In this prospective, randomized, double-masked, and placebo-controlled study including a cohort of patients with mild-to-moderate dry eyes, treatment with 0.05% CsN rapidly improved the associated signs and symptoms, as well as the objective and subjective parameters. The ocular surface findings were significantly better in the CsN group than in the HA group across all follow-up periods. These results demonstrate the superiority of 0.05% CsN over 0.15% HA treatment for dry eye, consistent with the findings from previous studies [7, 10, 22].

Dry eye is induced by the aqueous deficiency or hyper-evaporative state of the tear film, resulting in hyperosmolarity [23]. Hyperosmolarity triggers mitogen-activated protein kinase or nuclear factor kappa beta that promote secretion of inflammatory cytokines, which damage epithelial cells and mucin-secreting goblet cells in the conjunctiva. The tear film then becomes unstable, increasing ocular surface damage and undesirable ocular symptoms [23]. The anti-inflammatory effects of cyclosporine A have been demonstrated through various mechanisms, including inhibition of T-cell activation and cytokine production, and reduction of the expression of epithelial cell apoptosis marker, thereby increasing conjunctival goblet cell density and decreasing squamous metaplasia [24,25,26]. Meanwhile, topical cyclosporine A used in the present study uses a self-nanoemulsifying drug delivery system, which has a larger surface area and improved homogeneity and stability compared to the conventional anionic oil-in-water emulsions. In an animal study using a murine model of dry eye, the anti-inflammatory effect was confirmed within 2 weeks when CsN was used, and corneal epithelium and conjunctival goblet cell protection were more efficiently achieved than conventional agent [6]. In a clinical study conducted on patients with Sjögren’s syndrome, CsN also showed faster improvement compared to conventional agent, suggesting that the induction time of the agent could be reduced by enhancing the physicochemical properties [22].

HA is a form of artificial tears that has been extensively used to treat dry eye disease for a long time. HA’s abundance of hydroxyl groups attracts water molecules, thickening and stabilizing the tear film, and its lubricating properties reduce mechanical trauma to the ocular surface [11, 13]. Additionally, HA contributes to re-epithelization of the corneal epithelium and prevents hyperosmolality of the tear film, thereby reducing ocular surface inflammation [11, 12]. Besides HA, mucin secretagogues such as diquafosol and rebamipide are increasingly used in South Korea to stabilize the tear film [27]. They stimulate mucin secretion from conjunctival goblet cells, increase the number of these cells, and improve mucosal epithelium [28]. However, achieving a sufficient anti-inflammatory effect using HA or mucin secretagogues alone is challenging. Topical cyclosporine is a single potent treatment tool targeting anti-inflammatory effects compared with other treatments for dry eye disease. Although comparisons between HA or mucin secretagogues and cyclosporine have been reported, data to prove a specific treatment’s superiority are insufficient, and follow-up studies are required.

Compared with baseline, TBUT and fluorescein staining scores at weeks 4, 8, and 12 significantly improved in the CsN group. Previous studies have shown that the effects of cyclosporine A typically begin after approximately 1 month, and the CsN group in this study has showed significant improvements in TBUT and fluorescein staining score from week 4, unlike the HA group, which is consistent with previous results [25, 29]. In Schirmer I test without anesthesia, a significant increase from baseline was observed at week 12 in the CsN group, and there was a statistically significant difference in the amount of change between the two groups at week 12. The Schirmer I test reflects improvement in sensory-stimulated reflex tearing associated with lacrimal gland tear production responses to ocular damage and is, therefore, appropriate for identifying long-term therapeutic effects [21]. A previous study demonstrated that eyes with detectable MMP-9 expression had significantly decreased tear production over time compared with those without detectable MMP-9 expression [30]. However, in the present study, all eyes remained negative for MMP-9 after 4 weeks of treatment. Tear production improved as shown in the Schirmer I test after 12 weeks.

