What Is B7-33? A Single-Chain Relaxin Analog Research Peptide

B7-33 is a synthetic single-chain peptide derived from the hormone H2 relaxin (relaxin-2), designed to retain relaxin’s anti-fibrotic signaling through the RXFP1 receptor while being far simpler and cheaper to synthesize than the native two-chain hormone. In preclinical and in-vitro tissue-fibrosis research, it has been characterized as a “biased” or functionally selective RXFP1 agonist. This article is educational only. B7-33 is a research compound discussed here strictly in the context of laboratory and animal studies — it is for research use only (RUO), is not sold by Bolt Peptide, and is not a drug, supplement, or treatment for any condition.

Quick facts

• Class: single-chain relaxin (H2 relaxin / relaxin-2) analog — based on the relaxin B-chain
• Primary receptor studied: RXFP1 (relaxin family peptide receptor 1), a G protein–coupled receptor
• Research focus: anti-fibrosis and tissue-remodeling signaling in cell and animal models
• Notable property: functionally selective (“biased”) agonism — preferential pERK1/2 over cAMP signaling

What is B7-33?

Native human relaxin-2 is a two-chain hormone whose A- and B-chains are joined by disulfide bonds, making it complex and costly to manufacture. B7-33 is a short single-chain analog built from the relaxin B-chain that was engineered to fold and bind RXFP1 without the A-chain. In structural terms, it trades the full hormone architecture for a minimal sequence that still engages the receptor — which is why researchers describe it as a streamlined relaxin “mimetic.” Because it is a single chain, it is more tractable for laboratory synthesis and structure–activity studies than the parent hormone.

What does the research show?

In the foundational 2016 study, B7-33 was reported to bind RXFP1 and prevent or reverse organ fibrosis and dysfunction across three preclinical rodent models of heart or lung disease, with potency broadly comparable to native H2 relaxin — while, unlike relaxin, not promoting prostate tumor growth in the animal models tested. A later 2023 study in an experimental rodent cardiomyopathy model reported that B7-33 retained relaxin’s cardioprotective and anti-fibrotic profile and reduced left-ventricular fibrosis more rapidly than the comparator drug perindopril in that animal setting. These are animal and in-vitro findings; they describe receptor-level and tissue-level biology in laboratory systems and do not establish any effect in humans.

Mechanisms studied in the lab

• RXFP1 activation: B7-33 binds and activates RXFP1, the primary relaxin receptor, in cells that endogenously express it.
• pERK1/2-biased signaling: it preferentially activates the phosphorylated ERK1/2 (pERK1/2) pathway over the cAMP pathway — the basis for calling it a functionally selective or “biased” agonist.
• Anti-fibrotic / matrix-remodeling pathways: in these models the anti-fibrotic actions were linked to RXFP1–angiotensin II type 2 receptor (AT2R) heterodimers and downstream induction of the collagen-degrading enzyme matrix metalloproteinase-2 (MMP-2).

Research status

B7-33 remains an investigational research peptide. The published work to date is preclinical — cell assays and animal models — and it has not completed the human clinical trials required for any regulatory approval. It is not a medicine and has no established human use. Bolt Peptide does not sell B7-33; this page exists for scientific education only.

Related research peptides

Researchers tracking tissue-repair and remodeling biology often compare relaxin-family analogs with other widely studied compounds. Browse our research peptide catalog, or read our educational overviews of BPC-157 and TB-500, two peptides studied in tissue-repair contexts.

FAQ

Is B7-33 the same as relaxin? No. B7-33 is a synthetic single-chain analog derived from the relaxin B-chain. It is designed to engage the same RXFP1 receptor as native relaxin-2 but has a simpler structure and a more selective (biased) signaling profile in laboratory studies.

Why is B7-33 called a “biased” agonist? In cell studies it preferentially activates the pERK1/2 signaling pathway over the cAMP pathway at RXFP1. Activating some receptor pathways more than others is called biased or functionally selective agonism.

Can I use B7-33 for fibrosis or a heart condition? No. All of the effects described above come from in-vitro and animal research. B7-33 is not approved for human or veterinary use and is not a treatment for any disease.

References

1. Hossain MA, et al. A single-chain derivative of the relaxin hormone is a functionally selective agonist of RXFP1. Chem Sci. 2016.
2. Alam F, et al. The single-chain relaxin mimetic B7-33 maintains cardioprotective effects and reduces LV fibrosis in experimental cardiomyopathy. Biomed Pharmacother. 2023.

For research use only. Not for human or veterinary use, consumption, or clinical application. All effects derive from preclinical studies. Bolt Peptide does not sell B7-33. Statements have not been evaluated by the FDA.