Osteoarthritis (OA) is the most prevalent musculoskeletal disease affecting about 27 million people in the United States, and is the leading cause of chronic disability in the United States. Current therapeutic options are limited to pain or symptom-modifying drugs and joint replacement surgery; no disease-modifying drugs are approved for clinical use. OA is characterized by progressive degeneration of the articular cartilage, resulting from abnormal activation, differentiation and death of cartilage cells. A unique and unexplored therapeutic opportunity exists to induce somatic stem cells to regenerate the damaged tissue and reverse the chronic destructive process. Because limited joints are affected in most OA patients, intra-articular (IA) drug injection is an attractive treatment approach that allows high local drug concentration with limited systemic exposure. Targeting resident stem cells pharmacologically also avoids the risks and costs associated with cell-based approaches.
Cartilage contains resident mesenchymal stem cells (MSCs) that can be differentiated in vitro to form chondrocytes. This observation suggests that intra-articular injection of a small molecule that promotes chondrogenesis in vivo will preserve and regenerate cartilage in OA-affected joints. Using an image-based screen, we identified a drug-like small molecule, kartogenin (KGN), that promotes efficient and selective chondrocyte differentiation from MSCs in vitro. Intra-articular injection of KGN also shows beneficial effects in surgery-induced acute and enzyme-induced chronic cartilage injury models in rodents, as well as positive effects in incapacitance pain models. This project is aimed at the development of new lead compounds with improved biological activity, the demonstration of efficacy of the lead compounds in rodent and dog OA models and the elucidation of the cellular mechanisms underlying the cartilage regeneration activities of KGN and its analogs.
Through medicinal chemistry efforts, we have designed and synthesized over 400 analogs of KGN. Using cell culture based assays, we assessed the chondrocyte differentiation activity of these analogs and identified 17 compounds exhibiting improved potency compared to KGN (EC50 < 100 nM). These compounds showed no obvious cytotoxicity at high concentrations (100 μM) when incubated with a variety of cells present in the joints including MSCs, chondrocytes, osteoblasts and synoviocytes. Up to date, we have assessed the efficacy of 7 compounds using a rat OA model (medial meniscal tear). Two of the tested compounds showed significantly improved cartilage repair at the end of the study. At the same time, no adverse effects, such as body weight loss, pain or impaired motor functions, were observed in any compound treated animals. We are currently studying the effects of another 10 analogs using the same OA model, which is expected to conclude within two to three months. Next, we will assess the efficacy of active compounds in a canine OA model (partial meniscectomy using beagles). Furthermore, full rodent pharmacokinetics and non-GLP toxicology studies will be performed for the lead compounds. To study the underlying mechanisms of KGN induced chondrogenesis, we designed and synthesized an affinity probe with biological activities comparable to that of KGN. Through affinity-based methods, we identified protein filamin A (FLNA) as the target of KGN. In MSCs, KGN binds to FLNA and disrupts its interaction with core binding factor β (CBFβ), which leads to the nuclear translocalization of CBFβ, subsequent activation of the RUNX1-CBFβ transcription program and, as a result, chondrocyte differentiation. This mechanism has been confirmed using cell biological methods including RNAi mediated gene silencing and cDNA overexpression of relevant genes such as FLNA, CBFβ and RUNX1. These studies have been published in the journal Science.