Possible nonlinear pharmacokinetics of desipramine after once daily dosing

Desipramine (DMI) plasma concentrations were measured in depressed outpatients treated with desipramine hydrochloride. Plasma level determinations were measured 24 hours after a single dose of DMI 50 mg, and then at weekly intervals thereafter while receiving once daily bedtime dosing with DMI 150 mg or 200 mg. The 24 hour DMI concentration was significantly, and rather closely correlated with steady state DMI levels (r = 0.74, p less than .001) (prediction coefficient [r2] = 55%). However, steady state plasma levels of DMI were higher than would be predicted based upon prior studies which also examined the relationship between steady state and 24 hour desipramine plasma concentrations. We speculate that the single bedtime administration of DMI in the present study may have led to saturation of metabolic hepatic enzymes during the first pass of the drug through the liver. The possibility of nonlinear DMI pharmacokinetics may be of clinical importance to some patients receiving a single, daily, high dose of medication.