Immuno-PET Imaging of Tumour PD-L1 Expression in Glioblastoma

Simple Summary Almost all patients with glioblastoma (GBM) eventually relapse, mainly due to adaptive and acquired resistance that results from tumour heterogeneity and its relatively immune-depleted (cold) microenvironment. High levels of programmed death ligand-1 (PD-L1) have been associated with GBM invasiveness and immuno-resistance. Presently, there is no standardised approach for the assessment of PD-L1 expression level that would help in predicting the response to immune checkpoint inhibitors. Therefore, we investigated the ability of a radiolabelled Z(PD-L1) affibody molecule to measure the expression level of PD-L1 in GBM xenograft models. There is no established method to assess the PD-L1 expression in brain tumours. Therefore, we investigated the suitability of affibody molecule (Z(PD-L1)) radiolabelled with F-18 ((AlF)-F-18) and Ga-68 to measure the expression of PD-L1 in xenograft mouse models of GBM. Mice bearing subcutaneous and orthotopic tumours were imaged 1 h post-radioconjugate administration. Ex vivo biodistribution studies and immunohistochemistry (IHC) staining were performed. Tumoural PD-L1 expression and CD4+/CD8+ tumour-infiltrating lymphocytes were evaluated in human GBM specimens. Z(PD-L1) was radiolabelled with radiochemical yields of 32.2 & PLUSMN; 4.4% (F-18) and 73.3 & PLUSMN; 1.8% (Ga-68). The cell-associated radioactivity in vitro was consistent with PD-L1 expression levels assessed with flow cytometry. In vivo imaging demonstrated that F-18-AlF-NOTA-Z(PD-L1) can distinguish between PD-L1 high-expressing tumours (U87-MGvIII) and PD-L1-negative ones (H292(PD-L1Ko)). The radioconjugate was quickly cleared from the blood and normal tissues, allowing for high-contrast images of brain tumours as early as 1 h post-injection. Ga-68-NOTA-Z(PD-L1) showed heterogeneous and diffuse accumulation that corresponded to the extensively infiltrating GCGR-E55 tumours involving contiguous lobes of the brain. Lastly, 39% of analysed GBM patient samples showed PD-L1+ staining of tumour cells that was associated with elevated levels of CD4+ and CD8+ lymphocytes. Our results suggest that the investigated radioconjugates are very promising agents with the potential to facilitate the future design of treatment regimens for GBM patients.

Automated summary

Almost all GBM patients relapse due to adaptive and acquired resistance from tumour heterogeneity and immune-depleted microenvironment. High PD-L1 levels are associated with GBM invasiveness and immuno-resistance. We investigated the ability of a radiolabelled Z(PD-L1) affibody molecule to measure PD-L1 expression in GBM xenograft models.