C8-Substituted Imidazotetrazine Analogs Overcome Temozolomide Resistance by Inducing DNA Adducts and DNA Damage

Temozolomide (TMZ) is the standard of care chemotherapeutic agent used in the treatment of glioblastoma multiforme. Cytotoxic O6-methylguaine lesions formed by TMZ are repaired by O6-methyl-guanine DNA methyltransferase (MGMT), a DNA repair protein that removes alkyl groups located at the O6-position of guanine. Response to TMZ requires low MGMT expression and functional mismatch repair. Resistance to TMZ conferred by MGMT, and tolerance to O6-methylguanine lesions conferred by deficient MMR severely limit TMZ clinical applications. Therefore, development of new TMZ derivatives that can overcome TMZ-resistance is urgent. In this study, we investigated the anti-tumor mechanism of action of two novel TMZ analogs: C8-imidazolyl (377) and C8-methylimidazole (465) tetrazines. We found that analogs 377 and 465 display good anticancer activity against MGMT-overexpressing glioma T98G and MMR deficient colorectal carcinoma HCT116 cell lines with IC₅₀ value of 62.50, 44.23, 33.09, and 25.37 μM, respectively. Analogs induce cell cycle arrest at G2/M, DNA double strand break damage and apoptosis irrespective of MGMT and MMR status. It was established that analog 377, similar to TMZ, is able to ring-open and hydrolyze under physiological conditions, and its intermediate product is more stable than MTIC. Moreover, DNA adducts of 377 with calf thymus DNA were identified: N7-methylguanine, O6-methylguanine, N3-methyladenine, N3-methylthymine, and N3-methylcytidine deoxynucleotides. We conclude that C8 analogs of TMZ share a mechanism of action similar to TMZ and are able to methylate DNA generating O6-methylguanine adducts, but unlike TMZ are able at least in part to thwart MGMT- and MMR-mediated resistance.

A novel low molecular weight nanocomposite hydrogel formulation for intra-tumoural delivery of anti-cancer drugs

Herein, an injectable formulation composed of a low molecular weight gelator (LMWG) based hydrogel and drug-loaded polymeric nanocapsules (NCs) is described. The NCs, made of hyaluronic acid and polyglutamic acid and loaded with C14-Gemcitabine (GEM C14), showed a size of 40 and 80 nm and a encapsulation efficiency >90%. These NCs exhibited a capacity to control the release of the encapsulated drug for >1 month. GEM C14-loaded NCs showed activity against various cancer cell lines in vitro; cell growth inhibition by 50% (GI₅₀) values of 15 ± 6, 10 ± 9, 13 ± 3 and 410 ± 463 nM were obtained in HCT 116, MIA PaCa-2, Panc-1 and Panc-1 GEM resistant cell lines respectively. Nanocomposite hydrogels were prepared using the LMWG - N4-octanoyl-2'-deoxycytidine and loaded for the first time with polymeric NCs. 2% and 4% w/v nanocapsule concentrations as compared to 8% w/v NC concentrations with 2% and 3% w/v gelator concentrations gave mechanically stronger gels as determined by oscillatory rheology. Most importantly, the nanocomposite formulation reformed instantly into a gel after injection through a needle. Based on these properties, the nanocomposite gel formulation has potential for the intratumoural delivery of anticancer drugs.

Cardiac glycoside cerberin exerts anticancer activity through PI3K/AKT/mTOR signal transduction inhibition

