Abstract Introduction: Allogeneic hematopoietic stem cell transplantation (allo-SCT) is the sole curative therapy for myelodysplastic syndromes (MDS). Several studies have addressed the efficacy of bridging therapy before allo-SCT including hypomethylating agents, combination chemotherapy (e.g. induction chemotherapy regimen for acute myeloid leukemia [AML]), and low-dose chemotherapy. However, these studies were retrospective, so selection bias was unavoidable, and the optimal bridging therapy prior to allo-SCT remains unclear. We analyzed the bridging therapy and outcomes of patients with MDS registered in a prospective observational study, the JALSG-AML/MDS/CMML Clinical Observational Study-11 (CS-11). Methods: We studied 393 patients with MDS-refractory anemia with excess blasts (MDS-EB) ≤70 years old at registration for CS-11 between 2011 and 2016. We collected new data using an online survey software program (Survey Monkey ®) and analyzed these data following their integration into the CS-11 data. This study was approved by the ethical committee of Kanazawa University (No. 2018-227) and all participating institutions. Results: A total of 371 patients (94.4%) had additional data available and who were included in this study. The median age of the patients at registration of CS-11 was 64 (range: 16-70) years, and 279 (75.2%) were male. The patients' cytogenetic subgroups were divided into very good (n=3 [0.8%]), good (n=117 [31.5%]), intermediate (n=60 [16.2%]), poor (n=35 [9.4%]), very poor (n=148 [39.9%]), and missing data (n=8 [2.1%]), according to Revised International Prognostic Scoring System (IPSS-R). A total of 188 patients (50.7%) were considered for allo-SCT at the diagnosis of MDS-EB, and 141 patients (75.0%) underwent allo-SCT, whereas 181 (48.9%) patients were not considered for allo-SCT at their diagnosis. The overall survival (OS) of the patients considered for allo-SCT at their diagnosis was significantly higher than that of the patients who were not considered for allo-SCT (3-year OS, 33.7% vs. 13.9%, P<0.001). The Cox regression analysis identified consideration for allo-SCT at the diagnosis, performance status, and cytogenetic subgroup according to IPSS-R as independent factors that affect the OS. In an intention-to-treat analysis, bridging therapy included azacytidine (n=90 [50%]), combination chemotherapy such as induction chemotherapy regimen for AML (n=33, [18%]), and low-dose chemotherapy (n=12 [7%]), while 32 patients (18%) received allo-SCT without bridging therapy. There was no correlation between the various bridging therapies and the transplantation rates or OS in patients considered for allo-SCT. The reasons for discontinuing allo-SCT included cancellation of transplant request (n=12 [30%]), complications or comorbidity developed during bridging therapy (n=10 [25%]), and uncontrolled MDS (n=7 [17%]). Conclusion: Transplant-eligible patients with MDS-EB should be considered for allo-SCT. Further studies will be required to determine the optimal bridging therapy prior to allo-SCT. Disclosures Usuki: MSD K.K.: Research Funding, Speakers Bureau; Eisai Co., Ltd.: Speakers Bureau; Alexion Pharmaceuticals, Inc.: Research Funding, Speakers Bureau; Pfizer Japan Inc.: Research Funding, Speakers Bureau; Kyowa-Kirin Co., Ltd.: Research Funding, Speakers Bureau; Nippon-Shinyaku Co., Ltd.: Research Funding, Speakers Bureau; Amgen-Astellas Biopharma K.K.: Research Funding; Mundipharma K.K.: Research Funding; Nippon-Boehringer-Ingelheim Co., Ltd.: Research Funding; Takeda Pharmaceutical Co., Ltd.: Research Funding, Speakers Bureau; Celgene K.K.: Research Funding, Speakers Bureau; Janssen Pharmaceutical K.K.: Research Funding; Ono Pharmaceutical Co., Ltd.: Research Funding, Speakers Bureau; Novartis Pharma K.K.: Research Funding, Speakers Bureau; Otsuka Pharmaceutical Co., Ltd.: Research Funding, Speakers Bureau; Sumitomo-Dainippon