Effect of Size on Solid Tumor Disposition of Protein Therapeutics

In this study, we evaluated the effect of size on tumor disposition of protein therapeutics, including the plasma and tumor pharmacokinetics (PK) of trastuzumab (∼150 kDa), FcRn-nonbinding trastuzumab (∼150 kDa), F(ab)₂ fragment of trastuzumab (∼100 kDa), Fab fragment of trastuzumab (∼50 kDa), and trastuzumab scFv (∼27 kDa) in both antigen (i.e., HER2)-overexpressing (N87) and antigen-nonexpressing (MDA-MB-468) tumor-bearing mice. The observed data were used to develop the maximum tumor uptake versus molecular weight and tumor-to-plasma area under the curve (AUC) ratio versus molecular weight relationships. Comparison of the PK of different sizes of FcRn nonbinding molecules in target-expressing tumor showed that ∼100 kDa is an optimal size to achieve maximum tumor uptake and ∼50 kDa is an optimal size to achieve maximum tumor-to-plasma exposure ratio of protein therapeutics. The PK data were also used to validate a systems PK model for tumor disposition of different-sized protein therapeutics. The PK model was able to predict a priori the PK of all five molecules in both tumor types reasonably well (within 2- to 3-fold). In addition, the model captured the bell-shaped relationships observed between maximum tumor uptake and molecular weight and between tumor-to-plasma AUC ratio and molecular weight. Our results provide an unprecedented insight into the effect of size and target engagement on the tumor PK of protein therapeutics. Our results also provide further validation of the tumor disposition model, which can be used to support discovery, development, and preclinical-to-clinical translation of different sizes of protein therapeutics. SIGNIFICANCE STATEMENT: This article highlights the importance of molecular size and target engagement on the tumor disposition of protein therapeutics. Our results suggest that ∼100 kDa is an optimal size to achieve maximum tumor uptake and ∼50 kDa is an optimal size to achieve maximum tumor-to-plasma exposure ratio for non-FcRn-binding targeted protein therapeutics. We also demonstrate that a systems pharmacokinetics model developed to characterize tumor disposition of protein therapeutics can predict a priori the disposition of different-sized protein therapeutics in target-expressing and target-nonexpressing solid tumors.