Optimisation of tetrahydroisoquinoline-based chimeric microtubule disruptors

Cell Fate following Irradiation of MDA-MB-231 and MCF-7 Breast Cancer Cells Pre-Exposed to the Tetrahydroisoquinoline Sulfamate Microtubule Disruptor STX3451

A tetrahydroisoquinoline (THIQ) core is able to mimic the A and B rings of 2-methoxyestradiol (2ME2), an endogenous estrogen metabolite that demonstrates promising anticancer properties primarily by disrupting microtubule dynamic instability parameters, but has very poor pharmaceutical properties that can be improved by sulfamoylation. The non-steroidal THIQ-based microtubule disruptor 2-(3-bromo-4,5-dimethoxybenzyl)-7-methoxy-6-sulfamoyloxy-1,2,3,4-tetrahydroisoquinoline (STX3451), with enhanced pharmacokinetic and pharmacodynamic profiles, was explored for the first time in radiation biology. We investigated whether 24 h pre-treatment with STX3451 could pre-sensitize MCF-7 and MDA-MB-231 breast cancer cells to radiation. This regimen showed a clear increase in cytotoxicity compared to the individual modalities, results that were contiguous in spectrophotometric analysis, flow cytometric quantification of apoptosis induction, clonogenic studies and microscopy techniques. Drug pre-treatment increased radiation-induced DNA damage, with statistically more double-strand (ds) DNA breaks demonstrated. The latter could be due to the induction of a radiation-sensitive metaphase block or the increased levels of reactive oxygen species, both evident after compound exposure. STX3451 pre-exposure may also delay DNA repair mechanisms, as the DNA damage response element ataxia telangiectasia mutated (ATM) was depressed. These in vitro findings may translate into in vivo models, with the ultimate aim of reducing both radiation and drug doses for maximal clinical effect with minimal adverse effects.

Nonsteroidal sulfamate derivatives as new therapeutic approaches for Neurofibromatosis 2 (NF2)

BACKGROUND

Neurofibromatosis 1 and 2, although involving two different tumour suppressor genes (neurofibromin and merlin, respectively), are both cancer predisposition syndromes that disproportionately affect cells of neural crest origin. New therapeutic approaches for both NF1 and NF2 are badly needed. In promising previous work we demonstrated that two non-steroidal analogues of 2-methoxy-oestradiol (2ME2), STX3451(2-(3-bromo-4,5-dimethoxybenzyl)-7-methoxy-6-sulfamoyloxy-1,2,3,4-tetrahydroisoquinoline), and STX2895 (7-Ethyl-6-sulfamoyloxy-2-(3,4,5-trimethoxybenzyl)-1,2,3,4-tetrahydroisoquinoline) reduced tumour cell growth and induced apoptosis in malignant and benign human Neurofibromatosis 1 (NF1) tumour cells. In earlier NF1 mechanism of action studies we found that in addition to their effects on non-classical hormone-sensitive pathways, STX agents acted on the actin- and myosin-cytoskeleton, as well as PI3Kinase and MTOR signaling pathways. Tumour growth in NF2 cells is affected by different inhibitors from those affecting NF1 growth pathways: specifically, NF2 cells are affected by merlin-downstream pathway inhibitors. Because Merlin, the affected tumour suppressor gene in NF2, is also known to be involved in stabilizing membrane-cytoskeletal complexes, as well as in cell proliferation, and apoptosis, we looked for potentially common mechanisms of action in the agents' effects on NF1 and NF2. We set out to determine whether STX agents could therefore also provide a prospective avenue for treatment of NF2.

METHODS

STX3451 and STX2895 were tested in dose-dependent studies for their effects on growth parameters of malignant and benign NF2 human tumour cell lines in vitro. The mechanisms of action of STX3451 and STX2895 were also analysed.

RESULTS

Although neither of the agents tested affected cell growth or apoptosis in the NF2 tumour cell lines tested through the same mechanisms by which they affect these parameters in NF1 tumour cell lines, both agents disrupted actin- and myosin-based cytoskeletal structures in NF2 cell lines, with subsequent effects on growth and cell death.

CONCLUSIONS

Both STX3451 and STX2895 provide new approaches for inducing cell death and lowering tumour burden in NF2 as well as in NF1, which both have limited treatment options.

