Pyrazole-based cathepsin S inhibitors with arylalkynes as P1 binding elements

A crystal structure of 1 bound to a Cys25Ser mutant of cathepsin S helped to elucidate the binding mode of a previously disclosed series of pyrazole-based CatS inhibitors and facilitated the design of a new class of arylalkyne analogs. Optimization of the alkyne and tetrahydropyridine portions of the pharmacophore provided potent CatS inhibitors (IC₅₀ = 40–300 nM), and an X-ray structure of 32 revealed that the arylalkyne moiety binds in the S1 pocket of the enzyme.     

Trifluoromethylphenyl as P2 for ketoamide-based cathepsin S inhibitors

The trifluoromethylphenyl P2 motif from previously reported heteroarylnitrile series has been successfully applied for the design and synthesis of highly potent novel ketoamide-based cathepsin S inhibitors. The key in this process is the change of the torsion angle between the P2 phenyl ring and the attached secondary amide by adding a small Cl, F, or Me group at the 2-position.     

Diazinones as P2 replacements for pyrazole-based cathepsin S inhibitors

A pyridazin-4-one fragment 4 (hCatS IC₅₀ = 170 μM) discovered through Tethering was modeled into cathepsin S and predicted to overlap in S2 with the tetrahydropyridinepyrazole core of a previously disclosed series of CatS inhibitors. This fragment served as a template to design pyridazin-3-one 12 (hCatS IC₅₀ = 430 nM), which also incorporates P3 and P5 binding elements. A crystal structure of 12 bound to Cys25Ser CatS led to the synthesis of the potent diazinone isomers 22 (hCatS IC₅₀ = 60 nM) and 27 (hCatS IC₅₀ = 40 nM).     

Synthesis and SAR of arylaminoethyl amides as noncovalent inhibitors of cathepsin S: P3 cyclic ethers

The synthesis and structure-activity relationship of a series of arylaminoethyl amide cathepsin S inhibitors are reported. Optimization of P3 and P2 groups to improve overall physicochemical properties resulted in significant improvements in oral bioavailability over early lead compounds. An X-ray structure of compound 37 bound to the active site of cathepsin S is also reported.     

Synthesis and evaluation of arylaminoethyl amides as noncovalent inhibitors of cathepsin S. Part 3: heterocyclic P3

A series of N(alpha)-2-benzoxazolyl-alpha-amino acid-(arylaminoethyl)amides were identified as potent, selective, and noncovalent inhibitors of cathepsin S. Structure-activity relationships including strategies for modulating the selectivities among cathepsins S, K, and L, and in vivo pharmacokinetics are discussed. A X-ray structure of compound 3 bound to the active site of cathepsin S is also reported.     

3,4-disubstituted azetidinones as selective inhibitors of the cysteine protease cathepsin K. Exploring P3 elements for potency and selectivity

The synthesis of a series of highly potent and selective inhibitors of cathepsin K based on the 3,4-disubstituted azetidin-2-one warhead is reported. A high degree of potency and selectivity was achieved by introducing a basic nitrogen into the distal part of the P3 element of the molecule. Data from kinetic and mass spectrometry experiments are consistent with the interpretation that compounds of this series transiently acylate the sulfhydrile of cathepsin K.     

3,4-disubstituted azetidinones as selective inhibitors of the cysteine protease cathepsin K. Exploring P2 elements for selectivity

A novel series of 3,4-disubstituted azetidinones based inhibitors of the cysteine protease cathepsin K (Cat K) has been identified. Although not optimized, some of these compounds show at least 100-fold selectivity against other cathepsins. The use of cyclic moieties as P2 elements has proven to be crucial to achieve a high degree of selectivity.     

Discovery and SAR of novel pyrazole-based thioethers as cathepsin S inhibitors. Part 2: Modification of P3, P4, and P5 regions

A novel class of tetrahydropyrido-pyrazole thioether amines that display potency against human Cathepsin S have been previously reported. Here, further SAR investigations of the P3, P4, and P5 regions are described. In particular, 4-fluoropiperidine is identified as a competent P3 binding element when utilized in conjunction with a (S)-2-hydroxypropyl linker-containing P5 moiety and oxamide or sulfonamide P4 substitution.     

The design of potent hydrazones and disulfides as cathepsin S inhibitors

The design and synthesis of dipeptidyl disulfides and dipeptidyl benzoylhydrazones as selective inhibitors of the cysteine protease Cathepsin S are described. These inhibitors were expected to form a slowly reversible covalent adduct of the active site cysteine of Cathepsin S. Formation of the initial adduct was confirmed by mass spectral analysis. The nature and mechanism of these adducts was explored. Kinetic analysis of the benzoyl hydrazones indicate that these inhibitors are acting as irreversible inhibitors of Cathepsin S. Additionally, the benzoylhydrazones were shown to be potent inhibitors of Cathepsin S processing of Class II associated invariant peptide both in vitro and in vivo.     

