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A New Method for Measuring Free Drug Concentration: Retinal Tissue as a Biosensor

¹From the Laboratory of Biomedical Engineering, Helsinki University of Technology, Espoo, Finland; and ²the Departments of Biological and Environmental Sciences and ³Chemistry, University of Helsinki, Helsinki, Finland.

PURPOSE. To develop a method of using isolated rat retina as a biosensor in experiments on controlled drug release for measuring the resultant concentration of free model drug in living tissue and for testing the biocompatibility of the polymers and polymeric nanostructures used as drug carriers.

METHODS. The method is based on the monotonic dependence of the photoresponse kinetics of retinal rods on the concentration of the membrane-permeable phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX). Changes in the time to peak (t_p) of linear-range rod photoresponses were followed by transretinal ERG mass potential recordings in the aspartate-treated, dark-adapted rat retina. The dependence of t_p on [IBMX] was measured, and the calibration curve thus obtained was used to determine the amount of IBMX released from polymeric structures. The biocompatibility of the carrier was first assessed by the degree to which rods retained stable function in the presence of the polymer or monomers alone.

RESULTS. The dependence of t_p on [IBMX] was well-described by a second-order polynomial. After each change of [IBMX], a new equilibrium state was reached within 6 to 9 minutes, depending on temperature. The amounts of IBMX released from biocompatible polymeric structures were measurable with good accuracy in the range 10 to 300 µM.

CONCLUSIONS. This method enables accurate concentration determinations of the model drug IBMX in retinal tissue in drug-release experiments. The concentration dependence of the photoresponse kinetics has to be calibrated for each retina and temperature. The same preparation can be used for rapid testing of possible bioincompatibility of various molecules.