Stereodefined and polyunsaturated inhibitors of histone deacetylase based on (2E,4E)-5-arylpenta-2,4-dienoic acid hydroxyamides

Have molecular hybrids delivered effective anti-cancer treatments and what should future drug discovery focus on?

INTRODUCTION

Cancer continues to be a big threat and its treatment is a huge challenge among the medical fraternity. Conventional anti-cancer agents are losing their efficiency which highlights the need to introduce new anti-cancer entities for treating this complex disease. A hybrid molecule has a tendency to act through varied modes of action on multiple targets at a given time. Thus, there is the significant scope with hybrid compounds to tackle the existing limitations of cancer chemotherapy.

AREA COVERED

This perspective describes the most significant hybrids that spring hope in the field of cancer chemotherapy. Several hybrids with anti-proliferative/anti-tumor properties currently approved or in clinical development are outlined, along with a description of their mechanism of action and identified drug targets.

EXPERT OPINION

The success of molecular hybridization in cancer chemotherapy is quite evident by the number of molecules entering into clinical trials and/or have entered the drug market over the past decade. Indeed, the recent advancements and co-ordinations in the interface between chemistry, biology, and pharmacology will help further the advancement of hybrid chemotherapeutics in the future.List of abbreviations: Deoxyribonucleic acid, DNA; national cancer institute, NCI; peripheral blood mononuclear cells, PBMC; food and drug administration, FDA; histone deacetylase, HDAC; epidermal growth factor receptor, EGFR; vascular endothelial growth factor receptor, VEGFR; suberoylanilide hydroxamic acid, SAHA; farnesyltransferase inhibitor, FTI; adenosine triphosphate, ATP; Tamoxifen, TAM; selective estrogen receptor modulator, SERM; structure activity relationship, SAR; estrogen receptor, ER; lethal dose, LD; half maximal growth inhibitory concentration, GI₅₀; half maximal inhibitory concentration, IC₅₀.

Incorporation of histone deacetylase inhibition into the structure of a nuclear receptor agonist

1,25-dihydroxyvitamin D(3) (1,25D) regulates gene expression by signaling through the nuclear vitamin D receptor (VDR) transcription factor and exhibits calcium homeostatic, anticancer, and immunomodulatory properties. Histone deacetylase inhibitors (HDACis) alter nuclear and cytoplasmic protein acetylation, modify gene expression, and have potential for treatment of cancer and other indications. The function of nuclear receptor ligands, including 1,25D, can be enhanced in combination with HDACi. We designed triciferol, a hybrid molecule in which the 1,25D side chain was replaced with the dienyl hydroxamic acid of HDACi trichostatin A. Triciferol binds directly to the VDR, and functions as an agonist with 1,25D-like potency on several 1,25D target genes. Moreover, unlike 1,25D, triciferol induces marked tubulin hyperacetylation, and augments histone acetylation at concentrations that largely overlap those where VDR agonism is observed. Triciferol also exhibits more efficacious antiproliferative and cytotoxic activities than 1,25D in four cancer cell models in vitro. The bifunctionality of triciferol is notable because (i) the HDACi activity is generated by modifying the 1,25D side chain without resorting to linker technology and (ii) 1,25D and HDACi have sympathetic, but very distinct biochemical targets; the hydrophobic VDR ligand binding domain and the active sites of HDACs, which are zinc metalloenzymes. These studies demonstrate the feasibility of combining HDAC inhibition with nuclear receptor agonism to enhance their therapeutic potential.

Structure–activity relationships of aryloxyalkanoic acid hydroxyamides as potent inhibitors of histone deacetylase

Syntheses of aryloxyalkanoic acid hydroxyamides are described, all of which are potent inhibitors of histone deacetylase, some being more potent in vitro than trichostatin A (IC(50)=3 nM). Variation of the substituents on the benzene ring as well as fusion of a second ring have marked effects on potency, in vitro IC(50) values down to 1 nM being obtained.

