Follistatin 344

Myostatin (GDF-8) limits how large muscle fibers can get by activating ActRIIB receptors and the SMAD2/3 pathway. Follistatin binds myostatin directly and pulls it out of circulation before it reaches the receptor, so the brake on muscle growth is released. It also blocks activin and GDF-11, which is why it produces larger effects than antibodies that target myostatin alone.

Almost entirely animals, and almost entirely gene therapy rather than injections. Transgenic mice, AAV gene delivery in mice, and one nonhuman-primate study drive the headline numbers. The only human data come from a small phase 1/2a gene-therapy trial in Becker muscular dystrophy (Mendell 2015), where the FS344 gene was delivered into muscle by viral vector and improved walking distance with increased fiber size. Injectable FST-344 protein has no published human efficacy or safety trials.

They are splice variants of the same follistatin gene. FST-288 has a strong heparin-binding tail that anchors it to cell surfaces, so it acts locally. FST-344 is processed to a circulating serum form (FS315) that moves more freely and binds activin with lower affinity. Research models use the two variants to separate local from systemic follistatin effects.

There is no validated human protocol. Vendor-suggested research schedules commonly cite roughly 100mcg per day subcutaneously over a few weeks, but this is empirical and not backed by dose-response data. The muscle literature rests on gene therapy that produces continuous local follistatin expression, not on fixed injectable doses.

Follistatin is roughly a 38 kDa glycoprotein that clears from circulation within hours, and there is no published pharmacokinetic profile for injectable FST-344 specifically. That short residence time is exactly why the successful animal and human studies used gene therapy: a viral vector makes muscle produce follistatin continuously, rather than relying on repeated bolus injections.

Bacteriostatic water is the standard diluent. For a 1mg vial, 1mL of BAC water gives 1mg/mL, or 100mcg per 0.1mL. Add the water slowly down the vial wall, swirl gently, never shake, and store refrigerated at 2-8°C after reconstitution.