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Introduction: BOS is approved for patients (pts) with Philadelphia chromosome-positive (Ph+) CML resistant/intolerant to prior therapy and pts with newly diagnosed Ph+ CP CML. Approval of first-line BOS was based on the primary results from the phase 3 BFORE trial, which showed superior efficacy vs imatinib (IMA) in the modified intent-to-treat (ITT) population (pop; Ph+ with e13a2/e14a2 transcripts) after ≥12 mo of follow-up. We report the final efficacy and safety results from the BFORE trial after 5 y of follow-up. Methods: In the open-label BFORE trial (NCT02130557), 536 pts with newly diagnosed CP CML were randomized 1:1 to receive BOS (n=268) or IMA (n=268; 3 untreated), both at 400 mg once daily. Efficacy was assessed in the ITT pop (all randomized pts). Long-term secondary endpoints included duration of response (DOR), on-treatment event-free survival (EFS) and overall survival (OS). Safety was assessed in the safety pop (all treated pts). This final analysis was based on an April 17, 2020 last pt last visit (June 12, 2020 database lock), 5 y (240 weeks) after the last enrolled pt. Results: At study completion in BOS and IMA arms, respectively, 59.7% and 57.4% were still on treatment, 86.6% and 86.2% completed 5 y on study. Median duration of treatment and time on study was 55.2 mo for pts receiving BOS or IMA; respective median (range) dose intensity was 394 (39-583) vs 400 (189-765) mg/d. Cumulative major molecular response (MMR) rate by 60 mo was higher with BOS vs IMA (73.9% vs 64.6%), as was cumulative molecular response (MR)4 (58.2% vs 48.1%) and MR4.5 rate (47.4% vs 36.6%; Table). Among evaluable pts, more pts in the BOS arm achieved BCR-ABL1 ≤10% at 3 months (Table); cumulative MMR by 60 mo was higher in pts with transcripts ≤10% vs >10% in both treatment arms (BOS, HR 2.67 [95% CI, 1.90-3.75]; IMA, HR 3.12 [2.19-4.45]). Pts in the BOS arm achieved responses earlier than pts in the IMA arm; cumulative incidence function for MMR, MR4 and MR4.5 was higher with BOS vs IMA (HR [95% CI]: MMR 1.34 [1.10-1.64], MR4 1.34 [1.07-1.69], MR4.5 1.41 [1.09-1.83]). Among responders, duration of MMR was similar for BOS and IMA (Table). Superior MRs with BOS vs IMA were consistent across Sokal risk groups, with the greatest difference seen in pts with high Sokal risk (Table). On-treatment transformations to accelerated/blast phase (AP/BP) occurred in 6 (AP 3; BP 3) BOS- and 7 (AP 6; BP 1) IMA-treated pts. No transformation occurred after the 24-mo follow-up. In all, 18 BOS- vs 25 IMA-treated pts had EFS events. There were no differences in EFS between treatment arms; cumulative incidence of on treatment progression/death at 60 mo was 6.7% for BOS vs 9.3% for IMA (Table). The 60-mo OS rates were similar (94.5% and 94.6%; Table); 14 BOS- and 14 IMA-treated pts died during the study period: 3 and 4 were CML-related, 0 and 1 were due to adverse events (AEs) related to study treatment. The most common reasons for permanent discontinuation were AEs (25.0% vs 12.5%) and lack of efficacy (4.9% vs 16.2%). Treatment-emergent AEs (TEAEs) occurred in 98.9% of pts in each arm; most common (>30%) were diarrhea (75.0%), nausea (37.3%), thrombocytopenia (35.8%) and increased alanine aminotransferase (ALT; 33.6%) with BOS, and diarrhea (40.4%), nausea (42.3%) and muscle spasms (30.6%) with IMA. Most TEAEs occurred during the first year of treatment. Grade 3/4 TEAEs occurred in 73.5% of BOS- vs 57.0% of IMA-treated pts; most common (>5%) were increased ALT (20.9%) and lipase (13.4%), thrombocytopenia (14.2%), increased aspartate aminotransferase (10.4%), diarrhea (9.0%) and neutropenia (7.5%) with BOS, and neutropenia (13.6%), thrombocytopenia (6.0%), anemia (5.7%) and increased lipase (5.7%) with IMA. No individual AE led to discontinuation in >5% of pts. The most frequent AEs leading to permanent treatment discontinuation were increased ALT (4.9%) with BOS vs thrombocytopenia (1.5%) with IMA; 1.5% vs 1.1% of pts discontinued due to diarrhea. Conclusions: At 5 y, first-line BOS continued to show superior efficacy vs IMA; BOS-treated pts achieved earlier and deeper molecular response. An improvement in MR with BOS was demonstrated across Sokal risk groups, with the greatest benefit vs IMA observed in Sokal high-risk pts. Long-term AEs were generally manageable, and consistent with previous reports and the known safety profiles of both drugs. These results confirm the use of BOS as a standard of care in pts with newly diagnosed CP