AlfaDAX-Derived ActRIIA/B Antibody with Semaglutide Enhances Fat Loss and Improves Weight-Loss Quality in DIO Mice

GLP-1 receptor agonists achieve weight loss but are associated with clinically significant reductions in lean mass. Activin type II receptors (ActRIIA and ActRIIB) mediate signaling of myostatin and activin A, both of which negatively regulate muscle growth, suggesting that dual blockade of these receptors may preserve or increase lean mass while promoting fat loss. In this study, we developed anti-ActRIIA/B antibodies using AI-driven platforms (AlfaDAX) and selected the lead candidate AB130-165 based on in vitro binding, functional blocking, and developability assessments. Compared with a laboratory-prepared bimagrumab analog, AB130-165 exhibited potent dual inhibition of ActRIIA/B signaling, with a 9.5-fold higher functional blocking activity against activin A-induced SMAD signaling and 1054-fold improvements in binding affinity for ActRIIA (K _D = 0.204 pM), 10-fold for ActRIIB (K _D = 0.243 pM), respectively. In diet-induced obese mice, combination therapy with AB130-165 and semaglutide resulted in a 33.4% body weight reduction, which was superior to semaglutide monotherapy (−24.3%) and the bimagrumab combination group (−25.5%). Moreover, the combination significantly improved body composition, reducing fat mass percentage by 77.8% ( vs . 65.0% in the bimagrumab combination group) and increasing the lean-to-body weight ratio to 67.3% ( vs . 62.3%), demonstrating superior fat loss with better preservation of lean mass. Collectively, these findings establish AB130-165 as a differentiated anti-ActRII antibody that enables high-quality weight loss, and its combination with semaglutide shows superior efficacy over bimagrumab-based regimens. With favorable developability and potential for long-acting subcutaneous administration, AB130-165 represents a promising next-generation therapeutic candidate for obesity and muscle-sparing weight management.