and folic acid in intracellular metabolism are summarized in Figure 53-6. Intracellular vitamin B12 is maintained as two active coenzymes: methylcobalamin and deoxyadenosylcobalamin. Deoxyadenosylcobalamin (deoxyadenosyl B12) is a cofactor for the mitochondrial mutase enzyme that catalyzes the isomerization of L-methylmalonyl CoA to succinyl CoA, an important reaction in carbohydrate and lipid metabolism. This reaction has no direct relationship to the metabolic pathways that involve folate. In contrast, methylcobalamin (CH3B12) supports the methionine synthetase reaction, which is essential for normal metabolism of folate (Weir and Scott, 1983). Methyl groups contributed by methyltetrahydrofolate (CH3H4PteGlu1) are used to form methylcobalamin, which then acts as a methyl group donor for the conversion of homocysteine to methionine. This folate-cobalamin interaction is pivotal for normal synthesis of purines and pyrimidines, and therefore of DNA. The methionine synthetase reaction is largely responsible for the control of the recycling of folate cofactors; the maintenance of intracellular concentrations of folylpolyglutamates; and, through the

synthesis of methionine and its product, S-adenosylmethionine, the maintenance of a number of methylation reactions.

Since methyltetrahydrofolate is the principal folate congener supplied to cells, the transfer of the methyl group to cobalamin is essential for the adequate supply of tetrahydrofolate (H4PteGlu1), the substrate for a number of metabolic steps. Tetrahydrofolate is a precursor for the formation of intracellular folylpolyglutamates; it also acts as the acceptor of a one-carbon unit in the conversion of serine to glycine, with the resultant formation of 5,10-methylenetetrahydrofolate (5,10-CH2H4PteGlu). The latter derivative donates the methylene group to deoxyuridylate (dUMP) for the synthesis of thymidylate (dTMP)an extremely important reaction in DNA synthesis. In the process, the 5,10-CH2H4PteGlu is converted to dihydrofolate (H2PteGlu). The cycle then is completed by the reduction of the H2PteGlu to H4PteGlu by dihydrofolate reductase, the step that is blocked by folate antagonists such as methotrexate (see Chapter 51). As shown in Figure 53-6, other pathways also lead to the synthesis of 5,10-methylenetetrahydrofolate. These pathways are important in the metabolism of formiminoglutamic acid (FIGLU) and purines and pyrimidines (see reviews byWeir and Scott, 1983; Chanarin et al. , 1985 ).

In the presence of a deficiency of either vitamin B12 or folate, the decreased synthesis of methionine and S-adenosylmethionine interferes with protein biosynthesis, a number of methylation reactions, and the synthesis of polyamines. In addition, the cell responds to the deficiency by redirecting folate metabolic pathways to supply increasing amounts of methyltetrahydrofolate; this tends to preserve essential methylation reactions at the expense of nucleic acid synthesis. With vitamin B12 deficiency, methylenetetrahydrofolate reductase activity increases, directing available intracellular folates into the methyltetrahydrofolate pool (not shown in Figure 53-6). The methyltetrahydrofolate then is trapped by the lack of sufficient vitamin B12 to accept and transfer methyl groups, and subsequent steps in folate metabolism that require tetrahydrofolate are deprived of substrate. This process provides a common basis for the development of megaloblastic anemia with deficiency of either vitamin B12 or folic acid.

The mechanisms responsible for the neurological lesions of vitamin B12 deficiency are less well understood (Weir and Scott, 1983). Damage to the myelin sheath is the most obvious lesion in this neuropathy. This observation led to the early suggestion that the deoxyadenosyl B12-dependent methylmalonyl CoA mutase reaction, a step in propionate metabolism, is related to the abnormality. However, other evidence suggests that the deficiency of methionine synthetase and the block of the conversion of methionine to S-adenosylmethionine are more likely to be responsible (Scott et al. , 1981 ).

