Synthesis of the Potent, Selective, and Efficacious β-Secretase (BACE1) Inhibitor NB-360

Abstract

Starting from lead compound 4, the 1,4-oxazine headgroup was optimized to improve potency and brain penetration. Focusing at the 6-position of the 5-amino-1,4-oxazine, the insertion of a Me and a CF3 group delivered an excellent pharmacological profile with a pK a of 7.1 and a very low P-gp efflux ratio enabling high central nervous system (CNS) penetration and exposure. Various synthetic routes to access BACE1 inhibitors bearing a 5-amino-6-methyl-6-(trifluoromethyl)-1,4-oxazine headgroup were investigated. Subsequent optimization of the P3 fragment provided the highly potent N-(3-((3R,6R)-5-amino-3,6-dimethyl-6-(trifluoromethyl)-3,6-dihydro-2H-1,4-oxazin-3-yl)-4-fluorophenyl)-5-cyano-3-methylpicolinamide 54 (NB-360), able to reduce significantly Aβ levels in mice, rats, and dogs in acute and chronic treatment regimens.

Discovery of Umibecestat (CNP520): A Potent, Selective, and Efficacious β-Secretase (BACE1) Inhibitor for the Prevention of Alzheimer’s Disease

“…With the various phenyl-5-amino-1,4-oxazine scaffolds, considerable P3 structure–activity relationship (SAR) had already been explored. − Known P3 motifs were therefore revisited to assess whether previously established SAR could be transferred to the 5-fluoropyridine-5-amino-1,4-oxazine scaffold. Key requirements were the preservation of the important hydrogen bond of the amide NH to the backbone carbonyl of Gly230 and the suppression of the amide hydrolysis by steric bulk next to the carbonyl, as for the phenyl scaffolds, leading to a focus on 3-substituted picolinamides and pyrazine-2-carboxamides (Table ).…”

“…Despite the excellent pharmacokinetic properties and brain penetration of the BACE1 inhibitor 8 ( NB-360 ) and its high efficacy in animal models, the development of this compound as a clinical candidate was discontinued. This decision was mainly driven by the appearance of gray patches in the black fur of mice chronically treated with NB-360 , − an effect also reported for several other BACE1 inhibitors. − We, like others, attributed this side effect to the inhibition of BACE2, which is involved in the processing of melanosome protein PMEL17, leading to improper melanin distribution in the hair follicle and hair depigmentation . The unknown physiological consequences of the impaired melanin processing, beyond the effect on the hair color, were internally rated a significant development risk for an AD drug, in particular when chronic treatment over several years is expected.…”

“…For these brain-penetrant and efficacious inhibitors, the optimization of additional parameters, that is, metabolic stability and selectivity over related proteases such as pepsin and cathepsin E (CathE), but in particular over cathepsin D (CathD), became important . The evolution of such an inhibitor series, the 5-amino-1,4-oxazine series, is exemplified in Table . − …”

“… a Values are means of at least three experiments. b Measured p K a . c P app is the permeability through a MDR1-MDCK cell monolayer [flux ( P AP–BL + P BL–AP )/2]. d ER is the efflux ratio ( P BL–AP / P AP–BL ) in MDCK cells transfected with human MDR1. e Ref . f Ref . …”

“…Rather, we went on to optimize the P1 and P3 fragments (nomenclature P1, P2, and P3 according to Schechter and Berger), for improved selectivity over BACE2 (schematic binding mode, see Figure ). Other research groups also pursued similar approaches improving the selectivity over BACE2 with large P3 substituents. − During our previous optimization of the P3 moiety, the lipophilicity (calculated and measured) for several inhibitors exceeded the CNS drug property space (clog P /log P < 4.0) . Therefore, our strategy was to first turn our attention to the P1 fragment with a focus on more polar moieties to provide more leeway in selecting P3 moieties while staying within the CNS drug property space.…”