Presenilin is implicated in the pathogenesis of Alzheimer's disease. It is thought to constitute the catalytic subunit of the ␥-secretase complex that catalyzes intramembrane cleavage of ␤-amyloid precursor protein, the last step in the generation of amyloidogenic A␤ peptides. The latter are major constituents of amyloid plaques in the brain of Alzheimer's disease patients. Inhibitors of ␥-secretase are considered potential therapeutics for the treatment of this disease because they prevent production of A␤ peptides. Recently, we discovered a family of presenilin-type aspartic proteases. The founding member, signal peptide peptidase, catalyzes intramembrane cleavage of distinct signal peptides in the endoplasmic reticulum membrane of animals. In humans, the protease plays a crucial role in the immune system. Moreover, it is exploited by the hepatitis C virus for the processing of the structural components of the virion and hence is an attractive target for anti-infective intervention. Signal peptide peptidase and presenilin share identical active site motifs and both catalyze intramembrane proteolysis. These common features let us speculate that ␥-secretase inhibitors directed against presenilin may also inhibit signal peptide peptidase. Here we demonstrate that some of the most potent known ␥-secretase inhibitors efficiently inhibit signal peptide peptidase. However, we found compounds that showed higher specificity for one or the other protease. Our findings highlight the possibility of developing selective inhibitors aimed at reducing A␤ generation without affecting other intramembranecleaving aspartic proteases. Alzheimer's disease (AD)1 is characterized by the formation of senile plaques in the brain. Major constituents of these plaques are the amyloidogenic 40-and 42-residue-long A␤ peptides A␤40 and A␤42, respectively (1). The amyloid cascade hypothesis casually links the generation of amyloid plaques with the neuropathological changes accompanying the symptoms typical of this disease (2). A␤ peptides are generated from the type I transmembrane protein ␤-APP (␤-amyloid precursor protein) by sequential proteolysis (3). The protein is first cleaved in the exoplasmic domain by the ␤-site APP-cleaving enzyme (BACE) to release the ectodomain (4, 5). The residual membrane-anchored stub of 99 residues (C99) is subsequently cleaved in the center of the transmembrane region by ␥-secretase (6). The resulting cleavage products, an A␤ peptide and the amyloid intracellular domain (AICD), are liberated from the lipid bilayer toward the exoplasm and cytosol, respectively (7-9).To date, the majority of characterized familial AD mutations are clustered along the presenilin-1 (PS1) gene (10, 11). They are thought to accelerate disease onset by increasing the A␤42/ A␤40 ratio (12). It is not well understood how these mutations, which are essentially scattered along the entire PS1 gene, can lead to a specific increase in the production of the 42-residuelong peptide that corresponds to the most amyloidogenic form of A␤ (13). ...

BackgroundAlzheimer’s disease (AD) is the most common form of dementia, the number of affected individuals is rising, with significant impacts for healthcare systems. Current symptomatic treatments delay, but do not halt, disease progression. Genetic evidence points to aggregation and deposition of amyloid-β (Aβ) in the brain being causal for the neurodegeneration and dementia typical of AD. Approaches to target Aβ via inhibition of γ-secretase or passive antibody therapy have not yet resulted in substantial clinical benefits. Inhibition of BACE1 (β-secretase) has proven a challenging concept, but recent BACE1inhibitors can enter the brain sufficiently well to lower Aβ. However, failures with the first clinical BACE1 inhibitors have highlighted the need to generate compounds with appropriate efficacy and safety profiles, since long treatment periods are expected to be necessary in humans.ResultsTreatment with NB-360, a potent and brain penetrable BACE-1 inhibitor can completely block the progression of Aβ deposition in the brains of APP transgenic mice, a model for amyloid pathology. We furthermore show that almost complete reduction of Aβ was achieved also in rats and in dogs, suggesting that these findings are translational across species and can be extrapolated to humans. Amyloid pathology may be an initial step in a complex pathological cascade; therefore we investigated the effect of BACE-1 inhibition on neuroinflammation, a prominent downstream feature of the disease. NB-360 stopped accumulation of activated inflammatory cells in the brains of APP transgenic mice. Upon chronic treatment of APP transgenic mice, patches of grey hairs appeared.ConclusionsIn a rapidly developing field, the data on NB-360 broaden the chemical space and expand knowledge on the properties that are needed to make a BACE-1 inhibitor potent and safe enough for long-term use in patients. Due to its excellent brain penetration, reasonable oral doses of NB-360 were sufficient to completely block amyloid-β deposition in an APP transgenic mouse model. Data across species suggest similar treatment effects can possibly be achieved in humans. The reduced neuroinflammation upon amyloid reduction by NB-360 treatment supports the notion that targeting amyloid-β pathology can have beneficial downstream effects on the progression of Alzheimer’s disease.

