Ondansetron (Zofran) vs. Alosetron (Lotronex)

Ondansetron is an anti-nausea medication used to prevent nausea and vomiting caused by cancer chemotherapy. Chemotherapy drugs increase serotonin production, which stimulates serotonin receptors in the brain, leading to nausea and vomiting. Ondansetron selectively blocks these receptors, reducing the effects of increased serotonin due to chemotherapy. It is also prescribed to prevent vomiting and nausea after surgery.

Alosetron is a serotonin-3 receptor antagonist used to treat diarrhea and abdominal discomfort in women with irritable bowel syndrome (IBS). It works similarly to other medications like granisetron, ondansetron, and dolasetron, which are used to prevent nausea and vomiting. Experts believe that IBS symptoms occur due to abnormal intestinal muscle activity and serotonin receptors. Alosetron reduces the actions of serotonin by blocking its receptors.

Side effects of Ondansetron and Alosetron

Ondansetron

Side effects may include an abnormal heart rate and rhythm.

Alosetron

WARNING

Alosetron was approved by the FDA in 2000 but withdrawn from the market due to serious gastrointestinal side effects. It was later approved again for restricted use in women with severe diarrhea-predominant IBS who have not responded to conventional treatment.

Common side effects include constipation, nausea, hemorrhoids, and abdominal distention. Severe constipation or intestinal inflammation (ischemic colitis) are rare but life-threatening side effects that require immediate discontinuation of alosetron and medical attention.

Dosage of Ondansetron vs. Alosetron

Ondansetron

For adults:

• Highly nauseating chemotherapy: 24 mg orally dissolved on the tongue 30 minutes before chemotherapy.
• Moderately nauseating chemotherapy: Take 8 mg tablet 30 minutes before chemotherapy and repeat in 8 hours, then 8 mg every 12 hours for 1 to 2 days after chemotherapy.
• Radiation-induced nausea and vomiting: Take 8 mg orally 1 to 2 hours before radiation and every 8 hours as needed.
• Post-surgery nausea and vomiting: 16 mg orally 1 hour before anesthesia.

For children:

• Moderately nauseating chemotherapy (12 years and older): 8 mg orally before chemotherapy and repeat in 8 hours, then 8 mg every 12 hours for 1 to 2 days after chemotherapy.
• Moderately nauseating chemotherapy (4 to 11 years): 4 mg orally 30 minutes before chemotherapy and repeat in 4 and 8 hours after the first dose, then every 8 hours for 1 to 2 days after chemotherapy.
• Not recommended for children under 4 years old.

Alosetron

The starting dose is 0.5 mg twice daily. If constipation develops, alosetron should be discontinued. It may be restarted at 0.5 mg once daily. If constipation persists at 0.5 mg once daily, alosetron should be discontinued. After 4 weeks, patients without adequate symptom control may receive up to 1 mg twice daily. If symptoms are still not adequately controlled after 4 weeks of 1 mg twice daily, alosetron should be discontinued.

Alosetron may be taken with or without food.

Drug interactions with Ondansetron and Alosetron

Ondansetron

Ondansetron should not be used with apomorphine due to a sudden lowering of blood pressure and loss of consciousness. Phenytoin, carbamazepine, and rifampin can decrease blood levels of ondansetron, potentially reducing its effectiveness. Dronedarone can increase blood levels of ondansetron, leading to an increased risk of abnormal heart rhythms.

Alosetron

Alosetron is primarily metabolized by certain enzymes (CYP1A2, CYP3A4, and CYP2C9), and drugs that affect these enzymes may change its clearance. Fluvoxamine and ketoconazole are strong inhibitors of these enzymes and should not be used with alosetron. The effects of other enzyme inhibitors on alosetron are unknown.

Safety of Ondansetron and Alosetron in Pregnancy and Breastfeeding

Ondansetron

There are no adequate studies of ondansetron in pregnant women, so its safe use during pregnancy is unknown. It is also unclear if ondansetron is excreted in breast milk, so caution is advised when using it in nursing mothers.

Alosetron

It is not known if alosetron is excreted in breast milk. Therefore, caution should be exercised by nursing mothers.

Summary

Ondansetron and alosetron are serotonin-3 receptor antagonists used to treat different conditions. Ondansetron is used to prevent chemotherapy-induced nausea and vomiting as well as post-surgery nausea and vomiting. Alosetron is used to treat diarrhea and abdominal discomfort in some women with IBS.