Small molecule targeting an epigenetic regulator for the treatment of osteoarthritis

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The goal of the project is to develop a disease-modifying treatment for Osteoarthritis (OA) by targeting USP16, a chromatin modifier involved in regulation of senescence and stem cell self-renewal. The treatment will consist of an intra-articular injection of a small molecule inhibiting USP16 in patients with moderate OA. USP16 is a deubiquitinase (DUB) enzyme that removes ubiquitin from histone H2A on lysine 119, a critical mark for the maintenance of multiple somatic tissues. Interestingly, triplication of USP16 is associated with Down syndrome (DS), a congenital disorder characterized by triplication of chromosome 21 (HSA21). Patients with DS show signs of accelerated aging, including early-onset Alzheimer’s, immune dysfunction, and osteoporosis. We have previously shown that reducing the levels of USP16 in DS mouse models or human tissues results in improved function of somatic stem cells and reduction in senescence, therefore alleviating the conditions associated with DS. This evidence makes USP16 an attractive target to ameliorate some of the aging-related pathologies. Notably, USP16 expression is highly upregulated in OA chondrocytes and synovial tissues. Moreover, the genetic locus of USP16 contains a SNP strongly associated with familial OA. We found that genetic downregulation of USP16 in patient-derived articular chondrocytes promotes cellular and mitochondrial health and reduces senescence associated markers, like SA-ꞵ-gal and p16Ink4a. Moreover, microarray analyses of OA chondrocytes treated with siRNA targeting USP16 showed an increase in markers of proliferation and collagen deposition, and a reduction of apoptosis and catabolism markers. Furthermore, USP16 knockdown in human bone marrow-derived mesenchymal stem cells promoted differentiation into mature chondrocytes and increased matrix deposition. Using a biochemical assay testing the enzymatic activity of recombinant human USP16, we identified small molecule inhibitors. We validated 19 hits with low IC50 and chose small molecule scaffolds with IC50 between 0.06 μM and 9.4 μM. During Phase I of the project, we will optimize them to increase potency, specificity, and solubility. During Phase II, we will move the two best compound series into lead optimization, followed by preclinical efficacy in OA models and early toxicity testing of the two best lead compounds. We will assess their safety, pharmacokinetics (PK), ability to engage target and modulate chromatin as a pharmacodynamic (PD) measure, and efficacy in preventing or reversing loss of cartilage in a surgical rat OA model, with the goal of establishing a relationship between time on target, dose and efficacy. We will also study the role of the rs6516886 SNP and understand if specific variants of this genomic locus are linked to differential expression of USP16 and different responses to treatment, potentially leading to stratification of the target patient population. The proposed studies will inform subsequent GLP studies to support an IND and clinical trial.