Section XI - Drugs Acting on the Blood and the Blood-Forming docx

Section XI. Drugs Acting on the Blood and the Blood-Forming
Organs
Overview
The short life span of mature blood cells requires their continuous replacement, a process termed
hematopoiesis. New cell production must be responsive to both basal needs and situations of
increased demand. For example, red blood cell production can vary over more than a fivefold range
in response to anemia or hypoxia. White blood cell production increases dramatically in response to
a systemic infection, and platelet production can increase severalfold when platelet destruction
results in thrombocytopenia.
The regulation of hematopoiesis is complex and involves cell–cell interactions within the
microenvironment of the bone marrow as well as both hematopoietic and lymphopoietic growth
factors. A number of these hormonelike glycoproteins now have been identified and characterized,
and, using recombinant DNA technology, their genes have been cloned and the proteins produced in
quantities sufficient for use as therapeutic agents. Clinical applications now are being developed,
ranging from treatment of primary hematological diseases to uses as adjunctive agents in the
treatment of severe infections and in the management of patients who are undergoing chemotherapy
or marrow transplantation.
Hematopoiesis also requires adequate supplies of minerals, both iron and copper, and a number of
vitamins, including folic acid, vitamin B
12
, pyridoxine, ascorbic acid, and riboflavin. Deficiencies of
these minerals and vitamins generally result in characteristic anemias and, less frequently, a general
failure of hematopoiesis. Therapeutic correction of a specific deficiency state depends on the
accurate diagnosis of the anemic state and knowledge as to the correct dose, the use of these agents
in various combinations, and the expected response.
This chapter deals with the growth factors, vitamins, minerals, and drugs that affect the blood and
blood-forming organs.
Hematopoietic Growth Factors
History
Modern concepts of hematopoietic cell growth and differentiation developed beginning in the 1950s
with the work of Jacobsen, Ford, and others (Jacobsen et al. , 1949 ; Ford et al. , 1956 ). These

investigators demonstrated the role that cells from the spleen and marrow play in the restoration of
hematopoietic tissue in irradiated animals. In 1961, Till and McCulloch were able to show that
individual hematopoietic cells could form macroscopic hematopoietic nodules in the spleens of
irradiated mice. Their work led to the concept of colony-forming stem cells. It also led to the
subsequent proof that stem cells present in human bone marrow are pluripotent—that is, they give
rise to granulocytes, monocytes, lymphocytes, megakaryocytes, and erythrocytes.
The role of growth factors in hematopoiesis was elucidated by Bradley, Metcalf, and others using
bone marrow culture techniques (Bradley and Metcalf, 1966). Individual growth factors were
isolated (Metcalf, 1985; Moore, 1991), and the target cells of these factors characterized. The
pluripotent stem cell gives rise to committed progenitors, which can be identified as single colony-
forming units, and to cells that are increasingly differentiated.
The existence of a circulating growth factor that controls erythropoiesis was first suggested by
experiments carried out by Paul Carnot in 1906 (Carnot and Deflandre, 1906). He observed an
increase in the red cell count in rabbits injected with serum obtained from anemic animals and
postulated the existence of a factor that he called hemapoietine. However, it was not until the 1950s
that Reissmann (1950), Erslev (1953), and Jacobsen and coworkers (1957) defined the origin and
actions of the hormone, now called erythropoietin. Subsequently, extensive studies of
erythropoietin were carried out in patients with anemia and polycythemia, culminating in 1977 with
the purification of erythropoietin from urine by Miyake and colleagues. The gene that encodes the
protein was subsequently cloned and expressed at a high level in a mammalian cell system (Jacobs
et al. , 1985 ; Lin et al. , 1985 ), producing a recombinant hormone that is indistinguishable from
human urinary erythropoietin. Similarly, complementary DNA and genomic clones for granulocyte,
macrophage, and, most recently, megakaryocyte colony-stimulating factors have been isolated and
sufficient quantities of biologically active growth factors produced for clinical investigation
(Kawasaki et al. , 1985 ; Lee et al. , 1985 ; Wong et al. , 1985 ; Yang et al. , 1986 ; Lok et al. , 1994 ; de
Sauvage et al. , 1994 ).
Growth Factor Physiology
Steady-state hematopoiesis involves the production of more than 200 billion (2 x 10
11
) blood cells

each day. This production is under delicate control, and, with increased demand, the rate can
increase severalfold. The hematopoietic organ also is unique in that several mature cell types are
derived from a much smaller number of pluripotent stem cells that are formed in early embryonic
life. These stem cells are capable of both maintaining their own number and differentiating under
the influence of cellular and humoral factors [stem cell factor (SCF), Flt3 ligand (FL), interleukin-3
(IL-3), and granulocyte/macrophage colony-stimulating factor (GM-CSF)] to produce a variety of
hematopoietic and lymphopoietic cells.
Stem cell differentiation can be described as a series of steps that produce so-called burst-forming
units (BFU) and colony-forming units (CFU) for each of the major cell lines (Quesenberry and
Levitt, 1979). Although these early progenitors (BFU and CFU) are not morphologically
recognizable as precursors of a specific cell type, they are capable of further proliferation and
differentiation, increasing their number by some 30-fold. Subsequently, colonies of
morphologically distinct cells form under the control of an overlapping set of additional growth
factors (G-CSF, M-CSF, erythropoietin, and thrombopoietin). Proliferation and maturation of the
CFU for each cell line can further amplify the resulting mature cell product by another 30-fold or
more, resulting in greater than 1000 mature cells produced for each committed stem cell (Lajtha et
al. , 1969 ).
Hematopoietic and lymphopoietic growth factors are produced by a number of marrow cells and
peripheral tissues. The growth factors are glycoproteins and are active at very low concentrations,
usually on more than one committed cell lineage. Most show synergistic interactions with other
factors, as well as "networking," wherein stimulation of a cell lineage by one growth factor induces
the production of additional growth factors. Finally, growth factors generally exert actions at
several points in the processes of cell proliferation and differentiation and in mature cell function
(Metcalf, 1985). Some of the overlapping effects of the more important hematopoietic growth
factors are illustrated in Figure 54–1 and listed in Table 54–1.

