4. In the absence of rapidly progressing disease, the interval from prior treatment to time of study drug administration should be at least 2 weeks for cytotoxic agents, or at least 5 half-lives for noncytotoxic agents. If the patient is on hydroxyurea to control peripheral blood leukemic cell counts, the patient must be off hydroxyurea for at least 48 hours before initiation of treatment on this protocol.