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Any feedback on the pre-print or questions around the work, let me know!

It was also a true model of collaborative science, with hundreds of scientists contributing to the wider project over the years, including collaborators at the NCI and Novartis. I was privileged to co-lead for a period at the end, but very much stood on the shoulders of giants here!

Hopefully, however, it still serves as informative for those working on RAS, particularly the new structural data and SAR around active & inactive RAS. There’s also learning on the power of fragment-based, structurally-enabled hit-finding for challenging targets, and how best to optimise them.

Nevertheless there are real challenges around these compounds. We were able to show cellular activity, but weaker binding to active RAS and their properties meant progressing this series was hard. We looked to address the high TPSA and issues around the phenols, but that’s a story for another day.

From a structural interest point of view, it is notable that these compounds bind proximal to the nucleotide binding site, forming an interaction with the Mg ion present here.

We discovered a number of analogues which formed a bidentate interaction with Asp38, pushing Switch I into a completely new orientation with Glu37 out of the pocket! Interestingly, some of these compounds seem to show a slight preference for binding to the active forms of RAS c.f. inactive.

By swapping out the phenol for an amino-pyridine, we were able to find compounds which did just that. Despite the affinity towards the inactive forms of RAS dropping off with this series, they maintained their single digit micromolar affinity to the active forms, making them roughly equipotent.

As well as the strategy of designing compounds that fit more readily into the constraints of this smaller pocket in the active form, we considered an other approach. Can we push deeper into this site, and pick up interactions with a different residue, recovering the affinity towards the active form?

We think this may be due to the preferred orientation of the Glu37 residue in this active form – despite being distal to the nucleotide binding site, in a number of our ligand-free structures, it seems happier moving into the area occupied by this series:

This worked! It also delivered molecules with single-digit nanomolar binding affinity to the GDP-loaded, inactive form of a range of RAS mutants and isoforms. However, we experienced a big drop-off of binding affinity to active RAS proteins, and this was borne out in the activity of our compounds.

The big step forward came when we solved the crystal structure of compound A below, which suggested that forming a macrocycle between the amide nitrogen and the one on the benzothiazole ring could constrain our molecules in the bioactive conformation:

Using structure-guided design principles, we were able to optimise these hits, which were shown to bind in the Switch I/II pocket (similar to compounds from the Fesik group, and Boehringer’s BI-2852)

One of the initial aims of our Programme was to establish a fragment-based platform to find hit matter for challenging targets previously considered “undruggable”, such as GTP-ases. From this, we discovered a number of novel, structurally-enabled fragment hits, weakly bound to KRAS:

Whilst this was written and published under the current Cancer Research Horizons affiliation of our group, the genesis of this work goes back 15 or so years to our previous incarnation, when we were the CRUK Beatson Institute (now CRUK Scotland Institute) Drug Discovery Programme.

Hi, could I be added to the Science feed please? I’m a medicinal chemist working in drug discovery- here’s the latest preprint from our group: chemrxiv.org/engage/chemr...