First Demonstration of Cerebrospinal Fluid and Plasma Aβ Lowering with Oral Administration of a β-Site Amyloid Precursor Protein-Cleaving Enzyme 1 Inhibitor in Nonhuman Primates

Abstract

β-Site amyloid precursor protein (APP)-cleaving enzyme (BACE) 1 cleavage of amyloid precursor protein is an essential step in the generation of the potentially neurotoxic and amyloidogenic Aβ42 peptides in Alzheimer's disease. Although previous mouse studies have shown brain Aβ lowering after BACE1 inhibition, extension of such studies to nonhuman primates or man was precluded by poor potency, brain penetration, and pharmacokinetics of available inhibitors. In this study, a novel tertiary carbinamine BACE1 inhibitor, tertiary carbinamine (TC)-1, was assessed in a unique cisterna magna ported rhesus monkey model, where the temporal dynamics of Aβ in cerebrospinal fluid (CSF) and plasma could be evaluated. TC-1, a potent inhibitor (IC₅₀ ∼ 0.4 nM), has excellent passive membrane permeability, low susceptibility to P-glycoprotein transport, and lowered brain Aβ levels in a mouse model. Intravenous infusion of TC-1 led to a significant but transient lowering of CSF and plasma Aβ levels in conscious rhesus monkeys because it underwent CYP3A4-mediated metabolism. Oral codosing of TC-1 with ritonavir, a potent CYP3A4 inhibitor, twice daily over 3.5 days in rhesus monkeys led to sustained plasma TC-1 exposure and a significant and sustained reduction in CSF sAPPβ, Aβ40, Aβ42, and plasma Aβ40 levels. CSF Aβ42 lowering showed an EC₅₀ of ∼20 nM with respect to the CSF [TC-1] levels, demonstrating excellent concordance with its potency in a cell-based assay. These results demonstrate the first in vivo proof of concept of CSF Aβ lowering after oral administration of a BACE1 inhibitor in a nonhuman primate.