Natural products possess a significant role in anticancer therapy and many currently-used anticancer drugs are of natural origin. Cerberin (CR), a cardenolide isolated from the fruit kernel of Cerbera odollam, was found to potently inhibit cancer cell growth (GI₅₀ values < 90 nM), colony formation and migration. Significant G2/M cell cycle arrest preceded time- and dose-dependent apoptosis-induction in human cancer cell lines corroborated by dose-and time-dependent PARP cleavage and caspase 3/7 activation, in addition to reduced Bcl-2 and Mcl-1 expression. CR potently inhibited PI3K/AKT/mTOR signalling depleting polo-like kinase 1 (PLK-1), c-Myc and STAT-3 expression. Additionally, CR significantly increased the generation of reactive oxygen species (ROS) producing DNA double strand breaks. Preliminary in silico biopharmaceutical assessment of CR predicted >60% bioavailability and rapid absorption; doses of 1-10 mg/kg CR were predicted to maintain efficacious unbound plasma concentrations (>GI₅₀ value). CR's potent and selective anti-tumour activity, and its targeting of key signalling mechanisms pertinent to tumourigenesis support further preclinical evaluation of this cardiac glycoside.

Development of novel apoferritin formulations for antitumour benzothiazoles

BACKGROUND

The benzothiazole structure is important in medicinal chemistry, and 5-fluoro-2-(3,4-dimethoxyphenyl) benzothiazole (GW 610) is of particular interest as it shows outstanding anticancer activity in sensitive breast and colorectal carcinoma cell lines via generation of lethal DNA adducts in sensitive cancer cells. Despite promising activity, poor water solubility limits its applications. The apoferritin (AFt) protein cage has been proposed as a robust and biocompatible drug delivery vehicle.

AIMS

Here, we aim to enhance solubility of GW 610 by developing amino acid prodrug conjugates and utilizing the AFt capsule as drug delivery vessel.

METHODS AND RESULTS

The potent experimental antitumour agent, GW 610, has been successfully encapsulated within AFt with more than 190 molecules per AFt cage. The AFt-GW 610 complex exhibits dose-dependent growth inhibition and is more potent than GW 610 alone in 5/7 cancer cell lines. To enhance both aqueous solubility and encapsulation efficiency, a series of amino acid esters of GW 608 prodrug were synthesized via N,N'-dicyclohexylcarbodiimide ester coupling to produce molecules with different polarity. A dramatic increase in encapsulation efficiency was achieved, with more than 380 molecules of GW 608-Lys molecules per AFt cage. Release studies show sustained release of the cargo over 12 hours at physiologically relevant pH. The AFt-encapsulated amino acid modified GW 608 complexes are sequestered more rapidly and exhibit more potent anticancer activity than unencapsulated agent.

CONCLUSION

These results indicate that AFt-encapsulation of GW 610 prodrug provides a biocompatible delivery option for this potent, selective experimental antitumour agent and for amino acid-modified GW 608. Of particular interest is the encapsulation efficiency and in vitro antitumour activity of AFt-GW 608-Lys, which warrants further preclinical evaluation.

In search of effective therapies to overcome resistance to Temozolomide in brain tumours

Glioblastoma multiforme is the most common and lethal brain tumour-type. The current standard of care includes Temozolomide (TMZ) chemotherapy. However, inherent and acquired resistance to TMZ thwart successful treatment. The direct repair protein methylguanine DNA methyltransferase (MGMT) removes the cytotoxic O6-methylguanine (O6-MeG) lesion delivered by TMZ and so its expression by tumours confers TMZ-resistance. DNA mismatch repair (MMR) is essential to process O6-MeG adducts and MMR-deficiency leads to tolerance of lesions, resistance to TMZ and further DNA mutations. In this article, two strategies to overcome TMZ resistance are discussed: (1) synthesis of imidazotetrazine analogues - designed to retain activity in the presence of MGMT or loss of MMR; (2) preparation of imidazotetrazine-nanoparticles to deliver TMZ preferably to the brain and tumour site. Our promising results encourage belief in a future where better prognoses exist for patients diagnosed with this devastating disease.

