Kinase crystal identification and ATP-competitive inhibitor screening using the fluorescent ligand SKF86002

Cocrystallization of the Src-Family Kinase Hck with the ATP-Site Inhibitor A-419259 Stabilizes an Extended Activation Loop Conformation

Hematopoietic cell kinase (Hck) is a member of the Src kinase family and is a promising drug target in myeloid leukemias. Here, we report the crystal structure of human Hck in complex with the pyrrolopyrimidine inhibitor A-419259, determined at a resolution of 1.8 Å. This structure reveals the complete Hck active site in the presence of A-419259, including the αC-helix, the DFG motif, and the activation loop. A-419259 binds at the ATP-site of Hck and induces an overall closed conformation of the kinase with the regulatory SH3 and SH2 domains bound intramolecularly to their respective internal ligands. A-419259 stabilizes the DFG-in/αC-helix-out conformation observed previously with Hck and the pyrazolopyrimidine inhibitor PP1 (PDB: 1QCF). However, the activation loop conformations are distinct, with PP1 inducing a folded loop structure with the tyrosine autophosphorylation site (Tyr416) pointing into the ATP binding site, while A-419259 stabilizes an extended loop conformation with Tyr416 facing out into the solvent. Autophosphorylation also induces activation loop extension and significantly reduces the Hck sensitivity to PP1 but not A-419259. In cancer cells where Hck is constitutively active, the extended autophosphorylation loop may render Hck more sensitive to inhibitors like A-419259 which prefer this kinase conformation. More generally, these results provide additional insight into targeted kinase inhibitor design and how conformational preferences of inhibitors may impact selectivity and potency.

Identified Isosteric Replacements of Ligands' Glycosyl Domain by Data Mining

Biologically equivalent replacements of key moieties in molecules rationalize scaffold hopping, patent busting, or R-group enumeration. Yet, this information may depend upon the expert-defined space, and might be subjective and biased toward the chemistries they get used to. Most importantly, these practices are often informatively incomplete since they are often compromised by a try-and-error cycle, and although they depict what kind of substructures are suitable for the replacement occurrence, they fail to explain the driving forces to support such interchanges. The protein data bank (PDB) encodes a receptor-ligand interaction pattern and could be an optional source to mine structural surrogates. However, manual decoding of PDB has become almost impossible and redundant to excavate the bioisosteric know-how. Therefore, a text parsing workflow has been developed to automatically extract the local structural replacement of a specific structure from PDB by finding spatial and steric interaction overlaps between the fragments in endogenous ligands and particular ligand fragments. Taking the glycosyl domain for instance, a total of 49 520 replacements that overlap on nucleotide ribose were identified and categorized based on their SMILE codes. A predominately ring system, such as aliphatic and aromatic rings, was observed; yet, amide and sulfonamide replacements also occur. We believe these findings may enlighten medicinal chemists on the structure design and optimization of ligands using the bioisosteric replacement strategy.

ATP-site inhibitors induce unique conformations of the acute myeloid leukemia-associated Src-family kinase, Fgr

The Src-family kinase Fgr is expressed primarily in myeloid hematopoietic cells and contributes to myeloid leukemia. Here, we present X-ray crystal structures of Fgr bound to the ATP-site inhibitors A-419259 and TL02-59, which show promise as anti-leukemic agents. A-419259 induces a closed Fgr conformation, with the SH3 and SH2 domains engaging the SH2-kinase linker and C-terminal tail, respectively. In the Fgr:A-419259 complex, the activation loop of one monomer inserts into the active site of the other, providing a snapshot of trans-autophosphorylation. By contrast, TL02-59 binding induced SH2 domain displacement from the C-terminal tail and SH3 domain release from the linker. Solution studies using HDX MS were consistent with the crystal structures, with A-419259 reducing and TL02-59 enhancing solvent exposure of the SH3 domain. These structures demonstrate that allosteric connections between the kinase and regulatory domains of Src-family kinases are regulated by the ligand bound to the active site.

