Clinical potency of methotrexate, aminopterin, talotrexin and pemetrexed in childhood leukemias

Purpose

Renewed interest in antifols for the treatment of childhood cancers has resulted from identification of novel antifols with broad spectrums of anti-cancer activity and from re-evaluation of the original clinical antifol, aminopterin. In this pre-clinical study we evaluated the in vitro activity of both traditional antifols (methotrexate, aminopterin) and novel antifols (pemetrexed, talotrexin) in childhood acute leukemias and lymphomas.

Methods

We compared the in vitro cytotoxicity of methotrexate, aminopterin, pemetrexed, and talotrexin in a panel of six pediatric leukemia and lymphoma cell lines using the sulforhodamine B assay. In addition to defining a 50% growth inhibitory concentration (IC₅₀) for a 120-h drug exposure, we contrasted the activity of the drugs in the context of clinically achievable (tolerable) drug exposures using the area under the plasma concentration–time curve (AUC). We defined each agent’s clinical potency index (CPI) as the AUC achieved with standard pediatric dosing regimens divided by the in vitro IC₅₀.

Results

Across all cell lines, talotrexin (median IC₅₀ 7 nM) and aminopterin (median IC₅₀ 17 nM) had lower IC₅₀’s than methotrexate (median IC₅₀ 78 nM) and pemetrexed (median IC₅₀ 155 nM). However, the CPI for methotrexate (median 0.9) was significantly greater than that for aminopterin (median 0.4). In contrast, pemetrexed had a significantly better CPI (median 13) than the traditional antifols.

Conclusions

Aminopterin does not appear to offer any advantage over methotrexate for the treatment of childhood ALL. Further study of pemetrexed in childhood leukemias is warranted.