Transcript Phar 722 Pharmacy Practice III

Phar 722
Pharmacy Practice III
VitaminsFolic Acid
Spring 2006
Folic Acid Study Guide
• The applicable study guide items in the Vitamin
Introduction
• History
• Structure of the vitamin and conversion to the
cofactor forms
• Role of conjugase in processing the vitamin
• Function of the cofactors including the specific
types of reactions
• Deficiency conditions and how they may occur
• Importance in pregnancy
• Role in controlling excess blood homocysteine
levels and cardiovascular disease
• Drug interactions
• Dietary and commercial forms of the vitamin
What is Anemia?
• Definition
– Below normal levels of red blood cells or
hemoglobin, or both, which can be caused by
many different conditions.
• Symptoms may include fatigue, weakness,
headache, and dizziness.
• Appropriate blood tests will confirm the
diagnosis of anemia and shed light on its
etiology.
• Anemia is a symptom. Classification and
cause are essential for proper treatment.
Simplified Classification of Anemias-1
• Size of erythrocyte
– Normocytic
• Deficiency of normal sized erythrocytes
– Iron deficiency
– Hemorrhage
– Hemolysis (Hemolytic Anemia)
– Anemia of chronic disease (autoimmune diseases,
cancer, etc.)
– Microcytic
• Inability to make adequate amount of hemoglobin
leading to small sized erythrocytes
– Iron deficiency
Simplified Classification of Anemias-2
– Megaloblastic-Macrocytic
• Larger than normal sized erythrocytes
• May or may not include larger than normal
precursor cells (megaloblasts).
– Folic Acid
– Cobalamin (Cyanocobalamin)
Simplified Classification of Anemias-3
– Aplastic Anemia
• Decreased ability of the bone marrow to make
new erythrocytes.
• May include deficiency in platelets and immune
system cells.
– Toxins
– Drug therapy
– Radiation
– Defective Hemoglobins
• Sickling Disease (Sickle Cell Anemia)
• Thalessemias
Folic Acid History
• More patients are seen with folic acid deficiencies
than any other vitamins.
• 1931
– First recognized as a treatment for megaloblastic anemia.
– It was named by Professor Snell (U of Texas).
• Folic is derived from folium, the Latin word for leaf because it
was isolated in quantities sufficient enough to characterize
from four tons of spinach.
• 1993
– Initial recommendations that women contemplating
pregnancy should take 0.4 mg to 1 mg per day of folic acid
in order to prevent neural tube defects (NTDs) such as spina
bifida and anencephaly.
Folic Acid Chemistry and Nomenclature
• The synthetic vitamin is composed of the
heterocyclic pteridine ring linked to a paminobenzoic acid which, in turn, is connected to a
glutamic acid by a peptide-like bond.
– In this form it is called folic acid
• The natural vitamin found in foods is made up of a
family of polyglutamates all connected to the initial
glutamic acid at the γ-carboxyl group.
– From that point on the glutamic acids are linked by normal
peptide bonds between α-amino and α-carboxyl moieties.
– The length of this polyglutamate chain varies with the
source of the vitamin, but lengths of 3, 5, and 7 amino acids
are seen.
– The various forms found in food are called folate or folates.
Folic Acid Uptake
• The dietary polyglutamates are cleaved to
the monoglutamate vitamin by a γ-Lglutamylcarboxy peptidase commonly called
conjugase.
• Folic acid and tetrahydrofolate is absorbed
as the monoglutamate.
• Conjugase is found in the brush border of
the intestine.
– Chronic inflammatory conditions in the intestine
lead to low conjugase activity which can result in
significant decreased folic acid absorption.
• At least one drug, phenytoin, can inhibit folic
acid uptake.
Folic Acid Metabolism
• The absorbed vitamin must be converted to the several
cofactor forms.
– The first step requires adding back a five to seven member
glutamate chain.
– Next there is a two step reduction to the active cofactor
tetrahydrofolic acid.
• There are two common abbreviations for the cofactor form, FH4 and
THF.
