SAN DIEGO—Preliminary trial results suggest re-treatment with dasatinib is feasible and safe for a second attempt at tyrosine kinase inhibitor (TKI) discontinuation in chronic myeloid leukemia (CML) patients who fail to achieve treatment-free remission (TFR) after discontinuing imatinib.
However, investigators reported the rate of second TFR (TFR2) was 21% at 6 months, which was not enough to confirm, at this time, that dasatinib could improve the TFR2 rate after imatinib discontinuation failure.
Dennis Kim, MD, of the University of Toronto in Ontario, Canada, presented these results at the 2018 ASH Annual Meeting (abstract 787).
The design of this trial (NCT02268370) includes three phases: the imatinib discontinuation phase, the dasatinib re-challenge phase to achieve a molecular response of ≥ 4.5-log reduction in BCR-ABL1 transcripts (MR4.5), and the dasatinib discontinuation phase.
The primary objective of the trial is to determine the proportion of patients who remain in deep molecular remission (> MR4.5) after discontinuing dasatinib following a failed attempt at discontinuation of imatinib.
If patients had a confirmed molecular relapse after discontinuing imatinib, they were started on 100 mg of dasatinib daily, and, after achieving MR4.5 or greater for 12 months, they discontinued dasatinib for a try at the second TFR.
Investigators defined loss of molecular response, or relapse, as a loss of a major molecular response (MMR) once or loss of MR4.0 on two consecutive occasions.
The 131 enrolled CML patients were a median age of 61 (range, 21 to 84), and 62% were male.
Patients had a median 9.36 years of disease duration, 9.18 years of imatinib treatment, 6.82 years of MR4 duration, and 5.08 years of MR4.5 duration.
“The cohort has a very long history of imatinib treatment as well as MR4 duration,” Dr. Kim pointed out, “which also can affect our TFR1 rate, and I think, also, it can affect our TFR2 rate.”
TFR1 and TFR2 rates
As of October 25, the TFR1 rate using loss of MMR as the measure was 69.9% at 12 months from imatinib discontinuation. Relapse-free survival was 57.2% at that time.
Of the 53 patients who lost response, 51 patients received dasatinib. At 3 months of treatment, 97.7% achieved an MMR, 89.9% achieved MR4, and 84.6% achieved MR4.5.
Twenty-five of 51 patients treated with dasatinib attained MR4.5 for 12 months or longer and discontinued treatment for a second attempt at TFR.
Twenty-one patients are still receiving dasatinib and have attained MR4.5, but not for the 12-month duration yet.
Dr. Kim noted that the median time to achievement of molecular response after dasatinib re-challenge ranged from 2.76 months for MR4.5 to 1.71 months for MMR.
Twenty-one of 25 patients (84.0%) who discontinued dasatinib lost their molecular response at a median of 3.7 months.
The estimated TFR2 rate after dasatinib discontinuation is 21.0% to 24.4% at 6 months, which means the investigators cannot reject the null hypothesis of 28% or more patients remaining in remission.
Patients who lost response rapidly after dasatinib discontinuation also tended to lose response rapidly after imatinib discontinuation, Dr. Kim pointed out.
“However, you see some patients who do not lose their response after dasatinib discontinuation or who lose the response but later after the dasatinib discontinuation, they tend to lose their imatinib response also in a later time point,” he said. “So we started to look at the risk factors.”
Risk factor analysis
Out of seven potential risk factors, the investigators were able to demonstrate that time to molecular relapse after imatinib discontinuation, molecular relapse pattern after imatinib discontinuation, and BCR-ABL1 quantitative polymerase chain reaction (qPCR) value prior to dasatinib discontinuation “seemed to be very significant,” Dr. Kim said.
Time to molecular relapse after discontinuation of imatinib correlates with TFR2. The group of patients who relapsed in 3 to 6 months of stopping imatinib had a significantly longer TFR2 than patients who relapsed within 3 months of stopping imatinib (P=0.018).
The molecular relapse pattern also correlates with TFR2. The group with a single loss of MMR after imatinib discontinuation had a significantly shorter TFR2 than those who lost MR4 twice after imatinib discontinuation (P=0.043).
And 0% of the patients who had qPCR transcript levels between a 4.5 and 5.4 log reduction maintained TFR2 at 6 months. However, 28.7% who had qPCR deeper than 5.5 logs prior to dasatinib discontinuation had TFR2 at 6 months (P=0.017).
The risk factor analysis shed light, in part, on the reason the trial thus far failed to satisfy the null hypothesis.
“In other words, because we have selected a really good-risk group for TFR1, the remaining patients are actually a high-risk group for TFR2,” Dr. Kim said. “Because of that, the TFR2 rate might be somewhat lower than we had expected.”
“Or is it related to our conservative treatment with dasatinib, which is 12 months after achieving MR4.5 or deeper response? That may affect our TFR2 rate. We still have to think about that.”
Dr. Kim suggested stricter criteria be considered for attempting TFR2, such as achieving a 5.5 log reduction or deeper in BCR-ABL1 qPCR levels prior to the second TKI discontinuation attempt, and/or an MR4 duration of more than 12 months.
Dr. Kim disclosed receiving honoraria and research funding from Novartis and Bristol-Myers Squibb and serving as a consultant for Pfizer, Paladin, Novartis, and Bristol-Myers Squibb.