Intravitreal Injection of Proinsulin-Loaded Microspheres Delays Photoreceptor Cell Death and Vision Loss in the rd10 Mouse Model of Retinitis Pigmentosa

PURPOSE. The induction of proinsulin expression by transgenesis or intramuscular gene therapy has been shown previously to retard retinal degeneration in mouse and rat models of retinitis pigmentosa (RP), a group of inherited conditions that result in visual impairment. We investigated whether intraocular treatment with biodegradable poly (lactic-co-glycolic) acid microspheres (PLGA-MS) loaded with proinsulin has cellular and functional neuroprotective effects in the retina. METHODS. Experiments were performed using the Pde6b(ra10) mouse model of RP Methionylated human recombinant proinsulin (hPI) was formulated in PLGA-MS, which were administered by intravitreal injection on postnatal days (P) 14 to 15. Retinal neuroprotection was assessed at P25 by electroretinography, and by evaluating outer nuclear layer (ONL) cellular preservation. The attenuation of photoreceptor cell death by hPI was determined by TUNEL assay in cultured P22 retinas, as well as Akt phosphorylation by immunoblotting. RESULTS. We successfully formulated hPI PLGA-MS to deliver the active molecule for several weeks in vitro. The amplitude of b-cone and mixed b-waves in electroretinographic recording was significantly higher in eyes injected with hPI-PLGA-MS compared to control eyes. Treatment with hPI-PLGA-MS attenuated photoreceptor cell loss, as revealed by comparing ONL thickness and the number of cell rows in this layer in treated versus untreated retinas. Finally, hPI prevented photoreceptor cell death and increased Akt(Thr308) phosphorylation in organotypic cultured retinas. CONCLUSIONS. Retinal degeneration in the rd10 mouse was slowed by a single intravitreal injection of hPI-PLGA-MS. Human recombinant proinsulin elicited a rapid and effective neuroprotective effect when administered in biodegradable microspheres, which may constitute a future potentially feasible delivery method for proinsulin-based treatment of RP.