Results From The Korean Imatinib Discontinuation Study (KIDS): Updated Data With 14-Month Median Follow Up

Approximately 50% of CP CML patients achieve undetectable molecular residual disease (UMRD) at 6-7 years of first-line imatinib (IM) therapy. Although imatinib therapy is effective in chronic myeloid leukemia (CML) patients and a substantial portion of patients achieve UMRD with prolonged IM therapy, up to 10^7 leukemic cells can still be present in the absence of detectable BCR-ABL1 in RQ-PCR assay due to the sensitivity limit of current RQ-PCR technology. The recent several reports to assess whether IM can be discontinued in CML patients have shown that IM discontinuation can be employed based on clinical study in patients who had enough IM therapy and UMRD durations prior to IM discontinuation. In our previous report, the 12-month probability of sustained UMRD of 48 patients was 80.8% (78.4% in 37 patients with at least 12 months follow-up), a higher rate than that reported by the ‘STIM’ (Stop Imatinib) trial. To identify predictors for safer, successful IM discontinuation and to explore additional contributing factors for sustained molecular responses (MRs), this multicenter prospective Korean Imatinib Discontinuation Study (KIDS) is on-going.

Patients who were treated with IM for more than 3 years and whose BCR-ABL1 was undetectable in RQ-PCR for at least 2 years were enrolled in this study. After discontinuation, molecular response was monitored using RQ-PCR assay every month up to 6 month follow-up, every 2 months up to 12 month follow-up, and every 3 months thereafter. In case of relapse, defined as loss of MMR on 2 consecutive assessments, IM therapy was re-introduced and molecular response after resumption was observed every month using RQ-PCR. Our primary objectives were to evaluate the probability of persistent UMRD at 12 month follow-up after discontinuation, and to measure the duration of persistent UMRD after discontinuation. The secondary objective was to evaluate the probability of major molecular response (MMR) loss and the time taken to lose MMR after discontinuation.

Of the 100 patients who were screened for KIDS, 22 patients have failed because of positive result of RQ-PCR tested in the central laboratory (N= 15, 69%), failure to meet inclusion criteria (N=4, 18%) and informed consents withdrawal (N=3, 13%).As of data cut-off date of 10 July 2013, a total of 78 patients (42 females, 36 males) who were diagnosed between 26 Feb 1998 and 11 Dec 2009 were enrolled on KIDS, with a median age of 45 years (range, 19 – 82), the percentages of patients with low, intermediate and high Sokal risk scores were 37%, 29% and 13%, respectively with unknown Sokal risk scores in 18%. And the median time on IM therapy and the median duration of sustained UMRD were 85 months (range, 38 – 141) and 44 months (range, 23 – 114), respectively, prior to discontinuation. With a median follow-up of 14 months (range, 0.3 – 33.5), the 12-month probability of sustained MMR and UMRD were 78.5% ± 5.3% and 78.5% ± 5.1%, respectively. All patients with MMR loss were in non-transplant group, whereas none of the 21 patients in the transplant group experienced MMR loss. The probability of sustained MMR and UMRD in non-transplant group were 67.3% ± 7.7% and 68.5% ± 7.0%, respectively. Nine of 13 patients who lost MMR were resumed IM with a median time of 15 months (range, 0.1 - 22.3 months). All 9 patients, who resumed IM, re-achieved MMR at a median of 3 months (range, 1.4 - 4.7 months) after resuming IM therapy and re-achieved UMRD at a median of 7.5 months (range, 3.3 - 13.3 months). Univariate analysis of factors affecting loss of MMR showed that IM therapy duration and UMRD duration before treatment discontinuation had a higher 12-month probability of sustained MMR.

Our updated data shows lower relapse rate after discontinuation compared to previous discontinuation studies. In particular, the rate of screening failure due to positive result of RQ-PCR tested in the central laboratory implied that strict PCR sensitivity criteria is important for safer, successful IM discontinuation.

No relevant conflicts of interest to declare.