Silent NK/T Cell Reactions Against Dasatinib during Sustained Deep Molecular Response before Discontinuation Are Associated with Longer Treatment-Free Remission: A Multicenter, Single-Arm, Phase 2 Study

Introduction

Tyrosine kinase inhibitors (TKIs) markedly enhance the prognosis of chronic myelogenous leukemia (CML), potentially enabling the attainment of deep molecular response (DMR). Subsequently, the discontinuation of TKI became imperative to evade adverse events and financial burden of TKI therapy. In several studies, beginning with the STIM1 (Lancet Oncol 2010;11:1029), nearly 40%-60% of patients with chronic CML who sustained long DMR could discontinue TKI and attain long-term treatment-free remission (TFR). Remarkably, initial dasatinib specifically induces the elevation in lymphocytes, including large granular lymphocytes (LGL), NK cells, and cytotoxic T cells, as well as the decline in regulatory T cells (Tregs) in the early phase of treatment, related to early clinical responses.(Int J Hematol 2014;99:41) Nevertheless, lymphocyte variations by dasatinib during sustained DMR before cessation is an area of growing interest. In the Japanese multicenter prospective D-STOP trial (NCT01627132), we discontinued dasatinib following 2-year consolidation to sustain DMR in chronic CML to assess the TFR rate. We here present the final results of the D-STOP trial, including the peripheral NK/T cell change during dasatinib consolidation associated with successful TFR.

Methods:

Chronic phase CML patients treated with TKIs who had undetectable BCR-ABL1 mRNA were enrolled. After confirmation of undetectable BCR-ABL1 mRNA (International Scale <0.01%) using a real-time quantitative polymerase chain reaction (RQ-PCR) in the central laboratory, the patient received additional dasatinib treatment for another 2 years as consolidation therapy. Patients who maintained DMR during the consolidation therapy proceeded to discontinue dasatinib. BCR-ABL1 mRNA was monitored every month in year 1 and every 3 months in year 2. Molecular relapse was defined as two successive positive RQ-PCRs for BCR-ABL1 within 1 month. The relapsed patients restarted dasatinib treatment. The primary endpoint was treatment-free survival after 12 months of discontinuation. Lymphocyte subsets were analyzed using flow cytometry during and after the consolidation therapy.

Results:

Sixty-five patients received consolidation therapy, and 54 discontinued dasatinib treatment after maintenance of DMR for 2 years. Estimated treatment-free remission (TFR) rates at 12 and 36 months were 63.0% [95% confidence interval (CI): 48.7-74.3] and 59.3% (95% CI: 45.0-71.0), respectively.(Figure 1) CD3^-CD56⁺NK, CD16⁺CD56⁺NK, CD57⁺CD56⁺NK large granular lymphocytes (NK-LGL), CD8⁺CD4^- cytotoxic T, and CD57⁺CD3⁺T-LGL cell numbers were relatively elevated throughout 24-month consolidation only in failed patients who molecularly relapsed within 12 months. In successful patients (TFR >12-months), these subsets elevated transiently after 12 months but returned to basal levels after 24-month consolidation. (Figure 2)There were no differences in CD8-CD4+ helper T and Treg. cell numbers during consolidation between 2 groups. Therefore, smaller changes in the NK/T sebsets, particularly NK subset throughout consolidation, exhibited higher TFR rates. TFR rates of those exhibiting elevation in CD3^-CD56⁺NK > 376 cells/mL, CD16⁺CD56⁺NK > 241 cells/mL,CD57⁺CD56⁺NK-LGL > 242 cells/mL or CD8+CD4- cytotoxic T cells > 212 cells/mL during consolidation compared with others were 26.7% (8.3%-49.6%) versus 78.3% (55.4%-90.3%), HR 0.032 (0.0027-0.38; P = 0.0064) (Figure 3), 31.2% (11.4%-53.6%) versus 85.0% (60.4%-94.9%), HR 0.039 (0.0031-0.48; P = 0.011), 36.8% (16.5%-57.5%) versus 77.3% (53.7%-89.8%), HR 0.21 (0.065-0.69; P = 0.010), or 41.2% (18.6%-62.6%) versus 76.5% (48.8%-84.9%), HR 0.18 (0.019-1.77; P = 0.14), respectively.

Conclusion

Silent responses of the T/NK subsets to dasatinib throughout consolidation was significant for longer TFR. Elevated NK/T, particularly NK lymphocytes responsive to dasatinib, could be immature cells with little immunosurveillance; their decline subsequently replaced by altered lymphocyte population with less response to dasatinib during sustained DMR might be immunologically significant.

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