- Anti-obesity medication
Anti-obesity medication or weight loss drugs are all pharmacological agents that reduce or control weight. These drugs alter one of the fundamental processes of the human body, weight regulation, by either altering appetite, metabolism, or absorption of calories. It is common for them to be tried and if there is little or no benefit from them to discontinue treatment. The main treatment modalities for overweight and obesity remain dieting and physical exercise.
Only one anti-obesity medications orlistat (Xenical) is currently approved by the FDA for long term use. It reduces intestinal fat absorption by inhibiting pancreatic lipase. Rimonabant (Acomplia), a second drug, works via a specific blockade of the endocannabinoid system. It has been developed from the knowledge that cannabis smokers often experience hunger, which is often referred to as "the munchies". It had been approved in Europe for the treatment of obesity but has not received approval in the United States or Canada due to safety concerns. The European Medicines Agency in October 2008 recommended the suspension of the sale of rimonabant as the risks seem to be greater than the benefits. Sibutramine (Meridia), which acts in the brain to inhibit deactivation of the neurotransmitters, thereby decreasing appetite was withdrawn from the United States and Canadian markets in October 2010 due to cardiovascular concerns.
- 1 Mechanisms of action
- 2 History
- 3 Contemporary anti-obesity drugs
- 4 Alternative medicine
- 5 Side effects
- 6 Limitations of current knowledge
- 7 Future developments
- 8 Research
- 9 References
- 10 Further reading
- 11 External links
Mechanisms of action
Anti-obesity drugs operate through one or more of the following mechanisms:
- Suppression of the appetite. Catecholamines and their derivatives (such as phentermine and other amphetamine-based drugs) are the main tools used for this, although other classes of drugs such as anti-depressants and mood stabilizers have been anecdotally used for appetite suppression (see: bupropion and topiramate). Drugs blocking the cannabinoid receptors may be a future strategy for appetite suppression.
- Increase of the body's metabolism.
- Interference with the body's ability to absorb specific nutrients in food. For example, Orlistat (also known as Xenical and Allī) blocks fat breakdown and thereby prevents fat absorption. The OTC fiber supplements glucomannan and guar gum have been used for the purpose of inhibiting digestion and lowering caloric absorption
Anorectics are primarily intended to suppress the appetite, but most of the drugs in this class also act as stimulants (dexedrine, e.g.), and patients have abused drugs "off label" to suppress appetite (e.g. digoxin).
The first described attempts at producing weight loss are those of Soranus of Ephesus, a Greek physician, in the second century AD. He prescribed elixirs of laxatives and purgatives, as well as heat, massage, and exercise. This remained the mainstay of treatment for well over a thousand years. It was not until the 1920s and 1930s that new treatments began to appear. Based on its effectiveness for hypothyroidism, thyroid hormone became a popular treatment for obesity in euthyroid people. It had a modest effect but produced the symptoms of hyperthyroidism as a side effect, such as palpitations and difficulty sleeping. Dinitrophenol (DNP) was introduced in 1933; this worked by uncoupling the biological process of oxidative phosphorylation in mitochondria, causing them to produce heat instead of ATP. The most significant side effect was a sensation of warmth, frequently with sweating. Overdose, although rare, lead to a rise in body temperature and, ultimately, fatal hyperthermia. By the end of 1938 DNP had fallen out of use because the FDA had become empowered to put pressure on manufacturers, who voluntarily withdrew it from the market.
Amphetamines (marketed as Benzedrine) became popular for weight loss during the late 1930s. They worked primarily by suppressing appetite, and had other beneficial effects such as increased alertness. Use of amphetamines increased over the subsequent decades, culminating in the "rainbow pill" regime. This was a combination of multiple pills, all thought to help with weight loss, taken throughout the day. Typical regimens included stimulants, such as amphetamines, as well as thyroid hormone, diuretics, digitalis, laxatives, and often a barbiturate to suppress the side effects of the stimulants. In 1967/1968 a number of deaths attributed to diet pills triggered a Senate investigation and the gradual implementation of greater restrictions on the market. This culminating in 1979 with the FDA banning the use of amphetamines, then the most effective of the diet drugs, in diet pills.
