Synthesis and biological evaluation of biaryl alkyl ethers as inhibitors of IDO1.

Starting from the dialkylaniline indoleamine 2,3-dioxygenase 1 (IDO1) inhibitor lead 3 (IDO1 HeLa IC50 = 7.0 nM), an iterative process of synthesis and screening led to cyclized analog 21 (IDO1 HeLa IC50 = 3.6 nM) which maintained the high potency of 3 while addressing issues of lipophilicity, cytochrome P450 (CYP) inhibition, hERG (human potassium ion channel Kv11.1) inhibition, Pregnane X Receptor (PXR) transactivation, and oxidative metabolic stability. An x-ray crystal structure of a biaryl alkyl ether 11 bound to IDO1 was obtained. Consistent with our earlier results, compound 11 was shown to bind to the apo form of the enzyme.

Development of BET inhibitors as potential treatments for cancer: A new carboline chemotype.

We describe our efforts to introduce structural diversity to a previously described triazole-containing N1-carboline series of bromodomain and extra-terminal (BET) inhibitors. N9 carbolines were designed to retain favorable binding interactions that the N1-carbolines possess. A convergent synthetic route enabled modifications to reduce clearance, enhance physicochemical properties, and improve the overall in vitro profile. This work led to the identification of a potent BET inhibitor, (S)-2-{8-fluoro-5-[(3-fluoropyridin-2-yl)(oxan-4-yl)methyl]-7-[4-(2H3)methyl-1-methyl-1H-1,2,3-triazol-5-yl]-5H-pyrido[3,2-b]indol-3-yl}propan-2-ol (10), a compound with enhanced oral exposure in mice. Subsequent evaluation in a mouse triple-negative breast cancer tumor model revealed efficacy at 4 mg/kg of N9-carboline 10.

Discovery and Preclinical Pharmacology of an Oral Bromodomain and Extra-Terminal (BET) Inhibitor Using Scaffold-Hopping and Structure-Guided Drug Design.

Inhibition of the bromodomain and extra-terminal (BET) family of adaptor proteins is an attractive strategy for targeting transcriptional regulation of key oncogenes, such as c-MYC. Starting with the screening hit 1, a combination of structure-activity relationship and protein structure-guided drug design led to the discovery of a differently oriented carbazole 9 with favorable binding to the tryptophan, proline, and phenylalanine (WPF) shelf conserved in the BET family. Identification of an additional lipophilic pocket and functional group optimization to optimize pharmacokinetic (PK) properties culminated in the discovery of 18 (BMS-986158) with excellent potency in binding and functional assays. On the basis of its favorable PK profile and robust in vivo activity in a panel of hematologic and solid tumor models, BMS-986158 was selected as a candidate for clinical evaluation.

Development of BET inhibitors as potential treatments for cancer: A search for structural diversity.

We describe our efforts to identify structurally diverse leads in the triazole-containing N1-carboline series of bromodomain and extra-terminal inhibitors. Replacement of the N5 "cap" phenyl moiety with various heteroaryls, coupled with additional modifications to the carboline core, provided analogs with similar potency, improved pharmacokinetic properties, and increased solubility compared to our backup lead, BMS-986225 (2). Rapid SAR exploration was enabled by a convergent, synthetic route. These efforts provided a potent BET inhibitor, 3-fluoropyridyl 12, that demonstrated robust efficacy in a multiple myeloma mouse tumor model at 1 mg/kg.

Using yeast surface display to engineer a soluble and crystallizable construct of hematopoietic progenitor kinase 1 (HPK1).

Hematopoietic progenitor kinase 1 (HPK1) is an intracellular kinase that plays an important role in modulating tumor immune response and thus is an attractive target for drug discovery. Crystallization of the wild-type HPK1 kinase domain has been hampered by poor expression in recombinant systems and poor solubility. In this study, yeast surface display was applied to a library of HPK1 kinase-domain variants in order to select variants with an improved expression level and solubility. The HPK1 variant with the most improved properties contained two mutations, crystallized readily in complex with several small-molecule inhibitors and provided valuable insight to guide structure-based drug design. This work exemplifies the benefit of yeast surface display towards engineering crystallizable proteins and thus enabling structure-based drug discovery.

Application of the Gyrolab microfluidic platform to measure picomolar affinity of a PD-L1-binding Adnectin™ radioligand for positron emission tomography.

