A 32-year-old woman undergoing chemotherapy for breast cancer (AC regimen) presents with severe nausea and vomiting 2 hours after the first cycle. She has no history of GERD, gastroparesis, or CNS pathology. Standard antiemetics (5-HT3 antagonist + dexamethasone) were given prophylactically but failed. Which of the following is the most appropriate next agent to add to her regimen?

## Chemotherapy-Induced Nausea and Vomiting (CINV) Management ### Pathophysiology of CINV Highly emetogenic chemotherapy (AC regimen) triggers emesis via multiple pathways: 1. **Acute phase (0–24 hrs):** Chemotherapy damages GI mucosa → serotonin release from enterochromaffin cells → 5-HT3 receptor activation on vagal afferents 2. **Delayed phase (24 hrs–7 days):** Substance P release → NK1 receptor activation in chemoreceptor trigger zone and vomiting centre **Key Point:** Breakthrough CINV despite 5-HT3 antagonist + corticosteroid indicates inadequate NK1 blockade, especially in the delayed phase. ### Antiemetic Classes and Mechanisms | Agent Class | Mechanism | Best For | Limitations | |---|---|---|---| | **5-HT3 antagonists** (ondansetron, granisetron) | Block serotonin receptors on vagal afferents | Acute CINV | Ineffective for delayed phase; tachyphylaxis | | **NK1 antagonists** (aprepitant, fosaprepitant) | Block substance P in chemoreceptor trigger zone | Delayed CINV; breakthrough emesis | Longer onset; CYP3A4 inhibitor | | **Corticosteroids** (dexamethasone) | Reduce inflammatory mediators | Delayed CINV (synergistic with NK1) | Hyperglycaemia, immunosuppression | | **Metoclopramide** | D2 antagonist + weak 5-HT4 agonist | Gastric dysmotility; low-dose CINV | Ineffective for high-dose chemotherapy; tardive dyskinesia | | **Phenothiazines** (promethazine) | H1 + muscarinic antagonism | Mild nausea; post-op nausea | Sedation; extrapyramidal effects; weak in CINV | **High-Yield:** NCCN and ASCO guidelines recommend **NK1 antagonist + 5-HT3 antagonist + dexamethasone** for highly emetogenic chemotherapy. Aprepitant is the gold standard for breakthrough CINV. ### Why Aprepitant Is Correct - **Mechanism:** Crosses blood–brain barrier; binds NK1 receptors in chemoreceptor trigger zone and nucleus tractus solitarius - **Timing:** Most effective for delayed emesis (24 hrs–7 days), which is the failure pattern here - **Synergy:** Additive effect with 5-HT3 antagonist and dexamethasone - **Dosing:** 125 mg on day 1, then 80 mg on days 2–3 (given with chemotherapy) **Clinical Pearl:** Aprepitant is a potent CYP3A4 inhibitor — monitor for interactions with concurrent medications (e.g., warfarin, oral contraceptives). ### Why Other Options Fail - **Metoclopramide:** D2 antagonist; effective only for gastric dysmotility and low-dose CINV. Ineffective for highly emetogenic chemotherapy and does not address NK1 pathway. Risk of tardive dyskinesia with prolonged use. - **Promethazine:** First-generation antihistamine with anticholinergic properties. Weak antiemetic in CINV; causes significant sedation and extrapyramidal side effects. Not guideline-recommended for chemotherapy. - **Domperidone:** Peripheral D2 antagonist; does not cross BBB. Ineffective for CINV; used only for gastric dysmotility in non-CNS causes. ## Treatment Algorithm for CINV ```mermaid flowchart TD A[Highly emetogenic chemotherapy planned]:::outcome --> B[Prophylactic: 5-HT3 antagonist + dexamethasone]:::action B --> C{Nausea/vomiting controlled?}:::decision C -->|Yes| D[Continue regimen]:::action C -->|No - breakthrough CINV| E[Add NK1 antagonist aprepitant]:::action E --> F{Response?}:::decision F -->|Yes| G[Continue triple therapy]:::action F -->|No| H[Consider olanzapine or benzodiazepine]:::action ``` **Mnemonic:** **SANK** = **S**erotonin (5-HT3) + **A**prepitant (NK1) + **N**K1 + **K**orticosteroid (dexamethasone) for highly emetogenic chemotherapy.