In conclusion starting from the weakly potent GSM

In conclusion, starting from the weakly potent GSM , exploration of various spacer groups between the triazole ring and the aromatic Torin 1 led to the discovery of a new series of potent gamma secretase modulators, with an improved drug-like profile—compared with the originally reported series around . In this anilinotriazole series, there was a disconnection between the in vitro potency and the in vivo activity of some of the analogs. The presence of an -substituent on the aniline ring proved to be crucial for obtaining in vivo activity. This could be the result of the higher free fraction in brain tissue of most of the ortho-substituted anilines in comparison to the - and or -analogs. In order to better understand the discrepancy between the in vitro and the in vivo activity observed in the anilinotriazole series, the coverage ratio was examined. This parameter represents the ratio between the free brain concentration and the potency, given by IC. Some compounds already showed a significant effect in modulating Aβ peptide levels in brain at a coverage ratio ∼0.1. A higher coverage ratio routinely translated into robust reduction in Aβ42 peptide levels. Due to the general low of all analogs in this series, high exposures of the compounds in the mouse brain were generally required for in vivo activity. Compound was identified, combining good in vitro efficacy with in vivo potency across species. The lower lipophilicity of versus led to an improvement of the overall ADME profile versus . In dog, treatment with resulted in significant changes in Aβ levels and, more importantly, single and repeated dosing did not affect the liver function. Compound was therefore progressed towards further evaluation. Acknowledgments
Presenilins and the γ-secretase protease complex The presenilin 1 (PS1) and presenilin 2 (PS2) proteins that form the catalytic core of the γ-secretase protease complex were initially discovered in genetic screens for mutations causing early onset forms of familial Alzheimer's disease (FAD) [1], [2]. γ-Secretase is a multisubunit transmembrane proteolytic complex belonging to the family of intramembrane cleaving proteases (ICliPs) of which there are four classes; the serine proteases of the rhomboid class, the metalloproteases in the Site-2-proteases (S2P) class and the GXGD aspartyl proteases, signal peptide peptidases (SPP) and γ-secretase [3]. Of all the ICliPs studied to date γ-secretase is unique as it is a four-protein complex, consisting of the presenilins, anterior pharynx-defective 1 (Aph-1), presenilin enhancer 2 (Pen-2) and Nicastrin, which is only found in multicellular organisms [3], [4]. In humans, two forms of presenilin (PS1 and PS2) and two forms of Aph-1 have been identified, one of the Aph-1 homologues is also expressed in two isoforms via alternative splicing, leading to the possible existence of at least six different γ-secretase complexes that may have tissue- or cell type specificity [5]. The presenilins are highly conserved transmembrane proteins that are synthesized as 50kDa holoproteins that undergo endoproteolysis to generate heterodimeric presenilin N-terminal and C-terminal fragments (NTF/CTF), a required activation step in their role as the catalytic subunit of the γ-secretase protease complex [6]. The structure of the PS1-γ-secretase protease complex has revealed the intricate interaction between each subunit and an overall horseshoe-shaped structure composed of the 20 transmembrane domains of presenilin, Pen2, Aph1 and nicastrin [7], [8]. To date more than 90 γ-secretase substrates have been identified including APP and a large number of cell surface receptors and adhesion molecules such as Notch, ErbB4, CD44 and E-cadherin [9], [10]. The sequential proteolysis of APP culminating in intramembrane cleavage by γ-secretase now serves as a model for the regulated intramembrane proteolysis of numerous transmembrane proteins (Fig. 1) [3], [11]. Cleavage of APP by α-secretase or β-secretase results in the release of respective soluble ectodomains (sAPPα or sAPPβ), generation of the APP intracellular domain (AICD) and P3 or Aβ peptides that represent the remaining transmembrane domain of APP [13]. The sAPPα and sAPPβ fragments have been shown to promote neurite growth in cortical neurons in rats and mice and to regulate cell signalling pathways [11], [12], [14], [15], while the AICD fragment has been shown to have a number of cellular functions such as regulating Wnt signalling, gene transcription and apoptosis [16], [17].