CML. Disclosures Brümmendorf: Takeda: Consultancy; Janssen: Consultancy; Pfizer: Consultancy, Honoraria, Other: Travel, Accommodation, Expenses, Research Funding; Merck: Consultancy; Novartis: Consultancy, Other: travel, accommodation, expenses, Research Funding. Cortes:Astellas: Research Funding; Arog: Research Funding; Merus: Research Funding; Daiichi Sankyo: Consultancy, Research Funding; BioPath Holdings: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Takeda: Consultancy, Research Funding; Pfizer: Consultancy, Research Funding; Amphivena Therapeutics: Research Funding; Immunogen: Research Funding; Telios: Research Funding; BiolineRx: Consultancy, Research Funding; Bristol-Myers Squibb: Research Funding; Jazz Pharmaceuticals: Consultancy, Research Funding; Sun Pharma: Research Funding; Novartis: Consultancy, Research Funding. Milojkovic:Incyte: Consultancy, Honoraria; Pfizer: Consultancy, Honoraria; Novartis: Consultancy, Honoraria; Bristol-Myers Squibb: Consultancy, Honoraria. Gambacorti-Passerini:Pfizer: Honoraria, Research Funding; Bristol-Myers Squibb: Consultancy. Clark:Pfizer: Honoraria, Research Funding; Novartis: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Jazz Pharmaceuticals: Membership on an entity's Board of Directors or advisory committees; Ariad/Incyte: Honoraria; Bristol-Myers Squibb: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; AbbVie: Membership on an entity's Board of Directors or advisory committees. le Coutre:Incyte: Honoraria; Pfizer: Honoraria; Novartis: Honoraria. Garcia-Gutiérrez:Novartis: Consultancy, Other: Travel, Accommodation, Expenses, Research Funding; Bristol-Myers Squibb: Consultancy, Other: Travel, Accommodation, Expenses, Research Funding; Pfizer: Consultancy, Other: Travel, Accommodation, Expenses, Research Funding; Incyte: Consultancy, Other: Travel, Accommodation, Expenses, Research Funding. Chuah:Novartis: Honoraria; Bristol-Myers Squibb: Honoraria, Research Funding; Pfizer: Other: Travel, Research Funding; Korea Otsuka Pharmaceutical: Honoraria. Kota:Novartis: Consultancy, Honoraria; Pfizer: Consultancy, Honoraria; Millennium Pharmaceuticals, Inc., a wholly owned subsidiary of Takeda Pharmaceutical, Company Ltd, Cambridge, MA, USA: Honoraria; Ariad: Honoraria; Incyte: Honoraria; Xcenda: Honoraria. Lipton:BMS: Consultancy, Research Funding; Novartis: Consultancy, Research Funding; Bristol-Myers Squibb: Honoraria; Takeda: Consultancy, Honoraria, Research Funding; Ariad: Consultancy, Research Funding; Pfizer: Consultancy, Honoraria, Research Funding. Rousselot:Incyte: Consultancy, Research Funding; Takeda: Consultancy; Pfizer: Consultancy, Research Funding; Bristol-Myers Squibb: Consultancy; Novartis: Consultancy. Mauro:Sun Pharma/SPARC: Research Funding; Novartis: Consultancy, Honoraria, Other: Travel, Accommodation, Expenses, Research Funding; Bristol-Myers Squibb: Consultancy, Honoraria, Other: Travel, Accommodation, Expenses, Research Funding; Takeda: Consultancy, Honoraria, Other: Travel, Accommodation, Expenses, Research Funding; Pfizer: Consultancy, Honoraria, Other: Travel, Accommodation, Expenses, Research Funding. Hochhaus:MSD: Research Funding; Takeda: Honoraria; Pfizer: Honoraria, Research Funding; Novartis: Honoraria, Research Funding; Incyte: Honoraria, Research Funding; Bristol-Myers Squibb: Honoraria, Research Funding. Hurtado Monroy:Incyte: Consultancy; Pfizer: Consultancy. Leip:Pfizer: Current Employment, Current equity holder in publicly-traded company. Purcell:Pfizer: Current Employment, Current equity holder in publicly-traded company. Yver:Pfizer: Current Employment, Current equity holder in publicly-traded company. Viqueira:Pfizer: Current Employment, Current equity holder in publicly-traded company. Deininger:Gilead Sciences: Research Funding; Celgene: Research Funding; Takeda: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Ariad: Consultancy, Honoraria, Other; SPARC: Research Funding; DisperSol: Consultancy; Pfizer: Honoraria, Other, Research Funding; Bristol-Myers Squibb: Consultancy, Honoraria, Other, Research Funding; Fusion Pharma: Consultancy; Blueprint Medicines Corporation: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: part of a study management committee, Research Funding; Leukemia & Lymphoma Society: Research Funding; Incyte: Consultancy, Honoraria, Other, Research Funding; Novartis: Consultancy, Other, Research Funding; Medscape: Consultancy; Sangamo: Consultancy, Membership on an entity's Board of Directors or advisory committees; Galena: Consultancy, Honoraria, Other.