Abstract C90: An MRI and histological investigation of the acute response of orthotopic PC3 prostate tumors to the HIF pathway inhibitor NSC-134754 in vivo

Abstract

Abstract Solid tumors typically exhibit hypoxic regions which impact negatively on chemotherapy and radiotherapy. A key player in cellular adaptation to the hypoxic microenvironment is the transcription factor hypoxia inducible factor-1 (HIF-1) [1]. HIF-1 target genes mediate a wide range of actions promoting changes to cell metabolism, angiogenesis, proliferation, invasion and metastasis, hence HIF pathway inhibition represents an attractive therapeutic strategy. Recently, the small molecule HIF pathway inhibitor NSC-134754 was identified from an in vitro screen of the NCI Diversity Set [2]. In this study we have investigated the acute response of orthotopic PC3 prostate tumors to NSC-134754 in vivo using magnetic resonance imaging (MRI). Ex vivo histological markers of tumor hypoxia and perfusion were also obtained, in addition to tumor VEGF, a pro-angiogenic factor downstream of HIF-1. Orthotopic prostate tumors were propagated by injecting 1×105 PC3 cells into the ventral prostate gland of male NCr nude mice. Once tumors reached approximately 1cm in diameter mice were administered with 100mg/kg NSC-134754 i.p. and MRI was performed 6 or 24 hours later. An untreated control group was also included. Diffusion-weighted MRI was used to determine the tumor apparent diffusion coefficient (ADC), sensitive to tissue cellularity, and dynamic contrast-enhanced MRI data acquired to provide estimates of Ktrans (dependent on vascular permeability, blood flow and volume) and ve (the fraction of extravascular, extracellular volume). At study end, mice were administered pimonidazole and Hoechst 33342, enabling the degree of hypoxia and perfused (functional) vasculature to be subsequently quantified by immunohistochemistry and fluorescence microscopy. VEGF expression in tumor homogenates was assayed by ELISA. A significant increase in ADC was observed 24 hours post treatment with NSC-134754 (6.6 ± 0.3×10−4mm2/s, n=6) compared to treated tumors evaluated 6 hours after treatment (5.8 ± 0.2×10−4mm2/s, n=7, p<0.03). No significant differences in Ktrans or ve were found. Tumor hypoxia was significantly increased 24 hours post treatment compared to controls (14.9% versus 8.8%, p<0.05). Tumor perfusion however remained unchanged between the treatment groups. A trend towards a reduction in tumor VEGF concentrations was observed 6 hours post-treatment, however due to intracohort variability this did not reach significance. The data suggest that ADC provides a non-invasive imaging biomarker of response 24 hours following treatment with NSC-134754 in orthotopic PC3 prostate tumors, is associated with an increase in tumor hypoxia and independent of any changes in tumor blood flow/vascular permeability. Histological analysis of cellular necrosis is currently being performed to provide qualification of the increased tumor ADC and evidence of anti-proliferative effects of NSC-134754. Citation Information: Mol Cancer Ther 2009;8(12 Suppl):C90.