DETERMINATION OF EXTRACELLULAR METHOTREXATE TISSUE-LEVELS BY MICRODIALYSIS IN A RAT MODEL

We used a microdialysis technique to determine tissue methotrexate (MT X) levels during steady state in a rodent model. Two different approac hes were employed to measure the actual extracellular MTX concentratio ns in muscle, liver, and kidney tissues of anesthetized Wistar rats. W ith the reduced-perfusion-rate technique, the flow in the microdialysi s perfusate was gradually decreased toward zero to permit calculation of zero-flow intercepts. Using the net change technique, microdialysis probes were perfused with different MTX concentrations to allow an as sessment of equilibrium drug levels. For these two methods to be used, drug concentrations in the matrix to be analyzed must remain unchange d during the experimental procedure. In the animal model, steady state was attained after 1.5 h and maintained throughout the rest of the ex periments by the administration of MTX as continuous infusions through a venous catheter. In vitro and in vivo, both the reduced-perfusion-r ate and net change techniques gave reproducible data that permitted th e estimation of extracellular drug concentrations in the dialyzed tiss ue compartments. The data suggest that the level of unbound MTX in the circulation is fairly similar to the extracellular concentrations in the muscle and liver. In the kidney, MTX levels were measured to be 3- 8 times higher than those of unbound, circulating MTX, and a considera ble discrepancy between the two methods used for estimations was appar ent. These results demonstrate that both the net change and reduced-fl ow microdialysis techniques can produce reproducible and precise data. The results may constitute a basis for determining recoveries and, th us, true extracellular drug levels during in vivo microdialysis of MTX . This may be of importance in delineation of the relationship between tissue MTX levels and outcome in a variety of normally inaccessible c ompartments during cancer pharmacotherapy.