Inside the body with TB-500 (thymosin beta-4 fragment)

FDA identified one in-vivo pharmacokinetic study: thoroughbred geldings given a single subcutaneous injection had plasma TB-500 peaking at 0.05-0.08 ng/mL between 60 and 120 minutes, falling below the limit of quantification between 6 and 10 hours. Without intravenous data, absolute bioavailability could not be established. FDA found no human pharmacokinetic or pharmacodynamic studies by any route.[src][src]

Metabolites arise by sequential C-terminal truncation. Incubation with equine liver in vitro gave Ac-LKKTET, Ac-LKKTE, Ac-LKKT, Ac-LKK and Ac-LK plus stereoisomerised forms. In horses, plasma about 2 hours after a subcutaneous injection contained the parent peptide, a stereoisomer, Ac-LKKTE and Ac-LK; urine also contained Ac-LKKTET and Ac-LKK. Rat urine contained mainly Ac-LKKTE, Ac-LKK and Ac-LK. In vitro, human serum, kidney microsomes and liver fractions produced mainly C-terminally truncated metabolites, with Ac-LKKTE and Ac-LKK prominent in some systems. Ac-LK was the main early metabolite in rats and Ac-LKK persisted up to 72 hours. Human in-vivo metabolism has not been studied.[src][src][src]

Residues LKKTET of thymosin beta-4 form its actin-binding region; the full protein binds G-actin but not F-actin and buffers the monomer pool (the 4PL7 crystal structure shows full-length thymosin beta-4 on an actin monomer). FDA notes it is only hypothesised that LKKTETQ shares this buffering capacity, that it remains undetermined whether actin binding explains its reported pro-angiogenic and wound-healing effects, and that N-acetylation may change binding, so activity of the non-acetylated peptide cannot be extrapolated to TB-500.[src][src][src][src]

Full-length thymosin beta-4, given topically or intraperitoneally, increased re-epithelialisation in rat full-thickness wounds (42% over saline at day 4; up to 61% at day 7). The non-acetylated LKKTETQ heptapeptide matched full-length thymosin beta-4 for dermal repair in 26-month-old mice (Philp 2003). In a fibroblast scratch assay, TB-500 did not differ from vehicle, while Ac-LKKTE gave small but significant closure (Rahaman 2024), suggesting activity might depend on conversion to a metabolite. All of this is animal or cell evidence; FDA identified no nonclinical in-vivo studies of wound healing with TB-500 itself.[src][src][src][src]

In mice after coronary artery ligation, full-length thymosin beta-4 upregulated integrin-linked kinase and Akt activity, enhanced early myocyte survival and improved cardiac function (Bock-Marquette 2004); follow-up work in mouse embryonic hearts and cultured adult epicardial explants reported that it stimulated outgrowth of epicardial progenitor cells (Smart 2007). These are animal and tissue studies of the full-length protein. The review by Sosne et al. maps cell-survival (anti-apoptotic) activity to the N-terminal 15 amino acids rather than the LKKTETQ stretch, and lists ILK/PINCH/Akt activation among activities not yet localised in the molecule. No equivalent data exist for TB-500.[src][src][src]

A double-blind, placebo-controlled phase 2 dose-escalation trial of topical full-length thymosin beta-4 in 73 patients with venous stasis ulcers found safety comparable to placebo; the authors suggested that complete healing could be achieved within 3 months in about 25% of patients. A small phase 2 trial of thymosin beta-4 eye drops (nine patients with severe dry eye) reported 35.1% less ocular discomfort and 59.1% less corneal staining than vehicle at day 56. Both are investigational, unapproved, small studies of a different molecule and route from TB-500.[src][src][src]

FDA states that potential safety risks in humans are unknown: no acute, repeat-dose, genotoxicity, reproductive or carcinogenicity studies were identified. Its review flags a significant immunogenicity risk for an injectable peptide, potentially amplified by aggregation and synthesis-related impurities, and notes that a representative certificate of analysis did not control assay, impurities, endotoxin or aggregates.[src]