-Author name in bold denotes the presenting author
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Clinically Relevant Abstract denotes an abstract that is clinically relevant.

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648 Peripheral Blood CD26+ Leukemia Stem Cells Monitoring in Chronic Myeloid Leukemia Patients from Diagnosis to Response to TKIs: Interim Results of a Multicenter Prospective Study (PROSPECTIVE FLOWERS)Clinically Relevant Abstract

Program: Oral and Poster Abstracts
Type: Oral
Session: 632: Chronic Myeloid Leukemia: Therapy: CML: New and Beyond
Hematology Disease Topics & Pathways:
Diseases, CML, Technology and Procedures, Clinically relevant, Myeloid Malignancies, flow cytometry
Monday, December 7, 2020: 11:45 AM

Monica Bocchia1*, Anna Sicuranza, PhD2*, Paola Pacelli2,3*, Alessandra Iurlo, MD, PhD4*, Elisabetta Abruzzese, MD 5, Sara Galimberti, MD, PhD6*, Patrizia Pregno7*, Giovanni Caocci, MD8,9, Isabella Capodanno, MD10*, Monica Crugnola, MD11*, Antonella Gozzini, MD12*, Claudio Fozza, MD13*, Mario Annunziata, MD14*, Sabina Russo15*, Anna Marina Liberati16*, Olga Mulas17*, Daniele Cattaneo18*, Adele Santoni19*, Vincenzo Sammartano19*, Federico Caroni20* and Donatella Raspadori2*

1Hematology, University of Siena, Siena, Italy
2Hematology, University of Siena, Azienda Ospedaliera Universitaria Senese, Siena, Italy
3Department of Medical Biotechnologies, University of Siena, Siena, Italy
4Hematology Division, Foundation IRCCS Ca' Granda - Ospedale Maggiore Policlinico, Milano, Italy
5Department of Hematology S. Eugenio Hospital, Rome, Italy, Roma, Italy
6Department of Clinical and Experimental Medicine, Section of Hematology, University of Pisa, Pisa, Italy
8Ematologia-Centro Trapianti Midollo Osseo, Ospedale Businco, Dipartimento di Scienze mediche e Sanità Pubblica, Università di Cagliari, Cagliari, Italy
9Hematology Unit, Department of Medical Sciences and Public Health, University of Cagliari, Cagliari, ITA
10Hematology Unit, Azienda Unità Sanitaria Locale - IRCCS, Reggio Emilia, Italy
11Division of Hematology, Azienda Ospedaliero-Universitaria di Parma, Parma, Italy
12Hematology Unit, AOU Careggi, University of Florence, Florence, ITA
13Department of Clinical and Experimental Medicine, University of Sassari, Sassari, Italy
14Hematology Unit, Cardarelli Hospital, Naples, Italy
15Dipartimento di Medicina Interna, AOU Policlinico di Messina, Messina, Italy
16S.C. Oncoematologia, Università degli Studi di Perugia, A.O. Santa Maria, Terni, Italy
17Department of Medical Sciences and Public Health, University of Cagliari, Cagliari, Italy
19Hematology Unit, Azienda Ospedaliera Universitaria Senese, University of Siena, Siena, ITA
20Hematology Unit, University of Siena and Azienda Ospedaliera Universitaria Senese, Siena, Italy


We already showed that in CML pts peripheral blood CD34+/CD38-/CD26+ cell population represent a “CML specific” leukemia stem cell (LSC) circulating compartment. Indeed, we demonstrated that CD26+LSCs are measurable by flow-cytometry in 100% of CML pts at diagnosis the latter representing an alternative and rapid diagnostic tool. In addition, in a cross-sectional study we were able to spot peripheral blood CD26+LSCs also in about 65% of CML during TKI treatment regardless of type and length of TKI treatment and degree of molecular response. However, no prospective data are available regarding the behavior of PB CD26+LSCs in terms of rate and timing of reduction during TKI therapy and the correlation, if any, with the attainment of response according to ELN guidelines. Interestingly, even CML patients in stable TFR may harbor circulating CD26+LSCs thus suggesting a probable active role of the immune system in the control of residual disease. One hypothesis could reside in the presence or absence on the LSCs of molecules (such as PD-L1) able to hamper an anti-leukemic T cell response. From Jan 2018 we conducted a prospective multicenter Italian study including CML pts at diagnosis treated and managed by each of 15 participating center according to ELN guidelines. We here present the first interim analysis after a median time of treatment of 12 mos.


