3007 Background: Low-density lipoprotein receptor-related protein 1b (LRP1b) is a putative tumor suppressor and one of the most frequently altered genes in cancer. Our prior single-center work suggested that LRP1B alterations may enrich for responses to immune checkpoint inhibitors (ICI) in solid tumors including prostate cancer; however, validation of these findings is needed. Methods: We conducted a multicenter, retrospective analysis of patients with LRP1B alterations (on tissue-based next-generation sequencing panels) treated with ICI at Duke, Johns Hopkins (JHU), and University of Michigan (UM). The primary objective was to assess the association between objective response rate (ORR) to ICI and pathogenic LRP1B alterations, defined as deletions or truncating alterations, when compared with LRP1B variants of undetermined significance (VUS), defined as missense mutations not predicted to be pathogenic in COSMIC. Missense changes with a COSCMIC FATHMM score of > 0.8 were categorized separately as likely pathogenic. Summary statistics, ORR, progression free survival (PFS), and overall survival (OS) were calculated. Results: 101 patients (44 Duke, 35 JHU, 22 UM) with LRP1B alterations were treated with ICI. Median age was 61 (range 32-82). The most common tumor types by alteration (pathogenic or likely pathogenic/VUS%) were lung (33/47%), GI (17/13%), prostate (11/7%), sarcoma (2/9%), melanoma (11/0%), and others (26/24%). 93% of patients received single-agent PD-(L)1 inhibition. The ORR for patients with either pathogenic/likely pathogenic alterations, or VUS alterations was 57% and 18%, respectively. After excluding MSI-high or TMB-high ( > 10 mut/Mb) tumors, ORR was 14/25 (56%) and 6/36 (17%), respectively. Pathogenic or likely pathogenic LRP1B alterations were associated with longer PFS (HR 0.39, 95% CI 0.24-0.63) and OS (HR 0.58, 95% CI 0.36-0.95). Conclusions: This multicenter study shows impressive and durable objective response rates to ICI for patients harboring pathogenic LRP1B alterations when compared to those with LRP1B VUS, independent of TMB/MSI status. Further mechanistic insights and prospective validation studies are warranted. [Table: see text]