3.1 Inflammation

Inflammatory processes have been identified as key contributors to some DED sub-types, including DED in the setting of immune conditions (e.g., Sjögren’s syndrome [SS], graft versus host disease [GVHD]), anatomical abnormalities (e.g., conjunctivochalasis), and allergy (e.g., atopic dermatitis) [11]. Outside of these conditions, tear abnormalities themselves (e.g., hyperosmolarity, low tear production) can contribute to a pro-inflammatory state mediated by T cells, with downstream products including metalloproteinases, tumor necrosis factor-α (TNFα), and other pro-inflammatory cytokines, among others [11, 12]. In the US, most of the available anti-inflammatory therapies for DED target T cells, through various mechanisms [13]. Recently, research has focused on developing more potent T cell inhibitors with fewer side effects (e.g., instillation site reactions) and on focusing on other targets of inflammation.

Novel therapies are emerging, yet it is pertinent to highlight the availability of current anti-inflammatory treatments for DED. Two compounds that are currently available are cyclosporine and lifitegrast. There are two available ophthalmic formulations of cyclosporine, a calcineurin inhibitor and immunosuppressive agent [14, 15]. Cyclosporine 0.05% and 0.09%, trialed in adults with DED based on questionnaire scores and clinical features, most robustly increased tear production and less robustly, but still significantly, reduced DED-related symptoms compared to placebo across various trials [14,15,16,17,18]. Lifitegrast is a lymphocyte function-associated antigen-1 (LFA-1) antagonist whose anti-inflammatory mechanism of action comes from blocking the interaction of LFA-1 with its cognate ligand intercellular adhesion molecule-1 (ICAM-1) [19]. In clinical trials, lifitegrast 5% improved both eye dryness scores and ocular surface signs, including tear production and corneal staining, from baseline compared to vehicle [20,21,22]. In terms of available short-term treatment (up to 2 weeks) options, loteprednol etabonate is a topical anti-inflammatory corticosteroid indicated for dry eye flares [23]. Loteprednol etabonate ophthalmic suspension 0.25% was evaluated in clinical trials involving adult participants with ocular discomfort (based on Symptom Assessment in Dry Eye [SANDE]) and signs (non-anesthetized Schirmer tear test [STT], corneal staining, and conjunctival hyperemia) and was found to improve ocular discomfort severity and conjunctival hyperemia compared to vehicle [23, 24] (Table 1). Beyond the abovementioned products, several compounds are currently under investigation or have been recently approved and will be discussed below.

Table 1 Summary of FDA-approved pharmacological treatments for managing various DED categories, including inflammation, eyelid abnormalities, neuropathic conditions, and Neurotrophic Keratitis3.1.1 Vevye (Harrow Health, Inc. and Novaliq GmbH)

Vevye is a topical formulation of cyclosporine A (a calcineurin inhibitor that functions primarily by inactivating T cells and their functions) delivered in a novel tear-stabilizing water/preservative-free vehicle (perfluorobutylpentane) [25,26,27]. T cells play an essential role in mediating inflammation and amplifying immune responses related to DED [26]. Their diverse mechanisms include the release of a multitude of cytokines, some of which are responsible for cell death, reduced tear production, activation of antigen-presenting cells (APCs), or initiation of the release of matrix metalloproteinases (MMPs) [26, 28]. Correspondingly, higher levels of CD4+ T cells have been reported on flow cytometry from ocular surface wash samples in individuals with DED (diagnosis based on TFOS DEWS II) as compared to healthy individuals [29]. In this study, CD4+ T cell count correlated with Ocular Surface Disease Index (OSDI) (r = 0.28; p = 0.008) and tear break-up time (TBUT) scores (r = −0.31; p = 0.001) [29]. A randomized, double-masked, vehicle-controlled, phase 3 trial evaluated the efficacy of topical Vevye 0.1% (n = 423, administered twice daily) compared to a vehicle (n = 411) in individuals with DED (defined by symptoms, total cornea fluorescein staining [CFS] ≥ 10, and Schirmer 1–10 mm, among other requirements) [30]. After 4 weeks of therapy, dryness assessed with a visual analog scale (VAS) showed improvement from baseline in both groups (−12.2 vs. −13.6, respectively; p = 0.38; range 0–100) [30]. Additionally, the Vevye 0.1% group showed a greater improvement in total CFS assessed with the National Eye Institute (NEI) scale from baseline when compared to the vehicle group (−4.0 vs. −3.6, respectively; p = 0.03; range 0–15) [30]. As seen with previous cyclosporine formulations, ≥ 10 mm improvement in tear production (STT) was more frequently noted in individuals treated with Vevye compared to controls (11% vs 7%; p = 0.05) [31]. The US Food and Drug Administration (FDA) approved the medication on May 30th, 2023 [32]. Given its mechanism of action, Vevye may impact a DED sub-type associated with T cell-mediated inflammation and decreased tear production.

