In our recent studies on the synthesis of new antifolates, we prepared and evaluated 5,10-dideaza-5,6,7,8-tetrahydrofolic acid (DDATHF). This compound may be the first example of a new class of folate antagonists, active as antileukemic agents, whose primary target seems to be GAR transformylase, an enzyme in the de novo purine synthesis pathway. The purpose of this proposal is to further delineate the mechanism of action of DDATHF, and to determine what structural features are responsible for its activity. We will approach this goal as follows: (1) We will carry out additional biochemical studies aimed at delineating the mechanism of action of DDATHF. These will include investigations of (a) inhibition kinetics with purified mammalian GAR and AICAR transformylases; (b) effects of DDATHF on intracellular purine nucleotide pools and serum purines; (c) effects on the synthesis of FGAR in mammalian tumor cells; (d) intracellular conversion of DDATHF to polyglutamate forms; and (e) separation and individual study of the two diastereomers of DDATHF. (2) We have designed a number of specific analogs of DDATHF with which we will further probe the structural basis for its remarkable biological activity. (a) We predict that 10-formyl DDATHF will be an even more potent inhibitor of GAR transformylase because of its structural similarity to the natural substrate. We will prepare 10-formyl DDATHF and a number of its derivatives differing in physico-chemical properties such as lipophilicity, hydrogen bonding capability, and chemical reactivity. (b) In DDATHF both the 5- and 10-nitrogens of the natural cofactor have been replaced by carbon. In order to determine whether both replacements are necessary for activity, we propose the preparation of 5-deaza-5,6,7,8-tetrahydrofolic acid and selected derivatives. The compounds prepared above will be utilized as probes of the structural requirements for inhibition of folate-dependent processes and also evaluated as antitumor agents. Priorities in the ongoing synthetic work will be determined on the basis of these biochemical studies.