Synthesis and biological evaluation of arylated novobiocin analogs as Hsp90 inhibitors

Development of Hsp90 C-terminal inhibitors with noviomimetics that manifest anti-proliferative activities

Inhibition of the Hsp90 C-terminal domain offers a promising opportunity to treat numerous diseases/indications. Furthermore, the development of Hsp90 C-terminal inhibitors (CTIs) is advantageous over N-terminal inhibitors because it avoids the detriments associated with induction of the heat shock response (HSR). However, the lack of co-crystal structures of small molecules bound to the C-terminus have hindered their development. Therefore, structure-activity relationship (SAR) studies have been pursued to optimize such inhibitors. Noviose sugar surrogates, also known as noviomimetics have been prepared to investigate the size and nature of the C-terminal domain binding pocket. Herein, we report the synthesis and anti-proliferative activity manifested by this new series of Hsp90 C-terminal inhibitors.

A silicon-containing aryl/penta-1,4-dien-3-one/amine hybrid exhibits antiproliferative effects on breast cancer cells by targeting the HSP90 C-terminus without inducing heat-shock response

A pharmacophore-hybridized strategy based on previously reported HSP90 C-terminal inhibitors was utilized to prepare 32 aryl/penta-1,4-dien-3-one/amine hybrids. Among them, a silicon-containing compound 1z exhibited remarkable broad-spectrum antiproliferative effects on various human breast cancer cell lines. Through fluorescence polarization and AlphaScreen-based assays, we demonstrated that 1z specifically inhibited the HSP90 C-terminus without affecting HSP90 N-terminus. Furthermore, 1z effectively inhibited the HSP90 C-terminus without inducing heat-shock response (HSR), leading to the degradation of its client proteins HER2, pAKT, AKT, and CDK4, causing G1 arrest of MCF-7 and SKBr3 cells, and ultimately contributing to apoptosis of these cells through caspase-3, caspase-8, and caspase-9 activation. Additionally, the penta-1,4-dien-3-one linker in the hybrid, a large bulky lipophilic substitution in the aryl fragment at the 3'-site, and the presence of N-methylpiperazine as the amine fragment were identified as crucial factors that significantly contributed to the observed antiproliferative activity through structure-activity relationship (SAR) analysis. Lastly, we found that 1z exhibited superior thermostability compared to vibsanin B derivatives and good in vitro metabolic stability in simulated intestinal fluid, representing one of the few reported silicon-containing HSP90 C-terminal inhibitors.

Diaryl Ether: A Privileged Scaffold for Drug and Agrochemical Discovery

Diaryl ether (DE) is a functional scaffold existing widely both in natural products (NPs) and synthetic organic compounds. Statistically, DE is the second most popular and enduring scaffold within the numerous medicinal chemistry and agrochemical reports. Given its unique physicochemical properties and potential biological activities, DE nucleus is recognized as a fundamental element of medicinal and agrochemical agents aimed at different biological targets. Its drug-like derivatives have been extensively synthesized with interesting biological features including anticancer, anti-inflammatory, antiviral, antibacterial, antimalarial, herbicidal, fungicidal, insecticidal, and so on. In this review, we highlight the medicinal and agrochemical versatility of the DE motif according to the published information in the past decade and comprehensively give a summary of the target recognition, structure-activity relationship (SAR), and mechanism of action of its analogues. It is expected that this profile may provide valuable guidance for the discovery of new active ingredients both in drug and pesticide research.

Design, Synthesis, and Biological Activities of Vibsanin B Derivatives: A New Class of HSP90 C-Terminal Inhibitors

Previously, vibsanin B (ViB) was found to preferentially target HSP90β compared to HSP90α. In this study, multiple experiments, including pull-down assays of biotin-ViB with recombinant HSP90β-NTD, MD, CTD, and full-length HSP90β, molecular docking of ViB and its derivatives to the HSP90 CTD, and a inhibition assay of interaction of the HSP90β CTD with GST-tagged cyclophilin 40 (Cyp40) by ViB derivatives, suggest that ViB can directly bind to the HSP90 C-terminus. On the basis of the docking predictions and primary structure-activity relationships (SARs), a series of ViB analogues devised with focus on the C18 position, along with compounds derivatized at the C4, C7, and C8 positions, were designed and chemically synthesized. Compound 12f (IC₅₀ = 1.12 μM against SK-BR-3) exhibits great potency with drug-like properties. Overall, our findings demonstrate that compounds with the vibsanin B scaffold are a new class of HSP90 C-terminal inhibitors with considerable potential as anticancer agents.

