Population variability in the pharmacokinetics of terfenadine: the case for a pseudo-polymorphism with clinical implications.

: Terfenadine is nearly completely first pass biotransformed. Unmetabolized terfenadine plasma concentrations have been associated with altered cardiac repolarization. During previous drug interaction studies, 2 subjects were found to have quantifiable concentrations of unmetabolized terfenadine with accompanying electrocardiographic repolarization changes while on terfenadine alone. To determine whether these subjects were representative of the population, 150 healthy volunteers (109 males, 41 females, ages 19-49) were screened for their ability to metabolize terfenadine after achieving steady-state. Blood was obtained at known times of maximum terfenadine concentration after dosing. Eleven subjects had quantifiable concentrations of terfenadine demonstrating wide intersubject variability in terfenadine metabolism. Further studies to determine whether such subjects are more susceptible to untoward terfenadine-associated events are underway.