Treatment of depression with tricyclic drugs--pharmacokinetic and pharmacodynamic aspects.

A series of studies on the pharmacokinetic and pharmacodynamic properties of some tricyclic antidepressants is reviewed. During treatment with the same oral dose of these drugs, patients develop widely differing plasma levels. The importance of this variability for the clinical effects has been studied in detail for the monomethylated compound, nortriptyline. There is an association between side-effects and high plasma levels of this drug. In endogenously depressed patients, the relationship between plasma level and effect appears to be curvilinear. The tricyclic antidepressants differ in their capacity to inhibit transmitter uptake into noradrenaline- and serotonin neurons respectively. Nortriptyline is a preferential noradrenaline uptake inhibitor, while the dimethylated compound, chlorimipramine also has a profound influence on serotonin neurons. These differential effects are also reflected in changes in the levels of the transmitter metabolites in cerebrospinal fluid (CSF). The CSF studies have also supported the hypothesis of a biochemical heterogeneity of the depressive syndrome. The levels of the serotonin metabolite, 5-HIAA were bimodally distributed in CSF. In patients with a low level of 5-HIAA there was a significant correlation between the CSF metabolite level and the severity of the depression, and these patients also appeared to be more suicide-prone than those with higher 5-HIAA levels. These patients seemed to be less amenable to treatment with nortriptyline. The effect of chlorimipramine treatment in this subgroup of depressives is presently being explored.