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acid so that it could be utilized for the synthesis of thymine for incorporation intoDNA. Surprisingly, no other antifolate has emerged to rival methotrexate in thetreatment of either acute leukaemia or cancers.OH 2 N N NCOOHNHNNHCONHCHCH 2 CH 2nCONHCORCH 2 CH 2 COOH(17) n = 0, R = NHCH(COOH)CH2CH2COOH (18) n =2, R = OHOCOOHHNNNHCONHCHCORN H 2NNCH 2 CH 2nCONHCH 2 CH 2 COOH(19) R = H (20) R = MeOf the various halogenated pyrimidines which have been prepared, onlythe fluorinated pyrimidines and nucleosides have useful antitumour activity,and of these the most significant are 5-fluorouracil (FU) and it’s 2’-deoxyribonucleside (FUdR; Floxuridine). Both of these drugs have found wideuses in patients suffering from metastatic cancer, and have been administeredboth singly and in combination therapy. The presence of flourine, which cannotbe abstracted by base, halts the process. In contrast, the 5’-monophosphate of5-chloro-, 5-bromo- and 5- iodo- 2’-deoxyuridine, in which the halogen atomis bulkier and less electro- negative, have little or no useful inhibitory activity forthe enzyme. The drugs of53Pyrimidine…..