RXFP2 Small Molecule Agonists: Potential Therapeutics for Osteoporosis.

Osteoporosis is a chronic bone disease characterized by decreased bone mass and increased risk of developing fractures, predominantly observed in the elderly. Osteoporosis affects approximately 10 million people in the US, and the number is expected to increase exponentially as the elderly population continues to grow. The pathophysiological cause of the disease is a decrease in the activity of the bone-forming cells (osteoblasts) that alters bone remodeling in favor of bone resorption, leading to a decrease in bone mass. Recent studies identified the G protein-coupled receptor (GPCR) for insulin-like 3 peptide (INSL3), relaxin family peptide receptor 2 (RXFP2), as an attractive target for the treatment of osteoporosis. The goal of this study is to develop small molecule agonists of RXFP2, expressed in osteoblast cells, to promote bone growth and counteract the deleterious effects of osteoporosis. Currently, the most effective available treatment for osteoporosis is an expensive hormone therapy that requires daily injections. We aim to create drugs that are stable and can be delivered orally. Several low molecular weight compounds were identified as agonists of the RXFP2 receptor using a high-throughput screen of the NCATS small molecule library. The screening assay measured cAMP response in RXFP2-transfected HEK293T cells. An extensive structure-activity relationship campaign resulted in highly potent and efficient full RXFP2 agonists. The selectivity and specificity of the compounds for human and mouse RXFP2 was shown using orthogonal cAMP assays, counter-screening against the related relaxin receptor RXFP1, and a GPCRome screen using the PRESTO-Tango assay. The drug candidates were further tested in primary human osteoblasts to demonstrate that they promote mineralization by measuring hydroxyapatite deposition in the cell matrix. We showed that the compounds had low cytotoxicity in various cell types. Using a series of RXFP2/RXFP1 chimeric receptors, we established that the compounds are allosteric agonists of the RXFP2 receptor and identified the GPCR transmembrane domains as the specific region for compound interaction. The RXFP2 agonist with the highest activity in vitro was selected for pharmacokinetics (PK) profiling in mice, showing optimal oral bioavailability and bone exposure. An efficacy study using 2-month-old WT female mice treated orally with 10 mg/kg of compound 3 times a week for 8 weeks showed a significant increase of the vertebral trabecular number and thickness by micro-CT analysis compared to vehicle treated controls. We expect that the further characterization of these compounds may lead to the development of a new class of cost-effective drugs for the treatment of osteoporosis and other diseases associated with bone loss.