Preclinical TSPO Ligand PET to Visualize Human Glioma Xenotransplants: A Preliminary Study

<div><p>Current positron emission tomography (PET) imaging biomarkers for detection of infiltrating gliomas are limited. Translocator protein (TSPO) is a novel and promising biomarker for glioma PET imaging. To validate TSPO as a potential target for molecular imaging of glioma, TSPO expression was assayed in a tumor microarray containing 37 high-grade (III, IV) gliomas. TSPO staining was detected in all tumor specimens. Subsequently, PET imaging was performed with an aryloxyanilide-based TSPO ligand, [¹⁸F]PBR06, in primary orthotopic xenograft models of WHO grade III and IV gliomas. Selective uptake of [¹⁸F]PBR06 in engrafted tumor was measured. Furthermore, PET imaging with [¹⁸F]PBR06 demonstrated infiltrative glioma growth that was undetectable by traditional magnetic resonance imaging (MRI). Preliminary PET with [¹⁸F]PBR06 demonstrated a preferential tumor-to-normal background ratio in comparison to 2-deoxy-2-[¹⁸F]fluoro-D-glucose ([¹⁸F]FDG). These results suggest that TSPO PET imaging with such high-affinity radiotracers may represent a novel strategy to characterize distinct molecular features of glioma growth, as well as better define the extent of glioma infiltration for therapeutic purposes.</p></div

Side-by-side PET comparison of [¹⁸F]PBR06 and [¹⁸F]FDG in a primary human grade III astrocytoma xenotransplant.

<p>(A) T₂-weighted MRI (<i>coronal</i>) visualizes advanced tumor in the right hemisphere of the brain. (B) [¹⁸F]PBR06 uptake (<i>coronal</i>) is primarily confined to the tumor and co-localizes with tumor tissue visualized by MRI. Arrows indicate tumor and infiltration into left hemisphere. (C) [¹⁸F]FDG uptake (<i>coronal</i>) in normal brain is higher than tumor tissue, resulting in poor imaging contrast. (D) Correlative TSPO immunohistochemistry, Tumor + White Matter Tract (40X).</p

Primary human grade IV glioblastoma xenotransplant.

<p>(A) Dynamic PET image (<i>coronal</i>) of advanced tumor in the right hemisphere of the brain with [¹⁸F]PBR06 uptake confined primarily to the tumor. Arrows indicate tumor and infiltration into left hemisphere. (B) Time-activity curves of injected [¹⁸F]PBR06 in tumor (<i>green</i>) and contralateral brain (<i>blue</i>) [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0141659#pone.0141659.ref030" target="_blank">30</a>]. (C) Correlative TSPO immunohistochemistry, Tumor + White Matter Tract (40X).</p

[¹⁸F]PBR06 selectivity in primary human grade IV glioblastoma xenotransplant.

<p>(A) Dynamic PET image (axial) pre-infusion of cold analog, with [¹⁸F]PBR06 uptake primarily confined to the tumor. (B) Dynamic PET image (axial) post-infusion, showing nearly total displacement of [¹⁸F]PBR06. (C) Time-activity curves of injected [¹⁸F]PBR06 in tumor (<i>green</i>) and contralateral brain (<i>blue</i>). (D) Correlative TSPO immunohistochemistry, Tumor + White Matter Tract (40X).</p

Primary human grade III astrocytoma xenotransplant.

<p>(A) T₂-weighted MRI (coronal) visualizes advanced tumor in the right hemisphere of the brain. (B) Correlative dynamic PET image (coronal) of same advanced tumor in the right hemisphere of the brain, with [¹⁸F]PBR06 uptake primarily confined to the tumor and co-localizing with tumor tissue visualized by MRI. Top arrows indicate tumor and infiltration into left hemisphere. (C) Fused MRI (A) PET (B) image. (<b>D</b>) Time-activity curves of injected [¹⁸F]PBR06 in tumor (<i>green</i>) and contralateral brain (<i>blue</i>). Correlative vimentin immunohistochemistry: (E) Gross; (F) Tumor + White Matter Tract (40X); (G) Tumor (40X). (H) Correlative dynamic PET image, axial view along yellow line in (B); arrows indicate tumor and infiltration into left hemisphere. Correlative TSPO immunohistochemistry: (I) Gross; (J) Tumor + White Matter Tract (40X); (K) Tumor (40X). (L) Dynamic PET image (axial) of control cohort. Correlative CD68 immunohistochemistry: (M) Gross; (N) Tumor + White Matter Tract (40X); (O) Tumor (40X).</p

Human tumor microarrays (TMAs).

<p>Pilocytic astrocytoma TSPO immunohistochemistry (A) and TSPO immunohistochemical scoring (C). Glioblastoma multiforme TSPO immunohistochemistry (B) and TSPO immunohistochemical scoring (D).</p

Preclinical model of glioma: Human xenotransplantation into athymic rats.

<p>Primary human grade IV glioblastoma xenotransplant, vimentin immunohistochemistry: (A) Gross; (B) Pencil fibers (40X); (C) Tumor (40X).</p