- Phase IIa: single-agent palbociclib using the tolerable dose defined in the phase Ib part of the study is administered once daily for 21 days followed by 7 days of rest. Based on the optimal two-stage design of Simon, 21 patients are treated in the first stage. If results are positive, 29 additional patients will be recruited into the second stage of the study. An efficacy of the investigational therapy will be rejected in the first stage of 21 treated patients if two or less patients achieve complete remission (CR), CR with incomplete blood count recovery (CRi), partial remission (PR), or anti-leukemic effect (ALE). If three or more patients achieve CR, CRi, PR, or ALE during this first stage, the trial is intended to be continued in the second stage with a total sample size of 50 patients.