TB-500

What it is

TB-500 is a synthetic peptide based on a fragment of thymosin beta-4 (Tβ4), a naturally occurring 43-amino-acid peptide that is among the most abundant proteins in human cells. The synthetic fragment corresponds to the region of residues 17–23 and is sometimes formulated as a longer subset of the full Tβ4 sequence.

Thymosin beta-4 was first identified in calf thymus tissue in 1981 by Allan Goldstein and colleagues and was initially thought to be a thymic immune-modulating peptide (which is why “thymosin” is in the name). Later laboratory research described broader biological roles, particularly in cell migration and actin regulation. Tβ4 has been studied in human clinical trials for cardiac conditions, dry eye disease (FDA fast-track designation), and chronic wounds.

“TB-500” typically refers to the synthetic fragment; the full-length molecule is also sold under that name. TB-500 appears on most racing organizations’ prohibited substance lists and on the WADA prohibited list. TB-500 is not FDA-approved for any use in the United States.

Mechanism of action

Proposed mechanisms come from laboratory (cell) and animal research; they have not been confirmed in people. Tβ4 has been reported to bind G-actin (monomeric actin) and limit its polymerization into F-actin, a role in the actin dynamics involved in cell movement, shape change, and division. Other observations reported in research models include:

• Cell migration: in cell culture and animal models, Tβ4 has been reported to influence the migration of endothelial and progenitor cells.
• Blood vessel formation: in laboratory and animal studies, Tβ4 exposure was associated with changes in VEGF expression and markers of angiogenesis.
• Inflammatory signaling: in animal models, Tβ4 was associated with changes in levels of certain pro-inflammatory cytokines.
• Progenitor cell signaling: in rodent cardiac studies, Tβ4 was reported to affect the activity of certain progenitor cell populations; the relevance to humans is unknown.
• Cell survival signaling: in animal models of ischemic injury, Tβ4 was associated with changes in markers of programmed cell death.

How, or whether, these laboratory and animal observations apply to people has not been established.

Research findings

Animal studies: in rodent models of cardiac infarction, Tβ4 administration was associated with changes in ventricular function and scar size. Animal models of corneal injury, skin wounds, and central nervous system injury have also reported effects.

Human cardiac trials: a small Phase II trial (TIME-1) in patients after myocardial infarction explored Tβ4 and cardiac function. The development program did not proceed to approval.

Dry eye disease: RegeneRx studied a Tβ4 eye-drop formulation (RGN-259) for severe dry eye and neurotrophic keratopathy. Phase II and III trials have been published with mixed results; benefit in people has not been established. The FDA granted Fast Track designation, but the product has not been approved.

Chronic wounds: small Phase II trials of topical Tβ4 in venous stasis ulcers and pressure ulcers have been published; benefit in people has not been established.

These studies used thymosin beta-4 formulations, not the TB-500 fragment, and none established the injectable TB-500 products sold commercially as effective or safe. Well-controlled human trials establishing effectiveness and safety are lacking.

Side effects and contraindications

• Human safety of TB-500 has not been established in well-controlled trials, and long-term effects are unknown.
• Malignancy: because Tβ4 has been associated with cell migration and blood vessel formation in laboratory studies, researchers have raised concerns about possible effects on tumor growth or spread.
• Pregnancy and breastfeeding: no safety data are available.
• Products sold outside regulated channels (e.g., “research peptides”) may vary in purity, dose, and sterility.

What we don’t yet know

Despite the size of the preclinical literature and several human trials of thymosin beta-4, no human randomized controlled trial has established TB-500 as a treatment for any condition. Whether the TB-500 fragment and full-length thymosin beta-4 behave similarly in people is unknown. Long-term human safety has not been characterized. The relative contribution of each proposed mechanism (cell migration, angiogenesis, cell survival signaling) to any outcome remains an open question.