Nitrous oxide (N2O), used for anesthesia (see Chapter 13), can cause megaloblastic changes in the marrow and a neuropathy that resemble those of vitamin B12 deficiency (Chanarin et al. , 1985 ). Studies with N2O have demonstrated a reduction in methionine synthetase and reduced concentrations of methionine and S-adenosylmethionine. The latter is necessary for methylation reactions, including those required for the synthesis of phospholipids and myelin. Significantly, the neuropathy induced with N2O can be prevented partially by feeding methionine. A neuropathy similar to that occurring with vitamin B12 deficiency has been reported in dentists who are exposed to N2O used as an anesthetic.

Vitamin B12 Deficiency.

Vitamin B12 deficiency is recognized clinically by its impact on the hematopoietic and nervous systems. The sensitivity of the hematopoietic system relates to its high rate of cell turnover. Other tissues with high rates of cell turnover (e.g., mucosa and cervical epithelium) also have high requirements for the vitamin.

As a result of an inadequate supply of vitamin B12, DNA replication becomes highly abnormal. Once a hematopoietic stem cell is committed to enter a programmed series of cell divisions, the defect in chromosomal replication results in an inability of maturing cells to complete nuclear divisions while cytoplasmic maturation continues at a relatively normal rate. This results in the production of morphologically abnormal cells and death of cells during maturation, a phenomenon referred to as ineffective hematopoiesis. These abnormalities are readily identified by examination of the marrow and peripheral blood. Maturation of red cell precursors is highly abnormal (megaloblastic erythropoiesis). Those cells that do leave the marrow also are abnormal, and many cell fragments, poikilocytes, and macrocytes appear in the peripheral blood. The mean red cell volume increases to values greater than 110 fl. Severe deficiency affects all cell lines, and a pronounced pancytopenia results.

The diagnosis of a vitamin B12 deficiency usually can be made using measurements of the serum vitamin B12 and/or serum methylmalonic acid. The latter is somewhat more sensitive and has been used to identify metabolic deficiency in patients with normal serum vitamin B12 levels. As part of the clinical management of a patient with severe megaloblastic anemia, a therapeutic trial using very small doses of the vitamin can be used to confirm the diagnosis. Serial measurements of the reticulocyte count, serum iron, and hematocrit are performed to define the characteristic recovery of normal red cell production. The

Schilling test can be used to quantitate the absorption of the vitamin and delineate the mechanism of the disease. By performing the Schilling test with and without added intrinsic factor, it is possible to discriminate between intrinsic factor deficiency by itself and primary ileal cell disease.

Vitamin B12 deficiency can irreversibly damage the nervous system. Progressive swelling of myelinated neurons, demyelination, and neuronal cell death are seen in the spinal column and cerebral cortex. This causes a wide range of neurological signs and symptoms, including paresthesias of the hands and feet, decreased vibration and position senses with resultant unsteadiness, decreased deep tendon reflexes, and in the later stages, confusion, moodiness, loss of memory, and even a loss of central vision. The patient may exhibit delusions, hallucinations, or even overt psychosis. Since the neurological damage can be dissociated from the changes in the hematopoietic system, vitamin B12 deficiency must be considered in elderly patients with dementia or psychiatric disorders, even if they are not anemic (Lindenbaum et al. , 1988).

Vitamin B12 Therapy.

Vitamin B12 is available for injection or oral administration; combinations with other vitamins and minerals also can be given orally or parenterally. The choice of a preparation always depends on the cause of the deficiency. Although oral preparations may be used to supplement deficient diets, they are of limited value in the treatment of patients with deficiency of intrinsic factor or ileal disease. Even though small amounts of vitamin B12 may be absorbed by simple diffusion, the oral route of administration cannot be relied upon for effective therapy in the patient with a marked deficiency of vitamin B12 and abnormal hematopoiesis or neurological deficits. Therefore, the preparation of choice for treatment of a vitamin B12-deficiency state is cyanocobalamin, and it should be administered by intramuscular or subcutaneous injection.