Figure 54–1. Sites of Action of Hematopoietic Growth Factors in the
Differentiation and Maturation of Marrow Cell Lines. A self-sustaining pool of
marrow stem cells differentiates under the influence of specific hematopoietic
growth factors to form a variety of hematopoietic and lymphopoietic cells. Stem

cell factor (SCF), ligand (FL), interleukin-3 (IL-3), and granulocyte/macrophage
colony-stimulating factor (GM-CSF), together with cell–cell interactions in the
marrow, stimulate stem cells to form a series of burst-forming units (BFU) and
colony-forming units (CFU): CFU-GEMM, CFU-GM, CFU-Meg, BFU-E, and
CFU-E (GEMM, granulocyte, erythrocyte, monocyte, and megakaryocyte; GM,
granulocyte and macrophage; Meg, megakaryocyte; E, erythrocyte). After
considerable proliferation, further differentiation is stimulated by synergistic
interactions with growth factors for each of the major cell lines—granulocyte
colony-stimulating factor (G-CSF), monocyte/macrophage-stimulating factor (M-
CSF), thrombopoietin, and erythropoietin. Each of these factors also influences
the proliferation, maturation, and, in some cases, the function of the derivative
cell line (seeTable 54–1).
Erythropoietin
While erythropoietin is not the sole growth factor responsible for erythropoiesis, it is the most
important regulator of the proliferation of committed progenitors (BFU-E and CFU-E). In its
absence, severe anemia is invariably present. Erythropoiesis is controlled by a highly responsive
feedback system in which a sensor in the kidney can detect changes in oxygen delivery to increase
the secretion of erythropoietin, which then stimulates a rapid expansion of erythroid progenitors.
Erythropoietin is produced primarily by peritubular interstitial cells of the kidney under the control
of a single gene on human chromosome 7. The gene product is a protein containing 193 amino
acids, of which the first 27 are cleaved during secretion (Jacobs et al. , 1985 ; Lin et al. , 1985 ). The
final hormonal peptide is heavily glycosylated and has a molecular weight of approximately 30,000
daltons. Once released, erythropoietin travels to the marrow, where it binds to a receptor on the
surface of committed erythroid progenitors and is internalized. With anemia or hypoxemia, renal
synthesis rapidly increases by 100-fold or more, serum erythropoietin levels rise, and marrow
progenitor cell survival, proliferation, and maturation are dramatically stimulated. This finely tuned
feedback loop can be disrupted at any point—by kidney disease, marrow damage, or a deficiency in
iron or an essential vitamin. With an infection or an inflammatory state, erythropoietin secretion,
iron delivery, and progenitor proliferation are all suppressed by inflammatory cytokines.
Recombinant human erythropoietin (epoetin alfa), produced using a mammalian cell line (Chinese

hamster ovary cells), is virtually identical to endogenous hormone. Small differences in the
carbohydrate portion of the molecule do not appear to affect the kinetics, potency, or
immunoreactivity. Currently available preparations of epoetin alfa include EPOGEN and PROCRIT,
supplied in single-use vials of from 2000 to 10,000 U/ml for intravenous or subcutaneous
administration. When injected intravenously, epoetin alfa is cleared from plasma with a half-life of
10 hours. However, the effect on marrow progenitors is sufficiently sustained that it need not be
given more often than three times a week to achieve an adequate response. No significant allergic
reactions have been associated with the intravenous or subcutaneous administration of epoetin alfa,
and antibodies have not been detected, even after prolonged administration.
Therapeutic Uses
Recombinant erythropoietin therapy can be highly effective in a number of anemias, especially
those associated with a poor erythropoietic response. As first shown by Eschbach and coworkers in
1987, there is a clear dose-response relationship between the epoetin alfa dose and the rise in
hematocrit in anephric patients, with eradication of their anemia at higher doses. Epoetin alfa also
has been shown to be effective in the treatment of anemias associated with surgery, AIDS, cancer
chemotherapy, prematurity, and certain chronic inflammatory illnesses.
Anemia of Chronic Renal Failure
Patients with the anemia of chronic renal disease are ideal candidates for epoetin alfa therapy. The
response in predialysis, peritoneal dialysis, and hemodialysis patients is dependent on severity of
the renal failure, the erythropoietin dose and route of administration, and iron availability (Eschbach
et al. , 1989 ; Kaufman et al. , 1998 ; Besarab et al. , 1999 ). The subcutaneous route of administration
is preferred over the intravenous, since absorption is slower and the amount of drug required is
reduced by 20% to 40%. Iron supply is especially critical. Adequate iron stores, as reflected by an
iron saturation of transferrin of at least 30% and a plasma ferritin greater than 400 g/l, must be
maintained, usually by repeated injections of iron dextran (see"Therapy with Parenteral Iron").
The patient must be closely monitored during therapy, and the dose of epoetin alfa must be adjusted
to obtain a gradual rise in the hematocrit, over a 2- to 4-month period, until a final hematocrit of
33% to 36% is reached. Treatment to hematocrit levels greater than 36% is not recommended. A
study of patients treated to hematocrits above 40% showed a higher incidence of myocardial
infarction and death (Besarab et al. , 1998 ). Furthermore, the drug should never be used to replace

emergency transfusion in patients who need immediate correction of a life-threatening anemia.
It is currently recommended that the patient be started on a dose of 80 to 120 U/kg of epoetin alfa,
given subcutaneously, three times a week. It can be given on a once-a-week schedule, but
considerably more drug is required for an equivalent effect. If the response is poor, the dose should
be progressively increased. The final maintenance dose of epoetin alfa can vary from as little as 10
U/kg to more than 300 U/kg, with an average close to 75 U/kg, three times a week, in most patients.
Children under the age of 5 years generally require a higher dose. Resistance to therapy is
commonly seen in the patient who develops an inflammatory illness or becomes iron deficient, so
that close monitoring of general health and iron status is essential. Less common causes of
resistance include occult blood loss, folic acid deficiency, carnitine deficiency, inadequate dialysis,
aluminum toxicity, and osteitis fibrosa cystica secondary to hyperparathyroidism.
The most common side effect of epoetin alfa therapy is aggravation of hypertension, seen in 20% to
30% of patients and most often associated with a too-rapid rise in hematocrit. Blood pressure
control usually can be attained by either increasing antihypertensive therapy or ultrafiltration in
dialysis patients or by reducing the epoetin alfa dose to slow the hematocrit response. An increased
tendency to vascular access thrombosis in dialysis patients also has been reported, but this remains
controversial.
Anemia in AIDS Patients
Epoetin alfa therapy has been approved for the treatment of HIV-infected patients, especially those
on zidovudine therapy (Fischl et al. , 1990 ). Excellent responses to doses of 100 to 300 U/kg, given
subcutaneously three times a week, generally are seen in patients with zidovudine-induced anemia.
In the face of advanced disease, marrow damage, and elevated serum erythropoietin levels (greater
than 500 IU/L), therapy is less effective.
Cancer-Related Anemias
Epoetin alfa therapy, 150 U/kg three times a week or 450 to 600 U/kg once a week, can reduce the
transfusion requirement in cancer patients undergoing chemotherapy. It also has been used to treat
patients with multiple myeloma, with improvement in both their anemia and sense of well-being.
Here again, a baseline serum erythropoietin level may help to predict the response.
Surgery and Autologous Blood Donation
Epoetin alfa has been used perioperatively to treat anemia and reduce the need for transfusion.