Apoferritin encapsulation of cysteine protease inhibitors for cathepsin L inhibition in cancer cells

Cysteine proteases play a key role in tumorigenesis causing protein degradation and promoting invasive tumour growth. Cathepsin L is overexpressed in cancer cells and could provide a specific target for delivery of anticancer agents. We encapsulated novel dipeptidyl nitrile based cysteine protease inhibitors (Neq0551, Neq0554 and Neq0568) into biocompatible apoferritin (AFt) protein nanocages to achieve specific delivery to tumours and pH-induced drug release. AFt-encapsulated Neq0554 demonstrated ∼3-fold enhanced in vitro activity (GI₅₀ = 79 μM) compared to naked agent against MiaPaCa-2 pancreatic carcinoma cells. Selectivity for cancer cells was confirmed by comparing their activity to non-tumourigenic human fibroblasts (GI₅₀ > 200 μM). Transferrin receptor (TfR-1) expression, detected only in lysates prepared from carcinoma cells, may contribute to the cancer-selectivity. The G₁ cell cycle arrest caused by AFt-Neq0554 resulting in cytostasis was corroborated by clonogenic assays. Superior and more persistent inhibition of cathepsin L up to 80% was achieved with AFt-encapsulated agent in HCT-116 cells following 6 h exposure to 50 μM agent. The selective anticancer activity of AFt-encapsulated cysteine protease inhibitor Neq0554 reported here warrants further preclinical in vivo evaluation.

Novel apoferritin encapsulated cysteine protease inhibitors are developed with enhanced and selective uptake by cancer cells, and sustained pH-induced release of the agent. The persistent inhibition of cathepsin L is demonstrated in vitro.

Lipophilic activated ester prodrug approach for drug delivery to the intestinal lymphatic system

The intestinal lymphatic system plays an important role in the pathophysiology of multiple diseases including lymphomas, cancer metastasis, autoimmune diseases, and human immunodeficiency virus (HIV) infection. It is thus an important compartment for delivery of drugs in order to treat diseases associated with the lymphatic system. Lipophilic prodrug approaches have been used in the past to take advantage of the intestinal lymphatic transport processes to deliver drugs to the intestinal lymphatics. Most of the approaches previously adopted were based on very bulky prodrug moieties such as those mimicking triglycerides (TG). We now report a study in which a lipophilic prodrug approach was used to efficiently deliver bexarotene (BEX) and retinoic acid (RA) to the intestinal lymphatic system using activated ester prodrugs. A range of carboxylic ester prodrugs of BEX were designed and synthesised and all of the esters showed improved association with chylomicrons, which indicated an improved potential for delivery to the intestinal lymphatic system. The conversion rate of the prodrugs to BEX was the main determinant in delivery of BEX to the intestinal lymphatics, and activated ester prodrugs were prepared to enhance the conversion rate. As a result, an 4-(hydroxymethyl)-1,3-dioxol-2-one ester prodrug of BEX was able to increase the exposure of the mesenteric lymph nodes (MLNs) to BEX 17-fold compared to when BEX itself was administered. The activated ester prodrug approach was also applied to another drug, RA, where the exposure of the MLNs was increased 2.4-fold through the application of a similar cyclic activated prodrug. Synergism between BEX and RA was also demonstrated in vitro by cell growth inhibition assays using lymphoma cell lines. In conclusion, the activated ester prodrug approach results in efficient delivery of drugs to the intestinal lymphatic system, which could benefit patients affected by a large number of pathological conditions.

Autophagy modulation: a prudent approach in cancer treatment?

