Also known as: Single-chain Relaxin Analog, B-chain Relaxin-2
Half-life: ~2-3 hours
Last reviewed: · Published:
B7-33 is a synthetic single-chain peptide derived from the B-chain of human relaxin-2 (hormone-2). Relaxin-2 is a member of the insulin/relaxin superfamily and acts at the RXFP1 receptor to produce vasodilation, anti-fibrotic effects in heart, lung and kidney, and remodeling of connective tissue. Native relaxin-2 is a two-chain heterodimer that is difficult and expensive to manufacture; B7-33 is a 33-amino-acid single-chain analog designed to retain the receptor-binding and signaling properties of the full hormone in a much simpler peptide that can be made by standard solid-phase synthesis.
In preclinical studies, B7-33 activates RXFP1 and reproduces the key anti-fibrotic and tissue-remodeling effects of native relaxin-2, including reduced cardiac fibrosis after myocardial infarction in rodent models, reduced lung fibrosis in bleomycin-injured rats, and protection against renal fibrosis. Because B7-33 activates the cGMP signaling arm of RXFP1 but with reduced bias toward cAMP — the arm associated with relaxin's antitumor and angiogenic effects — it has a cleaner safety profile in cancer-adjacent research contexts.
B7-33 is in a different category from the typical "muscle growth" peptides: its primary mechanism is anti-fibrotic and tissue-remodeling rather than directly anabolic. It is included in this category here because vendors typically place it alongside other connective-tissue and recovery peptides, and because its anti-fibrotic action is relevant for musculoskeletal recovery and scar prevention. Human safety data is essentially absent.
B7-33 was designed by the team of Mohammed Akhter Hossain and Ross Bathgate at the Florey Institute (Melbourne, Australia) and reported in 2016. The design strategy was specifically to create a single-chain relaxin mimetic suitable for chemical synthesis and clinical translation, addressing the manufacturing barriers that had hampered the clinical development of native serelaxin (which was tested in acute heart failure as RELAX-AHF but did not show benefit in the larger RELAX-AHF-2 confirmatory trial).
Preclinical animal tolerability of B7-33 has been good, with no significant adverse events reported in published studies. Human tolerability is essentially unknown. By analogy with native relaxin-2 (serelaxin), the most likely acute effects would be vasodilation-related (hypotension, headache, flushing). Because relaxin family hormones have effects on reproductive tissues, B7-33 should be approached with particular caution by women of reproductive age and those who are pregnant.
Dose Range
1-2 mg
Frequency
Daily (SubQ or IV in preclinical work)
Duration
Per research protocol
Dosing information is for educational purposes only. Consult a healthcare professional before using any peptide.
Typical Vial Size
2 mg
Water Type
Bacteriostatic water (BAC water)
Mixing Volume
1-2 mL
Half-Life
~2-3 hours
Molecular Weight
~3,700 Da
Store reconstituted vial refrigerated at 2-8°C. Use within 14-21 days. Limited human stability data available.
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FDA Status
Not FDA approved.
Legal Status
Unregulated research chemical.
USA
Not approvedResearch-only; native serelaxin clinical program discontinued after RELAX-AHF-2
EU
Not approvedNot authorized as medicinal product
UK
Not approvedClassified as research chemical
Australia
Not approvedDespite Florey Institute origin, not authorized for human use
Canada
Not approvedNot authorized for human use
Hossain MA, Kocan M, Yao ST, Royce SG, Nair VB, Siwek C, Patil NA, Harrison IP, Rosengren KJ, Selemidis S, Summers RJ, Wade JD, Bathgate RA, Samuel CS
Chemical Science (2016)
Original report of B7-33 as a synthetic single-chain relaxin mimetic with biased agonism at RXFP1 favoring the cGMP pathway, showing anti-fibrotic efficacy in mouse models of cardiac and renal disease.
View Study →Devarakonda T, Salloum FN
British Journal of Pharmacology (2020)
Confirmatory study in larger animal models demonstrating that B7-33 reproduces the cardiac functional benefits of native relaxin in both rodent and porcine models of myocardial infarction.
View Study →Metra M, Teerlink JR, Cotter G, Davison BA, et al.
New England Journal of Medicine (2019)
Large confirmatory trial of native serelaxin in acute heart failure that failed to replicate the benefit observed in the earlier RELAX-AHF study, motivating continued search for optimized relaxin analogs like B7-33.
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