Abstract 4742: Comparison of enhancement of the antitumor effect of CDDP by PXR antagonists

Abstract We have already found that a nuclear receptor PXR can modulate the cytotoxicity of CDDP for cancer cells, and ketoconazole, a PXT antagonist, can be an agent enhancing the antitumor activity of CDDP due to increase of the intracellular platinum content as the mechanism. However, nuclear receptor ligands have a variety of functions, and there is a possibility that PXR antagonist may enhance the cytotoxicity of CDDP through the mechanism other than the inhibition of transporter-mediated efflux of CDDP. The goal of this study was to compare the enhancing ability of three PXR antagonists, ketoconazole (KTZ), phenethyl isothiocyanate (PEITC) and metformin (MFM), for the antitumor effect of CDDP and clarify the contribution of inhibition of the efflux transporter as the mechanism of PXR antagonist. Cell lines used in this study were human hepatoma cell line HepG2. Changes in mRNA expression were assessed by real-time RT-PCR, and the ability of apoptosis induction was assessed as caspase-3 activity. Total intracellular platinum contents were determined by ICP-AES. When HepG2 cells were treated for 24 hr with 10 μM of CDDP, no change of caspase-3 activity was observed when compared to control cells. Also, 30 μM of KTZ, 30 μM of PEITC or 5 mM of MFM had no affect on the caspase-3 activity in treated cells compared with control cells. When KTZ, PEITC or MFM was exposed to HepG2 cells from 24 hr before treatment and during 24-hr treatment with 10 μM of CDDP, the caspase-3 activity was significantly increased to 461%, 355% and 326% of the control, respectively, in comparison with that in CDDP alone. When HepG2 cells were treated for 8 hr with 25 μM of CDDP, the intracellular platinum content was 110 ng/mg protein. On the other hand when KTZ, PEITC or MFM was exposed to HepG2 cells from 24 hr before treatment and during 8-hr treatment with 25 μM of CDDP, the intracellular platinum content was apparently increased to 227%, 491% and 161% of CDDP alone, respectively. Three PXR antagonists resulted in an increase in the intracellular platinum content that seems to be due to inhibition of the platinum efflux transporter, but there is no correlation between increase in intracellular platinum content and enhancement of antitumor activity by PXR antagonists. These results concluded that PXR antagonist can be an agent enhancing the antitumor activity of CDDP, but the ability of PXR antagonist for enhancing antitumor activity of CDDP can not be determined from only the increase in the intracellular platinum content. Citation Format: Shuichi Kishimoto, Megumi Yasuda, Megumi Tomita, Yuki Yamane, Ryosuke Suzuki, Shoji Fukushima. Comparison of enhancement of the antitumor effect of CDDP by PXR antagonists. [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 4742.