Time Domian Electron Paramagnetic Resonance Imaging

A)肿瘤微环境的多模式代谢和生理评估：肿瘤微环境是选择治疗和治疗结果的重要决定因素。用于肿瘤代谢和生理成像的分子成像方法已经成为治疗计划和反应监测的重要手段。然而，肿瘤的生理和代谢方面之间的关系还没有完全了解。18F-脱氧葡萄糖正电子发射断层扫描(18FDG)被广泛用于识别体内的高代谢区域。18FDG在高表达GLT转运蛋白的细胞中转运，这在恶性肿瘤中是典型的，并使用己糖激酶捕获在细胞中。这是临床上广泛使用的一种依靠GLT转运蛋白和己糖激酶活性来识别恶性肿瘤的方法。肿瘤也具有高水平的LDHA活性。使用超极化13C标记丙酮酸的13C磁共振成像通过监测其向乳酸的转化率来检测恶性肿瘤。因此，这种方法依赖于单羧酸转运体和LDHA活性。在这项研究中，使用明确的胰腺导管腺癌异种移植模型，我们使用了超极化MRI和电子顺磁共振成像程序，可以更直接地定量评估肿瘤的糖酵解和氧合状态。我们研究了三种不同生理和代谢特征的人胰腺导管癌细胞异种移植瘤中缺氧、葡萄糖摄取和糖酵解之间的空间关系。在实体瘤水平，18F-FDG-PET与乳酸生成呈显著正相关，而PO2与乳酸生成及18F-2-氟-2-脱氧-D-葡萄糖(18F-FDG)摄取呈负相关。然而，整个肿瘤的新陈代谢并不是一致的，整个肿瘤结果掩盖了成像时变得明显的不同定位。18F-FDG摄取与肿瘤中心的氧分压呈负相关，与周边的氧分压呈正相关。与PO2和18F-FDG摄取相反，乳酸脱氢酶活性在整个肿瘤中分布相对均匀。每种测量方法所揭示的异质性表明，多模式分子成像方法可以改善肿瘤的特征，潜在地导致癌症治疗中更好的预后。B)低氧激活的前体药物依福福胺改善肿瘤的氧合。肿瘤有低位的区域。氧气水平被称为缺氧区，这些区域对放射治疗和化疗具有抵抗力。低氧激活的前体药物，如依福福胺，是为了特异性地杀死缺氧区域的细胞而开发的。在缺氧的肿瘤微环境中，强烈还原的氧化还原状态将依福福胺(TH-302)转化为还原形式，并释放出细胞毒性的溴异磷酰胺(BR-IPM)部分。因此，这种药物优先攻击肿瘤中的缺氧区，而其他标准的抗癌治疗，如化疗和放射治疗，往往无效。已经提出了各种与依福福胺的联合治疗方案，并在临床前和临床环境中进行了测试。然而，单用依福福胺对肿瘤缺氧的治疗效果尚不完全清楚，部分原因是缺乏定量方法来评估体内肿瘤的PO2。在这里，我们使用EPR的pO2定量成像，使用两个胰腺导管腺癌异种移植模型来评估肿瘤缺氧的变化；MIA Paca-2肿瘤对Eofosamide有反应，Su.86.86肿瘤没有反应。EPR成像显示，在低氧MIA Paca-2肿瘤治疗后，氧合功能总体上得到改善，这与体外低氧染色结果一致，与DCE MRI观察到的Ktras降低形成明显对比。观察到依福福胺不仅杀死了肿瘤的缺氧区，而且还改善了残留肿瘤区域的氧合作用，这为在依福福胺后使用放射和抗增殖药物以改善结果提供了理论基础。

a) Multi-modal metabolic and physiologic assessment of tumor microenvironment: Tumor microenvironment is an important determinant in the choice of treatments and treatment outcomes. Molecular imaging approaches for metabolic and physiologic imaging of tumors have become important for treatment planning and response monitoring. However, the relationship between the physiologic and metabolic aspects of tumors is not fully understood. Positron Emission Tomography using 18F-depxyglucose (18FDG) is widely used to identify hypermetabolic regions in vivo. 18FDG is transported in cells with high expression of GLT transporters typical in malignancies and trapped in cells using hexokinase. This is one of the widely used clinical method to identify malignancies relying on GLT transporters and hexokinase activity. Tumors also have high levels of LDHA activity. 13C MRI using hyperpolarized 13C labeled pyruvate is used to detect malignancies by monitoring its conversion rates to lactate. Thus this method relies on monocarboxylate transporters and LDHA activity. In this study, using well defined pancreatic ductal adenocarcinoma xenograft models Here, we used hyperpolarized MRI and electron paramagnetic resonance imaging procedures that allow more direct assessment of tumor glycolysis and oxygenation status quantitatively. We investigated the spatial relationship between hypoxia, glucose uptake, and glycolysis in three human pancreatic ductal adenocarcinoma tumor xenografts with differing physiologic and metabolic characteristics. At the bulk tumor level, there was a strong positive correlation between 18F-FDG-PET and lactate production, while pO2 was inversely related to lactate production and 18F-2-fluoro-2-deoxy- D-glucose (18F-FDG) uptake. However, metabolism was not uniform throughout the tumors, and the whole tumor results masked different localizations that became apparent while imaging. 18F-FDG uptake negatively correlated with pO2 in the center of the tumor and positively correlated with pO2 on the periphery. In contrast to pO2 and 18F-FDG uptake, lactate dehydrogenase activity was distributed relatively evenly throughout the tumor. The heterogeneity revealed by each measure suggests a multi- modal molecular imaging approach can improve tumor characterization, potentially leading to better prognostics in cancer treatment. b) 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.