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

Novobiocin analogues with second-generation noviose surrogates

Hsp90 is a promising therapeutic target for the treatment of cancer. Novobiocin is the first Hsp90 C-terminal inhibitor ever identified and recent structure-activity relationship studies on the noviose sugar identified several commercially available amines as suitable surrogates. In an effort to further understand this region of the molecule, analogues containing various N′-amino substituents were prepared and evaluated against two breast cancer cell lines for determination of their efficacy. Compound 37j manifested the most potent anti-proliferative activity from these studies and induced Hsp90-dependent client protein degradation at mid nano-molar concentrations

Synthesis and Evaluation of Radamide Analogues, A Chimera of Radicicol and Geldanamycin

Previously, we reported the Hsp90 inhibitory activity of radamide, an open chain amide chimera of geldanamycin and radicicol. Attempts to further expand upon structure–activity relationships for this class of Hsp90 inhibitors led to the preparation of a series of radamide analogues focused on differing tether lengths and quinone mimics. In addition, the cup-shaped conformation adopted by the two natural products when bound to the Hsp90 N-terminal ATP binding pocket suggests that conformationally biased compounds may demonstrate improved binding and inhibition. The preparation and evaluation of radamide analogues with cis/trans α,β-unsaturated amides yielded compounds that exhibit improved antiproliferative activity. In addition, several analogues demonstrated the ability to induce degradation of Hsp90-dependent oncogenic signaling proteins in vitro, a hallmark of Hsp90 N-terminal inhibition

Click Chemistry to Probe Hsp90: Synthesis and Evaluation of a Series of Triazole Containing Novobiocin Analogues

A series of triazole-containing novobiocin analogues has been designed, synthesized and their inhibitory activity determined. These compounds contain a triazole ring in lieu of the amide moiety present in the natural product. The anti-proliferative effects of these compounds were evaluated against two breast cancer cell lines (SKBr-3 and MCF-7), and manifested activities similar to their amide-containing counterparts. In addition, Hsp90-dependent client protein degradation was observed via western blot analysis, further supporting a common mode of Hsp90 inhibition for both structural classes

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

Synthesis and Structure activity relationships of EGCG Analogues, A Recently Identified Hsp90 Inhibitor

Epigallocatechin-3-gallate (EGCG), the principal polyphenol isolated from green tea, was recently shown to inhibit Hsp90, however structure-activity relationships for this natural product have not yet been produced. Herein, we report the synthesis and biological evaluation of EGCG analogues to establish structure-activity relationships between EGCG and Hsp90. All four rings as well as the linker connecting the C- and the D-rings were systematically investigated, which led to the discovery of compounds that inhibit Hs90 and display improvement in efficacy over EGCG. Anti-proliferative activity of all the analogues was determined against MCF-7 and SKBr3 cell lines and Hsp90 inhibitory activity of four most potent analogues was further evaluated by western blot analyses and degradation of Hsp90-dependent client proteins. Prenyl substituted aryl ester of 3,5-dihydroxychroman-3-ol ring system was identified as novel scaffold that exhibit Hsp90 inhibitory activity

Heat shock response and insulin-associated neurodegeneration

Dysfunctional insulin and insulin-like growth factor-I (IGF-I) signaling contributes to the pathological progression of diabetes, diabetic peripheral neuropathy (DPN), Alzheimer's (AD), Parkinson's (PD) and Huntington's diseases (HD). Despite their prevalence, there are limited therapeutic options available for the treatment of these neurodegenerative disorders. Therefore, establishing a link between insulin/IGF-I and the pathoetiology of these diseases may provide alternative approaches toward their management. Many of the heat shock proteins (Hsps) are well-known molecular chaperones that solubilize and clear damaged proteins and protein aggregates. Recent studies suggest that modulating Hsps may represent a promising therapeutic avenue for improving insulin and IGF-I signaling. Pharmacological induction of the heat shock response (HSR) may intersect with insulin/IGF-I signaling to improve aspects of neurodegenerative phenotypes. Herein, we review the intersection between Hsps and the insulin/IGF systems under normal and pathological conditions. The discussion will emphasize the potential of non-toxic HSR inducers as viable therapeutic agents

Exploiting conformational dynamics in drug discovery: design of C-terminal inhibitors of Hsp90 with improved activities

The interaction that occurs between molecules is a dynamic process that impacts both structural and conformational properties of the ligand and the ligand binding site. Herein, we investigate the dynamic cross-talk between a protein and the ligand as a source for new opportunities in ligand design. Analysis of the formation/disappearance of protein pockets produced in response to a first-generation inhibitor assisted in the identification of functional groups that could be introduced onto scaffolds to facilitate optimal binding, which allowed for increased binding with previously uncharacterized regions. MD simulations were used to elucidate primary changes that occur in the Hsp90 C-terminal binding pocket in the presence of first-generation ligands. This data was then used to design ligands that adapt to these receptor conformations, which provides access to an energy landscape that is not visible in a static model. The newly synthesized compounds demonstrated anti-proliferative activity at ~150 nanomolar concentration. The method identified herein may be used to design chemical probes that provide additional information on structural variations of Hsp90 C-terminal binding site

Novologues Containing a Benzamide Side Chain Manifest Anti-proliferative Activity Against Two Breast Cancer Cell Lines

The heat shock protein 90 (Hsp90) folding machinery is essential for the maturation of nascent polypeptides into their biologically active three-dimensional-structures and for the rematuration/clearance of misfolded proteins that form under cellular stress.1–3 As a prosurvival chaperone, Hsp90 overexpression is commonly observed in transformed cells, which is required to sustain the hostile tumor micro-environment associated with nutrient deprivation and hypoxia. Pharmacological inhibition of Hsp90 has been shown to induce the degradation of oncogenic proteins associated with all six hallmarks of cancer that rely upon Hsp90.4–8 Consequently, Hsp90 represents a highly sought after target for the treatment of cancer. In fact, 17 small molecules that bind competitively to the N-terminal ATP-binding pocket are under clinical evaluation against various cancers.9,10 However, heat shock factor 1 (HSF-1), the master regulator of the pro-survival heat shock response also binds Hsp90. Ultimately, Hsp90 N-terminal inhibition results in HSF-1 release, and upon phosphorylation, trimerizes and translocates to the nucleus wherein it binds the heat shock elements to activate the pro-survival, heat shock response (HSR). The HSR serves to expand the cellular buffering capacity and to assist in the maturation of mutated and oncogenic substrates.11 This concomitant heat shock response is detrimental to the treatment of cancer and may lead to drug resistance and tumor metastasis.12 Recent studies have demonstrated that allosteric modulation of the Hsp90 C-terminus can separate the pro-survival heat shock response from pro-apoptotic, client protein degradation.13–20 Two classes of small molecules derived from novobiocin 1, (Figure 1) the first identified Hsp90 C-terminal inhibitor, were discovered via the structure-activity relationship studies. KU-32 (2), which lacks a 4-hydroxyl, the 3’-carbamate, and contains an acetamide in lieu of a prenylated benzamide, represents a lead compound that induces the heat shock response at concentrations much lower than that needed for client protein degradation.2,21 Consequently, this class of analogues has been evaluated as neuroprotective agents to refold protein aggregates.22–24 In contrast, KU-174 (3) contains a biarylamide side chain in lieu of the acetamide, and induces Hsp90 client protein degradation without induction of the heat shock response.25–26 Therefore, this class of novobiocin analogues manifests optimal properties for the treatment of cancer, as no HSR is observed with such compounds