Quantitative preclinical imaging of TSPO expression in glioma using N,N-diethyl-2-(2-(4-(2- ¹⁸F-fluoroethoxy) phenyl)-5,7- dimethylpyrazolo[1,5-a]pyrimidin-3-yl)acetamide

There is a critical need to develop and rigorously validate molecular imaging biomarkers to aid diagnosis and characterization of primary brain tumors. Elevated expression of translocator protein (TSPO) has been shown to predict disease progression and aggressive, invasive behavior in a variety of solid tumors. Thus, noninvasive molecular imaging of TSPO expression could form the basis of a novel, predictive cancer imaging biomarker. In quantitative preclinical PET studies, we evaluated a high-affinity pyrazolopyrimidinyl-based TSPO imaging ligand, N,N-diethyl-2-(2-(4-(2- ¹⁸F-fluoroethoxy)phenyl)- 5,7- dimethylpyrazolo[1,5-a]pyrimidin-3-yl)acetamide ( ¹⁸F-DPA-714), as a translational probe for quantification of TSPO levels in glioma. Methods: Glioma-bearing rats were imaged with ¹⁸F-DPA- 714 in a small-animal PET system. Dynamic images were acquired simultaneously on injection of ¹⁸F-DPA-714 (130-200 MBq/0.2 mL). Blood was collected to derive the arterial input function (AIF), with high-performance liquid chromatography radiometabolite analysis performed on selected samples for AIF correction. Compartmental modeling was performed using the corrected AIF. Specific tumor cell binding of DPA-714 was evaluated by radioligand displacement of ³H-PK 11195 with DPA-714 in vitro and displacement of ¹⁸F-DPA-714 with an excess of DPA-714 in vivo. Immediately after imaging, tumor and healthy brain tissues were harvested for validation by Western blotting and immunohistochemistry. Results: ¹⁸F-DPA-714 was found to preferentially accumulate in tumors, with modest uptake in the contralateral brain. Infusion with DPA-714 (10 mg/kg) displaced ¹⁸F-DPA-714 binding by greater than 60% on average. Tumor uptake of ¹⁸F-DPA- 714 was similar to another high-affinity TSPO imaging ligand, ¹⁸F-N-fluoroacetyl-N-(2,5-dimethoxybenzyl)-2-phenoxyaniline, and agreed with ex vivo assay of TSPO levels in tumor and healthy brain. Conclusion: These studies illustrate the feasibility of using ¹⁸F-DPA-714 for visualization of TSPO-expressing brain tumors. Importantly, ¹⁸F-DPA-714 appears suitable for quantitative assay of tumor TSPO levels in vivo. Given the relationship between elevated TSPO levels and poor outcome in oncology, these studies suggest the potential of ¹⁸F-DPA-714 PET to serve as a novel predictive cancer imaging modality.