Excretion and Distribution of the 60-kDa PEG Moiety Used in BAY 94-9027 in Male Wistar Rats

Introduction: PEGylation is a widely used method to prolong the half-life of therapeutic proteins. Long-acting factor VIII (FVIII) products developed through PEGylation may potentially reduce the burden associated with prophylaxis in patients with hemophilia A. BAY 94-9027 is a prolonged-half-life FVIII product modified through site-specific addition of a 60-kDa (2 × 30-kDa branched) PEG via a maleimide linker to a cysteine amino acid. BAY 1025662 is the 60-kDa PEG moiety used for PEGylation including linker and cysteine (PEG-60-Mal-Cys). In toxicology studies, BAY 94-9027 and BAY 1025662 were not associated with any adverse events or PEG-related cellular vacuolation (Ivens et al. Haemophilia. 2013;19:11-20). The rate-limiting clearance of the PEG part of PEGylated proteins after metabolism is expected to be similar to that when the corresponding PEG is administered alone (Baumann et al. Drug Discov Today . 2014;19:1623-1631). The study objective was to investigate the pharmacokinetics and mass balance of total radioactivity in male Wistar rats after a single intravenous (IV) administration of [¹⁴C]-labeled BAY 1025662, including size exclusion chromatography profiles of urine at selected time points.

Methods: Radiolabeled BAY 1025662 ([prop-¹⁴C]BAY 1025662) was synthesized using a 2-step process resulting in a maleimide linker bearing the [¹⁴C] label at the propionic acid moiety. Radioactivity following a single IV dose of 11 mg/kg (the estimated maximal human PEG dose over ~30 years' treatment with BAY 94-9027) was determined in body fluids, organs and tissues, and in excreta by liquid scintillation counting after appropriate sample preparation and by quantitative whole-body autoradiography during and at the end of the study.

Results: Single-dose mass balance studies using [prop-¹⁴C]BAY 1025662 showed that 30.4% of administered radioactivity was excreted within 1 week (23.7% in urine and 6.3% in feces); cumulative excretion at day 168 via urine and feces was 82.6% of the administered dose (68.4% via urine, 13.8% via feces). High-performance liquid chromatography profiling analysis of rat urine suggested that BAY 1025662 is excreted nearly unchanged (74% of total radioactivity, corresponding to 17.6% of dose, present unchanged in urine in the 0- to 168-hour interval). Radioactivity was continuously, albeit slowly, eliminated throughout the study; elimination half-life was 23 and 24 days in blood and plasma, respectively. The corresponding elimination half-lives of radioactivity were 33, 31, 39, 54, and 68 days in the liver, skin, carcass, testes, and kidneys, respectively. Elimination was almost complete at the end of experiment, with a total recovery of radioactivity (cumulative excretion plus residues in animals) of 89% on day 168. Following [prop-¹⁴C]BAY 1025662 administration, radioactivity was slowly distributed from blood to peripheral organs, reaching maximum concentration 2 hours postdose; highest maximum concentration was observed in blood, followed by lung and liver. At day 168, only ~4% of the administered radioactive dose was still present in the animal body, partially localized in endocrine/exocrine glands and lymphatic organs. No residual radioactivity was observed in the brain, indicating no crossing of the blood-brain barrier.

Conclusions: Following single-dose administration of [prop-¹⁴C]BAY 1025662, elimination was almost complete by the end of the experiment on day 168. These results suggest that there are excretion processes in place for high-molecular-weight PEGs such as the PEG 60 moiety used in BAY 94-9027. Two clearance processes are proposed for large PEGs (>30 kDa), as indicated by a faster initial excretion over the first 1-2 weeks, followed by a nonspecific uptake into cells by pinocytosis leading to a longer elimination half-life. The PEG moiety can be recycled from inside the cells back to the plasma, possibly by exocytosis, to then undergo further (mainly renal) excretion (Baumann et al. Drug Discov Today . 2014;19:1623-1631).