Coadministration With Carbidopa Enhances the Antimyopic Effects of Levodopa in Chickens

Administration of Nicotine Can Inhibit Myopic Growth in Animal Models

Purpose

While previously investigating the mechanism by which atropine inhibits ocular growth, we observed that stimulation of nicotinic receptors can inhibit experimental myopia. This study expands on that preliminary finding and investigates the safety and efficacy of nicotinic stimulation in the inhibition of ocular growth.

Methods

Nicotine's ability to inhibit form-deprivation myopia (FDM), following intravitreal injection (9 chicks per group) or topical application (6 chicks per group), was investigated over three doses. The ability of nicotine to inhibit lens-induced myopia (LIM) was also tested (in 12 chicks). For ocular safety, following 4 weeks of topical treatment with nicotine (n = 10), pupillary reflex, intraocular pressure, corneal curvature/thickness, lens thickness, retinal health (retinal thickness/cell apoptosis), as well as retinal function (electroretinogram recordings) were assessed. We also examined the effects of nicotine on non-ocular autonomic functions in both chicks (n = 5) and mice (n = 5).

Results

Nicotine was observed to significantly inhibit the development of FDM in chicks when administered as an intravitreal injection (P < 0.05) or topical eye drops (P < 0.05), albeit not in a dose-dependent manner. Nicotine also inhibited LIM (P < 0.05) to a similar degree to that seen for FDM. Although ocular health was (for the most part) unaffected by nicotine, the highest topical dose induced a temporary reduction in cardiorespiratory output (P < 0.05).

Conclusions

Nicotine, administered as an intravitreal injection or topical eye drop, significantly inhibits the development of experimental myopia. Although the anti-myopic effects observed presently are interesting, the well-reported side effects (expanded on presently) and addictive properties of nicotine would preclude its clinical use.

Advances in myopia control strategies for children

Myopia has long been a global threat to public health. Timely interventions are likely to reduce the risk of vision-threatening complications. There are both established and rapidly evolving therapeutic approaches to slow myopia progression and/or delay its onset. The effective methods for slowing myopia progression include atropine eye-drops, defocus incorporated multiple segments (DIMS) spectacle lenses, spectacle lenses with highly aspherical lenslets target (HALT), diffusion optics technology (DOT) spectacle lenses, red light therapy (RLT), multifocal soft contact lenses and orthokeratology. Among these, 0.05% atropine, HALT lenses, RLT and +3.00 peripheral addition soft contact lenses yield over 60% reduction in myopia progression, whereas DIMS, DOT and MiSight contact lenses demonstrate at least 50% myopia control efficacy. 0.05% atropine demonstrates a more optimal balance of efficacy and safety than 0.01%. The efficacy of 0.01% atropine has not been consistent and requires further validation across diverse ethnicities. Combining atropine 0.01% with orthokeratology or DIMS spectacles yields better outcomes than using these interventions as monotherapies. Increased outdoor time is an effective public health strategy for myopia prevention while recent studies suggest that 0.05% low-concentration atropine and RLT therapy have promising potential as clinical myopia prevention interventions for high-risk groups. Myopia control spectacle lenses, being the least invasive, are safe for long-term use. However, when considering other approaches, it is essential to ensure proper instruction and regular follow-ups to maintain safety and monitor any potential complications. Ultimately, significant advances have been made in myopia control strategies, many of which have shown meaningful clinical outcomes. However, regular use and adequate safety monitoring over extended durations are imperative to foster confidence that can only come from extensive clinical experience.

