Effect of pH and Benzalkonium Chloride in Low-Dose Atropine on Pupil Dilation in Dutch Belted Rabbits

Purpose: To investigate the impact of altering common components of ophthalmic formulations, specifically pH and benzalkonium chloride (BAK), on efficacy and bioavailability of low-dose atropine compositions as measured by pupil dilation. Methods: Sixty Dutch Belted rabbits were screened for atropinesterase, and 20 were selected for functional testing with 0.02% atropine, 0.01% BAK, and pH 7. After a 7-day washout, 6 rabbits were randomized to group 1 (0.02% atropine, 0% BAK, H 2 O, pH 4) and 6 to group 2 (0.02% atropine, 0.01% BAK, D 2 O, pH 7). Rabbits received a single drop in each eye of the assigned treatment. Pupillometry was performed at baseline, and 30, 60, 120, 240, 360, 480, 600, and 720 min post-dose. After a 7-day washout, the rabbits received a single drop of the crossover treatment and repeated the same exam. Pupil diameters were averaged for both eyes and then averaged for the 6 rabbits at each time point, in group 1 and in group 2 for each treatment. Mean pupil diameters were also pooled for the same treatment. Change from baseline pupil diameter and standard deviations/errors were calculated. Between-group differences were evaluated with a paired t- test. Results: Change from baseline pupil diameter demonstrated that the eyes treated with 0.02% atropine, 0% BAK, H 2 O, and pH 4 were statistically significantly smaller at all post-baseline time points compared to those treated with 0.02% atropine, 0.01% BAK, D 2 O, and pH 7. Conclusions: Formulation, specifically the addition of BAK and near physiological pH, makes a profound difference to the efficacy of low-dose atropine eye drops.