B7-33

Overview

B7-33 is a synthetic peptide that does something serelaxin — the recombinant form of human relaxin-2 — has struggled to do economically: activate the RXFP1 receptor in a simpler, cheaper, and more manufacturable package. Where serelaxin requires a complex two-chain protein structure with three disulfide bridges, B7-33 is a single-chain analog derived from relaxin-2's B-chain that hits the same receptor and produces many of the same downstream effects.[1]

The research interest here is real. Relaxin-2 has decades of literature behind it showing anti-fibrotic, vasodilatory, and cardioprotective properties. The problem was always the molecule itself — difficult to synthesize at scale, short-lived in circulation, and expensive to produce. B7-33 was designed to solve that manufacturing problem without losing the biology that made relaxin-2 worth pursuing in the first place.[2]

That said, B7-33 is strictly a research compound. Every data point on this page comes from animal models or in-vitro work. There are no human clinical trials. If you're trying to understand what this peptide does and where the science stands, you're in the right place — but this is not a compound with any approved clinical application.

Mechanism of Action

Selective RXFP1 agonist; activates ERK1/2 signaling and nitric oxide synthesis

To understand B7-33, you need to understand what RXFP1 does — and why selectively activating it matters.

RXFP1 (relaxin family peptide receptor 1) is a G protein-coupled receptor expressed in the heart, blood vessels, kidneys, lungs, and uterus. When native human relaxin-2 binds RXFP1, it triggers a broad signaling cascade — cAMP production, nitric oxide synthesis, and ERK1/2 (extracellular signal-regulated kinase 1/2) phosphorylation. The result downstream is vasodilation, reduced collagen deposition, and suppressed inflammatory fibrosis.[2]

B7-33 is a functionally selective agonist, meaning it doesn't activate the full signaling suite that native relaxin-2 does. It preferentially drives ERK1/2 phosphorylation at RXFP1 while producing less cAMP signaling than serelaxin.[3] Whether that selectivity is an advantage or a limitation depends on what you're trying to achieve — for anti-fibrotic and vasoprotective applications, the ERK1/2 pathway appears to be the primary driver, so the selectivity may actually sharpen the effect rather than blunt it.

The nitric oxide component matters too. B7-33 stimulates nitric oxide synthesis in vascular tissue, which drives vasodilation — the same mechanism responsible for serelaxin's rapid blood pressure-lowering effects in early heart failure trials.[2] In rat studies, a single bolus injection of B7-33 at 13.3 μg/kg produced acute hemodynamic effects comparable to serelaxin, including reduced renal vascular resistance.[2]

One practical limitation: the peptide clears quickly. In-vitro serum stability studies show a short half-life, which limits its utility as a therapeutic without structural modification. A 2023 study from the same research group addressed this by attaching a lipid chain to B7-33, producing a derivative with meaningfully improved serum stability while preserving RXFP1 activity — though this lipidated version remains experimental.[4]

What the Evidence Shows

Every study on B7-33 is preclinical. That's not a caveat buried in the fine print — it's the defining fact about where this compound stands. Here's what the animal data actually shows.

The 2017 study that established B7-33's vascular profile compared it directly to serelaxin in male Wistar rats.[2] A bolus IV injection of B7-33 at 13.3 μg/kg produced acute reductions in renal vascular resistance and increased renal blood flow — effects that matched serelaxin's profile. The study also showed B7-33 stimulated nitric oxide production in isolated mesenteric artery rings in a concentration-dependent manner. This was the first evidence that the single-chain analog could replicate the vasoprotective functions of the much more complex parent molecule.

The most clinically relevant preclinical data comes from a 2020 study published in the Journal of the American Heart Association.[3] Researchers induced myocardial infarction in mice and treated them with B7-33 at 0.5 mg/kg/day — B7-33 dosing has not been established in humans; animal studies have employed doses in experimental models, but no clinical dosing recommendations are available — via osmotic minipump. Compared to vehicle-treated controls, B7-33-treated mice showed reduced infarct size, less adverse cardiac remodeling, and preserved cardiac function at 4 weeks post-infarction. The mechanism tracked with ERK1/2 activation in cardiac tissue. These results parallel what had previously been shown with serelaxin in similar models, which is the point — B7-33 appears to deliver the same biology through a simpler molecule.

A 2019 study took a different angle entirely.[1] Researchers embedded B7-33 into polymer coatings on implantable devices and tested whether sustained local release could reduce the foreign body response — the fibrotic capsule that forms around implants and eventually blocks their function. The B7-33-releasing coatings reduced fibrotic encapsulation compared to controls. This is a niche but genuinely interesting application: using B7-33 not as a systemic drug but as a local anti-fibrotic agent at an implant surface.

A 2021 FASEB Journal study used B7-33 as part of a methodology paper evaluating a novel second-harmonic generation imaging platform for quantifying renal fibrosis.[5] B7-33 served as the anti-fibrotic intervention in the model. While this study was primarily validating the imaging technology rather than B7-33 itself, it provides additional evidence of B7-33's anti-fibrotic activity in renal tissue.

Side Effects
There is no human safety data for B7-33. That's the honest answer.

In the animal studies conducted to date, no significant adverse effects were reported at the doses tested — but rodent tolerability data tells you very little about what a human would experience. The vasoprotective mechanism (nitric oxide-mediated vasodilation) raises an obvious theoretical concern: blood pressure reduction. Serelaxin's clinical trials documented hypotension as a dose-limiting consideration, and B7-33 produces the same vasodilatory effects in animal models.[2]

Beyond that, the side effect profile in humans is genuinely unknown. Anyone claiming otherwise is speculating.