The effects of phenindamine tartrate on sleepiness and psychomotor performance

Abstract

Phenindamine, an H 1-receptor antagonist that was developed almost 50 years ago, has been associated with both drowsiness and insomnia. Since its central nervous system profile has not been well characterized, we used a series of psychomotor tests to conduct two studies. In the first, 12 subjects received single oral doses of phenindamine (25 mg), diphenhydramine (50 mg). terfenadine (60 mg), or placebo in a four-way crossover study. Psychomotor tests included choice reaction time (CRT), tracking, and hand steadiness (HS). In the second trial, 15 subjects received single oral doses of phenindamine (25 mg), pseudoephedrine (60 mg), phenindamine and pseudoephedrine, diphenhydramine (50 mg), or placebo in a five-way crossover study. Psychomotor tests included CRT, HS, and a task that divided attention between tracking and reaction time. Introspective drowsiness was measured in both trials with use of a visual analog scale (VAS) and the Stanford Sleepiness Scale (SSS). All assessments were made before and 1, 3, and 5 hours after drug administration. In the first trial, diphenhydramine produced significant impairment relative to placebo ( p < 0.05) in CRT, tracking, and HS tasks and higher SSS and VAS scores, with peak effect noted at 3 hours. Phenindamine did not significantly differ from placebo or terfenadine. In the second trial, diphenhydramine produced significant impairment relative to placebo ( p < 0.05) in CRT, divided attention, HS, and VAS, and SSS, also peaking at 3 hours. Stanford Sleepiness Scale scores after phenindamine were greater than placebo at 3 hours ( p < 0.05) but significantly less than diphenhydramine ( p < 0.05). Psychomotor test scores after phenindamine and pseudoephedrine, either alone or in combination, were consistently and most often significantly better than diphenhydramine and not significantly different from placebo. Taken together, these data indicate that sleepiness after a single dose of phenindamine is rank ordered between placebo and diphenhydramine. We speculate that the relative lack of impairment reflected by the psychomotor tests from phenindamine may be related to its paradoxic sedative and stimulatory effects.