Dasatinib (SPRYCEL®) has demonstrated significant efficacy in a high proportion of imatinib-resistant or -intolerant chronic phase CML patients (pts) concerning achievement of hematologic and cytogenetic responses. We sought to establish a relationship between type of preexisting BCR-ABL mutations associated with imatinib resistance and achievement of major molecular response (MMR, BCR-ABL ≤0.1% according to International Scale, IS) after 12 months of dasatinib therapy in pts with chronic phase CML. We have investigated 1,605 peripheral blood samples from 202 pts (n=62 imatinib-intolerant, n=140 imatinib-resistant; 52% male, median age 60 yrs, range 21–78) who had been enrolled in an international phase II study (START-C study, CA180–013) investigating the activity of 70mg dasatinib BID after imatinib failure. Screening for BCR-ABL mutations was performed by D-HPLC combined with DNA sequencing. During follow up, pts were monitored in 3-monthly intervals by RQ-PCR for BCR-ABL mRNA transcripts and by mutation analysis to determine the quantitative course of the preexisting mutation or the emergence of new mutations. Prior to dasatinib therapy, 34 different BCR-ABL mutations involving 29 amino acids were detected in 85/202 pts (42%) with a striking predominance in imatinib-resistant (77/140 pts, 55%) over imatinib-intolerant pts (8/62 pts, 13%). 75 pts showed one, 8 pts two and 2 pts three mutations. RQ-PCR data after 12 months of therapy was available from 154 pts (76%), samples from 48 pts (24%) were not available for monitoring after one year due to progressive disease. MMR was achieved in 28 imatinib-intolerant (45%) and 19 imatinib-resistant pts (14%, p<0.0001). The overall rate of imatinib-resistant pts with mutations was comparable in pts achieving MMR (group 1) vs pts not achieving MMR within 12 months (group 2; 53% vs 57%, p=0.80). Several mutations in imatinib-resistant pts are associated with differential response (group 1 vs group 2 response) and are presented with their IC50 values to dasatinib: H396R (n=0 vs n=6; IC50 0.6–1.3nM), M351T (n=1 vs n=7; 1.1nM), G250E/V (n=1 vs n=8; 1.8nM), Y253H (n=0 vs n=5; 1.3–10nM), L387M (n=0 vs n=2; 2nM), F359I/V (n=0 vs n=4; 2.2nM), E255K/V (n=1 vs n=4; 5.6–13nM), F317L (n=0 vs n=3; 7.4–18nM), T315I (n=0 vs n=3; >1,000nM). Of 85 pts without sufficient molecular response to dasatinib (BCR-ABL IS >5%) after 12 months (median, range 9–15) mutation analysis revealed the emergence of new mutations in 17 formerly imatinib-resistant pts (T315I, n=2; T315A, n=1; F317L, n=6; V299L, n=2; M351T, n=2; L248V, n=1; G250E, n=1; K271R, n=1; Y320C, n=1). We conclude that dasatinib is capable of inducing high rates of major molecular response after one year treatment particularly in imatinib-intolerant pts. Response dynamics depend on the individual type of mutation which may be a basis for individual dose adjustment according to the mutation pattern.

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Disclosure:Research Funding: Research support from BMS, Novartis, Wyeth, Innovive and Merck. Honoraria Information: Honoraria from BMS and Novartis.