Evidence for a new mechanism of cytotoxicity of 1-beta-D arabinofuranosylcytosine.

Inhibition of DNA synthesis by a pulse of 1-beta-D-arabinofuranosylcytosine (ara-C) results in reinitiation of DNA replication in DNA segments replicated earlier in that S phase and hence double replication of some DNA segments. Experiments were with tissue culture cell lines of human origin. DNA replicated early in S phase was labeled with a pulse of [3H]deoxycytidine with the cells pulsed later in S phase with ara-C. The DNA replicated after the time of the ara-C pulse was density labeled with 5-bromodeoxyuridine. Reinitiation of DNA replication in the already replicated [3H]DNA segments was demonstrated using CsCl density gradient analysis by an increase in the 3H label present in the light-heavy peak of semiconservatively replicated DNA. Also, in DNA of control cells, all of the 3H was in the same strand of the light-heavy DNA duplex as was the 5-bromodeoxyuridine, as shown by alkaline CsCl density gradient analysis of purified light-heavy DNA. However, after a pulse of ara-C, utilization of [3H]DNA strands as template strands was demonstrated by the presence of 3H label at the density of unsubstituted DNA chains in alkaline CsCl gradients of the purified light-heavy DNA. This double replication phenomenon can explain certain chromosomal abnormalities induced by ara-C.