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PH6.2 | PH6.2 | Prokinetics and Antiemetics — SDL Guide — SDL Guide (Part 2)

Prokinetic Agents — Mechanisms and Clinical Use

Prokinetic agents restore or enhance the co-ordinated motility of the upper gastrointestinal tract by acting on the enteric nervous system. Understanding the mechanistic differences between the available prokinetics — particularly the CNS-penetration difference between metoclopramide and domperidone — is critical because it determines both the adverse effect profile and the appropriate clinical selection.

Metoclopramide is the most widely used prokinetic worldwide. Its dual mechanism — D₂ receptor antagonism (blocks the inhibitory effect of dopamine on myenteric cholinergic neurones, releasing co-ordinated peristalsis) and 5-HT₄ receptor agonism (directly stimulates ACh release from myenteric neurones, enhancing antral contractions and pyloric relaxation) — gives it both prokinetic and potent antiemetic properties. Critically, metoclopramide crosses the blood-brain barrier significantly, which explains its efficacy as a central antiemetic (D₂ block at the CTZ suppresses nausea) but also its most important adverse effects: extrapyramidal symptoms (EPS) arising from D₂ block in the nigrostriatal pathway — acute dystonia (oculogyric crisis, torticollis — more common in young women and children), parkinsonian features (tremor, rigidity — more common in elderly), akathisia (motor restlessness), and with prolonged use, tardive dyskinesia (potentially irreversible). Due to this risk, the European Medicines Agency and Tripathi recommend limiting metoclopramide courses to a maximum of 5 days. It also elevates prolactin (D₂ block in tuberoinfundibular pathway → galactorrhoea, gynaecomastia).

Domperidone is a peripheral D₂ antagonist that, unlike metoclopramide, does not cross the BBB to any significant extent under normal conditions (it is a P-glycoprotein substrate expelled from the CNS). This makes it a safer prokinetic in patients at risk for EPS — including those with Parkinson's disease who develop nausea on levodopa therapy (metoclopramide would worsen parkinsonism by blocking D₂ in the nigrostriatal pathway; domperidone can be used safely here). However, domperidone is not free of risk: it causes QT prolongation (hERG channel block similar to Class IA antiarrhythmics) and has been associated with sudden cardiac death in high-dose or long-term use. Doses should not exceed 30 mg/day, and it should be avoided in patients with cardiac arrhythmias or on other QT-prolonging drugs.

Prucalopride is a highly selective 5-HT₄ receptor agonist with high affinity for the colonic 5-HT₄ receptor; it promotes high-amplitude propagated contractions in the colon and is approved specifically for chronic constipation — not for nausea or upper GI dysmotility. It does not block D₂ and therefore carries no EPS risk; it has not been associated with the cardiac arrhythmias seen with cisapride.

Cisapride was a 5-HT₄ agonist prokinetic that was globally withdrawn in 2000 after reports of torsades de pointes and fatal ventricular fibrillation due to hERG potassium channel block (causing QT prolongation). It must not be prescribed. When asked about cisapride in an exam or clinical setting, the correct answer is always 'withdrawn — do not use.'

Drug	Mechanism	BBB	EPS risk	QT risk	Primary use
Metoclopramide	D₂ antagonist + 5-HT₄ agonist	Yes	High	Low	CINV, gastroparesis, GERD
Domperidone	Peripheral D₂ antagonist	Minimal	Very low	Moderate	Gastroparesis, Parkinson's nausea
Prucalopride	5-HT₄ agonist (colonic)	Minimal	None	None	Chronic constipation
Cisapride	5-HT₄ agonist	Moderate	None	Severe — WITHDRAWN	—

5-HT3 Antagonists and NK1 Antagonists

The 5-HT₃ antagonists and NK₁ antagonists are the two classes that transformed the management of chemotherapy-induced nausea and vomiting, making previously abandoned highly emetogenic chemotherapy regimens (cisplatin-based) tolerable for patients. Understanding their distinct mechanisms — and why their combination covers both the acute and delayed phases of CINV — is essential clinical pharmacology.

5-HT₃ antagonists (serotonin type 3 blockers): The prototype is ondansetron (also granisetron, tropisetron, and the second-generation palonosetron with higher receptor affinity and longer half-life). They act by competitively blocking 5-HT₃ receptors on vagal afferent neurones in the gut (which are activated by serotonin released from enterochromaffin cells during chemotherapy or radiation, or during anaesthetic stress in PONV) and at the CTZ in the area postrema. By blocking this signal, they prevent the vagal → NTS → vomiting motor output. They are highly effective for the acute phase of CINV (0–24 h) and for PONV.

Pharmacokinetics: Well absorbed orally; hepatic metabolism (CYP3A4, CYP1A2); half-life 3–9 hours for ondansetron (palonosetron has ~40-hour half-life, enabling single-dose administration). IV and oral formulations available. ADRs: Constipation (most common — reduced gut motility, as 5-HT₃ normally stimulates peristalsis), headache, and mild QT prolongation (particularly with IV ondansetron bolus — avoid in QTc >450 ms or with other QT-prolonging agents). They do not cause EPS (no D₂ block) and are generally well tolerated.

