Background Myelodysplastic syndromes (MDS) are characterized by frequent mutations in RNA splicing factor genes. Protein arginine methyltransferase 5 (PRMT5) regulates the activity of the splicing machinery through methylation of key spliceosome proteins. PRMT5 inhibitors have demonstrated preferential killing of acute myeloid leukemia cells with splicing factor mutations. JNJ-64619178 is a potent, selective, oral PRMT5 inhibitor that causes accumulation of splicing abnormalities in preclinical models. It is hypothesized that inhibition of PRMT5 by JNJ-64619178 may target splicing factor gene mutant MDS clones, leading to recovery of normal hematopoiesis. Here we present Part 2 of a phase 1 study of JNJ-64619178 evaluating the safety, pharmacokinetics (PK), pharmacodynamics (PD), and preliminary clinical activity of JNJ-64619178 in adult patients (pts) with lower risk MDS (International Prognostic Scoring System [IPSS] score Low or Intermediate-1) who are red blood cell (RBC) transfusion-dependent and relapsed or refractory to erythropoiesis-stimulating agent (ESA) treatment. Methods Dosing was initiated at 2 mg, 14 days on/7 days off (intermittent) on a 21-day cycle, a tolerable dose from the ongoing dose escalation in solid tumors and lymphomas in Part 1. Confirmation of tolerability followed a modified 3+3 design with de-escalation allowed based on the totality of the data. Enrollment was expanded at the selected, tolerable dose level to further evaluate safety, PK, PD, and preliminary clinical activity. Results As of 29 May 2021, 21 pts were enrolled. Median age was 71 (range 52-85). Revised IPSS score was Very Low (10%), Low (71%) or Intermediate (19%). Pts had a median of 1 prior line of therapy post-ESA (range 0-4), including lenalidomide (33%), hypomethylating agents (29%), and luspatercept (19%). SF3B1 mutations were identified in 71% of pts. Median treatment duration was 3.1 months (range 0.03-6.37). No dose-limiting toxicities were observed. While the 2 mg intermittent starting dose was tolerable, lower dose schedules of 1 mg intermittent and 0.5 mg continuous (once daily [QD]) dosing were evaluated to reduce myelosuppression, and enrollment was expanded at 0.5 mg QD (n=13 pts). Fifteen pts (71%) experienced ≥1 treatment emergent adverse event (TEAE) that was considered related to the study agent (TRAE). TRAE in >10% of pts were thrombocytopenia (52%), neutropenia (48%), anemia (19%), and dysgeusia (14%). Grade 3 or higher TRAE were reported for 57% of pts. Grade 3 or higher TRAE worsening from baseline in >1 pt were neutropenia (29%); anemia and thrombocytopenia (19.0% each); and diarrhea (9%). TEAEs contributed to treatment interruption, dose reduction, and treatment discontinuation in 57%, 14%, and 19% of pts, respectively. Anemia, thrombocytopenia, and neutropenia were the only TRAE leading to treatment modification in >1 pt. Hematologic toxicities were dose-dependent and reversible. Grade ≥3 thrombocytopenia and neutropenia were observed, respectively, in 67% and 50% of pts at 2 mg intermittent versus 0% and 31% at 0.5 mg QD. Serious AE considered possibly related to JNJ-64619178 by the investigator included 1 pt with Grade 3 anemia and 1 pt with Grade 1 atrial fibrillation, Grade 3 cardiac troponin I increase, and Grade 5 cardiac failure (>30 days from last dose). One other death of unknown cause was not considered related to treatment. Comparison of the multiple dose C trough across all dose levels suggests a dose-dependent increase in JNJ-64619178 plasma concentration. Robust target engagement, as measured by plasma symmetric dimethylarginine, was achieved at all dose levels. Serial analysis of variant allele frequency of clonal mutations in a subset of pts did not show significant reductions from baseline in peripheral blood or bone marrow. No pts experienced objective response or hematologic improvement according to International Working Group criteria, RBC transfusion independence, or meaningful reduction in transfusion requirements. Conclusion JNJ-64619178 demonstrated primarily hematologic toxicity in pts with transfusion-dependent lower-risk MDS, which was manageable at the selected expansion dose. Despite robust target engagement, clinical activity was not observed, and enrollment was stopped. The role of PRMT5 in MDS and the differential impact