Anti-emetic drugs

Phenothiazine tranquilizers antagonize the stimulatory effects of dopamine. These drugs are reported to have weak anticholinergic and antihistaminic action. The potential side effects are hypotension from peripheral alpha-adrenergic blockade. Other effects include sedation and possibly seizures by lowering the seizure threshold. Chlorpromazine (0.5 mg/kg TID to QID given IV, IM, SC) is the most common anti-emetic from this class.

Antihistaminic drugs block both cholinergic and histamine receptor mediated transmission to the vomiting center arising from the vestibular system. These drugs generally are used for management of motion sickness or vestibular disease, but will cause some sedation. A common antihistamine used for this purpose is diphenhydramine (2-4 mg/kg TID given PO).

Anti-emetic drugs and effects

Metoclopramide is rapidly metabolized through the liver, and the best results are observed when given at continuous-rate infusion (0.01 to 0.02 mg/kg every hour or 1 to 2 mg/kg every 24 hours). At high doses it causes CNS excitement from dopamine antagonism. It also has effects on GI motility. Metoclopramide should be avoided in epileptics or in animals receiving other drugs that are likely to cause extrapyramidal reactions and should not be used with phenothiazine tranquilizers that can cause an additive effect.

Serotonin antagonists are effective at blocking serotonin receptors found peripherally, in the CRTZ and in the vomiting center. They are not very effective in relieving motion sickness. Common drugs used include ondansetron (0.1-1.0 mg/kg SID-BID given PO) and dolasetron (0.6 mg/kg given IV q 24 hours). They also are used for prevention of chemotherapy-induced vomiting. Another serotonin antagonist sometimes used in cats is mirtazepine (1.25-2.5 mg per cat PO once every three days). This drug not only is a serotonin antagonist, but also has an agonist effect that functions as an appetite stimulant.

Maropitant (1 mg/kg Q 24 hours given SQ or 2 mg/kg Q 24 hours given PO) is a neurokinin-1 antagonist that blocks receptors found in the emetic center, CRTZ and in peripheral afferent nerves.

Maropitant appears to be a good broad-spectrum anti-emetic approved for use in the dog. At higher doses (8 mg/kg given orally up to two consecutive days) it also prevents motion sickness. It is effective in preventing vomiting and nausea associated with chemotherapy and in management of parvovirus enteropathy and pancreatitis, as well as many other causes of vomiting.

It is not yet approved for use in cats, but some have used it in cats. Maropitant does not appear to effect GI motility. Because of its hepatic metabolism, it should be used with caution in animals that have significant hepatic dysfunction.

Prokinetic disorders and their management

Abnormal gastrointestinal motility generally is associated with megaesophagus, gastroesophageal reflux, gastric hypomotility, food hypersensitivity problems and chronic constipation. Dogs with megaesophagus do not respond to prokinetic agents because the esophagus in the dog is predominately skeletal muscle and does not respond to agents that effect visceral smooth muscle. However, gastric prokinetic agents are used to prevent gastroesophageal reflux.

Metoclopramide acts on both dopaminergic and serotoninergic receptors. The prokinetic effects are through cholinergic stimulation of the upper gastrointestinal tract mediated by antagonism at the receptors and activation of the serotonin receptors located in neural and non-neural tissues of the GI tract.

Metoclopramide increases the tone and amplitude of gastric contractions, relaxes the pyloric sphincter and increases peristalsis of the duodenum and proximal jejunum, resulting in accelerated gastric emptying and intestinal transit. It also increases the resting tone of the lower esophageal sphincter.

It is given orally at a dosage of 0.2-0.4 mg/kg a half-hour before meals three or four times a day. This drug is contraindicated in gastric-outflow obstructions, with concurrent phenothiazine or narcotic therapy, or if the animal has epilepsy.

Side-effects with metoclopramide are common, and include central nervous system abnormalities associated with hyperexcitability or depression. If side effects occur, the drug should be discontinued for 48 hours and then given at a lower dosage. This drug also has an anti-emetic effect attributed to its central anti-dopaminergic activity through actions at the CRTZ.

Cisapride is a promotility agent that is more potent than metoclopramide. The drug binds to serotonin receptors on enteric postganglionic cholinergic neurons and stimulates contraction of gastrointestinal smooth muscle. Cisapride increases lower esophageal sphincter pressure, improves gastric emptying and promotes increased motility of both the small and large intestine.

In cats, cisapride increases distal esophageal motility and has become an important prokinetic agent for management of feline megacolon. Cisapride was marketed as Propulsid, but was taken off the market because of induced cardiac arrhythmias referred to as "torsades de pointes" that resulted in sudden death in some people. Cisapride caused QT interval prolongation and slowing of cardiac repolarization as an explanation for the fatal arrhythmia. This abnormality has not been observed in dogs or cats.

Cisapride is now available only through compounding pharmacies. A recommended dose is 0.1 to 0.5 mg/kg BID to TID given 30 minutes before meals. Higher doses of 1 mg/kg may be required in some cases. Cats appear to tolerate a 2.5-mg dose without problems. Side effects include vomiting, diarrhea and abdominal discomfort. Cisapride should not be used with anticholinergic drugs because they will counteract cisapride's effect on motility. Cisapride may affect absorption of some drugs through increased GI motility. It has few, if any, effects as an anti-emetic.

Erythromycin is a novel prokinetic associated with frequent gastrointestinal side effects, including nausea and vomiting. The gastrointestinal effects occur from the drugs binding to motilin receptors in both cholinergic nerves and smooth muscle. In the dog, antral contractions are due to a cholinergic mechanism and in cats it is a smooth-muscle motilin agonist.

When given at very low sub-microbiological doses, erythromycin stimulates migrating motor complexes in the stomach. This motility occurs during the fasting state with an empty stomach, but studies have shown that, if given during feeding, it accelerates gastric emptying of food. It also increases lower esophageal sphincter pressure and may be useful in management of gastroesophageal reflux.

Erythromycin will increase colonic activity in the dog but not the cat. In dogs, a dose of 0.5 to 1 mg/kg given TID is the recommended prokinetic dose. Erythromycin also appears to be a better prokinetic drug than metoclopramide.

H2 receptor antagonists ranitidine (Zantac) and nizatidine (Axid) stimulate gastrointestinal motility in addition to their ability to inhibit gastric acid secretion. They function by inhibiting acetylcholinesterase activity and increasing the amount of acetylcholine available to smooth-muscle muscarinic-cholinergic receptors in the gastrointestinal tract.

When given at the normal anti-secretory doses, they also stimulate gastric contractions and accelerate gastric emptying. These drugs could be useful in the management of gastric ulceration with concurrent gastric hypomotility. Their relative potency as a prokinetic compared to the other drugs discussed is unknown but believed to be weaker.

Domperidone is both an anti-emetic and prokinetic agent not yet available in the United States. Domperidone is a peripheral and CRTZ dopamine receptor antagonist. It appears to be a better anti-emetic than prokinetic agent, having weak effects on GI motility. It has little effect on sustaining lower esophageal sphincter pressure, and gastric motility does not appear to be significantly enhanced during the fed state.