Cyanocobalamin injection is safe when given by the intramuscular or deep subcutaneous route, but it should never be given intravenously. There have been rare reports of transitory exanthema and anaphylaxis after injection. If a patient reports a previous sensitivity to injections of vitamin B12, an intradermal skin test should be performed before the full dose is administered.

Cyanocobalamin is administered in doses of 1 to 1000 g. Tissue uptake, storage, and utilization depend on the availability of transcobalamin II (see above). Doses in excess of 100 g are cleared rapidly from plasma into the urine, and administration of larger amounts of vitamin B12 will not result in greater retention of the vitamin. Administration of 1000 g is of value in the performance of the Schilling test. After isotopically labeled vitamin B12 is administered orally, the compound that is absorbed can be quantitatively recovered in the urine if 1000 g of cyanocobalamin is administered intramuscularly. This unlabeled material saturates the transport system and tissue binding sites, so that more than 90% of the labeled and unlabeled vitamin is excreted during the next 24 hours.

A number of multivitamin preparations are marketed either as nutritional supplements or for the treatment of anemia. Many of these contain up to 80 g of cyanocobalamin without or with intrinsic factor concentrate prepared from the stomachs of hogs or other domestic animals. One oral unit of intrinsic factor is defined as that amount of material that will bind and transport 15 g of cyanocobalamin. Most multivitamin preparations supplemented with intrinsic factor contain 0.5 oral unit per tablet. While the combination of oral vitamin B12 and intrinsic factor would appear to be ideal for patients with an intrinsic factor deficiency, such preparations are not reliable. With prolonged therapy, some patients become refractory to oral intrinsic factor, perhaps related to production of an intraluminal antibody against the hog protein. Patients taking such preparations must be reevaluated at periodic intervals for recurrence of pernicious anemia.

Hydroxocobalamin given in doses of 100 g intramuscularly has been reported to have a more sustained effect than cyanocobalamin, with a single dose maintaining plasma vitamin B12 concentrations in the normal range for up to 3 months. However, some patients show reductions of the concentration of vitamin B12 in plasma within 30 days, similar to that seen after cyanocobalamin. Furthermore, the administration of hydroxocobalamin has resulted in the formation of antibodies to the transcobalamin II-vitamin B12 complex.

Vitamin B12 has an undeserved reputation as a health tonic and has been used for a number of disease states. Effective use of the vitamin depends on accurate diagnosis and an understanding of the following general principles of therapy:

Vitamin B12 should be given prophylactically only when there is a reasonable probability that a deficiency exists or will exist. Dietary deficiency in the strict vegetarian, the predictable malabsorption of vitamin B12 in patients who have had a gastrectomy, and certain diseases of the small intestine constitute such

indications. When gastrointestinal function is normal, an oral prophylactic supplement of vitamins and minerals, including vitamin B12, may be indicated. Otherwise, the patient should receive monthly injections of cyanocobalamin.

The relative ease of treatment with vitamin B12 should not prevent a full investigation of the etiology of the deficiency. The initial diagnosis usually is suggested by a macrocytic anemia or an unexplained neuropsychiatric disorder. Full understanding of the etiology of vitamin B12 deficiency involves studies of dietary supply, gastrointestinal absorption, and transport.

Therapy always should be as specific as possible. While a large number of multivitamin preparations are available, the use of "shotgun" vitamin therapy in the treatment of vitamin B12 deficiency can be dangerous. With such therapy, there is the danger that sufficient folic acid will be given to result in a hematological recovery. This can mask continued vitamin B12 deficiency and permit neurological damage to develop or progress.

Although a classical therapeutic trial with small amounts of vitamin B12 can help confirm the diagnosis, acutely ill, elderly patients may not be able to tolerate the delay in the correction of a severe anemia. Such patients require supplemental blood transfusions and immediate therapy with folic acid and vitamin B12 to guarantee rapid recovery.

Long-term therapy with vitamin B12 must be evaluated at intervals of 6 to 12 months in patients who are otherwise well. If there is an additional illness or a condition that may increase the requirement for the vitamin (e.g., pregnancy), reassessment should be performed more frequently.

