Analogs of 1-β- d-arabinofuranosylcytosine. Studies on mechanisms of action in Burkitt's cell culture and mouse leukemia, and in vitro deamination studies

Abstract

Analogs of 1-β- d-arabinofuranosylcytosine (ara-C), 1-β- d-arabinofuranosyl-5-fluorocytosine (ara-FC), cytidine (CR), and 2'-deoxycytidine (CdR) were studied for deamination and as deaminase inhibitors (ara-C as substrate) in human liver and mouse kidney enzyme systems. N 4 -Hydroxy, N 4-methyl, and 4-hydrazino derivatives of ara-C and ara-FC, and N 4-acetyl-ara-C were studied for deamination and as deaminase inhibitors and also for their effects on mouse leukemias L1210 and P815 and in Burkitt's tumor cells in cell culture. None of the N 4-substituted analogs of ara-C and ara-FC were deaminated by human liver and mouse kidney homogenates in vitro. N 4-Hydroxy analogs were active in mice but only slightly active in Burkitt's cell culture. The inhibitory effect against Burkitt's cell cultures in vitro was blocked by deoxycytidine but not by thymidine, and N 4-hydroxy-ara-C was active in vivo in a line of leukemia resistant to 5-fluorouracil but not in a line resistant to ara-C, suggesting this analog has a similar mechanism of action to ara-C. The N 4-methyl and 4-hydrazino derivatives of ara-C and ara-FC are inactive in mouse leukemia and Burkitt's cell culture. Only N 4-hydroxy-ara-C and N 4-methyl-ara-FC are deaminase inhibitors. N 4-Hydroxy-cytidine, N 4-hydroxy-5-methyl-deoxycytidine, and N 4 -hydroxy-5-fluoro-deoxycytidine are all deaminase inhibitors, but attempts at potentiating the effect of ara-C in mouse leukemia in vivo by administering the first two of these compounds in combination with ara-C resulted in partial blocking of the effect of ara-C, presumably due to reduction of the N 4-hydroxy derivative to deoxycytidine or to a derivative which acts like deoxycytidine. N 4-Hydroxy-deoxycytidine has minimal activity in Buikitt's cell culture, but its inhibitory effect was blocked completely at thirty times the ID 50 (50 per cent inhibitory dose) by thymidine at equimolar concentrations but only slightly by deoxycytidine. Although this analog is not deaminated, and is a strong deaminase inhibitor, it blocked rather than potentiated the effect of ara-C in L1210 leukemia, suggesting that in cell culture it acts as a thymine antimetabolite, whereas in vivo it is reduced to deoxycytidine. N 4Hydroxy-5-fluoio-deoxycytidine is active in Burkitt's cell culture, and its effect could be blocked by thymidine, and to a lesser degree, by deoxycytidine, suggesting that it too acts in this system as a thymine antimetabolite.