The inverse relationship between reduced folate carrier function and Pemetrexed activity in a human colon cancer cell line

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Abstract
Pemetrexed, a new generation antifolate recently approved for the treatment of mesothelioma and non–small cell lung cancer, is an excellent substrate for the reduced folate carrier (RFC). To explore the carrier's effect on pemetrexed activity, RFC was inactivated in HCT-15 colon cancer cells by mutagenesis and PT632 selective pressure. A clone (PT1) was obtained with a glycine to arginine substitution at amino acid 401, resulting in the loss of RFC function. PT1 cells were resistant to PT632 (178-fold), methotrexate (4-fold), and ZD1694 (Tomudex, raltitrexed; 20-fold), but were 3-fold collaterally sensitive to pemetrexed when grown in 25 nmol/L of 5-formyltetrahydrofolate. PT1 cells transfected with wild-type RFC had antifolate sensitivities comparable to that of wild-type HCT-15 cells, indicating that the RFC mutation was the sole basis for resistance. Folate pools were contracted in PT1 cells by 32% or 60%, as measured by radiolabeling intracellular folates or by an enzyme binding assay, respectively. This was reflected in marked (6.5-fold) collateral sensitivity to trimetrexate. The initial uptake of pemetrexed in PT1 cells was markedly reduced (∼85%) but intracellular pemetrexed levels increased to ∼60% and ∼70% to that of wild-type cells after 2 hours and 6 days, respectively. There was increased pemetrexed inhibition of glycinamide ribonucleotide transformylase and, to a lesser extent, thymidylate synthase in PT1 cells growing in 5-formyltetrahydrofolate based on nucleoside protection analyses. Hence, loss of RFC function leads to collateral sensitivity to pemetrexed in HCT-15 cells, likely due to cellular folate pool contraction resulting in partial preservation of pemetrexed polyglutamylation and increased target enzyme inhibition. [Mol Cancer Ther 2006;5(2):438–49]

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