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INTRODUCTION: Long-term survival of patients with chronic myeloid leukemia (CML) has significantly improved since the introduction of BCR-ABL1 tyrosine kinase inhibitors (TKIs). Several considerations about the side effects, risks and cost associated with the lifetime treatment, have led patients and physicians to explore the possibility of TKI discontinuation after achievement of a sustained deep molecular response (DMR), so-called treatment-free remission (TFR). Several clinical trials show that approximately half of patients who achieve a sustained DMR during TKI treatment maintain molecular remission after suspension of TKIs. There is currently no biomarker that reliably predicts TFR in CML, mainly due to different study designs that have generated inconsistent data. Thus, further investigations are needed to identify factors that consistently favor achievement of TFR. With the aim of developing a biomarker for TFR prediction we analyzed the phenotype of Natural Killer (NK) cells and their relation to successful TKI cessation. METHODS: This analysis was conducted as a substudy of the Argentina Stop Trial. Altogether, 50 consecutive chronic phase CML patients who participated in the clinical trial were recruited from 7 Argentinian centers. Peripheral blood samples were collected before stopping TKI treatment, at month 3, 12 and at any time when MR3.0 was lost. Freshly isolated mononuclear cells from 46 patients were immunophenotyped by staining with CD3, CD16, CD25, CD56, CD57, CD158, NKp30, NKp44, NKp46, NKG2A, NKG2C, NKG2D and PD-1 antibodies and NK cells subpopulations were analyzed by flow cytometry (BD FACS Canto™II). Molecular recurrence-free survival was estimated by the Kaplan-Meier method and compared within groups by the log-rank test. The cutoffs of the numerical variables were optimized according to the log-rank test. Quantitative variables were dichotomized according to receiver operating characteristics (ROC) curves in order to describe sensibility and specificity. Multivariate analysis was performed through Cox proportional hazards model. Main results are provided with hazard ratio (HR) at 6 months and 95% confidence intervals (95% CI). RESULTS: At the time of discontinuation the median proportion of NK cells (CD3-CD56+) among lymphocytes was significantly increased in patients compared with controls (15% vs 9%, P = 0.0016). A significant difference between molecular relapsed vs no-relapsed patients was observed when optimal cutoff (0.43) for CD56bright low and high was determined (at 6 months 74% vs 100% respectively, log rank test, p=0.023). At this time of follow up, no significant difference was observed for CD56dim NK cells. Phenotypic markers for adaptive-like NK cells were analyzed, however, no significant differences were observed between the non-relapsing and relapsing groups. Nevertheless, molecular non-relapsing patients had significantly higher frequencies of PD-1+ NK cells as compared with molecular relapsing patients (at 6 months 85% vs 64%, Log Rank test, P=0.009). Based on the ROC and Youden Index analysis, at 6 months the 1.2 cutoff shows an 80% specificity and 50% sensitivity. Moreover, after multivariable Cox proportional analysis, including age, time of treatment, deep molecular response time, Sokal risk, NK cells and PD-1+ NK cells, the last subpopulation was identified as an independent prognostic factor for molecular-relapse-free survival (Hazard ratio = 3.63; 95% CI 1.3 - 10.1; P=0.014). CONCLUSIONS: The clinical impact of NK cells in patients who have discontinued TKIs is controversial. The effects of TKIs against immune cells, including NK cell subsets, could depend on the type of TKIs; this aspect is particularly relevant in Argentinian treated patients real world, since many different copies of TKIs are routinely used in the clinical setting. Our study is the first, to our knowledge, to report a significant increase in PD-1 expression in NK cells at TKI cessation in patients who do not relapse. Accordingly to recent reports, PD-1 expression is more abundant on NK cells with an activated and more responsive phenotype and does not mark NK cells with an exhausted phenotype. To fully understand how PD-1 on NK cells modulates immune responses we are planning to carry out functional studies. In the future, we are also planning a comprehensive study of immune suppressors, including regulatory T cells and myeloid-derived suppressor cells. Disclosures Moiraghi: Novartis: Speakers Bureau; BMS: Speakers Bureau. Varela:Novartis: Consultancy, Speakers Bureau. Pavlovsky:Varifarma: Speakers Bureau; Astra Zeneca: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Abbvie: Membership on an entity's Board of Directors or advisory committees, Other: Travel grants, Speakers Bureau; Janssen: Membership on an entity's Board of Directors or advisory committees, Other: travel grants, Speakers Bureau. Pavlovsky:Pint Pharma: Speakers Bureau; Pfizer: Speakers Bureau; BMS: Speakers Bureau; Novartis: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau.