Important limitation: 5-HT₃ antagonists are not effective for the delayed phase of CINV (24–120 h), which is predominantly NK₁/substance P-mediated. They are also not effective for motion sickness (vestibular pathway — H₁/M₁, not 5-HT₃).

NK₁ antagonists (neurokinin-1/substance P blockers): Aprepitant (oral) and fosaprepitant (IV prodrug of aprepitant) are the primary agents. They block NK₁ receptors centrally in the NTS and CTZ (substance P/neurokinin-1 signalling is the dominant pathway in delayed CINV and in the central sensitisation phase). When added to a 5-HT₃ antagonist + dexamethasone for highly emetogenic chemotherapy (HEC — e.g. cisplatin, AC regimens), NK₁ antagonists dramatically reduce the delayed-phase nausea and vomiting that persists for days after treatment.

Pharmacokinetics: Aprepitant is highly protein-bound, metabolised by CYP3A4 (it is also a moderate CYP3A4 inhibitor — increases systemic exposure of dexamethasone used as antiemetic by ~2-fold, so dexamethasone dose is halved when aprepitant is co-prescribed). Dosing: 125 mg on day 1, 80 mg on days 2 and 3. ADRs: Generally well tolerated; hiccups, fatigue, constipation. Drug interaction with warfarin (CYP3A4 induction after aprepitant discontinuation may require INR monitoring).

Antihistamines, Anticholinergics, and Phenothiazines

This group of antiemetics acts primarily on the vestibular and cortical pathways rather than the gut-vagal 5-HT3 pathway, making them the drugs of choice for motion sickness and vestibular disorders — contexts where ondansetron and other gut-targeted antiemetics consistently fail. They are among the oldest antiemetic agents, predating the era of receptor-selective pharmacology, and they remain clinically indispensable precisely because the vestibular-mediated nausea they address is pharmacologically distinct from CINV and gastroparesis. Understanding why an H1 antihistamine or M1 anticholinergic prevents motion sickness while a 5-HT3 blocker does not requires returning to the receptor map of the vomiting arc and recognising that vestibular hair cell afferents project to the NTS via H1 and M1 synapses, not 5-HT3 synapses. This distinction is among the highest-yield clinical applications of receptor pharmacology in this subject.

This group of antiemetics acts primarily on the vestibular and cortical pathways rather than the gut-vagal 5-HT₃ pathway, making them the drugs of choice for motion sickness and vestibular disorders — contexts where ondansetron and other gut-targeted antiemetics fail. They are among the oldest antiemetics and remain clinically relevant, particularly for motion sickness, pregnancy nausea, and vertigo.

H₁ antihistamines with antiemetic properties:
- Promethazine (phenergan): First-generation H₁ antihistamine with additional M₁ muscarinic and D₂ antagonist properties, plus significant sedation (crosses BBB readily). Used for motion sickness, pregnancy nausea (historically, though concerns about neonatal effects limit use in late pregnancy), and general nausea. ADRs: sedation (often significant — avoid in drivers, pilots), anticholinergic effects (dry mouth, urinary retention, blurred vision, constipation), and EPS (rarely, due to weak D₂ block).
- Cyclizine and dimenhydrinate (diphenhydramine + 8-chlorotheophylline): H₁ antihistamines used for motion sickness and post-operative nausea; similar sedation profile.

M₁ anticholinergics:
- Hyoscine (scopolamine): A muscarinic M₁ antagonist that reduces vestibular afferent activity — the most effective single agent for motion sickness prophylaxis when given as a transdermal patch behind the ear (applied 4 hours before travel, effective for 72 hours). It is less effective once vomiting has begun. ADRs: classic anticholinergic effects (dry mouth, blurred vision, sedation, urinary retention, tachycardia). Contraindicated in glaucoma.

Phenothiazine D₂ antagonists:
- Prochlorperazine (stemetil): A phenothiazine antipsychotic used at low doses as an antiemetic via central D₂ blockade at the CTZ. Effective for acute nausea and vomiting of various causes and for vertigo-related nausea. ADRs: EPS (acute dystonia, akathisia, tardive dyskinesia with prolonged use), sedation, hypotension. Use with caution in Parkinson's disease (worsens disease by blocking the nigrostriatal D₂ receptors that levodopa is trying to stimulate).
- Haloperidol (butyrophenone): Low-dose haloperidol is used for opioid-induced nausea and refractory CINV via D₂ block at the CTZ. EPS risk similar to prochlorperazine.

Key distinction for practice: In a patient with Parkinson's disease who develops nausea on levodopa, the only safe antiemetics are domperidone (peripheral D₂ — does not block nigrostriatal D₂) and ondansetron (5-HT₃ — no D₂ block). All other D₂-blocking antiemetics (metoclopramide, prochlorperazine, haloperidol, promethazine) will worsen parkinsonism.