IRIS 6-Year Follow-Up: Sustained Survival and Declining Annual Rate of Transformation in Patients with Newly Diagnosed Chronic Myeloid Leukemia in Chronic Phase (CML-CP) Treated with Imatinib.
Authors: Hochhaus Andreas , Druker Brian J. , Larson Richard A. , O’Brien Stephen G. , Gathmann Insa , Guilhot Francois 
Published: April 2020

Background: The International Randomized study of Interferon versus STI571 (IRIS) study demonstrated that imatinib has superior safety and efficacy relative to interferon-α plus cytarabine (IFN+ara-C). Patients on the imatinib arm achieved an estimated 5-year OS of 89%. To monitor the long-term responses achieved by patients on imatinib, the 6-year follow-up of the IRIS patient population is summarized.

Methods: 1106 patients were randomly assigned to either imatinib or IFN+ara-C and evaluated for hematologic and cytogenetic responses, event-free survival, progression to accelerated-phase (AP) or blast crisis (BC), overall survival (OS), and frequency of adverse events and discontinuations.

Results: The downward trend in the risk of disease progression on imatinib has continued with a 0.4% event rate (including loss of response) and a 0% rate of transformation to AP/BC attained between years 5 and 6. Of 553 who were randomized to imatinib, 364 (65.8%) remain on study drug at 6 years: 14 (2.5%) crossed over to the IFN arm, and 175 (31.6%) pts have discontinued from imatinib study therapy for any reason. The following reasons were cited for discontinuation from the IRIS study: adverse events, 23 patients (4.2%); unsatisfactory therapeutic effect, 66 patients (11.9%); protocol violation, 15 patients (2.7%); withdrawal of consent, 32 patients (5.8%); administrative problems, 6 patients (1.1%); and 16 patients (2.9%) elected to undergo a stem cell transplantation (SCT). Death was the reason for discontinuation for 10 (1.8%) patients, and 7 patients (1.3%) were lost to follow-up. The best observed complete hematologic response rate among patients receiving first-line imatinib was 97%. The best observed major cytogenetic response (MCyR) and complete cytogenetic response (CCyR) rates were 89% and 83%, respectively, with 2 additional CCyR observed since the prior 5-year analysis. At the current 6-year follow-up, 325 patients are still in CCyR, another 24 had lost CCyR but regained it, 6 patients lost CCyR but remain in MCyR, and the remaining 9 patients never had a documented CCyR. Overall, an estimated 83% of patients were event-free, and 93% were free of progression to AP or BC at 6 years on imatinib study treatment, as patients were followed only for OS after discontinuation. After the second year on imatinib, the annual rate of events decreased every year, as did the annual relapse rate. Based on the current follow-up, a total of 66 (12%) patients have died (19 after SCT, 27 not due to CML). Estimated 6-year OS rate for all patients who received imatinib as initial therapy was 88%. When survival is censored at time of SCT for patients who proceeded to transplant, the estimated OS at 6 years is 91%. In an analysis of serious adverse events, no new safety issues were identified between the 5-year report and this analysis.

Conclusions: The 6-year follow-up analysis of the IRIS population indicates that continuous treatment of chronic-phase CML with imatinib induces durable responses in a high percentage of patients with a decreasing rate of relapse and a favorable long-term safety profile.