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Amiodarone is a unique antiarrhythmic agent originally developed as a vasodilator. Classified electrophysiologically as a Type III antiarrhythmic, it also has both nonspecific antisympathetic and direct, fast channel-membrane effects. Hemodynamic effects of orally administered amiodarone (a negative inotropic agent) are usually negligible, and are usually compensated for by induced vasodilation. Effects on thyroid and hepatic function may help to explain some of the unique pharmacologic as well as toxicologic effects of the drug. Amiodarone is poorly bioavailable (20-80%) and undergoes extensive enterohepatic circulation before entry into a central compartment. The principal metabolite, mono-n-desethyl amiodarone is also an antiarrhythmic. From this central compartment, it undergoes extensive tissue distribution (exceptionally high tissue/plasma partition coefficients). The distribution half-life of amiodarone out of the central compartment to peripheral and deep tissue compartments (t1/2 alpha) may be as short as 4 hours. The terminal half-life (t1/2 beta) is both long and variable (9-77 days) secondary to the slow mobilization of the lipophilic medication out of (primarily) adipocytes. A pharmacokinetically based loading scheme is described, and data suggesting a role for routine amiodarone plasma levels are presented.

Objectives: Methadone is a synthetic opioid, an analgesic and an antiaddictive. QT prolongation as well as torsade de pointes ventricular tachycardia and death have been reported with methadone. Methadone’s proarrhythmic toxicity is related to the inhibition of cardiac IKr channel and prolongation of the action potential. We hypothesized that the 2 isomers of methadone may have different effects on the IKr channel. Methods: The effects of the isomers on IKr were evaluated by using an oocytes system with heterogeneously expressed human ether-a-go-go-related gene (HERG) using the 2 electrode voltage clamp technique. r- and s-methadone were obtained by employing chiral high-performance liquid chromatography, separating methadone into 2 isolates, with optical rotations of –141 and +143 degrees. Results: At concentrations of 0.01, 0.03, 0.1, 1 and 3 mM, r/s-methadone produced a dose-dependent inhibition of HERG by 17 ± 5, 23 ± 4, 40 ± 4, 57 ± 3, 69 ± 3 and 80 ± 1%, respectively. The IC₅₀ of r/s-methadone was 0.21 ± 0.02 mM. At 0.1, 0.3 and 1 mM, s-methadone reduced HERG current by 50 ± 4, 76 ± 5 and 87 ± 5%, respectively, while r-methadone reduced HERG current by 26 ± 4, 53 ± 3 and 77 ± 3%, respectively. Conclusions: There was a significant difference (p < 0.01) in the percentage of current inhibition between r- and s-methadone, at 0.1 and 0.3 mM (52% reduction). Thus, r-methadone may be a safer agent due to less QT effect.