Disconnection between the early onset anorectic effects by C75 and hypothalamic fatty acid synthase inhibition in rodents

Utilization of an Active Site Mutant Receptor for the Identification of Potent and Selective Atypical 5-HT2C Receptor Agonists

Agonism of the 5-HT_2C receptor represents one of the most well-studied and clinically proven mechanisms for pharmacological weight reduction. Selectivity over the closely related 5-HT_2A and 5-HT_2B receptors is critical as their activation has been shown to lead to undesirable side effects and major safety concerns. In this communication, we report the development of a new screening paradigm that utilizes an active site mutant D134A (D3.32) 5-HT_2C receptor to identify atypical agonist structures. We additionally report the discovery and optimization of a novel class of nonbasic heterocyclic amide agonists of 5-HT_2C. SAR investigations around the screening hits provided a diverse set of potent agonists at 5-HT_2C with high selectivity over the related 5-HT_2A and 5-HT_2B receptor subtypes. Further optimization through replacement of the amide with a variety of five- and six-membered heterocycles led to the identification of 6-(1-ethyl-3-(quinolin-8-yl)-1H-pyrazol-5-yl)pyridazin-3-amine (69). Oral administration of 69 to rats reduced food intake in an ad libitum feeding model, which could be completely reversed by a selective 5-HT_2C antagonist.

Differential pharmacologic properties of the two C75 enantiomers: (+)-C75 is a strong anorectic drug; (-)-C75 has antitumor activity

C75 is a synthetic compound described as having antitumoral properties. It produces hypophagia and weight loss in rodents, limiting its use in cancer therapy but identifying it as a potential anti-obesity drug. C75 is a fatty acid synthase (FAS) inhibitor and, through its coenzyme A (CoA) derivative, it acts as a carnitine palmitoyltransferase (CPT) 1 inhibitor. Racemic mixtures of C75 have been used in all the previous studies; however, the potential different biological activities of C75 enantiomers have not been examined yet. To address this question we synthesized the two C75 enantiomers separately. Our results showed that (-)-C75 inhibits FAS activity in vitro and has a cytotoxic effect on tumor cell lines, without affecting food consumption. (+)-C75 inhibits CPT1 and its administration produces anorexia, suggesting that central inhibition of CPT1 is essential for the anorectic effect of C75. The differential activity of C75 enantiomers may lead to the development of potential new specific drugs for cancer and obesity.

The effects of C75, an inhibitor of fatty acid synthase, on sleep and metabolism in mice

Sleep is greatly affected by changes in metabolic state. A possible mechanism where energy-sensing and sleep-regulatory functions overlap is related to lipid metabolism. Fatty acid synthase (FAS) plays a central role in lipid metabolism as a key enzyme in the formation of long-chain fatty acids. We studied the effects of systemic administration of C75, an inhibitor of FAS, on sleep, behavioral activity and metabolic parameters in mice. Since the effects of C75 on feeding and metabolism are the opposite of ghrelin's and C75 suppresses ghrelin production, we also tested the role of ghrelin signaling in the actions of C75 by using ghrelin receptor knockout (KO) mice. After a transient increase in wakefulness, C75 elicited dose-dependent and long lasting inhibition of REMS, motor activity and feeding. Simultaneously, C75 significantly attenuated slow-wave activity of the electroencephalogram. Energy expenditure, body temperature and respiratory exchange ratio were suppressed. The diurnal rhythm of feeding was completely abolished by C75. There was significant correlation between the anorectic effects, the decrease in motor activity and the diminished energy expenditure after C75 injection. We found no significant difference between wild-type and ghrelin receptor KO mice in their sleep and metabolic responses to C75. The effects of C75 resemble to what was previously reported in association with visceral illness. Our findings suggest that sleep and metabolic effects of C75 in mice are independent of the ghrelin system and may be due to its aversive actions in mice.

Antidiabetic and antisteatotic effects of the selective fatty acid synthase (FAS) inhibitor platensimycin in mouse models of diabetes

Platensimycin (PTM) is a recently discovered broad-spectrum antibiotic produced by Streptomyces platensis. It acts by selectively inhibiting the elongation-condensing enzyme FabF of the fatty acid biosynthesis pathway in bacteria. We report here that PTM is also a potent and highly selective inhibitor of mammalian fatty acid synthase. In contrast to two agents, C75 and cerulenin, that are widely used as inhibitors of mammalian fatty acid synthase, platensimycin specifically inhibits fatty acid synthesis but not sterol synthesis in rat primary hepatocytes. PTM preferentially concentrates in liver when administered orally to mice and potently inhibits hepatic de novo lipogenesis, reduces fatty acid oxidation, and increases glucose oxidation. Chronic administration of platensimycin led to a net reduction in liver triglyceride levels and improved insulin sensitivity in db/+ mice fed a high-fructose diet. PTM also reduced ambient glucose levels in db/db mice. These results provide pharmacological proof of concept of inhibiting fatty acid synthase for the treatment of diabetes and related metabolic disorders in animal models.