Patients undergoing elective orthopedic and cardiac procedures have been treated with 150 to 300
U/kg of epoetin alfa once daily for the 10 days preceding surgery, on the day of surgery, and for 4
days after surgery. As an alternative, 600 U/kg can be given on days –21, –14, and –7 prior to
surgery, with an additional dose on the day of surgery. This can correct a moderately severe
preoperative anemia, hematocrit 30% to 36%, and reduce the need for transfusion. Epoetin alfa also
has been used to improve autologous blood donation (Goodnough et al. , 1989 ). However, as a
routine, the potential benefit is small while the expense is considerable. Patients treated for 3 to 4
weeks with epoetin alfa (300 to 600 U/kg twice a week), are able to donate only 1 or 2 more units
than untreated patients, and most of the time this goes unused. Still, the ability to stimulate
erythropoiesis for blood storage can be invaluable in the patient with multiple alloantibodies to
homologous red blood cells.
Other Uses
Epoetin alfa has been designated an orphan drug by the United States Food and Drug
Administration (FDA) for the treatment of the anemia of prematurity and patients with
myelodysplasia. In the latter case, even very high doses of more than 1000 U/kg 2 to 3 times a week
have had limited success. The possible use of very high dose therapy in other hematological
disorders, such as sickle cell anemia, is still under study. Highly competitive athletes have used
epoetin alfa to increase their hemoglobin levels ("blood doping") and improve performance.
Unfortunately, this misuse of the drug has been implicated in the deaths of several athletes, and it
should be discouraged.
Myeloid Growth Factors
The myeloid growth factors are glycoproteins that stimulate the proliferation and differentiation of
one or more myeloid cell lines. They also enhance the function of mature granulocytes and
monocytes. Recombinant forms of several of the growth factors have now been produced, including
GM-CSF (Lee et al. , 1985 ), G-CSF (Wong et al. , 1985 ), IL-3 (Yang et al. , 1986 ), M-CSF or CSF-1
(Kawasaki et al. , 1985 ), SCF (Huang et al. , 1990 ), and, most recently, thrombopoietin (Lok et al. ,
1994; de Sauvage et al. , 1994 ; Kaushansky et al. , 1994 ; Table 54–1).
The myeloid growth factors are produced naturally by a number of different cells including
fibroblasts, endothelial cells, macrophages, and T cells (Figure 54–2). They are active at extremely
low concentrations. GM-CSF is capable of stimulating the proliferation, differentiation, and

function of a number of the myeloid cell lineages (Figure 54–1). It acts synergistically with other
growth factors, including erythropoietin, at the level of the BFU. GM-CSF stimulates the CFU-
GEMM (granulocyte/erythrocyte/macrophage/megakaryocyte), CFU-GM, CFU-M, CFU-E, and
CFU-Meg (megakaryocyte) to increase cell production. It also enhances the migration,
phagocytosis, superoxide production, and antibody-dependent cell media toxicity of neutrophils,
monocytes, and eosinophils.

Figure 54–2. Cytokine–Cell Interactions. Macrophages, T cells, B cells, and
marrow stem cells interact via several cytokines [IL (interleukin)-1, IL-2, IL-3,
IL-4, IFN (interferon)- , GM-CSF, and G-CSF] in response to a bacterial or a
foreign antigen challenge. SeeTable 54–1 for the functional activities of these
various cytokines.
The activity of G-CSF is more focused. Its principal action is to stimulate the proliferation,
differentiation, and function of the granulocyte lineage. It acts primarily on the CFU-G, although it
can also play a synergistic role with IL-3 and GM-CSF in stimulating other cell lines. G-CSF
enhances phagocytic and cytotoxic activities of neutrophils. Unlike GM-CSF, G-CSF has little
effect on monocytes, macrophages, and eosinophils. At the same time, G-CSF reduces
inflammation by inhibiting IL-1, tumor necrosis factor, and interferon gamma.
Granulocyte/Macrophage Colony-Stimulating Factor (GM-CSF)
Recombinant human GM-CSF (sargramostim) is a 127–amino acid glycoprotein produced in yeast.
Except for the substitution of a leucine in position 23 and variable levels of glycosylation, it is
identical to endogenous GM-CSF. While sargramostim, like natural GM-CSF, has a wide range of
effects on cells in culture, its primary therapeutic effect is the stimulation of myelopoiesis. The
initial clinical application of sargramostim was in patients undergoing autologous bone marrow
transplantation. By shortening the duration of neutropenia, transplant morbidity was significantly
reduced without a change in long-term survival or risk of inducing an early relapse of the malignant
process (Brandt et al. , 1988 ; Rabinowe et al. , 1993 ). The role of GM-CSF therapy in allogeneic
transplantation is less clear. The effect of the growth factor on neutrophil recovery is less
pronounced in patients receiving prophylactic treatment for graft-versus-host disease (GVHD), and
studies have failed to show a significant effect on transplant mortality, long-term survival, the

appearance of GVHD, or disease relapse. However, it may improve survival in transplant patients
who exhibit early graft failure (Nemunaitis et al. , 1990 ). It also has been used to mobilize CD34-
positive progenitor cells for peripheral blood stem cell collection for transplantation following
myeloablative chemotherapy. Sargramostim has been used to shorten the period of neutropenia and
reduce morbidity in patients receiving intensive chemotherapy (Gerhartz et al. , 1993 ). It also will
stimulate myelopoiesis in some patients with cyclic neutropenia, myelodysplasia, aplastic anemia,
or AIDS-associated neutropenia (Groopman et al. , 1987 ; Vadhan-Raj et al. , 1987 ).
Sargramostim (LEUKINE) is administered by subcutaneous injection or slow intravenous infusion at
a dose of 125 to 500 g/m
2
per day. Plasma levels of GM-CSF rise rapidly after subcutaneous
injection and then decline, with a half-life of 2 to 3 hours. When given intravenously, infusions
should be maintained over 3 to 6 hours. With the initiation of therapy, there is a transient decrease
in the absolute leukocyte count secondary to margination and sequestration in the lungs. This is
followed by a dose-dependent, biphasic increase in leukocyte counts over the next 7 to 10 days.
Once the drug is discontinued, the leukocyte count returns to baseline within 2 to 10 days. When
GM-CSF is given in lower doses, the response is primarily neutrophilic, while at larger doses,
monocytosis and eosinophilia are observed. Following bone marrow transplantation or intensive
chemotherapy, sargramostim is given daily during the period of maximum neutropenia until a
sustained rise in the granulocyte count is observed. Frequent blood counts are essential to avoid an
excessive rise in the granulocyte count. The dose may be increased if the patient fails to respond
after 7 to 14 days of therapy. However, higher doses are associated with more pronounced side
effects, including bone pain, malaise, flulike symptoms, fever, diarrhea, dyspnea, and rash. Patients
can be extremely sensitive to GM-CSF, demonstrating an acute reaction to the first dose,
characterized by flushing, hypotension, nausea, vomiting, and dyspnea, with a fall in arterial oxygen
saturation due to sequestration of granulocytes in the pulmonary circulation. With prolonged
administration, a few patients may develop a capillary leak syndrome, with peripheral edema and
both pleural and pericardial effusions.
Granulocyte Colony-Stimulating Factor (G-CSF)
Recombinant human G-CSF (filgrastim, NEUPOGEN) is a 175–amino acid glycoprotein produced in

