Effect of the Size of Protein Therapeutics on Brain Pharmacokinetics Following Systematic Administration

Here, we have investigated the effect of size of protein therapeutics on brain pharmacokinetics (PK) following systemic administration in rats. All tested proteins were derived from trastuzumab that do not bind to any targets in rats. PK data generated with F(ab)₂ (100 kDa), Fab (50 kDa), and scFv (27 kDa) fragments of trastuzumab, along with published PK data for FcRn non-binding and wild-type trastuzumab (150 kDa), were used to establish a relationship between the protein size and brain exposure. A large-pore microdialysis system was used to measure the PK of proteins in the plasma, the interstitial fluid (ISF) at the striatum (ST), and the cerebrospinal fluid (CSF) at the lateral ventricle (LV) and cisterna magna (CM). Concentrations of all the proteins in plasma, brain homogenate, ISF, and CSF were measured using ELISA. When evaluating the effect of protein size in the absence of FcRn binding, we found a bell-shaped relationship between the size and ISF/plasma AUC ratio, where 100 kDa F(ab)₂ demonstrated the highest exposure. A similar bell-shaped relationship was observed for the brain homogenate/plasma AUC ratio, with a peak at 50 kDa. The CSF/plasma AUC ratio at LV increased monotonously with a decrease in the size of proteins. We observed that the exposure of protein therapeutics in different regions of the brain could be significantly different and there could be optimal sizes of protein therapeutics to accomplish maximum/selective exposure in selected brain regions following systemic administration.