Quantitative Analysis of Human Neonatal Fc Receptor (FcRn) Tissue Expression in Transgenic Mice by Online Peptide Immuno-Affinity LC-HRMS.

Neonatal Fc receptor (FcRn) is the homeostatic receptor responsible for the long half-life of endogenous IgG by protecting it from lysosomal degradation. Understanding systemic FcRn tissue expression is important to predict and design the half-life of therapeutic antibodies and Fc-coupled biotherapeutics. To this end, we measured human FcRn (hFcRn) tissue expression in Tg32, a human FcRn knock-in transgenic mouse model, for which a strong correlation of drug clearance to humans has been demonstrated. Building an hFcRn tissue expression profile in Tg32 was enabled by the development of a tissue preparation procedure composed of bead-based protein extraction and protein precipitation using acetone followed by pellet digestion with trypsin. Digests were then loaded onto an online peptide immuno-affinity flow configuration hyphenated with reversed phase nanoflow chromatography and coupled with high resolution mass spectrometry to quantify hFcRn derived peptides. The workflow allowed bypassing some of the challenges typically associated with membrane protein analysis. We demonstrated acceptable precision and bias for measuring hFcRn in tissue matrices, typically within 20% coefficient of variation and relative error. We also report hFcRn expression in several Tg32 tissues. We anticipate that establishing a quantitative approach for hFcRn in tissues will enable the systematic measurement of hFcRn concentrations to further increase the accuracy of physiologically based pharmacokinetic (PBPK) models for PK prediction of Fc-containing biotherapeutics. This is anticipated to improve the translation of pharmacokinetic data from preclinical model systems to humans.