Optimization of 1,2,4-Triazole-Based p97 Inhibitors for the Treatment of Cancer.

The AAA+ ATPase p97 (valosin-containing protein, VCP) is a master regulator of protein homeostasis and therefore represents a novel target for cancer therapy. Starting from a known allosteric inhibitor, NMS-873, we systematically optimized this scaffold, in particular, by applying a benzene-to-acetylene isosteric replacement strategy, specific incorporation of F, and eutomer/distomer identification, which led to compounds that exhibited nanomolar biochemical and cell-based potency. In cellular pharmacodynamic assays, robust effects on biomarkers of p97 inhibition and apoptosis, including increased levels of ubiquitinated proteins, CHOP and cleaved caspase 3, were observed. Compound (R)-29 (UPCDC-30766) represents the most potent allosteric inhibitor of p97 reported to date.

Structural Optimization and Anticancer Activity of Polo-like Kinase 1 (Plk1) Polo-Box Domain (PBD) Inhibitors and Their Prodrugs.

Polo-like kinase 1 (Plk1), a mitotic kinase whose activity is widely upregulated in various human cancers, is considered an attractive target for anticancer drug discovery. Aside from the kinase domain, the C-terminal noncatalytic polo-box domain (PBD), which mediates the interaction with the enzyme's binding targets or substrates, has emerged as an alternative target for developing a new class of inhibitors. Various reported small molecule PBD inhibitors exhibit poor cellular efficacy and/or selectivity. Here, we report structure-activity relationship (SAR) studies on triazoloquinazolinone-derived inhibitors, such as 43 (a 1-thioxo-2,4-dihydrothieno[2,3-e][1,2,4]triazolo[4,3-a]pyrimidin-5(1H)-one) that effectively block Plk1, but not Plk2 and Plk3 PBDs, with improved affinity and drug-like properties. The range of prodrug moieties needed for thiol group masking of the active drugs has been expanded to increase cell permeability and mechanism-based cancer cell (L363 and HeLa) death. For example, a 5-thio-1-methyl-4-nitroimidazolyl prodrug 80, derived from 43, showed an improved cellular potency (GI50 4.1 µM). As expected, 80 effectively blocked Plk1 from localizing to centrosomes and kinetochores and consequently induced potent mitotic block and apoptotic cell death. Another prodrug 78 containing 9-fluorophenyl in place of the thiophene-containing heterocycle in 80 also induced a comparable degree of anti-Plk1 PBD effect. However, orally administered 78 was rapidly converted in the bloodstream to parent drug 15, which was shown be relatively stable toward in vivo oxidation due to its 9-fluorophenyl group in comparison to unsubstituted phenyl. Further derivatization of these inhibitors, particularly to improve the systemic prodrug stability, could lead to a new class of therapeutics against Plk1-addicted cancers.

Identification of a New Heterocyclic Scaffold for Inhibitors of the Polo-Box Domain of Polo-like Kinase 1.

As a mitotic-specific target widely deregulated in various human cancers, polo-like kinase 1 (Plk1) has been extensively explored for anticancer activity and drug discovery. Although multiple catalytic domain inhibitors were tested in preclinical and clinical studies, their efficacies are limited by dose-limiting cytotoxicity, mainly from off-target cross reactivity. The C-terminal noncatalytic polo-box domain (PBD) of Plk1 has emerged as an attractive target for generating new protein-protein interaction inhibitors. Here, we identified a 1-thioxo-2,4-dihydro-[1,2,4]triazolo[4,3-a]quinazolin-5(1H)-one scaffold that efficiently inhibits Plk1 PBD but not its related Plk2 and Plk3 PBDs. Structure-activity relationship studies led to multiple inhibitors having ≥10-fold higher inhibitory activity than the previously characterized Plk1 PBD-specific phosphopeptide, PLHSpT (Kd ∼ 450 nM). In addition, S-methyl prodrugs effectively inhibited mitotic progression and cell proliferation and their metabolic stability was determined. These data describe a novel class of small-molecule inhibitors that offer a promising avenue for future drug discovery against Plk1-addicted cancers.

Optimization of Phenyl Indole Inhibitors of the AAA+ ATPase p97.

Optimization of the side-chain of a phenyl indole scaffold identified from a high-throughput screening campaign for inhibitors of the AAA+ ATPase p97 is reported. The addition of an N-alkyl piperazine led to high potency of this series in a biochemical assay, activity in cell-based assays, and excellent pharmaceutical properties. Molecular modeling based on a subsequently obtained cryo-EM structure of p97 in complex with a phenyl indole was used to rationalize the potency of these allosteric inhibitors.

Allosteric Indole Amide Inhibitors of p97: Identification of a Novel Probe of the Ubiquitin Pathway.

A high-throughput screen to discover inhibitors of p97 ATPase activity identified an indole amide that bound to an allosteric site of the protein. Medicinal chemistry optimization led to improvements in potency and solubility. Indole amide 3 represents a novel uncompetitive inhibitor with excellent physical and pharmaceutical properties that can be used as a starting point for drug discovery efforts.

Structure-Activity Study of Bioisosteric Trifluoromethyl and Pentafluorosulfanyl Indole Inhibitors of the AAA ATPase p97.

Exploratory SAR studies of a new phenyl indole chemotype for p97 inhibition revealed C-5 indole substituent effects in the ADPGlo assay that did not fully correlate with either electronic or steric factors. A focused series of methoxy-, trifluoromethoxy-, methyl-, trifluoromethyl-, pentafluorosulfanyl-, and nitro-analogues was found to exhibit IC50s from low nanomolar to double-digit micromolar. Surprisingly, we found that the trifluoromethoxy-analogue was biochemically a better match of the trifluoromethyl-substituted lead structure than a pentafluorosulfanyl-analogue. Moreover, in spite of their almost equivalent strongly electron-depleting effect on the indole core, pentafluorosulfanyl- and nitro-derivatives were found to exhibit a 430-fold difference in p97 inhibitory activities. Conversely, the electronically divergent C-5 methyl- and nitro-analogues both showed low nanomolar activities.

Relative substituent position on the strength of pi-pi stacking interactions.

It was observed that the relative position of the arene substituents have a profound influence on the strength of pi-pi stacking in the 9-benzyl substituted triptycene system. A new series of model compounds (3a-i) capable of revealing quantitatively pi-pi stacking interactions was studied. This series of compounds (3a-i) has an ortho substituted methyl group in one of the two interacting arenes and the syn/anti ratios were determined and compared to a series previously studied compounds (4a-i) that have a para methyl group on the corresponding arene. A greater than 50% increase in the strength of pi-pi stacking interactions was observed with the methyl group in the ortho position comparing to that in the para position. No difference in pi-pi stacking interactions was observed when the other aromatic ring was a pentafluorobenzoate group.