Determination of Free Extracellular Levels of Methotrexate by Microdialysis in Muscle and Solid Tumor of the Rabbit

Abstract

Purpose. To determine of the pharmacokinetic profile of methotrexate (MTX) in blood and extracellular fluid (ECF) of VX2 tumor and muscle in rabbits.

Methods. Microdialysis probes were inserted into VX2 tumor and in muscle tissue. Following intravenous administration of MTX (30 mg/ kg), serial collection of arterial blood samples and dialysates of muscle and tumor ECF for 4 h was carried out. Quantitation of MTX and determination of free plasma concentrations was performed by fluorescence polarization immunoassay and ultrafiltration, respectively. Correlations were established between the unbound plasma and ECF MTX concentrations.

Results. Total and free plasma concentrations exhibited a parallel three exponential decay in both healthy and tumorigenic animals. Total clearance (8.9 vs 6.5 ml⁻¹.min⁻¹.kg⁻¹) and volume of distribution (4.0 vs 2.9 1.kg⁻¹), however, tended to decrease in the tumor-bearing group. The ECF/plasma AUC ratio equaled 14.2 ± 8.8% in muscle and 23.9 ± 15.9% in tumor. The concentration-time profile of muscle ECF MTX was parallel and highly correlated (r = 0.97) to that determined in plasma. In contrast, free MTX plasma levels were not correlated with tumor ECF concentrations (r = 0.564).

Conclusions. In addition to the well-known pharmacological variability in the concentration-effect relationship, the important inter-individual variability in tumor exposure to MTX may partly explain that studies in patients with solid tumors have often failed to demonstrate firm correlations between MTX blood pharmacokinetics and the chemotherapeutic response.