Drug resistance of the tumor cells in cancer chemotherapy is very common. The objective of this proposal is to investigate the mechanisms of reversal of drug resistance of leukemic cells to Cytarabine (ara-C) by 5-Azacytidine (5-Aza-C) and its derivatives, dihydro-5-azacytidine (DHAC) and 2'-deoxy-5-Azacytidine (5-Aza-dCyd). The formation, accumulation and elimination kinetics of the triphosphates of these compounds will be determined in tumor (P388/0, L1210/ara-C, and P388/ara-C) and mouse host tissues (bone marrow (BM) and gastrointestinal mucosa (GI)) by high pressure liquid chromatography after in vivo injections of the maximally tolerated doses (MTD) to tumor bearing mice. The cellular concentrations of the nucleotide analogs will be correlated with their effects on inhibition of DNA synthesis and their incorporation into nucleic acids of the tumor cells. Our preliminary studies have indicated that partial reversal of resistance of L1210/ara-C to ara-C by expressing deoxycytidine kinase (dCyd kinase) is taking place after pretreatment of this tumor line with DHAC. This will be investigated in detail with all three drugs on the L1210/ara-C tumor line. Further studies of reversal of resistance to ara-C will examine whether it is a general effect of expressing dCyd kinase to a small percentage of control or a certain small number of L1210/ara-C cells are expressing the enzyme as the L1210/0 cells. Similar studies will be carried out with the human leukemia cell line CCRF/CEM/O and CCRF/CEM/dCK (-). The evolution of tumor resistance to ara-C will be investigated in relapsed pediatric leukemic patients who will receive one or more high dose ara-C (HDara-C) treatments. The pharmacodynamic parameters of HDara-C will be determined along with the activity of various enzymes that may indicate presence of resistance to ara-C, such as Cytidine deaminase and dCyd-kinase. Once resistance to ara-C has been established, the multiple relapsed patients will receive 5-Aza-C in vivo. The pharmacokinetic and biochemical parameters will then be investigated to determine whether 5-Aza-C treatment alters the tumor resistance to ara-C. The ultimate goal of this project is to examine the mechanism of reversal of both murine and human tumor that becomes resistant to ara-C.