– Dihydrofolate reductase is one of the most studied enzymes
because it is the site of cancer chemotherapy (methotrexate) in
humans, anti-bacterial activity in bacteria and anti-parasite activity
in malaria.
– Synthesis of FH4/THF occurs in a wide variety of tissues.
•
The fact that the dihydrofolate reductase inhibitor methotrexate is
active in so many tissues indicates that the enzyme is widely
distributed.
• The liver contains about a three to six months supply of the
vitamin, presumably in the polyglutamate form.
– Folic acid deficiencies are seen in patients who are chronic
alcoholics. This may be caused by alcohol interfering with storage
and processing of the vitamin in the liver.
As polyglutamates
Cofactor Forms-1
• There are at least five forms, four of
which are “cofactors” existing in each
of three different oxidation states.
– 5,10-methylene THF (N5,N10-methylene
THF)
• Methylation of dUMP forming dTMP
• Synthesis of serine from glycine
– 10-formyl THF
• Conversion aminoimidazole carboxamide
ribotide (AICAR) to Formamidoimidazole
carbxamide ribotide (FAICAR) in purine
biosynthesis
Cofactor Forms-2
– 5,10-methenyl THF (N5,N10-methenyl THF)
• Synthesis of formylglycine ribotide (FGAR)
from glycine amide ribotide (GAR) in purine
biosynthesis
– 5-methyl THF (N5-methyl THF)
• Methylation of homocysteine forming
methionine
• IMPORTANT:
– This is the one cofactor form which is not
interchangeable with any of the other cofactor
structures.
– It is the predominant cofactor form of the vitamin.
» This aspect of folic acid chemistry is important to
appreciate and will be discussed again during
cyanocobalamin (Vitamin B12) deficiencies.
A Fifth but Important Form
– 5-Formyl THF (folinic acid, citrovorum
factor)
• This compound is not a cofactor, but it can be
converted to any of the active cofactor forms.
– It is administered following methotrexate therapy to
return tetrahydrofolate activity to normal levels in
benign tissues.
H2N
H
N
N
CH2
Homocysteine
5
HN
CH
N
O
CH2
CH3
10
HN
R
N5-Methyl-THF
Met
Methyl
cobalamin
FAD
FADH2
H2N
H
N
N
CH2
5
HN
CH
N
O
CH2
H2C
N 10
Gly
N5,N10-Methylene-THF
R
NADP+
Ser
NADPH + H
Cofactor forms are
polyglutamates.
H2N
H
N
N
H2N
CH2
5
HN
N
H
O
H
N
N
CH2
His
CH
5
HN
CH2
HN
CH
N+
10
CH2
HC
O
N 10
R
THF
N5,N10-Methenyl-THF
R
HCOOH
ATP
H2N
H2O
H
N
N
CH2
ADP + Pi
H2N
5
HN
H
N
N
N
CH2
O
5
HN
N
H
HC
O
10
N -Formyl-THF
O
CH
CH
CH2
HN 10
O
CH
CH2
N 10
R
R
N5-Formyl-THF
(Folinic Acid;
citrovorum factor;
Leucovorum
,
O
O
H3 C
NH
NH
N
O
O
N
O
-O
P
O
O
CH2
H
-
O
-O
H
O
H
OH
P
O
CH2
H
-
Deoxyuridylic acid
O
H
H
OH
H
H
H
N5,N10-Methylene THF
DHF
+
NADPH + H
O
H
Deoxythymidylic acid
Dihydrofolate
reductase
NADP+
THF
H2 N
N
Site of methotrexate inhibition
N
CH3
N
N
N
CH2
H
N
NH2
Methotrexate
O
CO2-
CH2
C
CH2
CO2-
Folic Acid Deficiency-1
• Overview
– Folic acid is a very important vitamin for
biosynthetic reactions required for the
biosynthesis of purines and pyrimidines and
regeneration of methionine from homocysteine.
• The main deficiency is a characteristic megaloblastic
anemia due to a shortage of nucleotides required for the
production of erythrocyte precursors.
