Sudden blast phase in chronic myeloid leukemia developed during nilotinib therapy after major molecular response was achieved.

Sudden blast phase (SBP) is a rare event in which patients with chronic myeloid leukemia (CML) in complete cytogenetic response (CCyR) rapidly progress to the blast phase. Few patients on second-generation tyrosine kinase inhibitors (2nd TKIs) have been reported to develop SBP. Here, we report a 45-year-old man diagnosed with CML in the chronic phase in April 2008 and immediately started on imatinib therapy. He achieved CCyR 12 months after starting imatinib therapy. Imatinib was followed by treatment with the 2nd TKIs nilotinib and dasatinib from January 2011 to yield a better response. He achieved major molecular response (MMR) during dasatinib therapy in February 2012, but did not tolerate dasatinib well; hence, he was switched to nilotinib in July 2012. In December 2015, he presented at our hospital with fever and lumbago. A complete blood count revealed a white blood cell count of 30,500/µL with 60% blasts, leading to diagnosis of SBP. After dasatinib therapy and conventional chemotherapy, he again achieved MMR. This case demonstrates that SBP may occur after achieving MMR on treatment with 2nd TKIs. Continuous careful monitoring is required for the early detection of SBP, even in patients who have achieved MMR.