Jason A. Corsa

Dynamics of β-Amyloid Reductions in Brain, Cerebrospinal Fluid, and Plasma of β-Amyloid Precursor Protein Transgenic Mice Treated with a γ-Secretase Inhibitor

γ-Secretase inhibitors are one promising approach to the development of a therapeutic for Alzheimers disease (AD). γ-Secretase inhibitors reduce brain β-amyloid peptide (Aβ), which is believed to be a major contributor in the etiology of AD. Transgenic mice overexpressing the human β-amyloid precursor protein (APP) are valuable models to examine the dynamics of Aβ changes with γ-secretase inhibitors in plaque-free and plaque-bearing animals. BMS-299897 2-[(1R)-1-[[(4-chlorophenyl)sulfony](2,5-difluorophenyl)amino]ethyl]-5-fluorobenzenepropanoic acid, a γ-secretase inhibitor, showed dose- and time dependent reductions of Aβ in brain, cerebrospinal fluid (CSF), and plasma in young transgenic mice, with a significant correlation between brain and CSF Aβ levels. Because CSF and brain interstitial fluid are distinct compartments in composition and location, this correlation could not be assumed. In contrast, aged transgenic mice with large accumulations of Aβ in plaques showed reductions in CSF Aβ in the absence of measurable changes in plaque Aβ in the brain after up to 2 weeks of treatment. Hence, CSF Aβ levels were a valuable measure of γ-secretase activity in the central nervous system in either the presence or absence of plaques. Transgenic mice were also used to examine potential side effects due to Notch inhibition. BMS-299897 was 15-fold more effective at preventing the cleavage of APP than of Notch in vitro. No changes in the maturation of CD8+ thymocytes or of intestinal goblet cells were observed in mice treated with BMS-299897, showing that it is possible for γ-secretase inhibitors to reduce brain Aβ without causing Notch-mediated toxicity.

The Amyloid-β Rise and γ-Secretase Inhibitor Potency Depend on the Level of Substrate Expression

The amyloid-β (Aβ) peptide, which likely plays a key role in Alzheimer disease, is derived from the amyloid-β precursor protein (APP) through consecutive proteolytic cleavages by β-site APP-cleaving enzyme and γ-secretase. Unexpectedly γ-secretase inhibitors can increase the secretion of Aβ peptides under some circumstances. This “Aβ rise” phenomenon, the same inhibitor causing an increase in Aβ at low concentrations but inhibition at higher concentrations, has been widely observed. Here we show that the Aβ rise depends on the β-secretase-derived C-terminal fragment of APP (βCTF) or C99 levels with low levels causing rises. In contrast, the N-terminally truncated form of Aβ, known as “p3,” formed by α-secretase cleavage, did not exhibit a rise. In addition to the Aβ rise, low βCTF or C99 expression decreased γ-secretase inhibitor potency. This “potency shift” may be explained by the relatively high enzyme to substrate ratio under conditions of low substrate because increased concentrations of inhibitor would be necessary to affect substrate turnover. Consistent with this hypothesis, γ-secretase inhibitor radioligand occupancy studies showed that a high level of occupancy was correlated with inhibition of Aβ under conditions of low substrate expression. The Aβ rise was also observed in rat brain after dosing with the γ-secretase inhibitor BMS-299897. The Aβ rise and potency shift are therefore relevant factors in the development of γ-secretase inhibitors and can be evaluated using appropriate choices of animal and cell culture models. Hypothetical mechanisms for the Aβ rise, including the “incomplete processing” and endocytic models, are discussed.

