B7-33 vs Thymosin β-4.

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.

Thymosin beta-4 (Tβ4) is a small, naturally occurring 43-amino-acid acidic peptide found in nearly all mammalian cells and in high concentrations in platelets, wound fluid, and many tissues. It is the principal member of the beta-thymosin family and is one of the most abundant actin-binding proteins in the cytoplasm. The injectable "research peptide" TB-500 is widely marketed as thymosin beta-4, but it is typically a synthetic fragment or analog of the parent molecule rather than the full-length, naturally sequenced peptide; the two are not strictly interchangeable.

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.

Tβ4's best-characterized molecular function is sequestering monomeric (G-)actin: it binds G-actin in a roughly 1:1 ratio, buffers the pool of unpolymerized actin, and thereby regulates cytoskeletal assembly, cell migration, and motility. Beyond this structural role, Tβ4 has been reported to upregulate cell-survival signaling, notably activation of integrin-linked kinase (ILK) and the Akt pathway in cardiac cells, and to influence angiogenesis, inflammation, and the actin-binding protein laminin/myosin machinery during tissue remodeling. It also has downstream effects attributed to its N-terminal tetrapeptide (Ac-SDKP), a cleavage product with antifibrotic and anti-inflammatory activity. These mechanisms are largely defined in cell-culture and animal systems.

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.

The strongest human clinical data come from ophthalmology: the full-length peptide as RGN-259 (0.1% Tβ4 ophthalmic solution, RegeneRx/regional partners) was studied in a randomized, placebo-controlled, double-masked Phase III trial in neurotrophic keratopathy (Int J Mol Sci 2022, PMID 36613994), and in the ARISE-1/-2/-3 Phase III dry-eye program, where ARISE-3 missed its co-primary endpoints but showed significant improvement in some pre-specified secondary sign/symptom measures with a clean safety profile. Most other applications remain preclinical: the landmark cardiac work (Bock-Marquette et al., Nature 2004, PMID 15565145) showed Tβ4 promoted cardiomyocyte migration, survival, and improved cardiac function after injury in mice via ILK/Akt, and dermal/corneal wound-healing benefits are documented in animal models (e.g., Sosne et al., Exp Eye Res 2002, PMID 11950239). There is no FDA-approved Tβ4 product and no robust human evidence for the systemic "tissue repair," tendon/muscle recovery, or anti-aging uses for which TB-500 is informally promoted; that gap between animal data and proven human benefit is substantial.

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.

In the controlled ophthalmic trials, topical Tβ4 (RGN-259) was generally well tolerated with a safety profile comparable to placebo, but those data are limited to eye-drop administration and do not establish the safety of systemic injection. For injectable TB-500 sold as a research chemical, there are essentially no published controlled human safety data: long-term effects, immunogenicity, and risks are not characterized in humans. A specific theoretical concern is Tβ4's role in cell migration and angiogenesis, which has prompted caution about effects on tumor growth or metastasis; some preclinical studies link elevated Tβ4 to more aggressive tumor behavior. Material sold outside regulated channels also carries contamination, mislabeling, and dosing-uncertainty risks. No doses or protocols are provided here.

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.

Thymosin beta-4 is not FDA-approved for any indication; the full-length peptide (RGN-259) is investigational, completed Phase III trials in eye disease, and has held orphan-drug designation but no marketing approval, while injectable TB-500 is sold only as a research-use/unapproved compound. It is prohibited in sport at all times by WADA under section S2 (peptide hormones, growth factors, related substances and mimetics).