The main goals of this study were to prospectively monitoring PB CD26+LScs in CML pts during TKI treatment and to correlate the behavior of LSCs with molecular response. In a proportion of pts PD-L1 expression on CD26+ LSCs at diagnosis was also evaluated.


At diagnosis and during TKI treatment, pts have been centrally evaluated in Siena lab for flow-cytometry PB CD26+ LSCs (+3, +6, +12, +18, +24 mos) and PD-L1 expression (at diagnosis). At each time point molecular BCR-ABL/ABLIS ratio was monitored locally in each center.


176 consecutive CML pts (IMA 92; NILO 61; DASA 23) were enrolled in the study so far (table 1). PB CD26+LSCs were measured at time 0 (baseline) in all 176 CML pts and in 165/176 (94%), 142/176 (81%) and 112/176 (71%) at +3, +6, +12 mos of TKI treatment, respectively. Median CD26+LSCs absolute number/µL at baseline was 6.96/µL (range 0.0126-64429), at +3 mos 0.0137/µL (range 0-6,49), at +6 mos 0.0056/µL (range 0-1.188), and at +12 mos 0.0112/µL (range 0-0.1824). No significant correlation between number of CD26+LSC, degree of response and BCR-ABL copies was found (Table 2). Interestingly, median CD26+LSCs at diagnosis was found significantly higher in NILO and DASA treated pts (12, 48/µL and 17,48/µL, respectively) than in IMA pts (4,58/µL). So far, 20/176 (11.4%) pts switched to different TKIs, due to failure/suboptimal response: of note, median CD26+ LSCs of this cohort at diagnosis was the highest (23.12/µL).

Starting from Jun 2019, 44/176 (25%) CML pts have been evaluated also for PD-L1 expression at diagnosis: of these, 23/44 (52%) resulted PD-L1 positive and 21/44 (48%) resulted negative with a median of CD26+LSCs of 15.39/µL (range 1.28-635.5) and 4.45/µL (range 0.234-113.9), respectively.


After a sensible drop observed at 3 mos of any TKI treatment, CD26+LSCs are fluctuating and measurable at low level in most of pts (> 65%) even at 18 and 24 mos. We confirmed no correlation between the absolute number of persisting CD26+LSCs and BCR-ABL copies. However, pts with failure or suboptimal response showed the highest level of CD26+ at diagnosis. CD26+LSCs were found PD-L1+ in about half of 44 pts tested. At diagnosis higher CD26+LSCs number, PD-L1 positivity or both may correlate with a lower probability to achieve an optimal response; interim data of this first report will be presented; enrolment and follow up are ongoing.

Disclosures: Bocchia: Incyte: Honoraria; CELGENE: Honoraria. Abruzzese: Pfizer: Consultancy, Membership on an entity's Board of Directors or advisory committees; Novartis: Consultancy, Membership on an entity's Board of Directors or advisory committees; Bms: Honoraria; Incyte: Consultancy, Membership on an entity's Board of Directors or advisory committees. Galimberti: Novartis: Speakers Bureau; Incyte: Honoraria. Pregno: Incyte-Italy,: Membership on an entity's Board of Directors or advisory committees, Other: conference reports; Novartis-Italy: Membership on an entity's Board of Directors or advisory committees, Other: conference reports; Pfizer-Italy: Membership on an entity's Board of Directors or advisory committees, Other: conference reports. Crugnola: BMS: Honoraria; Janssen: Honoraria; Celgene: Honoraria; Novartis: Honoraria. Liberati: MORPHOSYS: Honoraria, Research Funding; ONCONOVA: Honoraria, Research Funding; INCYTE: Honoraria; VERASTEM: Honoraria, Research Funding; ROCHE: Honoraria, Research Funding; PFIZER: Honoraria, Research Funding; ONCOPEPTIDES AB: Honoraria, Research Funding; TAKEDA: Honoraria, Research Funding; FIBROGEN: Honoraria; BIOPHARMA: Honoraria; ARCHIGEN: Honoraria; BEIGENE: Honoraria; BMS: Honoraria; AMGEN: Honoraria; CELGENE: Honoraria; JANSSEN: Honoraria; ABBVIE: Honoraria, Research Funding; NOVARTIS: Honoraria, Research Funding; KARYOPHARM: Honoraria, Research Funding.

*signifies non-member of ASH