Correct Answer: A. NK-1 antagonist
Aprepitant is a selective neurokinin-1 (NK-1) receptor antagonist that crosses the blood-brain chemoreceptor trigger zone barrier to block substance P, the primary neuropeptide mediating chemotherapy-induced nausea and vomiting (CINV). Unlike 5-HT3 antagonists (ondansetron) and corticosteroids, which address acute and delayed CINV through different pathways, aprepitant uniquely targets the emetic center in the chemoreceptor trigger zone where substance P concentration is highest. This mechanism is particularly effective for late-onset CINV (occurring >24 hours post-chemotherapy), where tachyphylaxis to 5-HT3 blockers occurs. Aprepitant is a prodrug metabolized by CYP3A4, requiring dose adjustment with strong CYP3A4 inhibitors/inducers—a critical point in Indian oncology practice where drug interactions with antiretrovirals (in HIV+ cancer patients) or antifungals are common. The drug is typically given as part of a triple antiemetic regimen (aprepitant + 5-HT3 antagonist + corticosteroid) in Indian cancer centers for highly emetogenic chemotherapy (cisplatin, doxorubicin). Its efficacy in late-onset CINV has made it standard of care in tertiary oncology units across India.
Why the other options are wrong
B. NK-2 antagonist — NK-2 receptors are not the primary mediators of CINV; substance P acts predominantly via NK-1 receptors in the chemoreceptor trigger zone. NK-2 antagonists have shown minimal efficacy in CINV prevention in clinical trials. This is a distractor exploiting confusion between neurokinin receptor subtypes. C. NK-1 agonist — An NK-1 agonist would enhance substance P signaling and worsen nausea and vomiting, not prevent it. Aprepitant is an antagonist (blocks the receptor), not an agonist. This trap tests whether students understand the direction of receptor modulation in CINV pathophysiology. D. NK-3 antagonist — NK-3 receptors are involved in neuroendocrine and behavioral functions, not CINV. While NK-3 antagonists have been studied for other indications (depression, anxiety), they lack efficacy in chemotherapy-induced emesis. This is a distractor using a plausible but incorrect neurokinin subtype.
High-Yield Facts
- Aprepitant is an NK-1 receptor antagonist, not NK-2 or NK-3—substance P is the ligand.
- Late-onset CINV (>24 hours post-chemo) is the primary indication; aprepitant is superior to 5-HT3 antagonists alone in this window.
- Triple antiemetic regimen: aprepitant + ondansetron + dexamethasone is standard in Indian oncology for highly emetogenic agents (cisplatin).
- CYP3A4 metabolism requires dose adjustment with ketoconazole, ritonavir, and other strong inhibitors—critical in HIV+ cancer patients.
- Substance P concentration in the chemoreceptor trigger zone is the discriminating target; 5-HT3 antagonists block serotonin, a different pathway.
Mnemonics
NK-1 = Nausea Killer-1 NK-1 blocks substance P → stops nausea. NK-2 and NK-3 are not involved in CINV. Remember: 1 = One drug for CINV (aprepitant). CINV Timing Rule Acute (0–24 h) → 5-HT3 blocker + steroid. Delayed (>24 h) → Add NK-1 antagonist. Aprepitant shines in the delayed phase.
NBE Trap
NBE pairs "NK-1 antagonist" with "late-onset CINV" to test whether students confuse NK-1 with NK-2/NK-3 or mistake antagonism for agonism—a common error in receptor pharmacology questions.
Clinical Pearl
In Indian tertiary cancer centers, aprepitant is reserved for highly emetogenic regimens (cisplatin, doxorubicin) due to cost; it is often unavailable in tier-2 facilities, making 5-HT3 antagonists + steroids the practical standard. However, when available, aprepitant dramatically improves quality of life in late-onset CINV, which is the most distressing phase for patients.
_Reference: KD Tripathi Pharmacology Ch. 72 (Anticancer Drugs); Harrison Ch. 100 (Nausea and Vomiting)_