Synovial Microdialysis for Evaluating Methotrexate Disposition in Male Sprague-dawley Rats: A Translational Approach to Anti-rheumatoid Drug Discovery.

INTRODUCTION

The efficacy of Rheumatoid Arthritis (RA) treatment depends on adequate drug exposure within the joint; however, intra-articular pharmacokinetics remain poorly understood. Synovial microdialysis enables continuous measurement of unbound drug concentrations in joints. This study aimed to evaluate the applicability of techniques like microdialysis and lavage collection as preclinical tools for measuring unbound intra-articular drug exposure by characterizing methotrexate (MTX) pharmacokinetics in rats.

METHODS

In this study, male Sprague-Dawley (SD) rats were used for evaluating the kinetics of MTX. A linear probe was implanted in the knee joint of anesthetized rats to perform synovial microdialysis. An intravenous dose of 1 and 3 mg/kg was administered for synovial microdialysis, while lavage was performed in a parallel group at 3 mg/kg for comparison. Plasma protein binding was determined using the equilibrium dialysis method. Pharmacokinetic parameters were derived using non-compartmental analysis, and inter-method agreement was evaluated.

RESULTS

MTX showed moderate plasma protein binding (~51% unbound) and dose-proportional systemic exposure at tested doses. Microdialysis data revealed rapid joint penetration (Tmax ≈ 0.25 h) and synovial half-life comparable to unbound plasma kinetics. At 3 mg/kg, microdialysis and lavage demonstrated close agreement (2.9% difference, ~2% CV), with microdialysis yielding slightly higher concentrations, likely due to its ability to measure unbound drug without dilution bias.

DISCUSSION: The strong concordance between microdialysis and lavage confirms both methods capture intra-articular MTX exposure. However, microdialysis offers superior physiological relevance by continuously quantifying unbound drug without dilutional artifacts. The alignment of synovial and plasma half-life supports rapid equilibration between systemic and joint compartments, reinforcing its utility for mechanistic and PK-PD studies.

CONCLUSION

Synovial microdialysis provides a robust and physiologically relevant approach for quantifying active drug exposure in the joint and offers clear advantages over synovial lavage for preclinical evaluation of anti-rheumatic therapies.