Purpose: To evaluate the efficacy of FCR in improving complete remission (CR), disease-free survival (DFS) and overall survival (OS) rates in patients previously treated with chlorambucil-prednisolone and untreated CLL patients. Patients and methods: A total of 45 CLL patients started FCR. Forty-one patients completed treatment: 16 following previous relapse and 25 previously untreated with progressive disease. Four patients are still receiving treatment. Median patient age was 63 years (range 34–88 years). Binet’s stage were: A: 8%, B: 34% and C: 58%. CD38 expression was positive (> 7% of cells) in 56% of patients and negative in 44%. FCR consisted of: fludarabine (25 mg/m2/day × 3); cyclophosphamide (250 mg/m2/day × 3) and rituximab (375 mg/m2/day × 1), all given intravenously, every 4 weeks for 4–6 cycles. CR was defined by CLL/NCI-WG criteria. Minimal residual disease (MRD) negativity was < 1% CD19, CD5 positive cells in peripheral blood and bone marrow. Results: The results of this study are detailed in Table 1. To summarise: CR: 69%; nodular partial remission (PR): 15%; PR: 7% and stable disease: 2%. Grade 3–4 neutropenia occurred in 33% of patients and 34% required hospitalization due to infections. Table 1: Patient details following FCR treatment Patients % CR % MRD negative % DFS 36 mo P % OS 36 mo P Untreated 25 87 96 90 95 Treated 16 53 87 75 0.10 89 0.90 Total 41 74 92 83 93 CD38 Positive 23 71 90 71 0.02 87 0.27 CD38 Negative 18 76 94 100 100 Conclusions: FCR induces a high CR (74%) and DFS (83%) rate and increases MRD negativity (92%). Significantly higher DFS rates at 36 months were observed in patients who were CD38 negative vs. CD38 positive (100 % vs. 71% P=0.02).

Introduction: During the last decade, strong prognostic factors and new compounds emerged for the treatment of CLL. The initial experience with the combination of FCR at MDACC as salvage therapy and later in untreated patients (pts) promote us to use these regimen since March 2001. Purpose: To evaluate the efficacy of FCR in improving the complete remission (CR), disease free survival (DFS) and overall survival (OS) rates in pts previously treated with chlorambucil-prednisone as a single therapy and untreated CLL pts. Patients and methods: A total of 32 CLL pts, 16 after relapse and 16 untreated with progressive disease completed treatment and evaluation, 3 pts are still on treatment. FCR consisted of Fludarabine 25 mg/m2 iv on days 1 to 3, Cyclophosphamide 250 mg/m2 iv on day 1 to 3 and Rituximab 375 mg/m2 iv on day 1 given at 4 weeks interval up to 4–6 cycles. Response was assessed 4–8 weeks after treatment. CR was defined by CLL/NCI-WG. Minimal residual disease (MRD) negative was defined as 7% of cells) in 53% and negative in 47% of the cases. Results: The CR was obtained in 66% of the pts, nodular partial remission (NPR) in 19%, PR in 9% and stable disease in 6%. Characteristics Pts. # % CR % MRD negative % DFS 24 mo P= # OS 24 mo P= Untreated Pts 16 81 100 81 0.64 90 0.81 Treated Pts 16 50 81 76 87 CD38 Positive 17 65 82 65 0.01 80 0.28 CD38 Negative 15 67 87 100 100 Total 32 66 84 78 89 Neutropenia grade 3–4 was observed in 31% of the courses, 5% of the pts required hospitalisation due to infeccious episodes. Conclusion: In patients with CLL, FCR induce a high CR, MRD negative and DFS rate. Patients with CD38 negative expression correlated with a better DFS.