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Overview :
When a pharmacist receives an order to dispense a dose of an
active drug moiety, s/he must take into account the constituents in the actual
drug compound prepared by the pharmaceutical company. This may include absorbed
water, water of hydration, acid/base elements, and other factors that affect
the weight of the drug complex. The amount of drug compound to dispense can be
calculated once these factors are taken into account.
Parameters:
(1) weight of drug complex to dispense
(2) weight of the active or functional drug moiety in the
prescription
(3) chemical formula weight of the drug complex
(4) fraction dry weight
(5) formula weight of the active or functional drug moiety
in the drug complex
weight of drug complex to dispense in grams =
= (weight of active drug moiety prescribed) * (formula
weight of drug complex) / ((fraction dry weight) * (formula weight drug moiety
in drug complex))
where:
• It is
essential to know all of the components in the drug complex. This information
may be found in the Merck Index or other pharmaceutical reference
sources.
• The chemical
formula weight of the drug complex includes the number of moles of functional
drug moiety, water of hydration, acid/base in salts, etc.
• The fraction
dry weight is based on the percent moisture reported after drying the drug. The
fraction = 1.00 - (fraction of moisture). This takes into account the absorbed
water that adds to the weight of the drug complex. Absorbed water is different
from water of hydration in the drug complex.
• The formula
weight of the drug moiety in the drug complex is the formula weight of the
moiety times the number of moles in the drug complex.
Examples of drug complexes:
• Netilimicin
sulfate is prepared as (2 (netilimicin)) • (5(sulfate)). The chemical formula
weight of the drug complex is 1442 grams, which consists of 2 moles netilimicin
(formula weight 475.6 grams, total 951.2 grams) and 5 moles sulfate (98 grams,
total 490). Since there are 2 moles of netilimicin in the complex, the formula
weight of the active drug moiety in the drug complex is 951.2.
Limitations:
• The amount of
water absorbed depends on how well the drug container is sealed and the local
humidity.
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