Histone deacetylase inhibitor, trichostatin A induces ubiquitin-dependent cyclin D1 degradation in MCF-7 breast cancer cells

BACKGROUND

Cyclin D1 is an important regulator of G1-S phase cell cycle transition and has been shown to be important for breast cancer development. GSK3beta phosphorylates cyclin D1 on Thr-286, resulting in enhanced ubiquitylation, nuclear export and degradation of the cyclin in the cytoplasm. Recent findings suggest that the development of small-molecule cyclin D1 ablative agents is of clinical relevance. We have previously shown that the histone deacetylase inhibitor trichostatin A (TSA) induces the rapid ubiquitin-dependent degradation of cyclin D1 in MCF-7 breast cancer cells prior to repression of cyclin D1 gene (CCND1) transcription. TSA treatment also resulted in accumulation of polyubiquitylated GFP-cyclin D1 species and reduced levels of the recombinant protein within the nucleus.

RESULTS

Here we provide further evidence for TSA-induced ubiquitin-dependent degradation of cyclin D1 and demonstrate that GSK3beta-mediated nuclear export facilitates this activity. Our observations suggest that TSA treatment results in enhanced cyclin D1 degradation via the GSK3beta/CRM1-dependent nuclear export/26S proteasomal degradation pathway in MCF-7 cells.

CONCLUSION

We have demonstrated that rapid TSA-induced cyclin D1 degradation in MCF-7 cells requires GSK3beta-mediated Thr-286 phosphorylation and the ubiquitin-dependent 26S proteasome pathway. Drug induced cyclin D1 repression contributes to the inhibition of breast cancer cell proliferation and can sensitize cells to CDK and Akt inhibitors. In addition, anti-cyclin D1 therapy may be highly specific for treating human breast cancer. The development of potent and effective cyclin D1 ablative agents is therefore of clinical relevance. Our findings suggest that HDAC inhibitors may have therapeutic potential as small-molecule cyclin D1 ablative agents.

Aromatic Sulfide Inhibitors of Histone Deacetylase Based on Arylsulfinyl-2,4-hexadienoic Acid Hydroxyamides

The synthesis of a novel series of potent inhibitors of histone deacetylases is described, based on arylsulfinyl-2,4-hexadienoic acid hydroxyamides and their derivatives. In vitro IC(50) values down to 40 nM were obtained, and several compounds showed inhibition of CEM (human leukemic) cell viability with IC(50) of approximately 1.5 microM, comparable to or better than that of suberoylanilide hydroxamic acid, an inhibitor of histone deacetylase currently in clinical trials.

Plasma pharmacokinetics and metabolism of the histone deacetylase inhibitor trichostatin a after intraperitoneal administration to mice

Trichostatin A is a potent and specific histone deacetylase inhibitor with promising antitumor activity in preclinical models. Plasma pharmacokinetics of trichostatin A were studied following single-dose intraperitoneal administration of 80 mg/kg (high dose) or 0.5 mg/kg (low dose) to female BALB/c mice. Plasma trichostatin A concentrations were quantified by high performance liquid chromatography (HPLC)-UV assay (high dose) or by HPLC-multiple reaction monitoring assay (low dose). Trichostatin A was rapidly absorbed from the peritoneum and detectable in plasma within 2 min. Cmax of 40 microg/ml and 8 ng/ml occurred within 5 min, followed by rapid exponential decay in plasma trichostatin A concentration with t1/2 of 6.3 min and 9.6 min (high and low doses, respectively). Phase I metabolites at the high dose were identified by simultaneous UV and positive ion electrospray mass spectrometry. Trichostatin A underwent extensive metabolism: primary metabolic pathways were N-demethylation, reduction of the hydroxamic acid to the corresponding trichostatin A amide, and oxidative deamination to trichostatic acid. N-Monomethyl trichostatin A amide was the major plasma metabolite. No didemethylated compounds were identified. Trichostatic acid underwent further biotransformation: reduction and beta-oxidation of the carboxylic acid, with or without N-demethylation, resulted in formation of dihydro trichostatic acid and dinor dihydro trichostatic acids. HPLC fractions corresponding to trichostatin A and N-demethylated trichostatin A exhibited histone deacetylase-inhibitory activity; no other fractions were biologically active. We conclude that trichostatin A is rapidly and extensively metabolized in vivo following intraperitoneal administration to mice, and N-demethylation does not compromise histone deacetylase-inhibitory activity.