NIMG-26. UPTAKE OF [18F]GE-180 IN TSPO PET IS ASSOCIATED WITH SURVIVAL IN PATIENTS WITH RECURRENT GLIOMA

OBJECTIVE

The 18 kDa translocator protein (TSPO) is expressed in both activated microglia and glioma cells. Elevated expression of TSPO has been reported to be associated with higher WHO grade. Here we analyze whether TSPO positron emission tomography (PET) signal using the tracer [18F]GE-180 is correlated with clinical outcome in a cohort of patients with recurrent glioma.

METHODS

Patients with suspected glioma recurrence received a [18F]GE-180 TSPO PET. All recurrent tumors were confirmed either by stereotactic biopsy or resection. Maximum standard uptake value SUVmax as well as tumor volume in MRI and, if available, in [18F]FET PET were evaluated together with patient characteristics (age, sex, Karnofsky-Performance score) and neuropathological features (WHO grade, IDH-mutation status). Uni- and multivariate Cox regression and Kaplan-Meier survival analyses were performed to identify prognostic factors for post-recurrence survival (PRS) and time to treatment failure (TTF).

RESULTS

88 consecutive patients were evaluated. TSPO tracer uptake correlated with tumor grade at recurrence (p< 0.05), with no significant differences between IDH-wildtype and IDH-mutant tumors. Within the subgroup of IDH-mutant glioma (n= 46), patients with low SUVmax (median split, ≤ 1.60) had a significantly longer PRS (median 41.6 vs. 25.3 months, p= 0.031) and TTF (32.2 vs 8.7 months, p= 0.001). Also among IDH-wildtype tumors (n= 42), patients with low SUVmax (≤ 1.89) had a significantly longer PRS (median not reached vs 8.2 months, p= 0.002). SUVmax remained an independent prognostic factor for PRS in a multivariate analysis including WHO grade, IDH status and age. Tumor volume defined by [18F]FET PET or contrast-enhanced MRI correlated weakly with TSPO tracer uptake. Treatment regimen did not differ between the subgroups.

CONCLUSION

Our data suggest that uptake of [18F]GE-180 in TSPO PET can add prognostic information in patients with recurrent glioma even in molecular defined subgroups and might serve as an imaging biomarker.

RBIO-05. QUANTIFICATION OF TSPO EXPRESSION AND TSPO RADIOLIGAND UPTAKE IN PET IN HUMAN AND MURINE GLIOBLASTOMA ON CELLULAR LEVEL

OBJECTIVE

Translocator protein (TSPO) PET imaging is used in different neurological diseases and emerges as new tool for the diagnosis and therapy planning of glioma patients. Various signal sources of the TSPO-PET signal, however, hamper image interpretation and biological understanding. Here we assess the TSPO expression and ligand uptake in a murine model and human tissue derived from glioma patients.

METHODS

Mice with implanted GFP+ glioblastoma (n=20) or after sham implantation (n=14) were injected with the TSPO tracer [18F]GE-180 and received PET imaging. Tumors were harvested and processed to single cell suspension. Subsequently, tumors were either stained for TSPO, CD11b, ACSA2, GFAP and analyzed by flowcytometry or else magnetic cell sorting was performed to isolate tumor and CD11b+ cells that were measured by gamma counter. Multiple regression was used to determine contributions of GFP, CD11b, ACSA2 and GFAP positive cells to the total uptake. In TSPO PET-scans of glioma patients, regions with different GE-180 uptake were defined. Tumor samples (n=12) of these predefined areas were collected during surgery and analyzed by FACS as described above.

RESULTS

The expression of TSPO and PET signal in mice was increased in the tumor compared to tumor-free brain (SUVRGBM: 2.06 / SUVRSHAM: 0.92). FACS detected TSPO in tumor cells, microglia/macrophages and astrocytes. Tumor cells showed higher tracer uptake per cell when compared to CD11b+ cells (+31%). The contribution to the whole TSPO tracer uptake was high for tumor (β=0.41, P=0.018) and microglia/macrophages (β=0.35, P=0.042), and lowest for astrocytes (β=0.28, P=0.011). Patients with high-grade glioma indicated higher TSPO tracer uptake in GFAP+ cells when compared to CD11b+ cells and single cell uptake correlated with PET signal.

