{'fraction_non_alphanumeric': 0.026474769012082445, 'fraction_numerical': 0.016346837242359632, 'mean_word_length': 4.869655891553702, 'pattern_counts': {'":': 0, '<': 0, '<?xml version=': 0, '>': 1, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 0, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
Another One (of the "Undruggable" Targets) Bites the Dust: Discovery of a Potent and Selective Inhibitor of the Histone Acetyl Transferase p300/CBP.

T reversible acetylation of lysine residues in a protein is known to occur broadly throughput the proteome and has a wide array of regulatory functions. This process is of particular interest when the substrate protein is a histone, because the acetylation state of particular lysine residues in the histones (and particularly in the N-terminal tails) has a profound effect on the regulation of gene expression in eukaryotic cells. Lysine acetylation is catalyzed by a class of proteins known as histone acetyl transferases (HATs). These proteins mediate the transfer of an acetyl group from acetyl-CoA to the substrate lysine. There are many HATs in the human proteome. The opposing reaction, lysine deacetylation, is mediated by histone deacetylases (HDACs), a class of amide hydrolases specific for this linkage. Because of their central role in the regulation of gene expression, considerable effort has been focused on the development of chemical probes and drug leads for these proteins. These efforts have borne fruit with respect to HDACs. While selectivity between family members remains an issue with respect to the development of probes, several HDAC inhibitors are now used clinically against certain hematological cancers. Curiously, the same has not been true of efforts to “drug” HATs, which appear to be more recalcitrant targets. Indeed, some investigators consider HATs to be a member of the dreaded “undruggable target” class of proteins that also includes Myc, K-Ras, and a variety of other targets that have frustrated drug developers for years. Of course, no protein is a molecular billiard ball, or else it would not be able to bind anything in a cell and perform its function. So everyone understands that “undruggable” really means “quite difficult”. Indeed, some progress is being made on a number of these fronts, even with K-Ras. The HAT paralogs p300 and CBP, which are particularly important regulators of gene expression, have now been snatched away from the list of undruggables, as well. A group of investigators from AbbVie reported in Nature recently the development of A-485 (Figure 1), a potent and selective inhibitor of p300/CBP. They performed a virtual screen of 800000 compounds by docking these compounds in silico with p300, specifically the CoA binding pocket. This effort encouraged them to examine 1300 of these compounds experimentally in an assay that monitors the p300-mediated acetylation of a peptide substrate in vitro. Only two of the 1300 predicted hits demonstrated significant inhibitory activity, hydantoin 1 and thiazolidenedione 2 (Figure 1). Interestingly, these structures were quite similar to hits from a previously reported in silico screening effort (vide infra). Neither compound was potent; 1 and 2 had IC50 values of ∼5 and 11 μM, respectively, in the biochemical assay. At this point, the medicinal chemists engaged, focusing on analogues of compound 1, and eventually creating compound R (Figure 1), which was >100-fold more potent than 1. More medicinal chemistry then provided A-485, which displayed a potency similar to that of compound R, but showed better microsomal stability. This effort also resulted in a useful negative control molecule, A-486, which had no measurable activity despite the fact that it differs from A-485 only in the absence of the sulfur atom at the junction of the two fivemembered rings and a different regiochemistry of the urea side chain. The authors went on to show, using a variety of biochemical, biophysical, and cellular assays, that A-485 is indeed a useful probe molecule for p300/CBP. It showed good selectivity over a number of other HAT family members, which the authors rationalized on the basis of a crystal structure of the A-485− p300 complex. Analysis of the effect of A-485 on the proliferation of 124 different cancer cell lines revealed significant activity in mantle cell lymphoma, multiple myeloma, and non-Hodgkin’s lymphoma cells, as well as in androgen receptor (AR)-positive prostate cancer cells. Activity in a mouse model for this disease was also demonstrated, and data for the mechanism of action were presented. So what can we learn from this impressive study? The most important conclusion is that HATs are druggable targets. The best previous effort had been the report of a 400 nM compound, which these authors claimed was inactive in their hands, though it has been used widely. A-485 would appear to be a major step up. With respect to informing new approaches to HAT inhibitor discovery, it is less clear what lessons can be taken away from this paper. The in silico screen was not innovative and indeed provided scaffolds similar to compounds that had arisen from similar efforts previously. The secret sauce in this study was clearly a major medicinal chemistry effort that, unfortunately, is described in only the most superficial terms. The leap from primary screening hit 1 to spiro-containing compound R is non-obvious, and the insertion of sulfur into the molecule at that position in going from compound R to A-485 also seems unusual. It seems quite important, however, because A-486, which is completely inactive, lacks this unusual functional group. One suspects that many, many compounds were evaluated along the way and that obtaining the potent A485 was the end of a long and hard road. One hopes that story is told at some point to provide some guidance to future investigators. In an academic laboratory lacking the chemical