Vitamin B 12 Substances
Vitamin B 12 is the name generally used for a group
of related cobalt-containimcg compounds, also
known as cobalamins, of which cyanocobalamin
and hydroxocobalamin are the principal forms in
clinical use.
Vitamin B 12, a water-soluble vitamin, occurs in the
body mainly as methylcobalamin (mecobalamin)
and as adenosylcobalamin (cobamamide) and hy-
droxocobalamin.
Cyanocobalamin
Dark red crystals or crystalline or amorphous powder. In the
anhydrous form it is very hygroscopic and when exposed to
air it may absorb about 12% of water.
Soluble I in 80 of water: the Ph. Eur. states that it is sparimcgly
soluble, and the USP that it is soluble, in alcohol; practically
insoluble in acetone, in chloroform, and in ether. Store in air-
tight containers. Protect from light.
Hydroxocobalamin
also includes Hydroxocobalamin Chloride and Hydroxo
cobalamin Sulphate.
Hydroxocobalamin occurs either as the acetate
& the chloride or the sulphate . The hydrated form of
hydroxocobalamin has been referred to as aquocobalamin.
Dark red. odourless or almost odourless crystals or crystalline
powder. Some decomposition may occur on dryimcg. The an-
hydrous form is very hygroscopic.
Soluble I in 50 of water and I in 100 of alcohol: sparimcgly
soluble in methyl alcohol: practically insoluble in acetone, in
chloroform, and in ether. A 2% solution in water has a pH of
8 to 10. The USP has store at a temperature not exceedimcg 15
and the Ph. Eur. between 2Β° and 8Β°. Store in airtight contain
Adverse Effects and Precautions
Allergic hypersensitivity reactions have occurred
rarely followimcg the parenteral administration of the
vitamin B~ compounds cyanocobalamin and hy-
droxocobalamin.
Arrhythmias secondary to hypokalaemia have oc-
curred at the beginnimcg of parenteral treatment with
hydroxocobalamin.
Cyanocobalamin or hydroxocobalamin should, if
possible, not be given to patients with suspected vi-
tamin B 12 deficiency without first confirmimcg the di-
agnosis, and should not be used to treat
megaloblastic anaemia of pregnancy. Regular mon-
itorimcg of the blood is advisable. Administration of
doses greater than 10 mcg daily may produce a hae-
matological response in patients with folate defi-
ciency; indiscriminate use may mask the precise
diagnosis. Conversely, folate may mask vitamin B 12
deficiency.
Cyanocobalamin should not be used for Leber's dis-
ease or tobacco amblyopia since these optic neu-
ropathies may degenerate further.
Antibodies to hydroxocobalamin-transcobalamin II
complex have developed durimcg hydroxocobalamin
therapy.
Analysis, by the Boston Collaborative Drug Surveillance Pro-
gram of data on 15 438 patients hospitalised between 1975
am 1982 detected 3 allergic skin reactions attributed to cy-
anocobalamin amomcg 168 recipients of the drug. For the pur-
poses of the study, reactions were defined as beimcg
generalised morbilliform exanthems, urticaria or generalised
pruritus only.
Interactions
Absorption of vitamin B 12 from the gastro-intestinal
tract may be reduced by neomycin, aminosalicylic
acid, histamine Hi-receptor antagonists, and colchi-
cine. Serum concentrations may be decreased by
concurrent administration of oral contraceptives.
Many of these interactions are unlikely to be of clin-
ical significance but should be taken into account
when performimcg assays for blood concentrations.
Parenteral chloramphenicol may attenuate the effect
of vitamin B 12 in anaemia.
Pharmacokinetics
Vitamin B 12 substances bind to intrinsic factor, a
glycoprotein secreted by the gastric mucosa. and are
then actively absorbed from the gastro-intestinal
tract. Absorption is impaired in patients with an ab-
sence of intrinsic factor, with a malabsorption syn-
drome or with disease or abnormality of the gut, or
after gastrectomy. Absorption from the gastro-intes-
tinal tract can also occur by passive diffusion, little
of the vitamin present in diets is absorbed in this
manner although the process becomes increasimcgly
important with larger amounts such as those used
therapeutically. Vitamin B 12 is extensively bound to
specific plasma proteins called transcobalamins:
transcobalamin II appears to be involved in the rapid
transport of the cobalamins to tissues. Vitamin B12
is stored in the liver, excreted in the bile. and under-
goes extensive enterohepatic recyclimcg; part of an
administered dose is excreted in the urine, most of it
in the first 8 hours: urinary excretion, however, ac-
counts for only a small fraction in the reduction of
total body stores acquired by dietary means. Vita-
min Bi; diffuses across the placenta and also ap-
pears in breast milk.
