Subretinal Injection Volume Correlates to Persistent Outer Retinal Thinning in the Pig Eye

Transgenic pig models are a valuable model for preclinical testing of gene and cell therapies. Subretinal injection (SRI) is a common drug delivery method but has been associated with retinal thinning and atrophy. This study examined whether SRI volume correlates with retinal thinning in the pig eye and compared the effects of balanced salt solution (BSS) and adeno-associated virus (AAV) injections.

Ten eyes from five transgenic pigs were included in this study. Eight eyes received escalating subretinal injection volumes (SRVs) (50, 100, 150, and 200 µL) of either BSS or AAV [5 × 1011 vg/mL], targeting the cone-rich area centralis of the pig eye. After six months, outer retinal thickness inside versus outside the bleb area (ΔORT) was quantified using optical coherence tomography (OCT). Histology was performed to confirm OCT findings.

Treated eyes showed clinically relevant (-21.5 ± 2.7 µm) outer retinal thinning inside the bleb area (P = 0.0001). A strong, statistically significant, linear correlation (R² = 0.73 P = 0.0068) was found between SRV and ORT loss. There was a 1 µm loss of ORT for every 9 µL of SRV. ORT loss was similar between AAV and BSS, except at the highest volume (200 µL, 1 × 1011 vg), where greater thinning occurred with AAV over BSS (Δ11 µm).

This study supports the notion that SRV could be an independent factor in development of outer retinal thinning in the pig eye. Modifying surgical technique to favor the placement of multiple smaller blebs of <100 µL might mitigate retinal thinning because of volumetric stress and enhance the preclinical safety profile of investigational therapies.