Johanna Senger

Synthesis and Biological Investigation of Oxazole Hydroxamates as Highly Selective Histone Deacetylase 6 (HDAC6) Inhibitors

Histone deacetylase 6 (HDAC6) catalyzes the removal of an acetyl group from lysine residues of several non-histone proteins. Here we report the preparation of thiazole-, oxazole-, and oxadiazole-containing biarylhydroxamic acids by a short synthetic procedure. We identified them as selective HDAC6 inhibitors by investigating the inhibition of recombinant HDAC enzymes and the protein acetylation in cells by Western blotting (tubulin vs histone acetylation). The most active compounds exhibited nanomolar potency and high selectivity for HDAC6. For example, an oxazole hydroxamate inhibits HDAC6 with an IC50 of 59 nM and has a selectivity index of >200 against HDAC1 and HDAC8. This is the first report showing that the nature of a heterocycle directly connected to a zinc binding group (ZBG) can be used to modulate subtype selectivity and potency for HDAC6 inhibitors to such an extent. We rationalize the high potency and selectivity of the oxazoles by molecular modeling and docking.

Targeting histone deacetylase 8 as a therapeutic approach to cancer and neurodegenerative diseases

Histone deacetylase 8 (HDAC8), a unique class I zinc-dependent HDAC, is an emerging target in cancer and other diseases. Its substrate repertoire extends beyond histones to many nonhistone proteins. Besides being a deacetylase, HDAC8 also mediates signaling via scaffolding functions. Aberrant expression or deregulated interactions with transcription factors are critical in HDAC8-dependent cancers. Many potent HDAC8-selective inhibitors with cellular activity and anticancer effects have been reported. We present HDAC8 as a druggable target and discuss inhibitors of different chemical scaffolds with cellular effects. Furthermore, we review HDAC8 activators that revert activity of mutant enzymes. Isotype-selective HDAC8 targeting in patients with HDAC8-relevant cancers is challenging, however, is promising to avoid adverse side effects as observed with pan-HDAC inhibitors.

Total synthesis of (18S)- and (18R)-homolargazole by rhodium-catalyzed hydrocarboxylation.

Homolargazole derivatives, in which the macrocycle of natural largazole is extended by one methylene group, were prepared by the recently developed rhodium-catalyzed hydrocarboxylation reaction onto allenes. This strategy gives access to both the (18S)- and (18R)-stereoisomers in high stereoselectivity under ligand control.

4‐Biphenylalanine‐ and 3‐Phenyltyrosine‐Derived Hydroxamic Acids as Inhibitors of the JumonjiC‐Domain‐Containing Histone Demethylase KDM4A

Overexpression of the histone lysine demethylase KDM4A, which regulates H3K9 and H3K36 methylation states, has been related to the pathology of several human cancers. We found that a previously reported hydroxamate‐based histone deacetylase (HDAC) inhibitor (SW55) was also able to weakly inhibit this demethylase with an IC50 value of 25.4 μm. Herein we report the synthesis and biochemical evaluations, with two orthogonal in vitro assays, of a series of derivatives of this lead structure. With extensive chemical modifications on the lead structure, also by exploiting the versatility of the radical arylation with aryldiazonium salts, we were able to increase the potency of the derivatives against KDM4A to the low‐micromolar range and, more importantly, to obtain demethylase selectivity with respect to HDACs. Cell‐permeable derivatives clearly showed a demethylase‐inhibition‐dependent antiproliferative effect against HL‐60 human promyelocytic leukemia cells.

Alkoxyurea-Based Histone Deacetylase Inhibitors Increase Cisplatin Potency in Chemoresistant Cancer Cell Lines

The synthesis and biological evaluation of potent hydroxamate-based dual HDAC1/6 inhibitors with modest HDAC6 preference and a novel alkoxyurea connecting unit linker region are described. The biological studies included the evaluation of antiproliferative effects and HDAC inhibitory activity in the human ovarian cancer cell line A2780, the human squamous carcinoma cell line Cal27, and their cisplatin resistant sublines A2780CisR and Cal27CisR. The three most potent compounds 1g-i showed IC50 values in the low μM and sub-μM range. 1g-i revealed low nM IC50 values for HDAC6 with up to 15-fold preference over HDAC1, >3500-fold selectivity over HDAC4, and >100-fold selectivity over HDAC8. Furthermore, their ability to enhance cisplatin sensitivity was analyzed in Cal27 and Cal27CisR cells. Notably, a 48 h preincubation of 1g-i significantly enhanced the antiproliferative effects of cisplatin in Cal27 and Cal27CisR. 1g-i interacted synergistically with cisplatin. These effects were more pronounced for the cisplatin resistant subline Cal27CisR.

