Follistatin-344
FS-344, FST-344
A myostatin-binding protein that produced dramatic muscle growth in monkeys, by gene therapy. The injectable version sold online is a different proposition entirely.
Preclinical only
Not approved anywhere. The published muscle work used viral gene delivery in animals, not injected protein. Sold as research material.
What it is
Follistatin is a naturally occurring protein that binds and neutralises myostatin, the principal brake on muscle growth, along with several activins. Remove the brake and muscle grows. FS-344 is one of the naturally occurring isoforms.
The animal results are spectacular, and they are the reason this compound has the reputation it does.
The delivery method is the entire story
The landmark study, published in Science Translational Medicine in 2009, injected an adeno-associated viral vector carrying the follistatin gene into the quadriceps of nonhuman primates. The muscle then produced follistatin continuously, locally, for months. Muscle size and strength increased substantially and durably.
That is gene therapy. A single viral injection turns the muscle into a follistatin factory.
What is sold online is recombinant follistatin protein in a vial, injected subcutaneously, cleared within hours, repeated daily for a few weeks. The published evidence does not speak to that at all. It is not a lower dose of the same intervention; it is a different intervention that happens to involve the same protein.
Later work has continued in the gene delivery direction, including studies of muscle follistatin gene delivery and protein synthesis, which is where the field actually is.
The selectivity problem
Follistatin is not a myostatin-specific inhibitor. It binds activin A and other TGF-beta family members, and those ligands do a great deal besides restraining muscle: reproductive signalling, inflammatory regulation, and tumour suppression in several tissues.
This is where bimagrumab is instructive. It targets the activin type II receptor with an antibody, has randomised human trials, has had its cardiac safety specifically examined, and is still not approved. Broad ligand sequestration by an injected protein with no human data is a less careful version of the same idea.
Practical notes
- Gene therapy results do not transfer to injected protein. This is the single most important thing to understand about follistatin marketing, and it is almost never stated.
- Local expression is not systemic exposure. The primate work produced follistatin inside one muscle. Systemic injection is a different exposure with different consequences.
- If the interest is myostatin inhibition, bimagrumab is where the human evidence lives. It is not obtainable either, which is a more honest summary of the state of this field than the vials suggest.
Side effects and warning signs
Commonly reported
- Injection-site irritation
- Unknown otherwise. No human safety study exists
Stop and get medical help
- Follistatin binds activins as well as myostatin, and activin signalling is involved in reproductive function, inflammation and tumour suppression. Broad inhibition has consequences nobody has mapped in humans
- Any active or suspected malignancy, given the above
Sources
From the community
Discussion for this compound concentrates in r/Peptides.
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