B7-33
Not medical advice. PepTutor summarizes fallible research and community signal for trained practitioners; some compounds are research-only, unapproved, controlled, jurisdiction-dependent, or labeled not for human consumption.
Preclinical anti-fibrotic relaxin analog for cardiac, renal, hepatic, and pulmonary scarring questions; mainly interesting for organ-protection and tissue-remodeling experiments, not validated human recovery outcomes.
Preclinical studies have not surfaced a major toxicity signal, but human safety is untested and the practical risk is unknown rather than proven low.
Preclinical anti-fibrotic relaxin analog for cardiac, renal, hepatic, and pulmonary scarring questions; mainly interesting for organ-protection and tissue-remodeling experiments, not validated human recovery outcomes.
Unknown human dose-response, sourcing/product-identity risk, theoretical additive blood-pressure lowering with antihypertensives, and localized injection-site reactions in sparse community discussion.
Very niche research peptide: the mechanism is unusually specific, but cost, sourcing uncertainty, and no human outcome data make it a high-uncertainty bet.
The strongest signal is measured mouse cardioprotection and anti-fibrotic biology. Human efficacy is still unproven, and community confidence comes from mechanism transfer rather than repeated user logs.
No established class or mechanism conflicts documented; avoid stacking with other vasodilatory peptides (serelaxin, if available) due to redundant RXFP1 targeting with no additive benefit.
Intro
B7-33 is a single-chain synthetic peptide derived from the B-chain of human relaxin-2 (H2 relaxin), a naturally occurring hormone involved in cardiovascular regulation, tissue remodeling, and organ protection.
It was developed at Monash University in Australia by researchers including Mohammed Akhter Hossain and Ross Bathgate as a functionally selective alternative to serelaxin — the recombinant form of H2 relaxin that showed promise in cardiovascular disease but failed late-stage trials partly due to its vasodilatory side-effect profile.
The core engineering insight behind B7-33 is biased agonism at RXFP1 (relaxin family peptide receptor 1). Full relaxin-2 activates two primary intracellular pathways: the cAMP-PKA cascade, which drives systemic vasodilation and blood pressure reduction, and the ERK1/2 (extracellular signal-regulated kinase 1/2) pathway, which mediates anti-fibrotic and cytoprotective effects. B7-33 selectively engages ERK1/2 while largely bypassing cAMP-PKA. The practical result is anti-fibrotic and organ-protective activity without the hemodynamic effects that made serelaxin difficult to use clinically. The 2017 European Journal of Pharmacology paper by Marshall et al. confirmed that B7-33 replicates serelaxin's vasoprotective functions in isolated vessel preparations despite this pathway selectivity.
The highest-quality preclinical data comes from Devarakonda et al. (J Am Heart Assoc, 2020), which showed B7-33 reduced infarct size by approximately 51% (from 45.32% to 21.99%) and preserved cardiac fractional shortening (29% vs 20% vehicle at 7 days post-MI) in a mouse ischemia-reperfusion model. These numbers drove early community interest — particularly among enhanced bodybuilders concerned about long-term cardiac consequences of anabolic steroid use, and among longevity-focused self-experimenters interested in organ fibrosis reversal.
As of 2026, B7-33 remains strictly preclinical. No phase 1 or phase 2 human trials have been published. Community use exists but is extremely limited, and genuine first-person experience reports are absent from publicly indexed sources. The evidence base is almost entirely mouse model data, and any human application involves significant extrapolation.
Observed Effects
In animal models (all evidence):
The most reproducible finding is cardioprotection after ischemia-reperfusion injury. In adult male CD1 mice subjected to 30-minute LAD ligation followed by reperfusion, B7-33 given at onset of reperfusion reduced infarct size from 45.32% to 21.99% — approximately a 51% reduction (P=0.02, Devarakonda et al. 2020). Cardiac fractional shortening was preserved at 29% vs 23% at 24 hours post-MI (P=0.02), and the functional advantage widened at 7 days: 29% B7-33 vs 20% vehicle, indicating ongoing protective remodeling rather than just acute rescue.
