B7-33

B7-33 is a synthetic single-chain peptide derived from H2 relaxin, one of the human relaxin hormones best known for its roles in tissue remodeling and connective-tissue softening. It was designed to retain relaxin's anti-fibrotic signaling while simplifying the parent hormone's more complex two-chain structure.

B7-33 is an investigational, animal-stage research compound. It is not an approved medicine in any major jurisdiction, and the effects described in the scientific literature come almost entirely from preclinical cell and animal models rather than from established human therapeutics.

Interest in B7-33 centers on its ability to engage the same receptor system as full-length relaxin while producing a distinct downstream signaling pattern. High-level mechanistic themes studied in the laboratory include:

• Binding to the relaxin family peptide receptor 1 (RXFP1)

• Preferential activation of the ERK1/2 signaling pathway, which is associated with anti-fibrotic and tissue-remodeling effects

• An apparent bias away from some of relaxin's other signaling outputs, which researchers have proposed may narrow its biological footprint

Whether this signaling profile translates into meaningful, durable outcomes in intact organisms — and eventually in humans — remains an open research question rather than an established fact.

B7-33 has no approved medical indication. In the research setting it has been explored as a tool for studying fibrosis, the process in which tissues accumulate excess scar-like extracellular matrix. Preclinical work has examined models of fibrosis in the heart, lung, and kidney, as well as models related to preeclampsia.

Because these investigations are exploratory and confined to laboratory systems, any discussion of B7-33 should be framed as basic and translational research rather than as a validated treatment for any human condition.

B7-33 is not specifically named on prominent public anti-doping lists. However, athletes and support personnel should be aware that anti-doping frameworks such as the WADA Prohibited List include a broad category (S0) for substances that are not approved for human therapeutic use by any government health authority. Investigational peptides can fall under such catch-all provisions.

Anyone subject to sport testing should verify current status directly with the relevant anti-doping authority rather than relying on general educational summaries.

Because B7-33 has not undergone the structured human safety evaluation that approved drugs require, its side-effect profile, tolerability, and long-term effects in people are not established. Relaxin biology touches on cardiovascular tone, connective tissue, and vascular function, so effects on those systems are areas researchers would want to characterize carefully.

The absence of documented adverse effects in early research is not evidence of safety. It reflects how little human data exist. Product quality and sourcing for research compounds also vary widely.

B7-33 is a short single-chain peptide engineered from the B-chain region of H2 relaxin, designed to preserve receptor engagement in a smaller, more stable format than the native two-chain hormone.

This summary describes concepts discussed in research and does not constitute medical advice or any suggestion of how the compound might be used in people.

B7-33 is best understood next to the two other forms of the same signal:

• H2 relaxin / serelaxin. The natural hormone and its recombinant version — a two-chain, insulin-like molecule held together by three disulfide bridges. That structure is what makes it expensive and laborious to produce, and it was administered in trials as a 48-hour intravenous infusion (Metra et al., NEJM 2019).

• B7-33. A single linear chain built from the B-chain region alone, with no A chain and no disulfide scaffolding to assemble — far simpler to synthesise, and biased toward pERK1/2 signalling rather than cAMP (Hossain et al., Chem Sci 2016).

• Lipidated B7-33 analogues. The originating laboratory's own follow-up work, because B7-33 survives roughly six minutes in serum in vitro. Conjugating a fatty acid with an appropriate spacer extended that to about 60 minutes without changing activity (Praveen et al., Int J Mol Sci 2023).

That last point is the practical one: the version of B7-33 available as a research powder is the short-lived one. The people who designed it consider its stability an unsolved problem, and the improved analogues had not yet been characterised in animals for pharmacokinetics at the time of that report.

The published evidence on B7-33 comes from preclinical studies exploring anti-fibrotic signaling in models of organ fibrosis and related conditions. This work has generated scientific interest in relaxin-derived peptides as research tools for understanding tissue remodeling.

At the same time, the evidence base is early-stage and animal-centric. Findings in cell and animal models frequently do not carry over to humans, so claims about B7-33 should be interpreted with caution. High-level overviews are not a substitute for critical appraisal of the primary literature.

For deeper dives on this peptide, see:

• B7-33 areas of research interest
• B7-33 safety context and unknowns
• B7-33 pharmacology concepts (educational)
• B7-33 research and evidence overview

What is B7-33, in one sentence? It is the B chain of human gene-2 relaxin, on its own, rebuilt as a single linear peptide — the first reported functionally selective agonist of the relaxin receptor RXFP1, preferentially activating the pERK pathway over cAMP in cells that natively express the receptor (Hossain et al., Chem Sci 2016). Removing the A chain removed most of the synthetic difficulty of making relaxin.

Has B7-33 ever been given to a human? No. There is no registered clinical trial of B7-33 on ClinicalTrials.gov, and no published human pharmacokinetic, safety, or efficacy data of any kind. Everything known about the peptide comes from cell assays, isolated blood vessels, and rodents. This is an unusually clean case of a compound sold to consumers that has never been studied in one.

What did the animal work actually show? In its introductory paper, B7-33 prevented or reversed fibrosis and organ dysfunction in three preclinical rodent models of heart or lung disease with potency comparable to full H2 relaxin, working through RXFP1–angiotensin II type 2 receptor heterodimers to drive pERK1/2 and the collagen-degrading enzyme MMP-2. Notably, unlike H2 relaxin, it did not promote prostate tumour growth in vivo (Hossain et al., 2016). A separate group found B7-33 reproduced serelaxin's acute vasoprotective effects in rat mesenteric arteries at equimolar doses (Marshall et al., Eur J Pharmacol 2017).

If relaxin biology is so promising, why is there no relaxin drug? Because the parent hormone was tested properly and failed. RELAX-AHF-2 randomised 6,545 patients hospitalised with acute heart failure to a 48-hour serelaxin infusion or placebo. Cardiovascular death at 180 days was 8.7% versus 8.9% (HR 0.98, p = 0.77) and worsening heart failure at day 5 was 6.9% versus 7.7% (HR 0.89, p = 0.19) — both primary endpoints missed (Metra et al., NEJM 2019). Encouraging rodent fibrosis data is exactly what serelaxin had too.

How long does B7-33 last in the body? Very briefly. Its in-vitro serum half-life is about six minutes, which the originating laboratory describes as a limitation it is actively trying to engineer around; fatty-acid conjugation raised it to roughly 60 minutes (Praveen et al., Int J Mol Sci 2023). Any account of B7-33 producing sustained effects from occasional injection sits awkwardly against that number.

Where does B7-33 stand in anti-doping terms? It is not named on the 2026 Prohibited List. That is not clearance: category S0 covers any pharmacological substance not addressed by another section and not approved by any governmental regulatory health authority for human therapeutic use, which describes B7-33 precisely (WADA Prohibited List). Anyone subject to testing should confirm status with their own anti-doping authority.