Time Domian Electron Paramagnetic Resonance Imaging

Division Of Basic Sciences - Nci

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Abstract

Dynamic range extension of pO2 imaging: In the preceding research, we examined and confirmed the utility of spin probe Ox071, deuterated version of Ox063, for both R1 and R2* based EPR oximetry. Due to the narrow line width and slower signal decay, it was expected that pO2 estimation using Ox071 is suitable not only in hypoxic tissue but also in less hypoxic tissue such as solid organs. In this work, we present our first 3D in vivo EPR oximetry study using Ox071 in comparison with Ox063 oximetry. The R2* change with [Ox071] and pO2 was calibrated using standard phantom solutions at 1,2,5,10 mM and 0, 2, 5, 10, 21%, respectively. In vivo EPR imaging of a mouse bearing MIA Paca-2 tumor was performed on successive days by using either Ox071 or Ox063. The spin density, pO2 maps, and pO2 histograms in the tumor regions marked by co-registration with MRI were similar between Ox063 and Ox071. Healthy kidney imaging was also performed on athymic mice using both Ox071 and Ox063. Ox071 oximetry showed more homogeneous pO2 profile in kidney compared with Ox063, suggesting that Ox071 is suitable for oximetry in tissue at higher pO2 range. Tumor microenvironment determinants in evofosfamide efficacy: Pancreatic ductal adenocarcinomas (PDACs) form hypovascular and hypoxic tumors which are difficult to treat with current chemotherapy regimens. Gemcitabine (GEM) is often used as a first line treatment for PDACs, but has issues with chemoresistance and penetration in the interior of the tumor. Evofosfamide, a hypoxia activated prodrug, has been shown to be effective in combination with GEM, although the mechanism of each drug on the other has not been established. We used two mouse xenografts from two cell lines (MIA Paca-2 and SU 86.86) with different tumor microenvironmetal characteristics to probe the action of each drug on the other. GEM treatment enhanced survival times in mice with SU.86.86 xenografts (HR =0.35, 95% CI=0.13 to 0.90 p=0.03) but had no effect on MIA Paca-2 mice (HR =0.91, 95% CI=0.37 to 2.25, p=0.84). Conversely, evofosfamide had no effect on SU86.86 mice and did not improve survival times to a statistically significant degree (HR=0.57, 95% CI=0.23 to 1.42, p=0.22). In MIA Paca-2 tumors, which were initially poorly perfused, electron paramagnetic resonance (EPR) imaging showed that oxygenation worsened when treated with GEM, providing a direct mechanism for the activation of evofosfamide by GEM and the effectiveness of evofosfamide and GEM combinations. Sublethal amounts of either treatment enhanced the toxicity of other treatment in vitro in Su86.86 but not in MIAPaca-2. Repair of double stranded DNA lesions was enhanced in the combination treatment in Su86.86 but not MIA Paca-2. A possible mechanism for the synergy between evofosfamide and GEM has been proposed. The synergy between GEM and evofosfamide appears to stem from the dual action of GEM's effect on tumor vasculature and the GEM inhibition of the homologous recombination DNA repair process. The relative importance of each pathway is dependent on the tumor microenvironment and merits further study. The hypoxia activated prodrug Evofosfamide improves tumor oxygenation. Tumors have regions with low. Levels of oxygen called hypoxic zones, These regions are resistant to radiation therapy and chemotherapy. Hypoxia activated prodrugs such as Evofosfamide are developed to specifically kill cells in hypoxic regions. In hypoxic tumor microenvironments, the strongly reducing redox state converts evofosfamide (TH-302) to a reduced form and releases a cytotoxic bromo-isophosphoramide (Br-IPM) moiety. This drug therefore preferentially attacks hypoxic regions in tumors where other standard anti-cancer treatments such as chemotherapy and radiation therapy are often ineffective. Various combination therapies with evofosfamide have been proposed and tested in preclinical and clinical settings. However, the treatment effect of evofosfamide monotherapy on tumor hypoxia has not been fully understood, partly due to the lack of quantitative methods to assess tumor pO2 in vivo. Here, we use quantitative pO2 imaging by EPR to evaluate the change in tumor hypoxia in response to evofosfamide treatment using two pancreatic ductal adenocarcinoma xenograft models; MIA Paca-2 tumors responding to evofosfamide and Su.86.86 tumors which do not respond. EPR imaging showed oxygenation improved globally after evofosfamide treatment in hypoxic MIA Paca-2 tumors, in agreement with the ex vivo results obtained from hypoxia staining by pimonidazole and in apparent contrast to the decrease in Ktrans observed in DCE MRI. The observation that evofosfamide not only kills the hypoxic region of the tumor but also improves oxygenation in the residual tumor regions provides a rationale for combination therapies using radiation and anti-proliferatives post evofosfamide for improved outcomes.