Metoclopramide is a substituted benzamide, which, prior to the introduction of serotonin (5-HT3) receptor antagonists, was considered the most effective single antiemetic agent against highly emetogenic chemotherapy such as cisplatin. Although metoclopramide is a competitive antagonist at dopaminergic (D2) receptors, it is most effective against acute vomiting when given IV at high doses (e.g., 0.5–3 mg/kg/dose), probably because it is a weak competitive antagonist (relative to other serotonin antagonists) at 5-HT3 receptors. It may act on the CTZ and the periphery. Metoclopramide also increases lower esophageal sphincter pressure and enhances the rate of gastric emptying, which may factor into its overall antiemetic effect. It can be administered IV at the U.S. Food and Drug Administration (FDA)–approved dose of 1 to 2 mg/kg every 2 hours (or less frequently) for three to five doses. Metoclopramide has also been safely given by IV bolus injection at higher single doses (up to 6 mg/kg) and by continuous IV infusion, with or without a loading bolus dose, with efficacy comparable to multiple intermittent dosing schedules.[13,14,15]
Metoclopramide is associated with akathisia and dystonic extrapyramidal effects; akathisia is seen more frequently in patients older than 30 years, and dystonic extrapyramidal effects are seen more commonly in patients younger than 30 years. Diphenhydramine, benztropine mesylate, and trihexyphenidyl are commonly used prophylactically or therapeutically to pharmacologically antagonize EPRs.[7,16] While cogwheeling rigidity, acute dystonia, and tremor are responsive to anticholinergic medications, akathisia—the subjective sense of restlessness or inability to sit still—is best treated by the following:
- Switching to a lower potency neuroleptic for vomiting, if possible.
- Lowering the dose.
- Adding a benzodiazepine (e.g., lorazepam) or beta blocker (e.g., propranolol).
5-HT3 Receptor Antagonists
Four serotonin receptor antagonists—ondansetron, granisetron, dolasetron, and palonosetron—are available in the United States. Tropisetron, while not approved by the FDA, is available internationally. Agents in this class are thought to prevent N&V by preventing serotonin, which is released from enterochromaffin cells in the gastrointestinal (GI) mucosa, from initiating afferent transmission to the CNS via vagal and spinal sympathetic nerves. The 5-HT3 receptor antagonists may also block serotonin stimulation at the CTZ and other CNS structures.