Donepezil primarily attenuates scopolamine-induced deficits in psychomotor function, with moderate effects on simple conditioning and attention, and small effects on working memory and spatial mapping

RationaleAlzheimer’s dementia (AD) patients have profound deficits in cognitive and social functions, mediated in part by a decline in cholinergic function. Acetylcholinesterase inhibitors (AChEI) are the most commonly prescribed treatment for the cognitive deficits in AD patients, but their therapeutic effects are small, and it is still not clear if they primarily affect attention, memory, or some other cognitive/behavioral functions.ObjectivesThe objective of the present experiments was to explore the effects of donepezil (Aricept™), an AChEI, on behavioral deficits related exclusively to cholinergic dysfunction.Materials and methodsThe effects of donepezil were assessed in Sprague–Dawley rats with scopolamine-induced deficits in a battery of cognitive/behavioral tests.ResultsScopolamine produced deficits in contextual and cued fear conditioning, the 5-choice serial reaction time test, delayed nonmatching to position, the radial arm maze, and the Morris water maze. Analyses of the pattern and size of the effects revealed that donepezil produced very large effects on scopolamine-induced deficits in psychomotor function (∼20–50% of the variance), moderate-sized effects on scopolamine-induced deficits in simple conditioning and attention (∼3–10% of the variance), but only small effects on scopolamine-induced deficits in higher cognitive functions of working memory and spatial mapping (∼1% of the variance).ConclusionsThese results are consistent with the limited efficacy of donepezil on higher cognitive function in AD patients, and suggest that preclinical behavioral models could be used not only to determine if novel treatments have some therapeutic potential, but also to predict more precisely what the pattern and size of the effects might be.

P-Glycoprotein Efflux and Other Factors Limit Brain Amyloid β Reduction by β-Site Amyloid Precursor Protein-Cleaving Enzyme 1 Inhibitors in Mice

Alzheimers disease (AD) is a progressive neurodegenerative disease. Amyloid β (Aβ) peptides are hypothesized to cause the initiation and progression of AD based on pathologic data from AD patients, genetic analysis of mutations that cause early onset forms of AD, and preclinical studies. Based on this hypothesis, β-site amyloid precursor protein (APP)-cleaving enzyme 1 (BACE1) inhibitors are an attractive therapeutic approach for AD because cleavage of the APP by BACE1 is required to form Aβ. In this study, three potent BACE1 inhibitors are characterized. All three inhibitors decrease Aβ formation in cultured cells with IC50 values less than 10 nM. Analysis of APP C-terminal fragments by immunoblotting and Aβ peptides by mass spectrometry showed that these inhibitors decreased Aβ by inhibiting BACE1. An assay for Aβ1–40 in mice was developed and used to show that these BACE1 inhibitors decreased plasma Aβ1–40, but not brain Aβ1–40, in wild-type mice. Because these BACE1 inhibitors were substrates for P-glycoprotein (P-gp), a member of the ATP-binding cassette superfamily of efflux transporters, these inhibitors were administered to P-gp knockout (KO) mice. These studies showed that all three BACE1 inhibitors decreased brain Aβ1–40 in P-gp KO mice, demonstrating that P-gp is a major limitation for development of BACE1 inhibitors to test the amyloid hypothesis. A comparison of plasma Aβ1–40 and brain Aβ1–40 dose responses for these three compounds revealed differences in relative ED50 values, indicating that factors other than P-gp can also contribute to poor brain activity by BACE1 inhibitors.

Soluble Aβ and cognitive function in aged F-344 rats and Tg2576 mice

Recent findings suggest that Alzheimers dementia may be mediated by soluble beta amyloid (Abeta) more than the deposits of aggregated, insoluble Abeta, and vulnerability to cognitive deficits after scopolamine challenge may help identify AD even in patients that are still pre-symptomatic. The objectives of the present experiments were to determine if vulnerability to cognitive deficits after scopolamine challenge is related to levels of soluble Abeta, and if levels of soluble Abeta are more closely related to cognitive deficits than levels of insoluble Abeta, even in aged, transgenic mice, after they have developed very high levels of insoluble Abeta. Aged F-344 rats and young mice over-expressing the Swedish mutation in the human amyloid precursor protein (APPsw; Tg2576+) had elevated levels of soluble Abeta, and were more vulnerable to scopolamine challenge in the Morris water maze (MWM), relative to young rats and Tg2576- mice; but, among individual animals, higher levels of soluble Abeta were not correlated with vulnerability to scopolamine. On the other hand, in aged Tg2576+ mice, cognitive deficits were related to levels of soluble Abeta, not insoluble Abeta, despite the fact that the levels of insoluble Abeta were thousands of times higher than the levels of soluble Abeta. The results of the present experiments suggest that vulnerability to cognitive deficits after scopolamine challenge is not related to elevated levels of soluble Abeta, but that high levels of soluble Abeta are more closely correlated with cognitive deficits than the amount insoluble Abeta, even after large amounts of aggregated, insoluble Abeta have been deposited.

