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

Abstract

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 microPET system. Dynamic images were acquired simultaneously upon injection of [18F]DPA-714 (130 – 200 MBq/0.2 mL). Arterial blood was collected to derive the input function (AIF), with HPLC radiometabolite analysis performed upon select 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 excess DPA-714 in vivo. Immediately following 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 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]PBR06, and agreed with ex vivo assay of TSPO protein levels in tumor and healthy brain. Conclusions 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.

The Translocator Protein Radioligand 18F-DPA-714 Monitors Antitumor Effect of Erufosine in a Rat 9L Intracranial Glioma Model

“…As TSPO expression in glioma has been correlated to malignancy, proliferation, apoptosis, and survival, it was explored as a prognostic marker (6)(7)(8). Here, PET imaging of TSPO with 18 F-DPA-714, a newly proposed marker for glioma detection with rapid blood-brain barrier penetration (10,12,13), was implemented and used for the assessment of treatment efficacy.…”

“…Our group and others have successfully shown highly specific TSPO imaging using 18 F-DPA-714 in rat models of brain cancer and demonstrated its potential as a promising imaging marker for glioma (12,13). In addition, several studies have reported a correlation between TSPO protein levels measured using Western blot and TSPO radioligand binding ( 3 H-PK11195, 18 F-DPA-714, 18 F-PBR06) in rat models of glioma and multiple sclerosis (11,(13)(14)(15).…”

“…Given the reported ability of 18 F-DPA-714 to accurately reflect TSPO density in normal or diseased brain (11,13), we investigated its performance to noninvasively and quantitatively assess treatment efficacy of ErPC3 in vivo in an orthotopic 9L glioma model. In addition, histologic and immunohistochemical analyses were used to evaluate the changes in tumor volume, intratumoral level of TSPO expression, and presence of TSPO-positive microglia/macrophages and astrocytes in ErPC3-and mock-treated animals.…”

The 18-kDa Mitochondrial Translocator Protein in Human Gliomas: An 11C-(R)PK11195 PET Imaging and Neuropathology Study

“…The evidence that the single nucleotide polymorphism in exon 4 of the TSPO gene affects the binding of second-generation ligands (11) led us to opt for 11 C-(R)PK11195 (1-(2-chlorophenyl)-N-methyl-N-(1-methylpropyl)-3-isoquinoline carboxamide). In addition, most second-generation TSPO ligands have been used only in experimental animals and are not fully characterized in humans (12)(13)(14). In our previous study, we have demonstrated that 11 C-(R)PK11195 is a suitable radiotracer for glioma imaging (15).…”

TSPO PET using 18F-GE-180: a new perspective in neurooncology?

“…Further studies, however, were hampered by partially nonspecific binding and low tumour-to-brain contrast of 11 C-PK11195. In recent years, the development of so-called second generation TSPO ligands with lower nonspecific binding characteristics has initiated more preclinical studies exploiting the utility of TSPO imaging for diagnosis of brain tumours [4,5]. Unfortunately, these new ligands are sensitive to a single-nucleotide polymorphism (rs6971 polymorphism, A147T) which is present in the human, but not the rodent, TSPO gene, resulting in variable binding affinities to the protein [6].…”