B7-33
A short, single-chain relaxin-2 mimic designed to be cheaper to make than serelaxin. Anti-fibrotic and heart-protective in rodents; never given to people.
B7-33 is a synthetic single-chain peptide modelled on the B-chain of human relaxin-2, designed as a simpler alternative to the two-chain recombinant hormone serelaxin. It activates relaxin family peptide receptor 1, preferentially through ERK signalling. In rats it reproduced serelaxin's vascular effects, in mice it reduced infarct size and adverse remodelling after heart attack, and coatings releasing it reduced fibrotic capsules around implants. No study has given it to people. It is not a drug.
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Recommended Dosing
No dosing data yet
This page has no dosing data yet.
Mechanism of action
A functionally selective agonist of RXFP1, the relaxin-2 receptor, that preferentially activates ERK (2020); it enhanced endothelium-dependent relaxation in rat mesenteric arteries as serelaxin did (2017).
Primary uses
- Research: cardiac remodelling and fibrosis (mice)
- Research: vascular function (rats)
- Research: anti-fibrotic implant coatings
Typical dosing
No human dose: the studies read used rats and mice.
Educational information, not medical advice. The doses on this page come from approved labels, published studies and, where no study exists, amounts reported by users. None is a recommendation for you. An unapproved compound has no established safe or effective human dose, and products sold for “research use only” are not made or tested for people. Talk to a doctor before acting on anything on this site, including before you start, stop or change any medicine or dose. Full medical disclaimer.
Regulatory status
Not a medicine: animal and laboratory studies only. Drugs@FDA and the EMA medicines list hold no record (searched October 28, 2026).
Status as of this entry's last review, October 29, 2026. Approvals, compounding rules and sport rules change; the regulatory tracker is checked more often.
What we checked
B7-33 is a laboratory relaxin mimic; this section records what can be checked.
Identity, computed
| Design | single-chain analogue of the relaxin-2 B-chain (2019 review; 2020 study) |
| Target | relaxin family peptide receptor 1 (RXFP1) |
No sequence, formula or weight is given: the abstracts read describe its design but not its full sequence, and PubChem returned no record for the name.
Regulatory status, read October 28, 2026
Drugs@FDA: 0 records for “B7-33”. EMA medicines list: 0 records for “B7-33”. Run for semaglutide, the same queries return 6 and 5 records (and the registry search 80 trials), so a zero here is an absence, not a broken search. Not named on the WADA 2026 and 2027 Prohibited Lists. As a substance with no current approval for human therapeutic use, it falls under section S0 (non-approved substances), which is prohibited at all times; the lists were read on October 28, 2026.
What the studies show
The design paper reported that B7-33 bound RXFP1, favoured ERK over cAMP signalling, and prevented or reversed fibrosis in three rodent models of heart or lung disease about as potently as relaxin (Hossain et al., 2016).
In rats a single injection improved endothelium-dependent relaxation of the mesenteric artery as serelaxin did (Marshall et al., 2017), and in mice after a simulated heart attack it reduced infarct size and preserved heart function (Devarakonda et al., 2020).
In mouse cardiomyopathy and kidney-obstruction models it reduced fibrosis where an ACE inhibitor did not (Alam et al., 2023; Bhuiyan et al., 2021), coatings releasing it produced thinner capsules around implants (Welch et al., 2019), and injected into the brain it briefly reduced pain behaviour (Abboud et al., 2021).
What we looked for and did not find
No study in people: no clinical trial, no registry record, no human pharmacokinetics and no human dose. That is why this entry stays a stub rather than a mid-tier page, which on this site requires at least one human study of the molecule itself. Trials of the two-chain hormone it mimics, serelaxin, are on that entry.
| Source | Query | Results, October 28, 2026 | What it was |
|---|---|---|---|
| PubMed | "B7-33"[tiab] | 15 | |
| ClinicalTrials.gov | B7-33 | 0 | |
| Drugs@FDA (openFDA) | B7-33 | 0 | |
| EMA medicines list | B7-33 | 0 |
What the studies show
B7-33 is a synthetic single-chain peptide modelled on the B-chain of human relaxin-2 and built to be cheaper to make than the recombinant hormone. Every study below is in rats, mice or cultured cells; the peptide has never been given to a person, and no trial of it is registered.
