Unraveling the dynamics of B7-H3-targeting therapeutic antibodies in cancer through PET imaging and antibody pharmacokinetics.

B7-H3, an immunomodulatory protein overexpressed in many cancers, is associated with tumor aggressiveness and poor prognosis, making it a crucial target for imaging to elucidate its role in cancer progression and guide therapeutic interventions. This study employed PET imaging to investigate the in vivo delivery and pharmacokinetics of two anti-B7-H3 antibodies, Ab-1 and Ab-2, in mouse xenograft models with varying B7-H3 expression levels. The antibodies were radiolabeled with [Zr]Zr and evaluated through PET imaging, biodistribution studies, and in vitro assays to assess binding, tumor uptake, and retention. [Zr]Zr-Ab-1 demonstrated high initial tumor uptake in B7-H3 positive xenografts but exhibited unexpected decreasing retention over time. This clearance was likely attributed to proteolytic cleavage mediated by matrix metalloproteinases in the tumor and the tumor microenvironment. Conversely, [Zr]Zr-Ab-2 showed more stable tumor retention but lower overall uptake. Further investigation revealed that Ab-1 had affinity for both 4Ig and 2Ig B7-H3 isoforms, while Ab-2 bound exclusively to the 4Ig isoform. This differential binding to B7-H3 isoforms may explain the observed variations in tumor uptake and retention between the two antibodies. The study provides insights into the complex dynamics of B7-H3 targeted antibodies in vivo, highlighting how antibody characteristics, including isoform-specific binding, and tumor factors influence their behavior. These findings have potential implications for optimizing radiotherapy strategies, suggesting the possibility of tailored approaches based on antibody properties and tumor biology.