[Critical comparison of various studies on the blocking effect of cytosine arabinoside (cytarabine)]

The therapeutic effect of phase-specific cytostatic drugs depends to a high degree on the time schedule. If the drug has a blocking effect too, then it is important to know, whether the block acts before or after the sensitive phase of the cell cycle. Concerning the S-phase specific drug cytarabine (cytosine arabinoside, 1-beta-D-arabinofuranosylcytosine, ara-C), different and even contradicting answers have been given to this question. In this paper we assume, that the reported experimental facts are reliable, and show that one experiment has been misinterpreted, giving rise to a controversial statement. If this experiment is interpreted well, it urges the conclusion, that ara-C blocks the transition from G1 to S. This is maintained also by experiments with quite a different design.