Thermogenesis and related metabolic targets in anti-diabetic therapy

Exercise, together with a low-energy diet, is the first-line treatment for type 2 diabetes type 2 diabetes . Exercise improves insulin sensitivity insulin sensitivity by increasing the number or function of muscle mitochondria mitochondria and the capacity for aerobic metabolism, all of which are low in many insulin-resistant subjects. Cannabinoid 1-receptor antagonists and β-adrenoceptor agonists improve insulin sensitivity in humans and promote fat oxidation in rodents independently of reduced food intake. Current drugs for the treatment of diabetes are not, however, noted for their ability to increase fat oxidation, although the thiazolidinediones increase the capacity for fat oxidation in skeletal muscle, whilst paradoxically increasing weight gain.There are a number of targets for anti-diabetic drugs that may improve insulin sensitivity insulin sensitivity by increasing the capacity for fat oxidation. Their mechanisms of action are linked, notably through AMP-activated protein kinase, adiponectin, and the sympathetic nervous system. If ligands for these targets have obvious acute thermogenic activity, it is often because they increase sympathetic activity. This promotes fuel mobilisation, as well as fuel oxidation. When thermogenesis thermogenesis is not obvious, researchers often argue that it has occurred by using the inappropriate device of treating animals for days or weeks until there is weight (mainly fat) loss and then expressing energy expenditure energy expenditure relative to body weight. In reality, thermogenesis may have occurred, but it is too small to detect, and this device distracts us from really appreciating why insulin sensitivity has improved. This is that by increasing fatty acid oxidation fatty acid oxidation more than fatty acid supply, drugs lower the concentrations of fatty acid metabolites that cause insulin resistance. Insulin sensitivity improves long before any anti-obesity effect can be detected.

Discovery of pyrazine carboxamide CB1 antagonists: The introduction of a hydroxyl group improves the pharmaceutical properties and in vivo efficacy of the series

Structure-activity relationships for a series of pyrazine carboxamide CB1 antagonists are reported. Pharmaceutical properties of the series are improved via inclusion of hydroxyl-containing sidechains. This structural modification sufficiently improved ADME properties of an orally inactive series such that food intake reduction was achieved in rat feeding models. Compound 35 elicits a 46% reduction in food intake in ad libidum fed rats 4-h post-dose.

Hypothalamic fatty acid metabolism: a housekeeping pathway that regulates food intake

The hypothalamus is a specialized area in the brain that integrates the control of energy homeostasis. More than 70 years ago, it was proposed that the central nervous system sensed circulating levels of metabolites such as glucose, lipids and amino acids and modified feeding according to the levels of those molecules. This led to the formulation of the Glucostatic, Lipostatic and Aminostatic Hypotheses. It has taken almost that much time to demonstrate that circulating long-chain fatty acids act as signals of nutrient surplus in the hypothalamus. Moreover, pharmacological and/or genetic inhibition of fatty acid synthase, AMP-activated protein kinase and carnitine palmitoyltransferase 1 results in profound decrease in feeding and body weight in rodents. The molecular mechanism behind these actions depends on changes in the cellular pool of malonyl-CoA and fatty acyl-CoAs. Current evidence also suggests that this pathway may play a major role in the physiological regulation of feeding, by integrating hormonal and nutrient-derived signals in the hypothalamus. Here, we summarize what is known about hypothalamic fatty acid metabolism and feeding control and provide future directions for research. Understanding these molecular mechanisms could provide new targets for the treatment of obesity and related disorders.

Thermogenic and metabolic antiobesity drugs: rationale and opportunities

Antiobesity drugs that target peripheral metabolism may avoid some of the problems that have been encountered with centrally acting anorectic drugs. Moreover, if they cause weight loss by increasing fat oxidation, they not only address a cause of obesity but also should promote loss of fat rather than lean tissue and improve insulin sensitivity. Weight loss may be slow but more sustained than with anorectic drugs, and thermogenesis may be insufficient to cause any discomfort. Some thermogenic approaches are the activation of adrenergic, thyroid hormone or growth hormone receptors and the inhibition of glucocorticoid receptors; the modulation of transcription factors [e.g. peroxisome proliferator-activated receptor delta (PPARdelta) activators] or enzymes [e.g. glutamine fructose-6-phosphate amidotransferase (GFAT) inhibitors] that promote mitochondrial biogenesis, and the modulation of transcription factors (PPAR alpha activators) or enzymes (AMP-activated protein kinase) that promote fatty acid oxidation. More surprisingly, studies on genetically modified animals and with enzyme inhibitors suggest that inhibitors of fatty acid synthesis [e.g. ATP citrate lyase, fatty acid synthase, acetyl-CoA carboxylase (ACC)], fatty acid interconversion [stearoyl-CoA desaturase (SCD)] and triglyceride synthesis (e.g. acyl-CoA : diacylglycerol acyltransferase) may all be thermogenic. Some targets have been validated only by deleting genes in the whole animal. In these cases, it is possible that deletion of the protein in the brain is responsible for the effect on adiposity, and therefore a centrally penetrant drug would be required. Moreover, whilst a genetically modified mouse may display resistance to obesity in response to a high fat diet, it requires a tool compound to demonstrate that a drug might actually cause weight loss. Even then, it is possible that differences between rodents and humans, such as the greater thermogenic capacity of rodents, may give a misleading impression of the potential of a drug.

