Occurrence and significance of D-methotrexate as a contaminant of commercial methotrexate.

Methotrexate from various commercial sources has been found to contain 0.5 to 48% (w/w) of the enantiomer D-methotrexate. The two methotrexate enantiomers were separated by using chiral high-performance liquid chromatography with an octadecyl silica column and a mobile phase containing L-proline and cupric nitrate. For the assay of D-methotrexate impurity in commercial methotrexate, L-methotrexate was hydrolyzed with carboxypeptidase G1, and the remaining D-methotrexate was quantitated by high-performance liquid chromatography. The biological effects of D-methotrexate were investigated and compared to that of L-methotrexate. D-Methotrexate was found to be a good inhibitor of dihydrofolate reductase from both murine and human tumor cells, but was a poor inhibitor of L1210 and CCRF-CEM cell growth. In animal experiments with dogs and mice, D-methotrexate was rapidly absorbed from the intestine and excreted by the kidneys.