Autophagy is a tightly controlled process comprising lysosomal degradation and recycling of cellular proteins and organelles. In cancer, its paradoxical dual role of cytoprotection and cytotoxicity is context-dependent and controversial. Autophagy primarily acts as a mechanism of tumour suppression, by maintenance of genomic integrity and prevention of proliferation and inflammation. This, combined with immune-surveillance capabilities and autophagy's implicated role in cell death, acts to prevent tumour initiation. However, established tumours exploit autophagy to survive cellular stresses in the hostile tumour microenvironment. This can lead to therapy resistance, one of the biggest challenges facing current anti-cancer approaches. Autophagy modulation is an exciting area of clinical development, attempting to harness this fundamental process as an anti-cancer strategy. Autophagy induction could potentially prevent tumour formation and enhance anti-cancer immune responses. In addition, drug-induced autophagy could be used to kill cancer cells, particularly those in which the apoptotic machinery is defective. Conversely, autophagy inhibition may help to sensitise resistant cancer cells to conventional chemotherapies and specifically target autophagy-addicted tumours. Currently, hydroxychloroquine is in phase I and II clinical trials in combination with several standard chemotherapies, whereas direct, deliberate autophagy induction remains to be tested clinically. More comprehensive understanding of the roles of autophagy throughout different stages of carcinogenesis has potential to guide development of novel therapeutic strategies to eradicate cancer cells.

Sustainable Syntheses of (-)-Jerantinines A & E and Structural Characterisation of the Jerantinine-Tubulin Complex at the Colchicine Binding Site

The jerantinine family of Aspidosperma indole alkaloids from Tabernaemontana corymbosa are potent microtubule-targeting agents with broad spectrum anticancer activity. The natural supply of these precious metabolites has been significantly disrupted due to the inclusion of T. corymbosa on the endangered list of threatened species by the International Union for Conservation of Nature. This report describes the asymmetric syntheses of (-)-jerantinines A and E from sustainably sourced (-)-tabersonine, using a straight-forward and robust biomimetic approach. Biological investigations of synthetic (-)-jerantinine A, along with molecular modelling and X-ray crystallography studies of the tubulin-(-)-jerantinine B acetate complex, advocate an anticancer mode of action of the jerantinines operating via microtubule disruption resulting from binding at the colchicine site. This work lays the foundation for accessing useful quantities of enantiomerically pure jerantinine alkaloids for future development.

Temozolomide analog PMX 465 downregulates MGMT expression in HCT116 colorectal carcinoma cells

The efficacy of temozolomide (TMZ) treatment for cancers is currently limited by inherent or the development of resistance, particularly, but not exclusively, due to the expression of the DNA repair enzyme O6-methylguanine-DNA methyltransferase (MGMT) in a significant proportion of tumors. We have found that TMZ analog C8-methyl imidazole tetrazine (PMX 465) displayed good anticancer activity against the colorectal carcinoma HCT116 cells which are MGMT-overexpressing and mismatch repair (MMR)-deficient. In this study, we found that PMX 465 could downregulate the expression of MGMT in HCT116 cells at the protein and mRNA levels. We found that PMX 465 could reduce MGMT expression by increasing the binding of wild-type p53 to the MGMT promoter and reducing the binding of Sp1 to the MGMT promoter.

Abstract P1-03-11: Withdrawn

This abstract was withdrawn by the authors.

Discovery of a highly active anticancer analogue of cardamonin that acts as an inducer of caspase-dependent apoptosis and modulator of the mTOR pathway

Cardamonin is a natural chalcone that has been shown to exhibit high anticancer activity. In an attempt to discover analogues of cardamonin with enhanced anticancer activity, 19 analogues were synthesized and tested against A549 and HK1 cell lines. Results of the MTS cell viability assay showed that several derivatives possessed cytotoxic activities that were several-fold more potent than cardamonin. SAR analysis showed the importance of the ketone and alkene groups for bioactivity, while substituting cardamonin's phenolic groups with more polar moieties resulted in activity enhancement. As part of the SAR study and further exploration of chemical space, the effect of metal coordination on cytotoxicity was also investigated, but it was only possible to successfully obtain the Cu (II) complex of cardamonin (19). Compound 19 was the most active analogue possessing IC₅₀ values of 13.2μM and 0.7μM against A549 and HK1 cells, corresponding to a 5- and 32-fold increase in activity, respectively. It was also able to significantly inhibit the migration of A549 and HK1 cells. Further mode of action studies have shown that the most active analogue, 19, induced DNA damage resulting in G2/M-phase cell- cycle arrest in both cell lines. These events further led to the induction of apoptosis by the compound via caspase-3/7 and caspase-9 activation, PARP cleavage and downregulation of Mcl-1 expression. Moreover, 19 inhibited the expression levels of p-mTOR and p-4EBP1, which indicated that it exerted its anticancer activity, at least in part, via inhibition of the mTOR signalling pathway.

