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Starting from peptidomimetic BACE-1 inhibitors, the P2 amino acid including the P2/P3 peptide bond was replaced by a rigid 3-aminomethyl cyclohexane carboxylic acid. Co-crystallization revealed an unexpected binding mode with the P3/P4 amide bond placed into the S3 pocket resulting in a new hydrogen bond interaction pattern. Further optimization based on this structure resulted in highly potent BACE-1 inhibitors with selectivity over BACE-2 and cathepsin D., (Copyright 2010 Elsevier Ltd. All rights reserved.)

A series of macrocyclic peptidic BACE-1 inhibitors was designed. While potency on BACE-1 was rather high, the first set of compounds showed poor brain permeation and high efflux in the MDRI-MDCK assay. The replacement of the secondary benzylamino group with a phenylcyclopropylamino group maintained potency on BACE-1, while P-glycoprotein-mediated efflux was significantly reduced and brain permeation improved. Several compounds from this series demonstrated acute reduction of Abeta in human APP-wildtype transgenic (APP51/16) mice after oral administration., (Copyright 2009 Elsevier Ltd. All rights reserved.)

The macrocyclic peptidic BACE-1 inhibitors 2a-c show moderate enzymatic and cellular activity. By exchange of the hydroxyethylene- to ethanolamine-transition state mimetic the peptidic character was reduced, providing the highly potent and selective inhibitor 3. Variation of the P' moiety resulted in the macrocyclic inhibitor 14. Both macrocycles show inhibition of BACE-1 in the brain of APP51/16 transgenic mice, 3 (NB-544) after intravenous and 14 (NB-533) after oral application.

The hydroxyethylene octapeptide inhibitor OM99-2 served as starting point to create the tripeptide inhibitor 1 and its analogues 2a and b. An X-ray co-crystal structure of 1 with BACE-1 allowed the design and syntheses of a series of macrocyclic analogues 3a-h covalently linking the P1 and P3 side-chains. These inhibitors show improved enzymatic potency over their open-chain analogue. Inhibitor 3h also shows activity in a cellular system.

Based on the X-ray cocrystal structure of the Tang-Ghosh heptapeptide inhibitor 1 (OM00-3), a series of macroheterocyclic analogues were designed and synthesized. Analogues containing dithia, dioxa, oxathia, and carbathia macrocycles were synthesized by methods relying on ring-closing olefin metathesis for the dioxa analogues and by alkylation of thiolates or bisthiolates for the others. Molecular modeling suggested that the incorporation of piperidine units appended to the macrocycles improved interactions through additional H-bonds and introduced further rigidity. These were synthesized in enantiomerically pure form using enzyme-catalyzed desymmetrization and diastereomer separation. Inhibitory activity on beta-site amyloid precursor protein cleaving enzyme (BACE) was observed with several macroheterocyclic inhibitors and structure-activity relationship (SAR) correlations were deduced. Cocrystal structures of two synthetic analogues revealed interesting and unexpected binding interactions.

Molecular modeling based on the X-ray crystal structure of the Tang-Ghosh heptapeptide inhibitor 1 (OM99-2) of BACE led to the design and synthesis of a series of constrained P(1)' analogues. A cyclopentane ring was incorporated in 1 spanning the P(1)' Ala methyl group and the adjacent methylene carbon atom of the chain. Progressive truncation at the P(2)'-P(4)' sites led to a potent truncated analogue 5 with good selectivity over Cathepsin D. Using the same backbone replacement concept, a series of cyclopentane, cyclopentanone, tetrahydrofuran, pyrrolidine, and pyrrolidinone analogues were synthesized with considerable variation at the P and P' sites. The cyclopentanone and 2-pyrrolidinone analogues 45 and 57 showed low nM BACE inhibition. X-ray cocrystal structures of two analogues 5 and 45 revealed excellent convergence with the original inhibitor 1 structure while providing new insights into other interactions which could be exploited for future modifications.