Toxicity of Transition Metal Complex-based Nanophotoswitches in Retina

Purpose: Nanophotoswitches (NPSs) based on transition metal complexes offer a new tool for optical stimulation of neural activity in photoreceptor degenerated retina. We previously reported robust light-elicited neural activity in degenerate retinae exposed to ruthenium bipyridine based NPSs (Rubpy-C17) and its iridium analog (Irbpy-C17). Irbpy-C17 was developed as an alternative to Rubpy-C17 for the biosafety properties of the iridium complexes. Here we present a study of the toxicity of both NPSs in rodent retinae. Methods: Toxicity of Rubpy-C17 was tested in wildtype C57BL/6J mice and Irbpy-C17 in wildtype Long Evans rats. Animals were intravitreally injected with the test molecules (up to 50 µM) and sacrificed at different time points post injection: 3, 7, 14 and 28 days, respectively. Retinae were obtained, fixed and sliced for histological analysis immediately after animal euthanization. H&E staining was performed to examine morphological integrity of retina and TUNEL staining performed to detect apoptosis of retinal cells. For comparison, Ru(bpy)3]Cl2 injection and sham surgery were included for control. Results: H&E staining revealed no detectable sign of morphological or structural changes in the retinae after prolonged exposure to either Rubpy-C17 or Irbpy-C17 versus the control. There was no significant reduction in the thickness of different nuclear and plexiform retinal layers or the density of retinal neurons (p<0.05), nor was there evidence of significant aggregation of immune cells. TUNEL staining showed minimal occurrence of cell apoptosis in the NPS treated retinae, similar to the control (p