POTENTIATION OF ANTIMETABOLITE ACTION BY URIDYLATE TRAPPING DIETRICH KEPPLER, AXEL HOLSTEGE,GISBERT WECKBECKER, JOACHIM FAULER and THOMAS GASSER BtochemlschesInstltut, Unwerslty of Frelburg lm Brelsgau, D-7800 Frelburg LBr., West Germany INTRODUCTION Several analogs of D-galactose and D-glucose that are modified at carbon atom 2 induce a trapping of cytoplasmic uridylate (1-4). The uridylate moiety of UDP-glucose or UTP is diverted to the respective UDP-sugar analog that accumulates while UTP and related pyrimidine nucleotide pools are depleted in various tumor cells and tissues. This effect of some sugar analogs enhances or potentiates the growth inhibitory and cytotoxic action of inhibitors of de novo pyrimidine synthesis (5--7) and of pyrimidine ribonucleoside analogs such as 5-fluorouridine (8- I 1). The uridylate-trapping action of sugar analogs is decisively influenced by the properties of the respective tumor tissue: (i) the enzyme pattern of the tumor cell determines whether analogs of D-galactose or of D-glucose are preferentially taken up and metabolized to the UDP- derivatives; (ii) a high rate of de novo pyrimidine synthesis can compensate for the uridylate-trapping action and prevent the depletion of UTP pools; and (iii) an active salvage of extracellular and/or intracellular uridine furthermore counteracts UTP deficiency (7, 12, 13). MATERIALS AND METHODS Tumors and cultured cells. TA3-Ha ascites mammary tumor cells were carried in female NMRI mice and incubated in suspension as described earlier (14, 15). Solid TA3 tumors were induced by injection of 40 #1 ofTA3 ascites cell suspension bilaterally into the thigh muscle. The mean tumor weight was 0.27 + 0.09 g 5 days after inoculation of the tumor cells. The solid tumor was epitheloid. AS-30D hepatoma cells (16) were transplanted into the peritoneal cavity (15) or into the liver of female Sprague-Dawley rats (11); AS-30D cells were cultured in suspension as primary cultures derived from the ascites hepatoma (7). Primary cultures of rat hepatocytes were kindly provided by Dr. T.-A. Tran-Thi and Prof. K. Decker (17). The hepatocytes had been cultured for 48 hr (17) when experiments were initiated. 417