Ex Vivo Occupancy of γ-Secretase Inhibitors Correlates with Brain β-Amyloid Peptide Reduction in Tg2576 Mice

Reduction of brain β-amyloid peptide (Aβ) synthesis by γ-secretase inhibitors is a promising approach for the treatment of Alzheimers disease. However, measurement of central pharmacodynamic effects in the Alzheimers disease patient will be a challenge. Determination of drug occupancy may facilitate the analysis of efficacy of γ-secretase inhibitors in a clinical setting. In this study, the relationship of γ-secretase site occupancy and brain Aβ40 reduction by γ-secretase inhibitors was examined in Tg2576 mice. [3H](2R,3S)-2-Isobutyl-N1-((S)-1-methyl-2-oxo-5-phenyl-2,3-dihydro-1H-benzo[e][1,4]diazepin-3-yl)-3-propylsuccinamide (IN973) was used as a γ-secretase radioligand, since it has been shown to bind to γ-secretase in rat, rhesus, and human brains with high affinity and specificity. We extended these findings by showing that [3H]IN973 bound to γ-secretase in Tg2576 brains with an affinity, specificity, and regional localization very similar to the other species. To quantify γ-secretase occupancy by γ-secretase inhibitors, an ex vivo binding assay was developed using [3H]IN973 and frozen brain sections from drug-treated mice. γ-Secretase occupancy and brain Aβ40 reduction were found to be highly correlated in animals dosed with either 2-[(1R)-1-[[4-chlorophenyl)-sulfonyl](2,5-difluorophenyl)amino] ethyl]-5-fluoro-benzenepropanoic acid (BMS-299897) or (S)-2-((S)-2-(3,5-difluorophenyl)-2-hydroxyacetamido)-N-((S,Z)-3-methyl-4-oxo-4,5-dihydro-3H-benzo[d][1,2]diazepin-5-yl)propanamide (BMS-433796) over a wide range of doses and times postdose, with the exception of the earliest times postdose. This lag in Aβ40 response to γ-secretase inhibition is probably related to the delayed clearance of previously produced Aβ40. The excellent correlation between brain Aβ40 and γ-secretase occupancy suggests that a positron emission tomography ligand for γ-secretase could be a valuable biomarker to determine whether γ-secretase inhibitors bind to their target in humans.

Ex vivo Occupancy of γ-Secretase Inhibitors Correlates with Brain Aβ Reduction in Tg2576 Mice

Reduction of brain β-amyloid peptide (Aβ) synthesis by γ-secretase inhibitors is a promising approach for the treatment of Alzheimers disease. However, measurement of central pharmacodynamic effects in the Alzheimers disease patient will be a challenge. Determination of drug occupancy may facilitate the analysis of efficacy of γ-secretase inhibitors in a clinical setting. In this study, the relationship of γ-secretase site occupancy and brain Aβ40 reduction by γ-secretase inhibitors was examined in Tg2576 mice. [3H](2R,3S)-2-Isobutyl-N1-((S)-1-methyl-2-oxo-5-phenyl-2,3-dihydro-1H-benzo[e][1,4]diazepin-3-yl)-3-propylsuccinamide (IN973) was used as a γ-secretase radioligand, since it has been shown to bind to γ-secretase in rat, rhesus, and human brains with high affinity and specificity. We extended these findings by showing that [3H]IN973 bound to γ-secretase in Tg2576 brains with an affinity, specificity, and regional localization very similar to the other species. To quantify γ-secretase occupancy by γ-secretase inhibitors, an ex vivo binding assay was developed using [3H]IN973 and frozen brain sections from drug-treated mice. γ-Secretase occupancy and brain Aβ40 reduction were found to be highly correlated in animals dosed with either 2-[(1R)-1-[[4-chlorophenyl)-sulfonyl](2,5-difluorophenyl)amino] ethyl]-5-fluoro-benzenepropanoic acid (BMS-299897) or (S)-2-((S)-2-(3,5-difluorophenyl)-2-hydroxyacetamido)-N-((S,Z)-3-methyl-4-oxo-4,5-dihydro-3H-benzo[d][1,2]diazepin-5-yl)propanamide (BMS-433796) over a wide range of doses and times postdose, with the exception of the earliest times postdose. This lag in Aβ40 response to γ-secretase inhibition is probably related to the delayed clearance of previously produced Aβ40. The excellent correlation between brain Aβ40 and γ-secretase occupancy suggests that a positron emission tomography ligand for γ-secretase could be a valuable biomarker to determine whether γ-secretase inhibitors bind to their target in humans.