Advances in in vitro display and protein engineering yield therapeutics with affinities in the picomolar range. The Gyrolab® microfluidics platform uses the kinetic exclusion assay principle to measure subnanomolar solution affinities. This work describes application of the Gyrolab solution affinity module and the new multi-curve analysis feature to determine affinity of the PD-L1 Adnectin™ positron emission tomography radioligand, which was measured as 20 pM for human PD-L1. We also report key parameters that affect assay signal-to-background ratio and data quality, such as detection reagent concentration. Gyrolab offers the necessary throughput for rapid assay development with low sample consumption, as demonstrated in this study, which also provides helpful tips for assay optimization for solution affinity measurement.

An improved protocol for amino acid type-selective isotope labeling in insect cells.

An improved expression protocol is proposed for amino acid type-specific [13C], [15N]-isotope labeling of proteins in baculovirus-infected (BV) insect cell cultures. This new protocol modifies the methods published by Gossert et al. (J Biomol NMR 51(4):449-456, 2011) and provides efficient incorporation of isotopically labeled amino acids, with similar yields per L versus unlabeled expression in rich media. Gossert et al. identified the presence of unlabeled amino acids in the yeastolate of the growth medium as a major limitation in isotope labeling using BV-infected insect cells. By reducing the amount of yeastolate in the growth medium ten-fold, a significant improvement in labeling efficiency was demonstrated, while maintaining good protein expression yield. We report an alternate approach to improve isotope labeling efficiency using BV-infected insect cells namely by replacing the yeast extracts in the medium with dialyzed yeast extracts to reduce the amount of low molecular weight peptides and amino acids. We report the residual levels of amino acids in various media formulations and the amino acid consumption during fermentation, as determined by NMR. While direct replacement of yeastolate with dialyzed yeastolate delivered moderately lower isotope labeling efficiencies compared to the use of ten-fold diluted undialized yeastolate, we show that the use of dialyzed yeastolate combined with a ten-fold dilution delivered enhanced isotope labeling efficiency and at least a comparable level of protein expression yield, all at a scale which economizes use of these costly reagents.

Development and Fit-for-Purpose Validation of a Soluble Human Programmed Death-1 Protein Assay.

Programmed death-1 (PD-1) protein is a co-inhibitory receptor which negatively regulates immune cell activation and permits tumors to evade normal immune defense. Anti-PD-1 antibodies have been shown to restore immune cell activation and effector function-an exciting breakthrough in cancer immunotherapy. Recent reports have documented a soluble form of PD-1 (sPD-1) in the circulation of normal and disease state individuals. A clinical assay to quantify sPD-1 would contribute to the understanding of sPD-1-function and facilitate the development of anti-PD-1 drugs. Here, we report the development and validation of a sPD-1 protein assay. The assay validation followed the framework for full validation of a biotherapeutic pharmacokinetic assay. A purified recombinant human PD-1 protein was characterized extensively and was identified as the assay reference material which mimics the endogenous analyte in structure and function. The lower limit of quantitation (LLOQ) was determined to be 100 pg/mL, with a dynamic range spanning three logs to 10,000 pg/mL. The intra- and inter-assay imprecision were ≤15%, and the assay bias (percent deviation) was ≤10%. Potential matrix effects were investigated in sera from both normal healthy volunteers and selected cancer patients. Bulk-prepared frozen standards and pre-coated Streptavidin plates were used in the assay to ensure consistency in assay performance over time. This assay appears to specifically measure total sPD-1 protein since the human anti-PD-1 antibody, nivolumab, and the endogenous ligands of PD-1 protein, PDL-1 and PDL-2, do not interfere with the assay.

Oxadiazole-isopropylamides as potent and noncovalent proteasome inhibitors.

Screening of the 50000 ChemBridge compound library led to the identification of the oxadiazole-isopropylamide 1 (PI-1833) which inhibited chymotrypsin-like (CT-L) activity (IC50 = 0.60 µM) with little effects on the other two major proteasome proteolytic activities, trypsin-like (T-L) and postglutamyl-peptide-hydrolysis-like (PGPH-L). LC-MS/MS and dialysis show that 1 is a noncovalent and rapidly reversible CT-L inhibitor. Focused library synthesis provided 11ad (PI-1840) with CT-L activity (IC50 = 27 nM). Detailed SAR studies indicate that the amide moiety and the two phenyl rings are sensitive toward modifications. Hydrophobic residues, such as propyl or butyl in the para position (not ortho or meta) of the A-ring and a m-pyridyl group as B-ring, significantly improve activity. Compound 11ad (IC50 = 0.37 µM) is more potent than 1 (IC50 = 3.5 µM) at inhibiting CT-L activity in intact MDA-MB-468 human breast cancer cells and inhibiting their survival. The activity of 11ad warrants further preclinical investigation of this class as noncovalent proteasome inhibitors.

