Fatty-acid siRNA silences the muscle-wasting gene myostatin for months in mice

Muscle loss with age, disease and injury is a leading driver of frailty, and myostatin (MSTN) is one of the best-known brakes on muscle growth. Silencing it with RNA drugs has been hard because delivering them to muscle and heart is difficult. Antibody-siRNA conjugates are in clinical development but are complex to manufacture. Simple lipid-linked siRNAs are easy to synthesize at scale, but earlier versions needed high doses (>50 mg/kg), caused toxicity, or silenced under 50%. This study asked whether careful chemical engineering of a docosanoic acid (DCA)-conjugated siRNA could fix that.

The team first screened 184 siRNAs, 88 against mouse Mstn and 96 against human MSTN, in muscle cell lines. Several sequences gave 80%-95% silencing. The best new leads were Mstn1928 (IC50 26 nM) in mouse and MSTN2155 (IC50 75 nM) in human. A previously reported benchmark sequence had IC50 values of 29 nM in mouse and 142 nM in human. Mouse leads were then built into the DCA-siRNA scaffold with a stabilizing 5'-vinylphosphonate and an extended nucleic acid (exNA) modification at the 3' end of the guide strand.

In mice given 20 mg/kg subcutaneously on days 0 and 3 (n=4-5 per group), exNA boosted silencing of the benchmark sequence by about 50% in all tissues tested. The new Mstn1928 sequence with exNA reached 80%-90% silencing in skeletal muscle and about 75% in the heart one week after dosing. The authors then compared three 2'-O-methyl/2'-fluoro modification patterns, each with four or seven phosphorothioate linkages. The alternating 50:50 pattern and the methyl-rich (~75%) pattern outperformed the original pattern. They kept silencing steady over four weeks, while the original pattern faded after week one. All patterns accumulated similarly in tissue, so durability reflected stability and activity, not distribution. The alternating pattern with four phosphorothioates was the strongest.

Per the abstract, a single subcutaneous dose silenced MSTN about 80% for up to 6 weeks and about 30% at 14 weeks. Biweekly dosing reduced circulating MSTN more than 95% for half a year with no observed toxicity. Mice showed muscle growth, higher lean mass and better grip strength, and benefits extended beyond direct target silencing, which suggests prolonged effects. Optimized dosing extended durability to 20 weeks without losing phenotypic benefit. The scaffold worked across several muscle groups and for three additional targets (JAK1, MECP2, HTT). In a cardiotoxin muscle-injury model (40 mg/kg), MSTN inhibition combated muscle wasting.

The implications are practical for drug development. A chemically defined, scalable siRNA that reaches muscle and heart after an injection under the skin, and acts for months, could reduce dosing frequency and manufacturing burden compared with biologic conjugates. Targeting myostatin is relevant to sarcopenia, cachexia and muscular disease, and the modular design suggests heart and muscle genes could be tuned for longer.

Important caveats apply. All work was in mice, and the in vivo studies shown used female mice only (FVB, plus C57BL/6J for the injury model). Groups were small, and only a subset of blood chemistry was shown. Myostatin inhibitors have had mixed clinical results in humans, and human safety, immunogenicity and long-term effects of near-complete myostatin suppression are untested.