B7-33 vs TB-500.

B7-33 is an experimental single-chain peptide derived from the B-chain of the human hormone relaxin-2 (H2 relaxin). Native relaxin is a complex two-chain molecule linked by disulfide bonds and is difficult and costly to manufacture; B7-33 is a simplified 24-amino-acid single-chain mimetic intended to keep the useful anti-fibrotic activity while being easier to make. It is a laboratory research compound only, studied in cells and animals for fibrosis of the heart, kidney, lung, and blood vessels. It has never been tested in humans and has no approved use. It is categorized under healing because its studied effects are anti-fibrotic and tissue-remodeling.

TB-500 is a synthetic peptide sold for research use and widely marketed as "thymosin beta-4 (Tβ4)." Naturally occurring Tβ4 is a 43-amino-acid, ~4.9 kDa actin-sequestering peptide found in nearly all mammalian cells and at high concentration in platelets and wound fluid. Notably, many products labeled "TB-500" are actually a shorter synthetic fragment built around the active actin-binding motif (LKKTET/LKKTETQ) rather than the full-length Tβ4 molecule, so the name is used loosely in the research-chemical market and the exact identity of a given vial is often unverified.

B7-33 is a functionally selective (biased) agonist of the relaxin family peptide receptor 1 (RXFP1). Rather than strongly activating the cAMP pathway like native relaxin, it preferentially signals through the pERK pathway at RXFP1. This biased signaling is proposed to drive anti-fibrotic effects, notably increased activity of matrix-degrading enzymes such as MMP-2, which break down excess collagen, while potentially avoiding some effects tied to broader relaxin signaling. In preclinical models it has shown vasoprotective and cardioprotective actions consistent with relaxin biology. The single-chain design is meant to preserve receptor engagement without the manufacturing burden of the two-chain hormone.

The best-characterized function of thymosin beta-4 is binding monomeric G-actin in a 1:1 complex, acting as the principal intracellular actin-sequestering peptide that buffers the pool of unpolymerized actin and thereby modulates cytoskeletal assembly, cell shape, and migration. Through the LKKTET motif and downstream signaling, Tβ4 has been reported in preclinical models to promote keratinocyte and endothelial cell migration, angiogenesis, anti-inflammatory and anti-apoptotic effects, and activation of integrin-linked kinase (ILK)/Akt and laminin-5 pathways relevant to wound repair. These mechanisms are mostly established in cell and animal systems; the degree to which a synthetic LKKTET-type fragment reproduces full-length Tβ4 biology is not firmly established.

All evidence is preclinical: cell-culture and animal studies, with no clinical trials. B7-33 has never been administered to a human being in a registered study, so no efficacy or safety claim about people can be drawn from the literature that exists. The founding paper (Hossain et al., Chemical Science, 2016) described the design of B7-33 and showed it binds RXFP1 and preferentially activates pERK over cAMP, with anti-fibrotic activity in cell and animal models of heart, lung, and kidney fibrosis. That work combined receptor pharmacology in transfected cell lines with short rodent experiments, using tissue collagen content and enzyme activity as endpoints, in small groups, without the randomization and blinding conventions that govern clinical research. Subsequent work reported vascular and cardiac benefits, including a study finding that B7-33 maintained relaxin's cardioprotective effects and reduced left-ventricular fibrosis more rapidly than the ACE inhibitor perindopril in an experimental model of cardiomyopathy (Alam et al., Biomedicine and Pharmacotherapy, 2023). Additional reports describe effects in myocardial-infarction remodeling and in hypertrophic-scar fibroblasts in vitro. These are early-stage animal and laboratory findings; no human efficacy or safety has been demonstrated, and translation to people is unproven. The cautionary comparison is the parent molecule. Serelaxin, recombinant human relaxin-2, travelled the whole distance: it produced encouraging earlier-phase signals in acute heart failure, then failed to improve clinical outcomes in the large RELAX-AHF-2 trial, and analyses of that dataset have continued to examine why its biomarker and end-organ signals did not convert into patient benefit. A biased RXFP1 agonist that performs well in rodent fibrosis models is therefore starting from a class whose flagship compound already failed a definitive human outcome trial. For B7-33 itself there is no human pharmacokinetic data, no dose-finding work, no formal toxicology package, no immunogenicity assessment, and no registered clinical trial.

