Resistance to 5-azacytidine, a potent inhibitor of the growth of mouse lymphoid leukemia cells in vivo, was achieved in an inbred strain of A K R mice in the third transplant period. The incorporation of orotic acid into the livers of such resistant mice was more than six times lower than in animals carrying the 5-azacytidine-sensitive parent strain of leukemia. The incorporation of orotic acid by 5-azacytidine-sensitive mice was inhibited b.y 5-azacytidine twice as much as in 5-azacytidine-resistant animals. The incorporation of uridine, cytidine, thymidine, deoxycytidine, and of guanosine into nucleic acids was also significantly depressed in 5-azacytidine-resistant leukemic cells. Uridine kinase activity was decreased by 20 x, while uridine phosphorylase activity was diminished by 31-50 "/,. The resistant leukemic mice were cross-resistant to 5-fluorouracil; in contrast, however, their sensitivity to aminopterin was increased significantly.
The antileukemic (Sorm and Vesely, 1964; Sorm et al., 1964) and bacteriostatic properties of 5-azacytidine (Piskala and Sorm, 1964), a pyrimidine analogue synthesized in these laboratories, have been described in previous papers. In mice, its effects are directed primarily against lymphoid tissue and bone marrow, in which considerable depression of deoxyribonucleic acid and ribonucleic acid synthesis is seen, especially in large lymphocytes and in more mature myeloid cells (Vesely and Sorm, 1965). Furthermore, it has been shown that 5-azacytidine is converted by mammalian cells to its 5'-polyphosphates, which are incorporated into nucleic acids (JurovEik et al., 1965).
The present report describes the development of resistance to 5-azacytidine in AKR leukemic mice. This phenomenon is accompanied by distinct biochemical changes in the metabolism of the nucleic acid precursors of the resistant cells, as well as by changes in their sensitivity toward various antinietabolites. The observed Approved for publication: