EVALUATION OF METHOTREXATE TISSUE EXPOSURE BY IN-SITU MICRODIALYSIS IN A RAT MODEL

The feasibility of using a microdialysis technique to obtain pharmacok inetic data on tissue exposure to methotrexate (MTX) was investigated. Microdialysis probes were implanted in the jugular vein, femoral musc le, and liver of anesthetized male Wistar rats. MTX (100 mg/ kg) was g iven as a bolus injection through an indwelling venous catheter, and b lood samples were obtained through a second venous access and by micro dialysis for a total of 6 h. Heparinized plasma, ultrafiltered plasma, and microdialysis effluent from tissue and venous probes were analyze d by high-performance liquid chromatography. Centrifugal ultrafiltrati on of rat plasma spiked in vitro with MTX (1-100 mu M) revealed a mean binding to plasma proteins of 21%. In vitro microdialysis of this spi ked plasma resulted in 23% relative recovery of the unbound fraction. In rats receiving MTX, plasma protein binding was 23% and the relative drug recovery as assessed with venous microdialysis probes was 18%. P lotting of unbound (i. e., ultrafiltrate) MTX concentrations in the bl ood against venous microdialysis perfusate values in the blood gave a good linear correlation with a coefficient of correlation (r(2)) of 0. 98. There was also a linear correlation between the total MTX concentr ations in venous blood and the drug levels in microdialysis samples fr om muscle and liver (r(2) = 0.93 and 0.74, respectively). Area under t he curve estimations were consistent with an MTX exposure of 30% and 4 6% for the muscle and liver as compared with the circulation. The pres ent study demonstrates that the microdialysis technique can provide re producible data on tissue exposure to MTX in an animal model and indic ates that the methodology is adaptable to clinical settings.