Novel Phenyl-Substituted 5,6-Dihydro-[1,2,4]triazolo[4,3‑a]pyrazine P2X7 Antagonists with Robust Target Engagement in Rat Brain

Novel 5,6-dihydro-[1,2,4]­triazolo­[4,3-a]­pyrazine P2X7 antagonists were optimized to allow for good blood-brain barrier permeability and high P2X7 target engagement in the brain of rats. Compound 25 (huP2X7 IC50 = 9 nM; rat P2X7 IC50 = 42 nM) achieved 80% receptor occupancy for 6 h when dosed orally at 10 mg/kg in rats as measured by ex vivo radioligand binding autoradiography. Structure–activity relationships within this series are described, as well as in vitro ADME results. In vivo pharmacokinetic data for key compounds is also included.