The renaissance of alpha-methylene-gamma-butyrolactones: new synthetic approaches

The amount of research activity concerning alpha-methylene-gamma-butyrolactones and alpha-alkylidene-gamma-butyrolactones has increased dramatically in recent years. This Review summarizes the structural types, biological activities, and biosynthesis of these compounds, concentrating on publications from the past 10 years. Traditional approaches to alpha-methylene-gamma-butyrolactones and alpha-alkylidene-gamma-butyrolactones are then reviewed together with novel approaches, including those from our own research group, reported more recently.

Learning from nature: advances in geldanamycin- and radicicol-based inhibitors of Hsp90

Natural products have been fundamental in the development of new therapeutic agents predicated on the inhibition of heat shock protein 90 (Hsp90). This Perspective describes the influential role of the benzoquinone ansamycin geldanamycin and the resorcylic acid macrolactone radicicol not only in driving forward drug discovery programs but also in inspiring organic chemists to develop innovative methodology for the synthesis of natural products and analogues with improved properties.

A Novel Hsp90 Inhibitor Activates Compensatory Heat Shock Protein Responses and Autophagy and Alleviates Mutant A53T α-Synuclein Toxicity

A potential cause of neurodegenerative diseases, including Parkinson's disease (PD), is protein misfolding and aggregation that in turn leads to neurotoxicity. Targeting Hsp90 is an attractive strategy to halt neurodegenerative diseases, and benzoquinone ansamycin (BQA) Hsp90 inhibitors such as geldanamycin (GA) and 17-(allylamino)-17-demethoxygeldanamycin have been shown to be beneficial in mutant A53T α-synuclein PD models. However, current BQA inhibitors result in off-target toxicities via redox cycling and/or arylation of nucleophiles at the C19 position. We developed novel 19-substituted BQA (19BQA) as a means to prevent arylation. In this study, our data demonstrated that 19-phenyl-GA, a lead 19BQA in the GA series, was redox stable and exhibited little toxicity relative to its parent quinone GA in human dopaminergic SH-SY5Y cells as examined by oxygen consumption, trypan blue, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-tetrazolium bromide (MTT), and apoptosis assays. Meanwhile, 19-phenyl-GA retained the ability to induce autophagy and potentially protective heat shock proteins (HSPs) such as Hsp70 and Hsp27. We found that transduction of A53T, but not wild type (WT) α-synuclein, induced toxicity in SH-SY5Y cells. 19-Phenyl-GA decreased oligomer formation and toxicity of A53T α-synuclein in transduced cells. Mechanistic studies indicated that mammalian target of rapamycin (mTOR)/p70 ribosomal S6 kinase signaling was activated by A53T but not WT α-synuclein, and 19-phenyl-GA decreased mTOR activation that may be associated with A53T α-synuclein toxicity. In summary, our results indicate that 19BQAs such as 19-phenyl-GA may provide a means to modulate protein-handling systems including HSPs and autophagy, thereby reducing the aggregation and toxicity of proteins such as mutant A53T α-synuclein.

Synthesis of macrolactam analogues of radicicol and their binding to heat shock protein Hsp90

A series of macrolactam analogues of the naturally occurring resorcylic acid lactone radicicol have been synthesised from methyl orsellinate in 7 steps, involving chlorination, protection of the two phenolic groups, and hydrolysis to the benzoic acid. Formation of the dianion and quenching with a Weinreb amide results in acylation of the toluene methyl group that is followed by amide formation and ring closing metathesis to form the macrocyclic lactam. Final deprotection of the phenolic groups gives the desired macrolactams whose binding to the N-terminal domain of yeast Hsp90 was studied by isothermal titration calorimetry and protein X-ray crystallography.

