Dose-Dependent Corneal Epithelial and Neural Toxicity of Topical Proparacaine: An In Vivo and In Vitro Study

To evaluate the dose-dependent toxicity of topical proparacaine (PPC) on the corneal epithelium and nerve fibers using both in vitro and in vivo models.

Human corneal epithelial cells were used for cell viability and scratch assays. Male Sprague-Dawley rats received topical PPC (0.5%, 0.1%, 0.01%) for 2 weeks. Apoptosis was assessed by TUNEL assay, intraepithelial nerve density by anti-βIII tubulin immunohistochemistry, and corneal NGF, substance P, and CGRP levels by ELISA. Corneal epithelial wound healing was also evaluated.

PPC demonstrated dose-dependent cytotoxicity, with the CC₅₀ between 2^-4% (0.0625%) and 2^-3% (0.125%). PPC concentrations ≥ 2^-5% (0.03125%) significantly inhibited epithelial wound gap closure in vitro. In vivo, PPC 0.5% induced significant apoptosis in corneal epithelium and markedly reduced intraepithelial nerve density compared with controls (P = 0.015 and 0.001, respectively). Corneal NGF levels were significantly reduced in the PPC 0.1% and 0.5% groups, whereas corneal substance P levels were significantly reduced only in the PPC 0.5% group. Corneal epithelial wound healing was significantly delayed with PPC 0.5% (P = 0.01), but not with PPC 0.1% or 0.01%.

Repeated application of PPC at the clinical concentration (0.5%) induced corneal epithelial and neural toxicity, whereas diluted concentrations (0.1% and 0.01%) showed no significant adverse effects. These findings highlight the risk of toxicity associated with repeated use of clinical concentration PPC, while also suggesting the potential utility of dilute formulations in select clinical scenarios.