Dynamics of Cytogenetic Aberrations in Philadelphia Chromosome Positive and Negative Hematopoiesis during Dasatinib Therapy of Chronic Myeloid Leukemia Patients after Imatinib Failure.

Clonal cytogenetic aberrations of the Philadelphia chromosome (Ph) positive hematopoiesis have been associated with the natural evolution of CML to advanced disease. Clonal aberrations of Ph negative metaphases have been described after treatment with interferon alpha or imatinib in a minority of patients (pts) with cytogenetic response. Conflicting data suggest selection of preexisting clones vs. induction of aneuploidy by tyrosine kinase inhibitors. The prognostic impact of aberrations in the Ph negative hematopoiesis for the individual pts remains to be determined. Dasatinib is a multitargeted agent inhibiting both ABL and SRC tyrosine kinases. The efficacy and safety of dasatinib has been demonstrated in phase I and II studies in pts with Ph positive CML after failure of imatinib therapy. We sought to evaluate the effect of dasatinib on Ph positive clones with additional cytogenetic aberrations and the frequency of novel aberrations in Ph positive and Ph negative metaphases during dasatinib therapy. 71 pts (40 m, 31 f) with Ph positive CML after failure of imatinib therapy have been evaluated. Median age was 58 years (range, 28–78), median time from diagnosis 73 months (range, 14–231). Pts were in chronic phase (CP, n=50), accelerated phase (AP, n=6), myeloid (n=8) or lymphoid (n=7) blast crisis (BC). Pretreatment consisted of imatinib (n=71), hydroxyurea (n=48), interferon alpha (n=49), cytosine arabinoside (n=11) and nilotinib (n=2). Dasatinib therapy was commenced at a dose of 100–140 mg/day, median duration of dasatinib therapy was 9 months, (range, 1–16). Prior to dasatinib therapy, 22 pts (31%) demonstrated additional chromosomal aberrations in the Ph positive clone indicating BCR-ABL independent mechanisms of resistance. Of these, 8 pts were in CP (16%), 5 in AP (83%), and 9 in BC (60%). In 35 pts (49%), BCR-ABL mutations associated indicating a BCR-ABL dependent mechanism of resistance were observed. 9 pts (13%) showed both clonal evolution and BCR-ABL mutations. Two pts (3%) had trisomy 8 as an aberration of the Ph negative clone at baseline. During dasatinib therapy, 33 pts (46%) achieved major cytogenetic remission, 26 (37%) being complete. From pts with clonal evolution, 3 (14%) achieved a major with 2 (9%) complete cytogenetic remissions. Novel aberrations of the Ph positive clone appeared during dasatinib therapy in 6 pts (8%), aberrations of the Ph negative clone in 3 pts (4%). In total, 5/71 pts (7%) showed clonal aberrations of Ph negative metaphases after consecutive imatinib/dasatinib therapies. None of these pts had morphological indications for secondary neoplastic changes, like myelodysplasia or acute leukemia. In conclusion, dasatinib was efficacious in pts with clonal cytogenetic aberrations as a marker of BCR-ABL independent imatinib resistance. However, cytogenetic response was delayed compared to pts without additional aberrations. Preexisting clonal aberrations of the Ph negative hematopoiesis were uncovered by a simultaneous gradual elimination of the Ph positive clone. In addition, a minority of pts demonstrated novel aberrations in Ph negative cells. The prognostic significance of aneuploidy of the Ph negative hematopoiesis remains to be evaluated by long term observation.