Nonlinear kinetics of imipramine in low and medium plasma level ranges.

Steady state plasma concentrations of imipramine and desipramine were studied at three to nine different imipramine dose levels in 17 extensive metabolizers of sparteine and at two dose levels in two poor metabolizers of sparteine, all treated for diabetic neuropathy symptoms. The imipramine doses were changed stepwise from doses yielding plasma concentrations of imipramine plus desipramine below 150 nM, up to doses yielding therapeutic drug levels of at least 300-500 nM. The imipramine doses required to achieve therapeutic drug levels was 20 or 25 mg/day in the two poor metabolizers and 50-350 mg/day in the extensive metabolizers. In the extensive metabolizers, the concentration/dose ratio increased for imipramine and desipramine with increasing dose. Dose adjustments based on a simple linear prediction from drug levels at initial dose (50 or 75 mg imipramine/day) thus would result in 0-130% (median, 20%) overestimates, most pronounced in patients with initial low steady state levels. The nonlinear kinetics of imipramine thus may be a significant clinical problem in patients treated for diabetic neuropathy symptoms.