July 2001

Aim: LeadDiscovery is a company of industrial scientists dedicated to identifying areas of research with pharmaceutical/biotech potential. We use our experience to help academic scientists or biotech companies highlight this potential. Equally we provide an impartial and non-commission based service to industry identifying field leaders and by suggesting how their research areas can be adapted to product development.

Abstract: Control of food intake represents a major objective of the pharmaceutical industry given the large increase in incidence of obesity. To meet this aim a number of opioid receptor antagonists have been investigated as anorectics. Likewise the lead cannabinoid receptor antagonist, SR141716 is in development for this indication. Field-leading researchers at the University of Reading have shown that low doses of SR141716 and the reference opioid antagonist, naloxone, do not significantly suppress food intake in rats. When given in combination however, the same doses of the two antagonists evoked a dramatic anorectic response, reducing food intake by over 70%. These exciting data suggest that mixed cannabinoid/opioid antagonists may represent a novel target for the treatment of obesity. Furthermore this target may selectively reduce the intake of snack foods, a significant contributor to obesity, without altering hunger or satiety signals. Collaboration with this center of excellence may be useful to companies involved in molecular modeling, combinatorial chemistry and/or HTS.

Background: The definition of obesity is the accumulation of excess body fat sufficient to endanger health, and results from energy intake exceeding energy expenditure. In both the US and the UK the incidence of obesity is increasing by about 1% per year, a tendency related to the rise in other serious co-morbitities (see Quantumhcp or obesity on-line). As a general guide, obesity increases the likelihood of death from all causes by 20%, and more specifically a 25% increase in death from coronary artery disease, a 10% increase from stroke, twice the risk of diabetes, and a 40% increase in gall bladder disease. These statistics increase dramatically as the level of obesity increases. In addition to these diseases, obese patients suffer an increased incidence of cancer, pulmonary disorders and arthritis. Not surprisingly therefore, obesity has been suggested to account for 300,000 deaths annually making it the second leading cause of preventable death, second only after smoking (which accounted for 400,000 deaths).

The exact relationship of energy intake to obesity is complicated. This area was excellently reviewed by McCrory et al (2000) who described how energy density, palatability and dietary variety are all important dietary components, and that these dietary variables may play a more significant role than dietary fat in determining energy intake and body fatness. The number of high energy food products introduced into the US food market in the past 30 years that are classified as condiments, candy, snacks and bakery foods parallels the increasing prevalence of obesity and is strikingly out of proportion to the number of new vegetable and fruit products. Tipping the nutritional balance back towards a high variety of vegetables and a low variety of sweets, snacks, condiments, entrees and carbohydrates may promote a long-term reduction in voluntary energy intake and body fatness without the need to resort to a conscious restriction of energy intake. Clinical strategies that can correct this balance are multi-faceted including change in life-styles and pharmaceutical intervention. A general consideration of treatment is offered in Guidelines on Overweight and Obesity, produced by the NIH. The general guideline for the use of pharmaceutical strategies is that they "may only be used as part of a comprehensive weight loss program, including dietary therapy and physical activity, for patients with a BMI of ³ 30 with no concomitant obesity-related risk factors or diseases, and for patients with a BMI of ³ 27 with concomitant obesity-related risk factors or diseases. Weight loss drugs should never be used without concomitant lifestyle modifications. Concomitant assessment of drug therapy for efficacy and safety is necessary. If the drug is efficacious in helping the patient lose and/or maintain weight loss and there are no serious adverse effects, it can be continued. If not, it should be discontinued". Pharmacotherapy of obesity has developed greatly over the past decade. Based on the conclusion of a trial reported by Weintraub in 1992, phentermine and fenfluramine, drug therapy began to change from short-term to long-term use. Both dexfenfluramine and fenfluramine alone, as well as the combination of phentermine/fenfluramine, were used long term. However, concerns about unacceptable side effects, such as valvular lesions of the heart causing significant insufficiency of the valves, led to the withdrawal of these drugs from the market in September 1997. No drugs remained that were approved by the FDA for use longer than 3 months. In November 1997, the FDA approved a new drug, sibutramine, for use in obesity. In April 1999, orlistat was approved by FDA for weight loss. Three classes of anorectic drugs have been developed, all of which affect neurotransmitters in the brain: those that affect catecholamines, those that affect serotonin, and those that affect both. They work by increasing the secretion of dopamine, norepinephrine, or serotonin into the synaptic neural cleft, or by inhibiting the reuptake of these neurotransmitters back into the neuron, or by both mechanisms. The new agent sibutramine has norepinephrine and serotonin effects. Another new agent, orlistat, has a different mechanism of action, the blockage of fat absorption.

