BACKGROUND: Economic differences among currently available proteasome inhibitors (PI)-based lenalidomide-dexamethasone (Rd)-backbone triplet regimens-ixazomib (I), bortezomib (V), and carfilzomib (K) plus Rd-remain poorly understood. OBJECTIVE: To assess health care resource utilization (HCRU) and health care costs of patients with relapsed/refractory multiple myeloma (RRMM) in the United States treated with IRd, VRd, and KRd. METHODS: This retrospective longitudinal cohort study using IQVIA PharMetrics Plus adjudicated claims US data (January 1, 2015, to September 30, 2020) included adult patients with all available data who initiated IRd, VRd, or KRd in second line of therapy or later (LOT2+) on or after September 1, 2015. The index date was the treatment initiation date for each LOT (multiple LOTs per patient were included) and the baseline was 6 months pre-index. MM-related and all-cause HCRU/costs were assessed during follow-up and reported per patient per month (PPPM; 2020 US Dollars). For MM-related costs only, treatment administration costs were excluded from outpatient (OP) costs and instead summed with pharmacy costs. HCRU/costs were compared between treatment groups using generalized linear models (GLMs). Cost variables were compared using 2-part models and GLM with log transformation and γ distribution. Inverse probability of treatment weighting (IPTW) adjusted for imbalance of baseline confounders across treatment groups. RESULTS: The study included 511 patients contributing 542 LOTs (IRd: n = 153; VRd: n = 262; KRd: n = 127). Before IPTW, mean observed time spent on therapy was 8.5, 9.3, and 7.3 months for the IRd, VRd, and KRd cohorts, respectively. During follow-up and after IPTW, IRd and VRd were associated with significantly fewer OP visits vs KRd. Post-IPTW comparisons of MM-related costs for IRd vs KRd yielded lower OP costs for IRd (mean diff. PPPM: -$3,428; P < 0.001), contributing to lower total medical costs (-$3,813; P < 0.001) and total health care cost savings with IRd vs KRd (-$5,813; P = 0.001). MM-related OP costs were lower for VRd (mean diff. PPPM: -$3,543; P < 0.001) than KRd, reducing its total MM-related medical costs (-$3,997; P = 0.002), leading to total MM-related health care cost savings with VRd vs KRd (-$12,357; P < 0.001). All-cause cost comparisons yielded similar results (total health care cost savings for IRd and VRd vs KRd: -$6,371 and -$13,629, respectively; all P < 0.001). CONCLUSIONS: From the US insurance-payer perspective, patients treated with IRd and VRd had significant medical cost savings vs KRd due to lower OP costs when excluding treatment administration costs. The differential economic impacts of PI-Rd regimens in this study may help to inform treatment decisions for patients with MM. DISCLOSURES: This study and article were supported by Takeda Development Center Americas, Inc. Dr Sanchez has no conflicts to declare. Dr Chari has the following relationships: Research Support/Principal Investigator: Amgen, Array Biopharma, Celgene, Glaxo Smith Klein, Janssen, Millenium/Takeda, Novartis Pharmaceuticals, Oncoceutics, Pharmacyclics, Seattle Genetics; Consultant: Amgen, Bristol-Myers Squibb, Celgene, Millenium/Takeda, Janssen, Karyopharm; Scientific Advisory Board: Amgen, Celgene, Millenium/Takeda, Janssen, Karyopharm, Sanofi, Seattle Genetics. Drs Cherepanov, Huang, Dabora, and Noga are current employees of Takeda, while Drs Stull and Young are ex-employees of Takeda; Drs Cherepanov and Huang also own stocks in Takeda. Dr DerSarkissian, Ms Cheng, Ms Zhang, Mr Banatwala, and Dr Duh are employees of Analysis Group, Inc. (AG), a consulting firm that received funding from Takeda to conduct this study. Ms Pi was an employee of AG at the time of the study. Dr Ailawadhi has the following relationships to declare: Research Support and Consulting for BMS, GSK, and Janssen; Research Support from AbbVie, Arch Oncology, Cellectar, Medimmune, Pharmacyclics, and Xencor; Consulting for Beigene, Oncopeptides, Regeneron, Sanofi, and Takeda.