IKr Blockade

The Channel: hERG/KV11.1

IKr (the rapid component of the delayed rectifier potassium current) is carried through channels encoded by the KCNH2 gene (also called hERG - human ether-a-go-go related gene). These channels open during late phase 2 and phase 3 of the cardiac action potential, driving rapid repolarization back toward the resting membrane potential. Their distinctive structural feature - a large inner vestibule - is why an enormous variety of drugs block them as an off-target effect, making IKr blockade the most common drug-induced cause of QT prolongation.

Electrophysiological Consequences

When IKr is blocked, repolarization is slowed. The result, at the cellular level, is prolongation of the action potential duration (APD), and at the surface ECG level, QT interval prolongation. This creates the substrate for early afterdepolarizations (EADs) - spontaneous membrane oscillations during the prolonged plateau phase that can trigger ventricular ectopy.

The most feared arrhythmia is torsades de pointes (TdP) - a polymorphic ventricular tachycardia that can degenerate into ventricular fibrillation. TdP is characteristically:

Initiated by a "short-long-short" RR sequence (pause-dependent)
Seen more with slow heart rates (the IKr block worsens at slow rates - so-called reverse use-dependence)
Promoted by hypokalemia and hypomagnesemia

Repolarization Reserve

A key concept is repolarization reserve: the heart maintains multiple, partially redundant repolarizing currents (IKr, IKs, IK1, Ito) so that impairment of one does not immediately cause pathological APD prolongation. IKr blockade may be subclinical at baseline but becomes dangerous when additional hits occur:

Hypokalemia - directly reduces IKr conductance AND increases binding affinity of IKr-blocking drugs (e.g., dofetilide), compounding the blockade. Also impairs Na+/K+-ATPase, increasing intracellular Ca²+ and afterdepolarization risk
Genetic predisposition - 10-15% of patients with drug-induced TdP harbor latent ion channel mutations (e.g., KCNH2 polymorphisms like p.K897T). These variants reduce repolarization reserve, sensitizing the patient to IKr-blocking drugs
Additive drug combinations - multiple QT-prolonging agents together
Drug interactions - CYP3A4 inhibitors increasing plasma levels of IKr blockers
Bradycardia - worsens IKr block due to reverse use-dependence
Female sex - women have intrinsically lower repolarization reserve and greater TdP risk

Drugs That Block IKr

Intentional (Class 3 antiarrhythmics)

These drugs use IKr blockade therapeutically to prolong the refractory period and suppress re-entrant arrhythmias:

Drug	Primary Mechanism	Notes
Dofetilide	Selective, dose-dependent IKr blockade	Block increases in hypokalemia; slow rate of recovery from block means little frequency dependence; requires in-hospital initiation with QTc monitoring
Ibutilide	IKr block + late INa activation	IV only; for acute AF/flutter cardioversion
Sotalol	IKr block + beta-blockade	TdP risk ~6% at highest doses; renally cleared
Quinidine	IKr + INa + alpha/cholinergic blockade	Paradoxically used to suppress TdP in Brugada syndrome via sodium channel effects
Amiodarone	IKr + INa + ICa + beta-blockade (all 4 classes)	Rarely causes TdP despite prominent QT prolongation, due to multi-channel effects that offset pure IKr block

Off-Target (non-cardiac drugs)

Because the hERG channel has an unusually wide inner vestibule, it binds diverse molecules. Clinically important examples include:

Antibiotics: macrolides (erythromycin, azithromycin), fluoroquinolones (moxifloxacin)
Antipsychotics: haloperidol, ziprasidone, prochlorperazine
Antihistamines: terfenadine, astemizole (withdrawn from market)
Antifungals: fluconazole
GI drugs: cisapride, domperidone
Oncologics: multiple targeted therapies, anthracyclines

Nuances Beyond Simple hERG Block

Braunwald's Heart Disease highlights that hERG/IKr blockade alone does not fully explain DI-TdP risk. Emerging evidence shows:

PI3K inhibition is a second mechanism: drugs like dofetilide, sotalol, and azithromycin that also inhibit PI3K and increase late Na+ current (INaL) are more torsadogenic than drugs that cause only hERG blockade. This means the in vitro hERG IC50 used in pre-clinical screening may underestimate true TdP liability
Ranolazine illustrates the complexity: it blocks both INaL and IKr. In normal cells the net effect is slight APD prolongation; in LQT mutant cells it actually shortens APD - making it potentially anti-arrhythmic despite IKr block

Pre-clinical Screening

Regulatory authorities (FDA/ICH E14) require hERG IC50 testing for all new drugs. Screening for hERG block and potential QT prolongation is a standard part of pre-clinical toxicology. Despite this, TdP risk is idiosyncratic - it depends heavily on the individual's genetic repolarization reserve, making population-level screening imperfect.

Clinical Monitoring Points

When prescribing IKr-blocking drugs:

Obtain baseline QTc - withhold if QTc >450 ms (>500 ms with bundle branch block)
Check and correct K+ and Mg2+ before initiation
Avoid bradycardia (<50 bpm is a relative contraindication for dofetilide)
Check for drug interactions (especially CYP3A4 inhibitors that raise plasma drug levels)
For dofetilide: repeat QTc before each of the first 5 doses; if QTc rises to ≥500 ms, reduce dose or discontinue

Sources: Braunwald's Heart Disease (Chapters 62, 63, 64); Katzung's Basic and Clinical Pharmacology, 16th ed.; Goodman & Gilman's Pharmacological Basis of Therapeutics; The Washington Manual of Medical Therapeutics

This is a shared conversation. Sign in to Orris to start your own chat.