Human evidence comes almost entirely from the full-length Tβ4 molecule developed pharmaceutically by RegeneRx and partners, not from research-chemical "TB-500." Topical Tβ4 (RGN-137) was tested in completed Phase 2 dermal-wound trials including a randomized, placebo-controlled study in venous stasis ulcers (ClinicalTrials.gov NCT00832091), and ophthalmic Tβ4 (RGN-259) has advanced to Phase 3 for neurotrophic keratopathy (e.g., the SEER-2 trial, NCT05555589). Preclinical support for dermal, corneal, and cardiac repair is substantial and replicated across labs, as reviewed by Kleinman and Sosne (Vitamins and Hormones, 2016). However, there are no controlled human trials of injectable "TB-500" as sold in the peptide market for musculoskeletal healing, tendon/ligament injury, or athletic recovery; those uses rest on animal data and extrapolation, and the human-vs-preclinical gap is wide.

There are no human safety data for B7-33 because it has not entered clinical trials; all information comes from cell and animal experiments. Preclinical reports have not flagged prominent toxicities in the models studied, but absence of reported harm in a handful of animal studies is not evidence of human safety. Those studies were small, short, and designed to detect efficacy signals rather than toxicity, and none included the systematic histopathology, reproductive testing, or repeat-administration escalation that regulators require before a first human exposure. Relaxin biology indicates where problems would be looked for. Relaxin is a vasodilator and a systemic tissue-remodeling hormone, so blood pressure effects, renal hemodynamic changes, and unwanted matrix degradation in tissues that were not the target are the plausible concerns for any RXFP1 agonist. The anti-fibrotic mechanism that is desirable in a scarred heart is not obviously desirable everywhere else in the body. Clinical experience with serelaxin showed that an RXFP1 agonist can be given to acutely ill patients without an alarming adverse-event profile, but serelaxin is a different molecule with different signaling bias and different pharmacokinetics, and that experience does not transfer to a single-chain mimetic. Purity, dosing, and long-term effects in humans are entirely unknown, and immunogenicity against a synthetic single-chain sequence has never been assessed. Products marketed online as B7-33 are research chemicals, not medicines, and are not intended for human use, with no verified identity, sterility, or endotoxin testing behind them. This entry is educational only and does not provide any usage guidance.

There is no established human safety profile for research-chemical "TB-500"; it is not a licensed drug and is not manufactured to pharmaceutical quality, so identity, purity, sterility, and endotoxin content of marketed vials are unverified. Pharmaceutical full-length Tβ4 has been reasonably well tolerated in controlled topical and ophthalmic trials, but those findings do not transfer to unregulated injectable products. A recurring theoretical concern is that a peptide promoting angiogenesis and cell migration could be undesirable in the setting of occult malignancy, though this is not established as a clinical harm. Contamination, dosing errors, and injection-related risks are the most concrete real-world hazards.

B7-33 is a preclinical research compound with no FDA or other regulatory approval and no approved indication. It has not been evaluated in human clinical trials. It is sold, where sold, only as a research reagent labeled not for human consumption.

TB-500/thymosin beta-4 is not approved by the FDA or EMA for any indication; full-length Tβ4 remains investigational (RegeneRx/ReGenTree ophthalmic and dermal programs), and material sold as "TB-500" is research-use-only and not a dietary supplement or medicine. It is prohibited in sport at all times by the World Anti-Doping Agency under class S2 (peptide hormones, growth factors, related substances and mimetics).