Escherichia coli. Unlike natural G-CSF, it is not glycosylated and carries an extra N-terminal
methionine. The principal action of filgrastim is the stimulation of CFU-G to increase neutrophil
production (Figure 54–1). It also enhances the phagocytic and cytotoxic functions of neutrophils.
Filgrastim has been shown to be effective in the treatment of severe neutropenia following
autologous bone marrow transplantation and high-dose chemotherapy (Lieschke and Burgess,
1992). Like GM-CSF, filgrastim shortens the period of severe neutropenia and reduces morbidity
secondary to bacterial and fungal infections. When used as a part of an intensive chemotherapy
regimen, it can decrease the frequency of both hospitalization for febrile neutropenia and
interruptions in the chemotherapy protocol. G-CSF also has proven to be effective in the treatment
of severe congenital neutropenias. In patients with cyclic neutropenia, G-CSF therapy, while not
eliminating the neutropenic cycle, will increase the level of neutrophils and shorten the length of the
cycle sufficiently to prevent recurrent bacterial infections (Hammond et al. , 1989 ). Filgrastim
therapy can improve neutrophil counts in some patients with myelodysplasia or marrow damage
(moderately severe aplastic anemia or tumor infiltration of the marrow). The neutropenia of AIDS
patients receiving zidovudine also can be partially or completely reversed. Filgrastim is now
routinely used in the patient undergoing peripheral blood stem cell (PBSC) collection and a stem
cell transplant. It encourages the release of CD34+ progenitor cells from the marrow, reducing the
number of collections necessary for transplant. Moreover, filgrastim-mobilized PBSCs appear more
capable of rapid engraftment. PBSC-transplanted patients require fewer days of platelet and red
blood cell transfusions and a shorter duration of hospitalization than do patients receiving
autologous bone marrow transplants.
Filgrastim is administered by subcutaneous injection or intravenous infusion over at least 30
minutes at a dose of 1 to 20 g/kg per day. A usual starting dose in a patient receiving
myelosuppressive chemotherapy is 5 g/kg per day. The distribution and clearance rate from
plasma (half-life of 3.5 hours) are similar for both routes of administration. A continuous 24-hour
intravenous infusion can be used to produce a steady-state serum concentration of the growth factor.
As with GM-CSF therapy, filgrastim given daily following bone marrow transplantation or
intensive chemotherapy will increase granulocyte production and shorten the period of severe
neutropenia. Frequent blood counts should be obtained to determine the effectiveness of the
treatment. The dosage may need to be adjusted according to the granulocyte response, and the

duration of therapy will depend on the specific application. In marrow transplantation and intensive
chemotherapy patients, continuous daily administration for 14 to 21 days or longer may be
necessary to correct the neutropenia. With less intensive chemotherapy, fewer than 7 days of
treatment may be needed. In AIDS patients on zidovudine or patients with cyclic neutropenia,
chronic G-CSF therapy often will be required.
Adverse reactions to filgrastim include mild to moderate bone pain in those patients receiving high
doses over a protracted period, local skin reactions following subcutaneous injection, and, rarely, a
cutaneous necrotizing vasculitis. Patients with a history of hypersensitivity to proteins produced by
E. coli should not receive the drug. Marked granulocytosis, with counts greater than 100,000/ l, can
occur in patients receiving filgrastim over a prolonged period of time. However, this is not
associated with any reported clinical morbidity or mortality and rapidly resolves once therapy is
discontinued. Mild to moderate splenomegaly has been observed in patients on long-term therapy.
The therapeutic roles of other growth factors still need to be defined. M-CSF may play a role in
stimulating monocyte and macrophage production, though with significant side effects, including
splenomegaly and thrombocytopenia. Because of their primary effect on primitive marrow
precursors, IL-3 and FL may be used in combination with GM-CSF and G-CSF. Administration of
IL-3 followed by GM-CSF has been shown to give a greater neutrophil response than GM-CSF
alone (Ganser et al. , 1992 ). This combination also may be more effective in promoting the release
of marrow CD34+ stem cells in patients undergoing stem cell pheresis. SCF, IL-1, IL-6, IL-9, and
IL-11 need to be studied alone and in combination with each other, as well as with both GM-CSF
and G-CSF. The combination of IL-3 followed by GM-CSF also needs to be studied in protocols
that include the reinfusion of harvested stem cells for their growth-promoting activity.
Thrombopoietin
The cloning and expression of a recombinant human thrombopoietin, a cytokine that selectively
stimulates megakaryocytopoiesis, is another major milestone in the development of hematopoietic
growth factors as therapeutic agents (Lok et al. , 1994 ; de Sauvage et al. , 1994 ; Kaushansky et al. ,
1994). If future clinical trials live up to the early promise of the demonstrated ability of this new
cytokine to increase rapidly the platelet count in animals (Harker, 1999), the combined use of
thrombopoietin with G-CSF or GM-CSF together with erythropoietin will have a great impact in the
treatment of primary hematological diseases and the anemia, neutropenia, and thrombocytopenia

associated with high-dose chemotherapy. In a study of a small number of patients with
gynecological cancers receiving carboplatin (Vadhan-Raj et al. , 2000 ), recombinant human
thrombopoietin (rHuTPO) therapy reduced the duration of severe thrombocytopenia as well as the
need for platelet transfusions. Larger, randomized, controlled trials are now under way to define
fully the clinical merits and safety of rHuTPO. The optimal dose and schedule of administration in
various clinical settings also need to be worked out. Both rHuTPO and pegylated recombinant
human megakaryocyte growth and development factor (PEG-rHyMGDF) give delayed platelet
responses. Following a single bolus injection, platelet counts show a detectable increase by day 4
and a peak response by 12 to 14 days. The platelet count then returns to normal over the next 4
weeks. The peak platelet response follows a log-linear dose response. Platelet activation and
aggregation are not affected, and patients are not at increased risk of thromboembolic disease,
unless the platelet count is allowed to rise to very high levels. These kinetics need to be taken into
account when planning therapy in a chemotherapy patient.
Drugs Effective in Iron Deficiency and Other Hypochromic Anemias
Iron and Iron Salts
Iron deficiency is the most common cause of nutritional anemia in human beings. 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. However, 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 well reflect a reduction in the activity of iron-
dependent mitochondrial enzymes. Iron deficiency also has been associated with behavioral and
learning problems in children and with abnormalities in catecholamine metabolism and, possibly,
heat production (Pollit and Leibel, 1982; Martinez-Torres et al. , 1984 ). Awareness of the ubiquitous
role of iron has stimulated considerable interest in the early and accurate detection of iron
deficiency and in its prevention.
History
Iron has been used in the treatment of illness since the Middle Ages and the Renaissance. However,

it was not until the sixteenth century that iron deficiency was recognized as the cause of "green
sickness," or chlorosis, in adolescent women. Sydenham subsequently proposed iron as a preferred
therapy over bleedings and purgings, and in 1832, the French physician Pierre Blaud recognized the
need to use adequate doses of iron to successfully treat chlorosis. Blaud's nephew later distributed
the "veritable pills of Blaud" throughout the world. The treatment of anemia with iron followed the
principles enunciated by Sydenham and Blaud until the end of the nineteenth century. At that time
the teachings of Bunge, Quincke, von Noorden, and others cast doubt on their treatment of
chlorosis. The dose of iron employed was reduced, and the resulting lack of efficacy brought
discredit on the therapy. It was not until the third and fourth decades of the twentieth century that
the lessons taught by the earlier physicians were relearned.
The modern understanding of iron metabolism began in 1937 with the work of McCance and
Widdowson on iron absorption and excretion and Heilmeyer and Plotner's measurement of iron in
plasma. Then in 1947, Laurell described a plasma iron transport protein that he called transferrin.
Hahn and coworkers (1943) were the first to use radioactive isotopes to quantitate iron absorption
and define the role of the intestinal mucosa to regulate this function. In the next decade, Huff and
associates (1950) initiated isotopic studies of internal iron metabolism. The subsequent
development of practical clinical measurements of serum iron, transferrin saturation, plasma
ferritin, and red cell protoporphyrin permitted the definition and detection of the body's iron store
status and iron-deficient erythropoiesis.
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 it is solubilized by acid or chelating agents. For example, to
meet their needs, 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.
Human beings, 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. From a quantitative standpoint, hemoglobin dominates the essential

