Small molecule inhibitors for cancer immunotherapy and associated biomarkers - the current status.

Following the success of cancer immunotherapy using large molecules against immune checkpoint inhibitors, the concept of using small molecules to interfere with intracellular negative regulators of anti-tumor immune responses has emerged in recent years. The main targets for small molecule drugs currently include enzymes of negative feedback loops in signaling pathways of immune cells and proteins that promote immunosuppressive signals within the tumor microenvironment. In the adaptive immune system, negative regulators of T cell receptor signaling (MAP4K1, DGKα/ζ, CBL-B, PTPN2, PTPN22, SHP1), co-receptor signaling (CBL-B) and cytokine signaling (PTPN2) have been preclinically validated as promising targets and initial clinical trials with small molecule inhibitors are underway. To enhance innate anti-tumor immune responses, inhibitory immunomodulation of cGAS/STING has been in the focus, and inhibitors of ENPP1 and TREX1 have reached the clinic. In addition, immunosuppressive signals via adenosine can be counteracted by CD39 and CD73 inhibition, while suppression via intratumoral immunosuppressive prostaglandin E can be targeted by EP2/EP4 antagonists. Here, we present the status of the most promising small molecule drug candidates for cancer immunotherapy, all residing relatively early in development, and the potential of relevant biomarkers.

In Vitro Characterization of Sphingosine 1-Phosphate Receptor 1 (S1P1) Expression and Mediated Migration of Primary Human T and B Cells in the Context of Cenerimod, a Novel, Selective S1P1 Receptor Modulator.

Cenerimod is a potent, selective sphingosine 1-phosphate receptor 1 (S1P1) modulator currently investigated in a Phase IIb study in patients with systemic lupus erythematosus (SLE) (NCT03742037). S1P1 receptor modulators sequester circulating lymphocytes within lymph nodes, thereby reducing pathogenic autoimmune cells (including T and B lymphocytes) in the bloodstream and inflamed tissues, making them an effective therapeutic concept for autoimmune disorders. Although the effect of S1P receptor modulators in reducing circulating lymphocytes is well documented, the precise molecular role of the S1P1 receptor on these cell types is not fully understood. In this study, the mode of action of cenerimod on human primary lymphocytes in different activation states was investigated focusing on their chemotactic behavior towards S1P in real-time, concomitant to S1P1 receptor expression and internalization dynamics. Here, we show that cenerimod effectively prevents T and B cell migration in a concentration-dependent manner. Interestingly, while T cell activation led to strong S1P1 re-expression and enhanced migration; in B cells, an enhanced migration capacity and S1P1 receptor surface expression was observed in an unstimulated state. Importantly, concomitant treatment with glucocorticoids (GCs), a frequently used treatment for autoimmune disorders, had no impact on the inhibitory activity of cenerimod on lymphocytes.

Regulation of CYLD activity and specificity by phosphorylation and ubiquitin-binding CAP-Gly domains.

Non-degradative ubiquitin chains and phosphorylation events govern signaling responses by innate immune receptors. The deubiquitinase CYLD in complex with SPATA2 is recruited to receptor signaling complexes by the ubiquitin ligase LUBAC and regulates Met1- and Lys63-linked polyubiquitin and receptor signaling outcomes. Here, we investigate the molecular determinants of CYLD activity. We reveal that two CAP-Gly domains in CYLD are ubiquitin-binding domains and demonstrate a requirement of CAP-Gly3 for CYLD activity and regulation of immune receptor signaling. Moreover, we identify a phosphorylation switch outside of the catalytic USP domain, which activates CYLD toward Lys63-linked polyubiquitin. The phosphorylated residue Ser568 is a novel tumor necrosis factor (TNF)-regulated phosphorylation site in CYLD and works in concert with Ser418 to enable CYLD-mediated deubiquitination and immune receptor signaling. We propose that phosphorylated CYLD, together with SPATA2 and LUBAC, functions as a ubiquitin-editing complex that balances Lys63- and Met1-linked polyubiquitin at receptor signaling complexes to promote LUBAC signaling.

Design of pyrido[2,3-d]pyrimidin-7-one inhibitors of receptor interacting protein kinase-2 (RIPK2) and nucleotide-binding oligomerization domain (NOD) cell signaling.

