What it is
B7-33 is a piece of a hormone. Relaxin-2 is the human hormone best known for loosening ligaments and remodelling tissue in pregnancy; it also widens blood vessels and breaks down the collagen that builds up as scar tissue. Like insulin, it is built from two chains, A and B, held together by disulfide bridges — which makes it awkward and expensive to manufacture.
B7-33 keeps only the B chain — its name refers to residues 7 to 33 of that chain — as one short, linear peptide. It was designed at the Florey Institute and Monash University in Melbourne and first reported in Chemical Science in 2016.
What it was designed to do
Relaxin acts through a receptor called RXFP1, and that receptor can send more than one signal inside the cell. The full hormone strongly triggers one of them, cAMP, and the designers linked that signal to reports that relaxin can promote tumour growth. B7-33 binds the same receptor but preferentially triggers a different signal, ERK — making it, in the authors' words, the first "functionally selective" agonist of that receptor.
The 2016 paper reported two results that set the direction for everything since:
- In three rodent models of heart or lung disease, B7-33 prevented or reversed organ fibrosis with potency similar to the full hormone.
- In mice, it did not promote prostate tumour growth, where relaxin-2 did.
What the later papers add
PubMed holds 11 records for B7-33 with relaxin, from 2016 to 2025. Every one is chemistry, cell or animal work. The most-quoted:
| Year | Model | Finding |
|---|---|---|
| 2017 | Blood vessels, laboratory | Replicated the vessel-protecting effects of relaxin-2 |
| 2019 | Mouse implants | A coating releasing B7-33 reduced the scar capsule that forms around implanted material |
| 2020 | Mouse heart attack | Infarct size 22.0% vs 45.3% on vehicle at 24 hours; heart pumping better preserved at day 7 |
| 2023 | Heart fibrosis model | Reduced left-ventricular fibrosis faster than the blood-pressure drug perindopril |
| 2025 | Nanoparticles | Linked to particles taken up by immune cells, to extend its action and allow oral dosing |
ClinicalTrials.gov returned zero studies naming B7-33 on 2026-09-23. No paper describes giving it to a person.
The problem with the parent
There is one large human result in this family, and it is a caution. Serelaxin, the full recombinant relaxin-2, went through a phase 3 trial in 6,545 people hospitalised with acute heart failure (New England Journal of Medicine, 2019). Cardiovascular death by day 180: 8.7% on serelaxin, 8.9% on placebo. Worsening heart failure, all-cause death and rehospitalisation did not differ either.
That trial tested a different molecule for a different purpose, so it says nothing directly about B7-33. What it shows is the distance between a strong animal result in this pathway and a result in people — a distance the full hormone did not cross.
What it is sold for
Online, B7-33 is sold beside BPC-157 and TB-500 for injury and tissue healing. It is an unrelated molecule — a relaxin fragment, not a stomach-protein or thymosin fragment — and the tendon and injury claims have no published study at all, animal or human. The research use is organ fibrosis, and even there the record stops at rodents.
Where it stands
Preclinical only. No approval, no application (Drugs@FDA returned nothing on 2026-09-23, with a control query returning 38 applications for desmopressin), no registered trial.
Related reading
For the healing claims it is sold beside, see peptides for healing and peptides for inflammation. For another receptor-selective peptide designed to keep one signal and drop another, see ecnoglutide, which is built the same way at the GLP-1 receptor and has reached approval.