Retinal toxicity in rabbits administered intravitreal TSG-6-rabbit FAb fusion proteins

Abstract

Purpose : Hyaluronic acid binding protein-antibody fragment fusions (HABP-FAb) are designed to have increased residence time in the vitreous chamber compared to FAbs and may provide a platform for long-acting delivery of biologic therapeutics. We aimed to characterize the intravitreal (IVT) safety profile of a species-matched, non-targeted HABP-FAb, TSG6-RabFab, in the New Zealand White rabbit (NZW).

Methods : Three male NZW were administered a single IVT dose of 2 mg/eye 1x-TSG6-RabFab (1:1 ratio) bilaterally. The animals were assessed by serial ocular exam, serum toxicokinetics (TK) and anti-drug-antibodies (ADA), and eyes were examined microscopically (H&E and immunohistochemistry for GFAP, vimentin, and Ki67) at day 30. Subsequently, four male NZW were administered a single IVT dose of 2.5 mg/eye Alexa Fluor 488 (AF488) labeled 2x-TSG6-RabFab (2:1 ratio) bilaterally. In addition to the parameters above, ocular test article (TA) concentration was assessed via fluorophotometer (FPT). The eyes of two animals were examined day 8 and day 30 each for H&E histopathology (OD) and distribution of TA in frozen sections by fluorescence microscopy (FM) (OS).

Results : Serum TK confirmed exposure in all dosed animals. Mild to severe inflammation was noted from day 15 in all animals in both studies and posterior cataracts were noted from day 3 in NZW administered AF488-TSG6-RabFab. Inflammation was associated with high ADA titers (up to 5.82; log₁₀ of last serum dilution above the cutpoint), which were first detectable on day 8, and correlated to the microscopic finding of mononuclear inflammatory cell infiltrates in the ciliary body and posterior segment. Outer retinal degeneration of varying severity and hypertrophy and peripheral migration of Müller cells (vimentin, GFAP and Ki67 positive) were present in all animals at both day 8 and 30. Presence of TA in the vitreous chamber up to day 8 was confirmed by FPT and FM. Assessment of retinal distribution was hampered due to poor morphology in frozen sections.

Conclusions : Retinal degeneration following administration of IVT TSG6-RabFab precludes further development of TSG6 fusions. The role of immunogenicity in the pathogenesis of this finding despite the species-matched Fab moiety, previously been shown to be non-immunogenic in NZW, remains incompletely understood. However, it is concerning that protein-Human Fab fusions may be similarly immunogenic in patients.

This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.