Discovery and pharmacological evaluation of growth hormone secretagogue receptor antagonists.

The discovery and pharmacological evaluation of potent, selective, and orally bioavailable growth hormone secretagogue receptor (GHS-R) antagonists are reported. Previously, 2,4-diaminopyrimidine-based GHS-R antagonists reported from our laboratories have been shown to be dihydrofolate reductase (DHFR) inhibitors. By comparing the X-ray crystal structure of DHFR docked with our GHS-R antagonists and GHS-R modeling, we designed and synthesized a series of potent and DHFR selective GHS-R antagonists with good pharmacokinetic (PK) profiles. An amide derivative 13d (Ca2+ flux IC50 = 188 nM, [brain]/[plasma] = 0.97 @ 8 h in rat) showed a 10% decrease in 24 h food intake in rats, and over 5% body weight reduction after 14-day oral treatment in diet-induced obese (DIO) mice. In comparison, a urea derivative 14c (Ca2+ flux IC50 = 7 nM, [brain]/[plasma] = 0.0 in DIO) failed to show significant effect on food intake in the acute feeding DIO model. These observations demonstrated for the first time that peripheral GHS-R blockage with small molecule GHS-R antagonists might not be sufficient for suppressing appetite and inducing body weight reduction.

Chronic treatment with either dexfenfluramine or sibutramine in diet-switched diet-induced obese mice.

Dexfenfluramine (DEX) and sibutramine (SIB) are effective antiobesity agents. Their effects on weight control and hormone profile have not been previously studied in diet-switched diet-induced obese (DIO) mice, in which treatment is initiated upon cessation of a low-fat diet and resumption of a high-fat diet. Furthermore, their effects on circulating ghrelin in obese humans or in animal models of obesity have not yet been reported. Male C57Bl/6J DIO mice after 16 wk on a high-fat diet (HF, 60 kcal% fat) were switched to a low-fat diet (LF, 10 kcal% fat) for 50 d. HF diet resumed concurrently with treatment for 28 d with DEX 3 and 10 mg/kg, twice a day (BID); SIB 5 mg/kg BID; or vehicle. Rapid weight regain ensued in vehicle-treated DIO mice. DEX or SIB treatment significantly blunted the body weight gain. Caloric intake was decreased acutely by DEX or SIB vs vehicle during the first 2 d treatment, but returned to control after 5 d. At the end of study, epididymal fat weight and whole body fat mass determined by DEXA scan were decreased by DEX 10 mg/kg, and whole body lean mass decreased with DEX 3 mg/kg treatment. Circulating ghrelin on d 28 was increased with either DEX 3 or 10 mg/kg treatment, while growth hormone and insulin were decreased. Leptin was also decreased in the DEX 10 mg/kg group. SIB did not significantly affect fat mass, ghrelin, growth hormone, insulin, or leptin. Mice chronically fed LF diet maintained a lower caloric intake, gained less weight and fat mass than diet-switched mice, and had higher ghrelin and lower insulin and leptin. In summary, weight regain in diet-switched DIO mice is delayed with either DEX or SIB treatment. DEX treatment of diet-switched DIO mice decreased growth hormone, insulin, leptin, fat mass, lean mass, and increased ghrelin, while SIB only decreased body weight.

2,4-diaminopyrimidine derivatives as potent growth hormone secretagogue receptor antagonists.

Ghrelin, a gut-derived orexigenic hormone, is an endogenous ligand of the growth hormone secretagogue receptor (GHS-R). Centrally administered ghrelin has been shown to cause hunger and increase food intake in rodents. Inhibition of ghrelin actions with ghrelin antibody, peptidyl GHS-R antagonists, and antisense oligonucleosides resulted in weight loss and food intake decrease in rodents. Here we report the effects of GHS-R antagonists, some of which were potent, selective, and orally bioavailable. A structure-activity relationship study led to the discovery of 8a, which was effective in decreasing food intake and body weight in several acute rat studies.