Exploring the Interplay Between Radioimmunoconjugates and Fcγ Receptors in Genetically Engineered Mouse Models of Cancer

Fcγ receptors (FcγR) are responsible for many of the interactions between immunoglobulins (IgG) and immune cells. In biomedicine, this interplay is critical to the activity of several types of immunotherapeutics; however, relatively little is known about how FcγRs affect the in vivo performance of radiolabeled antibodies. A handful of recent preclinical studies suggest that binding by FcγR-and particularly FcγRI-can affect the pharmacokinetic profiles of ⁸⁹Zr-labeled radioimmunoconjugates, but there are no extant studies in immunocompetent or genetically engineered mouse models of cancer. In the investigation at hand, we synthesized and characterized ⁸⁹Zr-labeled probes based on wild-type and aglycosylated variants of the CA19-9-targeting antibody 5B1 and evaluated their in vivo behavior in several murine models of cancer, including immunocompetent and FcγR-humanized mice. The aglycosylated desferrioxamine (DFO)-bearing immunoconjugate DFO-^N297A5B1 displayed identical binding to CA19-9-expressing cells compared to its wild-type analogue (DFO-5B1) but exhibited dramatically attenuated affinity for several FcγR. Positron emission tomography imaging and biodistribution studies with [⁸⁹Zr]Zr-DFO-5B1 and [⁸⁹Zr]Zr-DFO-^N297A5B1 were subsequently performed in several strains of mice bearing CA19-9-expressing BxPC3 human pancreatic ductal adenocarcinoma and B16F10-FUT3 murine melanoma xenografts. Significant differences in the pharmacokinetics of the two radioimmunoconjugates were observed in tumor-bearing immunocompromised NSG mice, but these differences failed to materialize in immunocompetent C57BL/6 and FcγR-humanized C57BL/6 mice with B16F10-FUT3 xenografts. We hypothesize that these observations are related to the presence or absence of endogenous IgG. NSG mice completely lack endogenous IgG, and thus their mFcγR are free to bind radioimmunoconjugates and alter their pharmacokinetic behavior. In contrast, C57BL/6 and FcγR-humanized C57BL/6 mice both have endogenous IgG that occupy their FcγR (murine for the former and human for the latter), precluding interactions with radioimmunoconjugates. Ultimately, these data suggest that understanding the interplay between radiolabeled antibodies and FcγR is critical during the preclinical evaluation of radioimmunoconjugates.