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 tumour growth is driven by activating mutations in BRAF such as V600E. Despite these successes, resistance often emerges after several months. ERK1/2 inhibitors are therefore of key interest as an alternative approach to block this pathway, and several are already in early clinical trials. The tool inhibitor SCH772984 has been shown to reduce the phosphorylation of ERK itself in addition to inhibiting the phosphorylation of downstream substrates such as RSK. We sought to understand the structural link to this pharmacological effect and to develop a series of orally bioavailable ERK inhibitors to assess this profile in vivo. We screened our fragment library by high throughput X-ray crystallography, which provided a selection of hit fragments binding at the hinge of the kinase. One fragment was found to bind in a second pocket also occupied by the inhibitor SCH772984. We postulated that binding in this region of the protein, which is associated with a conformational change in the glycine-rich loop, might correlate with reduced levels of phospho-ERK, and initiated a programme of fragment elaboration to target this binding mode. Structural information from fast turnaround X-ray crystallography experiments was key to optimising the growth of the fragments into potent lead molecules. A key challenge was also to identify a series with suitable oral bioavailability. 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 anti-tumour activity upon oral dosing in a range of sub-cutaneous xenograft models including the mutant BRAF colorectal line Colo205, providing a promising basis for further optimization towards clinical pERK1/2 modulating ERK1/2 inhibitors.

Fragment-based discovery of an orally bioavailable ERK1/2 inhibitor which reduces the level of phosphorylated ERK

256th National Meeting (2018)

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