fraction (Table 54–2). This protein, with a molecular weight of 64,500 daltons, contains four atoms
of iron per molecule, amounting to 1.1 mg of iron per milliliter of red blood cells (20 mM). Other
forms of essential iron include myoglobin and a variety of heme and nonheme iron-dependent
enzymes. Ferritin is a protein-iron storage complex, which exists as individual molecules or in an
aggregated form. Apoferritin has a molecular weight of about 450,000 daltons and is composed of
24 polypeptide subunits; these form an outer shell within which resides a storage cavity for
polynuclear hydrous ferric oxide phosphate. Over 30% of the weight of ferritin may be iron (4000
atoms of iron per ferritin molecule). Aggregated ferritin, referred to as hemosiderin and visible by
light microscopy, constitutes 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 (Bothwell et al. , 1979 ).
Internal exchange of iron is accomplished by the plasma protein transferrin (Aisen and Brown,
1977). This
1
-glycoprotein has a molecular weight of about 76,000 daltons and 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 a
pH-dependent fashion in an 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 ).
Human 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. 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 stores. Isolation of the genes for the human transferrin receptor and ferritin has permitted a
better definition of the molecular basis of this regulation. Apoferritin synthesis is regulated by a
system of cytoplasmic binding proteins (IRP-1 and -2) and an iron-regulating element on mRNA
(IRE). When iron is in short supply, IRP binds to mRNA IRE and inhibits the translation of
apoferritin. 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 the reticuloendothelial cell (Figure 54–3). 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 reticuloendothelium. 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 the reticuloendothelial cell and is 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. When there are abnormalities in maturation of red
cells, the predominant portion of iron assimilated by the erythroid marrow may be rapidly localized
in the reticuloendothelial cell as defective red cell precursors are broken down; this is termed
ineffective erythropoiesis. With red cell aplasia, the rate of turnover of iron in plasma may be
reduced by one-half or more, with all the iron now going to the hepatocyte for storage.

Figure 54–3. Pathways of Iron Metabolism in Human Beings (Excretion
Omitted).
The most remarkable feature of iron metabolism is the degree to which the body store is 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 normal menstruating women lose over 2 mg per day. Pregnancy
imposes a requirement for iron of even greater magnitude (Table 54–3). Other causes of iron loss
include the donation of blood, the use of antiinflammatory drugs that cause bleeding from the
gastric mucosa, and gastrointestinal disease with associated bleeding. Much rarer are the
hemosiderinuria that follows intravascular hemolysis and pulmonary siderosis, wherein iron is

deposited in the lungs and becomes unavailable to the rest of the body.
The limited physiological losses of iron point to the primary importance of absorption as the
determinant of the body's iron content. Unfortunately, the biochemical nature of the absorptive
process is understood only in general terms. After acidification and partial digestion of food in the
stomach, its content of 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 separate transporters: DCT1, which controls uptake from the
intestinal lumen, and a second transporter, which 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 are both determined by the HFE gene, a novel MHC
class 1 molecule localized to chromosome 6 (Peters et al. , 1993 ). 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. Normal absorption is only about 1 mg per day in
the adult man and 1.4 mg per day in the adult woman, and 3 to 4 mg of dietary iron is the most that
can be absorbed under normal conditions. Increased iron absorption is seen whenever iron stores are
depleted or when erythropoiesis is increased and ineffective. Patients with hereditary
hemochromatosis, secondary to a defective HFE gene, also demonstrate increased iron absorption,
as well as loss of the normal regulation of iron delivery to transferrin by reticuloendothelial cells.
The resulting increased saturation of transferrin opens the door to 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, the adult man has a requirement of only 13 g/kg per day (about 1 mg), whereas the
menstruating woman requires 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 the infant has similar
requirements due to its rapid growth. These requirements (Table 54–4) 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 the adult male of between 12 and 20 mg and for the adult

female of between 8 and 15 mg. Foods high in iron (greater than 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 (less than 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 is obviously important, of greater nutritional significance is the
bioavailability of iron in food (Hallberg, 1981). Heme iron is far more available, and its absorption
is independent of the composition of the diet. Heme iron, which constitutes only 6% of dietary iron,
represents 30% of iron absorbed. Nevertheless, it is the availability of the nonheme fraction that
deserves the greatest attention, since it represents by far the largest amount of dietary iron that is
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
(Layrisse and Martinez-Torres, 1971). Two substances are known to facilitate the absorption of
nonheme iron—ascorbic acid and meat. Ascorbate forms complexes with and/or reduces ferric to
ferrous iron. While meat facilitates the absorption of iron by stimulating production of gastric acid,
it is possible that some other effect, not yet identified, also is involved. Either of these substances
can increase availability severalfold. Thus, assessments of available dietary iron should include not
only the amount of iron ingested but also an estimate of its availability based on the intake of
substances that enhance or inhibit its absorption and iron stores (Figure 54–4; Monsen et al. , 1978 ).

Figure 54–4. Effect of Iron Status on the Absorption of Nonheme Iron in
Food. The percentages of iron absorbed from diets of low, medium, and high
bioavailability in individuals with iron stores of 0, 250, 500, and 1000 mg are
portrayed. (After Monsen et al. , 1978 . ©American Journal of Clinical Nutrition.
Courtesy of American Society for Clinical Nutrition. With permission.)
A comparison of iron requirements with available dietary iron is made in Table 54–4. Obviously,
pregnancy and infancy represent periods of negative balance. The menstruating woman also is at
risk, whereas iron balance in the adult man and nonmenstruating woman is reasonably secure. The
difference between dietary supply and requirements is reflected in the size of iron stores. These will
be low or absent when iron balance is precarious and high when iron balance is favorable (seeTable

53–2). Thus, in the infant after the third month of life and in the pregnant woman after the first
trimester, stores of iron are negligible. Menstruating women have approximately one-third the
stored iron found in the adult man, indicative of the extent to which the additional average daily loss
of about 0.5 mg of iron affects iron balance.
Iron Deficiency
The prevalence of iron-deficiency anemia depends on the economic status of the population and on
the methods used for evaluation. In developing countries, as many as 20% to 40% of infants and
pregnant women may be affected (WHO Joint Meeting, 1975), while studies in the United States
suggest that the prevalence of iron-deficiency anemia in adult men and women is as low as 0.2% to
3% (Cook et al. , 1986 ). Better iron balance has been achieved by 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 a dietary intake of iron that is inadequate to meet normal
requirements (nutritional iron deficiency), blood loss, or some interference with iron absorption.
Most nutritional iron deficiency in the United States is mild. Moderate-to-severe iron deficiency is
usually the result of blood loss, either from the gastrointestinal tract or, in the woman, from the
uterus. Impaired absorption of iron from food results most often from partial gastrectomy or
malabsorption in the small intestine.
Iron deficiency in infants and young children can lead to behavioral disturbances and
developmental delays. Chronic developmental defects 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 increases 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). 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 54–

5). In adults, depletion of iron stores may be recognized by a plasma ferritin of less than 12 g per
liter and the absence of reticuloendothelial hemosiderin in the marrow aspirate. Iron-deficient
erythropoiesis, defined as a suboptimal supply of iron to the erythron, 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 (Cook et al. , 1976 ). Moreover, "normal" hemoglobin and iron values in infancy and
childhood are different because of the more restricted supply of iron in young children (Dallman et
al. , 1980 ).

