Fragment-based screening targeting an open form of the SARS-CoV-2 main protease binding pocket.

To identify starting points for therapeutics targeting SARS-CoV-2, the Paul Scherrer Institute and Idorsia decided to collaboratively perform an X-ray crystallographic fragment screen against its main protease. Fragment-based screening was carried out using crystals with a pronounced open conformation of the substrate-binding pocket. Of 631 soaked fragments, a total of 29 hits bound either in the active site (24 hits), a remote binding pocket (three hits) or at crystal-packing interfaces (two hits). Notably, two fragments with a pose that was sterically incompatible with a more occluded crystal form were identified. Two isatin-based electrophilic fragments bound covalently to the catalytic cysteine residue. The structures also revealed a surprisingly strong influence of the crystal form on the binding pose of three published fragments used as positive controls, with implications for fragment screening by crystallography.

The crystallization additive hexatungstotellurate promotes the crystallization of the HSP70 nucleotide binding domain into two different crystal forms.

The use of the tellurium-centered Anderson-Evans polyoxotungstate [TeW6O24]6- (TEW) as a crystallization additive has been described. Here, we present the use of TEW as an additive in the crystallization screening of the nucleotide binding domain (NBD) of HSP70. Crystallization screening of the HSP70 NBD in the absence of TEW using a standard commercial screen resulted in a single crystal form. An identical crystallization screen of the HSP70 NBD in the presence of TEW resulted in both the "TEW free" crystal form and an additional crystal form with a different crystal packing. TEW binding was observed in both crystal forms, either as a well-defined molecule or in overlapping alternate positions suggesting translational disorder. The structures were solved by both molecular replacement and single wavelength anomalous diffraction (SAD) using the anomalous signal of a single bound molecule of TEW. This study adds one more example of TEW binding to a protein and influencing its crystallization behavior.

Discovery and Optimization of Isoquinoline Ethyl Ureas as Antibacterial Agents.

Our strategy to combat resistant bacteria consisted of targeting the GyrB/ParE ATP-binding sites located on bacterial DNA gyrase and topoisomerase IV and not utilized by marketed antibiotics. Screening around the minimal ethyl urea binding motif led to the identification of isoquinoline ethyl urea 13 as a promising starting point for fragment evolution. The optimization was guided by structure-based design and focused on antibacterial activity in vitro and in vivo, culminating in the discovery of unprecedented substituents able to interact with conserved residues within the ATP-binding site. A detailed characterization of the lead compound highlighted the potential for treatment of the problematic fluoroquinolone-resistant MRSA, VRE, and S. pneumoniae, and the possibility to offer patients an intravenous-to-oral switch therapy was supported by the identification of a suitable prodrug concept. Eventually, hERG K-channel block was identified as the main limitation of this chemical series, and efforts toward its minimization are reported.

Structure-guided design, synthesis and biological evaluation of novel DNA ligase inhibitors with in vitro and in vivo anti-staphylococcal activity.

A series of 2-amino-[1,8]-naphthyridine-3-carboxamides (ANCs) with potent inhibition of bacterial NAD(+)-dependent DNA ligases (LigAs) evolved from a 2,4-diaminopteridine derivative discovered by HTS. The design was guided by several highly resolved X-ray structures of our inhibitors in complex with either Streptococcus pneumoniae or Escherichia coli LigA. The structure-activity-relationship based on the ANC scaffold is discussed. The in-depth characterization of 2-amino-6-bromo-7-(trifluoromethyl)-[1,8]-naphthyridine-3-carboxamide, which displayed promising in vitro (MIC Staphylococcus aureus 1 mg/L) and in vivo anti-staphylococcal activity, is presented.