Evaluation of the antitumour activity of novel flavonoids on pre-clinical models of breast and ovarian cancer

Abstract

New drugs are needed for better cancer management. Clinical trials are currently underway to assess the use of flavonoids (natural polyphenols) as anticancer agents. Among them, myricetin has been shown to induce cell cycle arrest and apoptosis in pre-clinical cancer models. We hypothesised that myricetin-derived novel flavonoids designed to enhance this natural potential and improve on the drug-likeness limitations of myricetin might have increased potential for their application in the management of breast and ovarian cancer. The effect of a library of novel flavonoids was screened on 3 panels of breast and ovarian cancer cell lines, representing different molecular subtypes and phenotypes, to assess their potency. The second-generation bi-methoxylated analogue AO-1530-OMe (Oncamex) was identified as the most effective candidate in the library, with sub-micromolar concentrations exerting a strong antiproliferative effect across almost all models studied. Results suggested that changes in the hydroxylation profile, the addition of methoxylations and a decyl alkyl chain were some of the structure-activity relationships contributing to this improved efficacy. Plate assays showed 8 h treatment with Oncamex reduced cell viability and induced cytotoxicity and apoptosis, concomitant with caspase activation and PARP cleavage. Pre-incubation with an antioxidant partially blocked these effects, suggesting the possible involvement of ROS modulation in the mechanism of action of Oncamex. Fluorescence microscopy reported the quick and stable delivery of Oncamex to the mitochondria. Fluorescent probes showed that Oncamex can induce mitochondrial superoxide production at concentrations associated with its antiproliferative effects. Study of the electrochemical properties of Oncamex by cyclic voltammetry supported this. Differential gene expression analysis following a microarray experiment showed Oncamex induces changes in the expression of genes controlling cell cycle and apoptosis. Together with previous results, the findings from this analysis led to the postulation of a model for the mechanism of action of Oncamex: due to its enhanced reactivity and mitochondrial targeting, Oncamex can generate mitochondrial superoxide, leading to mitochondrial dysfunction, membrane permeabilisation and the activation of the JNK pathway and the transcription factor FOXO3, which together contribute to the induction of intrinsic apoptosis and the inhibition of proliferation. Further proliferation assays on cell culture models also reported enhanced effect of Oncamex when administered in combination with paclitaxel and TRAIL. These improved responses were observed in breast and ovarian cancer models, including cells lines characterised by their treatment-resistant phenotype. Cotreatment with Oncamex also improved the effect of tamoxifen on anti-oestrogen resistant LCC9 breast cancer cells. Results from preliminary in vivo studies in mice implanted with the MDA-MB-231 breast cancer xenograft were consistent with an antiproliferative effect of Oncamex (25mg/kg/day) in vivo, as treatment inhibited tumour growth and reduced the expression of the marker of proliferation Ki-67 without signs of systemic toxicity. Tissues from this experiment also allowed for preliminary in vivo validation of the proposed mechanism of action of Oncamex by immunohistochemistry. The in vivo cytostatic effect of Oncamex was confirmed in a second in vivo experiment, which also investigated the effect of Oncamex at higher doses or in combination with paclitaxel. In conclusion, the novel flavonoid Oncamex has shown a promising antiproliferative effect in pre-clinical models of breast and ovarian cancer, including models of treatment-resistant cancers. Preliminary in vivo studies have demonstrated a partial recapitulation of the effect of Oncamex. A mechanistic model has been proposed by which Oncamex induces intrinsic apoptosis through its redox reactivity and mitochondrial targeting. These results support the potential of this prototypic candidate, although possible work in the structure and formulation of this candidate and further study and validation of its mechanism of action is needed for its continued development as an anticancer agent.