D1 x fei

D12 = < X fe2 Dividing the first equation by the second gives:

Assuming that fei = fe2, we have

D1 D

Du D

D1 D0


U (extravascular)

Absolute bioavailability (47)

Thus, if it is assumed that the same fraction of absorbed drug always reaches the urine unchanged, the bioavailability can be calculated as the ratio of total amounts of unchanged drug recovered in urine.

Example. When a drug was administered IV to a group of volunteers, 80% of the 500-mg dose (i.e., 400 mg) was recovered unchanged in urine. When the same drug was administered orally to the same volunteers, 280 mg was recovered unchanged in urine. What is the absolute bioavailability of the drug following oral administration? From equation (47),

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