Phase 1A safety assessment of intravenous amitriptyline

Abstract

The antidepressant amitriptyline is used as an adjuvant in the treatment of chronic pain. Among its many actions, amitriptyline blocks Na+ channels and nerves in several animal and human models. As perioperative intravenous lidocaine has been suggested to decrease postoperative pain, amitriptyline, because of its longer half-life time, might be more useful than lidocaine. However, the use of intravenous amitriptyline is not approved by the US Food and Drug Administration. We therefore investigated the adverse effects of preoperative intravenous amitriptyline in a typical phase 1A trial. After obtaining written Food and Drug Administration and institutional review board approval, we obtained written consent for preoperative infusion of amitriptyline in an open-label, dose-escalating design (25, 50, and 100 mg, n=5 per group). Plasma levels of amitriptyline/nortriptyline were determined, and adverse effects were recorded in a predetermined symptom list. Infusion of 25 and 50 mg amitriptyline appears to be well tolerated; however, the study was terminated when 1 subject in the 100-mg group developed severe bradycardia. Intravenous infusion of amitriptyline (25 to 50 mg over 1 hour) did not create side effects beyond dry mouth and drowsiness, or dizziness, in 2 of our 10 otherwise healthy participants receiving the 25- to 50-mg dose. An appropriately powered future trial is necessary to determine a potential role of amitriptyline in decreasing postoperative pain.

Perspective: Amitriptyline potently blocks the persistently open Na+ channels, which are known to be instrumental in various pain states. As this occurs at very low plasma concentrations, a single preoperative intravenous infusion of amitriptyline could provide long-lasting pain relief and decrease the incidence of chronic pain.

Fig. 1

Plasma levels of amitriptyline after 1-hour infusions of 25, 50, and 100 mg (n=5 per group) or 2-hour infusion of 25 mg (n=5). Even after infusion of the high 100-mg dose and with the relatively rapid infusion over 1 hour, serum levels were well below toxic levels (the toxic range of amitriptyline commences at 500 ng/ml). Significant differences (p < 0.05) in plasma amitriptyline levels were seen at all time points. Dunn’s post hoc comparison was used to identify which doses resulted in serum levels of amitriptyline different from the control condition; at baseline, 20 minutes, 1 hour, and 3 hours, 50-mg and 100-mg dose groups differed from controls; at 5 hours, only the 100-mg group differed from controls (p < 0.05).

Fig. 2

Plasma levels of nortriptyline, the metabolite of amitriptyline. As with amitriptyline, nortriptyline did not reach toxic levels (the toxic range of nortriptyline commences at 500 ng/ml). Levels of nortriptyline showed significant differences from baseline at 1, 3, and 5 hours (p < 0.05).

Fig. 3

Percentage change in QTc, the corrected QT interval, at the end of the 1-hour infusion, compared with baseline (immediately before start of the infusion).