An improved route to 19-substituted geldanamycins as novel Hsp90 inhibitors--potential therapeutics in cancer and neurodegeneration

19-Substituted geldanamycin derivatives are efficient Hsp90 inhibitors, without the toxicity associated with the other benzoquinone ansamycins, thus giving them potential for use as molecular therapeutics in cancer and neurodegeneration. Here a new method of synthesising these important compounds is reported, eliminating the need for toxic metals and metalloids.

19-substituted benzoquinone ansamycin heat shock protein-90 inhibitors: biological activity and decreased off-target toxicity

The benzoquinone ansamycins (BQAs) are a valuable class of antitumor agents that serve as inhibitors of heat shock protein (Hsp)-90. However, clinical use of BQAs has resulted in off-target toxicities, including concerns of hepatotoxicity. Mechanisms underlying the toxicity of quinones include their ability to redox cycle and/or arylate cellular nucleophiles. We have therefore designed 19-substituted BQAs to prevent glutathione conjugation and nonspecific interactions with protein thiols to minimize off-target effects and reduce hepatotoxicity. 19-Phenyl- and 19-methyl-substituted versions of geldanamycin and its derivatives, 17-allylamino-17-demethoxygeldanamycin and 17-(dimethylaminoethylamino)-17-demethoxygeldanamycin (17-DMAG), did not react with glutathione, whereas marked reactivity was observed using parent BQAs. Importantly, although 17-DMAG induced cell death in primary and cultured mouse hepatocytes, 19-phenyl and 19-methyl DMAG showed reduced toxicity, validating the overall approach. Furthermore, our data suggest that arylation reactions, rather than redox cycling, are a major mechanism contributing to BQA hepatotoxicity. 19-Phenyl BQAs inhibited purified Hsp90 in a

NAD(P)H

quinone oxidoreductase 1 (NQO1)-dependent manner, demonstrating increased efficacy of the hydroquinone ansamycin relative to its parent quinone. Molecular modeling supported increased stability of the hydroquinone form of 19-phenyl-DMAG in the active site of human Hsp90. In human breast cancer cells, 19-phenyl BQAs induced growth inhibition also dependent upon metabolism via NQO1 with decreased expression of client proteins and compensatory induction of Hsp70. These data demonstrate that 19-substituted BQAs are unreactive with thiols, display reduced hepatotoxicity, and retain Hsp90 and growth-inhibitory activity in human breast cancer cells, although with diminished potency relative to parent BQAs.

2,6-Disubstituted 7-(naphthalen-2-ylmethyl)-7H-purines as a new class of potent antitubercular agents inhibiting DprE1

Phenotypic screening of an in-house library of small molecule purine derivatives against Mycobacterium tuberculosis (Mtb) led to the identification of 2-morpholino-7-(naphthalen-2-ylmethyl)-1,7-dihydro-6H-purin-6-one 10 as a potent antimycobacterial agent with MIC₉₉ of 4 μM. Thorough structure-activity relationship studies revealed the importance of 7-(naphthalen-2-ylmethyl) substitution for antimycobacterial activity, yet opened the possibility of structural modifications at positions 2 and 6 of the purine core. As the result, optimized analogues with 6-amino or ethylamino substitution 56 and 64, respectively, were developed. These compounds showed strong in vitro antimycobacterial activity with MIC of 1 μM against Mtb H₃₇Rv and against several clinically isolated drug-resistant strains, had limited toxicity to mammalian cell lines, medium clearance with respect to phase I metabolic deactivation (27 and 16.8 μL/min/mg), sufficient aqueous solubility (>90 μM) and high plasma stability. Interestingly, investigated purines, including compounds 56 and 64, lacked activity against a panel of Gram-negative and Gram-positive bacterial strains, indicating a specific mycobacterial molecular target. To investigate the mechanism of action, Mtb mutants resistant to hit compound 10 were isolated and their genomes were sequenced. Mutations were found in dprE1 (Rv3790), which encodes decaprenylphosphoryl-β-d-ribose oxidase DprE1, enzyme essential for the biosynthesis of arabinose, a vital component of the mycobacterial cell wall. Inhibition of DprE1 by 2,6-disubstituted 7-(naphthalen-2-ylmethyl)-7H-purines was proved using radiolabelling experiments in Mtb H₃₇Rv in vitro. Finally, structure-binding relationships between selected purines and DprE1 using molecular modeling studies in tandem with molecular dynamic simulations revealed the key structural features for effective drug-target interaction.

