Hypothetical potential pharmacokinetic profile of topically applied drug concentration in the tear film over time. (A) missed dose, the eye tissue without therapeutic levels of drug over time, (B) toxicity threshold, over administration of drug to its maximum level considered therapeutic, (C) the excessive volume of drug solution increases the drainage rate of instilled volume, and (D) controlled release drug delivery profile with a plateau level of drug distribution of topically applied formulations. Adapted, with permission, from (Ali and Byrne, 2008).