Spirit 2: Final 5 Year Analysis of the UK National Cancer Research Institute Randomized Study Comparing Imatinib with Dasatinib in Patients with Newly Diagnosed Chronic Phase CML

Objective. SPIRIT 2 is the largest phase 3 prospective randomized open-label trial comparing imatinib (I) 400mg with dasatinib (D) 100mg daily in newly diagnosed chronic phase CML. The primary endpoint was 5 year event-free survival. Methods. 812 (406 in each arm) of 814 patients recruited started study medication (median age 53.2, 275/812 (33.8%) were over 60 years old). Patients were recruited at 144 hospitals between August 2008 and March 2013 and randomized to receive either imatinib 400mg or dasatinib 100mg daily. Secondary endpoints included overall survival, rates of treatment failure, cytogenetic/molecular response - RT-PCR BCR-ABL/ABL ratio of <0.1%^IS(major molecular response (MMR), 3 log reduction, MR3) and deeper. To address the potential confounding effect on the primary endpoint of patients switching from randomized treatment to an alternative TKI or other treatment, exploratory per-protocol analyses were performed using the inverse probability of censoring weighting (IPCW) method.Results. All patients have now completed 5 years of follow-up. 424/812 (52.2%) patients completed the study whilst still taking first line medication: 230/406 (56.7%) on dasatinib, 194/406 (47.8%) on imatinib. Of the patients who discontinued first line therapy, more patients on the imatinib arm switched due to suboptimal PCR response (as decided by the local investigator) than on dasatinib (D7/406, 1.7%; I71/406, 17.4%) but more patients on the dasatinib arm discontinued due to intolerance (D123/406, 30.3%; I68/406, 16.7%). More patients went on to transplant in the imatinib arm than in the dasatinib arm (D 6/406, 1.5%; I 30/406 7.6%) and in the imatinib patients undergoing transplant the reason for first line treatment failure was disease progression in 5/30 and suboptimal molecular response in 11/30. Using an intention to treat analysis cumulative incidence of MR3 and MR4 on first line therapy within 5 years was higher in the dasatinib arm than the imatinib arm (MR3: D 83.0%, I 63.0% - difference 20.0%, p<0.0001; MR4: D 77.5%, I 57.2% - difference 20.3%, p<0.0001).At 24 months the complete cytogenetic response rate was D 42.6%, I 31.8% - difference 10.8%, Chi-square test p=0.001.At 5 years the probability of treatment failure-free survival was higher with dasatinib than imatinib(D60.9%, I52.9% - HR: 0.73 (95%-CI:0.59-0.90), p=0.004) but there were no significant differences in event free survival (D91.0%, I89.0% - HR:0.80 (95%-CI:0.51-1.25), p=0.319) or overall survival (D91.9%, I91.2% - HR: 0.90 (95%-CI:0.56-1.47), p=0.690). IPCW modelling results will be presented at the meeting.The overall rate of pleural effusion over 5 years in the dasatinib arm was 36.0% with a higher incidence in older patients. 13 patients developed their first pleural effusion after 3 years on study. Conclusions. In SPIRIT 2 we observed a higher molecular and cytogenetic response rate and also a higher pleural effusion rate with dasatinib but a higher treatment failure rate with imatinib often because investigators were concerned about sub-optimal PCR responses. More imatinib-treated patients proceeded to transplant. There were no statistically significant differences in event free survival (the primary endpoint) or overall survival. Imatinib remains a highly effective first line therapy though subgroup analyses are planned to explore whether dasatinib may have advantages in particular clinical scenarios.