Many compounds entering Phase I trials encounter unforeseen obstacles during their development, so individual drugs will not be detailed. Several classes of drugs are in active development.
    One group is made up of new Topoisomerase I inhibitors. Topoisomerase is a DNA-modifying enzyme. These drugs are derivatives of the drug camptothecin, originally tested in 1970.
    Another promising new set of drugs are antifolate compounds. These counteract some of the actions of the vitamin folic acid derivatives that act as co-factors in vital enzyme reactions. Except for edatrexate ( see "Group C Drugs" ), these new compounds have different enzyme targets than the original antifolate, methotrexate . Iometrexol is in Phase I trial, whereas ZD1694 has advanced to Phase III trials in colon cancer.