```html

Quizartinib‑Related QT Prolongation

---

Overview

QT prolongation is an abnormal lengthening of the QT interval on an electrocardiogram (ECG), which represents the time it takes for the heart’s ventricles to repolarize (reset) after each beat. When the QT interval is excessively prolonged, it can predispose a person to a dangerous ventricular arrhythmia called torsades de pointes (TdP) and, in rare cases, sudden cardiac death.

Quizartinib (brand name Vanflyta) is an oral, highly selective FLT3 inhibitor approved in several countries for the treatment of relapsed or refractory acute myeloid leukemia (AML) with FLT3‑ITD mutations. Because quizartinib blocks the cardiac potassium channel hERG (IKr), it can lengthen the QT interval. The FDA and EMA have placed a boxed warning on quizartinib for “QT‑interval prolongation and risk of torsades de pointes.”

Who is affected? All patients receiving quizartinib are at risk, but the likelihood is higher in individuals with:

• Pre‑existing cardiac disease (e.g., congenital long‑QT syndrome, heart failure, myocardial infarction)
• Electrolyte abnormalities (low potassium, magnesium, or calcium)
• Concomitant use of other QT‑prolonging drugs
• Renal or hepatic impairment that raises quizartinib plasma levels
• Elderly patients (≥65 years) who often have comorbidities

Prevalence: In the pivotal phase II trial (QuANTUM‑R), clinically significant QTc prolongation (≥500 ms) occurred in 7–10 % of patients on quizartinib, with TdP reported in ≤1 % (Miller et al., 2020). Real‑world registries echo similar rates, underscoring the need for vigilant monitoring.

Symptoms

QT prolongation itself is usually silent and is detected on routine ECG. However, when it leads to arrhythmias, patients may notice the following:

• Dizziness or Light‑headedness – Caused by transient drops in cerebral perfusion.
• Palpitations – Sensation of “fluttering” or “skipping” beats, often described as irregular heartbeats.
• Syncope (Fainting) – Sudden loss of consciousness lasting seconds to minutes.
• Chest Discomfort – May feel like pressure, tightness, or pain, especially if arrhythmia persists.
• Shortness of Breath – Particularly with rapid ventricular rates or when cardiac output falls.
• Seizure‑like Activity – Rare, usually secondary to prolonged cerebral hypoxia during TdP.
• Fatigue – Generalized tiredness that does not improve with rest, often a late manifestation of reduced cardiac output.

Because many of these signs overlap with AML‑related complications (anemia, infection, etc.), any new or worsening cardiovascular symptom while on quizartinib should prompt immediate evaluation.

Causes and Risk Factors

Mechanism of Quizartinib‑Induced QT Prolongation

Quizartinib blocks the hERG potassium channel (encoded by the KCNH2 gene), which is essential for the rapid component (I<sub>Kr</sub>) of the cardiac repolarizing current. Inhibition of I<sub>Kr</sub> slows the outward flow of potassium during phase 3 of the cardiac action potential, extending the QT interval. The effect is dose‑dependent; higher plasma concentrations produce a larger QT effect.

Key Risk Factors

• Baseline QTc ≥450 ms (men) or ≥470 ms (women) – Already prolonged intervals magnify the drug’s effect.
• Concomitant QT‑prolonging agents – e.g., azole antifungals (voriconazole), macrolide antibiotics (azithromycin), certain anti‑emetics (ondansetron), and some antidepressants (citalopram).
• Electrolyte disturbances – Hypokalemia (< 3.5 mmol/L), hypomagnesemia (< 0.7 mmol/L), or hypocalcemia.
• Renal or hepatic dysfunction – Reduced clearance raises drug levels.
• Genetic predisposition – Polymorphisms in CYP3A4/5 (affecting quizartinib metabolism) or in the KCNH2 gene.
• High-dose quizartinib – The standard dosing schedule (40 mg daily) may be reduced to 30 mg or 20 mg if QTc exceeds predefined thresholds.
• Age & comorbidities – Older age and chronic diseases (diabetes, hypertension) increase susceptibility.

Diagnosis

Diagnosis hinges on electrocardiographic findings, clinical context, and exclusion of other causes.

1. Baseline Assessment (Before Starting Quizartinib)

• 12‑lead ECG – Measure corrected QT (QTc) using Fridericia’s formula (QTc = QT/∛RR).
• Serum electrolytes – Potassium, magnesium, calcium.
• Renal & hepatic function tests.
• Medication review – Identify other QT‑prolonging drugs.

2. Ongoing Monitoring

• Day 1 (first dose) – ECG 2–4 hours after ingestion (peak plasma level).
• Day 3–7 – Serial ECGs every 48 hours until steady‑state (approximately 7 days).
• Monthly thereafter – ECG plus electrolytes, especially if dose modifications occur.

3. Diagnostic Criteria

Quizartinib‑related QT prolongation is confirmed when:

1. QTc increases ≥30 ms from baseline, or
2. QTc reaches ≥500 ms (or ≥60 ms increase in patients with baseline <450 ms),
3. Other causes (e.g., electrolyte abnormalities, other drugs) have been ruled out.

4. Additional Tests (if arrhythmia suspected)

• Continuous telemetry or Holter monitoring – Detect transient TdP episodes.
