Xianrui Zhao

Discovery of Dual Leucine Zipper Kinase (DLK, MAP3K12) Inhibitors with Activity in Neurodegeneration Models

Dual leucine zipper kinase (DLK, MAP3K12) was recently identified as an essential regulator of neuronal degeneration in multiple contexts. Here we describe the generation of potent and selective DLK inhibitors starting from a high-throughput screening hit. Using proposed hinge-binding interactions to infer a binding mode and specific design parameters to optimize for CNS druglike molecules, we came to focus on the di(pyridin-2-yl)amines because of their combination of desirable potency and good brain penetration following oral dosing. Our lead inhibitor GNE-3511 (26) displayed concentration-dependent protection of neurons from degeneration in vitro and demonstrated dose-dependent activity in two different animal models of disease. These results suggest that specific pharmacological inhibition of DLK may have therapeutic potential in multiple indications.

Mechanistic and Structural Understanding of Uncompetitive Inhibitors of Caspase-6

Inhibition of caspase-6 is a potential therapeutic strategy for some neurodegenerative diseases, but it has been difficult to develop selective inhibitors against caspases. We report the discovery and characterization of a potent inhibitor of caspase-6 that acts by an uncompetitive binding mode that is an unprecedented mechanism of inhibition against this target class. Biochemical assays demonstrate that, while exquisitely selective for caspase-6 over caspase-3 and -7, the compound’s inhibitory activity is also dependent on the amino acid sequence and P1’ character of the peptide substrate. The crystal structure of the ternary complex of caspase-6, substrate-mimetic and an 11 nM inhibitor reveals the molecular basis of inhibition. The general strategy to develop uncompetitive inhibitors together with the unique mechanism described herein provides a rationale for engineering caspase selectivity.

Design of Selective PAK1 Inhibitor G-5555: Improving Properties by Employing an Unorthodox Low-pK a Polar Moiety.

Signaling pathways intersecting with the p21-activated kinases (PAKs) play important roles in tumorigenesis and cancer progression. By recognizing that the limitations of FRAX1036 (1) were chiefly associated with the highly basic amine it contained, we devised a mitigation strategy to address several issues such as hERG activity. The 5-amino-1,3-dioxanyl moiety was identified as an effective means of reducing pK a and logP simultaneously. When positioned properly within the scaffold, this group conferred several benefits including potency, pharmacokinetics, and selectivity. Mouse xenograft PK/PD studies were carried out using an advanced compound, G-5555 (12), derived from this approach. These studies concluded that dose-dependent pathway modulation was achievable and paves the way for further in vivo investigations of PAK1 function in cancer and other diseases.

Potent, selective, and orally bioavailable inhibitors of mammalian target of rapamycin (mTOR) kinase based on a quaternary substituted dihydrofuropyrimidine.

A series of inhibitors of mTOR kinase based on a quaternary-substituted dihydrofuropyrimidine was designed and synthesized. The most potent compounds in this series inhibited mTOR kinase with K(i) < 1.0 nM and were highly (>100×) selective for mTOR over the closely related PI3 kinases. Compounds in this series showed inhibition of the pathway and antiproliferative activity in cell-based assays. Furthermore, these compounds had excellent mouse PK, and showed a robust PK-PD relationship in a mouse model of cancer.

Discovery and Biological Profiling of Potent and Selective mTOR Inhibitor GDC-0349.

Aberrant activation of the PI3K-Akt-mTOR signaling pathway has been observed in human tumors and tumor cell lines, indicating that these protein kinases may be attractive therapeutic targets for treating cancer. Optimization of advanced lead 1 culminated in the discovery of clinical development candidate 8h, GDC-0349, a potent and selective ATP-competitive inhibitor of mTOR. GDC-0349 demonstrates pathway modulation and dose-dependent efficacy in mouse xenograft cancer models.

Tailoring small molecules for an allosteric site on procaspase-6.

Although they represent attractive therapeutic targets, caspases have so far proven recalcitrant to the development of drugs targeting the active site. Allosteric modulation of caspase activity is an alternate strategy that potentially avoids the need for anionic and electrophilic functionality present in most active‐site inhibitors. Caspase‐6 has been implicated in neurodegenerative disease, including Huntington’s and Alzheimer’s diseases. Herein we describe a fragment‐based lead discovery effort focused on caspase‐6 in its active and zymogen forms. Fragments were identified for procaspase‐6 using surface plasmon resonance methods and subsequently shown by X‐ray crystallography to bind a putative allosteric site at the dimer interface. A fragment‐merging strategy was employed to produce nanomolar‐affinity ligands that contact residues in the L2 loop at the dimer interface, significantly stabilizing procaspase‐6. Because rearrangement of the L2 loop is required for caspase‐6 activation, our results suggest a strategy for the allosteric control of caspase activation with drug‐like small molecules.

A whole cell assay to measure caspase-6 activity by detecting cleavage of lamin A/C.

Caspase-6 is a cysteinyl protease implicated in neurodegenerative conditions including Alzheimers and Huntingtons disease making it an attractive target for therapeutic intervention. A greater understanding of the role of caspase-6 in disease has been hampered by a lack of suitable cellular assays capable of specifically detecting caspase-6 activity in an intact cell environment. This is mainly due to the use of commercially available peptide substrates and inhibitors which lack the required specificity to facilitate development of this type of assay. We report here a 384-well whole-cell chemiluminescent ELISA assay that monitors the proteolytic degradation of endogenously expressed lamin A/C during the early stages of caspase-dependent apoptosis. The specificity of lamin A/C proteolysis by caspase-6 was demonstrated against recombinant caspase family members and further confirmed in genetic deletion studies. In the assay, plasma membrane integrity remained intact as assessed by release of lactate dehydrogenase from the intracellular environment and the exclusion of cell impermeable peptide inhibitors, despite the induction of an apoptotic state. The method described here is a robust tool to support drug discovery efforts targeting caspase-6 and is the first reported to specifically monitor endogenous caspase-6 activity in a cellular context.

Chemically Diverse Group I p21-Activated Kinase (PAK) Inhibitors Impart Acute Cardiovascular Toxicity with a Narrow Therapeutic Window

p21-activated kinase 1 (PAK1) has an important role in transducing signals in several oncogenic pathways. The concept of inhibiting this kinase has garnered significant interest over the past decade, particularly for targeting cancers associated with PAK1 amplification. Animal studies with the selective group I PAK (pan-PAK1, 2, 3) inhibitor G-5555 from the pyrido[2,3-d]pyrimidin-7-one class uncovered acute toxicity with a narrow therapeutic window. To attempt mitigating the toxicity, we introduced significant structural changes, culminating in the discovery of the potent pyridone side chain analogue G-9791. Mouse tolerability studies with this compound, other members of this series, and compounds from two structurally distinct classes revealed persistent toxicity and a correlation of minimum toxic concentrations and PAK1/2 mediated cellular potencies. Broad screening of selected PAK inhibitors revealed PAK1, 2, and 3 as the only overlapping targets. Our data suggest acute cardiovascular toxicity resulting from the inhibition of PAK2, which may be enhanced by PAK1 inhibition, and cautions against continued pursuit of pan-group I PAK inhibitors in drug discovery.