Orlistat (Molecule of the Month for June 2007)
Xenical, Alli, Tetrahydrolipstatin
Orlistat is a drug designed to treat obesity. Its primary function is preventing the absorption of fats from the human diet, thereby reducing caloric intake. It is intended for use in conjunction with a physician-supervised reduced-calorie diet. Orlistat is the saturated derivative of lipstatin — a potent natural inhibitor of pancreatic lipases isolated from the bacterium Streptomyces toxytricini. Orlistat works by inhibiting pancreatic lipase, an enzyme that breaks down triglycerides in the intestine. Without this enzyme, triglycerides from the diet are prevented from being hydrolyzed into absorbable free fatty acids and are excreted undigested.
The amount of weight loss achieved with orlistat varies. In one-year clinical trials, between 35.5% and 54.8% of subjects achieved a 5% or greater decrease in body mass, although not all of this mass was necessarily fat. Between 16.4% and 24.8% achieved at least a 10% decrease in body mass. After orlistat was stopped, a significant number of subjects regained weight — up to 35% of the weight they had lost.
The primary side effects of the drug are gastrointestinal-related. Side effects are most severe when beginning therapy, and decrease in frequency with time; in clinical trials, nearly half of side effects lasted less than a week, but some may persist for over six months. Because orlistat's main effect is to prevent dietary fat from being absorbed, the fat is excreted unchanged in the feces and so the stool may become oily or loose (steatorrhea). Increased flatulence is also common. Bowel movements may become frequent or urgent, and rare occurrences of fecal incontinence have been seen in clinical trials. To minimize these effects, foods with high fat content should be avoided; the manufacturer advises consumers to follow a low-fat, reduced-calorie diet. Oily stools and flatulence can be controlled by reducing the dietary fat content to somewhere in the region of 15 grams per meal.
Update by Karl Harrison
(Molecule of the Month for June 2007 )