Discovery of Novel, Potent BACE Inhibitors with Central Activity for the Treatment of Alzheimer's Disease

One of the neuropathological hallmarks of Alzheimer's disease is the presence of β-amyloid (Aβ)-containing neuritic plaques. Aβ is produced by the proteolytic cleavage of the amyloid precursor protein (APP) by β- and γ-secretase enzymes. Thus, inhibition of β-secretase (BACE1) is a promising disease modifying approach for Alzheimer's disease. Several high affinity BACE inhibitors have been described that reduce plasma Aβ in vivo. Although this may be sufficient to provide clinical benefit (e.g., via peripheral sink), our goal was to identify BACE inhibitors that substantially reduced (>25%) Aβ in the brain. To support ongoing medicinal chemistry SAR efforts to identify BACE inhibitors that would reduce Aβ in the brain, we used lowering of Aβ1-40 in cerebrospinal fluid (CSF) as a marker of CNS BACE1 inhibition. We also switched our in vivo screening model from transgenic CRND8 mice expressing the Swedish and Indiana mutations in the human APP gene to non-transgenic rats and developed a sensitive rat Aβ1-40 immunoassay assay to support the in vivo screen. Our ex vivo binding studies and studies with heterozygous BACE1 mice supported the conclusion that BACE is not the rate limiting step for Aβ synthesis in the rodent brain and thus high occupancy of the BACE enzyme is likely required for brain efficacy. After focusing our SAR efforts around robust Aβ-lowering in rat CSF, several novel, potent, high affinity, orally available, brain penetrant BACE inhibitors were identified that substantially lowered CSF Aβ and more importantly, inhibited Aβ in the rat cortex. Based on our experience, brain exposures well in excess of inhibitor cell potency alone did not always result in substantial lowering of CSF or cortical Aβ, but that high brain exposure combined with sustained and substantial CSF efficacy was needed to observe Aβ-lowering in the cortex. Thus, we have identified several novel, potent BACE inhibitors with excellent Aβ-lowering activity in CSF and the brain which may be of use as disease modifying Alzheimer's disease therapeutics.