– Most deficiencies are caused by a medical
condition other than diet and usually are the
result of malabsorption.
• The main exception to the latter is inadequate diet during
pregnancy.
Folate Deficiency Causes-2
• Inadequate nutrition during periods of
increased requirements-1
– Megaloblastic anemia of pregnancy
– Neural tube defects (NTDs)
• spina bifida
• Anencephaly
– Doses of 4 mg (6.7X the RDA for pregnant women)
given to women with prior NTD-affected pregnancies
caused a 70% reduction of NTD-affected
pregnancies.
– Based on extrapolations from the few published
studies, doses of 0.4 – 0.8 mg (400 – 800 μg) will
result in an expected 50% reduction of NTDs.
– RDA (Pregnancy): 0.6 mg (600 μg)
Folate Deficiency Causes-3
• Inadequate nutrition during periods of increased
requirements-2
– Thus, the current recommendations call for women
contemplating pregnancy to take 0.4 – 0.8 mg daily of folic
acid.
– The main difference between the new recommendations and
the earlier ones is that women formerly began taking their
folic acid supplements (usually included in a standard
prenatal vitamin formulation) after the pregnancy was
confirmed.
– This could be one to three months following conception.
The damage from NTDs can occur early in the pregnancy.
Women not planning on becoming pregnant do not need a
folic acid supplement.
• Mutant mice models indicate that folic acid, in its cofactor
forms, is required for methylation of DNA.
– This is one of the regulatory mechanisms for turning genes on
and off.
Folate Deficiency Causes-4
• Inadequate nutrition during periods of
increased requirements-3
– Do we obtain enough folic acid from our
diets?
• Grain-based products are being fortified with
folic acid along with niacin, thiamine and
riboflavin.
• Earlier slide showed women were not obtaining
enough folic acid from their diets.
Uses of Dietary Reference Intakes-9
Folate Deficiency Causes-4
• Alcoholism
– This is considered the leading cause of
folic acid deficiency in the United States.
– There are two ways that excessive alcohol
consumption can interfere with folic acid
activity.
• Impairment of folic acid reduction to the active
THF forms
• Interference with folic acid storage and release
from the liver possibly caused by increased
lipid in the liver.
Folate Deficiency Causes-5
• Chronic inflammation of the intestinal
mucosa (malabsorption)
– There are two mechanisms that explain
how an inflamed mucosa can lead to a
folic acid deficiency.
• First, there can be reduced production of the
required conjugase enzyme which removes the
polyglutamate chain.
• Second, and some believe more important, an
inflamed mucosa inhibits folate transport.
Folate Deficiency Causes-6
• Drug-vitamin interactions
– Phenytoin
• The mechanism is uncertain, but there seems to be some type of
inhibition of folic acid uptake or utilization.
– Methotrexate
• This chemotherapeutic agent inhibits dihydrofolate reductase
preventing the conversion of the vitamin to its functional cofactor
forms.
• NOTE: The rationale for the development of methotrexate is of interest
because it illustrates how chance favors the prepared mind. It was
noted that administration of folic acid supplements hastened the death
of cancer patients. This led to the development of folic acid
antagonists, of which methotrexate is the standard in the United
States. (Trimethoprim is selective for bacterial dihydrofolate
reductase.)
• Uses
– Cancers
– Rheumatoid Arthritis
– Psoriasis
– Other autoimmune diseases
– Depending on the indication and dosing, folic acid or leucovorin
(folinic acid) will be part of the drug regimen with methotrexate.
Folate Deficiency Causes-7
• Too much sunlight (still a hypothesis)
– Increases ultraviolet light (sunlight)
destroys body stores of folate. Some
authorities discount this hypothesis.
• This is based on a correlation that more
problems are seen with fair skinned individuals
living in sunny areas.
Folate Deficiency??
• Elevated blood homocysteine levels
– The positive correlation between homocysteine
levels and cardiovascular disease is weakening.
• There may be a positive correlation between
homocysteine and osteoporosis.
