In a preceding report from our group we disclosed the development of a novel HE transition state isostere with a difluorophenoxymethyl side chain in the P1 position and a methoxy group in the P1’ position furnishing highly potent inhibitors of BACE-1 (i.e. lead compound 1), which moreover exhibit very promising selectivity over cathepsin D. In a continuation of this work with the aim at improving on the cell-based activity and pharmacokinetic properties, we have further developed the SAR for the P1 side chain of inhibitor 1 whereby the P1 side chain oxygen has been substituted for an amine, a carbon or a bond. The chemistry developed for the previous HE inhibitor structure 1 has now been extended to readily accommodate the introduction of new P1 side chains into this new HE scaffold. These modifications have given rise to several highly potent inhibitors where the most potent displayed a BACE-1 Ki value of 0.2 nM and a cell-based Aβ40 IC50 value of 9 nM. Thus, regarding the enzyme inhibition in the cell assay a more than 600-fold improvement compared to compound 1 was achieved via minor structural alterations.
Highly potent BACE-1 protease inhibitors derived from a novel hydroxyethylene-like core structure were recently developed by our group using X-ray crystal structure data and molecular modelling. In a continuation of this work guided by molecular modelling we have explored a truncated core motif where the P2’ amide group is replaced by an ether linkage resulting in a set of alkoxy, aryloxy and alkylaryl groups, with the overall aim to reduce molecular weight and the number of amide bonds to increase permeability and bestow the inhibitors with drug-like features. The most potent of these inhibitors displayed a BACE-1 IC50 value of 140 nM. The synthesis of these BACE-1 inhibitors utilizes readily available starting materials, furnishing the target compounds in good overall yields.
The synthesis and SAR of HIV-1 protease inhibitors containing novel P2 structural elements are presented. The inhibitors were designed having hydrogen bond accepting P2 substituents to probe potential favorable interactions to Asp-29/Asp-30 of the HIV-1 protease backbone utilizing inhibitor 3 as a model template. Several inhibitors were synthesized from an L-Val-methylamide P2 motif by appending hydrogen bonding moieties from either the isopropyl side chain or from the methylamide portion. The most promising inhibitors 4a and 4e displayed Ki values of 1.0 nM and 0.7 nM respectively and EC50 values in the MT4 cell-based assay of 0.17 µM and 0.33 µM respectively, a slight loss in potency compared to lead inhibitor 3. These inhibitors were also tested against an HIV protease inhibitor resistant strain carrying the M46I, V82F, and I84V mutations. Inhibitors 4a and 4e displayed a 3 and 4 fold change respectively compared with HIV wild type, whereas lead inhibitor 3 showed a higher 9 fold change. This study further demonstrate the chemical tractability of the approach where various P2 substituents can be introduced in just one chemical step from lactone x enabling facile modifications of the overall properties in this inhibitor class.
In a preceding study we have described the development of a new hydroxyethylene (HE) core motif displaying P1 aryloxymethyl and P1’ methoxy substituents delivering potent BACE-1 inhibitors. In a continuation of this work we have now explored the SAR of the S1’ pocket by introducing a set of P1’ alkoxy groups and evaluated them as BACE-1 inhibitors. Previously the P1 and P1’ positions of the classical HE template have been relatively little explored due to the complexity of the chemical routes involved in modifications at these positions. However, the chemistries developed for the current HE template renders substituents in both the P1 and P1’ positions readily available for SAR exploration. The BACE-1 inhibitors prepared displayed IC50 values in the range of 4-45 nM, where the most potent compounds featured small P1’ groups. The cathepsin D selectivity which was high for the smallest P1’ sustituents (P1’=ethoxy, fold selectively >600) dropped for larger groups (P1’=benzyloxy, fold selectivity of 1.6). We have also confirmed the importance of both the hydroxyl group and its stereochemistry preference for this HE transition state isostere by preparing both the deoxygenated analogue and by inverting the configuration of the hydroxyl group to the R-configuration, which as expected resulted in large activity drops. Finally substituting the hydroxyl group by an amino group having the same configuration (S), which previously have been described to deliver potent BACE-1 inhibitors with advantageous properties, surprisingly resulted in a large drop in the inhibitory activity.
Several highly potent BACE-1 protease inhibitors have been developed from an inhibitor series containing a novel hydroxyethylene (HE) core structure displaying aryloxymethyl or benzyloxymethyl P1 side chains and a methoxy P1’ side chain. The target molecules were readily synthesized from chiral carbohydrate starting materials, furnishing the inhibitor compounds in good overall yields. The inhibitors show both high BACE-1 potency and good selectivity against cathepsin D, where the most potent inhibitor furnish a BACE-1 IC50 value of 0.32 nM and displays > 3000 fold selectivity over cathepsin D.