Binding of imipramine to plasma protein and to brain tissue: relationship to CSF tricyclic levels in man.

The possible clinical significance of the plasma protein binding of tricyclic antidepressants has been evaluated using imipramine (IMI), a typical tricyclic. Using equilibrium dialysis, the in vitro binding of IMI to plasma was compared to that of brain tissue. Cerebrospinal fluid (CSF) IMI was used as an independent measure of 'free' drug in the central nervous system. Intrinsic metabolic clearances were calculated on the basis of steady-state plasma IMI concentrations. There were three significant results: (1) variations in plasma binding are not great; (2) plasma protein binding does not limit the entry of IMI into the CSF; (3) variations in metabolism (intrinsic clearance) account for almost all variations in CSF concentrations of drug. It is concluded that measurement of free tricyclic antidepressant is not indicated in studies of clinical efficacy.