Novobiocin Analogues That Inhibit the MAPK Pathway

Heat shock protein 90 (Hsp90) inhibition by modulation of its N- or C-terminal binding site has become an attractive strategy for the development of anticancer chemotherapeutics. The first Hsp90 C-terminus inhibitor, novobiocin, manifested a relatively high IC50 value of ∼700 μM. Therefore, investigation of the novobiocin scaffold has led to analogues with improved antiproliferative activity (nanomolar concentrations) against several cancer cell lines. During these studies, novobiocin analogues that do not inhibit Hsp90 were identified; however, these analogues demonstrated potent antiproliferative activity. Compound 2, a novobiocin analogue, was identified as a MAPK pathway signaling disruptor that lacked Hsp90 inhibitory activity. In addition, structural modifications of compound 2 were identified that segregated Hsp90 inhibition from MAPK signaling disruption. These studies indicate that compound 2 represents a novel scaffold for disruption of MAPK pathway signaling and may serve as a useful structure for the generation of new anticancer agents.

Alternative approaches to Hsp90 modulation for the treatment of cancer

Hsp90 is responsible for the conformational maturation of newly synthesized polypeptides (client proteins) and the re-maturation of denatured proteins via the Hsp90 chaperone cycle. Inhibition of the Hsp90 N-terminus has emerged as a clinically relevant strategy for anticancer chemotherapeutics due to the involvement of clients in a variety of oncogenic pathways. Several immunophilins, co-chaperones and partner proteins are also necessary for Hsp90 chaperoning activity. Alternative strategies to inhibit Hsp90 function include disruption of the C-terminal dimerization domain and the Hsp90 heteroprotein complex. C-terminal inhibitors and Hsp90 co-chaperone disruptors prevent cancer cell proliferation similar to N-terminal inhibitors and destabilize client proteins without induction of heat shock proteins. Herein, current Hsp90 inhibitors, the chaperone cycle, and regulation of this cycle will be discussed.

Synthesis and biological evaluation of coumarin replacements of novobiocin as Hsp90 inhibitors

Since Hsp90 modulates all six hallmarks of cancer simultaneously, it has become an attractive target for the development of cancer chemotherapeutics. In an effort to develop more efficacious compounds for Hsp90 inhibition, novobiocin analogues were prepared by replacing the central coumarin core with naphthalene, quinolinone, and quinoline surrogates. These modifications allowed for modification of the 2-position, which was previously unexplored. Biological evaluation of these compounds suggests a hydrophobic pocket about the 2-position of novobiocin. Anti-proliferative activities of these analogues against multiple cancer cell lines identified 2-alkoxyquinoline derivatives to exhibit improved activity.

Development of a high-throughput screening cancer cell-based luciferase refolding assay for identifying Hsp90 inhibitors

The 90 kDa heat-shock protein (Hsp90) and other cochaperones allow for proper folding of nascent or misfolded polypeptides. Cancer cells exploit these chaperones by maintaining the stability of mutated and misfolded oncoproteins and allowing them to evade proteosomal degradation. Inhibiting Hsp90 is an attractive strategy for cancer therapy, as the concomitant degradation of multiple oncoproteins may lead to effective anti-neoplastic agents. Unfortunately, early clinical trials have been disappointing with N-terminal Hsp90 inhibitors, as it is unclear whether the problems that plague current Hsp90 inhibitors in clinical trials are related to on-target or off-target activity. One approach to overcome these pitfalls is to identify structurally diverse scaffolds that improve Hsp90 inhibitory activity in the cancer cell milieu. Utilizing a panel of cancer cell lines that express luciferase, we have designed an in-cell Hsp90-dependent luciferase refolding assay. The assay was optimized using previously identified Hsp90 inhibitors and experimental novobiocin analogues against prostate, colon, and lung cancer cell lines. This assay exhibits good interplate precision (% CV), a signal-to-noise ratio (S/N) of ≥7, and an approximate Z-factor ranging from 0.5 to 0.7. Novobiocin analogues that revealed activity in this assay were examined via western blot experiments for client protein degradation, a hallmark of Hsp90 inhibition. Subsequently, a pilot screen was conducted using the Prestwick library, and two compounds, biperiden and ethoxyquin, revealed significant activity. Here, we report the development of an in-cell Hsp90-dependent luciferase refolding assay that is amenable across cancer cell lines for the screening of inhibitors in their specific milieu.

