In vitro stability and preclinical safety evaluation of high-dose intravitreal topotecan in rabbits: impact of dose and concentration

Abstract

Purpose: To assess the stability of concentrated solutions of topotecan and the ocular and systemic safety of administering repeated very high doses of intravitreal (IVi) topotecan in rabbits. Design: Experimental study. Subjects: Twenty four nontumor-bearing New Zealand White rabbits. Main Outcome Measures: In vitro stability of topotecan solutions, hematologic parameters, fundoscopic examination, electroretinography (ERG) response, fundoscopic photography, and histologic assessment. Methods: Three topotecan solutions (1-4 mg/ml) were assessed for stability at 4°C and –20°C for 1 month of drug reconstitution. Five groups of nontumor-bearing rabbits were used to evaluate systemic and ocular toxicity of 3 monthly doses of topotecan (50 μg, 100 μg, and 200 μg), injected in 50 μL or in 100 μL of diluent, or receiving only the vehicle. Ophthalmic, clinical, and electroretinographic evaluations were performed monthly. One month after the last injection, all eyes were enucleated for histological assessment. Electroretinographic parameters were compared among groups using a linear mixed-effects model (P < 0.05). Results: Topotecan solutions remained stable. During the study period, no hair loss, weight changes, or hematologic abnormalities were observed in any of the animal groups. Eyes treated with vehicle or injected with 3 doses of topotecan up to 100 μg per dose, delivered in either 50 μL or 100 μL diluent, showed no morphological, histological, or functional evidence of damage to the retina. However, eyes injected with 200 μg showed localized retinal alterations near the injection site on fundoscopy and histological analysis, and a significant decrease in the a- and b-wave amplitudes on ERG compared with the other groups (P < 0.05). Conclusions: Three monthly IVi injections of topotecan 50 or 100 μg (100 μg and 200 μg human equivalent doses) caused no systemic or ocular toxicity in a nontumor-bearing rabbit model. Repeated 200 μg doses (400 μg human-equivalent dose) resulted in localized retinal morphological alterations and small but detectable changes in electrophysiological parameters. The results of this study may have clinical utility in the assessment of very high-dose topotecan as a salvage treatment of highly compromised eyes with recurrent or relapsed subretinal seeds and retinal tumors.