After injection of cyanocobalamin a large proportion is ex-
creted in the urine within 24 hours; the body retains only 55%
of a 100-mcg dose and 15% of a 1000-mcg dose. Body stores of
vitamin B 12 amount to 2000 to 3000 mcg which is believed to
be enough for 3 to 4 years. If 1000 mcg is injected monthly, the
150 mcg retained lasts for about I month. Hydroxocobalamin
is better retained than cyanocobalamin: 90% of a 100-mcg dose
and 30 % of a 1000-mcg dose are retained and that ramcge is be-
lieved to be enough for 2 to 10 months.'
Intranasal absorption. For references to the intranasal ab-
sorption of cyanocobalamin and hydroxocobalamin. see Ad-
ministration, below.
Human Requirements
For adults, the daily requirement of vitamin B 12 is
probably about I to 2 mcg and this amount is present
in most normal diets. Vitamin B 12 occurs only in an-
imal products; it does not occur in vegetables, there-
fore strict vegetarian (vegan) diets that exclude dairy
products may provide an inadequate amount al-
though it has been said that many years of vegetari-
anism are necessary before a deficiency is produced,
if at all. Meats, especially liver and kidney, milk,
eggs, and other dairy products, and fish are good
sources of vitamin B 12.
In the United Kingdom' dietary reference values
have been published for vitamin B 12 and similarly in the
United States recommended dietary allowances (RDAs) have
been set.' Differing amounts are recommended for infants
and children of varying ages, adults and pregnant and lactat-
ing women. In the UK the reference nutrient intake (RNI) is
1.5 mcg daily for adult males and females and the estimated
average requirement (EAR) is 1.25 mcg daily. In the USA the
RDA for adults is 2.4 mcg daily.
Uses and Administration
Vitamin B 12, a water-soluble vitamin, occurs in the
body mainly as methylcobalamin (mecobalamin)
and as adenosylcobalamin (cobamamide) and hy-
droxocobalamin. Mecobalamin and cobamamide act as coenzymes in nucleic acid synthesis. Mecobalamin is also closely involved with folic acid in several important metabolic pathways.
Vitamin B 12 deficiency may occur in strict vegetari-
ans with an inadequate dietary intake, although it
has been said that it may take many years of an in-
adequate intake before a deficiency is produced.
Deficiency is more likely in patients with malab-
sorption syndromes or metabolic disorders, nitrous
oxide-induced megaloblastosis or followimcg gastr-
ectomy or extensive ileal resection. Deficiency leads
to the development of megaloblastic anaemias and
demyelination and other neurological damage. A
specific anaemia known as pernicious anaemia de-
velops in patients with an absence of the intrinsic
factor necessary for good absorption of the vitamin
from dietary sources.
Vitamin B 12 preparations are used in the treatment
and prevention of vitamin B 12 deficiency. It is desir-
able to identify the exact cause of deficiency before
commencimcg therapy. Hydroxocobalamin is gener-
ally preferred to cyanocobalamin: it binds more
firmly to plasma proteins and is retained in the body
longer (see under Pharmacokinetics, above). Cyano-
cobalamin and hydroxocobalamin are generally ad-
ministered by the intramuscular route, although
cyanocobalamin may be given by mouth and intra-
nasal formulations are being developed (see under
Administration, below). Oral cyanocobalamin may
be used in treating or preventing vitamin B 12 defi-
ciency of dietary origin.
In pernicious anaemia and other macrocytic
anaemias without neurological involvement, cyano-
cobalamin and hydroxocobalamin may be adminis-
tered in doses of 250 to 1000 mcg intramuscularly on
alternate days for I to 2 weeks, then 250 mcg weekly
until the blood count returns to normal. Mainte-
nance doses of 1000 mcg are administered monthly
(for cvanocobalamin) or every 2 to 3 months (for hy-
droxocobalamin). If there is neurological involve-
ment. cyanocobalamin or hydroxocobalamin may
be given in doses of 1000 mcg on alternate days and
continued for as long as improvement occurs. For
the prophylaxis of anaemia associated with vitamin
B 12 deficiency resulting from gastrectomy or malab-
sorption syndromes cyanocobalamin may be given
in doses of 250 to 1000 mcg intramuscularly each
month and hydroxocobalamin in doses of 1000 mcg
intramuscularly every 2 or 3 months. For vitamin
B 12 deficiency of dietary origin, cyanocobalamin 50
to 150 mcg may be taken daily by mouth between
meals. Lower doses for the administration of both
cyanocobalamin and hydroxocobalamin are recom-
mended in the LISA.
