Discovery of Potent and Centrally Active 6‑Substituted 5‑Fluoro-1,3-dihydro-oxazine β‑Secretase (BACE1) Inhibitors via Active Conformation Stabilization

β-Secretase
(BACE1) has an essential role in the production
of amyloid β peptides that accumulate in patients with Alzheimer’s
disease (AD). Thus, inhibition of BACE1 is considered to be a disease-modifying
approach for the treatment of AD. Our hit-to-lead efforts led to a
cellular potent 1,3-dihydro-oxazine 6, which however
inhibited hERG and showed high P-gp efflux. The close analogue of
5-fluoro-oxazine 8 reduced P-gp efflux; further introduction
of electron withdrawing groups at the 6-position improved potency
and also mitigated P-gp efflux and hERG inhibition. Changing to a
pyrazine followed by optimization of substituents on both the oxazine
and the pyrazine culminated in 24 with robust Aβ
reduction in vivo at low doses as well as reduced CYP2D6 inhibition.
On the basis of the X-ray analysis and the QM calculation of given
dihydro-oxazines, we reasoned that the substituents at the 6-position
as well as the 5-fluorine on the oxazine would stabilize a bioactive
conformation to increase potency.

β-分泌酶（β-Secretase, BACE1）在阿尔茨海默病（Alzheimer’s disease, AD）患者体内蓄积的淀粉样β肽（amyloid β peptides）的生成过程中发挥关键作用。因此，抑制BACE1被视为治疗AD的疾病修饰疗法策略。我们的从命中化合物到先导化合物（hit-to-lead）研发工作得到了一种细胞水平活性优异的1,3-二氢恶嗪类化合物6，但该化合物会抑制hERG通道并表现出较强的P-糖蛋白（P-gp）外排效应。5-氟代恶嗪类化合物8作为其密切结构类似物，降低了P-糖蛋白外排风险；进一步在恶嗪环6位引入吸电子基团，不仅提升了化合物的活性强度，同时缓解了P-糖蛋白外排与hERG通道抑制作用。将母核替换为吡嗪环，并对恶嗪环与吡嗪环上的取代基进行优化后，最终得到化合物24，其在低剂量下即可在体内实现强效的Aβ水平降低，同时减弱了对细胞色素P450 2D6（CYP2D6）的抑制作用。基于对目标二氢恶嗪类化合物的X射线晶体分析与量子力学（Quantum Mechanics, QM）计算结果，我们推断：恶嗪环6位取代基与5位氟原子可稳定化合物的生物活性构象，从而提升其活性强度。