Loss Of Major Molecular Response As a Trigger For Restarting Tyrosine Kinase Inhibitor Therapy In CP-CML Patients Who Have Stopped Imatinib After Durable Undetectable Minimal Residual Disease

More than half of patients with chronic phase chronic myelogenous leukemia (CP-CML) in complete molecular response (CMR) experience molecular relapse after imatinib discontinuation. We investigated loss of major molecular response (MMR) as a criterion for resuming therapy.

A multicenter observational study (A-STIM, According to STop IMatinib) evaluating MMR persistence was conducted in 80 CP-CML patients who had stopped imatinib after prolonged confirmed CMR (24 months or more). Patients with confirmed CMR with 1 or 2 occasional weak positive samples before study entry were also considered eligible. CMR was defined as undetectable BCR-ABL transcript with a sensitivity of at least 40000 amplified copies of the ABL control gene, in accordance with the level of sensitivity routinely applied within laboratories participating in the French GBMHM Network.

Median time from imatinib initiation to discontinuation was 79 months (range 30-145), median duration of CMR before imatinib discontinuation was 41 months (24-96), and median follow-up after discontinuation was 31 months (8-92). Twenty-nine patients (36%) lost MMR after a median of 4 months off-therapy (2-17). Cumulative incidence of MMR loss was estimated as 35% (95% CI, 25%-46%) at 12 months and 36% (95% CI, 26%-47%) at 24 months whereas probability of losing CMR was estimated as 51% at 12 months (95% CI, 41%-63%) and 54% at 24 months (95% CI, 44%-66%). Fivety two percent of the patients met the criteria for cCMR but experienced occasional BCR-ABL positivity (unstable cCMR). Those patients were not at higher risk of losing MMR as compared to patients with stable cCMR. This observation may potentially increase by two-fold the number of patients eligible for TKI discontinuation. Fluctuation of BCR-ABLtranscript levels below the MMR threshold (≥ 2 consecutive positive values) were observed in 31% of patients after imatinib discontinuation. Using cell sorting in three fluctuating patients, we were able to confirm that BCR-ABL signal was mostly present in the CD15 positive fraction, demonstrating first that BCR-ABL residual signal was not related to long living lymphoid cells and second that residual CML cells retain a clonogenic potential. Treatment-free remission was estimated as 64% (95% CI, 54%-75%) at 12 and 24 months and 61% (95% CI, 51%-73%) at 36 months. Treatment was resumed in 31 patients after loss of MMR. Twenty-three patients regained CMR4.5 and 8 patients are in MMR under therapy after 2+ to 17+ months. Median to time to a second CMR was estimated as 7.3 months in retreated patients.

The probability of losing MMR after imatinib discontinuation was estimated as 36% in the long-term. Loss of MMR is a practical and safe criterion for restarting therapy in CML patients with prolonged CMR and could be used for future discontinuation studies.

Rousselot:Novartis: Research Funding; BMS: Research Funding. Cony-Makhoul:BMS: Honoraria; Novartis: Research Funding. Nicolini:BMS, Teva, Ariad, Pfizer, Novartis: Consultancy, Honoraria, Membership on an entity’s Board of Directors or advisory committees, Research Funding, Speakers Bureau. Etienne:Novartis: Consultancy; Pfeizer: Consultancy; Novartis: Honoraria; BMS: Honoraria. Guerci-Bresler:Novartis: Membership on an entity’s Board of Directors or advisory committees; BMS: Membership on an entity’s Board of Directors or advisory committees. Turhan:BMS: Membership on an entity’s Board of Directors or advisory committees; Novartis: Membership on an entity’s Board of Directors or advisory committees; BMS: Honoraria; Novartis: Research Funding. Guilhot:BMS: Consultancy; Novartis: Consultancy; Ariad: Consultancy; Pfizer: Consultancy; BMS: Research Funding; Novartis: Research Funding. Mahon:Novartis: Consultancy, Honoraria, Membership on an entity’s Board of Directors or advisory committees, Research Funding; Brisol Myers Squibb: Consultancy, Honoraria; Ariad: Honoraria; Pfizer: Honoraria.