Investigation of the Role of FcγR and FcRn in mAb Distribution to the Brain

To evaluate the role of Fc receptors (FcR) on IgG distribution to the brain, the disposition of 8C2, a murine monoclonal IgG1 antibody, was investigated after intravenous administration in FcRn alpha-chain knockout mice, FcyRIIb knockout mice, Fc gamma RI/RIII knockout mice, and C57BL/6 control mice. I-125-8C2 was co-administered with Cr-51-labeled red blood cells to allow accurate assessment of residual blood content in brain samples. Blood and brain tissues were harvested from subgroups of three mice at several time-points up to 10 days, and radioactivity was counted. The blood and brain areas under 8C2 concentration vs time curves (AUCs) were calculated using the linear trapezoidal rule, and the associated standard deviations (SD) were assessed using a modified Bailer method. Concentration data were also analyzed with a semiphysiological population pharmacokinetic model. The brain/blood AUC ratios were comparable across all strains of mice (ratios +/- SD): 0.00774 +/- 0.000452, 0.00841 +/- 0.000535, 0.00636 +/- 0.000548, and 0.00917 +/- 0.000478 for C57BL/6 control mice, Fc gamma RI/RIII knockouts, Fc gamma RIIb knockouts, and FcRn alpha-chain knockout mice (p > 0.05). Statistically significant improvement in model fitting of the data was shown with incorporation of a strain-specific parameter for antibody clearance for FcRn knockout mice; however, no significant improvements in model fitting were found for strain effects on any other parameter, including the brain uptake clearance or efflux clearances for 8C2. The predicted 8C2 brain efflux clearance was found to be similar to 135-fold faster than the brain uptake clearance, consistent with the observed low ratio of brain-blood exposure. The experimental results and modeling results indicate that, in mice, FcRn and Fc gamma R do not contribute to the "blood-brain barrier" that limits mAb uptake into the brain.