Given the lack of association between the signs and symptoms of dry eye, various specific questionnaires have been developed. For example, some questionnaires evaluated the impact of dry eye on patient’s quality of life, whereas others focused on diagnosis, severity assessment, or screening [31]. In our study, both groups showed gradual, significant improvement in the SANDE score compared with baseline. The CsN group showed significant improvement in the SANDE score at 4 weeks, and both groups significantly improved at weeks 8 and 12. Although the Ocular Surface Disease Index (OSDI) is the most widely used questionnaire in dry eye clinical trials, it measures only the frequency, but not the severity, of dry eye symptoms, and shows poor correlation with the objective parameters in previous studies [19, 29]. However, SANDE as a short questionnaire based on VAS quickly reflected an improvement in the objective parameters, although it is not well refined. As a short and easily understandable questionnaire, SANDE has shown good reproducibility, repeatability, sensitivity, and specificity in assessing patients with symptoms of dry eye [19]. However, DEQS and ODAS did not show significant changes throughout the treatment period, which might reflect the differences in the characteristics of the questionnaires. DEQS was developed to assess the relationship between dry eye and quality of life in Japan [20]. Although it was based on the evaluation of quality of life and the multifaceted impact on patient daily life, the correlations between clinical parameters and questionnaire scores were low in a previous study [20].

Among the various questionnaires used in this study, SANDE, DEQS, and ODAS1-7 scores all showed significant correlations with each other, except for ODAS8 related to the onset time of dry eye symptoms. Although not developed for evaluating dry eye disease, the ocular discomfort analog scale during digital media usage correlated with other dry eye questionnaires and was related to decreased blinking rate and tear film instability [32]. The discrepancy between subjective symptoms and objective parameters in diagnosis and treatment assessment of dry eye can be explained by the natural variability of disease pathophysiology, symptom subjectivity, and the variability of cornea sensation [33,34,35]. According to the increasing importance of subjective and objective factors for dry eye treatment, various symptomatic questionnaires have been developed, while the patient-reported outcome guidance of FDA questionnaires recommends that they include objective indicators, subjective symptoms, and psychological feasibility. However, increasing the number of items in questionnaires may lead to a reduced response rate or recall bias due to patient input requirements [14]. Appropriate questionnaires with proper evaluating items should therefore be used according to their proper assessment; the impact of dry eye on quality of life, screening or diagnosis for dry eye disease, and severity assessment of dry eye disease should be evaluated.

Treatment-related adverse events following the use of 0.05% cyclosporine included burning eye, foreign body sensation, conjunctival hyperemia, visual disturbance, and eye pain [36]. In the present study, two participants (10.5%) discontinued CsN due to ocular discomfort after administration. Although the discontinuing rate in the present study was relatively high compared to the 2.2% in previous reports, further investigation of adverse effects was necessary considering the small number of participants in this study [37].

Our study has several limitations. First, the duration may not be sufficient to prove the long-term effects of cyclosporine A. Cyclosporine A eye drops may affect conjunctival inflammation by preventing recruitment of T cells, which may require between 3 and 6 months. However, previous studies have already revealed the long-term safety and efficacy outcomes of cyclosporin A treatment [38]. Second, commonly used questionnaires such as OSDI and Standard Patient Evaluation of Eye Dryness Questionnaire (SPEED) were not utilized for symptomatic evaluation. However, OSDI is copyrighted by Allergan Inc., which can limit its use for other industry concerns in clinical trials. SANDE scores correlated with those of the OSDI in patients with mild-to-moderate dry eyes [19]. Third, this study comprised relatively young patients (47.9-years-old in the HA group, 41.1-years-old in the CsN group), who were predominantly female, and exclusively Korean. To generalize these findings, data should be collected from patients of diverse age groups, sexes, and races.

In conclusion, both 0.05% CsN and 0.15% HA administration effectively improved the objective signs and subjective symptoms of dry eye. However, patients treated with 0.05% CsN improved more rapidly and effectively than patients treated with HA.