The
Mediator complex-associated cyclin-dependent kinase CDK8 has
been implicated in human disease, particularly in colorectal cancer
where it has been reported as a putative oncogene. Here we report
the discovery of 109 (CCT251921), a potent, selective,
and orally bioavailable inhibitor of CDK8 with equipotent affinity
for CDK19. We describe a structure-based design approach leading to
the discovery of a 3,4,5-trisubstituted-2-aminopyridine series and
present the application of physicochemical property analyses to successfully
reduce in vivo metabolic clearance, minimize transporter-mediated
biliary elimination while maintaining acceptable aqueous solubility.
Compound 109 affords the optimal compromise of in vitro
biochemical, pharmacokinetic, and physicochemical properties and is
suitable for progression to animal models of cancer.
WNT
signaling is frequently deregulated in malignancy, particularly
in colon cancer, and plays a key role in the generation and maintenance
of cancer stem cells. We report the discovery and optimization of
a 3,4,5-trisubstituted pyridine 9 using a high-throughput
cell-based reporter assay of WNT pathway activity. We demonstrate
a twisted conformation about the pyridine–piperidine bond of 9 by small-molecule X-ray crystallography. Medicinal chemistry
optimization to maintain this twisted conformation, cognisant of physicochemical
properties likely to maintain good cell permeability, led to 74 (CCT251545), a potent small-molecule inhibitor of WNT signaling
with good oral pharmacokinetics. We demonstrate inhibition of WNT
pathway activity in a solid human tumor xenograft model with evidence
for tumor growth inhibition following oral dosing. This work provides
a successful example of hypothesis-driven medicinal chemistry optimization
from a singleton hit against a cell-based pathway assay without knowledge
of the biochemical target.
Optimization of the imidazo[4,5-b]pyridine-based
series of Aurora kinase inhibitors led to the identification of 6-chloro-7-(4-(4-chlorobenzyl)piperazin-1-yl)-2-(1,3-dimethyl-1H-pyrazol-4-yl)-3H-imidazo[4,5-b]pyridine (27e), a potent inhibitor of Aurora
kinases (Aurora-A Kd = 7.5 nM, Aurora-B Kd = 48 nM), FLT3 kinase (Kd = 6.2 nM), and FLT3 mutants including FLT3-ITD (Kd = 38 nM) and FLT3(D835Y) (Kd = 14 nM). FLT3-ITD causes constitutive FLT3 kinase
activation and is detected in 20–35% of adults and 15% of children
with acute myeloid leukemia (AML), conferring a poor prognosis in
both age groups. In an in vivo setting, 27e strongly
inhibited the growth of a FLT3-ITD-positive AML human
tumor xenograft (MV4–11) following oral administration, with
in vivo biomarker modulation and plasma free drug exposures consistent
with dual FLT3 and Aurora kinase inhibition. Compound 27e, an orally bioavailable dual FLT3 and Aurora kinase inhibitor, was
selected as a preclinical development candidate for the treatment
of human malignancies, in particular AML, in adults and children.
Cellular stress in early mitosis activates the antephase checkpoint, resulting in the decondensation of chromosomes and delayed mitotic progression. Checkpoint with forkheadassociated and RING domains (CHFR) is central to this checkpoint, and its activity is ablated in many tumors and cancer cell lines through promoter hypermethylation or mutation. The interaction between the PAR-binding zinc finger (PBZ) of CHFR and poly(ADP-ribose) (PAR) is crucial for a functional antephase checkpoint. We determined the crystal structure of the cysteine-rich region of human CHFR (amino acids 425-664) to 1.9 Å resolution, which revealed a multizinc binding domain of elaborate topology within which the PBZ is embedded. The PBZ of CHFR closely resembles the analogous motifs from aprataxin-like factor and CG1218-PA, which lie within unstructured regions of their respective proteins. Based on co-crystal structures of CHFR bound to several different PARlike ligands (adenosine 5-diphosphoribose, adenosine monophosphate, and P 1 P 2 -diadenosine 5-pyrophosphate), we made a model of the CHFR-PAR interaction, which we validated using site-specific mutagenesis and surface plasmon resonance. The PBZ motif of CHFR recognizes two adenine-containing subunits of PAR and the phosphate backbone that connects them. More generally, PBZ motifs may recognize different numbers of PAR subunits as required to carry out their functions.
Aurora-A differs from Aurora-B/C
at three positions in the ATP-binding
pocket (L215, T217, and R220). Exploiting these differences, crystal
structures of ligand–Aurora protein interactions formed the
basis of a design principle for imidazo[4,5-b]pyridine-derived
Aurora-A-selective inhibitors. Guided by a computational modeling
approach, appropriate C7-imidazo[4,5-b]pyridine derivatization
led to the discovery of highly selective inhibitors, such as compound 28c, of Aurora-A over Aurora-B. In HCT116 human colon carcinoma
cells, 28c and 40f inhibited the Aurora-A
L215R and R220K mutants with IC50 values similar to those
seen for the Aurora-A wild type. However, the Aurora-A T217E mutant
was significantly less sensitive to inhibition by 28c and 40f compared to the Aurora-A wild type, suggesting
that the T217 residue plays a critical role in governing the observed
isoform selectivity for Aurora-A inhibition. These compounds are useful
small-molecule chemical tools to further explore the function of Aurora-A
in cells.
The AB (C 1 -C 13 ) and CD (C 17 -C 29 ) spiroketal of spongistatin 1 were prepared diastereoselectively from syn-and anti-3,5-dihydroxy-6-heptenoate derived from 2-deoxy-D-ribose.
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