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) concentrations in brain extracellular fluid of normal and RG-2 glioma-bearing rats. All animals received 50 mg/kg of MTX intraarterially following which serial blood and interstitial fluid samples were collected for 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-compartment model was fitted to the plasma MTX concentration-time data in both 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. The significant reductions in clearance and volume of distribution at steady-state were attributed in part to a cachectic state in the RG-2 animals 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) (brain tumor-bearing rats). The significantly higher AUC values obtained with RG-2 rats compared with control rats may have resulted from high plasma MTX concentrations and a more permeable blood-tumor barrier (BTB). A hybrid physiologically based pharmacokinetic model was used to characterize the mechanisms responsible for the high MTX brain tumor concentrations. In conclusion, a microdialysis technique was successfully utilized to examine the extracellular uptake of MTX in brain. This technique can be a powerful tool to evaluate intracerebral drug kinetics and the delivery of drugs to brain tumors.