Synthesis and growth-inhibitory activities of imidazo[5,1-d]-1,2,3,5-tetrazine-8-carboxamides related to the anti-tumour drug temozolomide, with appended silicon, benzyl and heteromethyl groups at the 3-position

A series of 3-(benzyl-substituted)-imidazo[5,1-d]-1,2,3,5-tetrazines (13) and related derivatives with 3-heteromethyl groups has been synthesised and screened for growth-inhibitory activity in vitro against two pairs of glioma cell lines with temozolomide-sensitive and -resistant phenotypes dependent on the absence/presence of the DNA repair protein O⁶-methylguanine-DNA methyltransferase (MGMT). In general the compounds had low inhibitory activity with GI₅₀ values >50 μM against both sets of cell lines. Two silicon-containing derivatives, the TMS-methylimidazotetrazine (9) and the SEM-analogue (10), showed interesting differences: compound (9) had a profile very similar to that of temozolomide with the MGMT+ cell lines being 5 to 10-fold more resistant than MGMT- isogenic partners; the SEM-substituted compound (10) showed potency across all cell lines irrespective of their MGMT status.

In Vitro Antitumor Effects of AHR Ligands Aminoflavone (AFP 464) and Benzothiazole (5F 203) in Human Renal Carcinoma Cells

We investigated activity and mechanism of action of two AhR ligand antitumor agents, AFP 464 and 5F 203 on human renal cancer cells, specifically examining their effects on cell cycle progression, apoptosis, and migration. TK-10, SN12C, Caki-1, and ACHN human renal cancer cell lines were treated with AFP 464 and 5F 203. We evaluated cytotoxicity by MTS assays, cell cycle arrest, and apoptosis by flow cytometry and corroborated a mechanism of action involving AhR signal transduction activation. Changes in migration properties by wound healing assays were investigated: 5F 203-sensitive cells show decreased migration after treatment, therefore, we measured c-Met phosphorylation by Western blot in these cells. A 5F 203 induced a decrease in cell viability which was more marked than AFP 464. This cytotoxicity was reduced after treatment with the AhR inhibitor α-NF for both compounds indicating AhR signaling activation plays a role in the mechanism of action. A 5F 203 is sequestered by TK-10 cells and induces CYP1A1 expression; 5F 203 potently inhibited migration of TK-10, Caki-1, and SN12C cells, and inhibited c-Met receptor phosphorylation in TK-10 cells. AhR ligand antitumor agents AFP 464 and 5F 203 represent potential new candidates for the treatment of renal cancer. A 5F 203 only inhibited migration of sensitive cells and c-Met receptor phosphorylation in TK-10 cells. c-Met receptor signal transduction is important in migration and metastasis. Therefore, we consider that 5F 203 offers potential for the treatment of metastatic renal carcinoma. J. Cell. Biochem. 118: 4526-4535, 2017. © 2017 Wiley Periodicals, Inc.

Jerantinine A induces tumor-specific cell death through modulation of splicing factor 3b subunit 1 (SF3B1)

