For laboratory and research use only. Not for human consumption.
Healing

B7-33

Also known as: B7-33 relaxin, Single-chain relaxin-2 analog, H2 relaxin B-chain 7-33, Relaxin B-chain peptide

Preclinical
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Key Facts: B7-33

Category
Healing
FDA Status
Not FDA Approved
Clinical Status
Preclinical - No FDA approval and no registered human trials. The parent hormone's drug version (serelaxin) failed phase 3 in acute heart failure and was not approved. WADA: not named on the 2026 Prohibited List; as a substance with no regulatory approval for human use it falls under S0 (Non-Approved Substances), prohibited at all times.
Banned in Sport (WADA)
Not named on the WADA list, but WADA's S0 catch-all bans any substance without human therapeutic approval, which covers it. Prohibited at all times.
Administration
Intraperitoneal or subcutaneous injection (including osmotic minipumps) in rodents
Typical Dose
No established human dose
Frequency
Once to twice daily in mouse studies
Duration
1-2 weeks in most rodent studies
Also Known As
B7-33 relaxin, Single-chain relaxin-2 analog, H2 relaxin B-chain 7-33, Relaxin B-chain peptide

How does B7-33 work?

B7-33 activates RXFP1, the relaxin receptor, but in an unusual way. In cells that naturally carry RXFP1 (like fibroblasts), it preferentially switches on the pERK1/2 pathway rather than the cAMP pathway, which made it the first described functionally selective (biased) RXFP1 agonist. Its anti-fibrotic action runs through RXFP1 paired with the angiotensin II type 2 receptor, increasing MMP-2, an enzyme that breaks down collagen. In other words, it pushes scar tissue to be broken down rather than built up. Two practical caveats: it binds RXFP1 poorly in standard engineered cell lines, so its potency depends heavily on the cell type, and it is short-lived in serum (roughly 6 minutes in lab tests), which is why researchers have tried fatty-acid versions to stretch its half-life.

What does the research show?

The founding paper (Hossain et al., Chemical Science, 2016) reported that B7-33 prevented or reversed organ fibrosis and dysfunction in three rodent models of heart or lung disease with potency similar to full relaxin-2, and, unlike native relaxin, it did not promote prostate tumor growth in mice. That last point matters because relaxin has been linked to tumor growth. Later mouse work extended the findings: Devarakonda et al. (Journal of the American Heart Association, 2020) gave 0.25 mg/kg twice daily by injection after a heart attack and found smaller infarcts, better heart function, and less fibrosis at 7 days. A 2023 follow-up from the original group confirmed that B7-33 cannot be shortened further without losing receptor binding. Important context: the full hormone (serelaxin) went all the way to a large phase 3 trial in acute heart failure (RELAX-AHF-2) and failed both primary endpoints, a reminder that strong animal anti-fibrotic data in this family have not translated easily to people. For B7-33 itself there are no human studies, no human pharmacokinetics, and no human safety data.

Trial Progress:Phase III
Pre
I
II
III
IV
FDA

What doses have been studied?

Preclinical·Rodent fibrosis and heart-attack models only; never tested in humans

Note: Animal study doses may not translate directly to humans.

Typical Dosingⓘ

Community experience

Common Dose

No established human dose

Range

Not established (mouse studies used 0.25 mg/kg per dose)

Frequency

Once to twice daily in mouse studies

There is no published human dosing, pharmacokinetic, or safety data for B7-33. The very short serum half-life seen in lab testing means once-daily dosing may not keep levels up, which is why the mouse studies used twice-daily injections or continuous pumps. Any human dose discussed online is a guess.

Research Dosingⓘ

Scientific studies

Doses observed in rodent research. No human dose has been studied.

Duration

1-2 weeks in most rodent studies

Administration

Intraperitoneal or subcutaneous injection (including osmotic minipumps) in rodents

When and how is B7-33 taken?

Best Time to Take

No evidence-based timing

Twice daily or continuous in animal studies

Food Recommendation

With or without food

Why This Timing?

Mouse studies used twice-daily injections or continuous infusion because the peptide clears quickly. No human timing data exist.

What are the side effects of B7-33?

Not everyone experiences these effects. Individual responses vary based on dosage, duration, and personal factors.

  • ●No human safety data of any kind
  • ●Very short half-life in serum (about 6 minutes in vitro)
  • ●Relaxin-family signaling relaxes connective tissue; long-term effects on joints and ligaments unknown
  • ●Theoretical cancer concern from the relaxin family (B7-33 did not promote prostate tumors in mice, but this is not tested in humans)
  • ●Injection site reactions possible
  • ●Product purity unknown outside of research labs

References

Research This Peptide Further

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Frequently Asked Questions

What does B7-33 do?

B7-33 is a 27-amino-acid, single-chain peptide cut down from the B-chain of human relaxin-2, the hormone that loosens tissue during pregnancy. Relaxin itself has strong anti-scarring effects but is a complex two-chain molecule that is hard and expensive to make, so an Australian group (Hossain and colleagues, 2016) designed B7-33 as a simpler version. In rodent models it reduced heart and lung fibrosis about as well as full relaxin. It has never been tested in humans.

How does B7-33 work?

B7-33 activates RXFP1, the relaxin receptor, but in an unusual way. In cells that naturally carry RXFP1 (like fibroblasts), it preferentially switches on the pERK1/2 pathway rather than the cAMP pathway, which made it the first described functionally selective (biased) RXFP1 agonist. Its anti-fibrotic action runs through RXFP1 paired with the angiotensin II type 2 receptor, increasing MMP-2, an enzyme that breaks down collagen. In other words, it pushes scar tissue to be broken down rather than built up. Two practical caveats: it binds RXFP1 poorly in standard engineered cell lines, so its potency depends heavily on the cell type, and it is short-lived in serum (roughly 6 minutes in lab tests), which is why researchers have tried fatty-acid versions to stretch its half-life.

Is B7-33 FDA approved?

No, B7-33 is not currently FDA approved. Current status: Preclinical - No FDA approval and no registered human trials. The parent hormone's drug version (serelaxin) failed phase 3 in acute heart failure and was not approved. WADA: not named on the 2026 Prohibited List; as a substance with no regulatory approval for human use it falls under S0 (Non-Approved Substances), prohibited at all times.

Is B7-33 banned in sport?

Not named on the WADA list, but WADA's S0 catch-all bans any substance without human therapeutic approval, which covers it. Prohibited at all times.

What are the side effects of B7-33?

Reported side effects include: No human safety data of any kind, Very short half-life in serum (about 6 minutes in vitro), Relaxin-family signaling relaxes connective tissue; long-term effects on joints and ligaments unknown, Theoretical cancer concern from the relaxin family (B7-33 did not promote prostate tumors in mice, but this is not tested in humans), Injection site reactions possible. Individual responses vary based on dosage, duration, and personal health factors.

What is the typical dose of B7-33?

Community-reported common dose: No established human dose (Once to twice daily in mouse studies). Range: Not established (mouse studies used 0.25 mg/kg per dose). Administration: Intraperitoneal or subcutaneous injection (including osmotic minipumps) in rodents. Community-reported doses. Not medical advice. Consult healthcare provider.

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Healing

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