The role of the serotonergic system in atropine's anti-myopic effects

The muscarinic cholinergic antagonist atropine is the most widely used pharmacological treatment for the visual disorder myopia (short-sightedness), the leading cause of low-vision worldwide. This study sought to better define the mechanism by which atropine inhibits myopic growth. Although classified as a muscarinic-cholinergic antagonist, atropine has been found to bind and modulate the activity of several non-cholinergic systems (e.g., serotonin). Thus, this study investigated whether the serotonergic system could underly atropine's anti-myopic effects. Using a chick model of myopia, we report that atropine's growth-inhibitory effects can be attenuated by pharmacological stimulation of the serotonin system. This may suggest that atropine can slow the development of myopia through inhibiting serotonergic receptor activity. We also observed that pharmacological antagonism of serotonergic receptors inhibits the development of experimental myopia in a dose-dependent manner, further demonstrating that modulation of serotonergic receptor activity can alter ocular growth rates. Finally, we found that neither experimental myopia, nor atropine treatment, induced a significant change in retinal serotonergic output (i.e., synthesis, transport, release and catabolism). This may suggest that, although myopic growth can be inhibited through modulation of serotonergic receptor activity (by atropine or serotonergic antagonists), this does not require a change in serotonin levels. These findings regarding a serotonergic mechanism for atropine may have significant ramifications for the treatment of human myopia. This includes assessing the use of atropine in patients who are also undergoing treatment to upregulate serotonergic signaling (e.g., serotonergic anti-depressants).

IMI 2023 Digest

Myopia is a dynamic and rapidly moving field, with ongoing research providing a better understanding of the etiology leading to novel myopia control strategies. In 2019, the International Myopia Institute (IMI) assembled and published a series of white papers across relevant topics and updated the evidence with a digest in 2021. Here, we summarize findings across key topics from the previous 2 years. Studies in animal models have continued to explore how wavelength and intensity of light influence eye growth and have examined new pharmacologic agents and scleral cross-linking as potential strategies for slowing myopia. In children, the term premyopia is gaining interest with increased attention to early implementation of myopia control. Most studies use the IMI definitions of ≤-0.5 diopters (D) for myopia and ≤-6.0 D for high myopia, although categorization and definitions for structural consequences of high myopia remain an issue. Clinical trials have demonstrated that newer spectacle lens designs incorporating multiple segments, lenslets, or diffusion optics exhibit good efficacy. Clinical considerations and factors influencing efficacy for soft multifocal contact lenses and orthokeratology are discussed. Topical atropine remains the only widely accessible pharmacologic treatment. Rebound observed with higher concentration of atropine is not evident with lower concentrations or optical interventions. Overall, myopia control treatments show little adverse effect on visual function and appear generally safe, with longer wear times and combination therapies maximizing outcomes. An emerging category of light-based therapies for children requires comprehensive safety data to enable risk versus benefit analysis. Given the success of myopia control strategies, the ethics of including a control arm in clinical trials is heavily debated. IMI recommendations for clinical trial protocols are discussed.

The safety and tolerability of levodopa eye drops for the treatment of ocular disorders: A randomized first-in-human study

Myopia is the leading cause of low vision worldwide and can lead to significant pathological complications. Therefore, to improve patient outcomes, the field continues to develop novel interventions for this visual disorder. Accordingly, this first-in-human study reports on the safety profile of a novel dopamine-based ophthalmic treatment for myopia, levodopa/carbidopa eye drops. This phase I, first-in-human, monocenter, placebo-controlled, double-blind, paired-eye, multidose, randomized clinical trial was undertaken in healthy adult males aged 18-30 years (mean age 24.9 ± 2.7) at the University of Canberra Eye Clinic, Australia. Participants were randomly assigned to receive either a low (1.4 levodopa:0.34 carbidopa [μmoles/day], n = 14) or standard dose (2.7 levodopa:0.68 carbidopa [μmoles/day], n = 15) of levodopa/carbidopa eye drops in one eye and placebo in the fellow eye once daily for 4 weeks (28 days). Over this 4-week trial, and after a 4-month follow-up visit, levodopa/carbidopa treatment had no significant effect on ocular tolerability and anterior surface integrity, visual function, ocular health, refraction/ocular biometry, and did not induce any non-ocular adverse events. These results indicate that topical levodopa/carbidopa is safe and tolerable to the eye, paving the way for future studies on the efficacy of this novel ophthalmic formulation in the treatment of human myopia. The findings of this study have implications not only for the treatment of myopia, but in a number of other visual disorders (i.e., amblyopia, diabetic retinopathy, and age-related macular degeneration) in which levodopa has been identified as a potential clinical intervention.