Figure 54–5. Sequential Changes (from Left to Right) in the Development of Iron
Deficiency in the Adult. Rectangles enclose abnormal test results. RE marrow Fe,
reticuloendothelial hemosiderin; RBC, red blood cells. (After Hillman and Finch,
1997, as modified from Bothwell and Finch, 1962. Courtesy of F.A. Davis Co.
With permission.)
The importance of mild iron deficiency lies more in identifying the underlying cause of the
deficiency than in any symptoms related to the deficient state. Because of the frequency of iron
deficiency in infancy and in the menstruating or pregnant woman, the need for exhaustive
evaluation of such individuals usually is determined by the severity of the anemia. However, iron
deficiency in the man or postmenopausal woman necessitates a search for a site of bleeding.
Although the presence of microcytic anemia is the most commonly recognized indicator of iron
deficiency, laboratory tests—such as quantitation of transferrin saturation, red cell protoporphyrin,
and plasma ferritin—are 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 is nonetheless limiting erythropoiesis. More
difficult is the differentiation of true iron deficiency from iron-deficient erythropoiesis due to
inflammation. In the latter condition, the stores of iron are actually 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 concentration of ferritin in plasma (Lipschitz et al. , 1974 ).
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 can be best measured from 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. Studies by Hillman and Henderson (1969)
demonstrated the importance of iron supply in governing erythropoiesis. Using phlebotomy to
induce a moderately severe anemia (hemoglobin 7 to 10 g/dl), erythropoiesis was reduced to less
than one-third of normal when the serum iron fell below 70 g/dl. In contrast, red cell production
levels increased to more than three times the basal rate when the serum iron was maintained
between 75 and 150 g/dl. Even higher levels of production were observed in patients with
hemolytic anemias or ineffective erythropoiesis.
As regards oral iron therapy, the ability of the patient to tolerate and absorb medicinal iron is a very
important factor in determining the rate of response. There are clear limits to the gastrointestinal
tolerance for iron. The small intestine regulates absorption and, in the face of increasing doses of
oral iron, limits the entry of iron into the bloodstream. Therefore, there is 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. Intrinsic disease of the marrow can, by decreasing the number of red cell precursors, 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. Since it takes time for the marrow to proliferate,
an increase in the reticulocyte count is not observed for 4 to 7 days or more 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. It also assumes that the patient has not been transfused during this time.
If the response to oral iron is inadequate, the diagnosis must be reconsidered. A full laboratory
evaluation should be carried out, and such factors as the presence of a concurrent inflammatory
disease or poor compliance by the patient must be assessed. 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 establish 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, the least expensive of iron preparations, is the treatment of
choice for iron deficiency (Callender, 1974; Bothwell et al. , 1979 ). Ferrous salts are absorbed about
three times as well as ferric salts, and the discrepancy becomes even greater at high dosage (Brise
and Hallberg, 1962). 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 (iron sulfate;FEOSOL, others) is the hydrated salt, FeSO

4
·7H
2
O, which contains 20%
iron. Dried ferrous sulfate (32% elemental iron) also is 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 (Cook
et al. , 1973 ), 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 (Viteri et al. , 1978 ).
The amount of iron, rather than the mass of the total salt in iron tablets, is important. It is also
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 different 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. One of the more
popular of these is ascorbic acid. 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 (Hallberg et al. , 1966 );
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 B
12
, folate, or cobalt, since the patient's response to
the combination cannot be easily 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 iron—for
example, 5 mg/kg. The dose used is a practical compromise between the therapeutic action desired
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 54–5.
However, 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 the
absorption of iron 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. The timing of the dose is important.
Sustained high rates of red cell production require an uninterrupted supply of iron. 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 is usually 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 is a different matter, requiring 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 of the individual.
The person with an inadequate diet may require continued therapy with low doses of iron. The
individual whose bleeding has stopped will require no further therapy after the hemoglobin has
returned to normal. For the individual with continued bleeding, long-term, high-dose therapy is
clearly indicated.
Untoward Effects of Oral Preparations of Iron
Intolerance to oral preparations of iron is primarily a function of the amount of soluble iron in the
upper gastrointestinal tract and of psychological factors. Side effects include heartburn, nausea,
upper gastric discomfort, constipation, and diarrhea. A good policy, particularly if there has been
previous intolerance to iron, 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
individuals, compared with an incidence of 13% among those receiving placebos; this increases to
approximately 40% when the dosage of iron is doubled. Nausea and upper abdominal pain are
increasingly common manifestations 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.
Toxicity caused by the long-continued administration of iron, with the resultant production of iron
overload (hemochromatosis), has been the subject of a number of case reports (for example,
seeBothwell et al. , 1979 ). Available evidence suggests that the normal individual 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 hemochromatosis. However,
recent data indicate that hemochromatosis may be a relatively common genetic disorder, present in
0.5% of the population.
Iron Poisoning
Large amounts of ferrous salts of iron are toxic but, in adults, fatalities are rare. Most deaths occur
in childhood, particularly between the ages of 12 and 24 months (Bothwell et al. , 1979 ). As little as

1 to 2 g of iron may cause death, but 2 to 10 g is usually 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, therefore, be kept in childproof bottles.
Signs and symptoms of severe poisoning may occur within 30 minutes or may be delayed for
several hours after ingestion. They consist largely of 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 the child who is 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 M (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 radioopaque). Iron in the
upper gastrointestinal 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 M; 3.5 mg per liter), deferoxamine should be
administered; dosage and routes of administration are detailed in Chapter 67: Heavy Metals and
Heavy-Metal Antagonists. 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
(Bothwell et al. , 1979 ). The rate of response to parenteral therapy is similar to that which follows
usual oral doses (Pritchard, 1966). Predictable indications are iron malabsorption (sprue, short
bowel, etc.), severe oral iron intolerance, as a routine supplement to total parenteral nutrition, and in
patients with renal disease 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 from 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 (Pritchard, 1966). 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 (Henderson and Hillman, 1969). Subsequently, however, the response is no better than that
seen with oral iron. This reflects the relative availability of the iron dextran stored in the
reticuloendothelial system. Furthermore, inflammatory cytokines suppress both sources of iron
supply equally, canceling any advantage.
Iron dextran injection (INFED, DEXFERRUM) is the parenteral preparation currently in general use in
the United States. It is a colloidal solution of ferric oxyhydroxide complexed with polymerized
dextran (molecular weight approximately 180,000), resulting in a dark brown, viscous liquid,
containing 50 mg/ml of elemental iron. It can be administered by either intramuscular or
intravenous injection. When given by deep intramuscular injection, it is gradually mobilized via the
lymphatics and transported to reticuloendothelial cells; the iron is then released from the dextran
complex. A variable portion (10% to 50%) may become locally fixed in the muscle for several
weeks or months, especially if there is a local inflammatory reaction. Intravenous administration
gives a more reliable response and is preferred. 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 level for 1 to 2 weeks.
Once the iron is released from the dextran within the reticuloendothelial cell, it is either
incorporated into stores or transported via transferrin to the erythroid marrow. The rate of release is
variable. While a portion of the processed iron is rapidly made available to the marrow, a significant
fraction is only gradually converted to usuable iron stores (Henderson and Hillman, 1969). All of
the iron is eventually released (Kernoff et al. , 1975 ), although many months are required before the

