Tizanidine (Zanaflex): A Muscle Relaxant That May Prolong the QT Interval by Blocking IKr

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Abstract

Background:

Tizanidine (Zanaflex) is a centrally acting imidazoline muscle relaxant that is structurally similar to clonidine (α2-adrenergic agonist) but not to other myorelaxants such as baclofen or benzodiazepines. Interestingly, cardiac arrhythmias and QT interval prolongation have been reported with tizanidine.

Objective:

To evaluate the effects of tizanidine on cardiac ventricular repolarization.

Methods:

(1) Whole-cell patch-clamp experiments: HERG- or KCNQ1+KCNE1-transfected cells were exposed to tizanidine 0.1-100 μmol/L (n = 29 cells, total) to assess drug effect on the rapid (IKr) and slow (IKs) components of the delayed rectifier potassium current. (2) Langendorff retroperfusion experiments: isolated hearts from male Hartley guinea pigs (n = 6) were exposed to tizanidine 1 μmol/L to assess drug-induced prolongation of monophasic action potential duration measured at 90% repolarization (MAPD90). (3) In vivo wireless cardiac telemetry experiments: guinea pigs (n = 6) implanted with radio transmitters were injected a single intraperitoneal (ip) dose of tizanidine 0.25 mg/kg and 24 hours electrocardiography (ECG) recordings were made.

Results:

(1) Patch-clamp experiments revealed an estimated IC50 for tizanidine on IKr above 100 μmol/L Moreover, tizanidine 1 μmol/L had hardly any effect on IKs (5.23% ± 4.54% inhibition, n = 5 cells). (2) While pacing the hearts at stimulation cycle lengths of 200 or 250 ms, tizanidine 1 μmol/L prolonged MAPD90 by 8.22 ± 2.03 (6.7%) and 11.70 ± 3.08 ms (8.5%), respectively (both P < .05 vs baseline). (3) Tizanidine 0.25 mg/kg ip caused a maximal 11.93 ± 1.49 ms prolongation of corrected QT interval (QTc), 90 minutes after injection.

Conclusion:

Tizanidine prolongs the QT interval by blocking IKr. Patients could be at risk of cardiac proarrhythmia during impaired drug elimination, such as in case of CYP1A2 inhibition during drug interactions.

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