Potential errors in the volume of distribution estimation of therapeutic proteins composed of differently cleared components.

The volume of distribution at steady state (Vss) of therapeutic proteins is usually assessed by non-compartmental or compartmental pharmacokinetic (PK) analysis wherein errors may arise due to the elimination of therapeutic proteins from peripheral tissues that are not in rapid equilibrium with the sampling compartment (usually blood). Here we explored another potential source of error in the estimation of Vss that is linked to the heterogeneity of therapeutic proteins which may consist of components (e.g. glycosylation variants) with different elimination rates. PK simulations were performed with such hypothetical binary protein mixtures where elimination was assumed to be exclusively from the central compartment. The simulations demonstrated that binary mixtures containing a rapid-elimination component can give rise to pronounced bi-phasic concentration-time profiles. Apparent Vss observed with both non-compartmental and 2-compartmental PK analysis, increased with increasing fraction as well as with increasing elimination rate k ( 10 ) of the rapid-elimination component. Simulation results were complemented by PK analysis of an in vivo study in cynomolgus monkeys with different lots of lenercept, a tumor necrosis factor receptor-immunoglobulin G1 fusion protein, with different heterogeneities. The comparative Vss data for the three lenercept lots with different amounts of rapidly cleared components were consistent with the outcome of our simulations. Both lots with a higher fraction of rapidly cleared components had a statistically significant higher Vss as compared to the reference lot. Overall our study demonstrates that Vss of a therapeutic protein may be overestimated in proteins with differently eliminated components.