– Elevated homocysteine can be corrected, at least
partially, with folate supplements, along with
pyridoxine and cyanocobalamin.
– Methionine metabolism review:
• Loss of the methyl group from methionine produces
homocysteine.
• Homocysteine is converted back to methionine by
methylation.
• The source of the methyl group is N5-methyl THF.
– Homocysteine may be an indication of inadequate
levels of pyridoxine, folic acid and possibly
cyanocobalamin.
Hypervitaminosis Folate
• This apparently is not a problem.
– Transport across the intestinal mucosa may be
regulated by a feedback mechanism or the rate of
hydrolysis of the polyglutamate chain or a
combination of both.
– What is very important is that taking the vitamin in
doses above 400 μg (800 μg in pregnant and
lactating women) can mask the macrocytic
anemia seen with pernicious anemia caused by a
cyanocobalamin deficiency.
– There are other tests for cyanocobalamin
deficiency besides examination of a blood smear.
– In 2005 there was a report that elevated intake of
folic acid caused mental deterioration in older
adults (65+ years).
• It may be early signs of pernicious anemia.
Folate Dosage Forms
• Stability
– Very unstable to sunlight, even in the dry state.
– Very heat labile such that it can be destroyed during
cooking.
• Folic Acid Solubility: 1.6 gm/ml
• Commercial Forms
– Folic Acid
– Sodium Folate
• Used in parenteral dosage forms.
– Leucovorin Calcium (5-formyl THF, folinic acid, citrovorum
factor)
• Used in orally and parenterally to
– Reverse the toxicity of methotrexate
– Administer an active form of folic acid when the intestinal uptake
or conversion to the tetrahydrofolate active form is impeded.
– Also co-administered with 5-flurouracil (5FU) which blocks the
methylation of dUMP to dTTP.
Folic Acid DRIs-1
• With the setting of the new DRIs, a new unit was
introduced.
– Dietary Folate Equivalent (DFE)
– It acknowledges the differences in bioavailability of
synthetic folic acid in supplements and fortified
foods versus naturally occurring folates.
• 1 mcg of food folate = 1 mcg of DFE
• 1 mcg of folic acid taken with meals or as fortified food = 1.7
mcg of DFE
• 1 mcg of folic acid supplement taken on an empty stomach =
2 mcg of DFE.
• The closest analogy to the DFE is the retinol
equivalent (RE) that takes into account the
differences in source of β-carotene.
Folic Acid DRIs-2
• AI
– Infants
65 - 80 μg DFE/day
• EAR
–
–
–
–
–
–
Children (1 - 8 years)
120 - 160 μg DFE/day
Children (9 - 13 years)
250 μg DFE/day
Adolescents (14 - 18 years) 330 μg DFE/day
Adults (19 - 50+ years)
320 μg DFE/day
Pregnancy
520 μg DFE/day
Lactation
450 μg DFE/day
Folic Acid DRIs-3
• RDA
– Children (1 - 8 years) 150 - 200 μg DFE/day
– Children (9 - 13 years)
300 μg DFE/day
– Adolescents (14 - 18 years) 400 μg DFE/day
– Adults (15 - 50+ years)
400 μg DFE/day
– Pregnancy
600 μg DFE/day
– Lactation
500 μg DFE/day
Folic Acid DRIs-4
• UL
–
–
–
–
Children (1 - 3 years)
Children (4 - 8 years)
Children (9 - 13 years
Adolescents (14 - 18 years)
300 μg DFE/day
400 μg DFE/day
600 μg DFE/day
800 μg DFE/day
• The UL is based on the amount of folate that
could mask the blood picture of
cyanocobalamin deficiency.
Food Sources
•
•
•
•
Deep green leafy vegetables
Liver
Kidney
Yeast
• Dietary forms are tetrahydrofolate
polyglutamates.
• Note:
– Even though the bacteria in the colon do produce
folic acid polyglutamates, this is well below the
upper third of the small intestine where most folic
acid absorption occurs.
– Therefore, it is doubtful if humans receive any of
this vitamin from their intestinal flora.