Human ␤-amyloid precursor protein (APP) transgenic mice are commonly used to test potential therapeutics for Alzheimer's disease. We have characterized the dynamics of ␤-amyloid (A␤) generation and deposition following ␥-secretase inhibition with compound LY-411575 Kinetic studies in preplaque mice distinguished a detergent-soluble A␤ pool in brain with rapid turnover (half-lives for A␤40 and A␤42 were 0.7 and 1.7 h) and a much more stable, less soluble pool. A␤ in cerebrospinal fluid (CSF) reflected the changes in the soluble brain A␤ pool, whereas plasma A␤ turned over more rapidly. In brain, APP C-terminal fragments (CTF) accumulated differentially. The half-lives for ␥-secretase degradation were estimated as 0.4 and 0.1 h for C99 and C83, respectively. Three different APP transgenic lines responded very similarly to ␥-secretase inhibition regardless of the familial Alzheimer's disease mutations in APP. Amyloid deposition started with A␤42, whereas A␤38 and A␤40 continued to turn over. Chronic ␥-secretase inhibition lowered amyloid plaque formation to a different degree in different brain regions of the same mice. The extent was inversely related to the initial amyloid load in the region analyzed. No evidence for plaque removal below baseline was obtained. ␥-Secretase inhibition led to a redistribution of intracellular A␤ and an elevation of CTFs in neuronal fibers. In CSF, A␤ showed a similar turnover as in preplaque animals demonstrating its suitability as marker of newly generated, soluble A␤ in plaque-bearing brain. This study supports the use of APP transgenic mice as translational models to characterize A␤-lowering therapeutics.Deposits of the A␤ peptide known as amyloid plaques are one of the defining pathological hallmarks of Alzheimer's disease, and aggregated A␤ species are considered to play a key role in disease pathogenesis (Hardy and Selkoe, 2002). Generation of A␤ from the membrane-bound ␤-amyloid precursor protein (APP) involves consecutive cleavage by the ␤-secretase BACE1 and the ␥-secretase complex (Wolfe, 2006). BACE catalyzes the cleavage at the N terminus of A␤, releasing a soluble form of APP (sAPP␤) and leaving a C-terminal fragment (C99) in the membrane. C99 is then processed by ␥-secretase, which possibly involves three successive cleavage steps (Zhao et al., 2005), finally yielding a set of A␤ peptides heterogeneous at the C terminus, with the most abundant ends at positions 40, 42, and 38. The other product of this cleavage is the APP intracellular domain thought to be a regulator of gene expression (Wolfe, 2006). The ␥-secretase is a complex composed of presenilins (PS1 or PS2), nicastrin, PEN-2 (presenilin enhancer-2), and anterior pharynx-defective protein 1 (APH-1). Considerable evidence suggests that the presenilins contain the active site of this intramembrane aspartyl protease (Wolfe, 2006). A large Article, publication date, and citation information can be found at

Structure-based design of a series of cyclic hydroxyethylamine BACE1 inhibitors allowed the rational incorporation of prime- and nonprime-side fragments to a central core template without any amide functionality. The core scaffold selection and the structure-activity relationship development were supported by molecular modeling studies and by X-ray analysis of BACE1 complexes with various ligands to expedite the optimization of the series. The direct extension from P1-aryl- and heteroaryl moieties into the S3 binding pocket allowed the enhancement of potency and selectivity over cathepsin D. Restraining the design and synthesis of compounds to a physicochemical property space consistent with central nervous system drugs led to inhibitors with improved blood-brain barrier permeability. Guided by structure-based optimization, we were able to obtain highly potent compounds such as 60p with enzymatic and cellular IC(50) values of 2 and 50 nM, respectively, and with >200-fold selectivity over cathepsin D. Pharmacodynamic studies in APP51/16 transgenic mice at oral doses of 180 μmol/kg demonstrated significant reduction of brain Aβ levels.

The beta‐site amyloid precursor protein cleaving enzyme‐1 (BACE‐1) initiates the generation of amyloid‐β (Aβ), and the amyloid cascade leading to amyloid plaque deposition, neurodegeneration, and dementia in Alzheimer's disease (AD). Clinical failures of anti‐Aβ therapies in dementia stages suggest that treatment has to start in the early, asymptomatic disease states. The BACE‐1 inhibitor CNP520 has a selectivity, pharmacodynamics, and distribution profile suitable for AD prevention studies. CNP520 reduced brain and cerebrospinal fluid (CSF) Aβ in rats and dogs, and Aβ plaque deposition in APP‐transgenic mice. Animal toxicology studies of CNP520 demonstrated sufficient safety margins, with no signs of hair depigmentation, retina degeneration, liver toxicity, or cardiovascular effects. In healthy adults ≥ 60 years old, treatment with CNP520 was safe and well tolerated and resulted in robust and dose‐dependent Aβ reduction in the cerebrospinal fluid. Thus, long‐term, pivotal studies with CNP520 have been initiated in the Generation Program.