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Early phase studies of alvocidib showed activity in relapsed CLL including patients with high risk genomic features and those refractory to fludarabine. A multi-center, international, phase II study of alvocidib in fludarabine refractory CLL was undertaken to validate these early results. Patients with fludarabine refractory CLL or prolymphocytic leukemia arising from CLL were treated with single agent alvocidib. The primary outcome measure was overall response rate, with secondary outcomes including survival, toxicity, and response duration. One hundred and sixty five patients were enrolled and 159 patients were treated. The median age was 61 years, the median number of prior therapies was 4, and 96% of patients were fludarabine refractory. The investigator-assessed overall response rate was 25%; the majority of responses were partial. Response rates were lower among patients with del(17p) (14%), but equivalent in patients with del(11q) or bulky lymphadenopathy. Median progression free and overall survival were 7.6 and 14.6 months, respectively. Tumor lysis occurred in 39 patients (25%), and 13 received hemodialysis. Diarrhea, fatigue, and hematologic toxicities were common. Alvocidib has clinical activity in patients with advanced, fludarabine refractory CLL. Future studies should focus on discovery of biomarkers of clinical response and tumor lysis, and enhanced supportive care measures.