PD-L1 Detection in Tumors Using [64Cu]Atezolizumab with PET

The programmed death protein 1 (PD-1) and programmed deathligand 1 (PD-L1) pair is a major immune checkpoint pathway exploited by cancer cells to develop and maintain immune tolerance. With recent approvals of anti-PD-1 and anti-PD-L1 therapeutic antibodies, there is an urgent need for noninvasive detection methods to quantify dynamic PD-Ll expression in tumors and to evaluate the tumor response to immune modulation therapies. To address this need, we assessed [Cu-64]atezolizumab for the detection of PD-Ll expression in tumors. Atezolizumab (MPDL3208A) is a humanized, human and mouse cross-reactive, therapeutic PD-Ll antibody that is being investigated in several cancers. Atezolizumab was conjugated with DOTAGA and radiolabeled with copper-64. The resulting [Cu-64]atezolizumab was assessed for in vitro and in vivo specificity in multiple cell lines and tumors of variable PD-L1 expression. We performed PET-CT imaging, biodistribution, and blocking studies in NSG mice bearing tumors with constitutive PD-Ll expression (CHO-hPD-L1) and in controls (CHO). Specificity of [Cu-64]atezolizumab was further confirmed in orthotopic tumor models of human breast cancer (MDAMB231 and SUM149) and in a syngeneic mouse mammary carcinoma model (4T1). We observed specific binding of [Cu-64]atezolizumab to tumor cells in-vitro, correlating with PD-L1 expression levels. Specific accumulation of [Cu-64]atezolizumab was also observed in tumors with high PD-L1 expression (CHO-hPD-L1 and MDAMB231) compared to tumors with low PD-L1 expression (CHO, SUM149). Collectively, these studies demonstrate the feasibility of using [Cu-64]atezolizumab for the detection of PD-Ll expression in different tumor types.