Open Access

Sandra Röhm

• The p38α mitogen-activated protein kinase (MAPK) is activated through stress stimuli such as heat shock or hypoxia. In the nucleus, p38α modulates the activity of other kinases and transcription factors, a process that regulates the expression of specific target genes, most importantly pro-inflammatory cytokines. Dysregulation of p38α therefore plays a major role in the development of inflammatory diseases such as rheumatoid arthritis. Despite many years of intensive research, no p38 small-molecule inhibitors have been approved yet. Several inhibitor design strategies have been reported, leading to >100-fold selective compounds for α/β over the γ and δ isoforms. Achieving such a selectivity among the two structurally most related α and β isoforms, however, remains a challenging task. Targeting an inactive DFG-out conformation offers another strategy for the development of potent kinase inhibitors (type-II), exemplified by the BCR/ABL-inhibitor Imatinib. Achieving selectivity with type-II binders is challenging, because many kinases can adopt an inactive DFG-out conformation. This is exemplified by the p38 type-II inhibitor BIRB-796, which exhibits picomolar on-target affinity but only a poor kinome-wide selectivity. A potent and selective type-II chemical probe for p38α/β was still lacking at the start of this thesis. The promising hit VPC-00628, was chosen for a combinatorial synthetic approach to develop a type-II chemical probe. The studies covered the optimization of the hinge-binding head group, the hydrophobic region I and the DFG-out deep pocket of the lead compound VPC-00628. Selectivity for the p38α and p38β isoforms was monitored during the optimization process, which identified several inhibitors with favorable isoform selectivity, providing valuable insights into the potential of isoform-selective inhibitor design for p38. A potent and highly selective p38 MAPK probe (SR-318) was discovered, which showed IC50 values in the low nanomolar range in HEK293T cells. An unusual P-loop conformation induced upon binding of SR-318 to p38α contributed most likely to the impressive selectivity profile within the kinome that surpassed both the parent compound and BIRB-796. A negative control compound, SR-321, was developed, to distinguish between on-target effects and non-specific effects due to cross-reactivity with other cellular proteins. Studies of the metabolic stability in human liver microsomes revealed a high stability of the compounds, with only a small amount of metabolites formed over several hours. Compound SR-318 also exhibited a good in vitro efficacy, quantitatively reducing the LPS-stimulated TNF-α release in whole blood. Taken together, SR-318 is a highly potent and selective type-II p38α/β chemical probe, which will help to gain a better understanding of the catalytic and non-catalytic functions of these key signaling kinases in physiology and pathology. The next studies focused on the exploration of the highly dynamic allosteric back pocket of p38 MAPK, and allosteric BIRB-796 derived compounds for targeting the αC- and DFG-out pockets were synthesized. Kinase activities of allosteric pyrazole-urea fragments were analyzed against a comprehensive set of 47 diverse kinases by differential scanning fluorimetry (DSF), revealing that BIRB-796 off-targets remain a problem when targeting this back-pocket binding motif. Revisiting the recently published compound MCP-081, which combines the allosteric part of BIRB-796 with the active-site directed part of VPC-00628, showed that it displays a clean selectivity profile in our kinase panel. Because the potency of MCP-081 was slightly reduced compared with VPC-00628 and the allosteric tert-butyl pyrazole moiety seemed suboptimal, a set of VPC-00628 derivatives for targeting the αC-out pocket region was synthesized. Through structure-guided extension of the terminal amide of VPC-00628 toward this allosteric site, the potent and selective compound SR-43 was developed, which showed excellent cellular activity on p38 MAPK in NanoBRETTM assays (IC50 [p38α/β] = 14.0 ± 0.1/ 16.8 ± 0.1 nM). SR-43 showed a dose-dependent inhibition of activating phosphorylation of p38 in HCT-15 cells as well as inhibition of phosphorylation of p38 downstream substrates MK2 and Hsp27. In addition, SR-43 induced an anti-inflammatory response by blocking TNF-α release in whole blood and displayed a high metabolic stability. Selectivity profiling of SR-43 revealed a narrow selectivity for additional targets such as the discoidin domain receptor kinases (DDR1/2). DDR kinases play a central role in fibrotic disorders, such as renal and pulmonale fibrosis, atherosclerosis and different forms of cancer. Since selective and potent inhibitors for these important therapeutic targets are largely lacking and the existing inhibitors are of low scaffold diversity, the next study focused on the optimization of SR-43 toward DDR1/2 kinase inhibition. The synthetic work covered the optimization of the hinge-binding head group and the allosteric part of SR-43 toward DDR1/2 kinase inhibition. These studies provided novel insights into the P-loop folding process of p38 MAPK and how targeting of non-conserved amino acids affects inhibitor selectivity. Importantly, they led to the development of a selective dual DDR/p38 inhibitor probe, SR-302, with picomolar affinity for DDR2. SR-302 was efficient in vitro and showed a destabilizing effect on the surface adhesion protein E-cadherin in epithelial cells. In summary, SR-302 and its negative control SR-301 provide a valuable tool set for studying the phenotypic effects of DDR1/2 signaling, e.g., in cancer cell lines.