Folate Deficiency. Folate deficiency is a common complication of diseases of the small intestine, which interfere with the absorption of folate from food and the recirculation of folate through the enterohepatic cycle. In acute or chronic alcoholism, daily intake of folate in food may be severely restricted, and the enterohepatic cycle of the vitamin may be impaired by toxic effects of alcohol on hepatic parenchymal cells; this is the most common cause of folate-deficient megaloblastic erythropoiesis. However, it also is the most amenable to therapy, inasmuch as the reinstitution of a normal diet is sufficient to overcome the effect of alcohol. Disease states characterized by a high rate of cell turnover, such as hemolytic anemias, also may be complicated by folate deficiency. Additionally, drugs that inhibit dihydrofolate reductase (e.g., methotrexate and trimethoprim) or that interfere with the absorption and storage of folate in tissues (e.g., certain anticonvulsants and oral contraceptives) can lower the concentration of folate in plasma and may cause a megaloblastic anemia (Stebbins and Bertino, 1976).

Folate deficiency has been implicated in the incidence of neural tube defects, including spina bifida, encephaloceles, and anencephaly. This is true even in the absence of folate-deficient anemia or alcoholism. A less-than-adequate intake of folate also can result in elevations in plasma homocysteine (Green and Miller, 1999). Since even moderate hyperhomocysteinemia is considered an independent risk factor for coronary artery and peripheral vascular disease and for venous thrombosis, the role of folate as a methyl donor in the homocysteine-to-methionine conversion is getting increased attention. Patients who are heterozygous for one or another enzymatic defect and have high normal to moderate elevations of plasma homocysteine may improve with folic acid therapy.

Folate deficiency is recognized by its impact on the hematopoietic system. As with vitamin B12, this fact reflects the increased requirement associated with high rates of cell turnover. The megaloblastic anemia that results from folate deficiency cannot be distinguished from that caused by vitamin B12 deficiency. This finding is to be expected because of the final common pathway of the major intracellular metabolic roles of the two vitamins. At the same time, folate deficiency is rarely if ever associated with neurological abnormalities. Thus the observation of characteristic abnormalities in vibratory and position sense and in motor and sensory pathways is incompatible with an isolated deficiency of folic acid.

After deprivation of folate, megaloblastic anemia develops much more rapidly than it does following interruption of vitamin B12 absorption (e.g., gastric surgery). This observation reflects the fact that body stores of folate are limited. Although the rate of induction of megaloblastic erythropoiesis may vary, a folate-deficiency state may appear in 1 to 4 weeks, depending on the individual's dietary habits and stores of the vitamin.

Folate deficiency is best diagnosed from measurements of folate in plasma and in red cells. However, an empiric trial of folate in cases of suspected deficiency has been proposed as more cost effective (Robinson and Mladenovic, 2001). Indeed, the concentration of folate in plasma is extremely sensitive to changes in dietary intake of the vitamin and the influence of inhibitors of folate metabolism or transport, such as alcohol. Normal folate concentrations in plasma range from 9 to 45 nmol (4 to 20 ng/ml); below 9 nmol is considered folate deficient. The plasma folate concentration rapidly falls to values indicative of

deficiency within 24 to 48 hours of steady ingestion of alcohol (Eichner and Hillman, 1971; Eichner and Hillman, 1973). The plasma folate concentration will revert quickly to normal once such ingestion is stopped, even while the marrow is still megaloblastic. Such rapid fluctuations detract from the clinical utility of the plasma folate concentration. The amount of folate in red cells or the adequacy of stores in lymphocytes (as measured by the deoxyuridine suppression test) may be used to diagnose a long-standing deficiency of folic acid. A positive result on either test shows that the deficiency must have existed for a sufficient time to allow the production of a population of cells with deficient folate stores.

Folic acid is marketed as oral tablets containing 0.4, 0.8, and 1 mg pteroylglutamic acid, as an aqueous solution for injection (5 mg/ml), and in combination with other vitamins and minerals.

