Lena Marie Berger

The tyrosine kinase expressed in hepatocellular carcinoma (TEC) family comprises five non-receptor tyrosine kinases – BTK, ITK, BMX, TXK, and TEC – with key roles in immune signalling. Although BTK and ITK have been extensively studied, selective inhibitors for TEC, TXK, and BMX remain scarce. Recently, we identified 7-azaindole-based covalent BMX inhibitors with potent inhibitory activity and robust cellular target engagement but limited selectivity across TEC family members, especially BTK. Here, we describe a new generation of BMX inhibitors designed to exploit subtle structural differences between BMX and BTK by incorporating diverse N-acylamino substituents at the azaindole 5-position or variations at the linker and warhead. Our compounds display subnanomolar BMX potency and improved selectivity over BTK and other TEC kinases. Key compound 11i showed strong cellular target engagement, good in vitro metabolic stability, a favourable kinome profile, and rapid covalent inactivation kinetics, positioning it among the best BMX chemical probes currently available.

The Tyrosine Kinase Expressed in Hepatocellular Carcinoma (TEC) family comprises five nonreceptor tyrosine kinases – BTK, ITK, BMX, TXK, and TEC – which play key roles in immunological signal transduction. Although BTK and ITK have been extensively studied, selective inhibitor development for TEC, TXK, and BMX remains scarce. In a recent study, we identified a new class of covalent BMX inhibitors with a 7-azaindole core. These compounds exhibit potent inhibitory activity and robust cellular target engagement but had limited selectivity across TEC family members, especially BTK, which is the closest relative of BMX. Here, we describe the next generation of BMX inhibitors that incorporate diverse N-acylamino substituents at the azaindole 5-position or variations in the linker and warhead region. These modifications were designed to exploit subtle structural differences between BMX and BTK in the pocket behind the gatekeeper residue as well as in the front pocket. Our compounds display excellent BMX potency in the sub-nanomolar range along with improved selectivity against BTK and other TEC family kinases. We identified compound 11i as a new BMX lead compound with strong cellular target engagement, good in vitro metabolic stability, a favourable kinome profile, and fast covalent inactivation kinetics. These properties position compound 11i among the best BMX chemical probes currently available.

Selectivity for closely related isoforms of protein kinases is a major challenge in the design of drugs and chemical probes. Covalent targeting of unique cysteines is a potential strategy to achieve selectivity for highly conserved binding sites. Here, we used a pan-LIMK inhibitor to selectively probe LIMK1 over LIMK2 by targeting the LIMK1-specific cysteine C349 located in the glycine-rich loop region. Binding kinetics of both noncovalent and covalent LIMK inhibitors were investigated, and the fast on-rate and small size of type-I inhibitors were used in the design of a covalent LIMK1 inhibitor. The developed cell-active, isoform-selective LIMK1 inhibitor showed excellent proteome-wide selectivity in pull-down assays, enabling studies of LIMK1 isoform-selective functions in cellular model systems and providing a versatile chemical tool for studies of the LIMK signaling pathway.

Selectivity for closely related isoforms of protein kinases is a major challenge in the design of drugs and chemical probes. Covalent targeting of unique cysteines is a potential strategy to achieve selectivity for highly conserved binding sites. Here, we used a pan-LIMK inhibitor to selectively probe LIMK1 over LIMK2 by targeting the LIMK1-specific cysteine C349 located in the glycine-rich loop region. Binding kinetics of both non-covalent and covalent LIMK inhibitors were investigated, and the fast on-rate and small size of type-I inhibitors were used in the design of a covalent LIMK1 inhibitor. The developed cell-active, isoform-selective LIMK1 inhibitor showed excellent proteome-wide selectivity in pull-down assays, enabling studies of LIMK1 isoform-selective functions in cellular model systems and providing a versatile chemical tool for studies of the LIMK signalling pathway.

Macrocyclization of acyclic compounds is a powerful strategy for improving inhibitor potency and selectivity. Here we have optimized 2-aminopyrimidine-based macrocycles to use these compounds as chemical tools for the ephrin kinase family. Starting with a promiscuous macrocyclic inhibitor, 6, we performed a structure-guided activity relationship and selectivity study using a panel of over 100 kinases. The crystal structure of EPHA2 in complex with the developed macrocycle 23 provided a basis for further optimization by specifically targeting the back pocket, resulting in compound 55, a potent inhibitor of EPHA2/A4 and GAK. Subsequent front-pocket derivatization resulted in an interesting in cellulo selectivity profile, favoring EPHA4 over the other ephrin receptor kinase family members. The dual EPHA2/A4 and GAK inhibitor 55 prevented dengue virus infection of Huh7 liver cells. However, further investigations are needed to determine whether this was a compound-specific effect or target-related.

