Example: Carbamazepine (CBZ) is an antiepileptic drug that is metabolized to several metabolites including CBZ-epoxide. CBZ half life is 15 hours and its Vd is 1.4 L/kg, while CBZ-epoxide half life is 7.5 hr and Vd is 1.1 L/kg. If 25% of CBZ dose is converted to CBZ-epoxide calculate the steady state CBZ-epoxide/CBZ concentration ratio during repeated administration of CBZ 100 mg every 8 hr and 200 mg every 8 hr.
The metabolite to drug concentration ratio is independent of the administered dose.
Increasing the drug dose will result in proportional increase in both drug and metabolite steady state concentrations, so the ratio will stay constant.
