The pharmacological action of imipramine hydrochloride was determined by a method involving the infusion of tritiated norepinephrine and measuring the tritiated metabolites in urine. The effects of similar dosages of imipramine on norepinephrine metabolism varied between subjects and were probably due to individual differences in drug metabolism.
A significant correlation was found between an index of the reuptake inhibition by imipramine on peripheral sympathetic nerve endings and the improvement in the clinical state of depressed patients.
Rosenblatt S, Chanley JD. Measuring the Pharmacological Action of Imipramine in the Treatment of Depressions. Arch Gen Psychiatry. 1974;30(4):456-460. doi:10.1001/archpsyc.1974.01760100026005