Ocular medications have an important role in the treatment of glaucoma. Medications are usually considered the first line of treatment for glaucoma, and in most glaucoma patients medications alone can control their disease. Glaucoma medications lower intraocular pressure (IOP) by either reducing aqueous production or increasing aqueous outflow through either the conventional or the unconventional pathways. Frequently, multiple glaucoma medications are used in combination to adequately lower IOP. A clear understanding of the pharmaco-kinetics of these medications is important to knowing several details:
1. Whether the drug itself or a metabolite is responsible for the therapeutic effect
2. The optimal route of drug administration
3. The optimal dosage regimen
4. The relationship between drug concentrations in tissues and their pharma-cologic or toxicologic response
Pharmacokinetics is the study of the time-course changes of drug concentrations and their metabolites in tissues. It involves the determination of the rates of four processes: absorption, distribution, metabolism, and excretion.1 Biopharmaceutics is the study of the effects of drug formulation on the pharmacologic and therapeutic activity.2 It deals with the relationships between the drug response and the drug's physical state, salt form, particle size, crystalline structure, surface area, dosage form, adjuvants, or preservatives present in the formulation. Pharmacokinetic and bio-pharmaceutic data are important for making informed judgments on drugs and their formulations—judgments that may allow the proper selection of an appropriate drug, dosage regimen, and method of drug delivery to achieve a desired therapeutic outcome.
Drug-receptor interactions determine the intensity of a pharmacologic response. These interactions are governed by laws of mass action; therefore, the greater the concentration of free drug at the receptor site, the higher the statistical probability that the drug will bind to the receptor and have a greater pharmacologic effect.3 As the drug concentration declines around the receptor site, the drug response declines proportionately. Drug dose, dosage regimen, and route of administration can influence the concentration of drug in the target tissue; usually, the clinician has some control over these variables.
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