Phenylpyrrole-based HDAC inhibitors: synthesis, molecular modeling and biological studies

AIM Histone deacetylases (HDACs) regulate the expression and activity of numerous proteins involved in the initiation and progression of cancer. Currently, three hydroxamate-containing HDAC pan-inhibitors have been approved as antitumor agents. RESULTS We herein present the development of a series of novel phenylpyrrole-based derivatives stemmed from combined computational and medicinal chemistry efforts to rationally modulate HDAC1/6 isoform selectivity. In vitro activity on HDAC1 and HDAC6 isoforms and the effects of selected analogs on histone H3 and α-tubulin acetylation levels were determined. Cell-based data evidenced, for selected compounds, a promising antitumor potential and low toxicity on normal cells. CONCLUSION The newly developed compounds represent a valuable starting point for the development of novel anticancer agents.

Hydroxamates of para-aminobenzoic acid as selective inhibitors of HDAC8

A series of hydroxamates (4a-4l) were prepared from p-aminobenzoic acid to inhibit HDAC8. The idea is to substitute rigid aromatic ring in place of less rigid piperazine ring of hydroxamates reported earlier by our group. It is expected to increase potency retaining the selectivity. Result obtained suggested that the modifications carried out retained the selectivity towards HDAC8 isoform and increasing the potency in very few cases. Increase in potency is also associated with variation in cap aryl region. Two compounds (4f &4l) were found to inhibit HDAC8 at concentrations (IC50) less than 20μM.

Design, multicomponent synthesis and anticancer activity of a focused histone deacetylase (HDAC) inhibitor library with peptoid-based cap groups

In this work, we report the multicomponent synthesis of a focused histone deacetylase (HDAC) inhibitor library with peptoid-based cap groups and different zinc-binding groups. All synthesized compounds were tested in a cellular HDAC inhibition assay and an MTT assay for cytotoxicity. On the basis of their noteworthy activity in the cellular HDAC assays, four compounds were further screened for their inhibitory activity against recombinant HDAC1-3, HDAC6, and HDAC8. All four compounds showed potent inhibition of HDAC1-3 as well as significant inhibition of HDAC6 with IC50 values in the submicromolar concentration range. Compound 4j, the most potent HDAC inhibitor in the cellular HDAC assay, revealed remarkable chemosensitizing properties and enhanced the cisplatin sensitivity of the cisplatin-resistant head-neck cancer cell line Cal27CisR by almost 7-fold. Furthermore, 4j almost completely reversed the cisplatin resistance in Cal27CisR. This effect is related to a synergistic induction of apoptosis as seen in the combination of 4j with cisplatin.

Isoform-selective HDAC1/6/8 inhibitors with an imidazo-ketopiperazine cap containing stereochemical diversity

A series of hydroxamic acids linked by different lengths to a chiral imidazo-ketopiperazine scaffold were synthesized. The compounds with linker lengths of 6 and 7 carbon atoms were the most potent in histone deacetylase (HDAC) inhibition, and were specific submicromolar inhibitors of the HDAC1, HDAC6 and HDAC8 isoforms. A docking model for the binding mode predicts binding of the hydroxamic acid to the active site zinc cation and additional interactions between the imidazo-ketopiperazine and the enzyme rim. The compounds were micromolar inhibitors of the MV4-11, THP-1 and U937 cancer cell lines. Increased levels of histone H3 and tubulin acetylation support a cellular mechanism of action through HDAC inhibition. This article is part of a discussion meeting issue ‘Frontiers in epigenetic chemical biology’.

Structure-Activity Relationship of Propargylamine-Based HDAC Inhibitors

As histone deacetylases (HDACs) play an important role in the treatment of cancer, their selective inhibition has been the subject of various studies. These continuous investigations have given rise to a large collection of pan‐ and selective HDAC inhibitors, containing diverse US Food and Drug Administration (FDA)‐approved representatives. In previous studies, a class of alkyne‐based HDAC inhibitors was presented. We modified this scaffold in two previously neglected regions and compared their cytotoxicity and affinity toward HDAC1, HDAC6, and HDAC8. We were able to show that R‐configured propargylamines contribute to increased selectivity for HDAC6. Docking studies on available HDAC crystal structures were carried out to rationalize the observed selectivity of the compounds. Substitution of the aromatic portion by a thiophene derivative results in high affinity and low cytotoxicity, indicating an improved drug tolerance.