Receptor interacting protein kinase-2 (RIPK2) is an enzyme involved in the transduction of pro-inflammatory nucleotide-binding oligomerization domain (NOD) cell signaling, a pathway implicated in numerous chronic inflammatory conditions. Herein, a pyrido[2,3-d]pyrimidin-7-one based class of RIPK2 kinase and NOD2 cell signaling inhibitors is described. For example, 33 (e.g. UH15-15) inhibited RIPK2 kinase (IC50 = 8 ± 4 nM) and displayed > 300-fold selectivity versus structurally related activin receptor-like kinase 2 (ALK2). This molecule blocked NOD2-dependent HEKBlue NF-κB activation (IC50 = 20 ± 5 nM) and CXCL8 production (at concentrations > 10 nM). Molecular docking suggests that engagement of Ser25 in the glycine-rich loop may provide increased selectivity versus ALK2 and optimal occupancy of the region between the gatekeeper and the αC-helix may contribute to potent NOD2 cell signaling inhibition. Finally, this compound also demonstrated favorable in vitro ADME and pharmacokinetic properties (e.g. Cmax = 5.7 µM, Tmax = 15 min, t1/2 = 3.4 h and Cl = 45 mL/min/kg following single 10 mg/kg intraperitoneal administration) further supporting the use of pyrido[2,3-d]pyrimidin-7-ones as a new structure class of RIPK2 kinase and NOD cell signaling inhibitors.

Receptor-interacting protein kinase 2 (RIPK2) and nucleotide-binding oligomerization domain (NOD) cell signaling inhibitors based on a 3,5-diphenyl-2-aminopyridine scaffold.

Receptor-interacting protein kinase 2 (RIPK2) is a key mediator of nucleotide-binding oligomerization domain (NOD) cell signaling that has been implicated in various chronic inflammatory conditions. A new class of RIPK2 kinase/NOD signaling inhibitors based on a 3,5-diphenyl-2-aminopyridine scaffold was developed. Several co-crystal structures of RIPK2•inhibitor complexes were analyzed to provide insights into inhibitor selectivity versus the structurally related activin receptor-like kinase 2 (ALK2) demonstrating that the inhibitor sits deeper in the hydrophobic binding pocket of RIPK2 perturbing the orientation of the DFG motif. In addition, the structure-activity relationship study revealed that in addition to anchoring to the hinge and DFG via the 2-aminopyridine and 3-phenylsulfonamide, respectively, appropriate occupancy of the region between the gatekeeper and the αC-helix provided by substituents in the 4- and 5-positions of the 3-phenylsulfonamide were necessary to achieve potent NOD cell signaling inhibition. For example, compound 18t (e.g. CSLP37) displayed potent biochemical RIPK2 kinase inhibition (IC50 = 16 ± 5 nM), >20-fold selectivity versus ALK2 and potent NOD cell signaling inhibition (IC50 = 26 ± 4 nM) in the HEKBlue assay. Finally, in vitro ADME and pharmacokinetic characterization of 18t further supports the prospects of the 3,5-diphenyl-2-aminopyridine scaffold for the generation of in vivo pharmacology probes of RIPK2 kinase and NOD cell signaling functions.

Small molecule inhibitors reveal an indispensable scaffolding role of RIPK2 in NOD2 signaling.

RIPK2 mediates inflammatory signaling by the bacteria-sensing receptors NOD1 and NOD2. Kinase inhibitors targeting RIPK2 are a proposed strategy to ameliorate NOD-mediated pathologies. Here, we reveal that RIPK2 kinase activity is dispensable for NOD2 inflammatory signaling and show that RIPK2 inhibitors function instead by antagonizing XIAP-binding and XIAP-mediated ubiquitination of RIPK2. We map the XIAP binding site on RIPK2 to the loop between ß2 and ß3 of the N-lobe of the kinase, which is in close proximity to the ATP-binding pocket. Through characterization of a new series of ATP pocket-binding RIPK2 inhibitors, we identify the molecular features that determine their inhibition of both the RIPK2-XIAP interaction, and of cellular and in vivoNOD2 signaling. Our study exemplifies how targeting of the ATP-binding pocket in RIPK2 can be exploited to interfere with the RIPK2-XIAP interaction for modulation of NOD signaling.

Requirement of GSK-3 for PUMA induction upon loss of pro-survival PI3K signaling.

Growth factor withdrawal induces rapid apoptosis via mitochondrial outer membrane permeabilization. We had previously observed that cell death of IL-3-dependent Ba/F3 cells, induced by removal of the growth factor, required the activity of the kinase GSK-3. Employing CRISPR/Cas9-mediated gene knockout, we aimed to identify pro-apoptotic GSK-3 regulated factors in this process. Knockout of either Puma or Bim demonstrated that the induction of Puma, but not Bim, was crucial for apoptosis induced by IL-3 deprivation. Thus, we aimed at identifying the GSK-3-dependent PUMA regulator. Loss of FOXO3A reduced the induction of Puma, while additional loss of p53 completely repressed induction upon growth factor withdrawal. A constitutively active mutant of FOXO3A, which cannot be controlled by AKT directly, still required active GSK-3 for the full transcriptional induction of Puma and cell death upon IL-3 withdrawal. Thus, the suppression of GSK-3 is the key function of PI3K signaling in order to prevent the induction of Puma by FOXO3A and p53 and thereby apoptosis upon growth factor withdrawal.

