Abstract
Treatment with second generation histamine H1 receptor antagonists has been associated with lengthening of the Q-T interval and proarrhythmia. Similarly, lengthening of the Q-T interval has been reported in patients after overdosing with diphenhydramine (DPH), a first generation agent. Therefore, our study was designed 1) to assess effects of DPH on cardiac repolarization and 2) to characterize effects of the drug on major voltage-dependent cardiac K+ currents. First, we noticed that oral administration of DPH at usual dosages to healthy volunteers or to patients (prior to angioplasty) was associated with prolongation of the Q-Tc interval. Although this effect was modest in most individuals, Q-Tc was increased more than 20 ms in 7 of 20 patients. Second, we noticed that exposure of isolated guinea pig hearts to DPH 10−5 M caused a lengthening of monophasic action potential duration. This effect was potentiated by the combined perfusion of other K+ channel blockers such as indapamide. Finally, experiments performed with the patch-clamp technique demonstrated unequivocal block of the rapid component of the delayed rectifier (IKr) by DPH; however, IC50 determined for block of IKr (3 · 10−5 M) is ∼40-fold greater than plasma concentrations of the drug measured at usual dosages (7 · 10−7 M). Consequently, in agreement with the long-term clinical use of the drug, prolongation of cardiac repolarization should be minimal in most patients at usual dosages but may be observed with overdosing. Nevertheless, caution remains since excessive lengthening of cardiac repolarization may occur after administration of DPH with other drugs due to 1) concomitant block of other ionic currents or 2) pharmacokinetic interactions leading to toxic concentrations of DPH.
Footnotes
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Send reprint requests to: Dr. Jacques Turgeon, Ph.D., Centre de Recherche, Hôpital Laval, 2725 Chemin Ste-Foy, Sainte-Foy, Québec, G1V 4G5, Canada. E-mail:phajtu{at}hermes.ulaval.ca
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↵1 This work was supported by grant MT 11876 from the Medical Research Council of Canada, by an operating grant from the Heart and Stroke Foundation of Canada, a studentship from the Quebec Heart Institute and the Faculty of Pharmacy, Laval University (M.K.), a research studentship from the Fonds pour la Formation de Chercheurs et l’Aide à la Recherche and a Merck Frosst award (B.D.), scholarships from the Fonds de la Recherche en santé du Québec (P.D., S.P., and B.A.H.), and a scholarship from the Joseph C. Edwards Foundation (J.T.).
- Abbreviations:
- IK
- delayed rectifier potassium current
- IKr
- rapidly activating component of IK
- IKs
- slowly activating component of IK
- MAPD90
- monophasic action potential duration at 90% repolarization
- NAPA
- N-acetylprocainamide
- DPH
- diphenhydramine
- NAPA
- N-acetylprocainamide
- PTCA
- percutaneous coronary angioplasty
- IND
- indapamide
- Received March 17, 1998.
- Accepted September 22, 1998.
- The American Society for Pharmacology and Experimental Therapeutics
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