Long-Term Follow-up Results after Imatinib Discontinuation in Chronic Myeloid Leukemia Patients with Undetectable Minimal Residual Disease

Introduction

Imatinib (IM) is an effective treatment in patients with chronic phase chronic myeloid leukemia (CML-CP). In the previous report (Leukemia research, 2012;36:689-693), we demonstrated that IM could be discontinued in CML patients achieved undetectable minimal residual disease (UMRD) after the treatment of front-line IM therapy. These observations were confirmed by prospective STIM1 and TWISTER studies. However, in both studies, approximately half of patients were treated with front-line interferon therapy, which might be a confounding factor when considering the impact of prior interferon on treatment free remission (TFR) in the previous IM discontinuation studies. Thus, the aim of this study was to investigate the long-term outcomes of IM discontinuation in patients with CML-CP, who have treated with front-line IM therapy and achieved UMRD.

Patients and methods

We consecutively enrolled patients with CML-CP, discontinued IM therapy after achieving UMRD for at least 12 months in 2 Korean institutions from June 2009 to Jan 2013. Patients with a prior history of any other treatment (>1 months) for CML before IM administration were excluded. UMRD was defined by undetectable levels of BCR-ABL transcript by RQ-PCR with sensitivity of at least 0.0046%^IS. After discontinuation, BCR-ABL/ABL ratio was monitored by RQ-PCR monthly during the first 6 months and every 3 months thereafter, and molecular relapse was defined by detectable levels of BCR-ABL transcript in two successive assays.

Results

Nineteen patients (8 male, 11 female) with a median age of 52 years (range, 29-78) were included. The reasons for discontinuing IM were shared decision between physicians and patients with long undetectable BCR-ABL transcript (n=9), chronic adverse events of IM (n=6), patient’s request (n=3), and wish for pregnancy (n=1). At initial diagnosis, the Sokal score was low to intermediate in 11 patients and high in 8 patients. All patients started IM at a dose of 400mg/day and median interval between IM initiation and UMRD was 21.5 months (range, 7.0-61.9). IM therapy was then maintained during a median of 34.8 months (range, 12.1-72.4).

With a median follow-up of 52.1 months (range, 17.5-60.5), the overall probability of UMRD persistence at 4-year was 22.1% (95% CI, 11.6-32.6). Fourteen patients (73%) lost UMRD after a median of 4.0 months (range, 1.1-22.8). 12 patients relapsed within first 9 months and 2 late relapse were identified at 20.5 and 22.8 months, respectively. No patients included in this analysis were progressed to advanced stage CML or died. IM therapy was resumed in all patients with molecular relapse, but 2 patients were switched to dasatinib owing to chronic adverse events of IM. At the time of this analysis, all patients were still sensitive to IM and dasatinib therapy. 12 patients re-achieved UMRD and 2 patients maintained stable major molecular response. In univariate analysis for molecular relapse, high risk of Sokal score (P<0.001), ≥24 months of interval between IM initiation and UMRD (P=0.016), and <34.8 months of duration of IM therapy after UMRD achievement (P=0.029) were significantly associated with frequent molecular relapse.

Conclusion

This study represents that IM discontinuation in patients who have received front-line IM therapy and achieved UMRD would be feasible, as approximately one-fourth of these patients could enjoy long-term TFR. Moreover, no molecular relapse was observed after 2 year of IM discontinuation. Although the Sokal score, time to deep molecular response, and duration of IM therapy were suggested as clinical factors for predicting molecular relapse in this analysis, further studies would be necessary to confirm the results in this population.