Pharma Co., Ltd.: Research Funding; Daiichi Sankyo Co., Ltd.: Research Funding, Speakers Bureau; SymBio Pharmaceuticals Ltd.: Research Funding, Speakers Bureau; Gilead Sciences, Inc.: Research Funding; AbbVie GK: Research Funding, Speakers Bureau; Astellas Pharma Inc.: Research Funding, Speakers Bureau; PharmaEssentia Japan KK: Research Funding, Speakers Bureau; Yakult Honsha Co., Ltd.: Research Funding, Speakers Bureau; Bristol-Myers-Squibb K.K.: Research Funding, Speakers Bureau; Apellis Pharmaceuticals, Inc.: Research Funding; Incyte Biosciences Japan G.K.: Research Funding; Chugai Pharmaceutical Co., Ltd.: Research Funding, Speakers Bureau; Sanofi K.K.: Speakers Bureau; Amgen K.K.: Research Funding. Handa: Ono: Honoraria; BMS: Honoraria; Janssen: Honoraria; Daiichi Sankyo: Research Funding; Celgene: Honoraria, Research Funding; Chugai: Research Funding; Kyowa Kirin: Research Funding; Takeda: Honoraria, Research Funding; Shionogi: Research Funding; Sanofi: Honoraria, Research Funding; Abbvie: Honoraria; MSD: Research Funding. Fujisawa: Novartis: Honoraria, Research Funding; Pfizer: Honoraria, Research Funding; Otsuka: Honoraria, Research Funding; Bristol Myers Squibb: Honoraria, Research Funding. Suehiro: Amgen BioPharma: Research Funding; Bayer: Research Funding; Bristol-Myers Squibb: Honoraria; Celgene: Research Funding; Chugai Pharmaceutical: Honoraria, Research Funding; Eisai: Honoraria, Research Funding; Incyte: Research Funding; Kyowa Kirin: Honoraria, Research Funding; Nippon Shinyaku: Honoraria; Novartis: Research Funding; Ono Pharmaceutical: Research Funding; Otsuka Pharmaceutical: Research Funding; Pfizer: Research Funding. Maeda: Bayer Yakuhin, Ltd.: Honoraria; Nippon Shinyaku Co., Ltd.: Honoraria; Novartis Pharmaceuticals: Honoraria, Research Funding; Chugai Pharmaceutical Co., Ltd.: Research Funding; Sumitomo Dainippon Pharma Co., Ltd.: Research Funding; Eisai Co., Ltd.: Research Funding; Astellas Pharma Inc.: Research Funding; Alexion Pharmaceuticals, Inc.: Research Funding. Yamamoto: Bristol-Myers Squibb Company: Honoraria; Chugai Pharmaceutical Co., Ltd.: Honoraria; Eisai Co., Ltd.: Honoraria; Kyowa Kirin Co., Ltd.: Honoraria; NIPPON SINYAKU CO., LTD: Honoraria; Novartis Pharma: Honoraria; ONO PHARMACEUTICAL CO.: Honoraria; Otsuka Pharmaceutical: Honoraria; Pfizer Japan Inc.: Honoraria; Takeda: Honoraria. Kiyoi: Astellas: Honoraria; celgene: Honoraria; Daiichi Sankyo: Honoraria; Dainippon Sumitomo: Honoraria; Eisai: Honoraria; Fijifilm: Honoraria; Kyowa Kirin: Honoraria; Otsuka: Honoraria; Perseus Proteomics: Honoraria; Pfizer: Honoraria; Pfizer: Honoraria; Sanofi: Honoraria; Takeda: Honoraria; Zenyaku Kogyo: Honoraria. Matsumura: Ono: Research Funding; Otsuka: Consultancy, Research Funding, Speakers Bureau; Pfizer: Research Funding, Speakers Bureau; Shionogi: Research Funding; Taiho: Research Funding; Takeda: Research Funding; Sumitomo Dainippon: Research Funding; Nihon Pharmaceutical: Research Funding; Daiichi Sankyo: Research Funding, Speakers Bureau; Japan Blood Products Organization: Research Funding; Mundipharma: Research Funding; AYUMI Pharmaceutical: Research Funding; Eli Lilly Japan: Research Funding; Novartis: Research Funding, Speakers Bureau; Nippon Shinyaku: Research Funding; MSD: Research Funding; Mitsubishi Tanabe: Research Funding; Amgen: Speakers Bureau; Bristol-Myers Squibb: Speakers Bureau; Janssen: Speakers Bureau; Kyowa Kirin: Research Funding; Eisai: Research Funding; Chugai: Research Funding; Astellas: Speakers Bureau; Asahi Kasei: Research Funding; Addvie: Research Funding. Miyazaki: Pfizer: Honoraria; Kyowa-Kirin: Honoraria; Abbvie: Honoraria; Bristol-Myers Squibb: Honoraria; Nippon-Shinyaku: Honoraria; Novartis: Honoraria; Astellas: Honoraria; Sumitomo-Dainippon: Honoraria, Research Funding; Janssen: Honoraria; Eisai: Honoraria; Daiichi-Sankyo: Honoraria; Takeda: Honoraria; Chugai: Honoraria; Sanofi: Honoraria.