Network Pharmacological Analysis of the Red Sea Sponge Hyrtios erectus Extract to Reveal Anticancer Efficacy of Corresponding Loaded Niosomes

In this study, the LC-HRMS-assisted chemical profiling of Hyrtios erectus sponge led to the annotation of eleven major compounds (1-11). H. erectus-derived crude extract (HE) was tested in vitro for its antiproliferative activity against three human cancer cell lines, Hep-G2 (human liver cancer cell line), MCF-7 (breast cancer cell line), and Caco-2 (colon cancer cell line), before and after encapsulation within niosomes. Hyrtios erectus extract showed moderate in vitro antiproliferative activities towards the studied cell lines with IC₅₀ values 18.5 ± 0.08, 15.2 ± 0.11, and 13.4 ± 0.12, respectively. The formulated extract-containing niosomes (size 142.3 ± 10.3 nm, PDI 0.279, and zeta potential 22.8 ± 1.6) increased the in vitro antiproliferative activity of the entrapped extract significantly (IC₅₀ 8.5 ± 0.04, 4.1 ± 0.07, and 3.4 ± 0.05, respectively). A subsequent computational chemical study was performed to build a sponge-metabolite-targets-cancer diseases network, by focusing on targets that possess anticancer activity toward the three cancer types: breast, colon, and liver. Pubchem, BindingDB, and DisGenet databases were used to build the network. Shinygo and KEGG databases in addition to FunRich software were used for gene ontology and functional analysis. The computational analysis linked the metabolites to 200 genes among which 147 genes related to cancer and only 64 genes are intersected in the three cancer types. The study proved that the co-occurrence of compounds 1, 2, 3, 7, 8, and 10 are the most probable compounds possessing cytotoxic activity due to large number of connections to the intersected cytotoxic genes with edges range from 9-14. The targets possess the anticancer effect through Pathways in cancer, Endocrine resistance and Proteoglycans in cancer as mentioned by KEGG and ShinyGo 7.1 databases. This study introduces niosomes as a promising strategy to promote the cytotoxic potential of H. erectus extract.

Toward an Understanding of Pan-Assay Interference Compounds and Promiscuity: A Structural Perspective on Binding Modes

Pan-assay interference compounds (PAINS) are promiscuous compound classes that produce false positive hits in high-throughput screenings. Yet, the mechanisms of PAINS activity are poorly understood. Although PAINS are often associated with protein reactivity, several recent studies have shown that they also mediate noncovalent interactions. Aiming at a deep understanding of PAINS promiscuity, we performed an analysis of the Protein Data Bank to characterize the binding modes of PAINS. We explored the binding mode conservation of 34 PAINS classes present in 871 ligands and among 517 protein targets. The two major findings of this work are the following: First, different PAINS classes exhibit different levels of binding mode conservation. Our novel classification of PAINS based on binding mode similarity enables a rational assessment of PAINS from a structural perspective. Second, PAINS classes with variable binding modes can bind with high affinity. The evaluation of noncovalent binding modes of PAINS-like compounds sheds light on the mechanisms of promiscuous binding. Our findings could facilitate the decisions on how to deal with PAINS and help scientists to understand why PAINS produce hits in their screenings.

In Vitro Evolution Reveals a Single Mutation as Sole Source of Src-Family Kinase C-Helix-out Inhibitor Resistance

Understanding cancer cell drug resistance to protein-tyrosine kinase inhibitors, which often arises from acquired mutations in the target kinase, is central to the development of more durable therapies. Experimental systems that reveal potential paths to resistance for a given inhibitor and kinase target have an important role in preclinical development of kinase inhibitor drugs. Here, we employed a codon mutagenesis strategy to define the mutational landscape of acquired resistance in HCK, a member of the SRC tyrosine kinase family and therapeutic target in acute myeloid leukemia (AML). Using PCR-based saturation mutagenesis, we created a cDNA library designed to replace each codon in the HCK open reading frame with all possible codons. This HCK mutant library was used to transform Rat-2 fibroblasts, followed by selection for resistant colonies with A-419259, a pyrrolopyrimidine HCK inhibitor and drug lead for AML. X-ray crystallography has shown that A-419259 binding induces outward rotation of the kinase domain αC-helix, a conformation incompatible with phosphotransfer. Remarkably, only a single resistance mutation evolved during A-419259 selection: histidine substitution for threonine at the gatekeeper position in the kinase domain. Deep sequencing confirmed representation of nearly all other missense mutations across the entire HCK open reading frame. This observation suggests that A-419259 and other C-helix-out Src-family kinase inhibitors may have a narrow path to acquired resistance in the context of AML cases where Hck is an oncogenic driver.