In vitro, primary cardiomyocytes treated with B7-33 at 50 nmol/L and 100 nmol/L showed improved cell survival after simulated ischemia-reoxygenation injury. At 100 nmol/L, B7-33 reduced tunicamycin-induced GRP78 upregulation (an endoplasmic reticulum stress marker) through an ERK1/2-dependent mechanism, suggesting ER stress attenuation as a component of its cardioprotective action.
Anti-fibrotic effects have been demonstrated across cardiac, renal, pulmonary, and hepatic tissue in preclinical models via MMP-2 upregulation and TGF-β1 pathway suppression. Collagen deposition is reduced and fibroblast-to-myofibroblast transition is inhibited. Specific quantitative endpoints for non-cardiac organ fibrosis were not available in the reviewed sources.
In animal models of kidney disease, B7-33 has been reported to increase renal blood flow, reduce pro-inflammatory protein expression, and improve filtration capacity.
Community-reported effects (human — anecdotal extrapolation only):
No genuine first-person human experience reports were found in the reviewed sources. Community expectations of effect are almost entirely derived from preclinical animal data. The only side effect consistently mentioned in community guides is minor localized injection-site reactions. Human efficacy, onset timing, dose-response, and long-term effects are unknown.
Field Reports
Genuine first-person B7-33 use reports are essentially absent from publicly indexed community sources as of 2026. That near-total absence matters: the article should be read as preclinical/mechanistic intelligence, not as a compound with settled real-world protocol norms.
What passes for community experience is mostly mechanism reasoning from preclinical data: "the mouse studies showed X, therefore it might do X in humans." That inference is understandable, but B7-33 has a larger translation gap than many research peptides because its clinical precursor, serelaxin, failed a phase 3 heart-failure trial. B7-33's pathway selectivity may matter, but it has not solved the human translation question yet.
The only practical observations in community sources are: - Minor localized injection-site reactions (redness, mild soreness) — the only side effect consistently mentioned - No clear reports of hypotension or blood pressure changes, consistent with the functional selectivity mechanism - No hormonal disruption reported, which fits the lack of androgenic, estrogenic, or GH-axis activity
The honest field read is narrow: low observed signal of harm in a tiny user base, no meaningful human efficacy confirmation, and no reliable way to separate a true non-response from a bad product or weak endpoint tracking.
Community Consensus
B7-33 occupies a narrow corner of the research peptide space, adopted mostly by two groups: enhanced athletes thinking about long-term organ remodeling, and longevity-focused self-experimenters interested in fibrosis reversal.
Community attention began after the 2020 mouse cardiac-remodeling study; the ~51% infarct-size reduction number is the fact that made the compound stand out.
The strongest community use narrative is harm reduction for AAS users. Oral anabolic steroids can create repeated hepatic stress, and long-term supraphysiologic androgen exposure is associated with cardiac remodeling. B7-33 is discussed as a fibrosis-focused support tool, especially alongside broader tissue-repair peptides, but that is still a mechanistic hypothesis rather than a proven human organ-protection protocol.
The broader peptide community's awareness of B7-33 is limited compared with established compounds. No stable practitioner consensus exists, and the common dosing ranges come mostly from research-chemical protocol norms rather than human dose-finding. That makes the community posture narrow but curious: the biology is coherent, the cardiac mouse data are interesting, and the human field report layer is still too thin to anchor confidence.
A real naming-confusion problem exists in low-quality market content: B7-33 the relaxin peptide is sometimes blurred with B7-H3, an immune checkpoint protein involved in cancer immunology. These are different molecules. Users sourcing B7-33 should verify they are purchasing the relaxin analog (amino acid sequence matching the B-chain of human relaxin-2), not a biologics immune-checkpoint product. This confusion is more likely to produce a non-functional product than a pharmacological danger, but it is a genuine sourcing risk for an already thin market.
