Allocation of TSPO-PET signals to cellular and extracellular compartments in dependence of blood-brain barrier disruption in experimental glioblastoma

Abstract

Abstract Background Translocator protein (TSPO)-PET imaging facilitates monitoring of glioblastoma in preclinical models and patients. However, specificity of TSPO-PET signals remains to be investigated. In this study, we aimed to decipher exact contributions of cellular and extracellular compartments including the impact of blood-brain barrier (BBB) disruption to TSPO-PET signals in an experimental glioblastoma mouse model. Methods Mice with implanted glioblastoma (SB28; n = 36 early-stage, n = 39 late-stage; GFP(+)) were injected with the TSPO tracer [ 18 F]GE-180 (21 MBq ± 1.5 MBq). Dynamic 60 min microPET scans were performed with and without prior blocking by excessive cold radiotracer. Tumors were dissociated, followed by cell sorting of tumor cells, tumor-associated microglia/macrophages (TAMs, CD11b(+)) and non-tumor/non-TAM (i.e. remaining) cells with subsequent gamma emission measures and flow cytometry (scRadiotracing) to calculate radioactivity per single cell. PET signals and single-cell tracer uptake in tumors and non-lesional hemispheres were compared between blocked and unblocked conditions. N = 10 mice were intravenously injected with fluorescent Dextran to investigate BBB disruption by confocal microscopy in correlation with TSPO-PET signals of the same animals. Results TSPO-PET indicated strong but incomplete signal reduction after blocking in tumors (early-stage: -72%; late-stage: -59%, p < 0.001) and contralateral hemispheres (early stage: -65%, late-stage: -63%, p < 0.001) compared to unblocked animals. We found nearly complete blocking of radiotracer uptake across all analyzed cell fractions (tumor cells: -95%, p = 0.0039; TAMs: -98%, p < 0.0001; remaining cells: -99%, p = 0.0033) compared to unblocked mice, regardless of tumor stages. There was a significant correlation of PET signal reduction ( R =-0.824, p = 0.0034) as well as residual PET signal upon blocking ( R = 0.809, p = 0.0046) with Dextran intensity. Conclusion Specificity of TSPO-PET signals in experimental glioblastoma reaches 72% in early-stage tumors, but decreases to 59% in late-stage SB28 tumors, due to progressive contributions of BBB disruption. Non-specific TSPO tracer uptake is driven by BBB disruption and fully allocated to the extracellular compartment, whereas cellular TSPO tracer uptake shows strong specificity.

The authors' abstract, as published at the source. European Journal of Nuclear Medicine and Molecular Imaging, 2026 · DOI ↗