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PH6.2 | PH6.2 | Prokinetics and Antiemetics — SDL Guide — SDL Guide

Learning Objectives

• Describe the vomiting reflex arc, identifying the receptor types at the CTZ, NTS, vestibular nucleus, and vagal afferents that drug classes target
• Classify prokinetic and antiemetic drugs by their primary receptor mechanism
• Distinguish metoclopramide from domperidone in terms of CNS penetration, EPS risk, and cardiac effects
• Select the appropriate antiemetic for a given clinical context: CINV, PONV, motion sickness, pregnancy nausea, and opioid-induced nausea
• Identify cisapride as a withdrawn prokinetic and explain why its use is contraindicated

INSTRUCTIONS

Nausea and vomiting are among the most distressing symptoms in clinical medicine — experienced by chemotherapy patients, post-operative patients, pregnant women, and travellers alike. The pharmacological management of these symptoms is nuanced because different aetiologies involve different receptor pathways, and the wrong drug chosen without understanding mechanism will be ineffective or harmful. This module builds on the neurophysiology of the vomiting reflex and equips you to match drug to mechanism and context with confidence.

References

• Tripathi KD. Essentials of Medical Pharmacology, 9th ed., Ch. 48 (Prokinetics and Antiemetics) (textbook)
• Goodman & Gilman's The Pharmacological Basis of Therapeutics, 13th ed., Ch. 50 (textbook)

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Version 2.0 | NMC CBUC 2024

Pathophysiology of Nausea, Vomiting, and Gastroparesis

Nausea and vomiting represent a coordinated protective reflex mediated by a distributed brainstem network, not a single anatomical vomiting centre. This distinction has profound pharmacological implications: different clinical causes of nausea engage different receptor populations at different nodes of the network, so an antiemetic that works brilliantly for one aetiology will be completely ineffective for another. The most common clinical error in antiemetic prescribing is selecting an agent without knowing which pathway is driving the symptom. Mapping the reflex arc onto its receptor pharmacology is therefore the direct foundation of every prescribing decision in this module.

Nausea and vomiting represent a coordinated protective reflex mediated by a distributed brainstem network, not a single 'vomiting centre.' Recognising the distinct afferent pathways and receptor populations at each node is the foundation of rational antiemetic prescribing — because blocking the wrong receptor at the wrong node achieves nothing.

The vomiting reflex arc has four principal afferent inputs that converge on the nucleus tractus solitarius (NTS), which integrates signals and co-ordinates the efferent motor output (diaphragmatic, abdominal wall, and pharyngeal contractions producing vomiting):

1. Chemoreceptor trigger zone (CTZ) / area postrema: Located outside the blood-brain barrier on the floor of the fourth ventricle, the CTZ is exposed to circulating emetic agents — drugs (opioids, cytotoxic agents, digoxin), uraemic toxins, and metabolic disturbances. It expresses high densities of D₂, 5-HT₃, and NK₁ receptors. Dopaminergic drugs and uraemia trigger emesis predominantly through this route.

1. Vagal and splanchnic afferents from the gut: The intestinal mucosa contains enterochromaffin cells that release serotonin (5-HT) during mechanical distension, mucosal irritation, or after cytotoxic chemotherapy/radiation. This 5-HT activates 5-HT₃ receptors on vagal afferents, which signal the NTS. This is the dominant pathway for chemotherapy-induced nausea and vomiting (CINV) — particularly the acute phase.

1. Vestibular system: Motion and positional changes stimulate vestibular hair cells, which project to the NTS via H₁ and M₁ receptor-expressing afferents. This is why antihistamines (promethazine) and anticholinergics (scopolamine) are effective for motion sickness but ondansetron (5-HT₃) is not — the vestibular pathway does not prominently involve 5-HT₃.

1. Higher cortical centres: Anticipatory nausea (conditioned response to chemotherapy environment), anxiety, and unpleasant stimuli (smell, sight) modulate the NTS via cortical input.

Gastroparesis — delayed gastric emptying without mechanical obstruction — results from impaired co-ordination of the interstitial cells of Cajal and enteric neurones. Dopamine (D₂) activation in the myenteric plexus reduces antral contractions and pyloric tone; prokinetics that block D₂ receptors (metoclopramide, domperidone) or activate 5-HT₄ receptors (metoclopramide, prucalopride) restore coordinated peristalsis.