3,17β-Bis-sulfamoyloxy-2-methoxyestra-1,3,5(10)-triene and Nonsteroidal Sulfamate Derivatives Inhibit Carbonic Anhydrase IX: Structure-Activity Optimization for Isoform Selectivity

3,17β-Bis-sulfamoyloxy-2-methoxyestra-1,3,5(10)-triene (STX140), a bis-sulfamate derivative of the endogenous steroid 2-methoxyestradiol, has shown promising anticancer potency both in vitro and in vivo, with excellent bioavailability. Its activity against taxane-resistant xenografts makes it a potential drug candidate against triple-negative breast cancer (TNBC). These properties are linked to the ability of STX140 to act in a multitargeting fashion in vivo as a microtubule disruptor, leading to cell cycle arrest and with both proapoptotic and anti-angiogenic activities. Carbonic anhydrase IX (CA IX) is a well-established biomarker for aggressive cancers, including TNBC. This study reports, for the first time, the inhibitory activities of a series of steroidal and nonsteroidal sulfamate derivatives against CA IX in comparison to the ubiquitous CA II, with some compounds demonstrating 100-200-fold selectivity for CA IX over CA II. X-ray crystallographic studies of four of the most promising compounds reveal that isoform-specific residue interactions are responsible for the high specificity.

Tetrahydroisoquinoline Sulfamates as Potent Microtubule Disruptors: Synthesis, Antiproliferative and Antitubulin Activity of Dichlorobenzyl-Based Derivatives, and a Tubulin Cocrystal Structure

Tetrahydroisoquinoline (THIQ) 6-O-sulfamate-based anticancer agents, inspired by the endogenous steroid 2-methoxyestradiol and its sulfamate derivatives, are further explored for antiproliferative and microtubule disruptor activity. Based on recently designed C3-methyl C7-methoxy-substituted THIQ derivatives, compounds with mono- and dichloro-substitutions on the pendant N-benzyl ring were synthesized and evaluated. Although improved antiproliferative activity was observed, for example, 4a versus 4b and 4b versus 8c, it was relatively modest. Compound 8c, a 2',5'-dichlorobenzyl derivative was, however, identified as a promising antiproliferative agent with in vitro activities exceeding that of the parent steroid (e.g., GI₅₀ 90 nM in DU-145 cells) and was highly potent against a range of tumor cell lines (e.g., GI₅₀ 26 nM for OVCAR-3). 8c inhibited the polymerization of tubulin in vitro with an IC₅₀ only twofold less potent than combretastatin A-4 and inhibited colchicine binding to tubulin. Tubulin polymerization assays showed the parent THIQ 4a to be only a very weak inhibitor, but a striking potency difference was seen between compounds with C2' methoxy and chloro substituents, whereas this was much smaller when these substituents were positioned at C5'. To confirm the target in atomic detail and because 8c is a racemic mixture, an achiral parent THIQ 6-O-sulfamate derivative 10 was successfully cocrystallized with the αβ-tubulin heterodimer. The derivative 10 binds at the colchicine site on tubulin, the first example of this compound class investigated in such detail, with its sulfamate group interacting with residues beyond the reach of colchicine itself, similar to a recently reported quinazolinone sulfamate derivative, 6a. The structure also suggests that for racemic C3-methyl-substituted THIQ derivatives, such as 8c, the (S)-enantiomer is likely to be preferentially accommodated within the colchicine site for steric reasons. The results further confirm the potential of nonsteroidal THIQ sulfamate derivatives for oncology and suggest that the mechanism of microtubule destabilization for the THIQ compound class is to prevent the curved-to-straight conformational transition of tubulin required for polymerization.

Modes of cell death induced by tetrahydroisoquinoline-based analogs in MDA-MB-231 breast and A549 lung cancer cell lines

Background

A and B rings of the steroidal microtubule disruptor, 2-methoxyestradiol, and its analogs can be mimicked with a tetrahydroisoquinoline (THIQ) core. THIQs are cytotoxic agents with potential anticancer activities. The aim of this in vitro study was to investigate the modes of cell death induced by four nonsteroidal THIQ-based analogs, such as STX 2895, STX 3329, STX 3451 and STX 3450, on MDA-MB-231 metastatic breast and A549 epithelial lung carcinoma cells.

Materials and methods

Cytotoxicity studies determined the half-maximal growth inhibitory concentration of the analogs to be at nanomolar concentrations without the induction of necrosis. Light and fluorescent microscopy determined that compounds caused microtubule depolymerization and displayed morphological hallmarks of apoptosis.

Results

Flow cytometric analyses confirmed apoptosis induction as well as an increased G₂/M phase on cell cycle analysis. Furthermore, intrinsic pathway signaling was implicated due to increased cytochrome c release and a decrease in mitochondrial transmembrane potential. Potential involvement of autophagy was observed due to increased acidic vacuole formation and increased aggresome activation factor.

Conclusion

Thus, it can be concluded that these four THIQ-based analogs exert anti-proliferative and antimitotic effects, induce apoptosis and involve autophagic processes. Further investigation into the efficacy of these potential anticancer drugs will be conducted in vitro and in vivo.

