Objective: Plasma methotrexate [MTX] concentrations monitoring is important in high-dose MTX [HDMTX]. However,
the availability of drug monitoring and its cost are problematic. The goal of this study was to study the MTX clearance in adult Thai patients and propose the simplified guideline.
Materials and Methods: This is a retrospective study examined osteosarcoma patients treated with multi-agent chemotherapy including HDMTX (12 g/m2) during 2011 to 2013. MTX serum levels were obtained every 24 h until the concentration fell below the threshold concentration of 0.1 mM. Leucovorin was given per standard protocol.
Results: A total of 326 MTX serum levels in 76 HDMTX courses were analyzed. The mean MTX serum peak values were 1,300 (980 to 1,618) mM. MTX serum level fell rapidly in the first 72 hours then gradually decreased over time. In 45% of the cases, leucovorin could be stopped at 96 hours after the infusion of MTX.
Conclusion: MTX serum level monitoring is mandatory in HDMTX regimen. Drug monitoring should start at 72 hours after HDMTX infusion and every 24 hours until the level is below 0.1 mM.

Keywords: Osteosarcoma, Methotrexate, Therapeutic drug monitoring