Meanwhile, phentermine had been FDA approved in 1959 and fenfluramine in 1973. The two were no more popular than other drugs until in 1992 a researcher reported that the two caused a 10% weight loss which was maintained for more than two years. Fen-phen was born and rapidly became the most commonly prescribed diet medication. Dexfenfluramine (Redux) was developed in the mid-1990s as an alternative to fenfluramine with less side-effects, and received regulatory approval in 1996. However, this coincided with mounting evidence that the combination could cause valvular heart disease in up to 30% of those who had taken it, leading to withdrawal of Fen-phen and dexfenfluramine from the market in September 1997.
Contemporary anti-obesity drugs
Some patients find that diet and exercise is not a viable option; for these patients, anti-obesity drugs can be a last resort. Some prescription weight loss drugs are stimulants, which are recommended only for short-term use, and thus are of limited usefulness for extremely obese patients, who may need to reduce weight over months or years.
Orlistat (Xenical) reduces intestinal fat absorption by inhibiting pancreatic lipase. Orlistat may cause frequent, oily bowel movements (steatorrhea), but if fat in the diet is reduced, symptoms often improve. Originally available only by prescription, it was approved by the FDA for over-the-counter sale in February 2007.  On May 26, 2010, the U.S. Food and Drug Administration (FDA) has approved a revised label for Xenical to include new safety information about cases of severe liver injury that have been reported rarely with the use of this medication.
Sibutramine (Reductil or Meridia) is an anorectic or appetite suppressant, reducing the desire to eat. Sibutramine may increase blood pressure and may cause dry mouth, constipation, headache, and insomnia.
Sibutramine has been withdrawn from the market in the United States, the UK, the EU, Australia, Canada and Hong Kong. Its risks (non-life threatening myocardial infarction and stroke) have been shown to outweigh the benefits.
Rimonabant (Acomplia) is a recently developed anti-obesity medication. It is cannabinoid (CB1) receptor antagonist that acts centrally on the brain thus decreasing appetite. It may also act peripherally by increasing thermogenesis and therefore increasing energy expenditure.
Weight loss with Rimonabant however has not been shown to be greater than other available weight-loss medication. Due to safety concerns, primarily psychiatric in nature, the drug has not received approval in the United States or Canada, either as an anti-obesity treatment or as a smoking-cessation drug.
Sanofi-Aventis has received approval to market Rimonabant as a prescription anti-obesity drug in the European Union, subject to some restrictions, however, in October 2008, the European Medicines Agency (EMEA) recommended that Acomplia no longer be available in UK. One month later, Sanofi-Aventis decided it would no longer study rimonabant for any indication.
Exenatide (Byetta) is a long-acting analogue of the hormone GLP-1, which the intestines secrete in response to the presence of food. Among other effects, GLP-1 delays gastric emptying and promotes a feeling of satiety. Some obese people are deficient in GLP-1, and dieting reduces GLP-1 further. Byetta is currently available as a treatment for Diabetes mellitus type 2. Some, but not all, patients find that they lose substantial weight when taking Byetta. Drawbacks of Byetta include that it must be injected subcutaneously twice daily, and that it causes severe nausea in some patients, especially when therapy is initiated. Byetta is recommended only for patients with Type 2 Diabetes. A somewhat similar drug, Symlin, is currently available for treating diabetes and is in testing for treating obesity in non-diabetics.
Pramlintide (Symlin) is a synthetic analogue of the hormone Amylin, which in normal people is secreted by the pancreas in response to eating. Among other effects, Amylin delays gastric emptying and promotes a feeling of satiety. Many diabetics are deficient in Amylin. Currently, Symlin is only approved to be used along with insulin by Type 1 and Type 2 diabetics. However, Symlin is currently being tested in non-diabetics as a treatment for obesity. A drawback is that Symlin must be injected at mealtimes.
ZGN-433, is undergoing initial human trials, starting in early 2011.
Human growth hormone is also said to increase fat loss and increase strength.
Other weight loss drugs have also been associated with medical complications, such as fatal pulmonary hypertension and heart valve damage due to Redux and Fen-phen, and hemorrhagic stroke due phenylpropanolamine. Many of these substances are related to amphetamine.
Unresearched nonprescription products or programs for weight loss are heavily promoted by mail and print advertising and on the internet. The US Food and Drug Administration recommends caution with use of these products, since many of the claims of safety and effectiveness are unsubstantiated. Individuals with anorexia nervosa and some athletes try to control body weight with laxatives, diet pills or diuretic drugs, although these generally have no impact on body fat. Products that work as a laxative can cause the blood's potassium level to drop, which may cause heart and/or muscle problems. Pyruvate is a popular product that may result in a small amount of weight loss. However, pyruvate, which is found in red apples, cheese, and red wine, has not been thoroughly studied and its weight loss potential has not been scientifically established.
