Contact Lenses Loaded with Melatonin Analogs: A Promising Therapeutic Tool against Dry Eye Disease.

Melatonin analogs topically administered evoke a potent tear secretagogue effect in rabbits. This route of drug administration requires high drug concentration and frequent dosing due to its reduced ocular surface retention. Therefore, contact lenses (CLs) have emerged as an alternative drug-delivery system that prolongs drug retention in the cornea, improving its therapeutic performance. This study explores the in vitro ability of five commercially available hydrogel CLs to act as a delivery system for melatonin analogs and the in vivo secretagogue effect of melatonin analog-loaded CLs. We soaked CLs with melatonin or melatonin analog solutions (1 mM) for 12 h. Spectroscopic assays showed that IIK7-loaded CLs led to the inadequate delivery of this compound. Conventional hydrogel lenses loaded with agomelatine released more agomelatine than silicone ones (16-33% more). In contrast, the CLs of silicone materials are more effective as a delivery system of 5-MCA-NAT than CLs of conventional materials (24-29%). The adaptation of CLs loaded with agomelatine or 5-MCA-NAT in rabbits triggered a higher tear secretion than the corresponding eye drops (78% and 59% more, respectively). These data suggest that CLs preloaded with melatonin analogs could be an adequate strategy to combat aqueous tear deficient dry eye disease.

Effect of Melatonin and Its Analogs on Tear Secretion.

Melatonin has been shown to enhance tear secretion associated with dinucleotide diadenosine tetraphosphate. This study investigated the isolated action of melatonin and its analogs, agomelatine, N-butanoyl-2-(2-methoxy-6H-isoindolo[2,1-a]indol-11-yl) ethanamine (IIK7), and 5-methoxycarbonylamino-N-cetyltryptamine (5-MCA-NAT) (10 µl at 100 µM), on tear secretion when applied topically in the rabbit cornea and its relationship with the melatonin MT1, MT2, and MT3/quinone reductase QR2 receptors. The results showed a significant increase in tear secretion, with a maximal effect at 60 minutes for the agonists (138.9% ± 6.5%, 128.9% ± 6.4%, and 120.0% ± 5.2%, respectively; P < 0.05; 100% control) but not for melatonin (101.6% ± 7.9%; P > 0.05). Agonist action was tested combined with the antagonists DH97 (MT2 selective), prazosin (MT3/QR2 inhibitor), and luzindole (nonselective MT membrane receptor) (10 µl at 100 µM). DH97 reversed the effect of agomelatine, IIK7, and 5-MCA-NAT up to 30.85% ± 7.6%,108% ± 7.2%, and 87.01% ± 7.6%, respectively (P < 0.05; 100% control). Luzindole antagonized agomelatine and 5-MCA-NAT up to 67.35% ± 7.6% and 92.12% ± 8%, respectively (P < 0.05). Prazosin only reversed 5-MCA-NAT action up to 84.2% ± 7.7% (P < 0.05). These results suggest different pathways for the agonists to act through MT membrane receptors. Therefore, agomelatine, IIK7, and 5-MCA-NAT act through MT membrane receptors as secretagogues of tear secretion, and these analogs could be considered excellent therapeutic candidates for dry eye treatment. SIGNIFICANCE STATEMENT: Currently, dry eye with aqueous deficit is treated by adding artificial tears palliatively. This study shows that topical installation of three melatonin analogs (agomelatine, IIK7, and 5-MCA-NAT), but not melatonin, in therapeutic doses in the rabbit cornea significantly increases tear production, acting through different melatonin membrane receptor subtypes. Therefore, this study suggests that melatoninergic compounds could be considered excellent therapeutic candidates for dry eye treatment and ocular surface diseases occurring with a reduction in tear production.