Background The TKIs Nilotinib and Dasatinib offer additional therapeutic options for patients with CML who are resistant or intolerant to Imatinib. These agents, active against the majority of Imatinib resistant BCR-ABL mutated clones, have a different pattern of kinase target selectivity, pharmacokinetics parameters, cell uptake, efflux properties and adverse events profiles. Preliminary results suggest that some patients may respond to a second TKI used as third line therapy ,but little is known about the long term benefit of such an approach.Aim of this collaborative Italian study was to verify the response (rate and duration) and the clinical outcome in patients with CML treated with a third TKI after sequential failure of the previous ones. \ud Methods. We evaluated 66 patients with CML, resistant/intolerant to Imatinib and treated with Dasatinib or Nilotinib, then switched to a third- line TKI after treatment failure. Of these, 29 patients were treated with dasatinib after imatinib/nilotinib failure and 37 with nilotinib after imatinib/dasatinib failure. Patients were monitored with complete blood counts, cytogenetic analysis, bone marrow aspiration RT-PCR and mutational analysis. \ud \ud Results. A total of 66 patients (median age 63 years, range, 33-85 years) were treated with sequential TKIs; 40 (61%) patients had received interferon-a before starting Imatinib; 26 (39%) patients received imatinib as first line therapy. The median time on imatinib therapy was 47.5 months (range 4-101 months). \ud \ud At the start of nilotinib as second line, 27/29 (93%) patients were in CP, 1 (3.5%) in AP, and 1 (3.5%) in BP. 9 patients (31%) had developed mutations before starting treatment. The median time on second line TKI was 8 months (range 2-36 months). In the resistant patients 4 new mutations were identified (F359V in two patients, T315I, Y253H+F359V). \ud \ud At the start of dasatinib as second line, 33/37 (89.2%) patients were in CP, 4 (10.8%) in AP. 7 patients (18.9%) had developed mutations before starting treatment. The median time on second line TKI was 14 months (range 4-59 months).In the resistant patients 5 new mutations were identified (F137L in three pts, M318T, M244V+F317L). \ud \ud At the start of the third TKI, 60/66 (90.9%) patients were in CP, 5 (7.6%) in AP, and 1 (1.5%) in BP. Of these, 7 patients (18.9%) on dasatinib and 7 (24.1%) on nilotinib had mutations before starting treatment. The best response to the third line treatment with TKI was 10 (15.2%) MMR, 10 (15.2%) CCyR, 8 PcyR (12.1%), 5 (7.5%) mCyR, 24 (36.4%) CHR and 9 (13.6%) No Response (NR). In the dasatinib group, 9 (31%) patients discontinued treatment because of toxicity versus 17 (45.9%) patients in the nilotinib group.Two new mutations (F317L, E255V) emerged with dasatinib as third line therapy.After a median follow up of 13 months (range 2-37 months) 50 patients (48 CP, 2 AP) are continuing therapy ( 33 on nilotinib, 17 on dasatinib).Since the start of the third TKI, 61 patients (92.4%) are still alive for a median overall survival of 110 months( range 15-300) (52 CP, 7 AP, 2 NA); the 5 deaths (7.6%) were caused by disease progression and spread of the gene mutation T315I. \ud \ud Discussion.In our study, about one third of patients derived benefit from the use of three sequential TKIs; patients with better, longer response (28.7%) to third TKI were the same patients with a better response to the Imatinib and 2TKIs therapy. All these patients had taken interferon therapy before the Imatinib. In this subset of patients (good responders: CCyR and MMR) 5 patients developed mutations that were sensitive to the sequential treatment.The lack of a durable cytogenetic remission could be explained by the emergence of new kinase domain mutations as patients are exposed to sequential TKI; a change of therapy resulted in an adequate response. In our series, patients with poor prognosis showed mutations not sensitive to the TKIs treatment. \ud \ud Conclusions. Although allogeneic SCT is the treatment of choice in all patients failing 2 TKIs who are suitable candidates for this approach, alternative strategies are required for ineligible patients. The use of a third TKI after failure of two previous TKIs induces response in some patients. Longer follow up of a larger series of patients is needed to determine the long term impact of the response
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