Abstract: [§ share last authorship] Background: In 2000-2010, the first large prospective trials in peripheral T-cell lymphoma (PTCL) showed outcomes burdened by high failure rates during induction. Concurrently, trials with the anti-CD52 monoclonal antibody alemtuzumab (ALZ) yielded promising responses in PTCL while demonstrating the feasibility of combining ALZ with CHOP. Hence, the Nordic Lymphoma Group initiated the randomized ACT-1 trial to test, in younger patients (pts) (18-65yrs), the addition of ALZ to CHOP + autologous stem cell transplant (ASCT). Primary endpoint was the 3 years event-free survival (EFS). Here, we present the final analysis of the ACT-1 trial (ClinicalTrials.gov: NCT00646854). Patients and Methods: Overall, 136 pts were randomized (43% of planned sample size due to slow accrual), five did not receive study treatment, and 131 were analyzed (ALZ-CHOP: 65; CHOP: 66). Due to lack of tumoral CD52 expression, anaplastic large cell lymphomas (ALCL) were not included in the ACT-1 trial. An amendment tapering ALZ dose from 360 mg (30 mg on days 1+2 of each CHOP course) to 120 mg (30 mg on day 1 of CHOP courses 1-4) was introduced early on due to systemic fungal infections in 2 pts. Of the 65 pts treated with ALZ-CHOP, 4 received the pre- and 61 (94%) the post-amendment dose. Monitoring for CMV- and EBV-DNA and antimicrobial prophylaxis were mandatory. Results: The median observation time for the Full Analysis Set was 66 months and the median age 51 yrs. The ALZ-CHOP and CHOP cohorts were well balanced with regard to classical prognostic factors and histological subtypes (PTCL-NOS 58% vs 54%, AILT 21% vs 25%, other 21% vs 21%). Feasibility: Neither CHOP nor ALZ-CHOP pts experienced substantial treatment delay. ALZ exposure did not affect stem cell harvest nor hematopoietic recovery. Grade 4 leucopenia was more frequent in ALZ-CHOP pts (73% vs 35%; p=0.001), whereas the occurrence of grade 3-4 anemia and thrombocytopenia did not differ significantly. After ALZ dose amendment, the frequency of bacterial and fungal infections of grade ≥3 was similar in both treatment arms. ALZ treated pts had more viral events (22/57=42% vs 4/23=17%), mainly due to asymptomatic CMV reactivations. The ratio of serious adverse events per ALZ-CHOP treated patient dropped markedly (from 3.25 to 0.86, comparable with 0.46 for CHOP) after dose amendment. Additional toxicity was mild and similar in both arms. Treatment related mortality was 4% (5% vs 3%). Efficacy: Complete remission (CR) was 52% in ALZ-CHOP vs 42% in CHOP. Primary refractory disease occurred for ALZ-CHOP and CHOP in 23% and 38% of pts, respectively. Overall, females had a significantly better outcome than males (p=0.004), also after adjustment for classical prognostic factors. Analyzing time-related endpoints without knowledge of CD52 expression, 3-years EFS, progression-free, and overall survival (PFS, OS) did not differ significantly between ALZ-CHOP and CHOP (EFS 35% vs 26%, PFS 37% vs 26%, OS 52% vs 50%). Fig.1A shows EFS by treatment arm, by gender, and by gender and treatment arm. Although not significantly different, EFS, PFS and OS values of ALZ-CHOP treated females in the ACT-1 trial were consistently higher than those of non-ALZ treated females or of males regardless of treatment group. RNA sequencing from evaluable pre-therapeutic tumor biopsies defined a signature of differentially expressed genes to be predictive of clinical outcome in ALZ-CHOP but not CHOP treated pts (n=33). Tumor microenvironment genes were prominent in determining response to ALZ. Tumors rich in B-cell milieu showed good responses, while the opposite was observed in tumors with signatures enriched with high endothelial cell genes (p 〈 0.001) (Fig.1B). The good risk signature was associated with a higher frequency of X-linked and the bad risk with a higher frequency of Y-linked transcripts (Fig.1B). Conclusion: In previously untreated younger non-anaplastic PTCL pts, the addition of ALZ to CHOP + ASCT was feasible. Overall, we did not find a significant outcome benefit. However, a gene expression signature predictive of ALZ response was identified and found predominantly in female patients. Carriers of this signature had an outcome benefit only if exposed to ALZ. A validation of this predictor of ALZ response is ongoing. Due to the limited sample size of the ACT-1 study cohort, both the negative and the positive findings of the trial should be interpreted with caution. Disclosures Leppä: Roche: Consultancy, Honoraria, Research Funding; Takeda: Consultancy, Research Funding; Bayer: Research Funding; Janssen: Consultancy, Research Funding; Celgene: Consultancy. Silva:Gilead Sciences: Research Funding; Abbvie, Gilead Sciences, Janssen, BMS: Consultancy, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Roche, Janssen, Celgene: Other: Travel Support; Roche, Janssen: Other: Institution's payment for consultancy. Hagberg:Roche: Honoraria. Lugtenburg:takeda: Consultancy, Research Funding; servier: Consultancy, Research Funding; roche: Consultancy; BMS: Consultancy; Celgene: Consultancy; Sandoz: Consultancy; GenMab: Research Funding. Walewski:Roche, GSK/Novartis, Takeda, and Janssen-Cilag: Research Funding; Roche, Celgene, Takeda, Janssen-Cilag, and Servier: Honoraria; Roche, Celegene, Takeda, Janssen-Cilag, and Servier: Membership on an entity's Board of Directors or advisory committees. Hopfinger:Janssen: Honoraria; Gilead: Honoraria, Research Funding; GlaxoSmithKline: Honoraria; Celgene: Honoraria; Novartis: Consultancy, Honoraria; Takeda: Consultancy, Honoraria; Roche: Consultancy, Honoraria. Jantunen:Amgen: Honoraria; Genzyme/Sanofi: Honoraria; Takeda: Honoraria. Steidl:Seattle Genetics: Consultancy; Juno Therapeutics: Consultancy; Tioma: Research Funding; Bristol-Myers Squibb: Research Funding; Nanostring: Patents & Royalties: patent holding; Roche: Consultancy. Gascoyne:NanoString: Patents & Royalties: Named Inventor on a patent licensed to NanoString Technologies. Scott:Celgene: Consultancy, Honoraria; Janssen: Research Funding; Roche: Research Funding; NanoString: Patents & Royalties: Named Inventor on a patent licensed to NanoString Technologies, Research Funding.