Efflux of monoclonal antibodies from rat brain by neonatal Fc receptor, FcRn.

Monoclonal antibody (mAb) engineering that optimizes binding to receptors present on brain vascular endothelial cells has enabled them to cross through the blood-brain barrier (BBB) and access the brain parenchyma to treat neurological diseases. However, once in the brain the extent to which receptor-mediated reverse transcytosis clears mAb from the brain is unknown. The aim of this study was to determine the contribution of the neonatal Fc-receptor (FcRn) in rat brain efflux employing two different in vivo drug delivery models. Two mAb variants with substantially different affinities to FcRn, and no known neuronal targets, (IgG1 N434A and H435A) were administered to rats via intranasal-to-central nervous system (CNS) and intra-cranial dosing techniques. Levels of full-length IgG were quantified in serum and brain hemispheres by a sensitive enzyme-linked immunosorbent assay (ELISA). Following intra-nasal delivery, low cerebral hemisphere levels of variants were obtained at 20min, with a trend towards faster clearance of the high FcRn binder (N434A); however, the relatively higher serum levels confounded analysis of brain FcRn contribution to efflux. Using stereotaxic coordinates, we optimized the timing and dosing regimen for injection of mAb into the cortex. Levels of N434A, but not H435A, decreased in the cerebral hemispheres following bilateral injection into the rat cortex and higher levels of N434A were detected in serum compared to H435A after 24h. Immunohistochemical staining of human IgG1 in sections of cortex was consistent with these results, illustrating relatively less intense immunostaining in N434A than H435A dosed animals. Using two in vivo methods with direct cranial administration, we conclude that FcRn plays an important role in efflux of IgG from the rat brain.