Resistance to the Bcr-Abl tyrosine kinase inhibitor (TKI) imatinib mesylate (IM) in patients (pts) with chronic myeloid leukemia (CML) and Philadelphia-positive (Ph+) acute lymphoblastic leukemia (ALL) is often caused by selection of point mutations in the Abl kinase domain (KD) altering residues that are directly or indirectly critical for IM binding. Several novel TKIs are now available for IM-resistant pts. In vitro studies have postulated that each of them is likely to retain its own distinct set of insensitive mutations, including novel inhibitor-specific mutations. To assess how Abl KD sequences evolve under the selective pressure of sequential therapy with one or more novel TKIs, we have monitored the mutation status of 101 IM-resistant pts before and during treatment with up to three consecutive novel TKIs (dasatinib, nilotinib, bosutinib). Forty-seven pts had CML in chronic phase; 54 pts had CML in accelerated/blastic phase (AP/BP) or Ph+ ALL. Overall, presence or emergence of TKI-resistant mutations accounted for 90% of cases of dasatinib, nilotinib or bosutinib failure. At the time of IM failure, 55/101 (54%) pts had KD mutations. After switching to a 2nd TKI (n=101 pts), 21/55 (38%) pts who had mutations at baseline as against 8/46 (17%; p=.02) pts who did not have mutations at baseline subsequently relapsed with newly acquired mutations. Median time to relapse was 8 months (range, 3-18). Cloning showed that these mutations could either be acquired by the pre-existing mutated subclone or arise in an independent one. In addition, 15/55 mutated pts did not respond to the 2nd TKI because of the mutation they were harboring at baseline. After switching to a 3rd TKI (n=20 pts), 11/16 mutated pts as against 0/4 non-mutated pts relapsed with newly acquired mutations. Median time to relapse was 3 months (range, 1–5). Switch to a 4th TKI has so far been attempted in 3 mutated pts, but observation time is still too short. Dasatinib failure was associated with the following mutations: T315I, F317L, V299L, T315A, F317I/S/V. Nilotinib failure was associated with the following mutations: Y253H, E255V, T315I, F359C. Bosutinib failure was associated with the following mutations: E255K, T315I, V299L. More detailed analyses will be presented. In summary, the Bcr-Abl kinase is a moving target and pts already harboring mutations, especially those with CML in AP/BP or with Ph+ ALL, have a higher likelihood of developing further mutations under the selective pressure of novel TKIs. It can be hypothesized that in these pts a higher genetic instability may foster rapid emergence of multiple mutations over time within the same or different Bcr-Abl-positive subclones, which are selected or de-selected depending on the sensitivity profile of the specific TKI employed. In this clinical setting combination therapies would probably be more effective than single-agent treatment for long-term disease control. Supported by European LeukemiaNet, AIL, FIRB, COFIN, Fondazione del Monte di Bologna e Ravenna.

Author notes

Disclosure: No relevant conflicts of interest to declare.