To understand the involvement of centrally acting pharmaceutical in eating it is first necessary to delineate the various stages of eating (see on-line review by Kirkham). Eating can broadly be partitioned into the initial approach to food and the onset of eating (latency) and then the maintenance of eating. Maintenance is a balance between two opposing control mechanisms, orosensory reward which is modified by taste, texture, temperature, etc. and satiety signals originating from the stomach and intestines. In addition energy balance also plays a role in eating. Modulation of orosensory reward offers a particularly attractive approach to obesity since it has the potential to preferentially target the intake of snacky foods without altering other signals such as hunger and satiety.

In addition to catecholamines and/or serotonin modulators, a number of other central neurotransmitters have been targeted as orosensory reward modulating anorectics including the opioids and the cannabinoids. A current overview of the opioid family can be found online (Corbett et al) and is strongly recommended for readers wishing to access detailed information of this field. In summary however, it is now clear that there are 3 well-defined or "classical" types of opioid receptor µ, d and k . Genes encoding for these receptors have been cloned. More recently, cDNA encoding an "orphan" receptor was identified which has a high degree of homology to the "classical" opioid receptors; on structural grounds this receptor is an opioid receptor and has been named ORL1 (opioid receptor-like). As would be predicted from their known abilities to couple through pertussis toxin-sensitive G-proteins, all of the cloned opioid receptors possess the same general structure of an extracellular N-terminal region, seven transmembrane domains and intracellular C-terminal tail structure. There is pharmacological evidence for subtypes of each receptor and other types of novel, less well-characterized opioid receptors, e , l , i , x , have also been postulated. The s -receptor, however, is no longer regarded as an opioid receptor. A current overview of the cannabinoid family can also be found online (Pertwee et al) and is strongly recommended for readers wishing to access detailed information on this field. In summary, two types of cannabinoid receptor have so far been identified, CB1, cloned in 1990, and CB2, cloned in 1993. These have now been detected in several species including man, rat and mouse. A spliced variant of CB1 cDNA, CB1a, has also been isolated, however, CB1a mRNA exists only as a minor transcript and there is no evidence for any notable difference between the pharmacology of CB1 and CB1a receptors. In parallel with our improved understanding of cannabinoid and opioid pathways, a wide range of tools have been developed. These are summarized in the table below, adapted from the website of Tocris, one of the leaders in the supply of cannabinoid and opioid research tools.

Table: Some common tools used in the research of cannabinoid- and opioid-mediated pathways

The role of opioids in food intake has been well documented (see review by Glass et al, 1999). The majority of the collected data related to opioids and feeding has led to the speculation that opioids are involved in meal maintenance and orosensory reward. These properties are highly attractive in the context of anorectic therapies since the ability to leave fluid intake, hunger and satiety unaltered would be expected to support good patient compliance. On the other hand an ability to exert a preferential effect on the intake of snack foods could be of immense benefit. This concept is based on the results of a number of studies including recent demonstration that opioids mediate reward without playing a role in satiety pathways (Stromberg et al, 1997; Kelley et al, 1996). Consequently mixed µ, d and k antagonists such as naltrexone effectively reduce food intake but this effect is considerably more potent for sweet foods (Giraudo et al, 1993) particularly as a result of binding to opioid receptors in the amygdala. The paraventricular nucleus also plays a role in the anorectic properties of the opiate antagonists although this region is less sensitive to naltrexone and effects are not related to specific foods (Glass et al, 2000). Consequently it has been suggested that opioids in the hindbrain might be involved in both sensory and metabolic aspects of food intake, those in the amygdala in processing of 'emotional' properties of foods, and those in the hypothalamus in energy needs (Glass et al, 1999). In view of this body of animal data it is not surprising that the administration of opioid antagonists affects human food intake. In the early 1980s it was shown that naltrexone reduces food intake in healthy human volunteers by as much as 28% although there was little change in the perception of satiety or hunger, or the intake of fluid (Trenchard & Silverstone, 1983; Cohen et al, 1985). This opioid effect was, as in animals, related to lower palatability (Yeomans & Wright, 1991;Drewnowski et al, 1995).

