Several β-blockers are metabolized by the polymorphic enzyme cytochrome P450 2D6 (CYP2D6). CYP2D6*4 is the main polymorphism leading to decreased enzyme activity. The clinical significance of impaired elimination of β-blockers is controversial, and most studies suffer from inclusion of small numbers of poor metabolizers (PMs) of CYP2D6. In this study, the association between CYP2D6*4 and blood pressure or heart rate was examined in 1,533 users of β-blockers in the Rotterdam Study, a population-based cohort study. In CYP2D6 *4/*4 PMs, the adjusted heart rate in metoprolol users was 8.5 beats/min lower compared with *1/*1 extensive metabolizers (EMs) (P < 0.001), leading to an increased risk of bradycardia in PMs (odds ratio = 3.86; 95% confidence interval 1.68-8.86; P = 0.0014). The diastolic blood pressure in PMs was 5.4 mm Hg lower in users of β-blockers metabolized by CYP2D6 (P = 0.017) and 4.8 mm Hg lower in metoprolol users (P = 0.045) compared with EMs. PMs are at increased risk of bradycardia.

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Journal Clinical Pharmacology and Therapeutics
Bijl, M.J, Visser, L.E, van Schaik, R.H.N, Kors, J.A, Witteman, J.C.M, Hofman, A, … Stricker, B.H.Ch. (2009). Genetic variation in the CYP2D6 gene is associated with a lower heart rate and blood pressure in β-blocker users. Clinical Pharmacology and Therapeutics, 85(1), 45–50. doi:10.1038/clpt.2008.172