Functional consequence of covalent reaction of phosphoenolpyruvate with UDP-N-acetylglucosamine 1-carboxyvinyltransferase (MurA).

The enzyme MurA has been an established antibiotic target since the discovery of fosfomycin, which specifically inhibits MurA by covalent modification of the active site residue Cys-115. Early biochemical studies established that Cys-115 also covalently reacts with substrate phosphoenolpyruvate (PEP) to yield a phospholactoyl adduct, but the structural and functional consequences of this reaction remained obscure. We captured and depicted the Cys-115-PEP adduct of Enterobacter cloacae MurA in various reaction states by X-ray crystallography. The data suggest that cellular MurA predominantly exists in a tightly locked complex with UDP-N-acetylmuramic acid (UNAM), the product of the MurB reaction, with PEP covalently attached to Cys-115. The uniqueness and rigidity of this "dormant" complex was previously not recognized and presumably accounts for the failure of drug discovery efforts toward the identification of novel and effective MurA inhibitors. We demonstrate that recently published crystal structures of MurA from various organisms determined by different laboratories were indeed misinterpreted and actually contain UNAM and covalently bound PEP. The Cys-115-PEP adduct was also captured in vitro during the reaction of free MurA and substrate UDP-N-acetylglucosamine or isomer UDP-N-acetylgalactosamine. The now available series of crystal structures allows a comprehensive view of the reaction cycle of MurA. It appears that the covalent reaction of MurA with PEP fulfills dual functions by tightening the complex with UNAM for the efficient feedback regulation of murein biosynthesis and by priming the PEP molecule for instantaneous reaction with substrate UDP-N-acetylglucosamine.

Discovery and synthesis of hydronaphthoquinones as novel proteasome inhibitors.

Screening efforts led to the identification of PI-8182 (1), an inhibitor of the chymotrypsin-like (CT-L) activity of the proteasome. Compound 1 contains a hydronaphthoquinone pharmacophore with a thioglycolic acid side chain at position 2 and thiophene sulfonamide at position 4. An efficient synthetic route to the hydronaphthoquinone sulfonamide scaffold was developed, and compound 1 was synthesized in-house to confirm the structure and activity (IC(50) = 3.0 ± 1.6 µM [n = 25]). Novel hydronaphthoquinone derivatives of 1 were designed, synthesized, and evaluated as proteasome inhibitors. The structure-activity relationship (SAR) guided synthesis of more than 170 derivatives revealed that the thioglycolic acid side chain is required and the carboxylic acid group of this side chain is critical to the CT-L inhibitory activity of compound 1. Furthermore, replacement of the carboxylic acid with carboxylic acid isosteres such as tetrazole or triazole greatly improves potency. Compounds with a thio-tetrazole or thio-triazole side chain in position 2, where the thiophene was replaced by hydrophobic aryl moieties, were the most active compounds with up to 20-fold greater CT-L inhibition than compound 1 (compounds 15e, 15f, 15h, 15j, IC(50) values around 200 nM, and compound 29, IC(50) = 150 nM). The synthetic iterations described here not only led to improving potency in vitro but also resulted in the identification of compounds that are more active such as 39 (IC(50) = 0.44 to 1.01 µM) than 1 (IC(50) = 3.54 to 7.22 µM) at inhibiting the proteasome CT-L activity in intact breast cancer cells. Treatment with 39 also resulted in the accumulation of ubiquitinated cellular proteins and inhibition of tumor cell proliferation of breast cancer cells. The hit 1 and its analogue 39 inhibited proteasome CT-L activity irreversibly.

Imidazopiperidine amides as dipeptidyl peptidase IV inhibitors for the treatment of diabetes.

A series of substituted imidazopiperidine amides has been prepared and evaluated for inhibition of dipeptidyl peptidase IV (DPP-4). Substitution at the 1- and 3-positions produced increased selectivity for DPP-4 relative to DPP-8 and DPP-9. Compounds in this series had IC(50) values as low as 5.8 nM for inhibition of DPP-4.