Risks & Monitoring
The adverse effect profile in humans is essentially unknown. All safety data comes from preclinical rodent studies and in vitro work.
The key advantage vs serelaxin — absence of significant vasodilation/hypotension — is mechanistically well-supported (ERK bias, minimal cAMP activation) but has not been confirmed in human subjects.
From preclinical data, B7-33 appears well-tolerated. No hepatotoxicity, renal toxicity, or immunogenicity signals were identified. The compound lacks the receptor-binding profile of growth factors (no GF receptor stimulation), which is why community guides have claimed it does not promote tumor growth — though this negative claim has not been formally tested.
From limited community experience: minor, localized injection-site reactions are the only consistently reported side effect. No cardiovascular events, no hormonal disruption, no metabolic changes have been reported — but the community use base is too small to draw meaningful safety conclusions.
A meaningful information risk exists around naming confusion: low-quality market content sometimes conflates B7-33 the relaxin peptide with B7-H3, an entirely different immune checkpoint protein. Users relying on low-quality market content may receive mislabeled or incorrect products. This is a product quality and identity risk rather than a pharmacological one.
For Women
Monitoring Panels
REQUIRED is a real safety gate. RECOMMENDED is the prudent default. OPTIONAL covers symptoms, risk factors, or tighter tracking.
Useful as a baseline for users trying to track organ-protection claims, but the article does not identify a known B7-33 toxicity that makes routine labs a safety gate.
B7-33 is being explored specifically for renal protection; monitoring renal function during use provides the clearest signal of whether the compound is having its intended effect in users with impaired kidneys.
Users specifically seeking cardiac remodeling protection (post-AAS cardiomyopathy or post-MI remodeling) should document baseline cardiac function. NT-proBNP is a practical biomarker proxy for users without access to echocardiography.
B7-33's anti-inflammatory mechanism (TGF-β1 modulation, reduced fibroblast activation) could in principle lower systemic inflammatory markers in users with active fibrotic disease. This would serve as a proxy efficacy signal in the absence of imaging.
Avoid With
Do not combine B7-33 with the following. Sorted highest-severity first.
Why:Both activate RXFP1. Serelaxin will drive cAMP-mediated vasodilation that B7-33 was specifically engineered to avoid. Combining them negates B7-33's functional selectivity advantage and introduces the hypotensive risk profile of full relaxin-2.
What to do:Serelaxin is not commercially available for self-experimenters; this conflict is theoretical. If serelaxin ever becomes accessible, do not combine.
Why:B7-33 has minor vasodilatory activity via residual NO production even at low cAMP activation. Combined with antihypertensive medications, additive blood pressure lowering is possible, though B7-33's effect is likely small.
What to do:Users on antihypertensive therapy should monitor blood pressure when initiating B7-33.
Protocols By Goal
AAS-related organ protection (cardiac fibrosis, liver fibrosis): This is the primary community use case.
Enhanced bodybuilders running long oral AAS cycles or heavy injectable AAS programs discuss B7-33 as a way to target downstream fibrosis rather than acute liver stress. Typical approach: 150-300 mcg/day SC during or after AAS cycles, 6-8 weeks, stacked with TUDCA or NAC for additional hepatoprotection. This is a harm-reduction hypothesis, not proven reversal of AAS-related organ remodeling in humans.
Post-MI / cardiac remodeling recovery: The preclinical rationale is strongest here. Protocol: 250 mcg SC every other day for 6 weeks. The Devarakonda 2020 paper used twice-daily dosing in mice; community protocols are less frequent. Stacked with BPC-157 for broader cardiac tissue repair. This use case is theoretical in humans — no one has verified cardiac remodeling benefit in human post-MI contexts.