Intracerebroventricular C75 decreases meal frequency and reduces AgRP gene expression in rats

3-Carboxy-4-alkyl-2-methylenebutyrolactone (C75), an inhibitor of fatty acid synthase and stimulator of carnitine palmitoyltransferase-1, reduces food intake and body weight in rodents when given systemically or centrally. Intracellular molecular mechanisms involving changes in cellular energy status are proposed to initiate the feeding and body weight reductions. However, effectors that lie downstream of these initial steps are not yet fully identified. Present experiments characterize the time courses of hypophagia and weight loss after single injections of C75 into the lateral cerebroventicle in rats and go on to identify specific meal pattern changes and coinciding alterations in gene expression for feeding-related hypothalamic neuropeptides. C75 reduced chow intake and body weight dose dependently. Although the principal effects occurred on the first day, weight losses relative to vehicle control were maintained over multiple days. C75 did not affect generalized locomotor activity. C75 began to reduce feeding after a 6-h delay. The hypophagia was due primarily to decreased meal number during 6–12 h without a significant effect on meal size, suggesting that central C75 reduced the drive to initiate meals. C75 prevented the anticipated hypophagia-induced increases in mRNA for AgRP in the arcuate nucleus at 22 h and at 6 h when C75 begins to suppress feeding. Overall, the data suggest that gene expression changes leading to altered melanocortin signaling are important for the hypophagic response to intracerebroventricular C75.

Characterization of the inactivation of rat fatty acid synthase by C75: inhibition of partial reactions and protection by substrates

C75, a synthetic inhibitor of FAS (fatty acid synthase), has both anti-tumour and anti-obesity properties. In this study we provide a detailed kinetic characterization of the mechanism of in vitro inhibition of rat liver FAS. At room temperature, C75 is a competitive irreversible inhibitor of the overall reaction with regard to all three substrates, i.e. acetyl-CoA, malonyl-CoA and NADPH, exhibiting pseudo-first-order kinetics of the complexing type, i.e. a weak non-covalent enzyme-inhibitor complex is formed before irreversible enzyme modification. C75 is a relatively inefficient inactivator of FAS, with a maximal rate of inactivation of 1 min(-1) and an extrapolated K(I) (dissociation constant for the initial complex) of approx. 16 mM. The apparent second-order rate constants calculated from these values are 0.06 mM(-1).min(-1) at room temperature and 0.21 mM(-1).min(-1) at 37 degrees C. We also provide experimental evidence that C75 inactivates the beta-ketoacyl synthase (3-oxoacyl synthase) partial activity of FAS. Unexpectedly, C75 also inactivates the enoyl reductase and thioesterase partial activities of FAS with about the same rates as for inactivation of the beta-ketoacyl synthase. In contrast with the overall reaction, the beta-ketoacyl synthase activity and the enoyl reductase activity, substrates do not protect the thioesterase activity of rat liver FAS from inactivation by C75. These results differentiate inactivation by C75 from that by cerulenin, which only inactivates the beta-ketoacyl synthase activity of FAS, by forming an adduct with an active-site cysteine. Interference by dithiothreitol and protection by the substrates, acetyl-CoA, malonyl-CoA and NADPH, further distinguish the mechanism of C75-mediated inactivation from that of cerulenin. The most likely explanation for the multiple effects observed with C75 on rat liver FAS and its partial reactions is that there are multiple sites of interaction between C75 and FAS.

The connections between C75 and obesity drug-target pathways

Obesity and its attendant disorders, such as Type II diabetes, have reached epidemic proportions in the USA, and their prevalence is increasing globally. C75 is a small-molecule inhibitor of fatty acid synthase (FAS) and a stimulator of carnitine palmitoyl 1 activity, which causes profound weight loss in mice. Although C75 is not a compound that is destined for human drug development, it has provided two potential pathways to target in obesity therapy: fatty acid synthesis and fatty acid oxidation. In this article, we discuss the latest data challenging the relationship between fatty acid synthase inhibition and C75-induced anorexia.