In a drug, an active drug moiety is the molecule or ion, responsible for physiological or pharmacological action it excludes those appended portions of the drug compound that cause the drug to be an ester, salt, or other noncovalent derivative (such as a complex, chelate, or clathrate) of the compound.
The following formula may be used to calculate the active drug moiety portion of a drug compound:
Active Drug Moiety (fraction) = Drug Moiety (g/mole) ÷ Drug compound (g/mole)
Example
The chemical formula of methadone hydrochloride is C21H27NO.HCl. Calculate the fraction of active moiety in the compound.
1. Calculate the molecular weights (MW) of the base and the salt form:
MW of methadone base:
C21 = 21 X 12.01 = 252.21
H27 = 27 X 1 = 27
N = 1 X 14.01 = 14.01
O = 1 X 16 = 16
Total = 309.22
MW of methadone salt:
H = 1 X 1 = 1
Cl = 1 X 35.45
Total = 309.22 + 36.45 = 345.67
2. Calculate the fraction of active moiety in the compound:
Fraction of methadone base = 309.22 ÷ 345.67 = 0.895
3. Calculate the percent of active moiety in the compound:
0.895 × 100 = 89.5%
4. Calculate the quantity of methadone in a 10 mg tablet of methadone hydrochloride:
(10 × 89.5) ÷ 100 = 8.95 mg
Practice Problem
Oral tablets of Bupropion HCl contain 150 mg of Bupropion HCl If the molecular weight of Bupropion is 239.7 and the molecular weight of Bupropion HCl is 276.2, calculate the amount of Bupropion in each tablet.
The amount of Bupropion in each tablet = (239.7 ÷ 276.2) × 150 mg = 130.178 mg
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