of PRMT5 inhibition on normal and malignant hematopoiesis require further study. Disclosures Platzbecker: Novartis: Honoraria; Janssen: Honoraria; AbbVie: Honoraria; Takeda: Honoraria; Celgene/BMS: Honoraria; Geron: Honoraria. Avivi: Kite, a Gilead Company: Speakers Bureau; Novartis: Speakers Bureau. Brunner: Agios: Consultancy; Keros Therapeutics: Consultancy; Aprea: Research Funding; AstraZeneca: Research Funding; GSK: Research Funding; Acceleron: Consultancy; Janssen: Research Funding; Takeda: Consultancy, Research Funding; BMS/Celgene: Consultancy, Research Funding; Novartis: Consultancy, Research Funding. Morillo: Janssen: Honoraria; Abbvie: Honoraria; Takeda: Honoraria. Patel: Acerta Pharma: Research Funding; Curis: Research Funding; Clovis: Research Funding; Celgene: Membership on an entity's Board of Directors or advisory committees, Research Funding; Aileron Therapeutics: Research Funding; ADC Therapeutics: Research Funding; Effector Therapeutics: Research Funding; Eli Lilly: Research Funding; EMD Serono: Membership on an entity's Board of Directors or advisory committees, Research Funding; Evelo Biosciences: Research Funding; Jacobio: Research Funding; Janssen: Membership on an entity's Board of Directors or advisory committees, Research Funding; Jounce Therapeutics: Research Funding; Klus Pharma: Research Funding; Kymab: Research Funding; Loxo Oncology: Research Funding; LSK Biopartners: Research Funding; Lycera: Research Funding; Mabspace: Research Funding; Macrogenics: Research Funding; Merck: Research Funding; Millennium Pharmaceuticals: Research Funding; Mirati Therapeutics: Research Funding; ModernaTX: Research Funding; ORIC Pharmaceuticals: Research Funding; Pfizer: Membership on an entity's Board of Directors or advisory committees, Research Funding; Phoenix Molecular Designs: Research Funding; Placon Therapeutics: Research Funding; Portola Pharmaceuticals: Research Funding; Prelude Therapeutics: Research Funding; Qilu Puget Sound Biotherapeutics: Research Funding; Revolution Medicines: Research Funding; Ribon Therapeutics: Research Funding; Seven and Eight Biopharmaceuticals: Research Funding; Syndax: Research Funding; Synthorx: Research Funding; Stemline Therapeutics: Research Funding; Taiho: Research Funding; Takeda: Research Funding; Tesaro: Research Funding; TopAlliance: Research Funding; Vedanta: Research Funding; Verastem: Research Funding; Vigeo: Research Funding; Xencor: Research Funding; Exelixis: Membership on an entity's Board of Directors or advisory committees; Bayer: Membership on an entity's Board of Directors or advisory committees; Pharmacyclics: Membership on an entity's Board of Directors or advisory committees; Abbvie: Membership on an entity's Board of Directors or advisory committees; Alexion, AstraZeneca Rare Disease: Other: Study investigator; Daiichi Sankyo: Research Funding; Cyteir Therapeutics: Research Funding; Ciclomed: Research Funding; Checkpoint Therapeutics: Research Funding; Calithera: Research Funding; Boehringer Ingelheim: Research Funding; Bicycle Therapeutics: Research Funding; AstraZeneca: Research Funding; Artios Pharma: Research Funding; Agenus: Research Funding; Florida Cancer Specialists: Research Funding; BioNTech: Research Funding; Incyte: Research Funding; Forma Therapeutics: Research Funding; Ignyta: Research Funding; Hutchinson MediPharma: Research Funding; Genentech/Roche: Membership on an entity's Board of Directors or advisory committees, Research Funding; Gilead: Research Funding; Hengrui: Research Funding; H3 Biomedicine: Research Funding; GlaxoSmithKline: Research Funding. Germing: Bristol-Myers Squibb: Honoraria, Other: advisory activity, Research Funding; Novartis: Honoraria, Research Funding; Celgene: Honoraria; Janssen: Honoraria; Jazz Pharmaceuticals: Honoraria. Lavie: AbbVie: Membership on an entity's Board of Directors or advisory committees, Other: Fees for lectures; BMS: Membership on an entity's Board of Directors or advisory committees; Takeda: Other: Fees for lectures; Roche: Other: Fees for lectures; Novartis: Other: Fees for lectures; Takeda: Consultancy. Lauring: Janssen Research and Development: Current Employment; Johnson and Johnson: Current holder of stock options in a privately-held company. Mistry: Janssen: Current Employment, Current equity holder in publicly-traded company.