Precursor mRNA (pre-mRNA) splicing is catalyzed by a large ribonucleoprotein complex known as the spliceosome. Numerous studies have indicated that aberrant splicing patterns or mutations in spliceosome components, including the splicing factor 3b subunit 1 (SF3B1), are associated with hallmark cancer phenotypes. This has led to the identification and development of small molecules with spliceosome-modulating activity as potential anticancer agents. Jerantinine A (JA) is a novel indole alkaloid which displays potent anti-proliferative activities against human cancer cell lines by inhibiting tubulin polymerization and inducing G2/M cell cycle arrest. Using a combined pooled-genome wide shRNA library screen and global proteomic profiling, we showed that JA targets the spliceosome by up-regulating SF3B1 and SF3B3 protein in breast cancer cells. Notably, JA induced significant tumor-specific cell death and a significant increase in unspliced pre-mRNAs. In contrast, depletion of endogenous SF3B1 abrogated the apoptotic effects, but not the G2/M cell cycle arrest induced by JA. Further analyses showed that JA stabilizes endogenous SF3B1 protein in breast cancer cells and induced dissociation of the protein from the nucleosome complex. Together, these results demonstrate that JA exerts its antitumor activity by targeting SF3B1 and SF3B3 in addition to its reported targeting of tubulin polymerization.

Design and Elaboration of a Tractable Tricyclic Scaffold To Synthesize Druglike Inhibitors of Dipeptidyl Peptidase-4 (DPP-4), Antagonists of the C-C Chemokine Receptor Type 5 (CCR5), and Highly Potent and Selective Phosphoinositol-3 Kinase δ (PI3Kδ) Inhibitors

A novel molecular scaffold has been synthesized, and its incorporation into new analogues of biologically active molecules across multiple target classes will be discussed. In these studies, we have shown use of the tricyclic scaffold to synthesize potent inhibitors of the serine peptidase DPP-4, antagonists of the CCR5 receptor, and highly potent and selective PI3K δ isoform inhibitors. We also describe the predicted physicochemical properties of the resulting inhibitors and conclude that the tractable molecular scaffold could have potential application in future drug discovery programs.

Cudraflavone C Induces Tumor-Specific Apoptosis in Colorectal Cancer Cells through Inhibition of the Phosphoinositide 3-Kinase (PI3K)-AKT Pathway

Cudraflavone C (Cud C) is a naturally-occurring flavonol with reported anti-proliferative activities. However, the mechanisms by which Cud C induced cytotoxicity have yet to be fully elucidated. Here, we investigated the effects of Cud C on cell proliferation, caspase activation andapoptosis induction in colorectal cancer cells (CRC). We show that Cud C inhibits cell proliferation in KM12, Caco-2, HT29, HCC2998, HCT116 and SW48 CRC but not in the non-transformed colorectal epithelial cells, CCD CoN 841. Cud C induces tumor-selective apoptosis via mitochondrial depolarization and activation of the intrinsic caspase pathway. Gene expression profiling by microarray analyses revealed that tumor suppressor genes EGR1, HUWE1 and SMG1 were significantly up-regulated while oncogenes such as MYB1, CCNB1 and GPX2 were down-regulated following treatment with Cud C. Further analyses using Connectivity Map revealed that Cud C induced a gene signature highly similar to that of protein synthesis inhibitors and phosphoinositide 3-kinase (PI3K)-AKT inhibitors, suggesting that Cud C might inhibit PI3K-AKT signaling. A luminescent cell free PI3K lipid kinase assay revealed that Cud C significantly inhibited p110β/p85α PI3K activity, followed by p120γ, p110δ/p85α, and p110α/p85α PI3K activities. The inhibition by Cud C on p110β/p85α PI3K activity was comparable to LY-294002, a known PI3K inhibitor. Cud C also inhibited phosphorylation of AKT independent of NFκB activity in CRC cells, while ectopic expression of myristoylated AKT completely abrogated the anti-proliferative effects, and apoptosis induced by Cud C in CRC. These findings demonstrate that Cud C induces tumor-selective cytotoxicity by targeting the PI3K-AKT pathway. These findings provide novel insights into the mechanism of action of Cud C, and indicate that Cud C further development of Cud C derivatives as potential therapeutic agents is warranted.

Correction: Development of a series of bis-triazoles as G-quadruplex ligands

Correction for ‘Development of a series of bis-triazoles as G-quadruplex ligands’ by Maysaa M. Saleh et al., RSC Adv., 2017, 7, 47297–47308.