No abstract

Production of amyloid ␤ peptides (A␤), followed by their deposition in the brain as amyloid plaques, contributes to the hallmark pathology of Alzheimer disease. The enzymes responsible for production of A␤, BACE1 and ␥-secretase, are therapeutic targets for treatment of Alzheimer disease. Two presenilin (PS) homologues, referred to as PS1 and PS2, comprise the catalytic core of ␥-secretase. In comparing presenilin selectivity of several classes of ␥-secretase inhibitors, we observed that sulfonamides in general tend to be more selective for inhibition of PS1-comprising ␥-secretase, as exemplified by ELN318463 and BMS299897. We employed a combination of chimeric constructs and point mutants to identify structural determinants for PS1-selective inhibition by ELN318463. Our studies identified amino acid residues Leu 172 , Thr 281 , and Leu 282 in PS1 as necessary for PS1-selective inhibition by ELN318463. These residues also contributed in part to the PS1-selective inhibition by BMS299897. Alanine scanning mutagenesis of areas flanking Leu 172 , Thr 281 , and Leu 282 identified additional amino acids that affect inhibitor potency of not only these sulfonamides but also nonsulfonamide inhibitors, without affecting A␤ production and presenilin endoproteolysis. Interestingly, many of these same residues have been identified previously to be important for ␥-secretase function. These findings implicate TM3 and a second region near the carboxyl terminus of PS1 aminoterminal fragment in mediating the activity of ␥-secretase inhibitors. Our observations demonstrate that PS-selective inhibitors of ␥-secretase are feasible, and such inhibitors may allow differential inhibition of A␤ peptide production and Notch signaling.

IntroductionInhibition of gamma-secretase presents a direct target for lowering Aβ production in the brain as a therapy for Alzheimer's disease (AD). However, gamma-secretase is known to process multiple substrates in addition to amyloid precursor protein (APP), most notably Notch, which has limited clinical development of inhibitors targeting this enzyme. It has been postulated that APP substrate selective inhibitors of gamma-secretase would be preferable to non-selective inhibitors from a safety perspective for AD therapy.MethodsIn vitro assays monitoring inhibitor potencies at APP γ-site cleavage (equivalent to Aβ40), and Notch ε-site cleavage, in conjunction with a single cell assay to simultaneously monitor selectivity for inhibition of Aβ production vs. Notch signaling were developed to discover APP selective gamma-secretase inhibitors. In vivo efficacy for acute reduction of brain Aβ was determined in the PDAPP transgene model of AD, as well as in wild-type FVB strain mice. In vivo selectivity was determined following seven days x twice per day (b.i.d.) treatment with 15 mg/kg/dose to 1,000 mg/kg/dose ELN475516, and monitoring brain Aβ reduction vs. Notch signaling endpoints in periphery.ResultsThe APP selective gamma-secretase inhibitors ELN318463 and ELN475516 reported here behave as classic gamma-secretase inhibitors, demonstrate 75- to 120-fold selectivity for inhibiting Aβ production compared with Notch signaling in cells, and displace an active site directed inhibitor at very high concentrations only in the presence of substrate. ELN318463 demonstrated discordant efficacy for reduction of brain Aβ in the PDAPP compared with wild-type FVB, not observed with ELN475516. Improved in vivo safety of ELN475516 was demonstrated in the 7d repeat dose study in wild-type mice, where a 33% reduction of brain Aβ was observed in mice terminated three hours post last dose at the lowest dose of inhibitor tested. No overt in-life or post-mortem indications of systemic toxicity, nor RNA and histological end-points indicative of toxicity attributable to inhibition of Notch signaling were observed at any dose tested.ConclusionsThe discordant in vivo activity of ELN318463 suggests that the potency of gamma-secretase inhibitors in AD transgenic mice should be corroborated in wild-type mice. The discovery of ELN475516 demonstrates that it is possible to develop APP selective gamma-secretase inhibitors with potential for treatment for AD.

Herein, we describe our strategy to design metabolically stable γ-secretase inhibitors which are selective for inhibition of Aβ generation over Notch. We highlight our synthetic strategy to incorporate diversity and chirality. Compounds 30 (ELND006) and 34 (ELND007) both entered human clinical trials. The in vitro and in vivo characteristics for these two compounds are described. A comparison of inhibition of Aβ generation in vivo between 30, 34, Semagacestat 41, Begacestat 42, and Avagacestat 43 in mice is made. 30 lowered Aβ in the CSF of healthy human volunteers.

A general method for the homologation of aldehydes to benzylic ketones has been developed. Aryldiazomethanes were generated in situ in the presence of an aldehyde by simply heating the tosylhydrazones of aromatic aldehydes in the presence of a stoichiometric amount of base in polar protic solvents. The resulting polar protic solvent promoted homologation afforded benzylic ketones in moderate to excellent yields with a variety of aldehydes. Isolation of the tosylhydrazones was not necessary; they could be prepared in ethanol and carried through the sequence without isolation. This methodology allows easy access to a wide variety of substituted aryldiazomethanes that would be difficult, or even impossible, to prepare via conventional methods and circumvents the toxicity and stability problems associated with the isolation and/or handling solutions of aryldiazomethanes.