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Background: Most IPSS low and int1 (lower) risk MDS patients with isolated del(5q) develop RBC TD or need treatment for symptomatic anemia early after diagnosis (median time to transfusion/treatment of 20 months, López Cadenas et al abstract 3180 ASH, 2016). Lenalidomide (LEN) is a reference treatment in MDS-del(5q) but approved in many countries only when RBC-TD occurs. LEN directly targets the del(5q) clone improving anemia and quality of life. Limited data also suggest a role of LEN in non-TD patients with del(5q) (Oliva et al Cancer Med 2015), but no randomized trial has assessed the efficacy and tolerance of early LEN treatment in this MDS subset. Material: The Sintra-Rev clinical trial is a phase III European multicenter study in low-risk MDS-del(5q) patients with anemia (Hb<12g/dL), without TD. Patients were randomized in a double-blind design to LEN (5mg/day continuously) vs placebo (2:1 randomization) for 2 years of treatment and 2y of follow-up. The primary endpoint was the time to TD, secondary endpoints included erythroid (HI-E) and cytogenetic response (CyR) (all according to IWG 2006 criteria), overall survival (OS), event free survival (EFS), time to AML and mutational analysis (TP53 and other myeloid mutations). Here, we report results of the interim analysis after completion of the treatment phase in all patients (March 2020). Results: Main clinical characteristics of the 61 patients (ITT population) included (Feb-2010 to Feb-2018) are summarized in Table 1: 82% females, median age 72 years (range 37-89), median time since diagnosis 3.6 months, median Hb at inclusion 9.8 g/dL (7.1 - 11.7) g/dL and 93% patients had isolated del(5q). Four patients were excluded due either to screening failure (1 pat) or failure to complete at least 12w of treatment (3 pat). Fifty-seven patients were included in the ITT evaluable population for efficacy and 59 for safety (2 patients did not receive any drug). Median time on treatment was 66 weeks (3-121), 95w in the LEN arm and 42w in the placebo arm (p=0.392). Forty-seven percent patients in the LEN arm successfully completed the study compared to 33% in the placebo arm. After a median follow up of 25.6 months (Q1 16-Q3 39), median OS was not achieved. Among the 57 patients evaluable for efficacy, median time to TD was 75.7 mo for the LEN patients and 25.9 mo for the placebo arm (HR 2.703, 95CI1.162-6.286, p=0.021, figure 1). HI-E response was observed in 72.5% of LEN patients compared to 0% in the placebo arm (p<0.001). Median Hb improvement in responders was 2.8 g/dL. Eighty percent of LEN patients achieved a cytogenetic response (CyR) compared to 4.8% of patients receiving placebo (p<0.001, complete CyR 70% in the LEN arm). OS was similar in both arms (not reached) while EFS was superior in the LEN arm (HR 2.274, 95CI 1.034-5.001, p=0.041). AML evolution was similar in both arms (5%). Mutational analysis will be presented at the meeting. Fifty-eight patients developed at least one adverse event (AE) during the trial (no differences between the LEN and placebo arm), related to the drug 86.8% in the LEN and 33.3% in the placebo arm, respectively (p<0.001). Hematological toxicity occurred in 40 patients in the LEN arm (50% grades 3/4) and only 4 patients in the placebo arm (25% grade 3/4). At least one SAE was seen in 31.6% of the LEN patients compared to 4.8% in the placebo arm (p=0.022), not related to the drug. Nineteen serious AE were reported in 13 patients, 4 of them (2nd neoplasia, pulmonary embolism, febrile neutropenia and blurred vision) potentially related with the study drug, with no related deaths. Conclusions: In this interim analysis we confirm that low dose LEN (5 mg) in anemic non-TD low risk MDS del(5q) patients prolongs the period of time to TD (75.7 mo vs 25.9 mo), improves Hb levels (72.5% of