General tissue repair and injury recovery: Athletes using B7-33 for muscle tear recovery or post-surgical scar reduction: 100-200 mcg SC daily for 4 weeks. Stacked with BPC-157 and GHK-Cu for synergistic ECM repair effects. The anti-fibrotic mechanism targets scar tissue specifically — the rationale being that flexible, healthy tissue heals better than fibrotic replacement tissue.
Chronic fibrotic disease (liver cirrhosis, pulmonary fibrosis, CKD): The most ambitious application. 100-250 mcg daily for 6-8 weeks, KPV + TB-500 stack for enhanced tissue repair. No human data supports this application — it is entirely mechanistic extrapolation from rodent fibrosis models.
Dosing Details
All dosing protocols are community-derived and not validated in human trials.
Observed research-use ranges: introductory reports cluster around 150 mcg/day subcutaneous, standard reports around 300 mcg/day, and intensive reports around 500 mcg/day. Frequency is usually once daily, often in the evening to mirror natural relaxin secretion patterns, though this has not been validated for the synthetic analog.
Anti-fibrotic reports: 100-250 mcg SC or IV daily appears in anti-fibrotic protocol discussion, usually in 6-8 week blocks with time off before resuming. IV use is not appropriate outside clinical supervision given the lack of human PK data.
Cardiovascular protection reports: 250 mcg SC or IV every other day appears in cardiovascular-protection discussion, usually around 6-week cycles.
Wound healing / acute recovery reports: 100-200 mcg SC daily for about 4 weeks appears in short acute-use discussion.
Preparation and storage details are not standardized and should not be treated as reader-specific injection or compounding guidance. Subcutaneous injection is the community-standard route; IV administration belongs only in clinician-supervised contexts.
Stacks & Alternatives
The most frequently cited B7-33 stack partner. BPC-157 promotes angiogenesis and broad tissue healing via multiple pathways; B7-33 contributes anti-fibrotic and collagen remodeling specificity. Together they cover wound healing comprehensively — BPC-157 for regeneration, B7-33 for preventing fibrotic replacement. Used in cardiac recovery and athletic injury contexts.
GHK-Cu supports extracellular matrix remodeling, collagen synthesis regulation, and vascular health. Stacked with B7-33 for vascular protection protocols and wound healing — GHK-Cu provides the 'build healthy matrix' signal; B7-33 provides the 'break down excess scar' signal. Complementary ECM regulation.
TB-500 (thymosin beta-4) promotes tissue repair and reduces inflammation via actin regulation. Combined with B7-33 and BPC-157 as part of a tissue repair stack. TB-500's systemic healing effects pair with B7-33's fibrosis-specific mechanism.
KPV (Lys-Pro-Val) is an anti-inflammatory tripeptide. Used with B7-33 in fibrotic disease protocols where inflammation drives ongoing organ damage. KPV handles the acute inflammatory component; B7-33 handles the downstream fibrotic remodeling.
For AAS-user organ protection protocols. TUDCA and NAC provide hepatoprotection from hepatotoxic orals; B7-33 addresses the downstream fibrotic consequences of chronic liver inflammation. These address different parts of the same problem — chemical stress vs. fibrotic scarring.
Cardiogen is a cardiac tetrapeptide that inhibits fibroblast formation in heart tissue. The practical comparison is anti-fibrotic cardiac support from a different mechanism. Some users stack both for additive cardiac remodeling coverage, though there is no stacking data to support this combination specifically.
Alternatives
Stack Cost
Minimal stack capacity cost — no hormonal, hepatic, or CNS lane involvement; primary obligation is route logistics (subcutaneous injection) and product identity verification.
Daily subcutaneous injection is commonly reported. Adds to injection burden in polypeptide stacks but does not create site conflicts or frequency issues.
No mandated labs beyond prudent baseline and endpoint organ function checks. No hormonal panels required. Low monitoring burden compared to AAS, SARMs, or GH-axis compounds.
