Site-specific ⁶⁸Ga-labeled nanobody for PET imaging of CD70 expression in preclinical tumor models

CD70-CD27 is a costimulatory ligand-receptor pair in the tumor necrosis factor receptor family. With only limited expression in normal tissues, CD70 is constitutively expressed in a variety of solid tumors and hematologic malignancies, facilitating immunosuppression through CD27 signaling in the tumor microenvironment by enhanced survival of regulatory T cells, induction of T cell apoptosis, and T cell exhaustion. Consequently, CD70 is an increasingly recognized target for developing antibody-based therapies, but its expression patterns vary among different tumor types in spatial distribution, magnitude of expression and percentage of positive cells. In that regard, individual confirmation of CD70 expression at screening and during treatment could enhance the successful implementation of anti-CD70 therapies. Here, we developed a gallium-68 (⁶⁸Ga) radiolabeled single-domain antibody-fragment targeting CD70 for in vivo positron emission tomography (PET) imaging.

An anti-CD70 VHH construct containing a C-direct-tag with a free thiol was developed to enable site-specific conjugation to a NOTA bifunctional chelator for ⁶⁸Ga radiolabeling. [⁶⁸Ga]Ga-NOTA-anti-CD70 VHH was obtained in good radiochemical yield of 30.4 ± 1.7% and high radiochemical purity (> 94%). The radiolabeled VHH showed excellent in vitro and in vivo stability. Specific binding of [⁶⁸Ga]Ga-NOTA-anti-CD70 VHH was observed on CD70^high 786-O cells, showing significantly higher cell-associated activity when compared to the blocking condition (p < 0.0001) and CD70^low NCl-H1975 cells (p < 0.0001). PET imaging showed specific radiotracer accumulation in CD70 expressing human tumor xenografts, which was efficiently blocked by prior injection of unlabeled anti-CD70 VHH (p = 0.0029). In addition, radiotracer uptake in CD70^high tumors was significantly higher when compared with CD70^low tumors (p < 0.0001). The distribution of the radioactivity in the tumors using autoradiography was spatially matched with immunohistochemistry analysis of CD70 expression.

[⁶⁸Ga]Ga-NOTA-anti-CD70 VHH showed excellent in vivo targeting of CD70 in human cancer xenografts. PET imaging using this radioimmunoconjugate holds promise as a non-invasive method to identify and longitudinally follow-up patients who will benefit most from anti-CD70 therapies.