Plasma levels of 7-hydroxymethotrexate after high-dose methotrexate treatment

Thirteen patients with cancer being treated with high-dose methotrexate (MTX) chemotherapy (350-5,000 mg/6 h IV were entered in this study. Plasma levels of MTX and 7-OHMTX, its main circulating metabolite, were measured by an HPLC technique. 7-OHMTX appears rapidly in the blood, reaching a maximum 6-12 h after the beginning of treatment. The elimination of 7-OHMTX is slower than that of MTX, but the elimination half-lives (24-48 h) are not significantly different: 25.2 h for 7-OHMTX versus 20.3 h for MTX. In all cases, 24 h after starting infusion plasma levels of 7-OHMTX exceeded those of MTX. There was a positive and significant correlation between the dose administered and peak plasma 7-OHMTX. Finally, 7-OHMTX formation was shown to be relatively stable throughout the treatment.