Abstract B161: Fragment-based discovery of a highly potent, orally bioavailable ERK1/2 inhibitor that modulates the phosphorylation and catalytic activity of ERK1/2
Authors: Heightman Tom D. , Berdini Valerio , Braithwaite Hannah , Buck Ildiko , Cassidy Megan , Castro Juan , Courtin Aurélie , Day James , East Charlotte , Fazal Lynsey , Graham Brent , Griffiths-Jones Charlotte , Lyons John , Martins Vanessa , Muench Sandra , Munck Joanne , Norton David , O'Reilly Marc , Palmer Nick , Pathuri Puja , Reader Mike , Rees David , Rich Sharna , Richardson Caroline , Saini Harpreet , Thompson Neil , Wallis Nicola , Walton Hugh , Wilsher Nicola , Woolford Alison , Murray Chris 
Published: August 2018

Abstract
The RAS-RAF-MEK-ERK signalling cascade is activated through mutations in RAS or RAF in over 30% of cancers. The successful development of inhibitors of BRAF and MEK kinases has led to effective treatment particularly of melanomas whose tumor growth is driven by activating mutations in BRAF such as V600E. Despite these successes, resistance emerges after several months, leading to increased signaling through ERK1/2. This has prompted the development of direct inhibitors of ERK1/2, several of which are in early clinical trials. The majority of clinical ERK1/2 inhibitors are ATP competitive, blocking ERK1/2 catalytic phosphorylation of downstream substrates such as RSK, but do not modulate phosphorylation of ERK1/2 by MEK. Crystal structural studies performed by us and others on the pERK1/2 modulating inhibitor SCH772984 suggested that it induces a conformational change in the glycine-rich loop of ERK2, which leads to Tyr36 becoming tucked under the loop and creating a new binding pocket. We hypothesized that this binding mode might underlie the ability of SCH772984 to block the phosphorylation of ERK1/2, and initiated a fragment-based approach to develop novel, orally bioavailable inhibitors that elicit a similar conformational change and also modulate the phosphorylation of ERK1/2. Using screening methods including high-throughput X-ray crystallography and biophysical assays, we identified fragments binding to both the hinge and the inducible pocket of ERK2. Progressive rounds of structure-guided fragment optimization and growing led to an understanding of inhibitor structure determinants required to induce the conformational change in ERK2. These efforts, together with iterative optimization in a screening cascade including measurement of pRSK and pERK levels and antiproliferative activity in RAS and BRAF mutant cells, led to the discovery of a novel series of pERK modulating ERK1/2 inhibitors. The lead compound shows low nanomolar potency in biochemical ERK1/2 assays and an excellent kinome selectivity profile. In BRAF and RAS mutant cell lines, the lead shows low nanomolar cell proliferation IC50 values, while sparing cell lines not driven by the MAPK pathway. The lead exhibits robust antitumor activity upon oral dosing in a range of subcutaneous xenograft models including the mutant BRAF colorectal line Colo205, providing a promising basis for further optimization towards clinical pERK1/2 modulating ERK1/2 inhibitors.
Citation Format: Tom D. Heightman, Valerio Berdini, Hannah Braithwaite, Ildiko Buck, Megan Cassidy, Juan Castro, Aurélie Courtin, James Day, Charlotte East, Lynsey Fazal, Brent Graham, Charlotte Griffiths-Jones, John Lyons, Vanessa Martins, Sandra Muench, Joanne Munck, David Norton, Marc O'Reilly, Nick Palmer, Puja Pathuri, Mike Reader, David Rees, Sharna Rich, Caroline Richardson, Harpreet Saini, Neil Thompson, Nicola Wallis, Hugh Walton, Nicola Wilsher, Alison Woolford, Chris Murray. Fragment-based discovery of a highly potent, orally bioavailable ERK1/2 inhibitor that modulates the phosphorylation and catalytic activity of ERK1/2 [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2017 Oct 26-30; Philadelphia, PA. Philadelphia (PA): AACR; Mol Cancer Ther 2018;17(1 Suppl):Abstract nr B161.