Bromodomain and extra-terminal domain (BET) proteins and histone deacetylases (HDACs) are prime targets in cancer therapy. Recent research has particularly focused on the development of dual BET/HDAC inhibitors for hard-to-treat tumors, such as pancreatic cancer. Here, we developed a new series of potent dual BET/HDAC inhibitors by choosing starting scaffolds that enabled us to optimally merge the two functionalities into a single compound. Systematic structure-guided modification of both warheads then led to optimized binders that were superior in potency to both parent compounds, with the best molecules of this series binding to both BRD4 bromodomains as well as HDAC1/2 with EC₅₀ values in the 100 nM range in cellular NanoBRET target engagement assays. For one of our lead molecules, we could also show the selective inhibition of HDAC1/2 over all other zinc-dependent HDACs. Importantly, this on-target activity translated into promising efficacy in pancreatic cancer and NUT midline carcinoma cells. Our lead molecules effectively blocked histone H3 deacetylation in pancreatic cancer cells and upregulated the tumor suppressor HEXIM1 and proapoptotic p57, both markers of BET inhibition. In addition, they have the potential to downregulate the oncogenic drivers of NUT midline carcinoma, as demonstrated for MYC and TP63 mRNA levels. Overall, this study expands the portfolio of available dual BET/class I HDAC inhibitors for future translational studies in different cancer models.

MST1, MST2, MST3, MST4, and YSK1 are conserved members of the mammalian sterile 20-like serine/threonine (MST) family that regulate cellular functions such as proliferation and migration. The MST3 isozyme plays a role in regulating cell growth and apoptosis, and its dysregulation has been linked to high-grade tumors. To date, there are no isoform-selective inhibitors that could be used for validating the role of MST3 in tumorigenesis. We designed a series of 3-aminopyrazole-based macrocycles based on the structure of a promiscuous inhibitor. By varying the moieties targeting the solvent-exposed region and optimizing the linker, macrocycle JA310 (21c) was synthesized. JA310 exhibited high cellular potency for MST3 (EC₅₀ = 106 nM) and excellent kinome-wide selectivity. The crystal structure of the MST3-JA310 complex provided intriguing insights into the binding mode, which is associated with large-scale structural rearrangements. In summary, JA310 demonstrates the utility of macrocyclization for the design of highly selective inhibitors and presents the first chemical probe for MST3.

Mammalian STE20-like (MST) kinases 1-4 play key roles in regulating the Hippo and autophagy pathways, and their dysregulation has been implicated in cancer development. In contrast to the well-studied MST1/2, the roles of MST3/4 are less clear, in part due to the lack of potent and selective MST3/4 inhibitors. Here, we re-evaluated literature compounds, and used structure-guided design to optimize the p21-activated kinase (PAK) inhibitor G-5555 ( 8 ) to selectively target MST3/4. These efforts resulted in the development of MR24 ( 24 ) and MR30 ( 27 ) with good kinome-wide selectivity, high potency for MST3/4, and selectivity towards the closely related MST1/2. In combination with the MST1/2 inhibitor PF-06447475 ( 2 ) the two MST3/4 inhibitors can be used to elucidate the multiple roles of MST kinases in cells. We found that MST3/4-selective inhibition caused a cell cycle arrest in the G1 phase, while MST1/2 inhibition resulted in accumulation of cells in the G2/M phase. These data point to distinct functions of these closely related kinases, which can now be addressed with subfamily-selective chemical tool compounds.

Bone morphogenetic protein (BMP) signaling is mediated by transmembrane protein kinases that form heterotetramers consisting of type-I and type-II receptors. Upon BMP binding, the constitutively active type-II receptors activate specific type-I receptors by transphosphorylation, resulting in the phosphorylation of SMAD effector proteins. Drug discovery in the receptor tyrosine kinase-like (TKL) family has largely focused on type-I receptors, with few inhibitors that have been published targeting type-II receptors. BMPR2 is involved in several diseases, most notably pulmonary arterial hypertension, but also contributes to Alzheimer's disease and cancer. Here, we report that macrocyclization of the promiscuous inhibitor 1, based on a 3-amino-1H-pyrazole hinge binding moiety, led to a selective and potent BMPR2 inhibitor 8a.

Salt-inducible kinases 1-3 (SIK1-3) are key regulators of the LKB1-AMPK pathway and play an important role in cellular homeostasis. Dysregulation of any of the three isoforms has been associated with tumorigenesis in liver, breast, and ovarian cancers. We have recently developed the dual pan-SIK/group I p21-activated kinase (PAK) chemical probe MRIA9. However, inhibition of p21-activated kinases has been associated with cardiotoxicity in vivo , which complicates the use of MRIA9 as a tool compound. Here, we present a structure-based approach involving the back-pocket and gatekeeper residues, for narrowing the selectivity of pyrido[2,3-d]pyrimidin-7(8 H )-one-based inhibitors towards SIK kinases, eliminating PAK activity. Optimization was guided by high-resolution crystal structure analysis and computational methods, resulting in a pan-SIK inhibitor, MR22, which no longer exhibited activity on STE group kinases and displayed excellent selectivity in a representative kinase panel. MR22-dependent SIK inhibition led to centrosome dissociation and subsequent cell-cycle arrest in ovarian cancer cells, as observed with MRIA9, conclusively linking these phenotypic effects to SIK inhibition. Taken together, MR22 represents a valuable tool compound for studying SIK kinase function in cells.