Niche RUO compound with limited supply consistency. Product identity risk (naming confusion with B7-H3) makes sourcing verification non-trivial.
- ·No hormonal lane impact — does not affect testosterone, estrogen, GH, or IGF-1
- ·Does not crowd out other compounds in the repair/healing peptide class
- ·Can be run alongside BPC-157, TB-500, GHK-Cu without meaningful lane overlap
- ·Bacteriostatic water for reconstitution
- ·Insulin syringes for subcutaneous injection
- ·Baseline organ function labs (CBC/CMP) prudent given entirely novel human profile
No established human safety concerns, but the complete absence of human clinical data means risk is unknown rather than low. Mechanistically benign, but users should understand they are extrapolating from mice.
- ·User expects validated human efficacy data before use
- ·User has cardiovascular disease and seeks this specifically for cardiac protection — consult cardiologist first
- ·User is on antihypertensive medications (minor interaction risk)
No dependency, no suppression, no rebound effects expected. Peptide half-life in mice is short; stopping returns baseline receptor activity promptly.
- ·Loss of any ongoing anti-fibrotic effect — progressive fibrosis, if present, would resume its trajectory
Verify the product is described as a relaxin-2 B-chain analog / RXFP1 agonist explicitly. Request COA. Avoid products using B7-H3 or immune checkpoint language.
Manage expectations upfront — the compound has no validated human efficacy signal. Outcome may not be detectable without imaging or specific biomarker tracking.
RXFP1 agonism during pregnancy interferes with endogenous relaxin signaling; pregnancy contraindicated
No human cardiac efficacy data; replacing or supplementing evidence-based cardiac therapy with a preclinical peptide creates therapeutic risk
Additive blood pressure reduction possible via residual NO activity
Practical Setup
Sourcing and product identity: The naming confusion in market content (B7-33 the relaxin peptide vs B7-H3 the immune checkpoint) creates real product-verification risk.
Confirm the compound is described as a single-chain analog of human relaxin-2 (H2 relaxin B-chain) that activates RXFP1; immune-checkpoint or B7-family immune-protein language signals the wrong target. Purity, sequence identity, sterility, and batch testing matter more here than brand familiarity because the market is small and easy to mislabel.
Handling: Preparation mechanics are not standardized and should not be treated as reader-specific injection guidance. Evening injection timing is suggested based on natural relaxin release patterns, though this has not been validated for the synthetic analog.
Drug interactions: No established interactions. Theoretical additive blood pressure reduction with antihypertensives (B7-33 retains minor NO/vasodilatory activity). No CYP enzyme involvement expected for a peptide compound.
When to adjust or stop: With no human clinical data, there are no validated biomarker thresholds for response or toxicity. Users monitoring for organ protection should track the markers that reflect their primary concern (LFTs for liver fibrosis, eGFR for kidney, echocardiography or NT-proBNP for cardiac remodeling). A clean panel at baseline and at cycle end is reasonable. Discontinue and investigate if any unexplained organ function decline appears during use.
Mechanism Deep Dive
Receptor biology: RXFP1 and functional selectivity
RXFP1 (relaxin family peptide receptor 1) is a G-protein-coupled receptor broadly expressed in cardiac tissue, kidney, liver, vasculature, and lung. Its endogenous ligand is human relaxin-2, a peptide hormone with diverse physiological roles including pregnancy-related tissue remodeling and cardiovascular regulation. When activated by full relaxin-2 or serelaxin, RXFP1 triggers at least two major downstream cascades: the cAMP-PKA pathway (mediating vasodilation, blood pressure reduction, renal blood flow increase) and the MAPK-ERK1/2 pathway (mediating anti-fibrotic and cytoprotective effects).
B7-33 was engineered as a biased agonist — it binds RXFP1 but preferentially activates the pERK1/2 arm while largely bypassing cAMP-PKA. This is the defining feature of B7-33 pharmacology. The ERK1/2 preference decouples the compound's organ-protective actions from the hemodynamic effects that caused serelaxin problems in clinical development.
