Abstract Background Lenalidomide is an effective treatment for myeloma and has been studied in a range of combination regimens worldwide. The results of these studies have suggested that prolonged exposure to lenalidomide is important to improve outcomes both as a maintenance agent post-transplant (Attal M et al NEJM 2012, McCarthy et al NEJM 2012) and in the transplant ineligible population (Palumbo A et al NEJM 2012, Benboubker L et al NEJM 2014). In the Myeloma XI study, the largest of its kind, we explored the use of oral lenalidomide continued to disease progression compared to no therapy in both newly diagnosed transplant eligible (TE) and transplant non-eligible (TNE) populations. Here we present the results of this maintenance randomization, which demonstrate the efficacy and safety of maintenance lenalidomide. Methods The Myeloma XI study is a Phase III, UK-based, multicenter, open-label, parallel group, randomized controlled trial for newly diagnosed symptomatic myeloma patients of all ages and includes a maintenance comparison of lenalidomide versus no maintenance. Newly diagnosed symptomatic myeloma patients both TE and TNE were enrolled to the study. Induction treatment in both pathways was with thalidomide or lenalidomide plus cyclophosphamide and dexamethasone, with appropriate dose reductions for TNE patients. TE patients proceded to a standard melphalan 200mg/m2 transplant. Patients were randomized to either maintenance lenalidomide or observation after achieving maximum response (TNE) or at 100 days after transplant (TE). Lenalidomide was administered at a dose of 10mg daily in 21/28 day cycles until disease progression. Dose adjustments for renal impairment and following AEs were permitted. The primary endpoints for the maintenance randomization were progression-free (PFS) and overall survival. Secondary endpoints included response, toxicity and PFS2. Time-to-event endpoints were measured from maintenance randomization. This abstract summarizes a preliminary analysis, final data will be presented at the meeting. The median follow up in this analysis is 26 months [IQR 12-41]. Results A total of 1550 patients, 828 TE and 722 TNE, median age 61 and 74 years, respectively, were randomized between lenalidomide (n=857) and no maintenance (n=693). The arms were well-balanced for clinical features and response to induction therapy (e.g. ISS stage III: 27% vs 23%, VGPR/CR: 73% vs 73%). The maintenance randomization has met its primary endpoint demonstrating a 55% reduction in risk of progression or death for lenalidomide compared to no maintenance (HR 0.45 [95%CI 0.39-0.52], median PFS 37 vs 19 months, p<0.0001) This significant improvement was observed in each pathway TE: HR 0.46 [95%CI 0.36-0.58], median PFS 60 vs 28 months, p<0.0001. TNE: HR 0.44 [95%CI 0.36-0.53], median PFS 26 vs 12 months, p<0.0001. The benefit of lenalidomide maintenance on PFS persisted across risk subgroups and was independent of induction therapy and response. An exploratory analysis of 132 patients stopping lenalidomide treatment for reasons other than disease progression (91 toxicity, 28 patient choice and 13 clinician choice) shows that patients receiving greater than 12 months of treatment have an improved median PFS compared to those stopping earlier (HR 0.35 [95%CI 0.18-0.68], 49 vs 31 months, p<0.0015). At this time 445 patients continue to receive lenalidomide maintenance on study. Of patients who have stopped therapy, only 21.5% did so due to toxicity. Relevant grade 3/4 adverse events were: neutropenia 35%, thrombocytopenia 7.4%, anaemia 4.4%, peripheral neuropathy 1.4%. Venous thromboembolism occurred in 2.3%. Second primary malignancy (SPM) data was collected and the relationship with maintenance therapy reviewed. 72 SPM were observed (24 no maintenance, 48 lenalidomide). Haematologic malignancy crude incidence was 0.3% vs. 0.9%. While we found a slight excess of SPM in older patients these were mostly non-invasive and did not impact the outcome benefit demonstrated. Conclusion The use of maintenance lenalidomide treatment results in highly significant improvements in PFS for patients of all ages and should be standard of care. On behalf of the NCRI Haem-Onc CSG Disclosures Jackson: Amgen: Consultancy, Honoraria, Speakers Bureau; Celgene: Consultancy, Honoraria, Other: Travel support, Research Funding, Speakers Bureau; Roche: Consultancy, Honoraria, Speakers Bureau; MSD: Consultancy, Honoraria, Speakers Bureau; Janssen: Consultancy, Honoraria, Speakers Bureau; Takeda: Consultancy, Honoraria, Other: Travel support, Research Funding, Speakers Bureau. Davies:Takeda: Consultancy, Honoraria; Celgene: Consultancy, Honoraria; Janssen: Consultancy, Honoraria. Pawlyn:Takeda Oncology: Consultancy; Celgene: Consultancy, Honoraria, Other: Travel Support. Jones:Celgene: Honoraria, Research Funding. Kishore:celgene: Other: travel grant. Garg:Janssen: Other: Travel support, Research Funding, Speakers Bureau; Takeda: Other: Travel support; Novartis: Other: Travel support, Research Funding. Williams:Takeda: Honoraria, Other: Travel support, Speakers Bureau; Amgen: Honoraria, Speakers Bureau; Novartis: Honoraria; Janssen: Honoraria, Other: Travel support, Speakers Bureau; Celgene: Honoraria, Other: Travel support, Speakers Bureau. Karunanithi:Celgene: Other: Travel support, Research Funding; Janssen: Other: Travel support, Research Funding. Lindsay:Janssen: Consultancy; Novartis: Other: Travel support; Takeda: Other: Travel support; BMS: Consultancy, Other: Travel support; Celgene: Honoraria, Other: Travel support. Jenner:Janssen: Consultancy, Honoraria, Other: Travel support, Research Funding; Takeda: Consultancy, Honoraria, Other: Travel support; Amgen: Consultancy, Honoraria, Other: Travel support; Celgene: Consultancy, Honoraria, Research Funding; Novartis: Consultancy, Honoraria. Cook:Celgene: Consultancy, Honoraria, Research Funding, Speakers Bureau; Janssen: Consultancy, Honoraria, Research Funding, Speakers Bureau; Glycomimetics: Consultancy, Honoraria; Amgen: Consultancy, Honoraria, Research Funding, Speakers Bureau; Takeda: Consultancy, Honoraria, Research Funding, Speakers Bureau; Bristol-Myers Squibb: Consultancy, Honoraria; Sanofi: Consultancy, Honoraria, Speakers Bureau. Kaiser:BMS: Consultancy, Other: Travel Support; Takeda: Consultancy, Other: Travel Support; Celgene: Consultancy, Honoraria, Research Funding; Amgen: Consultancy, Honoraria; Janssen: Consultancy, Honoraria; Chugai: Consultancy. Drayson:Abingdon Health: Equity Ownership, Membership on an entity's Board of Directors or advisory committees. Owen:Takeda: Honoraria, Other: Travel support; Celgene: Consultancy, Honoraria, Research Funding; Janssen: Consultancy, Other: Travel support. Morgan:Takeda: Consultancy, Honoraria; Bristol Meyers: Consultancy, Honoraria; Janssen: Research Funding; Univ of AR for Medical Sciences: Employment; Celgene: Consultancy, Honoraria, Research Funding.