process is complete. During this time, the appearance of visible 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 following a test dose of 0.5 ml (25
mg of iron). If no adverse reactions are observed, the injection can be given according to the
following schedule until the calculated total amount required has been reached. Each day's dose
should ordinarily not exceed 0.5 ml (25 mg of iron) for infants under 4.5 kg (10 lb), 1.0 ml (50 mg
of iron) for children under 9.0 kg (20 lb), and 2.0 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 prior to injection). However, local
reactions, including long-continued discomfort at the site of injection and local discoloration of the
skin, 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 dose injection also should precede intravenous administration of a therapeutic dose of iron
dextran. After establishing secure intravenous access, 0.5 ml of undiluted iron dextran or an
equivalent amount (25 mg of iron) diluted in saline should be administered. The patient should be
observed during the injection for signs of immediate anaphylaxis, and for an hour following
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
employed, a test dose injection should be given prior to each infusion, since hypersensitivity can
appear at any time. Furthermore, the patient should be closely monitored 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, 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. Factors to be taken into account are the hemoglobin deficit, the

need to reconstitute iron stores, and continued excess losses of iron, as seen with hemodialysis and
chronic gastrointestinal bleeding. A manufacturer-recommended formula for the calculation of the
total treatment dose for an iron deficient anemia patient is as follows:
[0.0476 x lean body weight in kg x hemoglobin deficit]+ 1 ml per 5 kg body weight (to maximum of
14 ml to reconstitute iron stores) = total dose in ml of iron dextran solution.
An alternative formula, which calculates the total dose in mg of iron, is as follows:
[0.66 x lean body weight in kg]x[100 – (patient's hemoglobin in g/dl x 100/14.8)]= Total dose in mg
of iron
Iron dextran solution (50 mg/ml of elemental iron) can then be administered undiluted in daily
doses of 2.0 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 is generally
insufficient to guarantee an optimal hemoglobin response. It is recommended, therefore, that
sufficient parenteral iron be given to maintain a plasma ferritin level between 100 and 800 g/l and
a percent saturation of transferrin between 20% and 50%. 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
dextran—especially multiple total-dose infusions, as sometimes used in the treatment of chronic
gastrointestinal blood loss—accumulations 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. Whether this is of any clinical importance is uncertain. While
disease-related iron overload (hemochromatosis) has been associated with an increased risk of both
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.
Sodium ferric gluconate complex in sucrose (FERRLECIT) was approved by the FDA for use in the
treatment of iron deficiency in patients undergoing chronic hemodialysis who are receiving
supplemental erythropoietin therapy.
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, however, is the rare
anaphylactic reaction, which may be fatal in spite of treatment. While only a few such deaths have
been reported, it remains a deterrent to the use of iron dextran. Thus, there must be specific
indications for the parenteral administration of iron.
Copper
Copper deficiency is extremely rare in human beings (Evans, 1973). The amount present in food is
more than adequate to provide the needed body complement of slightly more than 100 mg. There is
no evidence that copper ever needs to be added to a normal diet, either prophylactically or
therapeutically. Even in clinical states associated with hypocupremia (sprue, celiac disease,
nephrotic syndrome), effects of copper deficiency usually are not demonstrable. However, anemia
due to copper deficiency has been described in individuals who have undergone intestinal bypass
surgery (Zidar et al. , 1977 ), in those who are receiving parenteral nutrition (Dunlap et al. , 1974 ), in
malnourished infants (Holtzman et al. , 1970 ; Graham and Cordano, 1976), and in patients ingesting
excessive amounts of zinc (Hoffman et al. , 1988 ). While an inherited disorder affecting the
transport of copper in human beings (Menkes' disease; steely hair syndrome) is associated with
reduced activity of several copper-dependent enzymes, this disease is not associated with
hematological abnormalities.
Copper deficiency in experimental animals interferes with the absorption of iron and its release
from reticuloendothelial cells (Lee et al. , 1976 ). The associated microcytic anemia is related both to
a decrease in the availability of iron to the normoblasts and, perhaps even more importantly, to a
decreased mitochondrial production of heme. It may be that the specific defect in the latter case is a
decrease in the activity of cytochrome oxidase. Other pathological effects involving the skeletal,
cardiovascular, and nervous systems have been observed in copper-deficient experimental animals
(O'Dell, 1976). In human beings, the outstanding findings have been leukopenia, particularly
granulocytopenia, and anemia. Concentrations of iron in plasma are variable, and the anemia is not
always microcytic. When a low plasma copper concentration is determined in the presence of
leukopenia and anemia and in a setting conducive to a deficiency of the element, a therapeutic trial
with copper is appropriate. Daily doses up to 0.1 mg/kg of cupric sulfate have been given by mouth,

or 1 to 2 mg per day may be added to the solution of nutrients for parenteral administration. Copper
deficiency usually occurs concurrently with multiple nutritional deficiencies, so that its specific role
in the production of anemia may be difficult to ascertain.
Pyridoxine
The first case of pyridoxine-responsive anemia was described in 1956 by Harris and associates.
Subsequent reports suggested that the vitamin might improve hematopoiesis in up to 50% of
patients with either hereditary or acquired sideroblastic anemias (Horrigan and Harris, 1968).
Characteristically, these patients show an impairment in hemoglobin synthesis and an accumulation
of iron in the perinuclear mitochondria of erythroid precursor cells, so-called ringed sideroblasts.
Hereditary sideroblastic anemia is an X-linked recessive trait with variable penetrance and
expression. Affected men typically show a dual population of normal red cells and microcytic,
hypochromic cells in the circulation. In contrast, idiopathic acquired sideroblastic anemia and the
sideroblastosis seen in association with a number of drugs, inflammatory states, neoplastic
disorders, and preleukemic syndromes show a variable morphological picture. Moreover,
erythrokinetic studies demonstrate a spectrum of abnormalities, from a hypoproliferative defect
with little tendency to accumulate iron to marked ineffective erythropoiesis with iron overload of
the tissues (Solomon and Hillman, 1979a).
Oral therapy with pyridoxine is of proven benefit in correcting the sideroblastic anemias associated
with the antituberculosis drugs isoniazid and pyrazinamide, which act as vitamin B
6
antagonists. A
daily dose of 50 mg of pyridoxine completely corrects the defect without interfering with treatment,
and routine supplementation of pyridoxine is often recommended (seeChapter 48: Antimicrobial
Agents: Drugs Used in the Chemotherapy of Tuberculosis, Mycobacterium avium Complex
Disease, and Leprosy). In contrast, if pyridoxine is given to counteract the sideroblastic abnormality
associated with administration of levodopa, the effectiveness of levodopa in controlling Parkinson's
disease is decreased. Pyridoxine therapy does not correct the sideroblastic abnormalities produced
by chloramphenicol and lead.
Patients with idiopathic acquired sideroblastic anemia generally fail to respond to oral pyridoxine,
and those individuals who appear to have a pyridoxine-responsive anemia require prolonged

