Azithromycin Causes a Novel Proarrhythmic Syndrome

Abstract

Background: The widely used macrolide antibiotic azithromycin increases risk of cardiovascular and sudden cardiac death, although the underlying mechanisms are unclear. Case reports, including the one we document here, demonstrate that azithromycin can cause rapid, polymorphic ventricular tachycardia in the absence of QT prolongation, indicating a novel proarrhythmic syndrome. We investigated the electrophysiological effects of azithromycin in vivo and in vitro using mice, cardiomyocytes, and human ion channels heterologously expressed in human embryonic kidney (HEK 293) and Chinese hamster ovary (CHO) cells.

Methods and results: In conscious telemetered mice, acute intraperitoneal and oral administration of azithromycin caused effects consistent with multi-ion channel block, with significant sinus slowing and increased PR, QRS, QT, and QTc intervals, as seen with azithromycin overdose. Similarly, in HL-1 cardiomyocytes, the drug slowed sinus automaticity, reduced phase 0 upstroke slope, and prolonged action potential duration. Acute exposure to azithromycin reduced peak SCN5A currents in HEK cells (IC₅₀=110±3 μmol/L) and Na⁺ current in mouse ventricular myocytes. However, with chronic (24 hour) exposure, azithromycin caused a ≈2-fold increase in both peak and late SCN5A currents, with findings confirmed for I_Na in cardiomyocytes. Mild block occurred for K⁺ currents representing I_Kr (CHO cells expressing hERG; IC₅₀=219±21 μmol/L) and I_Ks (CHO cells expressing KCNQ1+KCNE1; IC₅₀=184±12 μmol/L), whereas azithromycin suppressed L-type Ca⁺⁺ currents (rabbit ventricular myocytes, IC₅₀=66.5±4 μmol/L) and I_K1 (HEK cells expressing Kir2.1, IC₅₀=44±3 μmol/L).

Conclusions: Chronic exposure to azithromycin increases cardiac Na⁺ current to promote intracellular Na⁺ loading, providing a potential mechanistic basis for the novel form of proarrhythmia seen with this macrolide antibiotic.

Keywords: calcium channel; mice; pharmacology; potassium channels; sodium channels.

Figure 1

Azithromycin-induced polymorphic VT in a 24 year old female with no structural heart disease and a normal ECG. The arrhythmias resolved with stopping the drug.

Figure 2

Effects of IP azithromycin (AZ) on the electrocardiogram of conscious adult mice. Mice received intraperitoneal injection of AZ (50 mg/kg, followed in 60 min by 100 mg/kg). Representative ECG tracings (and interval measurements) are shown in the upper panel, with parameter data for the group shown below (n=7). With both doses of AZ, heart rate declined, along with an increase in the PR, QRS, and QT/QTc intervals.

Figure 3

Concentration-dependent block of human and mouse cardiac Na⁺ currents with acute AZ exposure. A and B. In HEK cells expressing human SCN5A, acute exposure to AZ significantly reduced I_Na (from −967±54 to −769±49 and −512±38 pA/pF for AZ 50 and 100μM, respectively, at −20 mV [n=18, 14]; ^†P<0.01; representative current tracings in A and current-voltage plot of summary data in B). C and D. The concentration-response curve demonstrated an IC₅₀ of 110±3μM for SCN5A currents (C; n=6 for each concentration), with similar results obtained using mouse ventricular myocytes (D; VM; IC₅₀ 116±4 μM; n=5 for each concentration). E and F. Using the voltage clamp protocols shown in the insets, normalized steady-state inactivation (E) and activation (F) curves for SCN5A were generated and fit with a Boltzmann function. AZ caused small but significant shifts in the hyperpolarizing and depolarizing directions, respectively (n=14–32; ^†P<0.01 for AZ 100μM in E and F).

Figure 4

Increased peak Na⁺ current with chronic AZ exposure. A and B. For SCN5A currents in HEK cells, incubation with AZ (50μM) for 24 hr significantly increased I_Na (from −942±80 pA/pF to −1644±76 pA/pF at −20 mV; n=13 each; ^†P<0.01). C and D. In the same preparation, similar shifts were observed in the steady-state inactivation and activation curves, as those seen with acute exposure (for inactivation, −106.6±1.2 mV to −119.9±1.2 mV for control and AZ 50μM, respectively; for activation, −41.0±2 mV to −35.3±1 mV; n=14 and *P<0.05 for both).

Figure 5

Time- and concentration-dependent increases in late and peak I_Na. A. For SCN5A currents, examples of tetrodotoxin-sensitive Na⁺ current (top panel) are shown in the absence (control; middle panel) and presence of 24 hr incubation with AZ (50μM; bottom panel). B. Summary data demonstrate a significant increase in late I_Na (from 0.18±0.02 pA/pF to 0.48±0.03pA/pF; n=6; ^†P<0.01) with chronic (24 hr) but not acute exposure. C and D. The concentration-response relationships for the increase in peak (C) and late (D) SCN5A currents following a 24hr exposure to azithromycin are illustrated.

Figure 6

Effects of AZ on hERG currents (I_Kr). A. In HEK cells expressing human KCNH2 (hERG), K⁺ currents were recorded in the absence and presence of AZ 100 μM. B and C. Current-voltage relationships are shown for steady-state (B) and tail (C) current before and following acute exposure to AZ, with a reduction of 34 and 30% at +60 mV, respectively (n=15; IC₅₀ 219±21μM; ^†P<0.01).

Figure 7

Minimal reduction in I_Ks current. A. In CHO cells stably expressing KCNQ1 and KCNE1, K⁺ currents were recorded using the voltage clamp protocol shown in the inset before and following acute exposure to AZ 100μM. I_Ks steady-state current was suppressed by 30% at +80 mV (n=12; *P<0.05). B. Current-voltage plot of tail current demonstrated a 30% reduction at +80 mV (n=12; *P<0.05). C. The concentration-response curve demonstrated an IC₅₀ of 184±12μM.

Figure 8

Effects of AZ on Ca⁺⁺ currents in rabbit ventricular myocytes. A and B. Families of currents and peak current-voltage relationships are illustrated for nimodipine-sensitive Ca⁺⁺ currents, indicative of I_Ca-L, before and following acute exposure to AZ 100 μM, with a 65% reduction at +20 mV (n=7; ^†P<0.01; IC₅₀ 67±4μM). C and D. Similar results are shown for mibefradil-sensitive Ca⁺⁺ currents representing I_Ca-T, with 29% suppression at −20 mV (n=8; *P<0.05).