For obesity management including weight loss and weight maintenance when used in conjunction with a reduced-calorie diet. Also used to reduce the risk for weight regain after prior weight loss. Use of orlistat is pending revision due to reports of liver-related adverse events.
Orlistat is a lipase inhibitor for obesity management that acts by inhibiting the absorption of dietary fats. At the recommended therapeutic dose of 120 mg three times a day, orlistat inhibits dietary fat absorption by approximately 30%. It works by inhibiting pancreatic lipase, an enzyme that breaks down fat in the intestine. Without this enzyme, fat from the diet is excreted undigested and not absorbed by the body. Because some vitamins are fat soluble, the effect of orlistat is to reduce their body absorption. Therefore the drug should only be taken in conjuction with fatty meals, and a multivitamin tablet containing these vitamins (D E K and beta-carotene) should be taken once a day, at least 2 hours before or after taking the drug. In the March 15, 2004 issue of Cancer Research,  Steven J. Kridel et al. state that orlistat may also inhibit growth of prostate cancer, and in theory may be useful in treating other cancers, by interfering with the metabolism of fats.
Mode of Action:
Orlistat is a reversible inhibitor of lipases. It exerts its therapeutic activity in the lumen of the stomach and small intestine by forming a covalent bond with the active serine residue site of gastric and pancreatic lipases. The inactivated enzymes are thus unavailable to hydrolyze dietary fat in the form of triglycerides into absorbable free fatty acids and monoglycerides. As undigested triglycerides are not absorbed, the resulting caloric deficit may have a positive effect on weight control.
Metabolized primarily within the gastrointestinal wall forming relatively inactive metabolites. Metabolites M1 (4-member lactone ring hydrolyzed) and M3 (M1 with N-formyl leucine moiety cleaved) accounted for approximately 42% of total radioactivity in plasma. M1 and M3 have an open beta-lactone ring and extremely weak lipase inhibitory activity (1000- and 2500-fold less than orlistat, respectively).
The results of a massive overdose of Xenical are unknown, although the drug seems relatively harmless.
Torgerson JS, Hauptman J, Boldrin MN, Sjostrom L: XENical in the prevention of diabetes in obese subjects (XENDOS) study: a randomized study of orlistat as an adjunct to lifestyle changes for the prevention of type 2 diabetes in obese patients. Diabetes Care. 2004 Jan;27(1):155-61. Pubmed Mancini MC, Halpern A: Pharmacological treatment of obesity. Arq Bras Endocrinol Metabol. 2006 Apr;50(2):377-89. Epub 2006 May 23. Pubmed Menendez JA, Vellon L, Lupu R: Antitumoral actions of the anti-obesity drug orlistat (XenicalTM) in breast cancer cells: blockade of cell cycle progression, promotion of apoptotic cell death and PEA3-mediated transcriptional repression of Her2/neu (erbB-2) oncogene. Ann Oncol. 2005 Aug;16(8):1253-67. Epub 2005 May 3. Pubmed Garcia SB, Barros LT, Turatti A, Martinello F, Modiano P, Ribeiro-Silva A, Vespucio MV, Uyemura SA: The anti-obesity agent Orlistat is associated to increase in colonic preneoplastic markers in rats treated with a chemical carcinogen. Cancer Lett. 2006 Aug 28;240(2):221-4. Epub 2005 Dec 27. Pubmed Drew BS, Dixon AF, Dixon JB: Obesity management: update on orlistat. Vasc Health Risk Manag. 2007;3(6):817-21. Pubmed Wong NN, Cheng-Lai A: Orlistat. Heart Dis. 2000 Mar-Apr;2(2):174-81. Pubmed Hvizdos KM, Markham A: Orlistat: a review of its use in the management of obesity. Drugs. 1999 Oct;58(4):743-60. Pubmed Lucas KH, Kaplan-Machlis B: Orlistatãa novel weight loss therapy. Ann Pharmacother. 2001 Mar;35(3):314-28. Pubmed Curran MP, Scott LJ: Orlistat: a review of its use in the management of patients with obesity. Drugs. 2004;64(24):2845-64. Pubmed Ballinger A, Peikin SR: Orlistat: its current status as an anti-obesity drug. Eur J Pharmacol. 2002 Apr 12;440(2-3):109-17. Pubmed
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