therapy with large doses of the vitamin, 50 to 500 mg per day. Unfortunately, the early enthusiasm
for treatment with pyridoxine was not reinforced by results of later studies (Chillar et al. , 1976 ;
Solomon and Hillman, 1979a). Moreover, even when a patient with sideroblastic anemia responds,
the improvement is only partial, since both the ringed sideroblasts and the red cell defect persist,
and the hematocrit rarely returns to normal. However, in view of the low toxicity of oral pyridoxine,
a therapeutic trial with pyridoxine is appropriate.
As shown in studies of normal subjects, oral pyridoxine in a dosage of 100 mg three times daily
produces a maximal increase in red cell pyridoxine kinase and the major pyridoxal phosphate–
dependent enzyme glutamic-aspartic aminotransferase (Solomon and Hillman, 1978). For an
adequate therapeutic trial, the drug must be administered for at least 3 months while the response is
monitored by measuring the reticulocyte index and the concentration of hemoglobin. It has been
suggested that the occasional patient who is refractory to oral pyridoxine may respond to parenteral
administration of pyridoxal phosphate (Hines and Love, 1975). However, oral pyridoxine in doses
of 200 to 300 mg per day produces intracellular concentrations of pyridoxal phosphate equal to or
greater than those generated by therapy with the phosphorylated vitamin (Solomon and Hillman,
1979b). Pyridoxine is discussed further in Chapter 63: Water-Soluble Vitamins: The Vitamin B
Complex and Ascorbic Acid.
Riboflavin
A pure red cell aplasia that responds to the administration of riboflavin was reported in patients with
protein depletion and complicating infections (Foy et al. , 1961 ). Lane and associates (1964)
induced riboflavin deficiency in human beings and demonstrated that a hypoproliferative anemia
resulted within a month. The spontaneous appearance in human beings of red cell aplasia due to
riboflavin deficiency is undoubtedly rare, if it occurs at all. It has been described in combination
with infection and protein deficiency, both of which are capable of producing a hypoproliferative
anemia. However, it seems reasonable to include riboflavin in the nutritional management of
patients with gross, generalized malnutrition. Riboflavin is discussed further in Chapter 63: Water-
Soluble Vitamins: The Vitamin B Complex and Ascorbic Acid.
Vitamin B
12
, Folic Acid, and the Treatment of Megaloblastic Anemias

Vitamin B
12
and folic acid are dietary essentials. A deficiency of either vitamin results in defective
synthesis of DNA 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 a
megaloblastic anemia. Abnormal macrocytic red blood cells are produced, and the patient becomes
severely anemic. Recognition of this pattern of abnormal hematopoiesis—more than 100 years ago
—permitted the initial diagnostic classification of such patients as having "pernicious anemia" and
spurred investigations that subsequently led to the discovery of vitamin B
12
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.
History
The discovery of vitamin B
12
and folic acid is a dramatic story that starts more than 170 years ago
and includes two Nobel prize–winning discoveries. The first descriptions of what must have been
megaloblastic anemias came from the work of Combe and Addison, who published a series of case
reports beginning in 1824. It is still common practice to describe megaloblastic anemia as
Addisonian pernicious anemia. Although Combe suggested that the disorder might have some
relationship to digestion, it was Austin Flint who, in 1860, first described the severe gastric atrophy
and called attention to its possible relationship to the anemia. The name progressive pernicious
anemia was coined in 1872 by Biermer, and this colorful term has persisted.
Following the observation by Whipple in 1925 that liver is a source of a potent hematopoietic
substance for iron-deficient dogs, Minot and Murphy carried out their Nobel Prize–winning
experiments that demonstrated the effectiveness of the feeding of liver to reverse pernicious anemia.
Within a few years, Castle defined the need for both intrinsic factor, a substance secreted by the
parietal cells of the gastric mucosa, and extrinsic factor, the vitamin-like material provided by crude

liver extracts. However, nearly 20 years passed before Rickes and coworkers and Smith and Parker
isolated and crystallized vitamin B
12
; Dorothy Hodgkin then determined its crystal structure by X-
ray diffraction and subsequently received the Nobel prize for this work.
As attempts were being made to purify extrinsic factor, Wills and her associates described a
macrocytic anemia in women in India that responded to a factor present in crude liver extracts but
not in the purified fractions known to be effective in pernicious anemia (Wills et al. , 1937 ). This
factor, first called Wills' factor and later vitamin M, is now known to be folic acid. The actual term
folic acid was coined by Mitchell and coworkers in 1941, following its isolation from leafy
vegetables.
More recent work has shown that neither vitamin B
12
nor folic acid as purified from foodstuffs is the
active coenzyme for human beings. During extraction procedures, active, labile forms are converted
to stable congeners of vitamin B
12
and folic acid, cyanocobalamin and pteroylglutamic acid,
respectively. These congeners must then be modified in vivo to be effective. While a great deal has
been learned about the intracellular metabolic pathways in which these vitamins function as
required cofactors, many questions remain. The most important of these is the relationship of
vitamin B
12
deficiency to the neurological abnormalities that occur with this disorder (Chanarin et
al. , 1985 ).
Relationships between Vitamin B
12
and Folic Acid
The major roles of vitamin B
12

and folic acid in intracellular metabolism are summarized in Figure
54–6. Intracellular vitamin B
12
is maintained as two active coenzymes: methylcobalamin and
deoxyadenosylcobalamin (Linnell et al. , 1971 ). Deoxyadenosylcobalamin (deoxyadenosyl B
12
) is a
cofactor for the mitochondrial mutase enzyme that catalyzes the isomerization of L-methylmalonyl
CoA to succinyl CoA, an important reaction in both carbohydrate and lipid metabolism (Weissbach
and Taylor, 1968). This reaction has no direct relationship to the metabolic pathways that involve
folate. In contrast, methylcobalamin (CH
3
B
12
) supports the methionine synthetase reaction, which is
essential for normal metabolism of folate (Weir and Scott, 1983). Methyl groups contributed by
methyltetrahydrofolate (CH
3
H
4
PteGlu
1
) 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.

Figure 54–6. Interrelationships and Metabolic Roles of Vitamin B
12
and Folic
Acid. See text for explanation and Figure 54–9 for structures of the various folate
coenzymes. FIGLU is formiminoglutamic acid, which arises from the catabolism
of histidine. TcII is transcobalamin II.
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 (H
4
PteGlu
1
), the