Introduction <p>RM301B eye drops are a dexamethasone (DEX)-based formulation indicated for treating macular oedema associated with retinal vein occlusion (RVO). Non-clinical pharmacodynamic studies have demonstrated that RM301B eye drops have a certain inhibitory effect on retinal oedema. This study aimed to evaluate the safety, tolerability, and pharmacokinetic (PK) characteristics of RM301B eye drops following administration in healthy subjects.</p> Methods <p>A single-centre, randomized, double-blind, placebo-controlled, dose-escalation Phase I clinical study. In the single-dose study, ten subjects received once-daily RM301B or placebo. In the multiple ascending dose study, 30 subjects received 3, 6, or 8 daily instillations for 7&#xa0;days. Safety and tolerability were monitored throughout the study, with plasma samples collected for PK analysis.</p> Results <p>No serious adverse events (SAEs) occurred in the study. All mild adverse events (AEs) were determined to be unrelated to RM301B. PK results indicated detectable plasma concentrations (0.10–0.40&#xa0;ng/ml) only in individual subjects within 5&#xa0;h post-single dose and at specific time points following multiple dosing. Mean area under the curve from time zero to t (AUC_0–t) following single dosing was merely 0.48&#xa0;h·ng/ml. Area under the curve at steady state (AUC_ss) after multiple dosing was 1.69 and 1.40&#xa0;h·ng/ml for Cohorts 1 and 3, respectively. Minimal systemic exposure was observed across all cohorts post-administration.</p> Conclusions <p>RM301B eye drops demonstrated safety and good tolerability in subjects, characterized by minimal plasma exposure, a short half-life, and no significant accumulation. Subsequent trials will evaluate the safety and efficacy of RM301B in patients with macular oedema associated with RVO.</p> Trial Registration <p>ChiCTR2500111837 (retrospectively registered on November 6, 2025).</p>