Investigators have identified a possible mode of action for low dose atropine — via the muscarinic receptors in the posterior segment of the eye, according to research published in Optometry and Vision Science. As little as one drop of the treatment can produce a measurable concentration in both the aqueous and vitreous humor, according to the report.
Researchers enrolled patients who were undergoing either cataract surgery (n= 48; mean age, 54.4 years; 60.42% men) or vitreoretinal surgery (n= 30; mean age, 61.4 years; 33% men) in the cross-sectional interventional pilot study. The team obtained aqueous or vitreous humor samples in the cataract and vitreoretinal surgery groups, respectively, at various time intervals (cataract surgery group: 1 hour, 2 hours, 4 hours, 6 hours; n=12 for each interval; vitreoretinal surgery group: 2 hours, 4 hours, 6 hours; n=10 for each interval) following instillation of 1 drop of low dose atropine. A liquid chromatography-mass spectrometry analysis determined atropine concentration at each interval.
Atropine concentration in the aqueous samples were 1.33 ng/mL at 1 hour, 2.60 ng/mL at 2 hours, 1.615 ng/mL at 4 hours, and 1.46 ng/mL at 6 hours. Concentrations in the vitreous samples were 0.102 ng/mL at 2 hours, 0.1715 ng/mL at 4 hours and 0.2495 ng/mL at 6 hours, according to the report.
The study authors theorize that the measurable concentrations of low dose atropine detected in the aqueous and vitreous humor suggest that posterior segment muscarinic receptors may be a possible mode of action for the drug.
“Topical atropine may possibly act through these muscarinic receptors, especially the scleral muscarinic receptors, to regulate eye growth,” the researchers explain. “Therefore, these posterior ocular structures may be the target tissue for topically administered atropine in anti-myopia therapy.”
Study limitations include a short study duration and a failure to consider age, ethnicity, and drug formulation, which are factors that can affect drug penetration.
Kumar P, Saxena R, Dhiman R, et al. Evaluation of the levels of low dose topical atropine (0.01%) in aqueous and vitreous humor in human eyes. Optom Vis Sci. Published online July 27, 2023. doi:10.1097/OPX.0000000000002044