Candidate pathways for retina to scleral signaling in refractive eye growth

The global prevalence of myopia, or nearsightedness, has increased at an alarming rate over the last few decades. An eye is myopic if incoming light focuses prior to reaching the retinal photoreceptors, which indicates a mismatch in its shape and optical power. This mismatch commonly results from excessive axial elongation. Important drivers of the myopia epidemic include environmental factors, genetic factors, and their interactions, e.g., genetic factors influencing the effects of environmental factors. One factor often hypothesized to be a driver of the myopia epidemic is environmental light, which has changed drastically and rapidly on a global scale. In support of this, it is well established that eye size is regulated by a homeostatic process that incorporates visual cues (emmetropization). This process allows the eye to detect and minimize refractive errors quite accurately and locally over time by modulating the rate of elongation of the eye via remodeling its outermost coat, the sclera. Critically, emmetropization is not dependent on post-retinal processing. Thus, visual cues appear to influence axial elongation through a retina-to-sclera, or retinoscleral, signaling cascade, capable of transmitting information from the innermost layer of the eye to the outermost layer. Despite significant global research interest, the specifics of retinoscleral signaling pathways remain elusive. While a few pharmacological treatments have proven to be effective in slowing axial elongation (most notably topical atropine), the mechanisms behind these treatments are still not fully understood. Additionally, several retinal neuromodulators, neurotransmitters, and other small molecules have been found to influence axial length and/or refractive error or be influenced by myopigenic cues, yet little progress has been made explaining how the signal that originates in the retina crosses the highly vascular choroid to affect the sclera. Here, we compile and synthesize the evidence surrounding three of the major candidate pathways receiving significant research attention - dopamine, retinoic acid, and adenosine. All three candidates have both correlational and causal evidence backing their involvement in axial elongation and have been implicated by multiple independent research groups across diverse species. Two hypothesized mechanisms are presented for how a retina-originating signal crosses the choroid - via 1) all-trans retinoic acid or 2) choroidal blood flow influencing scleral oxygenation. Evidence of crosstalk between the pathways is discussed in the context of these two mechanisms.

Retinal Dopamine D2 Receptors Participate in the Development of Myopia in Mice

Purpose

To learn more about the locations of dopamine D2 receptors (D2Rs) that regulate form-deprivation myopia (FDM), using different transgenic mouse models.

Methods

One eye of D2R-knockout (KO) mice and wild-type littermates was subjected to four weeks of monocular FDM, whereas the fellow eye served as control. Mice in both groups received daily intraperitoneal injections of either the D2R antagonist sulpiride (8 µg/g) or vehicle alone. FDM was also induced in retina- (Six3^creD2R^fl/fl) or fibroblast-specific (S100a4^creD2R^fl/fl) D2R-KO mice. A subset of retina-specific D2R-KO mice and D2R^fl/fl littermates were also given sulpiride or vehicle injections. Refraction was measured with an eccentric infrared photorefractor, and other biometric parameters were measured by optical coherence tomography (n ≈ 20 for each group).

Results

FDM development was attenuated in wild-type littermates treated with sulpiride. However, this inhibitory effect disappeared in the D2R-KO mice, suggesting that antagonizing D2Rs suppressed myopia development. Similarly, the development of myopia was partially inhibited by retina-specific (deletion efficiency: 94.7%) but not fibroblast-specific (66.9%) D2R-KO. The sulpiride-mediated inhibitory effects on FDM also disappeared with retinal D2R-KO, suggesting that antagonizing D2Rs outside the retina may not attenuate myopia. Changes in axial length were less marked than changes in refraction, but in general the two were correlated.

Conclusions

This study demonstrates that D2Rs located in the retina participate in dopaminergic regulation of FDM in mice. These findings provide an important and fundamental basis for further exploring the retinal mechanism(s) involved in dopamine signaling and myopia development.