Clinical Lymphoma, Myeloma & Leukemia, Vol.21, Suppl.1 - September 2021

Abstracts Clinical Lymphoma, Myeloma & Leukemia September 2021 S324 Bellinzona, Switzerland, 5 Division of Hematology, Department of Translational Medicine, University of Eastern Piedmont, Novara, Italy, 6 Department of Hematology, ASST Grande Ospedale Metropolitano Niguarda, Niguarda Cancer Center, Milan, Italy, 7 Hematology Institute, Catholic University of the Sacred Hearth, Fondazione Policlinico Universitario A. Gemelli, Rome, Italy, 8 Division of Hematology, Ospedale di Circolo e Fondazione Macchi, University of Insubria, Varese, Italy, 9 Strategic Research Program on CLL, Division of Experimental Oncology, IRCCS San Raffaele Scientific Institute and Vita-Salute San Raffaele University, Milan, Italy, 10 Department of Haematology, University Hospital Basel, Basel, Switzerland, 11 Molecular Hematology, International Centre for Genetic Engineering and Biotechnology, Trieste, Italy, 12 Institute of Oncology Research, Università della Svizzera Italiana, Bellinzona, Switzerland, 13 Clinic of Oncology, Oncology Institute of Southern Switzerland, Bellinzona, Switzerland, 14 Division of Haematology and Central Haematology Laboratory, Cantonal Hospital Lucerne, Switzerland, 15 Faculty of Biomedical Sciences, Università della Svizzera Italiana, Lugano, Switzerland Co n text: Ibrutinib inhibits BTK with the most typical response in CLL being partial remission (PR) with measurable minimal residual disease (MRD) in blood. Remission is usually maintained until development of genetically-driven resistance. Objecti v e: Mechanism of adaptation and survival in MRD. Patie n ts a n d Methods: Pre-treatment CLL cells and under-treatment MRD were systematically collected from33 high-riskCLL patients. Samples were characterized by high-dimensional, single-cell flow cytometry; bulk genomic, transcriptomic, and chromatin accessibility profiling; and single-cell RNA-seq profiling. Results: The genetic composition of MRD remained constant across timepoints, indicating that ibrutinib did not have any impact in shaping its genomic diversity. Ibrutinib induced chromatin dynamics in MRD resulting in a predominantly closed state. Chromatin regions that became more accessible under ibrutinib were enriched of binding sites for transcription factor that lay downstream ERK signaling. Regions that turned into a less accessible state were enriched for the binding sites of TF emanating from the BCR. At the transcriptomic level, most of the differentially expressed genes between pre- and post-treatment samples were downregulated in MRD, while only a fraction was upregulated. IL4, NF-kB, and metabolism and proliferation signatures were downregulated in MRD, while MAPK and RAS signatures were upregulated. CLL spleen samples from TCL1 mice under ibrutinib treatment showed an upregulation of MAPK pathway genes compared to untreated mice. Single-cell transcriptomes revealed a distinct post-treatment cell population driven by MAPK activity that functionally pre-existed in a fraction of CLL cells of baseline samples before ibrutinib start. At the signaling level, ibrutinib had no effect on the integrity of RAS-BRAF-MAPK-ERK pathway protein expression upon crosslinking of the BCR with anti-IgM but not after stimulation of TLR9 or CD40. Functional validation on primary samples by western blotting shows that RAS/ERK can be activated by BCR stimulation, irrespective of ibrutinib treatment in ex vivo experiments. Pharmacological inhibition of MEK (trametinib) and ERK (ulixertinib) synergized with ibrutinib, leading to decreased cell viability. Co n clusio n s: MRD under ibrutinib adapts its phenotype in an epigenetic way to maintain functional competence of BCR signaling via the MAPK pathway, which turned to be a vulnerability of MRD persisting under ibrutinib. Keywords: CLL, epigenetics, MRD, ibrutinib CLL-376 Clinical Characteristics and Outcomes of Patients with Chronic Lymphocytic Leukemia (CLL), 80 Years of Age or Older Paul Hampel 1 *, Timothy Call 1 , Kari Rabe 2 , Sara Achenbach 2 , Eli Muchtar 1 , Saad Kenderian 1 , Yucai Wang 1 , Amber Koehler 1 , Jose Leis 3 , Susan Schwager 1 , Daniel Van Dyke 4 , Min Shi 5 , Curtis Hanson 5 , Tait Shanafelt 6 , Esteban Braggio 3 , Susan Slager 2 , Neil Kay 1 , Wei Ding 1 , Sameer Parikh 1 1 Division of Hematology, Department of Medicine, Mayo Clinic, Rochester, MN, USA, 2 Division of Biomedical Statistics & Informatics, Department of Health Sciences Research, Mayo Clinic, Rochester, MN, USA, 3 Department of Hematology and Oncology, Mayo Clinic, Phoenix, AZ, USA, 4 Division of Laboratory Genetics and Genomics, Department of Laboratory Medicine and Pathology, Mayo Clinic, Rochester, MN, USA, 5 Division Hematopathology, Department of Laboratory Medicine and Pathology, Mayo Clinic, Rochester, MN, USA, 6 Department of Hematology, Stanford University Medical Center, Stanford, CA, USA Co n text: Nearly 20% patients are  80 years of age at CLL diagnosis. Increased comorbidities often accompany advanced age. However, clinical trials enroll a disproportionally low number of people this age, providing limited evidence for guidance. Objecti v e: To determine clinical characteristics and outcomes of patients  80 years of age at the time of CLL diagnosis. D esig n : Retrospective. Setti n g: Academic. Patie n ts or Other Participa n ts: Between 1/1995–3/2020, we identified previously untreated CLL patients from the Mayo Clinic CLL Database  80 years of age at the time of CLL diagnosis and seen within 3 years of diagnosis. Mai n Outco m es Measures: Baseline characteristics, treatments, and time-to-first- treatment (TFT) were analyzed for all patients. Overall survival (OS) was measured from diagnosis date in all patients and separately from treatment date for treated patients. Time-to-next-treatment (TTNT) was measured in treated patients from the first treatment date to the date of second treatment or last known treatment. TFT and TTNT were analyzed accounting for competing risk of death. Cox multivariable regression models were used to determine which factors were associated with OS and TTNT. Results: Among 213 patients identified, the median age was 83 years (range 80–9 7 ). The median number of medical comorbidities among all patients was 4 (range 0–8). Fifty-six patients received treatment; median TFT 6. 7 years. Treatment approaches included monoclonal antibody alone (n=1 7 ), chemotherapy alone (n=1 7 ), chemoimmunotherapy (n=14), novel agents (n=3), and multi-agent Richter’s transformation regimens (n=5). Twenty-two of 56 treated patients received second- line therapy (median TTNT 2.8 years). Median OS among all