Ondasetron

# Ondasetron ## Overview - **Classification**: Serotonin 5-HT3 receptor antagonist, antiemetic. - **Mechanism**: Selectively blocks serotonin (5-HT3) receptors located on vagal nerve terminals and in the chemoreceptor trigger zone (CTZ). ## Primary Indications 1. **Chemotherapy-Induced Nausea & Vomiting (CINV)** - Prevention. 2. **Postoperative Nausea & Vomiting (PONV)** - Prevention and treatment. 3. **Radiation-Induced Nausea & Vomiting (RINV)** - Prevention. 4. **Gastroenteritis-related vomiting** (off-label, esp. pediatric). ## Adult Dosing ### Standard Dosing **CINV (Highly Emetogenic Chemotherapy)** - **Dose**: **16 mg** oral OR **8 mg** IV - **Frequency**: Single dose - **Route**: Oral, IV - **Special Considerations**: Administer 30 minutes prior to chemotherapy. Single IV dose > **16 mg** is not recommended due to increased QT prolongation risk. **CINV (Moderately Emetogenic Chemotherapy)** - **Dose**: **8 mg** oral OR **8 mg** IV - **Frequency**: Single dose - **Route**: Oral, IV - **Special Considerations**: Administer 30 minutes prior to chemotherapy. **PONV (Prevention)** - **Dose**: **4 mg** IV OR **16 mg** oral - **Frequency**: Single dose - **Route**: IV (administer over 2-5 min), Oral - **Special Considerations**: Administer immediately before induction of anesthesia (IV) or 1 hour prior to anesthesia (oral). **PONV (Treatment)** - **Dose**: **4 mg** - **Frequency**: Single dose - **Route**: IV (administer over 2-5 min) **RINV (Total Body Irradiation)** - **Dose**: **8 mg** - **Frequency**: Single dose, 1-2 hours before each fraction - **Route**: Oral ### Dose Adjustments - **Renal Impairment**: No dose adjustment required. - **Hepatic Impairment**: **Child-Pugh B or C**: Maximum total daily dose is **8 mg**. - **Elderly Patients**: No general adjustment. Avoid single IV doses > **16 mg** due to QT prolongation risk. ## Pediatric Dosing ### Neonates (0-28 days) - **Special Notes**: Not recommended due to lack of safety/efficacy data. Potential for serious adverse events (QT prolongation, TdP) in this age group. Use only if benefits outweigh risks significantly, under specialist guidance. ### Infants (1-12 months) **PONV** - **Dose**: **0.15 mg/kg** IV - **Frequency**: Single dose - **Maximum**: **4 mg/dose** - **Special Notes**: Administer slowly over 2-5 minutes. ### Children (1-12 years) **CINV** - **Dose**: **0.15 mg/kg** IV - **Frequency**: 3 doses (30 min before chemotherapy, then 4 and 8 hours after first dose) - **Maximum**: **16 mg/dose** for the first dose; **8 mg/dose** for subsequent doses. **PONV** - **Dose**: **0.15 mg/kg** IV OR **4 mg** oral (for children >40 kg) - **Frequency**: Single dose - **Maximum**: **4 mg/dose** (IV) - **Special Notes**: Administer slowly over 2-5 minutes (IV). Oral dose 1 hour pre-anesthesia. **Gastroenteritis-related vomiting (off-label)** - **Dose**: **0.15 mg/kg** oral (ODT or solution) - **Frequency**: Single dose, may repeat once after 4-6 hours if needed. - **Maximum**: **8 mg/dose** - **Special Notes**: Consider for prevention of dehydration/need for IV fluids. ### Adolescents (13-18 years) - **Dose**: Follow **adult dosing** recommendations. - **Maximum**: Follow **adult maximum doses**. ## Safety Information ### Contraindications - **Absolute**: Concomitant use with **apomorphine**. - **Absolute**: Hypersensitivity to ondansetron or any component. - **Relative**: Congenital long QT syndrome, uncorrected electrolyte abnormalities. ### Common Adverse Effects - **Very Common (>10%)**: Headache, constipation. - **Common (1-10%)**: Diarrhea, fatigue, dizziness, dry mouth, injection site reactions. - **Serious but Rare**: QT prolongation, Torsades de Pointes (TdP), serotonin syndrome, extrapyramidal symptoms, hypersensitivity (anaphylaxis). ### Key Drug Interactions - **Apomorphine**: Severe hypotension and loss of consciousness. **Concomitant use is absolutely contraindicated**. - **QT-prolonging drugs (e.g., amiodarone, sotalol, antipsychotics)**: Increased risk of QT prolongation. Monitor ECG, use with caution. - **Serotonergic drugs (e.g., SSRIs, SNRIs, MAOIs, TCAs, fentanyl)**: Increased risk of **serotonin syndrome**. Monitor for mental status changes, autonomic instability, neuromuscular changes. - **CYP3A4 inducers (e.g., rifampin)**: May decrease ondansetron plasma concentrations. Clinical significance usually low. ## Monitoring & Follow-up - **Before Treatment**: Baseline ECG if cardiac risk factors or on QT-prolonging drugs. Baseline electrolytes (potassium, magnesium). - **During Treatment**: Monitor for QTc prolongation, especially with IV doses > **16 mg** or in patients with risk factors. - **Clinical Signs**: Watch for constipation, headache, signs of serotonin syndrome (agitation, hyperthermia, tremor), extrapyramidal symptoms, allergic reactions. ## Clinical Pearls - 💡 **Tip 1**: Administer IV ondansetron slowly (**over 2-5 minutes**) to minimize headache/lightheadedness. - 💡 **Tip 2**: For CINV, administer **30 minutes prior** to the start of chemotherapy for optimal effect. - 💡 **Tip 3**: Oral disintegrating tablets (ODT) are excellent for patients with difficulty swallowing or active vomiting. - 💡 **Tip 4**: Always ensure adequate hydration, especially in patients with vomiting. - 💡 **Tip 5**: Be aware of additive QT-prolonging effects when prescribing with other medications that affect cardiac repolarization. > **⚠️ Important**: This information is for educational purposes only. Always consult current prescribing information, local guidelines, and clinical judgment before prescribing.