3.1.2 Reproxalap (Aldeyra Therapeutics, Inc.)

Reproxalap is a topical medication that inhibits reactive aldehyde species (RASP) [33]. Malondialdehyde and 4-hydroxynonenal are two RASP molecules, and major ocular biomarkers of oxidative stress [34, 35]. They result from lipid peroxidation and may be relevant molecules in DED pathogenesis due to their high cytotoxicity, ability to upregulate pro-inflammatory signaling cascades, and binding affinity to phosphatidylethanolamine, a component of the tear lipidome involved in retaining moisture on the ocular surface [36, 37]. Elevated levels of these molecules have been reported on the tear film and ocular surface in individuals with DED (defined by symptoms, TBUT ≤ 7 s, and Schirmer ≤ 7 mm, among other requirements) [38]. A randomized, double-blinded, vehicle-controlled, phase 2b trial looked at the efficacy of reproxalap (0.25% and 0.1%, n = 100 in each group, administered daily 4 times per day) compared to a vehicle (n = 100) in individuals with DED (defined by symptoms, TBUT ≤ 5 s, CFS ≥ 2 in ≥ 1 region, and Schirmer 1–10 mm, among other requirements) [33]. After 12 weeks of therapy with reproxalap 0.25%, all symptoms assessed with the Ora Calibra Ocular Discomfort & 4-Symptom Questionnaire (OD4SQ) improved from baseline when compared to reproxalap 0.1% and the vehicle group, especially for dryness (−0.9 vs. −0.6 vs. −0.5, respectively; p = 0.047 for comparison of 0.25% and vehicle; range 0–5) [33]. Additionally, reproxalap 0.25% improved all clinical signs, most robustly nasal CFS, from baseline when compared to reproxalap 0.1% and the vehicle group (−0.3 vs. ~ −0.28 vs. −0.1, respectively; p = 0.03 for comparison of 0.25% and vehicle; range 0–4) [33]. Aldeyra Therapeutics, Inc. submitted a Special Protocol Assessment (SPA) for a new clinical trial on November 16th, 2023, to address FDA feedback and gain regulatory approval [39]. Given its mechanism of action, reproxalap may impact a DED sub-type associated with inflammation, specifically high levels of RASP. However, a limitation in DED is that no current diagnostic tests evaluate ocular surface RASP levels. There are diagnostic tests that qualitatively assess MMP-9 (InflammaDry) but more diagnostics are needed to examine specific pathways of inflammation in an individual patient [40].

