Uptake and metabolism of 5,8-dideazaisofolic acid in human colon carcinoma cells

The uptake and metabolism of radiolabeled 5,8-dideazaisofolic acid (IAHQ) (N-[p-[(2-amino-4-hydroxy-6-quinazolinyl)] amino]methylbenzoyl]-l-glutamic acid), a new antifol targeted to thymidylate synthase, has been investigated in the human colon adenocarcinoma cell line HCT-8. [³H]IAHQ uptake was very slow, requiring days to achieve the intracellular level achieved in minutes by [³H]methotrexate. This slow transport of IAHQ was consistent with the long exposures required to achieve cytotoxicity. Intracellular [³H]IAHQ was converted in a concentration-dependent manner to polygamma-glutamate derivatives containing between two and four additional glutamate residues. These results are consistent with our hypothesis that IAHQ is a "pro-drug" which must be converted to polyglutamate derivatives before it is a sufficiently potent inhibitor of thymidylate synthase to induce a pyrimidineless state and cell death.