CONCLUSION

In experimental orthotopic glioblastoma, we could detect TSPO tracer signal at cellular resolution level. TSPO-PET can provide information about tumor and tumor-associated myeloid cells.

NIMG-51. 18KDA TRANSLOCATOR PROTEIN (TSPO) DECLINES IN TUMOR AND IMMUNE CELLS DURING PROGRESSION OF EXPERIMENTAL GLIOBLASTOMA

Glioblastoma combines a lack of immunogenicity with a highly immunosuppressive tumor microenvironment (TME), including both tumor and immune cells. However, biomarkers that allow monitoring of the immune phenotype are still lacking. Hence, we investigated the 18kDa translocator protein (TSPO) during tumor progression in an experiential glioblastoma mouse model (SB28) mimicking human TME. We used TSPO-PET imaging ([18F]GE-180) and in vivo measures of single cell tracer uptake between days 6 and 18 after inoculation to study alterations and dependence of TSPO in tumor and peripheral organs in SB28 mice (n= 27) in comparison to sham controls (n= 11). CSF1R inhibition was applied to deplete tumor associated microglia/macrophages (TAM) followed by withdrawal to induce immune cell rebound (PLX5622 day -20 to day 6, n= 3). Compared to sham, TSPO-PET signals were distinctly elevated in tumor (+83%, p< 0.001), heart (+35%, p< 0.05), lung (+42%, p< 0.01) and bone (+26%, p< 0.05) of SB28 mice at day 6. TSPO-PET increases were lower at day 18 (tumor: +52%, p< 0.05; all organs < +15%, n.s.). The tumor TSPO-PET signal was strongly coupled with TSPO-PET signal in peripheral organs (all R≥ 0.88, all p< 0.001). Single tumor cells and TAM showed strong early increases of TSPO tracer uptake at day 6 (tumor: 32-fold, TAM: 8.5-fold) and a decline of these increases at day 18 (tumor: 2.4-fold, TAM: 1.7-fold) when compared to sham microglia. Immune cell rebound restored TSPO tracer uptake in TAM (+120%, p< 0.01) but not in tumor cells (-24%, n.s.) when compared to therapy-naïve SB28 mice. TSPO declines in tumor cells and TAM during progression of experimental glioblastoma. TSPO in peripheral organs could serve as a supportive indicator of declining immune response in glioblastoma. CSF1R inhibition and reinitiation yields specific restoring of TSPO in TAM and could serve as immunomodulatory therapy strategy.

Local progenitor cells shape the neoplastic vasculature and promote brain tumor growth

e14044

Background: Aggressive brain tumors like glioblastoma depend on support by their local environment. While the role of tumor-associated myeloid cells on glioblastoma progression is well-documented, we have only partial knowledge of the pathological impact of glioblastoma -parenchymal progenitor cells. Methods: We investigated the glioblastoma microenvironment with transgenic lineage-tracing models ( nestin-creER2, R26-tdTomato and sox2-creER2,R26-tdTomato), intravital imaging, single-cell transcriptomics, immunofluorescence and flow-cytometry as well as histopathology and characterized a previously unknown tumor-associated progenitor cell. In functional experiments, we studied the knockout of the transcription factor SOX2 in these tumor-associated cells. Results: Lineage-traced cells from mouse glioblastoma were obtained by flow-cytometry and single cell transcriptomes compared to established gene expression data from brain tumor parenchymal cells. The traced tumor-associated cells had a transcriptomic profile largely resembling myeloid cells and expressed microglia-/macrophage-markers on the protein-level. However, transgenic models and bone-marrow chimera revealed that the traced cells were clearly distinct from microglia or macrophages. The traced tumor associated cells with a myeloid expression profile derived from a SOX2-dependent progenitor cell. Consequently, conditional Sox2-knockout ablated the entire myeloid-like cell population. Remarkably, this tumor-associated cell population had a large impact on disease-progression causing significant reduction of glioblastoma –vascularization to 53%, changing vascular function and leading to a decrease in tumor volume to 42% as compared to controls. The myeloid-like progenitor cells were identified in human brain tumors by immunofluorescence and in scRNA-seq data. Conclusions: We identified a previously unacknowledged population of tumor-associated progenitor cells with a myeloid-like expression profile that transiently appeared during glioblastoma growth. These progenitors have strong impact on glioblastoma progression and point towards a new and promising therapeutic target in order to support anti-angiogenic regimen in glioblastoma.