B7-33: Physiology and What Research Reports

What B7-33 Is

B7-33 is a laboratory-made peptide, not a hormone the human body produces. It is a single-chain sequence modelled on part of the B-chain of human relaxin-2 (also called H2 relaxin), the name reflecting the B-chain residues from which the sequence is drawn. Native relaxin-2 is a two-chain, disulfide-linked hormone that is difficult and costly to synthesise; B7-33 was developed as a simpler single-chain mimetic that still engages the relaxin family peptide receptor 1 (RXFP1). In the cardiac literature it has been characterised as a functionally selective RXFP1 agonist, meaning it appears to favour some receptor signalling pathways over others (Journal of the American Heart Association, 2020).

Because B7-33 is synthetic, the honest physiological framing is indirect: the peptide is studied as a tool that mimics relaxin signalling, so the relevant physiology is the physiology of relaxin-2 and its receptor.

Where the Underlying Hormone Sits in Physiology

Relaxin-2 is a peptide hormone described in the literature as being produced in reproductive tissues such as the corpus luteum and placenta, with expression also reported in non-reproductive tissues including the heart and vasculature. Its receptor, RXFP1, is a G-protein-coupled receptor expressed in blood vessels, kidney, heart and fibroblasts. Interest in relaxin outside pregnancy grew because of its vascular and connective-tissue actions: the recombinant form of the hormone, serelaxin, has been investigated for vasoprotective effects, and a 2017 pharmacology study reported that B7-33 replicated those vasoprotective functions of human relaxin-2 in the experimental systems tested (European Journal of Pharmacology, 2017).

Why "functional selectivity" matters in this literature

RXFP1 can couple to more than one downstream cascade. Classical relaxin signalling is often described through cAMP, while fibroblast-directed antifibrotic signalling has been linked to other pathways. Describing B7-33 as functionally selective is the reason researchers framed it as a possible way to retain tissue-remodelling effects while differing from the full hormone in other respects, a framing used in the mouse myocardial infarction work (JAHA, 2020).

What the Published Studies Report

The verified literature on B7-33 is preclinical — cells, isolated tissue, rodents and implanted materials. No human clinical trial of B7-33 appears among these papers.

Vascular findings

The earliest of these reports positioned B7-33 as a relaxin substitute rather than a novel mechanism. Its stated conclusion was that the single-chain peptide reproduced vasoprotective actions attributed to serelaxin, which is the basis for the peptide being described elsewhere as a relaxin mimetic (Eur J Pharmacol, 2017). Doses, routes and concentrations used in that work are stated in the primary paper and are not reproduced here.

Cardiac remodelling and fibrosis

Two rodent cardiac studies carry most of the current interest. In the infarction model, researchers reported that the functionally selective RXFP1 agonist attenuated adverse cardiac remodeling after myocardial infarction (JAHA, 2020). In the later cardiomyopathy model, the study reported that B7-33 maintained relaxin's cardioprotective effects and produced a more rapid reduction in left ventricular fibrosis than perindopril, an established antihypertensive comparator (Biomed Pharmacother, 2023). Both are animal endpoints — histological fibrosis and cardiac function measures in mice — not clinical outcomes in people.

Local delivery from materials

A different application appears in the biomaterials literature. Rather than systemic administration, coatings were engineered to release the peptide where an implant meets tissue, and the study reported reduced fibrotic encapsulation of the coated implants (ACS Appl Mater Interfaces, 2019). That work matters conceptually because it tests the antifibrotic hypothesis at a site of controlled injury.

How B7-33 Is Measured and Studied

Readers meeting the term in a paper will typically encounter a familiar toolkit:

• Receptor assays. RXFP1 binding and signalling readouts, used to establish that a synthetic sequence engages the relaxin receptor and to characterise pathway preference, as underpins the functionally selective description (JAHA, 2020).
• Isolated vessel and cell work. Endothelial and vascular preparations compared against relaxin-2/serelaxin (Eur J Pharmacol, 2017).
• Animal disease models. Surgical myocardial infarction and cardiomyopathy models with echocardiographic and histological fibrosis endpoints (Biomed Pharmacother, 2023).
• Materials science endpoints. Capsule thickness and fibrotic response around implants releasing the peptide (ACS Appl Mater Interfaces, 2019).

There is no routine clinical laboratory test for B7-33 in patients; it is a research compound quantified by analytical chemistry in study settings.

Safety and Tolerability: What Studies Report

The verified preclinical literature summarised here focuses on efficacy endpoints — vascular function, cardiac remodelling, fibrosis and implant encapsulation — rather than on systematic safety characterisation. Across these reports, researchers described B7-33 as reproducing relaxin-like actions in animals and tissue systems (Eur J Pharmacol, 2017), and as maintaining relaxin's cardioprotective effects in an experimental cardiomyopathy model (Biomed Pharmacother, 2023); no human adverse-event dataset exists in this set of papers. The absence of reported harms in animal efficacy studies is not evidence of human safety, and toxicology, immunogenicity and long-term exposure questions remain open. This page is for educational purposes only and is not medical advice; consult a licensed physician for any question about health, treatment or a specific compound.

Why the Term Matters to Readers

B7-33 appears in peptide discussions mainly because fibrosis is a shared thread across heart disease, implant rejection and organ scarring, and because a single-chain mimetic is far easier to make than two-chain relaxin. Its regulatory status is straightforward: it is an investigational research peptide, not an approved medicine for any indication, and the published evidence base is animal- and bench-level. Anyone reading claims about it should check whether a given statement traces back to a mouse, a coated implant, or a person — in the current literature, it is the first two (ACS Appl Mater Interfaces, 2019).

Limitations of the Current Evidence

1. All four cited reports are preclinical; species differences in relaxin biology limit extrapolation.
2. Endpoints are surrogate — histological fibrosis and cardiac measures in mice, capsule formation around implants — rather than symptoms or survival in people.
3. Comparator design varies; only the 2023 study set B7-33 against an established drug, perindopril (Biomed Pharmacother, 2023).
4. Exposure duration in these models is short relative to chronic human disease.

References

• B7-33 replicates the vasoprotective functions of human relaxin-2 (serelaxin) (European Journal of Pharmacology, 2017)
• Coatings Releasing the Relaxin Peptide Analogue B7-33 Reduce Fibrotic Encapsulation (ACS Applied Materials & Interfaces, 2019)
• B7-33, a Functionally Selective Relaxin Receptor 1 Agonist, Attenuates Myocardial Infarction-Related Adverse Cardiac Remodeling in Mice (Journal of the American Heart Association, 2020)
• The single-chain relaxin mimetic, B7-33, maintains the cardioprotective effects of relaxin and more rapidly reduces left ventricular fibrosis compared to perindopril in an experimental model of cardiomyopathy (Biomedicine & Pharmacotherapy, 2023)