Sparfosate (PALA) is a stable transition state analogue for an aspartate transcarbamylase- cartalyzed reaction with antineoplastic activity. PALA is a potent inhibitor of aspartate transcarbamylase (Ki about 10(-8) M for ACTases of various origins), which in whole cells blocks the de novo synthesis of pyrimidines. Thus PALA inhibits de novo pyrimidine biosynthesis and increases the extent to which fluorouracil metabolites are incorporated into RNA. In vivo, low doses of PALA inhibit whole body pyrimidine synthesis. While this action is cytotoxic in vitro, extensive human testing demonstrates that PALA alone is devoid of selective antitumor activity. Interest in the therapeutic action of PALA derives from the demonstration that its action potentiates the cytotoxicity of several cytotoxic drugs, notably 5-fluorouracil (5-FU). Development of Sparfosate for cancer and Hepatitis B treatment is assumed to have been discontinued.