ER) and induces clonal responses (70% complete CyR), ie potentially more responses than in historical series of MDS del(5q) treated with LEN at time of TD, with a manageable safety profile, and no increased progression rate. Longer follow up will be required to assess the effect of early treatment with LEN, and particularly the effect of the early reduction of the del(5q) clone size, on long-term outcomes. Disclosures Hernandez-Rivas: Janssen: Membership on an entity's Board of Directors or advisory committees; Gilead: Membership on an entity's Board of Directors or advisory committees; AstraZeneca: Membership on an entity's Board of Directors or advisory committees; Roche: Membership on an entity's Board of Directors or advisory committees; Abbvie: Membership on an entity's Board of Directors or advisory committees; Rovi: Membership on an entity's Board of Directors or advisory committees; Celgene/BMS: Membership on an entity's Board of Directors or advisory committees. Thepot:astellas: Honoraria; novartis: Honoraria; sanofi: Honoraria; celgene: Honoraria. Sanz:LaHoffman Roche Ltd.: Membership on an entity's Board of Directors or advisory committees; Abbvie Pharmaceuticals: Membership on an entity's Board of Directors or advisory committees; Takeda Pharmaceutical Ltd.: Membership on an entity's Board of Directors or advisory committees; Helsinn: Membership on an entity's Board of Directors or advisory committees. Giagounidis:AMGEN: Membership on an entity's Board of Directors or advisory committees; Novartis: Membership on an entity's Board of Directors or advisory committees. Platzbecker:BMS: Consultancy, Honoraria; Janssen: Consultancy, Honoraria, Research Funding; Novartis: Consultancy, Honoraria, Research Funding; Amgen: Honoraria, Research Funding; Geron: Consultancy, Honoraria; Takeda: Consultancy, Honoraria; AbbVie: Consultancy, Honoraria. Götze:Celgene: Research Funding. Fenaux:BMS: Honoraria, Research Funding; Novartis: Honoraria, Research Funding; Abbvie: Honoraria, Research Funding; Jazz: Honoraria, Research Funding. Diez-Campelo:Celgene-BMS: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding, Speakers Bureau; Novartis: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding, Speakers Bureau; Takeda: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding, Speakers Bureau. OffLabel Disclosure: LEN IN ANEMIC BUT NOT TD PATIENTS WITH LOW RISK MDS AND DEL(5Q)

Background. Immunotherapy represents one of the fundamental treatments in the management of some types of cancer, especially malignant melanoma. Toxicity derived from increased immune system activity can manifest in multiple organs and systems. We present a case of hematological toxicity, manifested as hemophagocytic syndrome (HPS), which was successfully treated with an anti-interleukin-6 antibody (tocilizumab). Case Report. This case presents a 75-year-old woman diagnosed with metastatic choroidal melanoma, refractory to several lines of treatment. After the failure of the previous lines, ipilimumab was started. After the third dose, she developed grade 2 thrombocytopenia and anemia accompanied by elevated levels of ferritin, triglycerides, and decreased fibrinogen. Hemophagocytosis was observed in the bone marrow biopsy, and a PET-CT showed splenomegaly with increased metabolism. Treatment was based on high doses of corticosteroids and tocilizumab. Four days after the start of treatment, progressive clinical and analytical improvement was observed, achieving total remission of the condition. Discussion. HPS induced by immunotherapy is due to an immunorelated cytokine storm syndrome (CSS). The administration of the anti-interleukin-6 receptor antibody drug acted on this cytokine cascade, leading to stabilization and subsequent remission. For this reason, the use of tocilizumab should be part of the immunotherapy-induced HPS treatment algorithm.