Aliphatic ketones are acetylcholinesterase inhibitors but not transition state analogs

A number of C4--C9 aliphatic ketones are acetylcholinesterase (acetylcholine hydrolase, EC 3.1.1.7) inhibitors, with Ki values in the 0.7--5 mM range. Comparison to analogous substrates would suggest that these ketones are transition state analogs; e.g. 2-pentanone binds to the enzyme approx. 550 times more tightly than ethylacetate. However, a number of other criteria contradict this conclusions: (1) the binding is insensitive to ketone structure: isomeric ketones, cycloalkanones, and sterically hindered ketones have similar inhibitory potencies. (2) Analogous alcohols are also good inhibitors even though they cannot form hemiketals with the enzyme. (3) Representative ketones are relatively ineffective at blocking inactivation of the enzyme by methylsulfonyl fluoride, indicating that ketones do not bind principally at the hydrolytic site. (4) A competition experiment shows that binding of tetramethylammonium chloride excludes binding of 2-pentanone, suggesting that ketones bind to the anionic rather than the hydrolytic site. Thus, observation of tight binding relative to a substrate is not a sufficient criterion to establish that an inhibitor is a transition state analog.

Abstract 2769: The development of 19-substituted benzoquinone ansamycins as potential anticancer drugs

Benzoquinone ansamycins (BQA's) were the first class of Hsp90 inhibitors developed but the use of the prototype agent geldanamycin was limited by hepatotoxicity. One major mechanism of toxicity of the BQA class is manifested via the electrophilic properties of the quinone and alkylation of cellular nucleophiles at the 19-position. We developed novel 19-substituted BQA's in the geldanamycin (GA), 17-AAG and 17-DMAG series as a means to prevent arylation of cellular nucleophiles. 19-substituted BQA's inhibited recombinant Hsp90 and did not react with thiols at the 19-position while marked thiol reactivity could be detected using their parent quinones. We examined the effects of different substitution patterns at the 19-position on the ability of BQAs to inhibit Hsp90 and induce growth inhibitory effects in breast cancer cell lines. We found that both 19-phenyl and 19-methyl BQA's in the 17-AAG, 17-DMAG and GA series were effective inhibitors of purified Hsp90 as demonstrated using an ATPase assay particularly in the presence of NQO1 to generate the hydroquinone ansamycin. Only 19-phenyl substituted BQAs were effective growth inhibitory agents in breast cancer cell lines using an MTT assay while 19-methyl substituted BQAs in the GA, 17-AAG and 17-DMAG series demonstrated only modest growth inhibition. The growth inhibitory effects of 19-phenyl BQAs were potentiated in cells containing high NQO1. Notably, 19-phenyl DMAG had similar growth inhibitory and apoptotic effects as its parent quinone, DMAG. These data demonstrate that the 19-phenyl BQAs had marked growth inhibitory, apoptotic, and client protein expression effect in breast cancer cells compared to 19-methyl BQAs (Supported by CA 51210).

