ImmunoPET Imaging with ⁸⁹Zr-Labeled Atezolizumab Enables in Vivo Evaluation of PD-L1 in Tumorgraft Models of Renal Cell Carcinoma

Purpose: Immune checkpoint inhibitors (ICI) targeting the programmed cell death protein 1 and its ligand (PD-1/PD-L1) have transformed the treatment paradigm for metastatic renal cell carcinoma (RCC). However, response rates to ICIs as single agents or in combination vary widely and predictive biomarkers are lacking. Possibly related to the heterogeneity and dynamic nature of PD-L1 expression, tissue-based methods have shown limited value. Immuno–positron emission tomography (immunoPET) may enable noninvasive, comprehensive, and real-time PD-L1 detection. Herein, we systematically examined the performance of immunoPET for PD-L1 detection relative to IHC in an RCC patient-derived tumorgraft (TG) platform. Experimental Design: Eight independent RCC TGs with a wide range of PD-L1 expression (0%–85%) were evaluated by immunoPET. Uptake of ⁸⁹Zr-labeled atezolizumab ([⁸⁹Zr]Zr-DFO-ATZ) was compared with PD-L1 expression in tumors by IHC through double-blind analyses. Clinical outcomes of ICI-treated patients whose TGs were examined were analyzed to evaluate the clinical role of immunoPET in RCC. Results: ImmunoPET with [⁸⁹Zr]Zr-DFO-ATZ (day 6/7 postinjection) revealed a statistically significant association with PDL1 IHC assays (P ¼ 0.0014; correlation r_XY ¼ 0.78). Furthermore, immunoPET can be used to assess the heterogeneous distribution of PD-L1 expression. Finally, studies in the corresponding patients (n ¼ 4) suggest that PD-L1 signal may influence ICI responsiveness. Conclusions: ImmunoPET with [⁸⁹Zr]Zr-DFO-ATZ may enable a thorough and dynamic assessment of PD-L1 across sites of disease. The power of immunoPET to predict ICI response in RCC is being explored in an ongoing clinical trial (NCT04006522).