Folinic acid (leucovorin calcium, citrovorum factor) is the 5-formyl derivative of tetrahydrofolic acid. The principal therapeutic uses of folinic acid are to circumvent the inhibition of dihydrofolate reductase as a part of high-dose methotrexate therapy and to potentiate fluorouracil in the treatment of colorectal cancer (see Chapter 51). It also has been used as an antidote to counteract the toxicity of folate antagonists such as pyrimethamine or trimethoprim. While it can be used to treat any folate-deficient state, folinic acid provides no advantage over folic acid, is more expensive, and therefore is not recommended. A single exception is the megaloblastic anemia associated with congenital dihydrofolate reductase deficiency. Leucovorin should never be used for the treatment of pernicious anemia or other megaloblastic anemias secondary to a deficiency of vitamin B12. Just as is seen with folic acid, its use can result in an apparent response of the hematopoietic system, but neurological damage may occur or progress if already present.

Untoward Effects. There have been rare reports of reactions to parenteral injections of folic acid and leucovorin. If a patient describes a history of a reaction before the drug is given, caution should be exercised. Oral folic acid usually is not toxic. Even with doses as high as 15 mg/day, there have been no substantiated reports of side effects. Folic acid in large amounts may counteract the antiepileptic effect of phenobarbital, phenytoin, and primidone, and increase the frequency of seizures in susceptible children (Reynolds, 1968). While some studies have not supported these contentions, the FDA recommends that oral tablets of folic acid be limited to strengths of 1 mg or less.

General Principles of Therapy. The therapeutic use of folic acid is limited to the prevention and treatment of deficiencies of the vitamin. As with vitamin B12 therapy, effective use of the vitamin depends on accurate diagnosis and an understanding of the mechanisms that are operative in a specific disease state. The following general principles of therapy should be respected:

Prophylactic administration of folic acid should be undertaken for clear indications. Dietary supplementation is necessary when there is a requirement that may not be met by a "normal" diet. The daily ingestion of a multivitamin preparation containing 400 to 500 g of folic acid has become standard practice before and during pregnancy to reduce the incidence of neural tube defects and for as long as a woman is breastfeeding. In women with a history of a pregnancy complicated by a neural tube defect, an even larger dose of 4 mg a day has been recommended (MRC Vitamin Study Research Group, 1991). Patients on total parenteral nutrition should receive folic acid supplements as part of their fluid regimen because liver folate stores are limited. Adult patients with a disease state characterized by high cell turnover (e.g., hemolytic anemia) generally require larger doses, 1 mg of folic acid given once or twice a day. The 1-mg dose also has been used in the treatment of patients with elevated levels of homocysteine.

As with vitamin B12 deficiency, any patient with folate deficiency and a megaloblastic anemia should be evaluated carefully to determine the underlying cause of the deficiency state. This should include evaluation of the effects of medications, the amount of alcohol intake, the patient's history of travel, and the function of the gastrointestinal tract.

Therapy always should be as specific as possible. Multivitamin preparations should be avoided unless there is good reason to suspect deficiency of several vitamins.

The potential danger of mistreating a patient who has vitamin B12 deficiency with folic acid must be kept in mind. The administration of large doses of folic acid can result in an apparent improvement of the megaloblastic anemia, inasmuch as PteGlu is converted by dihydrofolate reductase to H4PteGlu; this circumvents the methylfolate "trap." However, folate therapy does not prevent or alleviate the neurological defects of vitamin B12 deficiency, and these may progress and become irreversible.Task

A. Please answer :

1. Describe the normal mechanism of regulation of iron absoption and storage in the body2. List the anemias for wich iron supplementation is indicated and those for which it is

contraindicated3. Describe the acute and chronic toxity of iron4. Vitamin B12 and Folic acid cycle5. Describe the clinical application of B12 and Folic acid6. Describe the side effect of folic acid as sole therapy for megaloblastic anemia7. Name of major growth factors and their clinical uses

B. Classification & Dosing Iron, B12 and Folic acid (available in Indonesia)

Generic ®brand name

Presention Dosage Comment

ETC