Assay-dependent Artifactual Prolongation of Activated Partial Thromboplastin Time (aPTT) in Cynomolgus Monkeys: Potential Role of POEGMA

We identified an assay-dependent, artifactual prolongation of activated partial thromboplastin time (aPTT) during nonclinical safety studies of an enzyme biologic conjugated with a next-generation polyethylene glycol (PEG)-like stealth polymer, poly(oligoethylene glycol) methyl ether methacrylate (POEGMA), in cynomolgus monkeys. Two repeat-dose studies (8 and 13 weeks) with twice-weekly subcutaneous dosing evaluated pharmacokinetic/toxicokinetic (PK/TK) and pharmacodynamic (PD) assessments, clinical pathology, and gross/histopathology. The 8-week study used a single silica-based aPTT assay; the 13-week study used both silica-based and kaolin-based assays. In the 8-week study, the silica-based assay showed time- and dose-dependent aPTT prolongation at Weeks 4 and 8. PK/PD effects were most evident in Weeks 1-2, decreased thereafter, likely due to immunogenicity, and did not correlate with aPTT changes. Other coagulation parameters (eg, prothrombin time and fibrinogen) were unremarkable, and histopathology showed no bleeding or coagulopathy. In the 13-week study, aPTT prolongation again occurred only with the silica-based assay, whereas the kaolin-based assay showed no effect. Ex vivo aPTT testing used naïve monkey plasma spiked with either the biologic or POEGMA alone; neither component prolonged aPTT with either assay, indicating no direct interference. Collectively, these data indicate that the observed aPTT changes were assay-dependent artifacts related to the silica activator, rather than true pharmacologic or toxicologic effects. In contrast, such artifactual aPTT prolongation was not induced by the same enzyme biologic when conjugated with conventional 20-kDa PEG, indicating a POEGMA-conjugate-specific artifact.