IN-VIVO MICRODIALYSIS TO CHARACTERIZE DRUG TRANSPORT IN BRAIN-TUMORS - ANALYSIS OF METHOTREXATE UPTAKE IN RAT GLIOMA-2 (RG-2)-BEARING RATS

Brain microdialysis was applied to sample free methotrexate (MTX) conc entrations in brain extracellular fluid of normal and RG-2 glioma-bear ing rats. All animals received 50 mg/kg of MTX intraarterially followi ng which serial blood and interstitial fluid samples were collected fo r 3 h and measured for MTX by an HPLC assay. Retrodialysis was used to estimate the in vivo recovery of MTX from brain. A linear two-compart ment model was fitted to the plasma MTX concentration-time data in bot h the normal and RG-2 groups. The mean total body clearance and volume of distribution at steady state of MTX varied from 0.90 +/- 0.3 to 0. 24 +/- 0.02 l h(-1) kg(-1) (P < 0.05) and from 0.58 +/- 0.24 to 0.21 /- 0.16 l kg(-1) (P < 0.05) in control and tumor rats, respectively. T he significant reductions in clearance and volume of distribution at s teady-state were attributed in part to a cachectic state in the RG-2 a nimals in which total body water was reduced. The mean MTX area under the interstitial fluid concentration-time curve (AUC) was 171.6 +/- 69 .14 mu g min ml(-1)(control) and 583.5 +/- 296.7 mu g min ml(-1) (brai n tumor-bearing rats). The significantly higher AUC values obtained wi th RG-2 rats compared with control rats may have resulted from high pl asma MTX concentrations and a more permeable blood-tumor barrier (BTB) . A hybrid physiologically based pharmacokinetic model was used to cha racterize the mechanisms responsible for the high MTX brain tumor conc entrations. In conclusion, a microdialysis technique was successfully utilized to examine the extracellular uptake of MTX in brain. This tec hnique can be a powerful tool to evaluate intracerebral drug kinetics and the delivery of drugs to brain tumors.