Plasma and erythrocyte levels of tricyclic antidepressants in depressed patients.

Plasma and red blood cells (RBCs), amitriptyline, nortriptyline, imipramine, desipramine, doxepin, and desmethyl doxepin levels were measured in depressed inpatients during steady-state kinetics. A strong positive correlation between the drug levels in plasma and RBCs was found for amitriptyline, nortriptyline, desipramine, and desmethyl doxepin. However, at a given plasma level, up to a 6-fold interindividual variation in the RBC drug levels was found. The correlations between plasma and RBC imipramine and doxepin levels were low. The interindividual variation in the RBC-plasma tricyclic level ratios was large enough to warrant further clinical studies on the relationship between efficacy and pharmacokinetics of tricyclic antidepressants.