The lower bioavailability is attributed to first-pass metabolism. The absolute bioavailability of ondansetron after oral administration is approximately 60%(50%-70%). Ībsorption: Ondansetron undergoes rapid absorption from the GI tract, and the peak plasma concentration (Tmax) is approximately 1.5 hours after an 8 mg single oral dose. The peripheral actions of ondansetron are thought to be the predominant mechanism for its antiemetic effects. It forms synapses within the nucleus tractus solitarius of the brainstem, another region important in vomiting. The vagus nerve can sense nausea and vomiting triggers within the GI tract, such as stomach irritants. It works on the 5-HT3 receptors that can be found at the vagus nerve terminals. Ondansetron also has effects peripherally by acting on the vagus nerve. The area postrema, located on the fourth ventricle floor, contains the "chemoreceptor trigger zone." This zone senses neurotransmitters like serotonin, toxins, and other signals and plays a role in mediating the sensation of nausea and subsequent vomiting. Central effects are mediated by the antagonism of 5HT-3 serotonin receptors in the area postrema. Ondansetron acts both centrally and peripherally to prevent and treat nausea and vomiting. It is one of the four FDA-approved 5-HT3 serotonin-receptor antagonists used to combat nausea and vomiting, including granisetron, dolasetron, and the second-generation drug, palonosetron. Ondansetron is a selective 5-HT3 serotonin-receptor antagonist used for its antiemetic properties. Ondansetron is used off-label for severe refractory diarrhea associated with neuroendocrine tumors (carcinoid syndrome). Ondansetron is used in pediatric populations for the acute treatment of cyclic vomiting syndrome however, there is little information available on the efficacy of this disease. There are limited data available from pediatric populations. Ondansetron has minimal efficacy against nausea and vomiting caused by motion sickness, mediated by different control centers and pathophysiologic mechanisms. Off-label use for the prevention of nausea and vomiting associated with pregnancy. It is considered first-line therapy for the treatment of chemotherapy-induced and radiation-induced nausea and vomiting. The FDA-approved indications include the prevention of chemotherapy-induced nausea and vomiting(CINV), radiation-induced nausea and vomiting, and the prevention of postoperative nausea and vomiting (PONV). Ondansetron has widespread utility as an antiemetic drug and is effective against nausea and vomiting of various etiologies. In 2006 (the last year of its patent), the brand-name version of ondansetron was the 20th highest-selling brand-name drug in the United States, and its popularity continues today. Other antiemetics that appear on this list with ondansetron include dexamethasone and metoclopramide. Ondansetron is on the World Health Organization's (WHO) List of Essential Medicines, a list of medications considered effective and safe in meeting the essential needs of a health care system. Nausea and vomiting are common complaints seen by emergency department physicians and primary care clinicians daily.
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