Abstracts Clinical Lymphoma, Myeloma & Leukemia September 2023 S306 AML-481 A Multiomic, Single‑Cell Measurable Residual Disease (scMRD) Assay For Phasing DNA Mutations and Surface Immunophenotypes Charlie Murphy PhD, Kathryn Thompson BS, Lubna Nousheen MSc, Aaron Llanso BS, Todd Druley MD, PhD, Adam Sciambi PhD Mission Bio, South San Francisco, USA The small population of cancerous cells that remain following treatment, known as measurable residual disease (MRD), is the major cause of relapse in acute myeloid leukemia (AML). Usually, these refractory cells have gained additional resistance mutations or changed their surface immunophenotypes in ways that preclude detection and phasing by current gold standard flow cytometry or bulk next-generation sequencing assays. For this reason, a multiomic single-cell MRD (scMRD) assay could offer a more comprehensive indicator of relapse and the potential for faster response. Here, we present a new scMRD assay with a 0.01% limit of detection that provides single-cell clonal architecture and immunophenotyping to not only identify residual leukemia cells, but also identify putative DNA or protein targets for salvage therapy. The assay enables rarecell detection on a standard Mission Bio Tapestri run by adding (i) an upfront bead-based protocol to enrich for blast cells, (ii) a DNA and protein panel specifically designed for AML MRD diagnosis and treatment, and (iii) a new, automated analysis pipeline to evaluate single-cell multiomics output. By utilizing Mission Bio’s single-molecule DNA sensitivity for single cells, this pipeline can identify and correlate co-occurring de novo variants, thereby reducing false positive rates over bulk assays that do not correlate variants. It furthermore can create phylogenetic trees of the detected MRD cells and present their surface protein signature and arm-level copy number. In addition, the multiplexing of up to three patient samples combined in one run via germline identification further reduces per sample costs and increases throughput. To demonstrate these features on 0.01% MRD, we present data from samples constructed by titrating diseased cells into healthy bone marrow cells before processing them with the scMRD assay. We detected 0.01% spike-in (CD34+) and 0.1% spike-in (CD117+) in 6 of 6 samples, with an average enrichment of 41x and 13x, respectively. Further testing detected CD34+ 0.1% spike-ins in 10 of 10 samples (32x average enrichment), with further validation on spike-ins generated from clinical AML samples pending. By combining high sensitivity with multiomics, this assay offers a potential scalable solution for comprehensive MRD detection that guides therapeutic decision-making. Keywords: AML, measurable residual disease, single-cell DNA sequencing, proteogenomics, clonal architecture, immunophenotyping AML-497 Salvage Treatment With Venetoclax (Ven) and Hypomethylating Agents (HMA) for Relapsed/Refractory FLT3‑mutated Acute Myeloid Leukemia (AML) Patients: Clinical Characteristics and Outcomes Ahmad Ghorab MD, MSc1, Mark Litzow MD2, Naseema Gangat MD2, Aref Al-Kali MD2, Mithun Shah MD2, Hemant Murthy MD1, Hasan Alkhateeb MD2, Kebede Begna MD2, Mrinal Patnaik MD3, James Foran MD1, Talha Badar MD1 1Mayo Clinic, Jacksonville, USA. 2Mayo Clinic, Rochester, USA. 3Mayo Clinic, Rotchester, USA Introduction: FLT3 mutation (m) significantly impacts the prognosis of acute myeloid leukemia (AML), necessitating effective therapeutic strategies. Management of relapsed/refractory (R/R) FLT3m AML is challenging due to lower complete remission (CR) rates and shorter CR duration. This study assesses clinical outcome of R/R FLT3m AML patients undergoing salvage therapy with ventoclax (Ven) + hypomethylating agent (HMA) combination in a real-world setting. Methods: Thirty FLT3m AML patients with R/R disease, treated at Mayo Clinic with HMA+Ven (with/without gilteritinib [Gilt]) between 2019 and 2022, were analyzed. Responses were evaluated per 2022 ELN criteria, and overall survival (OS) was calculated from the salvage treatment starting date to the last followup or death. Results: The median age of the patients was 62 years [21-80], 53% were male, and 93% were White. De novo AML was the most common (91%) AML sub-type. Fifty-seven percent of patients had diploid cytogenetics (CG), and 43% had additional CG abnormalities. Patients with concurrent NPM1 mutation were 27%. Majority of the patients (n=26 [87%]) had the FLT3 ITD sub-type. Frequently reported mutations (≥ 5%) at diagnosis were NPM1 (37%), DNMT3A (27%), WT1 (20%), RUNX1 (17%), and TET2 (13%). Decitabine-Ven was used in (n=19) 63% and AzacitidineVen in (n=11) 37% of patients. HMA-Ven-Gilt triplet was used in (n= 6, [20%]) of patients. The CR with or without count recovery (i) (CR/CRi) was 50%. The median number of HMA+Ven cycles to achieve CR/CRi was 2 (range, 1-5). The median duration of response was 8 months (range, 1-23), and median OS for our cohort was also 8 months (range, 1-24 months). Univariate analysis for OS showed a significantly better OS with FLT3m ITD subtype (P<0.0001), FLT3 allelic ratio <0.5 (P=0.03), and TET2 mutation (P=0.007). No impact on OS with cytogenetics at diagnosis (P=0.38), NPM1 mutation (P=0.24), or HMA+Ven with/without Gilt (P=0.73). At the last follow-up, 37% of the patients were alive. Conclusion: Hypomethylating agent plus Ven based combination seem to be effective as salvage therapy in patients with FLT3m AML. Effective therapies are warranted to improve long-term outcome of these adverse-risk patient population. Keywords: Relapsed/Refractory AML, FLT3 mutation, Hypomethylating Agents, Venetoclax, Clinical Outcomes
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