TB-500 (Thymosin Beta-4) Research Peptide

A resting cell keeps much of its actin as single subunits rather than filaments, and thymosin beta-4 is the main thing holding them back. Each molecule grips one monomer, which then cannot join a filament, and concentrations may approach 0.5 mM in some cell types. A crystal structure published in 2004 showed how: the chain caps both ends of the actin subunit, with the LKKTETQ motif at residues 17 to 23 sitting at the centre of the contact (Learn more).

Release is handled by profilin, whose binding site on actin overlaps slightly with thymosin beta-4’s. Profilin passes monomers to growing filament ends, so the pair act as reservoir and delivery service, which matters to migration studies because a moving cell builds filaments fast at its leading edge.

One of the most cited studies, published in the Journal of Investigative Dermatology in 1999, used a rat full-thickness wound model. Thymosin beta-4 increased re-epithelialisation by 42% over controls at day 4 and by up to 61% at day 7, with more collagen deposition and new blood-vessel growth in the treated wounds (Learn more).

An NIH group then turned to hair. Thymosin beta-4 stimulated hair growth in normal rats and mice, and follicle keratinocytes originating in the bulge, the stem-cell niche, expressed it in step with the hair cycle. Isolated rat whisker-follicle keratinocytes migrated and differentiated more at nanomolar concentrations and secreted more of the matrix enzyme MMP-2 (Learn more).

In 2004, a study in Nature showed that thymosin beta-4 forms a complex with PINCH and integrin-linked kinase, activating the survival kinase Akt. In a mouse coronary-ligation model it improved early heart-muscle cell survival and cardiac function (Learn more). Paul Riley’s London group reported in 2007 that the peptide stimulated outgrowth from quiescent adult epicardial explants, and that cardiac knockdown disrupted coronary vessel development (Learn more). A 2011 follow-up found that priming mice before an infarct let Wt1-positive epicardial cells give rise to a small number of new cardiomyocytes.

Other laboratories added important caveats. Lineage tracing by William Pu’s team found that giving the peptide only after infarction thickened the epicardium and raised capillary density, yet produced no epicardium-derived heart muscle. Full knockout mice made in San Diego also developed normal hearts and coronary vessels (Learn more).

Thymosin beta-4’s first four residues have a research career of their own. In 1989 a French team purified the acetylated tetrapeptide Ac-Ser-Asp-Lys-Pro from fetal calf bone marrow as an inhibitor of haematopoietic stem-cell proliferation (Learn more). Kidney studies later showed meprin-α making the first cut in the parent peptide and prolyl oligopeptidase then releasing Ac-SDKP.

Its removal is just as interesting. Angiotensin-converting enzyme degrades Ac-SDKP, and in eight healthy volunteers a single exposure to the ACE inhibitor captopril raised plasma Ac-SDKP about 5.5-fold. Rodent studies of hypertensive heart and kidney damage have since examined Ac-SDKP as a brake on collagen build-up. The parent peptide has its own fibrosis literature in heart, liver and lung models, which raises the question of which effects belong to the whole chain and which to its fragment.

RegeneRx Biopharmaceuticals, where Allan Goldstein has served as board chairman and chief scientific adviser, took thymosin beta-4 into three formulations, and the record is mixed. The RGN-259 eye drop went through three Phase 3 ARISE dry-eye trials in more than 1,600 people; none met its pre-specified co-primary endpoints. Its Phase 3 neurotrophic keratopathy trial, SEER-1, closed early with only 18 patients: 6 of 10 on the peptide had complete corneal healing at four weeks against 1 of 8 on placebo, a difference that fell just short of significance by Fisher’s exact test.

The RGN-137 topical gel met its safety endpoints in 72-patient Phase 2 trials in pressure and venous stasis ulcers, without statistically significant healing differences from placebo. The injectable RGN-352 completed Phase 1 work in healthy volunteers before its Phase 2 heart-attack trial was put on FDA clinical hold in 2011 over a contract manufacturer’s compliance. No thymosin beta-4 medicine has been approved.