Follistatin-344 vs GHRP-2.

Follistatin-344 (FS-344) is an alternatively spliced isoform of human follistatin, a naturally occurring secreted glycoprotein that acts as a high-affinity antagonist of several TGF-beta superfamily ligands. The "344" refers to a 344-amino-acid precursor variant; relative to the longer FS-315 serum isoform, it lacks the C-terminal acidic tail and was selected for therapeutic use partly to reduce off-target heparin/cell-surface binding. In gene-therapy programs it is the FS344 transgene that is delivered, not an injected peptide product, although it is now marketed in gray-market channels as a "research peptide."

GHRP-2 (growth hormone-releasing peptide-2; international nonproprietary name pralmorelin; development codes KP-102/GPA-748) is a synthetic hexapeptide with the sequence D-Ala-D-2-Nal-Ala-Trp-D-Phe-Lys-NH2. It belongs to the "classical" growth hormone secretagogue (GHS) family pioneered by Cyril Bowers in the 1980s and is a synthetic agonist of the ghrelin receptor. Unlike GHRH, it is structurally unrelated to any hypothalamic releasing hormone and instead mimics the endogenous gut peptide ghrelin.

Follistatin works by binding and neutralizing myostatin (GDF-8) and related ligands such as activin A, GDF-11, and several BMPs, preventing them from engaging activin type II receptors. Because myostatin is a dominant negative regulator of skeletal muscle mass, removing this brake promotes satellite-cell activation, myofiber hypertrophy, and reduced fibrosis. Critically, follistatin neutralizes a broader set of ligands than myostatin-only blockade, which is why follistatin overexpression produces larger muscle gains in animals than myostatin knockout alone. The foundational biology traces to McPherron, Lawler and Lee (Nature, 1997), who showed myostatin loss roughly doubles muscle mass in mice.

GHRP-2 binds and activates the growth hormone secretagogue receptor type 1a (GHS-R1a), the same Gq/11-coupled receptor targeted by ghrelin, expressed on anterior-pituitary somatotrophs and in the hypothalamus (including the arcuate nucleus). Receptor activation drives phospholipase C signaling, IP3/DAG generation, and intracellular calcium release, evoking pulsatile GH secretion. Because it acts through a pathway distinct from (and synergistic with) GHRH, GHRP-2 and GHRH together produce a markedly larger GH pulse than either alone. Its action on hypothalamic ghrelin-receptor circuits also explains its orexigenic (appetite-stimulating) effect and a degree of off-target activation of the corticotroph and lactotroph axes.

Human evidence is limited to two small, open-label AAV1-delivered FS344 gene-therapy trials from Nationwide Children's Hospital (Mendell and colleagues), not to any injected-peptide product. A Phase 1/2a trial in Becker muscular dystrophy (6 subjects; Mol Ther 2015, PMID 25322757) reported six-minute-walk gains in some treated patients (e.g., +58 m and +125 m in two subjects) with histological evidence of reduced fibrosis and fiber hypertrophy. A companion sporadic inclusion body myositis trial (6 subjects; Mol Ther 2017, PMID 28279643) reported improved annualized six-minute-walk distance versus untreated controls, though responses were heterogeneous and the comparison used a non-randomized matched control group. These are early-phase, unblinded, very small studies; large-animal support comes from a nonhuman-primate follistatin gene-delivery study (Kota et al., Sci Transl Med 2009, PMID 20368179). No randomized controlled trial, and no trial of FS-344 as a standalone injectable peptide, has demonstrated efficacy. The sIBM functional claims also drew a published methodological critique in Molecular Therapy.

Human pharmacology is well characterized for acute, single-dose use: controlled studies show GHRP-2 reliably triggers a robust GH pulse, and in healthy men it increased subjective hunger and food intake, confirming it behaves as a ghrelin mimetic (Laferrère et al., JCEM 2005). On the strength of acute diagnostic data it is approved in Japan as a single-dose GH-deficiency provocation test, where the GH response separates GH-sufficient from GH-deficient subjects. Critically, the long-term therapeutic program failed: development for treating GH-deficient children/pituitary dwarfism reached Phase II but was not brought to market, in part because the GH response to GHRP-2 is blunted in people with GH deficiency relative to healthy individuals. There are essentially no rigorous long-term human trials demonstrating benefit for body composition, muscle, anti-aging, or performance. Claims in those areas rest on mechanism and short-term hormone changes, not proven clinical outcomes.

In the two small gene-therapy trials, intramuscular AAV1.FS344 was reported as generally well tolerated over follow-up exceeding two years, but these cohorts are far too small to characterize real risk. Because follistatin broadly inhibits TGF-beta/activin signaling, theoretical and preclinical concerns include effects on reproductive tissues (follistatin was first identified as an inhibitor of FSH secretion), the pituitary-gonadal axis, vascular and cardiac remodeling, and possible influence on tumor biology, none of which are adequately resolved in humans. Gray-market "follistatin-344 peptide" products carry the additional, unquantified hazards of unverified identity, purity, sterility, and the fundamental mismatch that human data come from a delivered gene, not an injected protein. There is no established human safety profile for self-administered FS-344.

Documented acute effects in human studies include off-target stimulation of ACTH and cortisol and a transient rise in prolactin, a feature that distinguishes GHRP-2 from the more selective secretagogue ipamorelin (Arvat et al., Peptides 1997). As a ghrelin-receptor agonist it predictably stimulates appetite, and sustained GH/IGF-1 elevation carries the theoretical risks associated with the GH axis (insulin resistance, fluid retention, joint discomfort). The fundamental safety gap is that GHRP-2 has been studied chiefly as a one-time diagnostic agent; the consequences of repeated or chronic non-clinical use, including effects on the HPA axis, glucose metabolism, and any proliferative risk from prolonged IGF-1 elevation, have not been established in controlled long-term human trials. Material sold for "research" use is unregulated and may differ in identity, purity, or sterility from pharmaceutical-grade product.

Follistatin-344 is not approved by the FDA (or any major regulator) for any indication; it has only been studied investigationally as an AAV-delivered gene therapy and is sold elsewhere strictly as a research-use-only chemical, not a medicine. Myostatin-pathway inhibition is also of interest to anti-doping bodies, and follistatin/myostatin inhibitors fall under WADA's prohibited categories.

Approved in Japan (marketed by Kaken Pharmaceutical as GHRP Kaken 100) solely as a single-dose diagnostic agent for assessing growth hormone deficiency, making it the only GHS ever granted national regulatory approval, and only for diagnosis, not therapy. It is not FDA-approved for any indication; in the United States and most jurisdictions it is investigational/research-use-only, and it is prohibited in sport under the WADA Prohibited List (S2, growth hormone secretagogues).