L-Moses

This is a good inhibitor developed against the PCAF bromodomain. Reasonably potent (Kd=100 nM by ITC) and selective against other BRDs by DSF. Although in this reviewer's opinion, this assay should ideally be run by BromoScan, which is the benchmark for bromodomain selectivity panel. Crystal structure confirmed binding mode (although not on the domain from humans), and cellular activity was confirmed by NanoBRET. Some DMPK parameters were reported. The chemistry was well done, and the compound appears of reasonably good quality to warrant use in cells. More cellular data are certainly warranted in future.

This is a newly discovered probe molecule and as such there is not much data yet. The publication shows that the compound has good potency and selectivity, and the cellular target engagement assay shows it can permeate into cells and engage with the target in that complex. It would be good to see a concentration-response curve for the cellular assay, and confirmation that the compound can act at the target in a more physiologically-relevant context. But I expect this will follow in future work around this target. From the data presented I am happy to recommend the use of this probe in cellular work. Studies should also use the inactive enantiomer (D-45) presented in the paper as a control.

L-Moses is an ~200 nM inhibitor of the bromodomains present in PCAF and GCN5, which are 78% identical, but it does not inhibit/bind to other bromodomain-containing proteins. In cells, L-Moses displaced the PCAF bromodomain from histone H3.3. When tested against full length protein, the response was not robust and the IC50 is estimated to be in the low micromolar range. The compound was non-toxic in PBMCs, but there is limited information on other biomarker readouts for this compound, making its overall utility unclear at this point. Nonetheless, this should be a useful tool to probe the role of the bromodomains in PCAF and GCN5 with the caveat that it will be difficult to distinguish between those two enzymes.

The data in the primary publication are insufficient to meet the criteria for a high-quality chemical probe. Selectivity within the bromodomain family is only determined by DSF, rather than a more sensitive technique such as bromoscan, or for the BET family, which is an off-target of particular concern, ITC. Additionally, the cellular potency in a nanoBRET assay is only presented at 5 uM, rather than as an IC50. It isn't clear that the potency in cells is less than 1 uM. No general pharmacology panel selectivity data was provided. This is especially important for cell surface GPCRs and ion channels that might be engaged by a lipophilic base, such as L-Moses. GSK4027 is a more potent and better characterized probe and should be used instead.

The cellular activity for this probe is significantly weaker than for GSK4027, which currently represents the best probe for PCAF-Brd/GCN5 due to greater confidence in its selective on-target-mediated mode of action in cells.