From Oxadiazole to Triazole Analogues: Optimization toward a Dual Orexin Receptor Antagonist with Improved in vivo Efficacy in Dogs

The orexin system is responsible for regulating the sleep‐wake cycle. Suvorexant, a dual orexin receptor antagonist (DORA) is approved by the FDA for the treatment of insomnia disorders. Herein, we report the optimization efforts toward a DORA, where our starting point was (5‐methoxy‐4‐methyl‐2‐[1,2,3]triazol‐2‐yl‐phenyl)‐{(S)‐2‐[5‐(2‐trifluoromethoxy‐phenyl)‐[1,2,4]oxadiazol‐3‐yl]‐pyrrolidin‐1‐yl}methanone ( 6 ), a compound which emerged from our in‐house research program. Compound 6 was shown to be a potent, brain‐penetrating DORA with in vivo efficacy similar to suvorexant in rats. However, shortcomings from low metabolic stability, high plasma protein binding (PPB), low brain free fraction (f u brain), and low aqueous solubility, were identified and hence, compound 6 was not an ideal candidate for further development. Our optimization efforts addressing the above‐mentioned shortcomings resulted in the identification of (4‐chloro‐2‐[1,2,3]triazol‐2‐yl‐phenyl)‐{(S)‐2‐methyl‐2‐[5‐(2‐trifluoromethoxy‐phenyl)‐4 H ‐[1,2,4]triazol‐3‐yl]‐pyrrolidin‐1‐yl}l‐methanone ( 42 ), a DORA with improved in vivo efficacy compared to 6 .