Abstract 17217: ActRIIA-Fc Rebalances Activin/GDF and BMP9 Signaling to Attenuate Experimental Pulmonary Hypertension

Introduction: Therapies targeting the underlying mechanisms of pulmonary arterial hypertension (PAH) might modify the natural history of disease. The genetics of heritable PAH implicate deficient BMP signaling as a pathogenetic driver. Lung tissues from experimental and human PAH exhibit deficient BMP & excessive TGFβ signaling. While previous studies suggest agonism of BMP9 signaling or inhibition of TGFβ signaling may prevent experimental PH, the roles of activin and growth and differentiation factor (GDF) ligand signaling are unclear. ACTRIIA-Fc (sotatercept) is an activin- and GDF8/11-specific ligand trap currently in Phase 2 studies for anemia and multiple myeloma. Hypothesis: ACTRIIA-Fc inhibits activin and GDF8/11 signaling in vascular cells to attenuate experimental PH and pulmonary vascular remodeling. Methods: Impact of ACTRIIA-Fc on BMP & TGFβ signaling was examined in human pulmonary microvascular endothelial (PMVEC) and pulmonary arterial smooth muscle cells (PASMC). ACTRIIA-Fc (15 mg/kg s.c. 2x/week), sildenafil (30 mg/kg p.o. BID) or vehicle were given prophylactically for 4 weeks to rats developing PH following a single injection of MCT (40 mg/kg s.c.), or via single injection of SU5416 (200 mg/kg s.c.) combined with hypoxia (FIO2=0.10, SU-Hx). Alternately, rats were allowed to develop PH following SU-Hx treatment for 3 weeks or MCT treatment for 4 weeks, and then treated with ACTRIIA-Fc (1, 3, or 10 mg/kg s.c. 2x/week) or isotype-matched control antibody (3 weeks under normoxia following SU-Hx, or 2 weeks following MCT). Invasive hemodynamics, RV mass and vascular remodeling were assessed. Results: In PMVEC, ACTRIIA-Fc enhanced BMP9-induced SMAD1/5/8 activation and transcriptional activity, and reversed inhibition of BMP9 signaling by GDF11. ACTRIIA-Fc inhibited GDF8-, GDF11- and activin A-mediated SMAD2 phosphorylation and myogenic differentiation of PASMC. In vivo , a standard dose of ACTRIIA-Fc prevented PH, RV hypertrophy and PV remodeling in MCT- and SU-Hx-rats, with more potency than sildenafil. All doses of ACTRIIA-Fc attenuated PH and RV hypertrophy when given in a delayed fashion after MCT- and SU-Hx-treatment, with graded effects on PV remodeling. Conclusions: ACTRIIA-Fc rebalances GDF11 vs. BMP9 signaling, with distinct effects in vascular endothelium vs. media, and potently inhibits experimental PH in several rodent models. Given its well-defined tolerability, dosing, and efficacy in previous human studies, ACTRIIA-Fc is an attractive candidate for PAH as a novel, mechanism-targeted therapy.