Ondensetron

# Ondensetron ## Overview - **Classification**: Selective 5-HT3 Receptor Antagonist (Serotonin Antagonist) - **Mechanism**: Blocks serotonin (5-HT3) receptors centrally in the chemoreceptor trigger zone (CTZ) and peripherally on vagal nerve terminals in the gut, preventing nausea and vomiting. ## Primary Indications 1. **Chemotherapy-Induced Nausea and Vomiting (CINV)** - Prevention 2. **Postoperative Nausea and Vomiting (PONV)** - Prevention and treatment 3. **Radiation-Induced Nausea and Vomiting (RINV)** - Prevention ## Adult Dosing ### Standard Dosing **Chemotherapy-Induced Nausea and Vomiting (CINV)** - **Highly Emetogenic**: - **Dose**: **16 mg** - **Frequency**: Once, 30 minutes before chemotherapy. - **Route**: Oral - **Special Considerations**: Often combined with dexamethasone. - **Moderately Emetogenic**: - **Dose**: **8 mg** - **Frequency**: Twice daily (q12h) for 1-2 days after chemotherapy. - **Route**: Oral - **Duration**: Up to 2 days post-chemo. **Postoperative Nausea and Vomiting (PONV)** - **Prevention**: - **Dose**: **4 mg** - **Frequency**: Single dose at induction of anesthesia. - **Route**: IV or Oral. - **Treatment**: - **Dose**: **4 mg** - **Frequency**: Single dose (can repeat once if needed). - **Route**: IV or Oral. - **Maximum**: **16 mg IV** (single dose) in surgical setting. **Radiation-Induced Nausea and Vomiting (RINV)** - **Total Body Irradiation**: - **Dose**: **8 mg** - **Frequency**: 1-2 hours before daily fractions. - **Route**: Oral. - **Fractionated Abdominal Radiation**: - **Dose**: **8 mg** - **Frequency**: Three times daily (q8h). - **Route**: Oral. - **Duration**: Throughout treatment course. ### Dose Adjustments - **Renal Impairment**: No dose adjustment necessary. - **Hepatic Impairment**: - **Severe Impairment (Child-Pugh score ≥10)**: - **Max Dose**: **8 mg/day**. - **Frequency**: Once daily. - **Elderly Patients**: - For **IV CINV prophylaxis**, a single dose > **16 mg** is not recommended due to increased QT prolongation risk. - **Oral dosing**: No general adjustment needed unless severe hepatic impairment. ## Pediatric Dosing ### Neonates (0-28 days) - **Use**: Not generally recommended due to limited data. - **Special Notes**: Use only if potential benefit outweighs risks, under specialist guidance. - **Safety**: Increased risk of QT prolongation in this age group. ### Infants (1-12 months) - **Use**: Often off-label, especially for younger infants. - **CINV (≥6 months)**: - **Dose**: **0.15 mg/kg/dose** (IV) - **Frequency**: 30 min before chemo, then 4 & 8 hrs after 1st dose. - **Maximum**: **16 mg/dose**. - **PONV (≥1 month)**: - **Dose**: **0.1 mg/kg/dose** (IV) - **Frequency**: Single dose postoperatively. - **Maximum**: **4 mg/dose**. ### Children (1-12 years) - **CINV**: - **Dose**: **0.15 mg/kg/dose** (IV) - **Frequency**: 30 min before chemo, then 4 & 8 hrs after 1st dose. - **Maximum**: **16 mg/dose**. - **Oral Alternative**: **4 mg** (if >12 kg and >4 years) **or 8 mg** (if >24 kg). - **Oral Frequency**: 30 min before chemo, then 4 & 8 hrs after 1st dose. - **PONV**: - **Dose**: **0.1 mg/kg/dose** (IV) - **Frequency**: Single dose postoperatively. - **Maximum**: **4 mg/dose**. - **Oral Alternative**: **4 mg** (if >40 kg) **or 8 mg** (if >80 kg). ### Adolescents (13-18 years) - **Dose**: Generally follows **adult dosing** guidelines. - **Maximum**: Standard adult maximum doses apply. - **Special Notes**: Consider lower IV single doses (e.g., **4-8 mg**) due to QT risk, especially in females. ## Safety Information ### Contraindications - **Absolute**: Concomitant use with **apomorphine** (risk of profound hypotension/loss of consciousness). - **Absolute**: Hypersensitivity to ondansetron or any component. - **Absolute**: Known long QT syndrome. - **Relative**: Uncorrected hypokalemia/hypomagnesemia. ### Common Adverse Effects - **Very Common (>10%)**: Headache, Constipation. - **Common (1-10%)**: Diarrhea, Dizziness, Fatigue, Dry mouth. - **Serious but Rare**: QT interval prolongation, Torsades de Pointes, Serotonin Syndrome (with other serotonergic drugs), Hypersensitivity reactions (anaphylaxis), Stevens-Johnson Syndrome. ### Key Drug Interactions - **Apomorphine**: **CONTRAINDICATED** (severe hypotension). - **QT-prolonging agents** (e.g., amiodarone, sotalol, quinidine, antipsychotics): Increased risk of Torsades de Pointes; **avoid concurrent use if possible**. - **Serotonergic drugs** (e.g., SSRIs, SNRIs, MAOIs, TCAs, fentanyl, tramadol): Increased risk of Serotonin Syndrome. **Monitor closely**. - **Phenytoin, Carbamazepine, Rifampin**: May decrease ondansetron plasma levels. **Monitor for reduced efficacy**. ## Monitoring & Follow-up - **Before Treatment**: Assess baseline ECG if patient has cardiac risk factors or is on QT-prolonging drugs. Check electrolytes (K, Mg). - **During Treatment**: Monitor for effective nausea/vomiting control. - **Clinical Signs**: Watch for signs of headache, constipation, or more serious adverse effects like dizziness, irregular heartbeat (QT prolongation), or altered mental status (Serotonin Syndrome). ## Clinical Pearls - 💡 **IV Rate**: Administer IV doses over **2-5 minutes**; rapid IV bolus can increase QT risk. - 💡 **ODT**: Ondansetron ODT (orally disintegrating tablet) is great for patients who cannot swallow tablets or have severe nausea. - 💡 **ECG Caution**: Use with caution in patients with uncorrected electrolyte abnormalities, heart failure, bradyarrhythmias, or those on diuretics. - 💡 **Constipation**: Prophylactic use of stool softeners may be beneficial in patients prone to constipation. > **⚠️ Important**: This information is for educational purposes only. Always consult current prescribing information, local guidelines, and clinical judgment before prescribing.