Design, synthesis, and biological evaluation of triazolopiperazine-based beta-amino amides as potent, orally active dipeptidyl peptidase IV (DPP-4) inhibitors.

Various beta-amino amides containing triazolopiperazine heterocycles have been prepared and evaluated as potent, selective, orally active dipeptidyl peptidase IV (DPP-4) inhibitors. These compounds display excellent oral bioavailability and good overall pharmacokinetic profiles in preclinical species. Moreover, in vivo efficacy in an oral glucose tolerance test in lean mice is demonstrated.

Discovery of potent, selective, and orally bioavailable oxadiazole-based dipeptidyl peptidase IV inhibitors.

A novel series of oxadiazole based amides have been shown to be potent DPP-4 inhibitors. The optimized compound 43 exhibited excellent selectivity over a variety of DPP-4 homologs.

Discovery of potent, selective, and orally bioavailable pyridone-based dipeptidyl peptidase-4 inhibitors.

anti-Substituted beta-methylphenylalanine derived amides have been shown to be potent DPP-IV inhibitors exhibiting excellent selectivity over both DPP8 and DPP9. The optimized compound exhibited good pharmacokinetic profiles in three preclinical species.

Discovery of potent and selective phenylalanine based dipeptidyl peptidase IV inhibitors.

anti-Substituted beta-methylphenylalanine derived amides have been shown to be potent DPP-IV inhibitors exhibiting excellent selectivity over both DPP8 and DPP9. These are among the most potent compounds reported to date lacking an electrophilic trap. The most potent compound among these is 5-oxo-1,2,4-oxadiazole 44, which is a 3 nM DPP-IV inhibitor.

Dipeptidyl peptidase IV inhibitors derived from beta-aminoacylpiperidines bearing a fused thiazole, oxazole, isoxazole, or pyrazole.

A series of beta-aminoacylpiperidines bearing various fused five-membered heterocyclic rings was synthesized as dipeptidyl peptidase IV inhibitors. Potent and relatively selective inhibition could be obtained, depending on choice of heterocycle, regioisomerism, and substitution. In particular, one analog (74, DPP-IV IC50=26 nM) exhibited good oral bioavailability and acceptable half-life in the rat, albeit with rather high clearance.

(2R)-4-oxo-4-[3-(trifluoromethyl)-5,6-dihydro[1,2,4]triazolo[4,3-a]pyrazin-7(8H)-yl]-1-(2,4,5-trifluorophenyl)butan-2-amine: a potent, orally active dipeptidyl peptidase IV inhibitor for the treatment of type 2 diabetes.

A novel series of beta-amino amides incorporating fused heterocycles, i.e., triazolopiperazines, were synthesized and evaluated as inhibitors of dipeptidyl peptidase IV (DPP-IV) for the treatment of type 2 diabetes. (2R)-4-Oxo-4-[3-(trifluoromethyl)-5,6-dihydro[1,2,4]triazolo[4,3-a]pyrazin-7(8H)-yl]-1-(2,4,5-trifluorophenyl)butan-2-amine (1) is a potent, orally active DPP-IV inhibitor (IC(50) = 18 nM) with excellent selectivity over other proline-selective peptidases, oral bioavailability in preclinical species, and in vivo efficacy in animal models. MK-0431, the phosphate salt of compound 1, was selected for development as a potential new treatment for type 2 diabetes.

Potent and selective proline derived dipeptidyl peptidase IV inhibitors.

In-house screening of the Merck sample collection identified proline derived homophenylalanine 3 as a DPP-IV inhibitor with modest potency (DPP-IV IC50=1.9 microM). Optimization of 3 led to compound 37, which is among the most potent and selective DPP-IV inhibitors discovered to date.

Discovery of potent and selective beta-homophenylalanine based dipeptidyl peptidase IV inhibitors.

Modification of in-house screening lead beta-aminoacyl proline 8 gave an equipotent thiazolidide 9. Extensive SAR studies on the phenyl ring of 9 led to the discovery of a novel series of potent and selective DP-IV inhibitors. Introduction of a fluorine at the 2-position proved to be crucial for the potency of this series. The 2,5-difluoro (22q) and 2,4,5-trifluoro (22t) analogues were potent inhibitors of DP-IV (IC(50)=270, 119nM, respectively).