Rational Design of Novel 1,3-Oxazine Based β-Secretase (BACE1) Inhibitors: Incorporation of a Double Bond To Reduce P-gp Efflux Leading to Robust Aβ Reduction in the Brain

Accumulation of A beta peptides is a hallmark of Alzheimer's disease (AD) and is considered a causal factor in the pathogenesis of AD. beta-Secretase (BACE1) is a key enzyme responsible for producing A beta peptides, and thus agents that inhibit BACE1 should be beneficial for disease-modifying treatment of AD. Here we describe the discovery and optimization of novel oxazine-based BACE1 inhibitors by lowering amidine basicity with the incorporation of a double bond to improve brain penetration. Starting from a 1,3-dihydrooxazine lead 6 identified by a hit-to-lead SAR following HTS, we adopted a pKa lowering strategy to reduce the P-gp efflux and the high hERG potential leading to the discovery of 15 that produced significant A beta reduction with long duration in pharmacodynamic models and exhibited wide safety margins in cardiovascular safety models. This compound improved the brain-to-plasma ratio relative to 6 by reducing P-gp recognition, which was demonstrated by a P-gp knockout mouse model.