Safety and efficacy of digoxin as a potential candidate agent for retinoblastoma treatment

Abstract

Purpose: Despite recent advances of local routes for chemotherapy delivery there have been few agents incorporated in the chemotherapy armamentarium for retinoblastoma treatment with almost no new schedules for drug administration. This study assessed the anti-tumor and antiangiogenic effect of digoxin in vitro under conventional and protracted schedules and the ocular and systemic toxicity of repeated intravitreal injections in rabbits.

Methods: Two retinoblastoma (Y79, WERI-RB1) and three endothelial cell types (HMEC, HUVEC, EPC) were exposed to increasing concentrations of digoxin after a single (1 day) or protracted (7 days) treatment fashion. Cytotoxicity was assessed with a vital dye and induction of apoptosis and the cell-cycle status were evaluated by flow cytometry. A cohort of 4 New Zealand rabbits (1.8-2.2 kg) received 4 bi-weekly doses of 1μg of intra-vitreal digoxin and the same volume (0.1 ml) of vehicle into the fellow eye. Animal controls included clinical, hematological and ocular evaluations (fundoscopy and electroretinograms). After 2 weeks of the last dose rabbits were euthanized and samples were obtained for retinal histology.

Results: Digoxin was cytotoxic in retinoblastoma and endothelial cells after a single exposure. Single and protracted treatments of all five cell types did not show a significant difference in terms of the IC50 (p>0.05). Both treatment schedules with digoxin at the IC50 induced apoptosis and cell cycle arrest at G0. Retinal toxicity was evident after the third intravitreal dose of digoxin with statistical changes of the ERG components against the control eye and considerable but local histologic damage of the retinas.

Conclusions: Digoxin showed cytotoxic effects in retinoblastoma cell lines while exerting an antiangiogenic activity in vitro at similar concentrations. A protracted treatment was assessed in retinoblastoma cells without an advantage in terms of the dose for antitumor effect with respect to single dose. Despite promising antitumor and antiangiogenic activity in vitro four bi-weekly injections of digoxin lead to significant but local toxicity to the retina of rabbits. Thus, a counterbalance between efficacy and toxicity should be taken into account if translated into the clinics for retinoblastoma treatment.