Steady-state Levels of Imipramine and Its Metabolites: Significance of Dose-dependent Kinetics

Seventeen hospitalized patients (age 39-66 years), received a loading dose of 100 mg imipramine HCl and then 50 mg b.i.d. The 12-h plasma concentration at steady-state varied between 40-637 nmol/l for imipramine, 49-1148 nmol/l for desipramine and 89-1603 nmol/l for imipramine + desipramine. Guided by plasma level monitoring, a final therapeutic plasma level between 548-910 nmol/l for imipramine + desipramine was achieved (therapeutic dose range: 50-400 mg/day). Mean time to reach the therapeutic level was 19 days. The mean 2-OH-imipramine/imipramine ratio was 0.24 and mean 2-OH-desipramine/desipramine ratio was 0.56. There was a significant intrapatient correlation between the two ratios, both during 100 mg imipramine/d and at the therapeutic dose level. A low ratio was associated with high imipramine and particularly with a high desipramine level. Well defined steady state levels were established at two different dose levels in 12 patients and at three dose levels in 5 patients. With increasing dose there was a marked and disproportionate rise in the desipramine level and to some extent in the imipramine level. Saturation of imipramine and desipramine hydroxylation appeared to be responsible for the dose-dependent kinetics. Concomitant treatment with levomepromazine and perphenazine in one patient resulted in a significant rise both in imipramine and desipramine concentration, apparently due to inhibition of the hydroxylation. Eleven out of twelve endogenously depressed patients responded completely to treatment, whereas the response was poor in the non-endogenously depressed patients despite optimal drug levels.