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 2769. doi:1538-7445.AM2012-2769

Geldanamycin, a Naturally Occurring Inhibitor of Hsp90 and a Lead Compound for Medicinal Chemistry

Geldanamycin remains a driver in the medicinal chemistry of heat shock protein 90 (Hsp90) inhibition, even half a century after its original isolation from nature. This Perspective focuses on the properties of the benzoquinone ring of the natural product that enable a range of functionalization reactions to take place. Therefore, inherent reactivity at C-17, where the methoxy group serves as a vinylogous ester, and at C-19 that demonstrates nucleophilic, enamide-type character toward electrophiles, and also as a conjugate acceptor to react with nucleophiles, has facilitated the synthesis of semisynthetic derivatives. Thus, a range of C-17-substituted amine derivatives has been investigated in oncology applications, with a number of compounds in this series reaching clinical trials. In contrast, the 19-position of geldanamycin has received less attention, although 19-substituted derivatives offer promise with markedly reduced toxicity compared to geldanamycin itself, while retaining Hsp90 inhibitory activity albeit with diminished potency in cellular studies.

Abstract B102: 19-Substituted benzoquinone ansamycins: Decreasing the toxicity of the benzoquinone ansamycin class of Hsp90 inhibitors

Benzoquinone ansamycins (BQAs) were the first class of Hsp90 inhibitors developed but members of this class, particularly geldanamycin, demonstrated hepatic toxicity. One major mechanism of toxicity of the BQA class is manifested via the electrophilic properties of the quinone and arylation of cellular nucleophiles at the 19-position of the ansamycin ring. Using selective halogenation and palladium-catalyzed coupling, we have synthesized a number of novel 19-substituted BQAs in the geldanamycin, 17-AAG and 17-DMAG series as a means to prevent arylation of cellular nucleophiles and have validated this approach using model thiols including N-acetylcysteine and glutathione. 19-Substituted BQAs did not react with thiols at the 19-position while marked reactivity could be detected using their parent quinones. 19-Substituted BQAs were tested for their ability to inhibit recombinant yeast Hsp90 and 19-substitution did not block the capacity of these novel molecules to inhibit the ATPase activity of the Hsp90 chaperone. This result was confirmed by molecular modeling of 19-substituted derivatives in the active site of human Hsp90 demonstrating that 19-substitution did not block entry of the molecule into the active site. The addition of NAD(P)H:quinone oxidoreductase 1 (NQO1) potentiated inhibition of recombinant yeast Hsp90 by 19-substituted BQAs confirming our previous data demonstrating increased inhibitory efficacy of the hydroquinone ansamycin relative to its parent quinone. Cellular effects of 19-substituted BQAs were examined in MDA468 breast cancer cells and the isogenic MDA468/NQ16 cell line which over-expresses NQO1. Growth inhibitory effects were observed using 19-substituted BQAs and were potentiated by the presence of NQO1 in the MDA468/NQ16 line. Hsp90 inhibition in MDA468 and MDA468/NQ16 cells was confirmed using decreases in the Hsp90 client protein Raf1 and a compensatory increase in Hsp70 as biomarkers. In summary, these data demonstrate that 19-substituted BQAs do not react with thiols at the 19-position but retain their Hsp90 inhibitory capacity using purified enzyme and in cells suggesting that they should undergo further translational evaluation as therapeutic candidates (Supported by CA51210 and the Parkinsons Disease Society UK).

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2011 Nov 12-16; San Francisco, CA. Philadelphia (PA): AACR; Mol Cancer Ther 2011;10(11 Suppl):Abstract nr B102.

Anticancer Effect of C19-Position Substituted Geldanamycin Derivatives Targeting NRF2-NQO1-activated Esophageal Squamous Cell Carcinoma

In esophageal squamous cell carcinoma, genetic activation of NRF2 increases resistance to chemotherapy and radiotherapy, which results in a significantly worse prognosis for patients. Therefore NRF2-activated cancers create an urgent clinical need to identify new therapeutic options. In this context, we previously identified the geldanamycin family of HSP90 inhibitors, which includes 17DMAG, to be synthetic lethal with NRF2 activity. As the first-generation of geldanamycin-derivative drugs were withdrawn from clinical trials due to hepatotoxicity, we designed second-generation compounds with C19-substituted structures in order to inhibit glutathione conjugation-mediated hepatotoxicity. In this study, using a variety of in vitro and in vivo cancer models, we found that C19-substituted 17DMAG compounds maintain their enhanced toxicity profile and synthetic lethal interaction with NRF2-NQO1-activated cancer cells. Importantly, using a xenograft mouse tumor model, we found that C19-substituted 17DMAG displayed significant anticancer efficacy against NRF2-NQO1-activated cancer cells without causing hepatotoxicity. These results clearly demonstrate the improved clinical potential for this new class of HSP90 inhibitor anticancer drugs, and suggest that patients with NRF2-NQO1-activated esophageal carcinoma may benefit from this novel therapeutic approach.