Carbamate-appended N-alkylsulfonamides as inhibitors of γ-secretase

The synthesis and gamma-secretase inhibition data for a series of carbamate-appended N-alkylsulfonamides are described. Carbamate 54 was found to significantly reduce brain Abeta in transgenic mice. 54 was also found to possess markedly improved brain levels in transgenic mice compared to previously disclosed 1 and 2.

2-(N-Benzyl-N-phenylsulfonamido)alkyl amide derivatives as γ-secretase inhibitors.

A series of (N-benzyl-N-phenylsulfonamido)alkyl amides were developed from classic and parallel synthesis strategies. Compounds with good in vitro and in vivo γ-secretase activity were identified and described.

P2-346: PGP efflux and other factors limit brain Aβ reduction by BACE1 inhibitors in mice

one protein located on the endoplasmic reticulum (ER) membrane, precisely at focal contacts between the ER and mitochondria (Hayashi & Su, Cell 131:596, 2007). The protein regulates the activity of different ER proteins, like IP3 receptors or ER stress sensors (GRP78/BiP, PERK, ATF-6). The 1 chaperone has the unique particularity to be sensitive to synthetic ligands, which therefore allow a very focal regulation of intracellular calcium homeostasis at ER and/or mitochondria contacts. As a consequence, 1 activators/agonists have been shown to induce acute modulation of transduction pathways, effective at the behavioral level. 1 Activators are indeed anti-amnesic, antidepressant and neuroprotective compounds. We validate new 1 activator compounds as neuroprotective agents against amyloid toxicity and analyze their mechanism of action. Methods: PRE-084 is a morpholine piperidine derivative acting as a high affinity and selective 1 activator. ANAVEX1-41 is a tetrahydro-furanmethanamine that shows high affinity for M1, M2, M4 muscarinic acetylcholine receptors and 1 protein. Both compounds are potent anti-amnesic drugs alleviating learning impairments observed in mice after the central (i.c.v.) injection of amyloid 25-35 peptide (A 25-35). Results: Central administration of A 25-35 induces within one week histological and biochemical changes, memory deficits, oxidative stress and ER stress in sensitive brain structures (hippocampus, cortex), highly reminiscent of the amyloid toxicity observed in Alzheimer’s disease. A 25-35 also provokes the induction of intracellular pro-apoptotic caspases and Bax-related proteins, markers of the induction of apoptosis. At the morphological level, A 25-35 induces a marked glial (astroglia, microglia) reaction and cell loss quantifiable in pyramidal layers of the hippocampus. Pre-administration of PRE-084 or ANAVEX1-41 prevents significantly all these pathological changes, showing that 1 activators are effective neuroprotectants. Part of the mechanism involves regulation of the expression of activity of IP3 receptors or ER stress sensors, in relation with the massive calcium overload induced by A 25-35. Conclusions: 1 Activators/agonists show potent neuroprotective and putatively disease-modifying activity against amyloid toxicity. Moreover, ANAVEX1-41 is active at 30-100 g/kg i.p., suggesting a cooperative action between muscarinic and 1 targets.