Abstract LB-233: Combination of metformin and valproic acid in personalized prostate cancer treatment: the role of p53 and androgen receptor signaling
Authors: Tran Linh N.k , Kichenadasse Ganessan , Morel Katherine L. , Ormsby Rebecca J. , Butler Lisa M. , Centenera Maggie M. , Sykes Pamela J. 
Published: July 2016

The anti-diabetic drug Metformin (MET) has been shown to reduce the incidence of prostate cancer, while the anti-epileptic drug Valproic Acid (VPA) has also been shown to have an anti-tumor effect. Here we aimed to determine if the combination of MET and VPA would induce at least an additive anti-tumour effect in prostate cancer due to their different mechanisms of action, and whether common mutations in prostate cancer (e.g. TP53 and AR genes) would affect the response.

In in vitro experiments, the proliferation and apoptosis responses of LNCaP (p53+, AR+), PC-3 (p53null, ARmut), DUCaP (p53mut, AR+) and Prostate Epithelial Cell (PrEC) lines to MET and/or VPA was assessed using an IncuCyte® FLR system. The Drug Combination Index (CI) was calculated using the Chou-Talalay method. mRNA knock-down of TP53 in LNCaP was performed using siRNA. p53 plasmid was transfected into PC3 using Lipofectamine 3000 and cell death was investigated using nuclear fragmentation index and cleaved-caspase-3 intensity in an Operetta® High Content Imaging System. AR was chemically inhibited using Enzalutamide. In a patient derived prostatic explant model, cell proliferation and apoptosis were determined by Ki67 and cleaved-caspase 3 immunohistochemistry.

The combination of MET+VPA synergistically inhibited proliferation in LNCaP (CI = 0.5) and PC-3 (CI = 0.6), with only an additive inhibition in DUCaP. The combination of MET and VPA also induced a significant decrease in Ki67 and increase in cleaved-caspase 3 compared to vehicle control in the human prostatic explant model (n = 7). Only LNCaP demonstrated a significantly higher (12-fold) apoptosis response to MET+VPA including at concentrations lower than the human toxic threshold (0.5 mM MET + 2.5 mM VPA), whereas the difference was not significant in PC3, DUCaP and PrEC cells. Knock-down of p53 in LNCaP reduced the synergistic apoptotic response, while maintaining the same anti-proliferative effect. p53 plasmid transfection into PC3 induced increased cell death in response to Met+VPA. AR inhibition in LNCaP reduced the synergistic apoptosis response and increased the anti-proliferative effect. AR inhibition had no effect in PC-3, an androgen-resistant cell line.

The concentration of MET and VPA used here are below the human plasma toxic threshold and may be a useful treatment for prostate cancer, particularly in tumours with functional p53 and/or AR signalling. As both drugs are currently used for non-cancer indications, repurposing of these drugs could be rapidly integrated into clinical use. Future functional studies include investigation of phosphorylation and acetylation changes of the p53 protein, and confirmation of the results in xenograft mice and a phase I clinical trial.

Citation Format: Linh N.k Tran, Ganessan Kichenadasse, Katherine L. Morel, Rebecca J. Ormsby, Lisa M. Butler, Maggie M. Centenera, Pamela J. Sykes. Combination of metformin and valproic acid in personalized prostate cancer treatment: the role of p53 and androgen receptor signaling. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr LB-233.