Influence of ABCB1 (P-glycoprotein) haplotypes on nortriptyline pharmacokinetics and nortriptyline-induced postural hypotension in healthy volunteers

Abstract

AIMS To investigate the influence of ABCB1 (1236-2677-3435) polymorphisms on nortriptyline pharmacokinetics and nortriptyline-induced postural hypotension in healthy volunteers. METHODS Genetic screening of 67 healthy volunteers identified eight CGC homozygotes and nine TTT homozygotes of ABCB1 (1236-2677-3435), who were administered a single dose of nortriptyline 25mg. Plasma exposure of nortriptyline and its active metabolites, E- and Z-10-hydroxynortriptyline, was determined over 72h. Heart rate and blood pressure responses to posture change (active standing and passive head-up tilt) were measured continuously using finger plethysmography. RESULTS There were no differences in plasma exposure between ABCB1 haplotype groups, as the geometric mean (95% CI) AUC(0,72h) ratios were 0.98 (0.94, 1.03), 1.02 (0.96, 1.09) and 0.95 (0.80, 1.10) for nortriptyline, E- and Z-10-hydroxynortriptyline, respectively. The pre dose heart rate response to standing was greater in the TTT than CGC homozygotes (mean (95% CI) difference 7.4 (1.5, 13.4) beatsmin -1, P=0.02). At t max at 8h post dose, nortriptyline increased the heart rate response to posture change in all subjects with mean (95% CI) Δ heart rate values of 7.4 (3.6, 11.3) beatsmin -1 on active standing (P=0.0009) and 4.8 (2.0, 7.6) beatsmin -1 on head-up tilt (P=0.002), but no difference was observed between haplotype groups. There was no difference in blood pressure response to posture change in either group. CONCLUSION The association between ABCB1 polymorphisms and nortriptyline-induced postural hypotension found in the previous study could not be confirmed. The results raise the possibility of a predisposition in heart rate response in the TTT homozygotes rather than an effect of nortriptyline. © 2011 The Authors. British Journal of Clinical Pharmacology © 2011 The British Pharmacological Society.