B7-33

B7-33 was rationally designed as a one-chain analog of human relaxin (H2 relaxin) retaining the B-chain motif required for RXFP1 engagement. The 27-residue format incorporates residues 7–29 of the H2 relaxin B-chain together with four C-terminal residues (KRSL) included to improve aqueous solubility. This compact linear architecture simplifies manufacture via solid-phase peptide synthesis compared to the native two-chain disulfide-linked hormone. Early structural and biochemical work, including high-affinity binding assays using a leucine-rich repeat receptor construct and structure-activity relationship studies (Bathgate and coworkers), confirmed that B7-33 engages RXFP1 at a site analogous to that used by natural relaxin. Binding studies established RXFP1 selectivity with no detected activity at RXFP2. These investigations guided the truncation strategy and characterized the minimal B-chain fragment needed for productive receptor–ligand interaction.

In vitro signaling studies characterize B7-33's functional selectivity at RXFP1. Cells expressing native RXFP1 were examined using cAMP reporter assays and ERK1/2 western blots to compare engagement of these two downstream pathways. Reporter-gene and second-messenger assays across multiple cell lines support a biased signaling profile, described in the IUPHAR database as preferential pERK pathway engagement over cAMP. In TGF-β-stimulated fibroblast cultures, MMP-2 secretion was quantified by zymography and ELISA in B7-33-treated and vehicle conditions; a signaling mechanism involving RXFP1 interaction with AT₂ receptor heterodimers has been proposed. B7-33's receptor selectivity was assessed alongside comparisons with the full relaxin hormone and with RXFP2 and glucocorticoid receptor readouts, supporting a narrow target profile.

B7-33 has been examined across several in vitro model systems. In fibroblast cultures exposed to TGF-β, assays measured MMP-2 secretion (zymography and ELISA) and cAMP levels in B7-33-treated versus control conditions. In neonatal rat cardiomyocytes subjected to hypoxia–reoxygenation, studies assessed ER stress markers including GRP78 levels and cell viability parameters. Assay platforms used across these models include calcium mobilization, ERK western blotting, cAMP reporter-gene systems, and second-harmonic generation collagen imaging. Welch et al. (2019) used a custom RXFP1-dependent reporter cell line to verify that peptide released from polymer biomaterial coatings retained signaling activity, confirming that the bioactive peptide structure was preserved through the encapsulation and release process.

In vivo studies have assessed B7-33 in rodent cardiac injury models. Devarakonda et al. (2020) used a mouse coronary ischemia–reperfusion model, employing echocardiography and histology (TTC staining, immunohistochemistry) to quantify cardiac function parameters and cardiomyocyte injury markers. Alam et al. (2023) examined a chronic isoproterenol-induced cardiomyopathy model comparing B7-33, relaxin, and a reference antihypertensive comparator; measurements included heart weight, myocyte cross-sectional area, blood pressure, tissue fibrosis indices, and vascular density markers. Injury biomarkers including TGF-β and MMP-2 were quantified across treatment groups and compared with vehicle controls. Histological endpoints (collagen staining, myocyte sizing) and functional measurements provided the primary experimental readouts.

Beyond cardiac tissue, B7-33 has been evaluated in other fibrosis research contexts. Bhuiyan et al. (2021) examined a mouse unilateral ureteral obstruction model of renal fibrosis; kidney sections from treated and untreated animals were analyzed by second-harmonic generation imaging to characterize collagen fiber architecture, and renal MMP-2 and TIMP-1 levels were assayed to measure extracellular matrix protease activity. In a biomaterial context, Welch et al. (2019) investigated subcutaneous PLGA polymer implants loaded with B7-33 in mice over a six-week observation period. Fibrotic capsule thickness was measured histologically and compared across coating formulations, alongside protein-release profiling and reporter-cell verification of released peptide activity. These studies explore B7-33 as a tool compound for examining extracellular matrix remodeling responses in vivo.

B7-33's linear peptide structure is subject to proteolytic clearance, and research groups have investigated structural modifications to extend its in vitro stability profile. Praveen et al. (2023) examined lipidated analogs attaching fatty acid chains (stearic or palmitic acid) at various positions via linkers; in vitro serum stability assays compared degradation kinetics of modified versus unmodified peptide, and RXFP1 signaling assays confirmed that ERK1/2 activation was maintained in the lipidated analogs. Additional approaches reported in the broader literature include peptide stapling and PEGylation to increase helical content and circulation time, with analogs assessed by comparing receptor signaling profiles and plasma persistence measurements. The research focus is on identifying structural modifications that extend in vitro stability while preserving the RXFP1-selective signaling profile.