COPENHAGEN – A year after stopping tyrosine kinase inhibitor therapy, more than half of patients with chronic myeloid leukemia (CML) in a large clinical trial remained in deep molecular remission.
Among 750 patients with CML in remission for at least 1 year before study entry, 62% retained a treatment response 6 months after stopping a tyrosine kinase inhibitor (TKI) such as imatinib (Gleevec) and 56% retained responses 1 year after being off their drugs, reported Dr. Johan Richter of Lund (Sweden) University at the annual congress of the European Hematology Association.
“About 6 years of therapy [with imatinib] would be optimal for therapy prior to a stop attempt,” he said at a briefing prior to the presentation of data at the congress.
Although in clinical practice patients with CML may remain on a TKI indefinitely, results from small clinical trials have suggested that in 40%-60% of patients with deep molecular responses (MR4.0 or better), TKIs can be safely stopped, Dr. Richter noted.
To get a better handle on when it might be safe to stop a TKI and under what conditions, EURO-SKI investigators enrolled 868 adults with CML in chronic phase from 11 countries, 750 of whom had complete data for the analysis.
In all, 94% of patients had received imatinib in the first line, 2% received dasatinib (Sprycel), and 4% had received nilotinib (Tasigna). Of this group, 115 had switched to a second-line agent due to intolerance of the first-line drug.
The median time from diagnosis was 7.7 years. The median duration of therapy was 7.6 years, and the median duration of MR4 before stopping was 4.7 years.
As noted, among 750 patients assessable for molecular relapse–free survival, 62% remained in remission at 6 months after stopping the TKI, as did 56% at 12 months, 52% at 24 months, and 49% at 36 months.
For patients who resumed therapy, the median time to restart was 4.1 months.
To see whether they could identify any factors prognostic for relapse after stopping a TKI, the investigators used data on 448 patients in the study who were treated with imatinib.
In univariate analysis there was no significant association between molecular relapse–free survival at 6 months and either age, gender, depth of molecular response, or any standard risk scores.
The only significant predictors of molecular remission status at 6 months were duration of imatinib therapy and duration of molecular response before stopping.
The odds ratio for treatment duration was 1.16, indicating that each additional year of imatinib treatment is associated with a 16% increase in the likelihood that a patient would remain in deep molecular remission 6 months after stopping.
The investigators used the minimal P value approach to determine the cutoff of approximately 6 years, based on a molecular relapse–free survival at 6 months of 65.5% for patients who remained on imatinib for more than 5.8 years, compared with 42.6% for those who were on it for 5.8 years or less.
Although the study is ongoing, to date more than 80% of patients who had a loss of deep molecular remission after stopping their TKI regained the remission after resuming therapy, Dr. Richter said.
Dr. Richter said in an interview that longer follow-up will be needed to confirm their findings, and that patients who were sensitive to TKIs prior to stopping therapy remained sensitive when restarting, suggesting that treatment interruption does not increase the likelihood of drug resistance.
Coprincipal investigator Dr. Francois-Xavier Mahon of Bordeaux University in France, noted that in the STIM (Stop Imatinib)–1 and –2 trials, the estimated annual savings to the French health care system were 20 million euros ($22.6 million).