Unfortunately though, despite the ability of opioid antagonist to reduce food intake in non-obese animals or humans, this class of drug was markedly less affective in obesity (Atkinson et al, 1985; Maggio et al, 1985; Cole et al, 1997) and it is not possible to increase the dose of currently approved opioid antagonists such as naltrexone since the risk of liver toxicity becomes significant (Mitchell et al, 1987). It should be noted however that many of these clinical studies were performed in ignorance of the subtle distinction between food intake and palatability. In other words opioid antagonists may preferentially reduce the intake of high-reward food-types without significantly affecting overall food intake (see Kirkham & Cooper, 1991). Furthermore the shift away from snack-foods represents a primary aim of anorectics and so early clinical studies may have missed important opioid antagonist effects. An alternative approach would be to identify pathways with which the opioids interact synergistically allowing more effective antagonists to be identified. This would not only be expected to improve efficacy but it would overcome any toxicology aspects. It should perhaps be noted that opioids antagonists have also been developed to modulate alcohol intake and since hepatotoxicity may be a particularly serious problem in alcoholics, dose lowering through the use of mixed antagonists may have wide-spread potential.

It has been known for a long time that cannabinoids affect food intake (for a recent review see Kirkham & Williams, 2001). In man, cannabis usage is generally associated with enhanced food intake, with one study reporting a weight gain of 3.7lbs after only 5 days of cannabis smoking (Greenberg et al, 1976). Later studies repeated this finding with calorific intake being increased by 40% due to prolific snacking, preferably on sweet items of food (Foltin et al, 1988). In the late 1990's following the development of improved cannabinoid ligands, clear effects of agonists and antagonists were reported in animal studies. The agonist, anandamide consistently caused increased food intake (Williams & Kirkham, 1999; Hao et al, 2000). On the other hand, the CB1 antagonist, SR141716 (0.3-3mg/kg) was found to reduce sucrose intake in rats although overall chow intake was only marginally affected, consistent with earlier studies suggesting that cannabinoids modulate brain reward systems (Arnone et al, 1997). Higher doses of SR141716 (3-10mg/kg) dose dependently reduced food intake and also body weight (Colombo et al, 1998). Clinical studies have not been published however SR141716 has now been advanced to phase II trials for obesity suggesting that this molecule displays clinical activity.

There is considerable evidence to show that the opioid and cannabinoid pathways impinge on one another, with data largely coming from the study of agonists. For example in 1992, tetrahydrocannabinol was shown to increase the antinociceptive efficacy of morphine by 10-fold (Welch & Stevens). Conversely the response to tetrahydrocanabinol was increased by morphine apparently activity through m and k opiate receptors (Reche et al, 1996). Similar suggestions have also been made regarding behavioral patterns with naloxone being able to block the effects of THC in a model of reward (Gardner et al, 1988), Due to the relatively late development of cannabinoid antagonists, few studies have been reported investigating their synergy with opioid receptor antagonists. Given current pharmaceutical interest in the development of these two classes of drug especially in relation to possible anorectic activity such studies are necessary. The identification of synergy could lead to the development of hyper-potent dual antagonists. In order to address this issue Tim Kirkham & Claire Williams recently conducted a ground breaking study to investigate antagonist synergy focussing on food intake (Kirham & Williams, 2001)

Profile of combined opioid-cannabinoid antagonism: A groundbreaking study was performed to determine the synergistic activity of cannabinoid and opioid antagonism in a model of feeding. For experimental detail the reader is referred to (Kirham & Williams, 2001). Of note however rats were non-obese and studies were performed using regular laboratory chow. Animals were non-starved. Reference antagonists were administered sc.

 

• Under the study condition animals ingested 6.3g of chow in a one hour period.

• Pretreatment with either 0.1mg/kg of the CB1 antagonist, SR141716 or 0.1mg/kg of the non-selective opioid antagonist, naloxone did not significantly reduce food intake (4% and 2% reduction respectively).

