Pharmacokinetics generally encompasses the process of absorption, distribution, metabolism, and excretion (ADME) of drugs. For the most part, ADME deals with systemically administered drugs that reach their pharmacological target by way of the systemic circulation, that is, blood, following oral or parenteral administration. Ocular pharmacokinetics embraces many features of ADME, but on a smaller scale and specifically applied to the eye. However, owing to the previously discussed unique anatomic, physiological, and barrier properties of the eye and surrounding tissue, ocular pharmacokinetics often can be as difficult to describe and predict as its systemic counterparts. The task is further confounded by the myriad of formulations, routes, and dosing regimens typically encountered in ophthalmology. For a more comprehensive discussion of ocular pharmacokinetics the reader is referred to various review articles (11,15-23). An overview of the principles regulating drug distribution is provided in section "Ocular Drug Transport and Delivery," but first we summarize the phenomenology.
While pharmacokinetics or ADME is often described as the action of the body on a drug, pharmacodynamics is the action of the drug on the body. More specifically, pharmacodynamics is the measurement of pharmacological response relative to dose or concentration. The pharmacological response induced by a drug in the eye can vary greatly from individual to individual because of differences in factors such as eye pigmentation, pathological state of the eye, tearing, and blink rate. The application of pharmacological endpoints is particularly useful in the study of drugs in the human eye, where, for obvious reasons, the ability to determine the ocular pharmacokinetics based on ocular tissue concentrations is severely limited.
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