A New Generation of Hsp90 Inhibitors: Addressing Isoform Selectivity and Heat Shock Induction

The 90 kDa heat shock proteins (Hsp90) are molecular chaperones that are upregulated in response to cellular stress and are responsible for the conformational maturation, activation and/or stability of more than 200 client proteins. Many of these clients are oncogenic and facilitate the progression of cancer. Disruption of Hsp90's inherent ATPase activity renders the chaperone inactive, leading to degradation of substrates and ultimately, apoptosis. Consequently, Hsp90 has become a highly sought after anti-cancer target and numerous pharmaceutical companies and academic labs are expending efforts to develop novel methods to regulate the Hsp90-mediated protein folding process. Included within the Hsp90 family are four isoforms, each of which exhibits a unique cellular localization, expression, function and clientele. Hsp90α (inducible) and Hsp90β (constitutive) both localize to the cytoplasm and share similar functions; however, recent studies have identified isoform specific substrates. Tumor necrosis factor receptor-associated protein (TRAP-1) is the Hsp90 isoform localized to the mitchondria and to date, no specific clients or selective inhibitors have been identified. The fourth isoform is glucose-regulated protein 94 kDa (Grp94), which is localized to the endoplasmic reticulum and is responsible for the maturation and stability of specific secreted and membrane bound proteins. Currently identified Hsp90 inhibitors exhibit pan-inhibition, resulting in the disruption of all four isoforms' ability to bind and hydrolyze ATP. This activity is believed responsible for the undesired toxicities related to Hsp90 inhibition in the clinic, as proteins that are critical to cardio function and the central nervous system are dependent upon yet to be determined Hsp90 isoforms. Another detriment arising from N-terminal Hsp90 inhibition is induction of the pro-survival, heat shock response. Specifically, induction of the target, Hsp90, has resulted in therapeutic resistance and complications with dosing and administration protocols. Presented herein is rationale for the development of Hsp90 isoform selective inhibitors and the first irreversible inhibitor of Hsp90 that mitigates induction of Hsp90; thus, providing key advancements towards addressing the detriments associated with Hsp90 inhibitors currently under clinical investigation

Hydrating for Resistance to Radicicol

Resistance to Hsp90 inhibition has become an important concern as several clinical trials are currently in progress for the treatment of cancer. A summary of known mechanisms of resistance to Hsp90 inhibitors is provided, including the recent solution of the Humicola fuscoatra Hsp90 structure, the organism responsible for the biosynthesis of radicicol. Through careful analyses of Hsp90 structures, a plausible mechanism for resistance to Hsp90 inhibitors has been obtained by single mutations about the N-terminal ATP-binding site

The cGAS-STING Pathway in Diabetic Retinopathy and Age-Related Macular Degeneration

Diabetic retinopathy and age-related macular degeneration are common retinal diseases with shared pathophysiology, including oxidative stress-induced inflammation. Cellular mechanisms responsible for converting oxidative stress into retinal damage are ill-defined but have begun to clarify. One common outcome of retinal oxidative stress is mitochondrial damage and subsequent release of mitochondrial DNA into the cytosol. This leads to activation of the cGAS-STING pathway, resulting in interferon release and disease-amplifying inflammation. This review summarizes the evolving link between aberrant cGAS-STING signaling and inflammation in common retinal diseases and provides prospective for targeting this system in diabetic retinopathy and age-related macular degeneration. Further defining the roles of this system in the retina is expected to reveal new disease pathology and novel therapeutic approaches

Synthesis and biological evaluation of arylated novobiocin analogs as Hsp90 inhibitors

Novobiocin analogs lacking labile glycosidic ether have been designed, synthesized and evaluated for Hsp90 inhibitory activity. Replacement of the synthetically complex noviose sugar with simple aromatic side chains produced analogs that maintain moderate cytotoxic activity against MCF7 and SkBR3 breast cancer cell-lines. Rationale for the preparation of des-noviose novobiocin analogs in addition to their synthesis and biological evaluation are presented herein

Design, synthesis, and biological activity of bicyclic radester analogues as Hsp90 inhibitors

Bicyclic radester analogues have been synthesized and evaluated for Hsp90 inhibitory activity. These analogues induce concentration-dependent degradation of Hsp90-dependent client proteins with the six-membered bicyclic analogues manifesting increased activity versus the five-membered counterparts

Grp94 Protein Delivers γ-Aminobutyric Acid Type A (GABAA) Receptors to Hrd1 Protein-mediated Endoplasmic Reticulum-associated Degradation

This research was originally published in the Journal of Biological Chemistry. Xiao-Jing Di, Ya-Juan Wang, Dong-Yun Han, Yan-Lin Fu, Adam S. Duerfeldt, Brian S. J. Blagg and Ting-Wei Mu.Grp94 Protein Delivers γ-Aminobutyric Acid Type A (GABAA) Receptors to Hrd1 Protein-mediated Endoplasmic Reticulum-associated Degradation. Journal of Biological Chemistry. 2016; 291, 9526-9539.Proteostasis maintenance of γ-aminobutyric acid type A (GABAA) receptors dictates their function in controlling neuronal inhibition in mammalian central nervous systems. However, as a multisubunit, multispan, integral membrane protein, even wild type subunits of GABAA receptors fold and assemble inefficiently in the endoplasmic reticulum (ER). Unassembled and misfolded subunits undergo ER-associated degradation (ERAD), but this degradation process remains poorly understood for GABAA receptors. Here, using the α1 subunits of GABAA receptors as a model substrate, we demonstrated that Grp94, a metazoan-specific Hsp90 in the ER lumen, uses its middle domain to interact with the α1 subunits and positively regulates their ERAD. OS-9, an ER-resident lectin, acts downstream of Grp94 to further recognize misfolded α1 subunits in a glycan-dependent manner. This delivers misfolded α1 subunits to the Hrd1-mediated ubiquitination and the valosin-containing protein-mediated extraction pathway. Repressing the initial ERAD recognition step by inhibiting Grp94 enhances the functional surface expression of misfolding-prone α1(A322D) subunits, which causes autosomal dominant juvenile myoclonic epilepsy. This study clarifies a Grp94-mediated ERAD pathway for GABAA receptors, which provides a novel way to finely tune their function in physiological and pathophysiological conditions

