Activin receptor type II ligands (ActRII ligands)

Activin A, Activin B, Growth Differentiation Factor 8 (GDF8, also known as Myostatin), and Growth Differentiation Factor 11 (GDF11) are key ligands within the Transforming Growth Factor-beta (TGF-β) superfamily that regulate diverse physiological processes including muscle growth, erythropoiesis, and vascular homeostasis [4, 18, 27]. These ligands primarily signal by binding to Activin Type II receptors (ActRIIA and ActRIIB), which then recruit Type I receptors to activate the intracellular SMAD2/3 signaling pathway [1, 3, 8]. GDF8 and Activin A are well-established negative regulators of skeletal muscle mass, while GDF11 and Activin B are involved in the regulation of red blood cell maturation and the maintenance of vascular tone [5, 21, 24]. Pathological elevation or dysregulation of these ligands is associated with muscle wasting (sarcopenia and cachexia), ineffective erythropoiesis in myelodysplastic syndromes and beta-thalassemia, and the development of pulmonary arterial hypertension (PAH) [1, 2, 5, 6]. Therapeutic targeting of this group of ligands is primarily achieved through "ligand traps"—recombinant fusion proteins such as sotatercept and luspatercept—which consist of the extracellular domain of ActRIIA or ActRIIB fused to an Fc fragment [1, 5, 12]. These traps sequester the ligands, preventing their interaction with cell-surface receptors and thereby inhibiting overactive SMAD2/3 signaling [3, 7, 16]. This mechanism helps restore the balance between the SMAD2/3 and SMAD1/5/8 pathways, leading to improved clinical outcomes such as increased hemoglobin levels in anemia or reduced pulmonary vascular resistance in PAH [2, 11, 26]. Other therapeutic approaches include monoclonal antibodies that either block the ActRII receptors directly (e.g., bimagrumab) or specifically neutralize individual ligands like GDF8 (e.g., apitegromab) [6, 14, 15, 23]. Notable safety concerns associated with these therapies include off-target effects such as skin telangiectasia, epistaxis, and erythrocytosis [2, 23, 27].

Other names

Activin AActivin BGrowth differentiation factor 8Growth differentiation factor 11MyostatinBone morphogenetic protein 11Inhibin beta AInhibin beta B

Ligand sequestration (ligand trap), receptor blockade (monoclonal antibody), and inhibition of SMAD2/3 signaling pathway.