Synthesis and biological evaluation of novobiocin analogues as potential heat shock protein 90 inhibitors

Recent studies have shown that novobiocin (NB), a member of the coumermycin (CA) family of antibiotics with demonstrated DNA gyrase inhibitory activity, inhibits Heat shock protein 90 (HSP90) by binding weakly to a putative ATP-binding site within its C-terminus. To develop more potent HSP90 inhibitors that target this site and to define structure-activity relationships (SARs) for this class of compounds, we have synthesized twenty seven 3-amido-7-noviosylcoumarin analogues starting from NB and CA. These were evaluated for evidence of HSP90 inhibition using several biological assays including inhibition of cell proliferation and cell cycle arrest, induction of the heat shock response, inhibition of luciferase-refolding in vitro, and depletion of the HSP90 client protein c-erbB-2/HER-2/neu (HER2). This SAR study revealed that a substantial increase in biological activity can be achieved by introduction of an indole-2-carboxamide group in place of 4-hydroxy-isopentylbenzamido group at C-3 of NB in addition to removal/derivatization of the 4-hydroxyl group from the coumarin ring. Methylation of the 4-hydroxyl group in the coumarin moiety moderately increased biological activity as shown by compounds 11 and 13. Our most potent new analogue 19 demonstrated biological activities consistent with known HSP90-binding agents, but with greater potency than NB.

A novel function of novobiocin: disrupting the interaction of HIF 1α and p300/CBP through direct binding to the HIF1α C-terminal activation domain

Hypoxia-inducible factor 1α (HIF1α) is an important cellular survival protein under hypoxic conditions, regulating the cellular response to low oxygen tension via recruitment of a transcriptional co-activator, p300/CBP. p300/CBP induces expression of multiple genes involved in cell survival, proliferation, angiogenesis, and tumor development. Thus, a strategy to inhibit hypoxic responses in tumors may be to target the protein-protein interaction between HIF1α and p300/CBP. Here, we document, for the first time, that the aminocoumarin antibiotic, novobiocin, directly blocks the protein-protein interaction between the HIF1α C-terminal activation domain (CTAD) and the cysteine-histidine rich (CH1) region of p300/CBP. Also, novobiocin down-regulated HIF1α-controlled gene expression, specifically CA9, which is related to tumorigenesis. In a monolayer cell culture, novobiocin inhibited cell proliferation and colony formation in the MCF-7 human breast adenocarcinoma cell line and the A549 human lung cancer cell line. Rescue experiments revealed that the recombinant CTAD fragment of HIF1α partially reversed novobiocin’s inhibitory effects on cell proliferation and colony formation in MCF-7 cells. These findings suggest a novel mechanism of action for novobiocin which has the potential for innovative therapeutic use in tumor treatment.

KU-32, a novel drug for diabetic neuropathy, is safe for human islets and improves in vitro insulin secretion and viability

KU-32 is a novel, novobiocin-based Hsp90 inhibitor that protects against neuronal glucotoxicity and reverses multiple clinical indices of diabetic peripheral neuropathy in a rodent model. However, any drug with potential for treating diabetic complications must also have no adverse effects on the function of pancreatic islets. Thus, the goal of the current study was to assess the effect of KU-32 on the in vitro viability and function of human islets. Treating human islets with KU-32 for 24 hours showed no toxicity as assessed using the alamarBlue assay. Confocal microscopy confirmed that with a minimum of 2-day exposure, KU-32 improved cellular viability by blocking apoptosis. Functionally, isolated human islets released more glucose-stimulated insulin when preincubated in KU-32. However, diabetic BKS-db/db mice, a model for type 2 diabetes, administered KU-32 for 10 weeks did not show any significant changes in blood glucose and insulin levels, despite having greater insulin staining/beta cell in the pancreas compared to untreated BKS db/db mice. In summary, KU-32 did not harm isolated human islets and may even be protective. However, the effect does not appear significant enough to alter the in vivo metabolic parameters of diabetic mice.