P2-P3 conformationally constrained ketoamide-based inhibitors of cathepsin K

An orally bioavailable series of ketoamide-based cathepsin K inhibitors with good pharmacokinetic properties has been identified. Starting from a potent inhibitor endowed with poor drug properties, conformational constraint of the P(2)-P(3) linker and modifications to P(1') elements led to an enhancement in potency, solubility, clearance, and bioavailability. These optimized inhibitors attenuated bone resorption in a rat TPTX hypocalcemic bone resorption model.     

Arylamine based cathepsin K inhibitors: investigating P3 heterocyclic substituents

A modification of novel cathepsin K inhibitors I was carried out. The structural design was aimed at reducing the lipophilic character of compounds I for obtaining better pharmacokinetic profiles. This modification afforded several less lipophilic compounds with good inhibitory activities and pharmacokinetic profiles, although the enzyme selectivity over cathepsin S was left at issue.     

Synthesis of proline analogues as potent and selective cathepsin S inhibitors

Cathepsin S is a potential target of autoimmune disease. A series of proline derived compounds were synthesized and evaluated as cathepsin S inhibitors. We discovered potent cathepsin S inhibitors through structure–activity relationship studies of proline analogues. In particular, compound 19-(S) showed promising in vitro/vivo pharmacological activities and properties as a selective cathepsin S inhibitor.     

Arylaminoethyl amides as noncovalent inhibitors of cathepsin S. Part 2: Optimization of P1 and N-aryl

A systematic study of anilines led to the discovery of a metabolically robust fluoroindoline replacement for the alkoxy aniline toxicophore in 1. Investigations of the P1 pocket resulted in the discovery of a wide tolerance of functionality leading to the discovery of 11 as a potent and selective inhibitor of cathepsin S.     

2-Phenyl-9H-purine-6-carbonitrile derivatives as selective cathepsin S inhibitors

Starting from previously disclosed equally potent cathepsin K and S inhibitor 4-propyl-6-(3-trifluoromethylphenyl)pyrimidine-2-carbonitrile 1, a novel 2-phenyl-9H-purine-6-carbonitrile scaffold was identified to provide potent and selective cathepsin S inhibitors.     

Exploration of the P2-P3 SAR of aldehyde cathepsin K inhibitors

The synthesis and biological activity of a series of aldehyde inhibitors of cathepsin K are reported. Exploration of the properties of the S2 and S3 subsites with a series of carbamate derivatized norleucine aldehydes substituted at the P2 and P3 positions afforded analogs with cathepsin K IC50s between 600 nM and 130 pM.     

Azepanone-based inhibitors of human cathepsin S: optimization of selectivity via the P2 substituent

A series of azepanone inhibitors of cathepsin S is described. Selectivity over both cathepsin K and cathepsin L was achieved by varying the P2 substituent. Ultimately, a balanced potency and selectivity profile was achieved in compound 39 possessing a 1-methylcyclohexyl alanine at P2 and nicotinamide as the P′ substituent. The cellular potency of selected analogs is also described.     

Arylaminoethyl carbamates as a novel series of potent and selective cathepsin S inhibitors

We report a novel series of noncovalent inhibitors of cathepsin S. The synthesis of the peptidomimetic scaffold is described and structure-activity relationships of P3, P1, and P1' subunits are discussed. Lead optimization to a non-peptidic scaffold has resulted in a new class of potent, highly selective, and orally bioavailable cathepsin S inhibitors.     

Pyrazole-based cathepsin S inhibitors with improved cellular potency

High potency pyrazole-based noncovalent inhibitors of human cathepsin S (CatS) were developed by modification of the benzo-fused 5-membered ring heterocycles found in earlier series of CatS inhibitors. Although substitutions on this heterocyclic framework had a moderate impact on enzymatic potency, dramatic effects on cellular activity were observed. Optimization afforded indole- and benzothiophene-derived analogues that were high affinity CatS inhibitors (IC(50)=20-40 nM) with good cellular potency (IC(50)=30-340 nM).     

Discovery of selective and nonpeptidic cathepsin S inhibitors

Nonpeptidic, selective, and potent cathepsin S inhibitors were derived from an in-house pyrrolopyrimidine cathepsin K inhibitor by modification of the P2 and P3 moieties. The pyrrolopyrimidine-based inhibitors show nanomolar inhibition of cathepsin S with over 100-fold selectivity against other cysteine proteases, including cathepsin K and L. Some of the inhibitors showed cellular activities in mouse splenocytes as well as oral bioavailabilities in rats.     

Discovery of piperazinylimidazo[1,2-a]pyridines as novel S4 binding elements for orally active factor Xa inhibitors

We have recently reported the discovery of orally active sulfonylalkylamide Factor Xa (FXa) inhibitors, as typified by compound 1 (FXa IC(50)=0.061 microM). Since the pyridylpiperidine moiety was not investigated in our previous study, we conducted detailed structure-activity relationship studies on this S4 binding element. This investigation led to the discovery of piperazinylimidazo[1,2-a]pyridine 2b as a novel and potent FXa inhibitor (FXa IC(50)=0.021 microM). Further modification resulted in the discovery of 2-hydroxymethylimidazo[1,2-a]pyridine 2e (FXa IC(50)=0.0090 microM), which was found to be a selective and orally bioavailable FXa inhibitor with reduced CYP3A4 inhibition.