Bombesin receptor subtype-3 (BRS-3) is an orphan G proteincoupled receptor implicated in the regulation of energy homeostasis. Here, we report the biologic effects of a highly optimized BRS-3 agonist, ( -5046). Single oral doses of MK-5046 inhibited 2-h and overnight food intake and increased fasting metabolic rate in wild-type but not Brs3 knockout mice. Upon dosing for 14 days, MK-5046 at 25 mg ⅐ kg Ϫ1 ⅐ day Ϫ1reduced body weight of diet-induced obese mouse by 9% compared with vehicle-dosed controls. In mice, 50% brain receptor occupancy was achieved at a plasma concentration of 0.34 Ϯ 0.23 M. With chronic dosing, effects on metabolic rate, rather than food intake, seem to be the predominant mechanism for weight reduction by MK-5046. The compound also effectively reduced body weight in rats and caused modest increases in body temperature, heart rate, and blood pressure. These latter effects on temperature, heart rate, and blood pressure were transient in nature and desensitized with continued dosing. MK-5046 is the first BRS-3 agonist with properties suitable for use in larger mammals. In dogs, MK-5046 treatment produced statistically significant and persistent weight loss, which was initially accompanied by increases in body temperature and heart rate that abated with continued dosing. Our results demonstrate antiobesity efficacy for MK-5046 in rodents and dogs and further support BRS-3 agonism as a new approach to the treatment of obesity.

Lanza T, Lin LS, Nargund RP, Guan XM, Strack AM, Reitman ML. Body temperature as a mouse pharmacodynamic response to bombesin receptor subtype-3 agonists and other potential obesity treatments. Am J Physiol Endocrinol Metab 299: E816 -E824, 2010. First published August 31, 2010; doi:10.1152/ajpendo.00404.2010.-Treatment of rodents with a bombesin receptor subtype-3 (BRS-3) agonist reduces food intake and increases fasting metabolic rate, causing weight loss with continued treatment. In small mammals, core body temperature (Tb) is regulated in part by nutritional status, with a reduced Tb during fasting. We report that fed Brs3 knockout mice have a lower T b, which is discordant with their nutritional status. Treatment of wild-type mice with a BRS-3 agonist increased Tb, more so when the baseline Tb was reduced such as by fasting or during the inactive phase of the light cycle. With repeated BRS-3 agonist dosing, the Tb increase attenuated despite continued weight loss efficacy. The increase in T b was not prevented by inhibitors of prostaglandin E (PGE) production but was partially reduced by a ␤-adrenergic blocker. These results demonstrate that BRS-3 has a role in body temperature regulation, presumably secondary to its effect on energy metabolism, including effects on sympathetic tone. By making use of this phenomenon, the reversal of the fasting Tb reduction was developed into a sensitive single-dose pharmacodynamic assay for BRS-3 agonism and other antiobesity compounds acting by various mechanisms, including sibutramine, cannabinoid-1, and melanin-concentrating hormone-1 receptor blockers, and melanocortin, ␤3-adrenergic, and cholecystokinin-1 receptor agonists. These drugs increased both the fasted Tb and the fasted, resting metabolic rates. The Tb assay is a robust, information-rich assay that is simpler and has a greater throughput than measuring metabolic rate and is a practical, effective tool for drug discovery. tachyphylaxis; metabolic rate; thermoregulation; fasting; drug discovery MAMMALS ARE HOMEOTHERMS, regulating their core body temperature (T b ) within a narrow range (30). T b impacts all facets of life, ranging from chemical reaction rates to defense against infection. Mammals typically live in environments below their thermoneutral range, so maintenance of T b involves generating and conserving heat. Heat is generated as a byproduct of metabolic processes and via dedicated heat generation ("facultative thermogenesis") that occurs principally in brown adipose tissue, a specialized tissue whose only known function is efficient heat generation (5). Heat is conserved through a variety of mechanisms, including behavioral (nests, huddling, choice of warm environment), anatomic (fur, increased body size), and physiological (vasoconstriction, regulation of energy expenditure) mechanisms (11).Small mammals maintain their T b , despite their increased heat loss to the environment, by burning a significant fraction of their energy intake for warmth. For example, about one-third of the food intake is ...

We report the development and characterization of compound 22 (MK-5046), a potent, selective small molecule agonist of BRS-3 (bombesin receptor subtype-3). In pharmacological testing using diet-induced obese mice, compound 22 caused mechanism-based, dose-dependent reductions in food intake and body weight.KEYWORDS MK-5046, bombesin receptor subtype-3 agonist, obesity T he mammalian bombesin G-protein-coupled receptor subfamily 1 comprises three structurally related members, the receptors for neuromedin B (NMBR or BB1), gastrin-releasing peptide (GRPR or BB2), 2 and bombesin receptor subtype-3 (BRS-3 or BB3). 3 BRS-3 is an orphan receptor whose natural ligand is not known and which, despite its name, does not bind bombesin with high affinity. BRS-3 is expressed predominantly in the central nervous system (CNS). Mice lacking functional BRS-3 develop mild obesity and insulin resistance, suggesting a role for the receptor in the regulation of energy homeostasis. 4 In addition, BRS-3 as well as the other mammalian family members are expressed by several human carcinoma cell lines, where they may play a role in growth regulation. 3 Until recently, there have been limited reports of peptide 5 or smallmolecule 6-9 pharmacological tools to further elucidate the role of the receptor and determine its potential as a drug target.Our laboratory has described the lead identification and initial optimization of the first reported series of smallmolecule BRS-3 agonists with efficacy in body weight lowering. [10][11][12] Exploratory medicinal chemistry efforts focused on lead 1. The potency at the human receptor was improved through the incorporation of branched substituents at the 4-position of the imidazole, while systemic and CNS exposure were increased through replacement of the benzoic acid moiety with pyridine. This work eventually led to the identification of 2 (Bag-1), the first key pharmacological tool in our program. Results of in vivo testing with this compound furnished a preclinical proof of concept for the BRS-3 mechanism: Administration of the BRS-3 agonist in established diet-induced obese (eDIO) mice caused a significant reduction of acute food intake and increase in fasting metabolic rate, and these effects were not evident in Brs3 null (KO) mice. 13 Furthermore, subchronic dosing (100 mpk BID, 8 days) led to mechanism-based body weight lowering in eDIO mice, while it had no significant effects in KO mice. 13 Undesirable features of 2 were its poor oral pharmacokinetics in preclinical species and a suboptimal off-target profile that included low micromolar binding to the human inward rectifying potassium ion channel (hERG, IC 50 = 1.2 μM) as well as the diltiazem (DLZ) site of the rabbit calcium ion channel (IC 50 = 0.5 μM).The low unbound exposure of compound 2 was shown to be due primarily to high intrinsic clearance. A bile duct cannulated rat study was performed to examine the fate of the compound after in vivo dosing. For this purpose, a tritium radiolabel was incorporated in the 5-position of th...