Structure-Activity Relationship Studies of Imidazo[4,5-b]pyrazine Derivatives as Mitochondrial Uncouplers and their Potential in the Treatment of Obesity

Mitochondrial uncouplers have the capacity of passively shuttling protons from the mitochondrial intermembrane space to the mitochondrial matrix, independent of ATP synthase. This results in the disruption of oxidative phosphorylation and increased rate of metabolism as a counter action from the mitochondria. Therefore, small molecule mitochondrial uncouplers have potential for the treatment of obesity, diabetes, non-alcoholic fatty liver disease (NAFLD), neurodegenerative disorders, amongst others.

A one-pot method for the synthesis of 1H-imidazo[4,5-b]pyrazines from [1,2,5]oxadiazolo[3,4-b]pyrazines is herein disclosed. In the presence of Fe, Yb(OTf)3, and the desired electrophile partner, in situ reduction of the oxadiazole fragment followed by cyclization afforded imidazolopyrazines in moderate to good yields. The selection of different orthoesters as electrophiles also allowed functionalization on the 2-position of the imidazole ring. This new method was used to synthesize 1H-imidazo[4,5-b]pyrazines to perform structure-activity relationship studies. Thus, a library of 75 compounds was synthesized and characterized for mitochondrial uncoupling activity. The biological activity of the compounds was demonstrated in oxygen consumption rate assays affording potent mitochondrial uncouplers. The method was further applied to the synthesis of 5-alkoxy-2-(trifluoromethyl)-1H-imidazo[4,5-b]pyrazin-6-amines, with over 50 derivatives synthesized. A structure-activity relationship study was performed using a variety of substituents to fine-tune the scaffold's potency. The installation of a methoxy group at the 5-position of the scaffold resulted in the discovery of compound 4.3.20, which exhibited the best activity with an EC50 of 3.6 ± 0.4 μM in rat L6 myoblasts and a half-life of 4.4 h in mice. Compound 4.3.20 displayed potential as an anti-obesity agent in a mouse model with an effective dose of 50 mg kg-1 without changes in food intake or lean mass. Tissue distribution studies revealed predominance in the liver and both white and brown adipose tissue. In addition, 4.3.20 improved serum markers of insulin sensitivity and hyperlipidemia such as insulin, glucose, triglycerides, cholesterol, and HOMA-IR. Taken together, compound 4.3.20 and related mitochondrial uncouplers show promise for further development in the treatment of obesity and other diseases.