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.
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