3.1.3 Thymosin β4 (RegeneRx Biopharmaceuticals, Inc. and HLB Therapeutics Co. Ltd)

Thymosin β4 is a topical G-actin–sequestering protein that is thought to reduce apoptosis, facilitate wound healing, and diminish inflammation [41]. The exact mechanism remains uncertain; however, it is thought to involve the suppression of nuclear factor kappa B (NF-kB) [42]. NF-kB is a transcription factor central to numerous immunobiological processes, including the regulation of multiple pro-inflammatory cytokines and chemokines associated with aspects of DED [43]. In a DED mouse model (induced by topical instillation of benzalkonium chloride), decreased corneal staining, corneal epithelial cell apoptosis, conjunctival NF-kB activation, and pro-inflammatory cytokine expression were all noted in mice receiving recombinant human thymosin β4 compared to controls [44]. A randomized, double-masked, vehicle-controlled, phase 2 trial looked at the efficacy of topical thymosin β4 0.1% (n = 36, administered twice daily) compared to a vehicle (n = 36) in individuals with DED (defined by symptoms, and CFS ≥ 2, among other requirements) [45]. Symptoms and signs were evaluated before and after exposure to a controlled adverse environment (CAE) chamber. After 28 days of therapy with thymosin β4 0.1%, ocular discomfort graded on a 0–4 scale showed a lower increment of discomfort post-CAE compared to placebo (1.6 vs. 2.2; p = 0.02; range 0–4) [45]. Additionally, thymosin β4 0.1% showed improvement in clinical signs, most robustly CFS in the superior region post-CAE from baseline when compared to vehicle (−0.14 vs. 0.32; p = 0.02; range 0–4) [45]. In October 2022, an SPA request for the fourth phase 3 clinical trial for DED (ARISE-4) was submitted to the US FDA [46]. Meanwhile, an ongoing phase 3 clinical trial (SEER-2) is investigating the potential of this compound for neurotrophic keratitis (NK) [47]. Given its mechanism of action, thymosin β4 may impact a DED sub-type associated with inflammation, specifically high levels of NF-kB. However, limitations are that current tests do not measure levels of NF-kB in tears.

3.1.4 Licaminlimab (Oculis Holding AG)

Licaminlimab is a topical single-chain antibody fragment that binds and inhibits the activity of human TNFα [48]. Primarily produced by macrophages, TNFα is a crucial cytokine that regulates immune responses, contributes to tissue degeneration and cell proliferation, and can influence the regulation of proinflammatory molecules, like NF-kB and mitogen-activated protein kinases, closely involved with DED pathogenesis [49, 50]. Elevated levels of TNFα messenger ribonucleic acid (mRNA) have been reported on conjunctival brush cytology in individuals with SS as compared to a non-SS DED and a control non-DED group [51]. A randomized, double-masked, vehicle-controlled, phase 2 trial evaluated the efficacy of topical licaminlimab 60 mg/mL (n = 69, administered daily 3 times per day) compared to a vehicle (n = 65) in individuals with DED (defined by symptoms, and hyperemia ≥ 1 in ≥ 2 quadrants, among other requirements) [48]. After 4 weeks of therapy with licaminlimab 60 mg/mL, ocular discomfort assessed with the SANDE questionnaire showed a significant improvement from baseline when compared to the vehicle group (−7.9 ± 1.45 vs. −3.6 ± 1.49; p = 0.04; range 0–100) [48]. Additionally, clinical evaluation of total CFS assessed in five regions showed improvement in both licaminlimab 60 mg/mL and the vehicle group (−1.1 vs. −1.4, range 0–20) [48]. An ongoing phase 2b clinical trial (RECOVER) is investigating the potential of this compound for DED [52]. Given its mechanism of action, licaminlimab may impact a DED sub-type associated with high levels of TNFα, but again, this metric cannot be currently measured using available diagnostic testing.

3.1.5 Tanfanercept (HanAll Biopharma Co. Ltd)

Tanfanercept is a topical solution made of a modified TNF receptor 1 (TNFR1), specifically from the TNFα binding fragment, that is able to block TNFα and its activity [53]. As previously mentioned, TNFα is a cytokine that plays a central role mediating the inflammatory cascade that contributes to the pathogenesis of multiple diseases, including DED, which suggest its downregulation as a potential treatment [54]. In a mouse model of DED (induced by subcutaneous injections of scopolamine hydrobromide and a controlled environmental chamber), topical tanfanercept suppressed inflammatory cytokines in corneal and lacrimal gland samples, in addition to improving goblet cell counts and corneal erosion scores [53]. A randomized, double-blinded, vehicle-controlled, phase 2 trial evaluated the efficacy of topical tanfanercept 0.25% (n = 50, administered twice daily) compared to a vehicle (n = 50) in individuals with DED (defined by symptoms, CFS ≥ 2 in ≥ 1 region, and Schirmer 1–10 mm, among others) [55]. Symptoms and signs were evaluated before and after exposure to a CAE chamber [55]. After 8 weeks of therapy all symptoms assessed with the OD4SQ improved from baseline in both groups, especially for discomfort (approximate change −1.30 vs. −1.26; p = 0.46; range 0–5) [55]. Additionally, for the change from pre- to post-CAE after 8 weeks, there was improvement on inferior CFS assessed with the Ora Calibra Corneal and Conjunctival Fluorescein Staining Scale in both groups (−0.61 ± 0.11 vs. −0.54 ± 0.11; p = 0.65; range 0–4) as well as in Schirmer test scores (1.87 ± 0.62 vs. 1.28 ± 0.62 mm; p = 0.50) [55]. On May 19, 2023, after completion of the clinical trial VELOS-3, HanAll Biopharma Co. Ltd projected intentions to advance into the next study, schedule to begin in 2024 [56]. Given its mechanism of action, tanfanercept may improve symptoms and signs in a DED sub-type characterized by high levels of TNFα.

