Clinical Pharmacology and Therapeutics
A Textbook of Clinical Pharmacology and ... - clinicalevidence
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392 ANAEMIA AND OTHER HAEMATOLOGICAL DISORDERS<br />
glycoprotein of molecular weight approximately 55 kDa which<br />
forms a stable complex with vitamin B 12 . The complex passes<br />
down the small intestine <strong>and</strong> binds to specific receptors on the<br />
mucosa of the terminal ileum (at neutral pH <strong>and</strong> in the presence<br />
of calcium) <strong>and</strong> is actively absorbed. Once in the circulation,<br />
vitamin B 12 is transported by the beta globulin transcobalamin<br />
II (TC II) to tissues, with its primary storage site being the<br />
liver (90% body stores). Vitamin B 12 complexed with other<br />
transcobalamins (TCI <strong>and</strong> III, which are alpha-globulins) probably<br />
represent the major intracellular storage form. The normal<br />
range of plasma vitamin B 12 concentration is 170–900 ng/L<br />
(150–660 pmol/L). Vitamin B 12 is secreted into the bile,<br />
but enterohepatic circulation results in most of this being<br />
reabsorbed via the intrinsic factor mechanism.<br />
FOLIC ACID<br />
Uses<br />
Folic acid is given to correct or prevent deficiency states <strong>and</strong><br />
prophylactically during pregnancy. It consists of a pteridine<br />
ring linked to glutamic acid via p-aminobenzoic acid (PABA).<br />
The richest dietary sources are liver, yeast <strong>and</strong> green vegetables.<br />
Folate deficiency may be due to:<br />
• poor nutrition – in children, the elderly or those with<br />
alcoholism;<br />
• malabsorption – caused by coeliac disease, sprue or<br />
diseases of the small intestine;<br />
• excessive utilization – in pregnancy, chronic haemolytic<br />
anaemias (e.g. sickle cell disease) <strong>and</strong> leukaemias;<br />
• anti-epileptic drugs (e.g. phenytoin).<br />
The normal requirement for folic acid is about 200 μg daily.<br />
In established folate deficiency, large doses (5–15 mg orally per<br />
day) are given. If the patient is unable to take folate by mouth, it<br />
may be given intravenously. Patients with severe malabsorption<br />
may be deficient in both folic acid <strong>and</strong> vitamin B 12 , <strong>and</strong> administration<br />
of folic acid alone may precipitate acute vitamin B 12 deficiency.<br />
Such patients require replacement of both vitamins<br />
concurrently. Many patients on chronic anticonvulsant therapy<br />
develop macrocytosis without frank folate deficiency. Treatment<br />
is by the addition of folic acid to the anticonvulsant regimen.<br />
Cellular mechanism of action<br />
Folic acid is required for normal erythropoiesis. Deficiency of<br />
folic acid results in a megaloblastic anaemia <strong>and</strong> abnormalities<br />
in other cell types. Folate acts as a methyl donor in biochemical<br />
reactions, including the methylation of deoxyuridylic acid to<br />
form thymidylic acid, as well as other reactions in purine <strong>and</strong><br />
pyrimidine synthesis.<br />
Pharmacokinetics<br />
Folate is present in food as reduced polyglutamates. These are<br />
hydrolysed to monoglutamate, reduced <strong>and</strong> methylated to<br />
methyltetrahydrofolate by the combined action of pteroylglutamyl<br />
carboxypeptidase <strong>and</strong> tetrahydrofolate reductase. This<br />
occurs in the proximal small intestine, the site of folate absorption<br />
into the portal blood. About one-third of total body folate<br />
(70 mg) is stored in the liver, representing only about four<br />
months supply. The normal range for serum folate concentration<br />
is 4–20 μg/L.<br />
IRON AND FOLIC ACID THERAPY IN PREGNANCY<br />
Pregnancy imposes a substantial increase in dem<strong>and</strong> on maternal<br />
stores of iron <strong>and</strong> folic acid. A pregnant woman during the<br />
last trimester therefore requires approximately 5 mg of iron<br />
daily. Most women are iron depleted by the end of the pregnancy<br />
if they do not receive supplements. Requirements for<br />
folic acid also increase by two- to three-fold during pregnancy.<br />
Folate deficiency is associated with prematurity, low birth<br />
weight for gestational age <strong>and</strong> neural-tube defects (Chapter 9).<br />
In the UK, the usual practice is to give iron <strong>and</strong> folic acid<br />
supplements throughout pregnancy. Folate supplementation<br />
should be also be given before conception to women who are<br />
attempting to become pregnant, in order to reduce the incidence<br />
of neural-tube defects. High-dose prophylaxis (folate,<br />
5 mg daily) is advised for women who have previously given<br />
birth to a child with a neural-tube defect.<br />
Key points<br />
Vitamin B 12 <strong>and</strong> folate therapy<br />
• Healthy subjects require 3–5 μg of vitamin B 12 <strong>and</strong><br />
200 μg of folate daily.<br />
• Body stores of vitamin B 12 are 3 mg; folate stores are<br />
approximately 200 mg.<br />
• Vitamin B 12 <strong>and</strong> folate are absorbed from the small<br />
intestine, <strong>and</strong> vitamin B 12 is specifically absorbed from<br />
the terminal ileum.<br />
• The most common cause of B 12 or folate deficiency is<br />
dietary or malabsorption, or due to gastric surgery.<br />
• Vitamin B 12 deficiency must not be inappropriately<br />
treated with folate alone, as any associated<br />
neurological damage may be irreversible.<br />
• Drugs may cause vitamin B 12 (e.g. metformin) or folate<br />
(e.g. phenytoin, other anti-epileptic drugs) deficiency.<br />
PYRIDOXINE, RIBOFLAVIN<br />
Sideroblastic anaemia with failure of incorporation of iron into<br />
haem in the mitochondria, may respond to long-term pyridoxine<br />
supplementation. Infrequently, red cell aplasia is due to<br />
riboflavin deficiency <strong>and</strong> will respond to supplementation<br />
with this vitamin (Chapter 35).<br />
HAEMATOPOIETIC GROWTH FACTORS<br />
Recombinant DNA technology has been used to synthesize<br />
several human haematopoietic growth factors (Figure 49.3<br />
shows an outline of haematopoiesis). Haematopoietic growth<br />
factors now have a clear role in the treatment of many forms of<br />
bone marrow dysfunction.<br />
RECOMBINANT HUMAN ERYTHROPOIETIN<br />
(ERYTHROPOIETIN AND DARBEPOETIN)<br />
Erythropoietin is secreted as a glycosylated protein with a<br />
mass of 34 kDa. About 90% of endogenous erythropoietin is