A 52-year-old woman undergoing chemotherapy with cisplatin develops severe nausea and vomiting. Among the following antiemetic agents, which is the most commonly used first-line agent for chemotherapy-induced nausea and vomiting (CINV) in current clinical practice?

Explanation

## Chemotherapy-Induced Nausea and Vomiting (CINV) Management ### Pathophysiology of CINV Chemotherapy agents (especially platinum compounds like cisplatin) trigger emesis through multiple mechanisms: 1. Direct activation of the chemoreceptor trigger zone (CTZ) 2. Vagal afferent stimulation from the GI tract → release of serotonin (5-HT) from enterochromaffin cells 3. Activation of 5-HT3 receptors on vagal afferents and in the nucleus tractus solitarius **Key Point:** The serotonergic pathway is the PREDOMINANT mechanism in CINV, making 5-HT3 antagonists the most effective and first-line agents. ### First-Line Antiemetic for CINV **High-Yield:** Ondansetron and other 5-HT3 antagonists (granisetron, tropisetron) are the gold-standard, most commonly used first-line agents for CINV. They are superior to dopamine antagonists in chemotherapy-induced emesis. ### Comparative Efficacy in CINV | Agent | Mechanism | Efficacy in CINV | Role | |---|---|---|---| | **Ondansetron** | **5-HT3 antagonist** | **Excellent (70–80%)** | **First-line, standard of care** | | Granisetron | 5-HT3 antagonist | Excellent (70–80%) | First-line alternative | | Metoclopramide | D2 antagonist | Poor (20–30%) | Not effective for CINV | | Promethazine | H1 antagonist + anticholinergic | Moderate (40–50%) | Adjunctive only | | Scopolamine | Anticholinergic | Poor (20–30%) | Motion sickness, not CINV | ### Mechanism of 5-HT3 Antagonists in CINV ```mermaid flowchart TD A[Chemotherapy agent<br/>e.g., Cisplatin]:::action --> B[Damage to GI mucosa] B --> C[Release of 5-HT from<br/>enterochromaffin cells]:::outcome C --> D[Activation of 5-HT3 receptors<br/>on vagal afferents]:::outcome D --> E{5-HT3 antagonist<br/>present?}:::decision E -->|Yes: Ondansetron| F[Blocks 5-HT3 receptors]:::action F --> G[Nausea/vomiting<br/>prevented]:::outcome E -->|No| H[Signal reaches<br/>vomiting centre]:::urgent H --> I[Severe CINV]:::urgent ``` ### Clinical Efficacy Data **Clinical Pearl:** Ondansetron achieves complete response (no nausea or vomiting) in 70–80% of patients receiving highly emetogenic chemotherapy when combined with dexamethasone and an NK1 antagonist (aprepitant). Metoclopramide, by contrast, is ineffective in CINV because the dopaminergic pathway is not the primary mechanism. ### Why Ondansetron is Preferred 1. **Mechanism fit:** Directly blocks the serotonergic pathway activated by chemotherapy 2. **Efficacy:** Superior to dopamine antagonists in CINV (70–80% vs. 20–30%) 3. **Safety:** Well-tolerated; no tardive dyskinesia risk (unlike metoclopramide) 4. **Guidelines:** Recommended as first-line by ASCO, NCCN, and ESMO **Mnemonic:** **5-HT3 = CINV's **H**ero** — Serotonin antagonism is the key to chemotherapy-induced nausea control. [cite:Harrison 21e Ch 297; KD Tripathi 8e Ch 16]