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-(18)F-fluoroethoxy)phenyl)-5,7-dimethylpyrazolo[1,5-a]pyrimidin-3-yl)acetamide ((18)F-DPA-714), as a translational probe for quantification of TSPO levels in glioma.

Methods: Glioma-bearing rats were imaged with (18)F-DPA-714 in a small-animal PET system. Dynamic images were acquired simultaneously on injection of (18)F-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 (3)H-PK 11195 with DPA-714 in vitro and displacement of (18)F-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: (18)F-DPA-714 was found to preferentially accumulate in tumors, with modest uptake in the contralateral brain. Infusion with DPA-714 (10 mg/kg) displaced (18)F-DPA-714 binding by greater than 60% on average. Tumor uptake of (18)F-DPA-714 was similar to another high-affinity TSPO imaging ligand, (18)F-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 (18)F-DPA-714 for visualization of TSPO-expressing brain tumors. Importantly, (18)F-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 (18)F-DPA-714 PET to serve as a novel predictive cancer imaging modality.

FIGURE 1

Chemical structure of DPA-714 (inset). Radioligand displacement of [³H]PK 11195 using DPA-714 in C6 glioma cell lysate (IC₅₀ = 10.9 nM).

FIGURE 2

(A) T₂-weighted MR image of C6 glioma-bearing rat in right hemisphere. (B) Transverse PET image obtained from dynamic scan of [¹⁸F]DPA-714 PET (summed dynamic scan, 0–90 min). (C) [¹⁸F]DPA-714 time–activity curves for tumor (blue), contralateral brain (green), and plasma (red). (D) Coronal PET image obtained from dynamic scan of [¹⁸F]DPA-714 PET (summed dynamic scan, 0–90 min). (E) Standard hematoxylin and eosin staining of serial tissue section. (F) Immunohistochemistry analysis of TSPO expression in typical C6 glioma. Tumor location is indicated with arrows in the images.

FIGURE 3

In vivo displacement of [¹⁸F]DPA-714 in C6 glioma–bearing rat. Relative [¹⁸F]DPA-714 uptake before (A) and after (B) intravenous infusion of excess DPA-714. (C) [¹⁸F]DPA-714 time–activity curves generated for tumor (blue), contralateral brain (green), and plasma (red).

FIGURE 4

Comparison of [¹⁸F]PBR06 and [¹⁸F]DPA-714 in the same glioma-bearing rat. (A) T₂-weighted MR image of rat bearing C6 glioma in right hemisphere. (B) [¹⁸F]PBR06 PET image (summed dynamic scan over the last 30 min). (C) [¹⁸F]DPA-714 PET image (summed dynamic scan over the last 30 min). (D) [¹⁸F]DPA-714 time–activity curves generated for tumor (blue), contralateral brain (green), and plasma (red). For comparison, [¹⁸F]PBR06 time–activity curves are shown in black.

FIGURE 5

Pharmacokinetic model fit of typical [¹⁸F]DPA-714 time-activity curves to 2-compartment, 2-kinetic parameter (A) and 3-compartment, 4-kinetic parameter models (B). Time–activity curves for tumor (blue), contralateral brain (green), and plasma (red) are shown with the associated model fit.

FIGURE 6

Representative graphical analysis of V_T for a subject from this investigation. The fit was carried out for normal brain (A) and for tumor (B). Solid grey line = Linear Regression; start time for linear regression, “t*”.