Patent position: A preliminary search was performed to identify patent activity surrounding mixed cannabinoid/opioid receptor ligands. A search of the PCT and US patent office databases was performed, however no applications have been filed to date claiming mixed ligands or indeed the use of a combination of specific ligands. This was true irrespective of indication.

Market size: It is estimated that obesity affects 34 million people in the US (Lehman Brothers, Genomic II, Jan 1998). In the UK the overweight and obese population increased by 15% between 1980 and 1992. In both the US and the UK the incidence of obesity is increasing by about 1% per year. Sales of anti-obesity pharmaceuticals in the US reached $422 million in 1996 (Scrip Reports, 1997). This dropped in 1997 when the FDA withdrew approval for the use of dexfenfluramine and fenfluramine in September 1997. The market was reestablished in November 1997, when the FDA approved sibutramine, for use in obesity, and still further in April 1999, orlistat was also approved. The regrowth of the obesity was paralleled in the European Union (inset), and in Japan, a region suffering from increased incidence of obesity as a result of dietary changes, the potential market for sibutramine is estimated as $150 million (Scrip Reports, 1998).

Market competition: Most anorectics suppress appetite via an effect on the central nervous system. They affect the dopamine, opioid, norepinephrine and 5-HT-mediated pathways of appetite control. Tolerance to anti-obesity agents develops rapidly and so these drugs are usually used to initiate weight loss. A table is provided below including key treatment in advanced development or on the market for obesity. Please note that although LeadDiscovery takes all measures to provide accurate information, drug development data comes from third parties and accuracy or completeness cannot be guaranteed. A total of 111 molecules are reported as being in various stages of development. In addition to the molecules reported below a further 14 are in Phase I and 83 are in preclinical development.

Table 1: Anorectic molecules on the market or in advanced stages of development. Click on products of interest to link through to drug profiles.

Comparison of cannabinoid or opioid antagonists with other therapeutic classes: The development of mixed cannabinoid/opioid receptor antagonists as a novel treatment of obesity remains conceptual and it is difficult to identify advantages of this approach over other candidate anorectics. Further comparative studies are therefore recommended. It should be noted however that SR 141716 is one of the most promising anorectics in advanced stage development and that the study reported in this dossier shows clear advantages of combinations of SR 141716 and naloxone over SR 141716 alone. Advantages not only include improved efficacy but they could also include reduced toxicity. These advantages are not restricted to use in food intake disorders rather they could extend to a variety of addictive disorders such as alcoholism. As mentioned above reduced toxicity may be particularly pertinent to use in alcohol abuse patients who may suffer from liver disfunction.

Strategic analysis, and suggested further studies: The results of the study presented in this dossier demonstrate clearly the potential of mixed opioid/cannabinoid antagonists in the treatment of obesity. This is clearly supported by a large body of animal data showing an involvement of these two pathways in food intake and also the therapeutic activity of cannabinoid antagonists. Cannabinoid and opioid antagonists are particularly exciting since in general they have little effect on fluid intake, hunger or satiety and they would therefore be expected to hold good patient compliance. Instead they are associated with a preferential effect on the desire to eat snack foods, the excessive ingestion of which is a major contributory factor to obesity. Despite the ability of opioid antagonist to reduce food intake in non-obese animals or humans, this class of drug was markedly less affective in obesity and consequently cannabinoid rather than opioid antagonists have progressed towards the market. Mixed opioid/cannabinoid antagonists are expected to demonstrate improved efficacy over pure cannabinoid antagonists, and also to allow the therapeutic targeting of the opioid pathway. A number of key questions need to be addressed before a proof of concept can be conclusively demonstrated:

• Does a combination of SR 141716 and naloxone reduce food intake in obese rats and if so is this protocol subject to tolerance?

• Is the effect of this combination biased towards snack foods since this may offer an improved drug profile, targeting a major cause of obesity, while simultaneously improving patient compliance by leaving hunger pathways unaffected?

• What receptor is responsible for mediating the effects of naloxone?

• How does the anorectic effect of combined opioid/cannabinoid antagonism compare to other centrally acting anorectics (eg serotonergic, noradrenergic, dopaminergic and gaberergic molecules)?