B7-33: Research Roundup

B7-33 is a single-chain peptide derived from the B-chain region of human relaxin-2 (H2 relaxin) — engineered to activate the relaxin family peptide receptor 1 (RXFP1) with functional selectivity for pERK1/2 signaling over cAMP pathways associated with native relaxin's two-chain structure. Hossain and colleagues reported in Chemical Science that B7-33 prevented or reversed organ fibrosis in rodent heart and lung disease models with potency comparable to H2 relaxin in those protocols, while avoiding prostate tumor promotion observed with relaxin's strong cAMP activation in published comparisons. Native H2 relaxin is a disulfide-rich two-chain hormone difficult to synthesize at scale; B7-33 represents a minimization strategy for fibrosis and cardiovascular research. It appears in recovery discussions alongside BPC-157, TB-500, and thymosin beta-4, but targets RXFP1 rather than actin-binding or gastric pentadecapeptide pathways. This roundup summarizes B7-33 literature and procurement standards. It is research information only; it contains no administration guidance.

What the literature describes

H2 relaxin completed phase 2 evaluation for acute heart failure, motivating interest in relaxin receptor pharmacology. B7-33 binds RXFP1 and preferentially activates pERK over cAMP in cells endogenously expressing the receptor — termed biased or functional selectivity. Anti-fibrotic actions in myocardial infarction, pulmonary fibrosis, and kidney fibrosis models involve RXFP1–angiotensin II type 2 receptor heterodimer signaling and matrix metalloproteinase activation in published mechanistic proposals.

Follow-on medicinal chemistry studies explored Aib substitution and hydrocarbon stapling to improve helicity and potency on minimal scaffolds. B7-33 showed reduced prostate tumor growth promotion versus H2 relaxin in cited in vivo comparisons — a research safety distinction, not a human clinical validation.

Mechanism and research context

Fibrosis research centers on TGF-β-driven myofibroblast activation and collagen deposition. Relaxin/RXFP1 signaling opposes these processes in preclinical models through multiple downstream effectors. B7-33's biased signaling profile is proposed to capture anti-fibrotic benefits while reducing cAMP-mediated off-target concerns raised for long-term native relaxin exposure.

ARA-290 engages innate repair receptor cytoprotection; teriparatide drives bone anabolism via PTH1 receptor — distinct targets. B7-33 should not be conflated with BPC-157 connective-tissue literature despite shared "repair" marketing language in catalog commerce.

Preclinical findings

Rodent cardiac, pulmonary, renal, and hepatic fibrosis models report reduced fibrosis markers and improved functional endpoints with B7-33 administration in protocol-defined doses. Vasodilatory and anti-inflammatory effects appear in secondary endpoints consistent with relaxin biology. All data remain animal-model dependent.

H2 relaxin's phase 2 serelaxin program in acute heart failure informed interest in simplified analogs, though serelaxin phase 3 outcomes were mixed — a reminder that parent-hormone trial results do not guarantee success for minimized biased agonists like B7-33.

Clinical and formal studies

B7-33 has no FDA approval and no completed phase 3 human program. Native serelaxin (H2 relaxin) clinical development provides adjacent context — including mixed phase 3 heart-failure outcomes for serelaxin itself — but does not transfer approval status to B7-33. Human translation of biased RXFP1 agonism remains speculative pending formal trials.

RXFP1 expression varies by tissue; B7-33 potency in fibroblasts with endogenous receptor expression may not predict activity in cell lines transfected with RXFP1 alone — a methodological point from Hossain et al. that affects how in vitro screening data relate to in vivo fibrosis models.

Material quality evaluation

B7-33 is a defined linear peptide sequence; incorrect synthesis, truncated analogs, or mislabeling as native relaxin or unrelated anti-fibrotic peptides undermine experimental validity. Mass spectrometry and HPLC on each batch are required per COA literacy and HPLC vs. MS.

Relaxin-family research benefits from peptide identity testing rigor because catalog naming is inconsistent. Vetting scorecards evaluate supplier documentation.

Related reading

Fibrosis and tissue repair context: thymosin beta-4, BPC-157, TB-500, ARA-290. Bone anabolic peptides: teriparatide, abaloparatide. Library entry: B7-33.

Limitations recap

Evidence is preclinical. Biased agonism conclusions depend on cell type and receptor expression context. Native relaxin clinical data do not automatically apply to B7-33. No human efficacy or safety database comparable to approved therapeutics. This page excludes dosing and personal-use instructions.

Samuel and colleagues' myocardial infarction model data with B7-33 complement Hossain's original RXFP1 selectivity work — together they form the core indexed preclinical record for this sequence. Liver and kidney fibrosis extensions in British Journal of Pharmacology papers suggest broader organ applicability in rodents, still awaiting human translation.

Samuel group follow-on chemistry (Aib substitution, stapled helices) pursues more potent minimal relaxin scaffolds — future analogs may differ from original B7-33 catalog sequences even when marketed under the same name.