Clinical evidence of an interaction between imipramine and acetylsalicylic acid on protein binding in depressed patients.

The binding of imipramine to plasma proteins was studied in 20 adult patients with endogenous depression, with the purpose of assessing the effect produced by its simultaneous administration with an analgesic. Patients were administered 150 mg/day imipramine for 5 days and the binding to plasma proteins was determined. This was repeated 2 days later, after simultaneous administration of imipramine with 1,000 mg/day acetylsalicylic acid (ASA). Adverse effects for each patient were registered during both phases and were classified as mild, moderate, or severe. Results showed 84.4 +/- 7.07% of imipramine bound to plasma proteins and 72.18 +/- 6.5% when imipramine was administered with ASA (p < 0.05). When imipramine was administered alone, 1.95 mild adverse events per patient were registered. When imipramine was administered with ASA, the mild adverse events increased to 3.1 (p < 0.01) and the severe adverse events increased from 0.6 to 1.5 (p < 0.01). The levels of free imipramine increased when ASA was administered, indicating a displacement on the binding to plasma proteins. When adverse events were compared for each treatment, the accumulation of the free fraction of imipramine caused an increase in adverse events as well as in their clinical severity.