Vomiting Reflex Arc and Antiemetic Receptor Targets

Therapeutic Goals in Nausea, Vomiting, and GI Dysmotility

The therapeutic goals in managing nausea, vomiting, and GI dysmotility vary substantially by clinical context, and the failure to match the goal to the context is the most common reason antiemetics fail. A drug that works brilliantly for CINV may be completely ineffective for motion sickness — not because it is a poor drug, but because the aetiology engages a different receptor pathway entirely.

For CINV (chemotherapy-induced nausea and vomiting), the goal is prophylaxis across three phases: the acute phase (0–24 hours post-chemotherapy, predominantly 5-HT₃-mediated), the delayed phase (24–120 hours, predominantly NK₁/substance P-mediated, worse than the acute phase for many patients), and the anticipatory phase (conditioned reflex, requires behavioural intervention ± benzodiazepine). For PONV (post-operative nausea and vomiting), the goal is prevention before emergence from anaesthesia using a multimodal approach (5-HT₃ antagonist + dexamethasone ± droperidol or transdermal scopolamine). For motion sickness, the goal is prophylaxis and early symptom relief using agents that block the vestibular pathway (H₁ antihistamines, scopolamine) — NOT 5-HT₃ antagonists. For pregnancy nausea (hyperemesis gravidarum), the goal is symptom control with the safest available agent for gestational age, avoiding teratogenic drugs. For gastroparesis, the goal is restoration of gastric emptying with the shortest effective course of a prokinetic, minimising EPS risk.

Classification of Prokinetics and Antiemetics

Prokinetics and antiemetics can be classified in two complementary ways: by their functional role (prokinetic vs antiemetic) and by their primary receptor mechanism (which determines clinical application and adverse effect profile). These two classification systems overlap in an important and instructive way, because several drugs serve both roles simultaneously. Most importantly, metoclopramide acts as a prokinetic through its 5-HT4 agonism on the myenteric plexus while simultaneously acting as a central antiemetic through its D2 antagonism at the CTZ. Mapping every drug to both its mechanism and its target receptor node is the key intellectual step that converts rote memorisation into clinical reasoning and prevents prescribing errors such as giving metoclopramide to a Parkinson's patient or ondansetron for motion sickness.

Prokinetics and antiemetics can be classified in two complementary ways: by their functional role (prokinetic vs antiemetic) and by their primary receptor mechanism (which determines clinical application and adverse effect profile). The functional and mechanistic classifications overlap, because several drugs serve both roles — most importantly metoclopramide, which is both a prokinetic (D₂ + 5-HT₄ action on myenteric plexus → improved gastric emptying) and an antiemetic (D₂ antagonism at the CTZ → suppresses vomiting). Mapping every drug to both its mechanism and its target receptor node is the key intellectual step that prevents prescribing errors.

Prokinetic agents (enhance coordinated GI motility):
- Metoclopramide — D₂ antagonist + 5-HT₄ agonist; prokinetic + antiemetic; crosses BBB
- Domperidone — peripheral D₂ antagonist; prokinetic; minimal CNS penetration
- Prucalopride — selective 5-HT₄ agonist; colon-selective; used for chronic constipation (not nausea)
- Itopride — D₂ antagonist + cholinesterase inhibitor; upper GI prokinetic
- Cisapride — 5-HT₄ agonist; WITHDRAWN globally in 2000 (fatal torsades de pointes from hERG channel block → QT prolongation → VF); do not prescribe under any circumstances

Antiemetics by mechanism (receptor-matched to aetiology):
- 5-HT₃ antagonists — ondansetron, granisetron, palonosetron; block vagal 5-HT₃ + CTZ; CINV/PONV
- NK₁ antagonists — aprepitant, fosaprepitant; block substance P at NK₁; delayed CINV
- D₂ antagonists — metoclopramide, prochlorperazine, haloperidol, domperidone; broad antiemetic
- H₁ antihistamines — promethazine, cyclizine, dimenhydrinate; motion sickness, pregnancy nausea
- M₁ anticholinergics — hyoscine/scopolamine; motion sickness (transdermal)
- Corticosteroids — dexamethasone; adjunct in CINV/PONV (mechanism not fully established)
- Cannabinoids — nabilone; refractory CINV (appetite stimulation + antiemetic)

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