Treatment usually results in rapid haematological
improvement and a striking clinical response. How-
ever, neurological symptoms respond more slowly
and in some cases remission may not be complete.
Hydroxocobalamin may also be given in the treat-
ment of tobacco amblyopia and Leber's optic atro-
phy; initial doses are 1000 mcg daily for 2 weeks
intramuscularly followed by 1000 mcg twice weekly
for as long as improvement occurs. Thereafter,
1000 mcg is administered every I to 3 months.
Cyanocobalamin and hydroxocobalamin are also
used in the Schilling test to investigate vitamin B 12
absorption and deficiency states. They are adminis-
tered in a non-radioactive form together with cyano-
cobalamin radioactively-labelled with cobalt-57 or
cobalt-58 and the amount bf radioactivity excreted
in the urine can be used to assess absorption status.
A differential Schilling test, in which the forms of
cyanocobalamin are given under different condi-
tions can provide information concernimcg the cause
of the malabsorption. Cobamamide and mecobala-
min may also be used for vitamin B12 deficiency.
Administration. The small amounts of vitamin B 12 present
in the diet are absorbed from the gastro-intestinal tract by an
active process which involves binding with intrinsic factor.
As intrinsic factor is absent in patients who have developed
pernicious anaemia it has often been assumed that oral treat-
ment with vitamin B 12 preparations will therefore be ineffec-
tive. This. however, is not necessarily so, as larger amounts of vitamin B 12 may be absorbed by passive diffusion although a
considerable amount does still remain totally unabsorbed.
Thus recent attention has been given again to the use of oral
cobalamins for the treatment of pernicious. It is
now considered that oral doses of 1000 mcg daily will provide
an adequate amount and will be a suitable alternative to injec-
tions given at monthly or so intervals. It has been stated that
cyanocobalamin is also effective when given intranasally.
with peak plasma concentrations greater than those achieva-
ble orally, and that this may offer another alternative to injection. The intranasal absorption of hydroxocobalamin has also
been studied.
Amino acid metabolic disorders. References to the use
of hydroxocobalamin in the treatment of inborn errors of vi-
tamin B 12 metabolism." Some patients with homocystinuria
or methylmalonic aciduria have responded to cobalamins.
Cyanide toxicity. Hydroxocobalamin combines with cya-
nide to form cyanocobalamin. and thus may be used as an
antidote to cyanide toxicity . Hydroxocobalamin is
reported to be effective in controllimcg cyanide toxicity due to
nitroprusside infusion.' and after exposure to inhaled com-
bustion products in residential fires'
Deficiency states. The emergence of new metabolic assays
for homocysteine and methylmalonic acid has led to the iden-
tification of subtle vitamin 812 deficiency without the overt
manifestations of megaloblastic anaemia or neurolog-
ical disease: this condition appears to be particularly common
in the elderly. At present, there is no clear clinical rationale
for treating subtle deficiency. However, preliminary findings
from a recent study suggest it may be linked to some immu-
nological impairment, identified as impaired antibody
responses to pneumococcal vaccine. Moreover, hyperhomo-
cysteinaemia has been identified as a risk factor for athero-
sclerosis and ischaemic heart disease, and there is increasing
interest in the potential of B vitamins, including B 12 to reduce
homocysteine concentrations and therefore atherosclerotic
outcomes, as discussed under Folic Acid.
Dietary vitamin B 12 deficiency in infants may lead to devel-
opmental abnormalities.
Neural tube defects. It has been found that an abnormality
in homocysteine metabolism is present in many women who
give birth to children with neural tube defects , the
enzyme methionine synthase, which converts homocysteine
to methionine, requires both folate and vitamin B 12 as cofac-
tors and low maternal vitamin B 12 concentrations may be an
independent risk factor for neural tube defects. If confirmed,
this would suggest that additional supplementation with co-
balamins may be warranted.