Background: Seribantumab (MM-121) is a human monoclonal IgG2 antibody that blocks the HRG domain of HER3. Preclinical data suggest that seribantumab reverses HRG mediated drug resistance across multiple cancer models. In prior retrospective analyses, addition of seribantumab to standard of care (SOC) appeared to improve outcomes in pts with HRG+ tumors. Here we tested if seribantumab plus SOC improved progression-free survival (PFS) in pts with HRG+ lung adenocarcinoma who had received prior platinum-based therapy. Methods: SHERLOC was a randomized, open-label, multicenter, Phase 2 study in pts with advanced HRG+ adenocarcinoma of the lung. Archival or pre-treatment tumor samples were assessed for HRG+ by RNA in situ hybridization. Eligibility criteria included prior platinum-based therapy for advanced disease with ≤ 2 total prior lines of therapy (prior IO was allowed) and no EGFR or ALK mutations. Pts were randomized 2:1 to receive seribantumab 3000 mg/docetaxel 75 mg IV q3w (experimental; exp) or docetaxel 75 mg IV q3w alone (control). Primary endpoint was PFS. Key secondary endpoints were overall survival (OS), objective response rate (ORR), and adverse event (AEs) profile. Results: At a pre-specified interim analysis of 75% of total PFS events, 108 pts were enrolled (exp n = 71, control n = 37). Median age was 62y (range 34-83y); female 34%; one prior treatment only 39%. Median PFS was 3.0m for exp and 4.0m for control, HR = 1.66m (p = 0.084). Median OS was 7.9m for exp and 8.4m for control, HR = 1.50 (p = 0.235). ORR was 19.7% for exp and 5.6% for control (p = 0.052). Serious AEs were more frequent in the exp arm (40.8%) vs control (24.3%). Most common treatment emergent AEs (TEAEs) in the exp arm were diarrhea (47%), fatigue (37%), and neutropenia (27%). Based on a determination of futility at interim analysis, the study was terminated early. Conclusions: Seribantumab failed to improve PFS when added to docetaxel among previously treated advanced HRG+ NSCLC pts. A higher response rate and a higher incidence of TEAEs were observed in the exp arm. No further study of seribantumab is planned in NSCLC. Clinical trial information: NCT02387216