Mechanistic analysis of fusion proteins: PBPK applied in an Albuferon case study
Christoph Niederalt(1), Lars Kuepfer(1), Thomas Wendl(1), Stefan Willmann(1), Jörg Lippert(1), Victoria Flores(2) & Piet van der Graaf(2)
(1)Systems Biology & Computational Solutions, Bayer Technology Services GmbH, 51368 Leverkusen, Germany; (2)Pfizer, New Opportunities Unit, Sandwich CT13 9NJ, UK
Objectives: The pharmacokinetics of protein therapeutics is governed by unspecific and specific processes[1,2]:
- Exchange across vascular endothelium by convection and diffusion.
- Return of drug from interstitial space in organs to circulation by lymph flow.
- Degradation mediated by neonatal Fc receptor (FcRn) in cellular endosome.
- Target-mediated deposition and clearance
A physiologically-based pharmacokinetics (PBPK) model for Albuferon, an albumin–interferon-fusion protein, has been developed using generic sub-models. Albuferon was chosen since it allowed to explore the influence of both unspecific endothelial exchange and lymph flow as well as the specific effects of FcRn-mediated recycling and target mediated deposition and clearance.
Methods: The model was built in a modular way using parameterization established for IgG-antibodies (FcRn related processes) and interferons (target mediated processes). All PBPK models were implemented in PK-Sim® and MoBi®. Competitive binding of endo- and exogenous ligands to FcRn takes place in the endosome of endothelial cells in each organ. To describe protection by FcRn the model Albuferon was coupled with a model of endogenous albumin. Target-mediated deposition and clearance is described by reversible binding of drug to interferon receptor (IFNAR2) and irreversible complex internalization. The PBPK model for Albuferon was build based on the models for the IgG-antibodies and interferon-α/β. The rate of formation of endogenous albumin was set to match steady state concentration of albumin.
Results: The PK of Albuferon is dose-linear and a model not taking the IFNAR2 receptor binding into account matches experimental data best. Target mediated deposition and clearance appears to have a negligible effect on the PK of Albuferon. The PBPK model was extended by a first order release rate into the interstitial space of the skin to describe s.c. applications [3,4,5]. For monkey, a good description of the data is obtained, if a release rate is chosen to be insensitively small. The rapid release indicates dominance of lymph flow rates for the absorption rate of Albuferon. For humans, however, the first order release rate has to be set to a half-life of 125 h to match experimental data and terminal half life is largely determined by subcutaneous release. The reason for the difference between monkeys and humans is not known.
Conclusion: Albuferon pharmacokinetics could be predicted using PBPK sub-models established independently from Albuferon, using benchmarking compounds and prior knowledge only. A detailed analysis revealed that IFNAR mediated deposition plays a minor role for Albuferon while it is highly relevant for “naked” reference interferons. Following s.c. application, terminal half-life is determined by subcutaneous release in human but not in monkey.
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