Challenges and Strategies in Drug Residue Measurement (Bioanalysis) of Ocular Tissues

Bioanalysis (quantification of drug/metabolite residue in biological fluids and tissues) plays an important role in support of drug efficacy and safety studies during drug development. Bioanalysis can be very challenging when the drug or the metabolites to be measured are unstable or are difficult to extract from biological matrices or when significant matrix interference is experienced during detection of the analytes of interest. Ocular tissue bioanalysis is particularly challenging because of the anatomical complexity of the eye. Furthermore, several heterogeneous tissues of the eye such as conjunctiva, sclera, cornea, and retina may act as barriers to drug absorption and distribution within the eye and therefore each ocular tissue must be collected individually and analyzed to determine drug and metabolite concentrations. Many ophthalmic drugs are administered topically and less than 5 % of the applied drug is likely to penetrate the eye. Most of the topically applied drug may be washed away by tears, cleared by other mechanisms, and/or absorbed into the systemic circulation. Because of the limited bioavailability, the drugs may not always reach targeted therapeutic concentrations in the ocular tissue(s). Without sensitive and accurate bioanalytical methods to measure and demonstrate that drug concentrations within the ocular tissues reach appropriate levels, following drug administration, erroneous conclusions may be drawn regarding efficacy and/or local safety. Pharmacokinetic measurements in blood/plasma for assessing systemic bioavailability, or residence time and drug distribution, are generally not useful in ocular studies since systemic exposure may not be directly related to exposure in the tissues of the eye after local ocular routes of administration. Systemic exposure, nonetheless, is very important in evaluating drug safety. Limited availability of control matrices needed for ocular bioanalysis studies poses additional challenges that may be partially addressed by using surrogate matrices. This chapter discusses the unique challenges and their resolution during eye tissue collection, method development, and the conduct of sample analysis for ocular bioanalytical studies.