Expression of myeloid Src-family kinases is associated with poor prognosis in AML and influences Flt3-ITD kinase inhibitor acquired resistance

Unregulated protein-tyrosine kinase signaling is a common feature of AML, often involving mutations in Flt3 and overexpression of myeloid Src-family kinases (Hck, Fgr, Lyn). Here we show that high-level expression of these Src kinases predicts poor survival in a large cohort of AML patients. To test the therapeutic benefit of Flt3 and Src-family kinase inhibition, we used the pyrrolopyrimidine kinase inhibitor A-419259. This compound potently inhibits Hck, Fgr, and Lyn as well as Flt3 bearing an activating internal tandem duplication (ITD). Flt3-ITD expression sensitized human TF-1 myeloid cells to growth arrest by A-419259, supporting direct action on the Flt3-ITD kinase domain. Cells transformed with the Flt3-ITD mutants D835Y and F691L were resistant to A-419259, while co-expression of Hck or Fgr restored inhibitor sensitivity to Flt3-ITD D835Y. Conversely, Hck and Fgr mutants with engineered A-419259 resistance mutations decreased sensitivity of TF-1/Flt3-ITD cells. To investigate de novo resistance mechanisms, A-419259-resistant Flt3-ITD+ AML cell populations were derived via long-term dose escalation. Whole exome sequencing identified a distinct Flt3-ITD kinase domain mutation (N676S/T) among all A-419259 target kinases in each of six independent resistant cell populations. These studies show that Hck and Fgr expression influences inhibitor sensitivity and the pathway to acquired resistance in Flt3-ITD+ AML.

Solvatochromism and linear solvation energy relationship of the kinase inhibitor SKF86002

We studied the spectroscopic characteristics of SKF86002, an anti-inflammatory and tyrosine kinase inhibitor drug candidate. Two conformers SKF86002A and SKF86002B are separated by energy barriers of 19.68kJ·mol(-1) and 6.65kJ·mol(-1) due to H-bonds, and produce the three major UV-Vis absorption bands at 325nm, 260nm and 210nm in cyclohexane solutions. This environment-sensitive fluorophore exhibited emission in the 400-500nm range with a marked response to changes in environment polarity. By using twenty-two solvents for the solvatochromism study, it was noticed that solvent polarity, represented by dielectric constant, was well correlated with the emission wavelength maxima of SKF86002. Thus, the SKF86002 fluorescence peak red shifted in aprotic solvents from 397.5nm in cyclohexane to 436nm in DMSO. While the emission maximum in hydrogen donating solvents ranged from 420nm in t-butanol to 446nm in N-methylformamide. Employing Lippert-Mataga, Bakhshiev and Kawski models, we found that one linear correlation provided a satisfactory description of polarity effect of 18 solvents on the spectral changes of SKF86002 with R(2) values 0.78, 0.80 and 0.80, respectively. Additionally, the multicomponent linear regression analysis of Kamlet-Taft (R(2)=0.94) revealed that solvent acidity, basicity and polarity accounted for 31%, 24% and 45% of solvent effects on SKF86002 emission, respectively. While Catalán correlation (R(2)=0.92) revealed that solvatochromic change of SKF86002 emission was attributed to changes in solvent dipolarity (71%), solvent polarity (12%), solvent acidity (11%) and solvent basicity (6%). Plot of Reichardt transition energies and emission energies of SKF86002 in 18 solvents showed also a linear correlation with R(2)=0.90. The dipole moment difference between excited and ground state was calculated to be 3.4-3.5debye.

Exploring the optical reporting characteristics of drugs: UV-Vis spectra and conformations of the tyrosine kinase inhibitor SKF86002

The ultimate understanding of drug–protein interactions relies on understanding drug behaviours in solution, at the molecular level.