Pharmacological profiling of NB-4 reveals the influence of brain region and Aβ isoform, but independence of age, on brain Aβ responses to BACE1 inhibition

The amyloid beta peptide (Aβ) remains central to the hypothesised mechanisms underlying Alzheimer’s disease (AD). The aspartyl protease BACE1 is the first enzyme in the synthetic pathway of Aβ. Therapeutic strategies for AD have thus targeted BACE1 with the aim of reducing Aβ and amyloid-related toxicity(1). A number of BACE inhibitors are currently in clinical testing, and a reliable, acute pharmacokinetic/pharmacodynamic relationship between inhibitor exposure and Aβ reduction has been observed across species, including in human CSF (1,2). These studies indicate that a significant reduction of Aβ in human brain with small molecule inhibitors is achievable. However, a number of questions remain about thepharmacology of BACE inhibition. For example, increasing age enhancesBACE1 activity (3), although it is unknown whether this influences the efficacy of BACE inhibitors. Similarly, the influence of BACE inhibition on Aβ levels in different brain regions is under-explored, as are direct comparisons of the effects of inhibitors on Aβ40 versus 42 isoforms. To this aim, we explored the in vivo pharmacological profile of NB-4 ((3S,4S,5R)-3-[4-amino-3-fluoro-5-((R)-2,2,2-trifluoro-1-methoxymethyl-ethoxy)-benzyl]-5-(3-tert-butyl-benzylamino)-1-oxo-hexahydro-1lambda*4*-thiopyran-4-ol)), a cyclic hydroyethelamine BACE inhibitor (4), in animal models of amyloidogenesis.

In pre-plaque APP51/16 mice (hAPPWT(751)+/-) a single acute dose of NB-4 (100 or 180μmol/kg, po.) produced a reliable dose- and time-dependent reduction in forebrain Aβ, C99 and CSF Aβ. The pattern of APP cleavage product responses demonstrated NB-4 acted as a BACE1 inhibitor in vivo. Comparison of younger (6 m old, pre-plaque) versus older (18 m old, plaque bearing) APP51/16 mice revealed an increase in forebrain C99 in the older mice, consistent with enhanced BACE1 activity. NB-4 produced an almost identical effect sizes on C99 and CSF Aβ40 in both age groups.

An acute study was conducted in beagle dogs (6) to examine effect of brain region on NB-4 efficacy. NB-4 produced a similar % reduction in C99 across the brain, and compound levels across the brain regions were not different. Aβ40 effect sizes, however, varied with different brain regions, the lowest effect (-52%) was seen in pre-frontal cortex (PFC), and the highest in cerebellum (-89%). This suggests that Aβ levels may have been influenced by region specific clearance rates. This is supported by data indicating that the PFC is the earliest region affected by amyloid plaques in the beagle brain (7).

Comparison of acute, forebrain Aβ ED50s in APP51/16 (Aβ40) and APP51/16xhPS1G384Amice (Aβ40 and Aβ42), indicated similar responses to BACE inhibition for both is oforms. After 8 weeks of dosing in APP51/16xhPS1G384A mice, however, efficacy against FA-extracted Aβ40 (-18%), Aβ42 (-18%) and plaque load (-12% and -14% for thalamus and cortex, respectively) was modest in comparison to C99 (-44%), CSF Aβ40 (-49%) and Aβ42 (-41%). This may be due to the high levels of proaggregant Aβ42 in this model (vehicle-treated mice, 6 m old: FA Aβ40 = 587 pmol/g, FA Aβ42 = 6998 pmol/g). Cognitive performance in the Y-maze was neither enhanced nor reduced by NB-4 treatment.

These studies with NB-4 indicate that BACE1 inhibition in the brain produces a robust efficacy independent of age, and revealed influences of brain region on Aβ kinetics and Aβ is oform on efficacy against aggregated Aβ. The latter finding suggests PS1 familial AD patients may require a different BACE inhibitor dosing regimen than those with sporadic AD.

(1) Vassar, 2014 Alzheimers Res Ther 6: 89-103;

(2) Neumann et al., 2015 Mol Neurodegener 10: 44-59;

(3) Fukumoto et al., 2004 Am J Pathol 164: 719-725;

(4) Rueeger et al., 2013 Bioorg Med Chem Lett 23: 5300-6;

(6) Mattsson et al., 2012 PLoS1; 7: e31084;

(7) Head et al., 2000 Neurobiol Aging; 21: 89-96.