Porcine Relaxin but Not Serelaxin Shows Residual Bioactivity after In Vitro Simulated Intestinal Digestion—Clues for the Development of New Relaxin Peptide Agonists Suitable for Oral Delivery

Despite human recombinant H2 relaxin or serelaxin holding promise as a cardiovascular drug, its actual efficacy in chronic treatment of heart failure patients was hampered by the need to be administered by multiple daily IV injections for a long time, with obvious drawbacks in terms of patients’ compliance. This in vitro study aimed at exploring the molecular background for a possible administration of the peptide hormone relaxin by the oral route. Serelaxin and purified porcine relaxin (pRLX) were subjected to simulated intestinal fluid (SIF) enzymatic digestion in vitro to mimic the behavior of gastroprotective formulations. The digestion time course was studied by HPLC, and the relative bio-potency of the intact molecules and their proteolytic fragments was assessed by second messenger (cAMP) response in RXFP1 relaxin receptor-bearing THP-1 human monocytic cells. Both intact proteins (100 ng/mL) induced a significant cAMP rise in THP-1 cells. Conversely, SIF-treated serelaxin showed a brisk (30 s) bioactivity decay, dropping down to the levels of the unstimulated controls at 120 s, whereas SIF-treated pRLX retained significant bioactivity for up to 120 s. After that, it progressively declined to the levels of the unstimulated controls. HPLC analysis indicates that this bioactivity could be ascribed to a minor component of the pRLX sample more resistant to proteolysis. When identified and better characterized, this peptide could be exploited for the development of synthetic relaxin agonists suitable for oral formulations.