Background: The efficacy of consolidation and maintenance in the context of salvage autologous haematopoietic stem-cell transplantation (HSCT) for relapsed multiple myeloma remains unclear. We aimed to assess whether consolidation after salvage autologous HSCT, using ixazomib, thalidomide, and dexamethasone, followed by maintenance with single agent ixazomib is superior to observation., Methods: This is an interim analysis of Myeloma XII (ACCorD; referred to as ACCorD hereafter), an open-label, randomised, controlled, phase 3 trial done at 79 hospitals in the UK. Eligible patients were aged 18 years or older, had relapsed multiple myeloma with measurable disease, an ECOG performance status of 2 or less with adequate renal, hepatobiliary, pulmonary, and cardiac function, and required treatment for first progressive disease occurring at least 12 months after first autologous HSCT. In a first randomisation, patients were assigned (1:1) to receive either conventional autologous HSCT with melphalan or augmented autologous HSCT with melphalan and ixazomib. In the second randomisation, reported here, patients were assigned (1:1) to consolidation using ixazomib, thalidomide, and dexamethasone (oral ixazomib 4 mg per day on days 1, 8, and 15, oral thalidomide 100 mg per day on days 1-28, and oral dexamethasone 40 mg per day on days 1, 8, 15 and 22 of 28-day cycles), followed by maintenance with single agent ixazomib (oral ixazomib 4 mg per day on days 1, 8, and 15 of 28-day cycles until disease progression or intolerance), or observation. The primary endpoint was progression-free survival, analysed by intention-to-treat. Safety was analysed per-protocol. This study is registered with ISRCTN, ISRCTN10038996, and EudraCT, 2016-000905-35, and recruitment is complete., Findings: Between Dec 12, 2017, and April 21, 2023, 206 patients entered the second randomisation (103 in the consolidation and maintenance group and 103 in the observation group). This prespecified interim analysis (data cutoff April 21, 2023), was done at a median follow-up of 27 months (IQR 13-38). Median progression-free survival was 20 months (95% CI 15-29) in the consolidation and maintenance group and 13 months (11-18) in the observation group (hazard ratio 0·55 [95% CI 0·39-0·78]; p=0·0006). Serious adverse events were reported in 29 (32%) of 92 patients in the consolidation and maintenance group compared with seven (7%) of 103 patients in the observation group. The most common serious adverse events were infections and infestations in both the consolidation and maintenance group and the observation group. The most common grade 3, 4, or 5 adverse events for patients in the consolidation and maintenance group were upper respiratory infection (seven [8%] of 92 patients). No deaths in the consolidation and maintenance group were deemed treatment related., Interpretation: ACCorD provides evidence that an orally administered, deliverable, and tolerable post-salvage autologous HSCT treatment regimen can improve the durability of response for transplantation-eligible patients at first relapse. The findings are of relevance to patients who had durable disease control from autologous HSCT in the first line, representing a viable alternative to continuous parentally-administered relapse therapies., Funding: Cancer Research UK, Takeda Oncology., Competing Interests: Declaration of interests GC reports research funding from Janssen, Takeda Oncology, Amgen, and BMS/Celgene; and consultancy fees from Janssen, Takeda, Sanofi, Oncopeptides, Karyopharm, Pfizer, Roche, and BMS/Celgene. AJA reports speaker fees and participation on advisory boards for Janssen and participation on advisory boards for Takeda. ES reports receipt of unrestricted educational grants to his institution from Takeda Oncology, Celgene Corporation, Merck, Amgen, and Sanofi. CO reports receipt of unrestricted educational grants to her institution from Takeda Oncology, Celgene Corporation, Merck, Amgen, and Sanofi. AH reports receipt of unrestricted educational grants to his institution from Takeda Oncology, Celgene Corporation, Merck, Amgen, and Sanofi; and speaker fees from Abbvie. JR reports receipt of unrestricted educational grants to her institution from Takeda Oncology, Celgene Corporation, Merck, Amgen and Sanofi. JS reports speaker fees from Janssen, Jazz, Mallinckrodt, and Sanofi; participation on advisory boards for Medac and Vertex; and Independent Data Monitoring Committee membership for a Kiadis Pharma clinical trial. MG reports participation on advisory boards for Amgen, Sanofi, Celgene, Stemline therapeutics, Janssen, and Novartis; research grants from Janssen; speaker honoraria from Janssen, Amgen and Alnylam; and travel expenses to attend educational meetings from Novartis, Janssen, and Takeda. KB reports participation on advisory boards at Takeda, Janssen, and Celgene/BMS; speaker honoraria from Janssen, Sanofi, and Celgene/BMS; and travel expenses to attend educational meetings from Janssen, GSK, and Takeda. SG reports receipt of unrestricted educational grants to her institution from Takeda Oncology, Celgene Corporation, Merck, Amgen, and Sanofi. DAC reports receipt of unrestricted educational grants to his institution from Takeda Oncology, Celgene Corporation, Merck, Amgen, and Sanofi; payment to their institution for educational lectures from Janssen; participation on a Data Safety Monitoring Board for a multiple myeloma study; and personal payment for meeting attendance from European Myeloma Network, Rotterdam. LR is a patient and public contributor. CP reports speaker honoraria from Janssen, Amgen, and Novartis; and participation on advisory boards at Pfizer and Sanofi. All other authors declare no competing interests., (Copyright © 2024 The Author(s). Published by Elsevier Ltd. This is an Open Access article under the CC BY 4.0 license. Published by Elsevier Ltd.. All rights reserved.)