Development of a Grp94 inhibitor

This document is the Accepted Manuscript version of a Published Work that appeared in final form in the Journal of the American Chemical Society, copyright © American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see http://doi.org/10.1021/ja303477g.Heat shock protein 90 (Hsp90) represents a promising therapeutic target for the treatment of cancer and other diseases. Unfortunately, results from clinical trials have been disappointing as off-target effects and toxicities have been observed. These detriments may be a consequence of pan-Hsp90 inhibition, as all clinically evaluated Hsp90 inhibitors simultaneously disrupt all four human Hsp90 isoforms. Using a structure-based approach, we designed an inhibitor of Grp94, the ER-resident Hsp90. The effect manifested by compound 2 on several Grp94 and Hsp90α/β (cytosolic isoforms) clients were investigated. Compound 2 prevented intracellular trafficking of the Toll receptor, inhibited the secretion of IGF-II, affected the conformation of Grp94, and suppressed Drosophila larval growth, all Grp94-dependent processes. In contrast, compound 2 had no effect on cell viability or cytosolic Hsp90α/β client proteins at similar concentrations. The design, synthesis, and evaluation of 2 are described herein

First-generation structure-activity relationship studies of 2,3,4,9-tetrahydro-1H-carbazol-1-amines as CpxA phosphatase inhibitors

Genetic activation of the bacterial two-component signal transduction system, CpxRA, abolishes the virulence of a number of pathogens in human and murine infection models. Recently, 2,3,4,9-tetrahydro-1H-carbazol-1-amines were shown to activate the CpxRA system by inhibiting the phosphatase activity of CpxA. Herein we report the initial structure-activity relationships of this scaffold by focusing on three approaches 1) A-ring substitution, 2) B-ring deconstruction to provide N-arylated amino acid derivatives, and 3) C-ring elimination to give 2-ethylamino substituted indoles. These studies demonstrate that the A-ring is amenable to functionalization and provides a promising avenue for continued optimization of this chemotype. Further investigations revealed that the C-ring is not necessary for activity, although it likely provides conformational constraint that is beneficial to potency, and that the (R) stereochemistry is required at the primary amine. Simplification of the scaffold through deconstruction of the B-ring led to inactive compounds, highlighting the importance of the indole core. A new lead compound 26 was identified, which manifests a ∼30-fold improvement in CpxA phosphatase inhibition over the initial hit. Comparison of amino and des-amino derivatives in bacterial strains differing in membrane permeability and efflux capabilities demonstrate that the amine is required not only for target engagement but also for permeation and accumulation in Escherichia coli

Development of Glucose Regularted Protein 94-Selective Inhibitors Based on the Bnlm and Radamide Scaffold

Glucose regulated protein 94 (Grp94) is the endoplasmic reticulum resident of the heat shock protein 90 kDa (Hsp90) family of molecular chaperones. Grp94 associates with many proteins involved in cell adhesion and signaling, including integrins, Toll-like receptors, immunoglobulins, and mutant myocilin. Grp94 has been implicated as a target for several therapeutic areas including glaucoma, cancer metastasis, and multiple myeloma. While 85% identical to other Hsp90 isoforms, the N-terminal ATP-binding site of Grp94 possesses a unique hydrophobic pocket that was used to design isoform-selective inhibitors. Incorporation of a cis-amide bioisostere into the radamide scaffold led to development of the original Grp94-selective inhibitor, BnIm. Structure–activity relationship studies have now been performed on the aryl side chain of BnIm, which resulted in improved analogues that exhibit better potency and selectivity for Grp94. These analogues also manifest superior antimigratory activity in a metastasis model as well as enhanced mutant myocilin degradation in a glaucoma model compared to BnIm

Total Syntheses of (−)-Pyrimidoblamic Acid and P‑3A

Total syntheses of (<b>−</b>)-pyrimidoblamic acid and P-3A are disclosed. Central to the convergent approach is a powerful inverse electron demand Diels–Alder reaction between substituted electron-deficient 1,2,3-triazines and a highly functionalized and chiral primary amidine, which forms the pyrimidine cores and introduces all necessary stereochemistry in a single step. Intrinsic in the convergent approach is the potential it provides for the late stage divergent synthesis of modified analogs bearing deep-seated changes in either the pyrimidine cores or the highly functionalized C2 side chain common to both natural products. The examination of the key cycloaddition reaction revealed that the inherent 1,2,3-triazine mode of cycloaddition (C4/N1 vs C5/N2) as well as the amidine regioselectivity were unaffected by introduction of two electron-withdrawing groups (−CO₂R) at C4 and C6 of the 1,2,3-triazine even if C5 is unsubstituted (Me or H), highlighting the synthetic potential of the powerful pyrimidine synthesis