Anti-fibrotic mechanism: TGF-β1 and MMP-2
Fibrosis occurs when chronic injury or inflammation activates fibroblasts to overproduce extracellular matrix proteins, primarily fibrillar collagen. The central driver is TGF-β1 (transforming growth factor beta-1), which promotes fibroblast-to-myofibroblast transition and upregulates collagen I and III synthesis.
B7-33 modulates TGF-β1 signaling downstream of RXFP1/ERK1/2 activation, suppressing fibroblast activation and reducing new collagen deposition. Simultaneously, it upregulates MMP-2 (matrix metalloproteinase-2), an enzyme that degrades existing collagen in the extracellular matrix. The dual action — reducing new fibrosis and degrading existing scar tissue — is what makes B7-33 theoretically attractive for established fibrotic diseases rather than just prophylaxis.
Cardioprotection: ER stress attenuation and cardiomyocyte survival
In the Devarakonda 2020 study, B7-33 protected cardiomyocytes against ischemia-reperfusion injury through at least two mechanisms. First, it improved cell survival directly at 50-100 nmol/L in simulated ischemia-reoxygenation. Second, it attenuated endoplasmic reticulum (ER) stress, as evidenced by reduced GRP78 (glucose-regulated protein 78) expression — a canonical ER stress marker. The GRP78 reduction was blocked by ERK1/2 inhibitors, confirming the pERK pathway as the operative mechanism. ER stress is a key driver of cardiomyocyte apoptosis in ischemic tissue; its attenuation helps preserve the surviving myocardium and limits infarct expansion.
The functional cardiac outcomes (infarct size reduction, preserved fractional shortening) in mice reflect the combined effect of acute cardiomyocyte rescue and ongoing inhibition of adverse fibrotic remodeling over 7 days of twice-daily treatment.
Vasoprotective functions without hypotension
The Marshall et al. 2017 paper specifically investigated whether B7-33 could replicate serelaxin's vasoprotective functions in isolated vessel preparations. It confirmed that B7-33 does replicate endothelium-dependent vasoprotection — likely through residual NO-mediated signaling — without the full cAMP-driven systemic vasodilation. This is the mechanistic basis for its potential use in cardiovascular disease without the blood pressure-lowering liability.
Distinction from serelaxin's clinical failure
Serelaxin failed its phase 3 RELAX-AHF-2 trial (acute heart failure) — the preclinical evidence for relaxin-2 did not translate to human benefit at the trial's primary endpoints. B7-33 is conceptually differentiated by its pathway selectivity, but this distinction has not been validated in humans. The failure of serelaxin should temper expectations that B7-33's ERK1/2 selectivity will be sufficient to solve the translational problem.
Evidence Index
Quantitative claims trace to these source studies. Population, dose, and study type matter — claims from HIV-lipodystrophy trials don't transfer cleanly to healthy adults; data from supraphysiologic doses doesn't apply at TRT.
B7-33 reduced infarct size from 45.32% to 21.99% (~51% reduction)
Mouse I/R model; human translational relevance unconfirmed; no human equivalent dose established.
B7-33 preserved cardiac fractional shortening at 29% vs 20% vehicle at 7 days post-MI
Echocardiographic measure in mice only; fractional shortening benchmarks differ between species.
B7-33 improved cardiomyocyte survival at 50 and 100 nmol/L under simulated ischemia-reoxygenation
In vitro cell culture; cell concentration does not translate directly to in vivo dosing.
Community dosing: 150-500 mcg/day subcutaneous, 4-8 week cycles
No human dose-finding study exists; these ranges are community estimates without validated efficacy or safety endpoints.
Not medical advice. PepTutor summarizes fallible research and community signal for trained practitioners; some compounds are research-only, unapproved, controlled, jurisdiction-dependent, or labeled not for human consumption.