Potent and durable gene modulation in heart and muscle with chemically defined siRNAs

Small interfering RNA (siRNAs) hold immense promise for treating cardiac and muscular diseases, but robust and scalable delivery to these tissues remains a challenge. Recent advances in delivery strategies to muscle include conjugation of biologics (antibody/antibody fragments, peptides), which are currently in clinical development. However, the manufacturing of biologic-siRNA conjugates is a challenging and complex process. By contrast, lipophilic siRNAs are readily chemically synthesized at scale and support sufficient cardiac and skeletal muscle delivery. In this work, we refine siRNA design elements to enhance potency and durability and support clinically relevant silencing in muscle. Applying this strategy for siRNAs targeting myostatin (MSTN), a key target in muscle-wasting conditions, we show that a single subcutaneous dose in mice achieved robust and durable silencing (~80% inhibition up to 6 weeks, ~30% at 14 weeks). Biweekly dosing resulted in >95% reduction of circulating MSTN for half a year, with no observed systemic or target-related toxicity. MSTN inhibition resulted in muscle growth and increased lean muscle mass, correlating with improved grip strength. Interestingly, the functional impact on muscle growth and strength significantly outlasts the target silencing, suggesting extended pharmacological effects. Systemic administration was equally efficacious in all muscle groups tested, including skeletal muscle, heart, tongue and diaphragm. The informational nature of the muscle-active chemically defined siRNA scaffold was confirmed by demonstrating muscle and heart efficacy with three additional targets. Our findings pave the way for potent and long-lasting gene modulation in muscle using chemically defined, lipophilic siRNAs, offering a new avenue for treating muscular diseases.