• Serum drug levels (if available) – Correlate high quizartinib concentrations with QTc spikes.
• Genetic testing – Consider in patients with unexplained severe prolongation.

Treatment Options

Treatment aims to correct the QT interval, manage arrhythmias, and allow continuation of effective AML therapy whenever possible.

Immediate Management of Significant QTc Prolongation (QTc ≥500 ms)

• Hold quizartinib – Discontinue until QTc falls < 480 ms.
• Correct electrolytes – IV potassium (target 4.5–5.0 mmol/L) and magnesium (target > 2.0 mg/dL).
• Evaluate drug interactions – Stop or substitute other QT‑prolonging meds.
• Initiate beta‑blocker therapy (e.g., propranolol 10–40 mg PO q6h) if TdP is documented, as it can suppress ventricular ectopy.
• Temporary pacing – In refractory TdP, overdrive pacing at 90–110 bpm may terminate arrhythmia (Cleveland Clinic, 2021).

Re‑initiation Strategies

1. Resume quizartinib at a reduced dose (e.g., 30 mg → 20 mg) once QTc < 480 ms and electrolytes are stable.
2. Implement stricter ECG monitoring (e.g., weekly for 4 weeks).
3. Consider co‑administration of a potassium‑sparingly diet and oral magnesium supplements.

Pharmacologic Alternatives for AML

If QT prolongation cannot be controlled, clinicians may switch to other FLT3 inhibitors with lower cardiac risk (e.g., gilteritinib) or to non‑targeted regimens, after weighing oncologic efficacy versus cardiac safety.

Supportive Lifestyle Measures

• Maintain adequate hydration.
• Avoid excessive alcohol or stimulants (caffeine, nicotine).
• Adopt a heart‑healthy diet rich in potassium (bananas, avocados) and magnesium (nuts, leafy greens).

Living with Quizartinib‑Related QT Prolongation

Even when QTc is mildly prolonged, patients can lead normal lives with a few practical steps:

• Adhere to ECG appointments – Missing monitoring visits can delay detection of dangerous changes.
• Track symptoms – Keep a daily log of palpitations, dizziness, or syncope; share with your oncology/ cardiology team.
• Medication diary – Write down all prescription, over‑the‑counter (OTC), and herbal products.
• Electrolyte vigilance – Request periodic labs; if K⁺ or Mg²⁺ drop below target, call your clinic.
• Emergency plan – Know the nearest emergency department and have a copy of your ECG records handy.
• Physical activity – Light‑to‑moderate exercise (e.g., walking, stationary biking) is generally safe; avoid extreme exertion that may trigger arrhythmias until cleared by your doctor.

Prevention

Prevention focuses on minimizing modifiable risk factors before and during quizartinib therapy.

1. Pre‑treatment screening – Baseline ECG, electrolytes, and review of concomitant meds.
2. Medication reconciliation – Substitute QT‑prolonging agents when possible (e.g., use levofloxacin instead of moxifloxacin).
3. Electrolyte optimization – Encourage a diet rich in potassium and magnesium; supplement when labs are low.
4. Renal/hepatic dose adjustment – Follow manufacturer guidelines for patients with CrCl < 30 mL/min or severe hepatic impairment.
5. Patient education – Provide written material about warning signs and the importance of timely ECGs.
6. Use of automated QT‑monitoring software – Many oncology centers integrate QTc alerts into electronic health records.

Complications

If QT prolongation is not recognized or treated, the following may occur:

• Torsades de pointes (TdP) – A polymorphic ventricular tachycardia that can degenerate into ventricular fibrillation.
• Sudden cardiac death – Estimated in <1 % of quizartinib‑treated patients but carries a high fatality rate.
• Syncope‑related injuries – Falls leading to fractures or head trauma, especially in older adults.
• Interruption of AML therapy – Discontinuation of quizartinib may reduce remission rates.
• Psychological stress – Anxiety about cardiac events can affect quality of life and adherence.

When to Seek Emergency Care

---

References

• Miller, A. et al. (2020). “QT‑interval prolongation with quizartinib in relapsed/refractory FLT3‑ITD AML.” Blood, 135(24): 2202‑2210. DOI:10.1182/blood.2020006210.
• U.S. Food & Drug Administration. (2022). “Quizartinib (Vanflyta) Prescribing Information.” Retrieved from fda.gov.
• Mayo Clinic. (2023). “Long QT syndrome.” Retrieved from mayoclinic.org.
• Cleveland Clinic. (2021). “Management of Torsades de Pointes.” Retrieved from clevelandclinic.org.
• World Health Organization. (2022). “Electrolyte disorders and cardiac arrhythmias.” WHO Guidelines.
• National Heart, Lung, and Blood Institute. (2024). “QT Prolongation.” Retrieved from nhlbi.nih.gov.

```