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.
