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

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.

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, H₂O, pH 4) and 6 to group 2 (0.02% atropine, 0.01% BAK, D₂O, 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.

Change from baseline pupil diameter demonstrated that the eyes treated with 0.02% atropine, 0% BAK, H₂O, and pH 4 were statistically significantly smaller at all post-baseline time points compared to those treated with 0.02% atropine, 0.01% BAK, D₂O, and pH 7.

Formulation, specifically the addition of BAK and near physiological pH, makes a profound difference to the efficacy of low-dose atropine eye drops.