Exploring the Interplay\nBetween Radioimmunoconjugates\nand Fcγ Receptors in Genetically Engineered Mouse Models of\nCancer

Fcγ receptors\n(FcγR) are responsible for many of the\ninteractions between immunoglobulins (IgG) and immune cells. In biomedicine,\nthis interplay is critical to the activity of several types of immunotherapeutics;\nhowever, relatively little is known about how FcγRs affect the\nin vivo performance of radiolabeled antibodies. A handful of recent\npreclinical studies suggest that binding by FcγRand\nparticularly FcγRIcan affect the pharmacokinetic profiles\nof <sup>89</sup>Zr-labeled radioimmunoconjugates, but there are no\nextant studies in immunocompetent or genetically engineered mouse\nmodels of cancer. In the investigation at hand, we synthesized and\ncharacterized <sup>89</sup>Zr-labeled probes based on wild-type and\naglycosylated variants of the CA19-9-targeting antibody 5B1 and evaluated\ntheir in vivo behavior in several murine models of cancer, including\nimmunocompetent and FcγR-humanized mice. The aglycosylated desferrioxamine\n(DFO)-bearing immunoconjugate DFO-<sup>N297A</sup>5B1 displayed identical\nbinding to CA19-9-expressing cells compared to its wild-type analogue\n(DFO-5B1) but exhibited dramatically attenuated affinity for several\nFcγR. Positron emission tomography imaging and biodistribution\nstudies with [<sup>89</sup>Zr]Zr-DFO-5B1 and [<sup>89</sup>Zr]Zr-DFO-<sup>N297A</sup>5B1 were subsequently performed in several strains of\nmice bearing CA19-9-expressing BxPC3 human pancreatic ductal adenocarcinoma\nand B16F10-FUT3 murine melanoma xenografts. Significant differences\nin the pharmacokinetics of the two radioimmunoconjugates were observed\nin tumor-bearing immunocompromised NSG mice, but these differences\nfailed to materialize in immunocompetent C57BL/6 and FcγR-humanized\nC57BL/6 mice with B16F10-FUT3 xenografts. We hypothesize that these\nobservations are related to the presence or absence of endogenous\nIgG. NSG mice completely lack endogenous IgG, and thus their mFcγR\nare free to bind radioimmunoconjugates and alter their pharmacokinetic\nbehavior. In contrast, C57BL/6 and FcγR-humanized C57BL/6 mice\nboth have endogenous IgG that occupy their FcγR (murine for\nthe former and human for the latter), precluding interactions with\nradioimmunoconjugates. Ultimately, these data suggest that understanding\nthe interplay between radiolabeled antibodies and FcγR is critical\nduring the preclinical evaluation of radioimmunoconjugates.