Abstract 1791: 19-Substituted benzoquinone ansamycin Hsp90 inhibitors: Effects on Hsp90 co-chaperones and Hsp90-Hsf1 complexes in cellular systems

The 19-substituted benzoquinone ansamycin (BQA) class of Hsp90 inhibitors were developed to decrease the off-target toxicity of their parent unsubstituted BQAs, geldanamcyin, 17-AAG, and 17-DMAG. We have shown that 19-BQAs do not react with thiols and show decreased toxicity to liver cell systems relative to parent BQAs. As a class, Hsp90 inhibitors exhibit anti-cancer activity by decreasing the levels of Hsp90 client proteins critical to cell growth and survival. Some Hsp90 inhibitors also disrupt interaction with Hsp90 co-chaperones, including cdc37, contributing to inhibited growth. Upon inhibition, the transcription factor heat shock factor 1 (Hsf1) dissociates from Hsp90 leading to a compensatory induction of other heat shock proteins, including Hsp70 which is commonly used as a molecular biomarker of cellular Hsp90 inhibition. We undertook a detailed study of the mechanism of action of 19-BQAs in HER2-positive breast cancer cells (BT474) using 19-phenyl- and 19-methyl-DMAG as model compounds. Their ability to disrupt association of Hsp90 with the co-chaperones p23 and cdc37, the kinetics of decreased Hsp90-Hsf1 levels, nuclear Hsf1 accumulation and increases in Hsp70 levels were defined. Both 19-phenyl- and 19-methyl -DMAG disrupted Hsp90-p23 association, confirming binding of the compounds at the N-terminal ATPase site of Hsp90. 19-Phenyl-DMAG and 19-methyl-DMAG also disrupted association of Hsp90 with cdc37, although 19-phenyl-DMAG was more potent. A detailed time course of decreased levels of Hsp90 client proteins and Hsp induction showed that increased Hsp70 was a more sensitive cellular marker as it could be detected at lower drug concentrations. Increases in Hsp70 did not appear to be a generalized stress response, since 19-substituted-BQAs caused dissociation of the Hsp90-Hsf1 dimer, nuclear translocation of Hsf-1, and subsequent increases in Hsp70 levels in a temporal manner. These data indicate that 19-substituted BQAs may exhibit their growth inhibitory effects in breast cancer cells through disruption of Hsp90 and its co-chaperones, particularly cdc37. The N-terminal ATPase site binding and Hsp90 inhibitory activity of these compounds was confirmed through disruption of Hsp90-p23 and Hsp90-Hsf1 associations, respectively (Supported by NCI grant CA51210).

Citation Format: Derek A. Drechsel, Chuan-Hsin Chang, Russell Kitson, David Siegel, Christopher J. Moody, David Ross. 19-Substituted benzoquinone ansamycin Hsp90 inhibitors: Effects on Hsp90 co-chaperones and Hsp90-Hsf1 complexes in cellular systems. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 1791. doi:10.1158/1538-7445.AM2014-1791

Both Nitro Groups Are Essential for High Antitubercular Activity of 3,5-Dinitrobenzylsulfanyl Tetrazoles and 1,3,4-Oxadiazoles through the Deazaflavin-Dependent Nitroreductase Activation Pathway