CDK9-mediated phosphorylation controls the interaction of TIP60 with the transcriptional machinery.

The acetyltransferase TIP60 is regulated by phosphorylation, and we have previously shown that phosphorylation of TIP60 on S86 by GSK-3 promotes p53-mediated induction of the BCL-2 protein PUMA. TIP60 phosphorylation by GSK-3 requires a priming phosphorylation on S90, and here, we identify CDK9 as a TIP60S90 kinase. We demonstrate that a phosphorylation-deficient mutant, TIP60S90A, exhibits reduced interaction with chromatin, histone 3 and RNA Pol II, while its association with the TIP60 complex subunit EPC1 is not affected. Consistently, we find a diminished association of TIP60S90A with the MYC gene. We show that cells expressing TIP60S90A, but also TIP60S86A, which retains S90 phosphorylation, exhibit reduced histone 4 acetylation and proliferation. Thus, our data indicate that, during transcription, phosphorylation of TIP60 at two sites has different regulatory effects on TIP60, whereby S90 phosphorylation controls association with the transcription machinery, and S86 phosphorylation is regulating TIP60 HAT activity.

SPATA2: more than a missing link.

The assembly of the TNFR1 signalling complex (TNF-RSC) depends on K63- and M1-linked ubiquitylation, promoting the recruitment of complex constituents and the stability of the complex. Ubiquitylation is a dynamic process, controlled by E3 ubiquitin ligases as well as deubiquitinases, such as CYLD and OTULIN. A novel molecule, SPATA2, which is crucial for recruiting and activating the deubiquitinase CYLD within the TNF-RSC, has now been identified by four different studies. Loss of SPATA2 was shown to result in increased TNF-, but also NOD2-mediated proinflammatory signalling. Importantly, SPATA2 is instrumental for TNF-induced cell death, and a closer look at these findings suggests that SPATA2 possibly has functions beyond promoting the activity of CYLD.

SPATA2 promotes CYLD activity and regulates TNF-induced NF-κB signaling and cell death.

K63- and Met1-linked ubiquitylation are crucial posttranslational modifications for TNF receptor signaling. These non-degradative ubiquitylations are counteracted by deubiquitinases (DUBs), such as the enzyme CYLD, resulting in an appropriate signal strength, but the regulation of this process remains incompletely understood. Here, we describe an interaction partner of CYLD, SPATA2, which we identified by a mass spectrometry screen. We find that SPATA2 interacts via its PUB domain with CYLD, while a PUB interaction motif (PIM) of SPATA2 interacts with the PUB domain of the LUBAC component HOIP SPATA2 is required for the recruitment of CYLD to the TNF receptor signaling complex upon TNFR stimulation. Moreover, SPATA2 acts as an allosteric activator for the K63- and M1-deubiquitinase activity of CYLD In consequence, SPATA2 substantially attenuates TNF-induced NF-κB and MAPK signaling. Conversely, SPATA2 is required for TNF-induced complex II formation, caspase activation, and apoptosis. Thus, this study identifies SPATA2 as an important factor in the TNF signaling pathway with a substantial role for the effects mediated by the cytokine.

GSK-3 - at the crossroads of cell death and survival.

Glycogen synthase kinase 3 (GSK-3) is involved in various signaling pathways controlling metabolism, differentiation and immunity, as well as cell death and survival. GSK-3 targets transcription factors, regulates the activity of metabolic and signaling enzymes, and controls the half-life of proteins by earmarking them for degradation. GSK-3 is unique in its mode of substrate recognition and the regulation of its kinase activity, which is repressed by pro-survival phosphoinositide 3-kinase (PI3K)-AKT signaling. In turn, GSK-3 exhibits pro-apoptotic functions when the PI3K-AKT pathway is inactive. Nevertheless, as GSK-3 is crucially involved in many signaling pathways, its role in cell death regulation is not uniform, and in some situations it promotes cell survival. In this Commentary, we focus on the various aspects of GSK-3 in the regulation of cell death and survival. We discuss the effects of GSK-3 on the regulation of proteins of the BCL-2 family, through which GSK-3 exhibits pro-apoptotic activity. We also highlight the pro-survival activities of GSK-3, which are observed in the context of nuclear factor κB (NFκB) signaling, and we discuss how GSK-3, by impacting on cell death and survival, might play a role in diseases such as cancer.