B7-33: A Functionally Selective Single-Chain Relaxin Analogue

Research summary. B7-33 is a synthetic single-chain peptide derived from the B-chain of human H2-relaxin. Engineered by the Hossain laboratory at the Florey Institute, it was designed to capture the anti-fibrotic activity of native H2-relaxin while simplifying production and biasing receptor signalling away from the cAMP pathway, which has been linked to undesirable off-target effects of full-length relaxin.

Molecular profile

• Sequence: B7-33 corresponds to residues 7–33 of the relaxin B-chain (single-chain construct, 27 residues)
• Approximate molecular weight: ~3.0 kDa
• Receptor target: RXFP1 (relaxin family peptide receptor 1)
• Signalling profile: Biased agonist favouring ERK1/2 phosphorylation over cAMP accumulation

Native H2-relaxin is a two-chain insulin-superfamily heterodimer connected by disulfide bonds, which makes it expensive and technically demanding to produce recombinantly. B7-33 was synthesised as a single-chain alternative containing only the B-chain core required for RXFP1 engagement, eliminating the disulfide-bridged A-chain entirely.

Mechanism of action

The relaxin family includes four endogenous peptides (relaxin, INSL3, H3-relaxin, INSL5) and four cognate receptors (RXFP1–4). RXFP1 is the principal target of H2-relaxin and is expressed in cardiac, vascular, renal, pulmonary, and reproductive tissues.

In the seminal characterisation of B7-33, Hossain and colleagues reported that the peptide retained binding affinity for RXFP1 comparable to full-length H2-relaxin but displayed biased agonism: it preferentially activated the ERK1/2 mitogen-activated protein kinase pathway downstream of RXFP1 while substantially reducing engagement of the canonical Gαs/cAMP pathway [1]. The ERK1/2 arm of RXFP1 signalling has been mechanistically tied to upregulation of matrix metalloproteinase-2 (MMP-2), the principal effector of relaxin's anti-fibrotic activity through degradation of extracellular collagen.

Preclinical research highlights

Cardiac fibrosis models. In rodent models of myocardial infarction-induced heart failure, administration of B7-33 has been reported to reduce cardiac fibrotic burden by approximately 50%, with corresponding improvements in left-ventricular function metrics. The reported effect size is comparable to that of full-length H2-relaxin in matched models, supporting the hypothesis that ERK1/2-driven MMP-2 activation is the primary fibrosis-relevant signalling arm [1].

Pulmonary and renal fibrosis. Mouse studies of bleomycin-induced lung fibrosis and chemically induced kidney fibrosis have similarly reported substantial reductions in collagen deposition with B7-33 treatment.

Oncology safety profile. A concern with full-length H2-relaxin has been its reported activity in promoting prostate cancer cell proliferation, attributed to the cAMP signalling arm. In murine prostate cancer xenograft studies, B7-33 administered at doses producing anti-fibrotic effects did not promote tumour growth, consistent with its biased-agonist profile.

Pre-eclampsia models. Rodent studies have explored B7-33 as a research candidate for studying placental vascular development. The peptide has been reported to upregulate VEGF expression in cytotrophoblasts via RXFP1 signalling, with downstream effects on trophoblast function and uterine vascular remodelling.

Lipidated B7-33. A subsequent generation of the molecule features fatty-acid conjugation to extend plasma half-life, with reported preservation of receptor activity. This work has supported continued interest in B7-33 derivatives as oral or longer-acting candidates.

Anti-fibrotic device coatings. A distinctive line of research has examined B7-33 as a coating for implantable medical devices, where local elution of the peptide reduced fibrotic capsule thickness around the implant by approximately 50% over multi-week trials in animal models.

Why the bias matters

H2-relaxin (recombinant form: serelaxin) reached Phase 3 clinical trials in acute heart failure under the name RELAX-AHF. The programme ultimately did not achieve regulatory approval, in part because the molecule's pleiotropic effects — including cAMP-driven vasodilation and tachycardia — complicated dosing and the safety profile. B7-33 represents a deliberate effort to retain the anti-fibrotic benefit while engineering out the cAMP-mediated liabilities. This pharmacological dissection is one of the more conceptually elegant examples of biased agonism in the peptide literature.

Current research status

B7-33 remains a preclinical research peptide. It is studied as:

• A tool compound for dissecting biased agonism at RXFP1
• A research candidate for cardiac, pulmonary, and renal anti-fibrotic studies
• A scaffold for further engineering, including lipidation and device-coating applications
• A reference molecule in the broader academic effort to develop next-generation relaxin-based therapeutics

It has not entered formal human clinical trials and is not approved as a therapeutic.

Key takeaways for researchers

• B7-33 is a single-chain, simplified analogue of the H2-relaxin B-chain that retains RXFP1 binding.
• It is a biased agonist favouring ERK1/2-MMP-2 signalling over cAMP accumulation.
• Anti-fibrotic effects in rodent cardiac, renal, and pulmonary models approach those of full-length H2-relaxin, with a reduced off-target profile.
• Production is substantially simpler than recombinant H2-relaxin owing to the single-chain architecture.
• All data are preclinical. B7-33 is not an approved therapeutic.

References

1. Hossain MA, Kocan M, Yao ST, et al. A single-chain derivative of the relaxin hormone is a functionally selective agonist of the G protein-coupled receptor, RXFP1. Chemical Science. 2016;7(6):3805–3819.

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This article is provided for educational and research purposes only. B7-33 is a research peptide. It is not an approved drug or therapeutic agent and is not intended for human consumption, diagnosis, treatment, cure, or prevention of any disease or condition. All work involving this peptide should be conducted by qualified personnel within an appropriate research setting and in compliance with applicable institutional and regulatory requirements.