3,5-Dinitrobenzylsulfanyl tetrazoles and 1,3,4-oxadiazoles, previously identified as having high in vitro activities against both replicating and nonreplicating mycobacteria and favorable cytotoxicity and genotoxicity profiles were investigated. First we demonstrated that these compounds act in a deazaflavin-dependent nitroreduction pathway and thus require a nitro group for their activity. Second, we confirmed the necessity of both nitro groups for antimycobacterial activity through extensive structure-activity relationship studies using 32 structural types of analogues, each in a five-membered series. Only the analogues with shifted nitro groups, namely, 2,5-dinitrobenzylsulfanyl oxadiazoles and tetrazoles, maintained high antimycobacterial activity but in this case mainly as a result of DprE1 inhibition. However, these analogues also showed increased toxicity to the mammalian cell line. Thus, both nitro groups in 3,5-dinitrobenzylsulfanyl-containing antimycobacterial agents remain essential for their high efficacy, and further efforts should be directed at finding ways to address the possible toxicity and solubility issues, for example, by targeted delivery.

Abstract 1732: Effect of 19-substituted benzoquinone ansamycin Hsp90 inhibitors on Hsp90/Cdc37/co-chaperone complexes and casein kinase 2 (CK2) activity

Benzoquinone ansamycin (BQA) Hsp90 inhibitors such as 17-DMAG and 17-AAG have off target toxicities including hepatotoxicity. Mechanisms underlying the toxicity of BQAs are a function of their ability to redox cycle and/or arylate cellular nucleophiles at the unsubstituted 19-position of the molecule. Therefore, we designed 19-substituted BQAs (19BQAs) to prevent conjugation with glutathione and protein thiols as an approach to reduce the hepatotoxicity and minimize off target effects of the BQA class of Hsp90 inhibitors. Our data demonstrated that 19BQAs had reduced toxicity to liver cells relative to their parent quinones while maintaining Hsp90 inhibitory activity, validating the overall approach. 19BQAs depend on tumor cell levels of NQO1 (NAD(P)H: quinone oxidoreductase 1) for optimal activity. We examined 19-phenyl-17-DMAG as a model compound in human isogenic breast cancer cell lines MDA468 (NQO1 null) and MDA468/NQ16 (NQO1 overexpressing) and in NQO1expressing BT474 breast cancer cells. 19-phenyl-17-DMAG induced growth inhibition, apoptosis and the molecular signature of Hsp90 inhibition including decreases in Hsp90 client protein levels and compensatory induction of Hsp70. In addition, treatment of MD468/NQ16 and BT474 cell lines with 19-phenyl-17-DMAG or 17-DMAG also caused the dissociation of Hsp90 from the co-chaperone Cdc37. In these studies we also observed that treatment with19-phenyl-17-DMAG or 17-DMAG resulted in inhibition of Cdc37 phosphorylation. A major kinase responsible for phosphorylation of Cdc37 is casein kinase 2 (CK2) and inhibitors of CK2 are currently under development as anticancer drugs. CK2 activity assays, immunoprecipitation and western blot analysis demonstrated that both 19-phenyl-17-DMAG and 17-DMAG inhibited CK2 kinase activity and reduced CK2α protein subunit expression in NQO1-expressing MD468/NQ16 and BT474 breast cancer cells. Decreased phosphorylation of Cdc37 led to dissociation of the Hsp90/Cdc37/client complex and resulted in the degradation of multiple kinase clients including Raf-1, Cdk4, Akt and HER2. These data suggest that inhibition of both Hsp90 and CK2 by Hsp90 inhibitors may play a role in their antitumor effects. (Supported by CA51210).

Citation Format: Chuan-Hsin Chang, David Ross, David Siegel, Christopher J Moody, Russell Kitson. Effect of 19-substituted benzoquinone ansamycin Hsp90 inhibitors on Hsp90/Cdc37/co-chaperone complexes and casein kinase 2 (CK2) activity. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 1732. doi:10.1158/1538-7445.AM2015-1732