Quinazolinone-Based Anticancer Agents: Synthesis, Antiproliferative SAR, Antitubulin Activity, and Tubulin Co-crystal Structure

Quinazolinone-based anticancer agents were designed, decorated with functional groups from a 2-methoxyestradiol-based microtubule disruptor series, incorporating the aryl sulfamate motif of steroid sulfatase (STS) inhibitors. The steroidal AB-ring system was mimicked, favoring conformations with an N-2 substituent occupying D-ring space. Evaluation against breast and prostate tumor cell lines identified 7b with DU-145 antiproliferative activity (GI₅₀ 300 nM). A preliminary structure-activity relationship afforded compounds (e.g., 7j GI₅₀ 50 nM) with activity exceeding that of the parent. Both 7b and 7j inhibit tubulin assembly in vitro and colchicine binding, and 7j was successfully co-crystallized with the αβ-tubulin heterodimer as the first of its class, its sulfamate group interacting positively at the colchicine binding site. Microtubule destabilization by 7j is likely achieved by preventing the curved-to-straight conformational transition in αβ-tubulin. Quinazolinone sulfamates surprisingly showed weak STS inhibition. Preliminary in vivo studies in a multiple myeloma xenograft model for 7b showed oral activity, confirming the promise of this template.

Accessing simply-substituted 4-hydroxytetrahydroisoquinolines via Pomeranz-Fritsch-Bobbitt reaction with non-activated and moderately-activated systems

Background: 1,2,3,4-Tetrahydroisoquinolines (THIQs) are common motifs in alkaloids and in medicinal chemistry. Synthetic access to THIQs via the Pomeranz–Fritsch–Bobbit (PFB) methodology using mineral acids for deactivated, electron-poor aromatic systems, is scarcely represented in the literature. Here, the factors controlling the regiochemical outcome of cyclization are evaluated.

Results: A double reductive alkylation was telescoped into a one-pot reaction delivering good to excellent yields of desired aminoacetals for cyclization. Cyclization of activated systems proceeded smoothly under standard PFB conditions, but for non-activated systems the use of HClO₄ alone was effective. When cyclization was possible in both para- and ortho-positions to the substituent, 7-substituted derivatives were formed with significant amounts of 5-substituted byproduct. The formation of the 4-hydroxy-THIQs vs the 4-methoxy-THIQ products could be controlled through modification of the reaction concentration. In addition, while a highly-activated system exclusively cyclized to the indole, this seems generally highly disfavored. When competition between 6- and 7-ring formation was investigated in non-activated systems, 5,7,8,13-tetrahydro-6,13-methanodibenzo[c,f]azonine was exclusively obtained. Furthermore, selective ring closure in the para-position could be achieved under standard PFB conditions, while a double ring closure could be obtained utilizing HClO₄.

Conclusion: Reactivity differences in aminoacetal precursors can be employed to control cyclization using the PFB methodology. It is now possible to select confidently the right conditions for the synthesis of N-aryl-4-hydroxy-1,2,3,4-tetrahydroisoquinolines.

Targeted NF1 cancer therapeutics with multiple modes of action: small molecule hormone-like agents resembling the natural anticancer metabolite, 2-methoxyoestradiol

Background:

Both the number and size of tumours in NF1 patients increase in response to the rise in steroid hormones seen at puberty and during pregnancy. The size of tumours decreases after delivery, suggesting that hormone-targeting therapy might provide a viable new NF1 treatment approach. Our earlier studies demonstrated that human NF1 tumour cell lines either went through apoptosis or ceased growth in the presence of 2-methoxyoestradiol (2ME2), a naturally occurring anticancer metabolite of 17-β estradiol. Previous reports of treatment with sulfamoylated steroidal and non-steroidal derivatives of 2ME2 showed promising reductions in tumour burden in hormone-responsive cancers other than NF1. Here we present the first studies indicating that 2ME2 derivatives could also provide an avenue for treating NF1, for which few treatment options are available.

Methods:

STX3451, (2-(3-Bromo-4,5-dimethoxybenzyl)-7-methoxy-6-sulfamoyloxy-1,2,3,4-tetrahydroisoquinoline), a non-steroidal sulphamate analogue of 2ME2, was tested in dose-dependent studies of malignant and benign NF1 human tumour cell lines and cell lines with variable controlled neurofibromin expression. The mechanisms of action of STX3451 were also analysed.

Results:

We found that STX3451-induced apoptosis in human malignant peripheral nerve sheath tumour (MPNST) cell lines, even in the presence of elevated oestrogen and progesterone. It inhibits both PI3 kinase and mTOR signalling pathways. It disrupts actin- and microtubule-based cytoskeletal structures in cell lines derived from human MPNSTs and in cells derived from benign plexiform neurofibromas. STX3451 selectively kills MPNST-derived cells, but also halts growth of other tumour-derived NF1 cell lines.

Conclusion:

STX3451 provides a new approach for inducing cell death and lowering tumour burden in NF1 and other hormone-responsive cancers with limited treatment options.

