Dynamics of BCR-ABL Kinase Domain Mutations in Patients with Chronic Myeloid Leukemia (CML) after Treatment with One, Two or Three Tyrosine Kinase Inhibitors (TKI).

Dasatinib (D) and nilotinib (N) are potent tyrosine kinase inhibitors (TKIs) with activity against many imatinib (IM) resistant BCR-ABL kinase domain mutants, except T315I. In vitro mutant models have selected specific mutations occurring after incubation with IM, D and N. Therapy with these new TKI may select for patients with T315I or other mutations relatively insensitive to them. We assessed the change in mutation status of the bcr-abl kinase domain (codons 220 to 500) in 113 patients (pts) with CML who received therapy with D and/or N after imatinib failure. Median age was 60 years (range, 21 to 82 years). Seventy-one (63%) pts received prior interferon (IFN). Median time on imatinib was 28 months (range, 2 to 78 months). At the time of imatinib failure, mutations were detected in 46 of 85 (54%) pts who had DNA sequencing. The evolution of mutations after a second TKI was as follows (Table 1).

Twenty pts received a third TKI after failing IM and a second TKI. The evolution of mutations in this cohort was as follows (Table 2).

Overall, 19 of 101 evaluable pts (19%), cases had new mutations emerge following TKI switch 17 after a 2^nd TKI (12 nilotinib, 5 dasatinib), and 2 after a 3^rd TKI (2 dasatinib). We analyzed whether these N- and D-associated new mutations were at sites that have been detected following D and N treatment in vitro (Burgess et al, PNAS 2005; Bradeen et al, Blood 2006; Von Bubnoff et al, Blood 2006). Only 14/46 (30%) kinase domain mutations that developed after D (7) or N (7) corresponded with an in vitro-identified site. Only 5 of 134 (4%) mutations identified were T315I (3 after dasatinib, 2 after nilotinib), but the mutation status of these patients was unknown after IM. We conclude that the spectrum of mutations that develops in vivo after TKI switch is broader and includes common imatinib-resistance sites as well. There appears to no marked increase in the incidence of T315I mutation after TKI switch.