Effects of Pretargeting and FcRn Binding Abrogation on the Biodistribution of a One-Armed Antibody-Based T Cell Imaging Agent.

Achieving a rapid image contrast is a critical attribute of successful imaging biomarkers of T-cell redirecting cancer immunotherapies, as even small shifts in the cluster of differentiation 8 (CD8) expressing T cell populations can be associated with meaningful therapeutic responses. However, T cell imaging agents, such as one-armed (OA) anti-CD8 monoclonal antibodies, are often limited by high renal uptake and poor resolution against the systemic blood signal. Herein we evaluate antibody pretargeting with and without the abrogation of binding to the neonatal Fc receptor (FcRn) as strategies to enhance the tumor contrast of OA antibodies targeting the minimally internalizing receptor, CD8. Single-photon emission computed tomography (SPECT) imaging of indium-111-labeled tracers in a solid CD8-expressing HPB-ALL tumor-bearing mouse model allowed the impact of FcRn binding to be assessed by both targeted and pretargeted imaging methods. We demonstrated that pretargeted imaging resulted in a higher tumor contrast within hours of tracer administration, irrespective of FcRn binding with reduced renal uptake relative to direct targeting. Abrogation of FcRn binding yielded a higher tumor contrast (relative to blood, kidney, and/or liver) at early time points (less than 2 h) for pretargeted imaging, but with reduced tumor enrichment and increased hepatic signal than in the presence of FcRn binding. Our findings demonstrate that pretargeted imaging with an OA anti-CD8 immunoSPECT tracer can overcome potential imaging liabilities associated with this molecule format, such as high renal uptake and poor resolution against the systemic pool.