Discovery and Development of the Aryl O-Sulfamate Pharmacophore for Oncology and Women's Health

In 1994, following work from this laboratory, it was reported that estrone-3-O-sulfamate irreversibly inhibits a new potential hormone-dependent cancer target steroid sulfatase (STS). Subsequent drug discovery projects were initiated to develop the core aryl O-sulfamate pharmacophore that, over some 20 years, have led to steroidal and nonsteroidal drugs in numerous preclinical and clinical trials, with promising results in oncology and women's health, including endometriosis. Drugs have been designed to inhibit STS, e.g., Irosustat, as innovative dual-targeting aromatase-steroid sulfatase inhibitors (DASIs) and as multitargeting agents for hormone-independent tumors, such as the steroidal STX140 and nonsteroidal counterparts, acting inter alia through microtubule disruption. The aryl sulfamate pharmacophore is highly versatile, operating via three distinct mechanisms of action, and imbues attractive pharmaceutical properties. This Perspective gives a personal view of the work leading both to the therapeutic concepts and these drugs, their current status, and how they might develop in the future.

Synthesis, anti-tubulin and antiproliferative SAR of steroidomimetic dihydroisoquinolinones

A SAR translation strategy adopted for the discovery of tetrahydroisoquinolinone (THIQ)-based steroidomimetic microtubule disruptors has been extended to dihydroisoquinolinone (DHIQ)-based compounds. A steroid A,B-ring-mimicking DHIQ core was connected to methoxyaryl D-ring mimics through methylene, carbonyl, and sulfonyl linkers, and the resulting compounds were evaluated against two cancer cell lines. The carbonyl-linked DHIQs in particular exhibit significant in vitro antiproliferative activities (e.g., 6-hydroxy-7-methoxy-2-(3,4,5-trimethoxybenzoyl)-3,4-dihydroisoquinolin-1(2H)-one (16 g): GI₅₀ 51 nm in DU-145 cells). The broad anticancer activity of DHIQ 16 g was confirmed in the NCI 60-cell line assay giving a mean activity of 33 nm. Furthermore, 6-hydroxy-2-(3,5-dimethoxybenzoyl)-7-methoxy-3,4-dihydroisoquinolin-1(2H)-one (16 f) and 16 g and their sulfamate derivatives 17 f and 17 g (2-(3,5-dimethoxybenzoyl)-7-methoxy-6-sulfamoyloxy-3,4-dihydroisoquinolin-1(2H)-one and 7-methoxy-2-(3,4,5-trimethoxybenzoyl)-6-sulfamoyloxy-3,4-dihydroisoquinolin-1(2H)-one, respectively) show excellent activity against the polymerization of tubulin, close to that of the clinical combretastatin A-4, and bind competitively at the colchicine binding site of tubulin. Compounds 16 f and 17 f were also shown to demonstrate in vitro anti-angiogenic activity. Additionally, X-ray and computational analyses of 17 f reveal that electrostatic repulsion between the two adjacent carbonyl groups, through conformational biasing, dictates the adoption of a “steroid-like” conformation that may partially explain the excellent in vitro activities.

EDL-360: A Potential Novel Antiglioma Agent

Glioma is a brain tumor that arises from glial cells or glial progenitor cells, and represents 80% of malignant brain tumor incidence in the United States. Glioblastoma multiforme (GBM) is the most aggressive primary brain tumor malignancy with fewer than 8% of patients with GBM surviving for more than 3 years. Over the past 10 years, despite improvement in diagnosis and therapies for cancer, the survival rate for high-grade glioma patients remains dismal. The main focus of our research is to identify potent novel antiglioma small molecules. We previously showed that EDL-360, a tetrahydroisoquinoline (THIQ) analog, as being highly cytotoxic to human glioma cell cultures. Here we show that EDL-360 significantly induced apoptosis in human glioma cell lines (U87 and LN18). However, in normal astrocytic cells, EDL-360 induced a modest G0/G1 cell cycle arrest but did not induce apoptosis. In an attempt to enhance EDL-360 induced cell death, we tested simultaneous treatment with EDL-360 and embelin (an inhibitor of the anti-apoptotic protein, XIAP). We found that, glioma cells had significant lower viability when EDL-360 and embelin were used in combination when compared to EDL-360 alone. We also used combination treatment of EDL-360 with decylubiquinone (dUb), a caspase-9 inhibitor, and found that the combination treatment induced a significant cell death when compared to treatment with EDL-360 alone. This is the first report that suggests that dUb has anticancer activity, and perhaps acts as a XIAP inhibitor. Finally, our in vivo data showed that EDL-360 treatment induced a partial regression in glioma tumorigenesis and induced cell death in the treated tumors as shown by H&E staining. Taken together these data suggests that EDL-360 has a potential therapeutic application for treating glioma, especially when combined with XIAP inhibitors.