3.1.6 SkQ1 (Essex Bio-Technology Ltd)

SkQ1 is a topical synthetic mitochondria-targeted antioxidant that functions to suppress reactive oxidative species (ROS) generation, oxidative stress propagation and increase intrinsic antioxidant defense [57]. ROS elevations can lead nuclear acids, lipids, and proteins to undergo irreversible oxidative modifications and elevate proinflammatory cytokine expression [57]. Higher levels of lipid oxidative stress markers have been observed in tear and conjunctival brush cytology of individuals with SS compared to healthy controls [58]. These levels were also correlated with worse tear stability and staining scores in SS [58]. As the primary source of ROS (~ 90%) is mitochondria, the SkQ1 molecule composition includes the naturally occurring antioxidant plastoquinone conjugated with the transport molecule triphenyl phosphonium, to enable cellular penetration and accumulation within the mitochondria [57, 59]. A randomized, double-blinded, vehicle-controlled, phase 2 trial evaluated the efficacy of SkQ1 (1.55 μg/mL vs. 0.155 μg/mL, n = 30 in each group, administered daily 2 times per day) compared to a vehicle (n = 31) in individuals with DED (defined by symptoms, CFS ≥ 2 in ≥ 1 region, and Schirmer 1–10 mm, among others) [60]. Symptoms and signs were evaluated before and after exposure to a CAE chamber [60]. After 29 days of therapy with SkQ1 1.55 μg/mL, symptoms of grittiness assessed with the OD4SQ improved post-CAE from baseline when compared to SkQ1 0.155 μg/mL and the vehicle group (−0.50 ± 0.90 vs. −0.40 ± 1.10 vs. 0.00 ± 1.17; range 0–5) [60]. Interestingly, post-CAE total CFS (assessed with the Ora Calibra Corneal and Conjunctival Fluorescein Staining Scale) improved from baseline in all groups (−0.70 vs. −1.23 vs. −0.95; range 0–20) [60]. After acquisition of SkQ1 from Mitotech on October 13th, 2022, Essex Bio-Technology is focused on completing the transfer of knowledge related to the drug and addressing potential risks with regulators before resuming clinical trials [61]. Given its mechanism of action, SkQ1 may impact a DED sub-type with high levels of ROS, but again, this metric cannot be currently measured using available diagnostic testing.

3.1.7 Summary

As above, many different aspects of inflammation including T cell activation, ROS generation, and cytokine-mediated inflammation have been noted in individuals with various sub-types of DED. Various compounds that target these aspects of inflammation have shown promising results in the developmental stage (reproxalap, thymosin β4, licaminlimab, tanfanercept, SkQ1) while Vevye has already secured approval. The success of precision-based DED therapies highlights a gap in the capabilities of our diagnostic tools, particularly in accurately identifying involved inflammatory profiles. Future research dedicated to advancing diagnostic technologies is the key to offering tailored treatment to individuals.

3.2 Eyelid Abnormalities: Demodex Blepharitis and Meibomian Gland Dysfunction

Demodex blepharitis and MGD are prevalent ocular conditions that can contribute to DED symptoms and signs [62, 63]. Demodex infestation, with cylindrical dandruff as its pathognomonic presentation, is a common cause of anterior blepharitis (with staphylococcal and seborrheic blepharitis as other variants) [63]. Two species of Demodex mites (folliculorum and brevis) can colonize human eyelash follicles, with the frequency of colonization increasing with age and impaired immunity [