Abstracts Clinical Lymphoma, Myeloma & Leukemia September 2023 S388 MPN-238 Evaluating the Efficacy of Therapeutic Plasma Exchange in the Management of HELLP Syndrome: A Single‑Center Experience İbrahim Halil Açar MD, Birol Güvenç MD Çukurova University Faculty of Medicine, Department of Haematology, Adana, Turkey Introduction and Objectives: HELLP syndrome is a severe pregnancy-related complication characterized by hemolysis, elevated liver enzymes, and low platelet count. Complement dysregulation contributes to the etiopathogenesis of HELLP syndrome. Therapeutic plasma exchange (TPE) removes abnormal complement pathway components and replaces them with normal physiological components. This study aimed to evaluate the impact of TPE on disease progression in HELLP syndrome patients who were unresponsive to supportive therapy and corticosteroids. Materials and Methods: This retrospective study involved 13 patients diagnosed with Class 1 HELLP syndrome based on the Mississippi system. These patients underwent TPE in the postpartum period between 2012 and 2015. Results: Of the thirteen patients, three succumbed to multiorgan failure. After TPE, hemoglobin and platelet counts increased, while AST, ALT, and LDH levels decreased. These changes were statistically significant (P<0.05). In patients who died after TPE, the duration between hospital admission and TPE initiation was longer. Conclusion: TPE is an effective treatment strategy that improves clinical outcomes in patients with complex postpartum HELLP syndrome who do not respond to conservative management. Early diagnosis and the role of TPE in disease management are increasingly important in such cases. Keywords: HELLP syndrome, therapeutic plasma exchange (TPE), pregnancy-related complication, complement dysregulation, disease management MPN-295 Avapritinib in Patients With Advanced Systemic Mastocytosis (AdvSM): Efficacy and Safety Analyses from the Phase 2 PATHFINDER Study With 2‑Year Follow‑Up Jason Gotlib MD, MS1, Andreas Reiter MD2, Deepti H. Radia MRCPI, FRCPath3, Michael W. Deininger MD, PhD4, Tracy I. George MD5, Jens Panse MD6, Alessandro M. Vannucchi MD, CRIMM7, Uwe Platzbecker MD8, Iván Alvarez-Twose MD, PhD9, Andrzej Mital MD, PhD10, Olivier Hermine MD, PhD11, Ingunn Dybedal Md, PhD12, Elizabeth O. Hexner MD13, Lambert Span MD, Phd14, Ruben Mesa MD, FACP15, Prithviraj Bose MD16, Kristen M, Pettit MD17, Stephen T. Oh MD, PhD18, Hui-Min Lin PhD19, Saša Dimitrijević PhD20, Javier I. Muñoz-González PhD20, Daniel J. DeAngelo Md, PhD21 1Division of Hematology, Stanford Cancer Institute/Stanford University School of Medicine, Stanford, CA, USA. 2Department of Hematology and Oncology, Heidelberg University, Mannheim, Germany. 3Guy’s & St Thomas’s NHS Foundation Trust, London, United Kingdom. 4Versiti Blood Research Institute, Milwaukee, WI, USA. 5ARUP Laboratories, University of Utah, Salt Lake City, UT, USA. 6Department of Oncology, Hematology, Hemostaseology and Stem Cell Transplantation, University Hospital Aachen, Medical Faculty, RWTH Aachen University, Aachen, Germany. 7Center for Research and Innovation of Myeloproliferative Neoplasms – CRIMM, Azienda Ospedaliera Universitaria Careggi, University of Florence, Florence, Italy. 8Leipzig University, Leipzig, Germany. 9Institute of Mastocytosis Studies of Castilla-La Mancha, Spanish Reference Center of Mastocytosis, Toledo, Spain. 10Department of Hematology and Transplantology, Medical University of Gdansk, Gdansk, Poland. 11Department of Hematology, CEREMAST, Necker–Enfants Malades Hospital, APHP, and Imagine Institute, INSERM U1163, Paris University, Paris, France. 12Department of Hematology, Oslo University Hospital, Oslo, Norway. 13Abramson Cancer Center, University of Pennsylvania, Philadelphia, PA, USA. 14Department of Hematology, University Medical Center Groningen, Groningen, Netherlands. 15Mays Cancer Center at UT Health San Antonio MD Anderson, San Antonio, TX, USA. 16The University of Texas MD Anderson Cancer Center, Houston, TX, USA. 17Division of Hematology/Oncology, University of Michigan, Ann Arbor, MI, USA. 18Siteman Cancer Center at Barnes-Jewish Hospital and Washington University School of Medicine, St. Louis, MO, USA. 19Blueprint Medicines Corporation, Cambridge, MA, USA. 20Blueprint Medicines (Switzerland) GmbH, Zug, Switzerland. 21Department of Medical Oncology, Dana-Farber Cancer Institute, Boston, MA, USA Background: AdvSM is a clonal mast cell (MC) disease driven by the KIT D816V mutation. Avapritinib is a potent, selective KIT D816V inhibitor approved for adults with AdvSM in the USA and Europe after ≥1 prior therapy. We present data from PATHFINDER with 2-years follow-up. Methods: The primary endpoint was overall response rate (ORR) per modified International Working GroupMyeloproliferative Neoplasms Research and Treatment-European Competence Network on Mastocytosis (mIWG) response criteria. Secondary endpoints were time to response (TTR), duration of response (DOR), progression-free survival (PFS), overall survival (OS), mean change from baseline objective disease burden measures (bone marrow [BM] MC burden, serum tryptase, blood KIT D816V variant allele fraction [VAF], spleen volume), and safety. Results: By September 9, 2022, 107 patients with centrally confirmed AdvSM initiated avapritinib (200 mg, n=105; 100 mg, n=2) QD. In 83 mIWG response-evaluable patients, the ORR (95% CI) was 73% (63–83), with 27% achieving complete remission (CR) or CR with partial hematologic recovery. ORR was 90% (74–98) in treatmentnaïve patients (n=27/30) with 40% achieving CR or CRh. The median TTR (range) was 2.3 months (0.3–15). The median DOR, PFS, and OS were not reached. Benefit was observed regardless of prior therapy or subtype. Reductions of ≥50% in BM MC burden (88%), serum tryptase (92%), KIT D816V VAF (81%), and ≥35% reduction in spleen volume (70%) were observed. Seventy percent of patients had a total clearance of MC aggregates, 61% had serum tryptase <20 ng/mL, 58% had KIT D816V VAF <1%, and 74% with palpable spleens became non-palpable. Treatment-related adverse events (TRAEs [≥25%], any Grade, Grade ≥3) were thrombocytopenia (39%, 18%), periorbital edema (39%, 6%), peripheral edema (38%,
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