Storage Stability
Relaxin-1 (RLN1) is the first member of the relaxin peptide family discovered, originally isolated from the corpus luteum of pregnant sows. It is a two-chain insulin-like peptide that activates RXFP1 and RXFP2 receptors, mediating collagen remodeling, reproductive tract softening, and cardiovascular vasodilation, with closest structural homology to relaxin-2 (H2 relaxin), the better-studied human isoform.
Mechanism of Action
- RXFP1 is a leucine-rich repeat-containing GPCR; relaxin-1 binding drives cAMP accumulation and downstream PKA signaling
- Inhibits collagen synthesis and promotes MMP-mediated ECM degradation, reducing fibrosis in multiple tissues
- Cervical and pubic symphysis remodeling during pregnancy: relaxes fibrocartilage via collagenase induction
- Vasodilatory via nitric oxide and endothelin-1 suppression; reduces systemic vascular resistance
- Anti-fibrotic in kidney, liver, heart, and lung via SMAD3 suppression and TGF-beta pathway inhibition
Research Findings
- Relaxin-1 and relaxin-2 share ~65% amino acid identity and similar RXFP1 agonism but have distinct tissue expression patterns
- RLN1 knockout mice show impaired cervical ripening at parturition and age-related fibrosis in multiple organs
- Recombinant RLN1 reduced renal fibrosis equivalently to RLN2 in unilateral ureteral obstruction mouse models
- Human RLN1 plasma levels peak in the first trimester of pregnancy (~1-2 ng/mL), slightly lower than RLN2 levels
- RLN1 expressed in prostate; may play local anti-proliferative and anti-fibrotic roles in benign prostatic hyperplasia
Research Protocols
- Most clinical data uses recombinant human RLN2 (serelaxin); RLN1 protocols are largely preclinical
- Recombinant RLN1: 0.5-1 mg/kg/day SC in rodent fibrosis models (comparable dosing to RLN2)
- In vitro: 10-100 ng/mL RLN1 for collagen synthesis inhibition assays in fibroblast cultures
- RXFP1 binding assay: RLN1 Ki ~0.1 nM; useful as reference agonist when comparing RXFP1 vs RXFP2 selectivity
Interactions
- Relaxin-2 (serelaxin): parallel RXFP1 agonist; RLN2 is the primary therapeutic candidate due to higher plasma levels
- TGF-beta1: RLN1 antagonizes TGF-beta-driven SMAD3 signaling and collagen upregulation
- Estrogen: synergizes with relaxin-1 in cervical remodeling during pregnancy
Safety Profile
Endogenous hormone; well tolerated in pregnancy. Recombinant RLN1 not clinically approved. Serelaxin (RLN2) was tested in Phase III heart failure (RELAX-AHF-2) with good short-term safety; RLN1 expected similar profile. Hypotension possible at supraphysiological doses.
Legal & Regulatory
Endogenous hormone; recombinant form investigational only
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