DUBs and their involvement in disease
Due to their roles in diverse cellular processes, modulation of ubiquitin-regulated pathways through DUB inhibition has critical relevance in many therapeutic areas (Clague et al., 2013). In particular, DUBs have reported roles in diseases such as cancer (reviewed in Fraile et al., 2012 and Sacco et al., 2010), inflammation (e.g. CYLD, A20), muscle wasting (e.g. USP19), infectious disease (e.g. USP25, USP15) and neurodegeneration (e.g. UCHL3, USP24).
Crucially, DUBs are true enzymes with catalytic activity and as such are chemically tractable, a fact which combined with their known involvement in cancer and other diseases makes them very attractive targets for drug-discovery. Indeed, a number of DUBs have been shown to have strong, intrinisic oncogenic potential driven by their catalytic activity, and certain DUBs have also been linked to development of chemoresistance or proposed as targets for chemosensitisation (e.g. Oliveira et al., 2012, Hussain et al., 2010, Williams et al., 2011 and Kon et al., 2010, Vendrell et al., 2007, Wiltshire et al., 2010 and Benassi et al., 2013).
Mission’s proprietary insights into DUBs target validation
Mission has developed rigorous target identification and validation cascades to ensure its efforts are focused on inhibiting the right DUBs. This includes synthetic lethal screens using an arrayed siRNA library of all known human DUBs to identify novel targets whose inhibition will selectively induce cell killing in cancer-relevant genetic backgrounds. This minimises the likelihood of toxicity to normal cells and incorporates a patient selection biomarker into the projects from inception. Mission strives for excellence and reflective of this, has received a number of Innovate UK grant awards for its internal target identification and validation work.
Mission’s DUB-specific screening cascade
Mission has developed a robust and thorough screening cascade, designed to eliminate false positives early and focus on genuine hit matter from the outset. Part of Mission’s philosophy is to develop and use biochemical assays as closely related as possible to physiological conditions. This involves purifying full-length DUBs from mammalian cells to ensure proper folding, post-translational modifications and co-factors, as well as using substrates closely related to the natural DUB substrates incorporating an isopeptide linkage between ubiquitin and, ideally, a peptide derived from the DUB’s cellular target. A number of orthogonal, confirmatory assays are also used to weed out false positives at an early stage and our proprietary, cellular, target engagment assays are performed as early as possible to confirm specific inhibition of DUBs in cells. The drug discovery cascade is fuelled by iterative design-make-test cycles, driven by our in-house medicinal chemistry team, incorporating molecular modelling and structural biology as guides.
Mission’s small-molecule DUB inhibitor library
Mission’s DUB-specific compound library was designed by our internal medicinal chemistry team. The chemotypes within the library were designed using a novel approach and are exclusive and proprietary to Mission. All the compounds are drug-like, contain no toxicophores and fully comply with ‘rule of 5’ criteria. This proprietary library asset has a high hit rate, has seeded all our DUB projects and has enabled rapid progress in the identification of optimised leads with candidate drug levels of potency, selectivity, pharmacokinetics and in vivo efficacy.
Clinical development and patient selection strategy
Clinical development and patient selection is at the heart of all Mission’s projects. Indeed, all of our home-grown oncology programmes have a patient selection biomarker incorporated from as early as the target identification stage. We engage with external clinical consulting companies at various stages of the discovery and development process to ensure our projects are aimed at viable markets in areas of unmet clinical need. In addition, Mission has an experienced, international, clinical advisory board (CAB) that guides our clinical strategies and has recently recruited an experienced Chief Medical Officer, Dr Michael Koslowski.