Some anti-obesity drugs have severe or life-threatening side effects, fen-phen being a famous example. These side effects are often associated with their mechanism of action. In general, stimulants carry a risk of high blood pressure, faster heart rate, palpitations, closed-angle glaucoma, drug addiction, restlessness, agitation, and insomnia.
Another drug, orlistat, blocks absorption of dietary fats, and as a result may cause oily spotting bowel movements (steatorrhea), oily stools, stomach pain, and flatulence. A similar medication, designed for patients with Type 2 diabetes, is Acarbose which partially blocks absorption of carbohydrates in the small intestine, and produces similar side effects including stomach pain, and flatulence.
Limitations of current knowledge
The limitation of drugs for obesity is that we do not fully understand the neural basis of appetite and how to modulate it. Appetite is clearly a very important instinct to promote survival. Arguably any drug that would abolish appetite may carry a high mortality risk and may be unsuitable for clinical use.
Because the human body uses various chemicals and hormones to protect its stores of fat (a reaction probably useful to our ancestors when food was scarce in the past,) there has not yet been found a 'silver bullet', or a way to completely circumvent this natural habit of protecting excess food stores. Because of this, anti-obesity drugs are not a practical long-term solution for people who are overweight.
In order to circumvent the number of feedback mechanisms that prevent most monotherapies from producing sustained large amounts of weight loss, it has been hypothesized that combinations of drugs may be more effective by targeting multiple pathways and possibly inhibiting feedback pathways that work to cause a plateau in weight loss. This was evidenced by the success of the combination of phentermine and fenfluramine or dexfenfluramine, popularly referred to phen-fen, in producing significant weight loss but fenfluramine and dexfenfluramine were pulled from the market due to safety fears regarding a potential link to heart valve damage. The damage was found to be a result of activity of fenfluramine and dexfenfluramine at the 5-HT2B serotonin receptor in heart valves. Newer combinations of SSRIs and phentermine, known as phenpro, have been used with equal efficiency as fenphen with no known heart valve damage due to lack of activity at this particular serotonin receptor due to SSRIs. There has been a recent resurgence in combination therapy clinical development with the development of 3 combinations: Qnexa (topiramate + phentermine), Empatic (bupropion + zonisamide) and Contrave (bupropion + naltrexone).
Other classes of drugs in development include lipase inhibitors, similar to orlistat. Another lipase inhibitor, called GT 389-255, is being developed by Peptimmune (licensed from Genzyme). This is a novel combination of an inhibitor and a polymer designed to bind the undigested triglycerides therefore allowing increased fat excretion without side effects such as oily stools that occur with orlistat. The development seems to be stalled as Phase 1 trials were conducted in 2004 and there has been no further human clinical development since then. In 2011, Peptimmune filed for Chapter 7 Liquidation.
Another potential long-term approach to anti-obesity medication is through the development of ribonucleic acid interference (RNAi). Animal studies have illustrated that the deletion of the RIP140 gene in mice by genetic knockdown results in the lack of fat accumulation, even when mice are fed a high fat diet. Experiments conducted by Professor Malcolm Parker of Imperial College show that by silencing RIP 140, a nuclear hormone co-repressor which regulates fat accumulation, animal models exhibit a lean profile throughout their life, are resistant to diet-induced obesity, and show an enhanced metabolic rate. CytRx Corporation is developing RNAi therapeutics against this drug target for the treatment of obesity and type 2 diabetes. Similarly, another nuclear hormone receptor co-repressor, SMRT, has demonstrated an opposing effect in genetically engineered mice. Dr. Russell Nofsinger and Dr. Ronald Evans of the Salk Institute showed that disruption of the molecular interaction between SMRT and their nuclear hormone receptor partners leads to increased adiposity and a decreased metabolic rate. These studies suggest that new drugs targeting the molecular interaction between nuclear hormone receptors and their regulatory cofactors could provide a useful new category of therapeutic targets to be developed in an effort to control obesity.
Another approach is to induce a sense of satiety by occupying space in the gastric and intestinal cavities. One clinical trial involves a hydrogel made of indigestible, food-grade materials. Another pilot study uses pseudobezoars.
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