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Dilated cardiomyopathy associated with lamin A/C (LMNA) gene variants (LMNA-related dilated cardiomyopathy [DCM]) is a life-threatening condition with a high unmet need, accounting for approximately 6% of idiopathic DCM cases. Currently, no disease-specific treatments target the underlying disease mechanism. ARRY-371797 (PF-07265803), a potent, selective, oral, small-molecule inhibitor of the p38α mitogen-activated protein kinase pathway, improved 6-minute walk test (6MWT) distance in 12 patients with symptomatic LMNA-related DCM in a 48-week, open-label, phase 2 study. This long-term extension study examined the safety and efficacy of ARRY-371797 in patients from the phase 2 study. 6MWT, N-terminal pro-B-type natriuretic peptide concentration, and 12-item Kansas City Cardiomyopathy Questionnaire score were assessed at weeks 48, 72, 96, 120, and 144 from phase 2 study baseline. Eight patients enrolled (mean [SD] age, 51 [10] years, 4 male). Mean 6MWT increased by30 m (10%) from phase 2 study baseline up to week 120. The decrease in N-terminal pro-B-type natriuretic peptide observed in the phase 2 study was maintained throughout the present study. Twelve-item Kansas City Cardiomyopathy Questionnaire Physical Limitation increased from baseline at all visits except week 96 (range: -0.8 [week 96] to 13.8 [week 120]); results for other domains were variable. Treatment was generally well tolerated; 2 patients discontinued because of causes not considered treatment-related. There were no deaths. ARRY-371797 was generally well tolerated over median (range) 155.7 (61 to 327)-week exposure; evidence suggested preserved exercise capacity over the study period. The ongoing, pivotal, phase 3, randomized, placebo-controlled study REALM-DCM investigates the efficacy and safety of ARRY-371797 (PF-07265803) in LMNA-related DCM. (ClinicalTrials.gov Identifier: NCT02351856).