Animal · 2016
Designed as a one-chain relaxin
Relaxin-2's two-chain, disulfide-rich structure makes it costly to produce, and its strong cAMP signalling has been linked to tumour-promoting effects. The authors built a B-chain-only analogue, B7-33, which bound RXFP1 and favoured ERK signalling over cAMP, and reported that it prevented or reversed fibrosis and organ dysfunction in three rodent models of heart or lung disease about as potently as relaxin itself.
Animal · 2017
Blood vessels in rats and mouse tissue
A single tail-vein injection of 13.3 µg/kg improved bradykinin-driven relaxation of the rat mesenteric artery as serelaxin did at an equivalent dose, with no effect in the renal artery or aorta. In mouse arteries incubated with placental trophoblast medium, it prevented the endothelial dysfunction that medium causes.
Animal · 2020
Heart attack model in mice
After 30 minutes of coronary artery ligation and reperfusion, B7-33 cut infarct size (22.0% against 45.3%) and preserved heart function at 24 hours and 7 days. In isolated heart muscle cells it improved survival after simulated ischaemia and reduced a marker of endoplasmic reticulum stress.
Animal · 2023
Compared with an ACE inhibitor in mice
In mice with isoprenaline-induced cardiomyopathy, B7-33 at 0.25 mg/kg/day and relaxin equally reduced left ventricular fibrosis, inflammation and cell enlargement and restored blood vessel density, while perindopril lowered blood pressure and inflammation but not fibrosis.
Animal · 2021
Kidney fibrosis, measured a new way
In mice with an obstructed ureter, a stain-free imaging platform showed that relaxin or B7-33 at 0.25 mg/kg/day for a week prevented fibrosis with thinner collagen fibres and less cross-linking, which conventional staining could not resolve.
Animal · 2019
Released from an implant coating
A biodegradable polymer coating released B7-33 steadily and the released peptide stayed active at RXFP1 in a reporter assay. Coated implants in mice developed capsules about 49% thinner over six weeks than uncoated controls.
Animal · 2021
Injected into the brain of mice in pain
Injected into a brain ventricle of mice with inflamed paws, B7-33 or relaxin briefly reduced mechanical and heat sensitivity, measurable only 30 minutes after injection; an RXFP1 blocker cancelled the mechanical effect but not the heat effect.
References
- [pubmed] Hossain MA, et al. "A single-chain derivative of the relaxin hormone is a functionally selective agonist of the G protein-coupled receptor, RXFP1." Chem Sci, 2016;7(6):3805-3819. PMID: 30155023.
- [pubmed] Marshall SA, et al. "B7-33 replicates the vasoprotective functions of human relaxin-2 (serelaxin)." Eur J Pharmacol, 2017;807:190-197. PMID: 28478069.
- [pubmed] Devarakonda T, et al. "B7-33, a Functionally Selective Relaxin Receptor 1 Agonist, Attenuates Myocardial Infarction-Related Adverse Cardiac Remodeling in Mice." J Am Heart Assoc, 2020;9(8):e015748. PMID: 32295457.
- [pubmed] Alam F, et al. "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." Biomed Pharmacother, 2023;160:114370. PMID: 36753958.
- [pubmed] Bhuiyan S, et al. "Assessment of renal fibrosis and anti-fibrotic agents using a novel diagnostic and stain-free second-harmonic generation platform." FASEB J, 2021;35(5):e21595. PMID: 33908676.
- [pubmed] Welch NG, et al. "Coatings Releasing the Relaxin Peptide Analogue B7-33 Reduce Fibrotic Encapsulation." ACS Appl Mater Interfaces, 2019;11(49):45511-45519. PMID: 31713411.
- [pubmed] Abboud C, et al. "Analgesic effect of central relaxin receptor activation on persistent inflammatory pain in mice: behavioral and neurochemical data." Pain Rep, 2021;6(2):e937. PMID: 34159282.
- [review] Praveen P, et al. "Single chain peptide agonists of relaxin receptors." Mol Cell Endocrinol, 2019;487:34-39. PMID: 30641102.
- [pubmed] Somanader-Livera DVN, et al. "Immune cell uptake of glycinated nanoparticles conjugated to anti-fibrotic peptides enables their prolonged activity and oral administration." J Biomed Sci, 2025;32(1):104. PMID: 41382190.
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This entry is for educational purposes only and does not constitute medical advice. Dosing information reflects published regulatory or research data and is not a recommendation. Many compounds described here are not approved for human use in the United States. Consult a licensed medical professional before considering any peptide therapy.