HPK1 as a novel target for cancer immunotherapy

Structure-Guided Optimization of 2-Aminoquinazoline Hematopoietic Progenitor Kinase 1 Inhibitors for Improved Oral Bioavailability and Synergistic Antitumor Immunity

Hematopoietic progenitor kinase 1 (HPK1), a pivotal T-cell immunity suppressor, offers transformative potential to overcome immune checkpoint resistance, yet existing inhibitors fail to balance potency, selectivity, and pharmacokinetics. We developed a spatially resolved strategy within a unified chemical framework of our 2-aminoquinazoline core, integrating (1) high-affinity engagement of the HPK1 hinge-region subpocket (Leu23/Phe93/Gly95) through bidentate hydrogen bonding and hydrophobic packing with (2) strategic occupation of a solvent-exposed allosteric site to sterically block CYP3A4/2C9/2D6-mediated oxidative metabolism. Optimized compound 39 demonstrated subnanomolar binding affinity (IC50 = 0.70 nM) with moderate selectivity, combined with high metabolic stability in human liver microsomes (CLint < 1 mL/min/kg) and favorable oral bioavailability (>100%) in mice. In CT26 models, compound 39 synergized with anti-PD-1 (60% tumor growth inhibition) by expanding IFN-γ+CD8+ tumor-infiltrating lymphocytes (7-fold) and enhancing splenic IFN-γ production (3-fold). This work validates 2-aminoquinazolines as a novel HPK1 chemotype addressing metabolic instability─a key hurdle in kinase drug discovery.

Discovery of a New and Selective HPK1 PROTAC for Enhancing Tumor Immunotherapy through Eliminating GLK Degradation

HPK1 is an attractive therapeutic target for tumor immunotherapy. Nevertheless, the formidable challenge selectivity over GLK and limited antitumor efficacy of HPK1 inhibitors and PROTACs impeded their developments. Here, we demonstrated that blocking GLK alone or simultaneous blocking both GLK and HPK1 could reduce immune activation through siRNA, which underscores the necessity for designing selective HPK1 degraders. Given the differences in spatial tolerance between HPK1 and GLK, we successfully obtained a degrader, designated E3, which exhibits a DC50 of 3.16 nM for HPK1 and demonstrates at least a 1000-fold selectivity over GLK. Notably, E3 displayed a superior capacity for promoting immune activation. Oral administration of E3 combined with PDL-1 antibody showed significant antitumor activity. In conclusion, the availability of E3 provides a structural foundation for the development of selective HPK1 PROTACs.

Discovery and Optimization of Pyrazine Carboxamide AZ3246, a Selective HPK1 Inhibitor

Hematopoietic progenitor kinase 1 (HPK1) is a negative regulator of the T cell receptor signaling pathway and is therefore a target of interest for immunooncology. Nonselective HPK1 inhibitors may affect other kinase components of T cell activation, blunting the beneficial impact of enhanced T cell activity that results from HPK1 inhibition itself. Here, we report the discovery of pyrazine carboxamide HPK1 inhibitors and their optimization through structure-based drug design to afford a highly selective HPK1 inhibitor, compound 24 (AZ3246). This compound induces IL-2 secretion in T cells with an EC50 of 90 nM without inhibiting antagonistic kinases, exhibits pharmacokinetic properties consistent with oral dosing, and demonstrates antitumor activity in the EMT6 syngeneic mouse model.

An Oral PROTAC Targeting HPK1 Degradation Potentiates Anti-Solid Tumor Immunity

Hematopoietic progenitor pinase1 (HPK1) knockout has been identified as an efficient route to enhance anti-tumor immune response. Here, this work develops an oral proteolysis targeting chimera (PROTAC) targeting HPK1 to efficiently and selectively degrade HPK1 to augment immunotherapeutic outcomes. In a postoperative tumor model of human cervical cancer in NSG mice, the orally-administrated PROTAC can reach tumors, down-regulate HPK1 levels in locally-administrated CAR-T cells, and promote their efficiency in inhibiting solid tumor recurrence, achieving 50% partial response (PR) and 50% complete response (CR). In addition, oral administration of PROTAC can amplify the suppression capability of the anti-PD-L1 antibody on the growth of CT26 solid tumors in BALB/c mice by promoting the infiltration of CD45-positive immune cells from 0.7% to 1.5% and CD3-positive T cells from 0.2% to 0.5% within the tumors.

Discovery of PF-07265028, A Selective Small Molecule Inhibitor of Hematopoietic Progenitor Kinase 1 (HPK1) for the Treatment of Cancer

Hematopoietic progenitor kinase 1 (HPK1/MAP4K1) represents a high interest target for the treatment of cancer through an immune-mediated mechanism. Herein we present highlights of the drug discovery campaign within the lactam/azalactam series of inhibitors that yielded a small molecule (21, PF-07265028), which was advanced to a phase 1 clinical trial (NCT05233436). Key components of the discovery effort included optimization of potency through mitigation of ligand strain as guided by the use of cocrystal structures, mitigation of ADME liabilities (plasma instability and fraction metabolism by CYP2D6), and optimization of kinase selectivity, particularly over immune-modulating kinases with high homology to HPK1. Structure-based drug design via leveraging cocrystal structures and lipophilic efficiency analysis proved to be valuable tools that ultimately enabled the delivery of a clinical-quality small molecule inhibitor of HPK1.

An updated review of small-molecule HPK1 kinase inhibitors (2016-present)

Hematopoietic progenitor kinase 1 (HPK1) is a serine-threonine kinase specific to hematopoiesis and a member of the MAP4K family of Ste20-related protein kinases. Targeting HPK1 to ameliorate T cell exhaustion and enhance T cell functions is a promising strategy for clinical immunotherapies. Numerous studies have reported the progress in developing effective HPK1 inhibitors and elucidating their mechanisms of action. However, most inhibitors affect multiple signaling pathways, resulting in unintended side effects that limit their clinical development and application. Herein, we reviewed HPK1-related signaling pathways, clinical candidates and recent advances in small-molecule inhibitors targeting HPK1. Additionally, we present our perspectives on current challenges and potential future research field, hoping to provide inspiration for the development of novel HPK1 inhibitors.

Discovery of BAY-405: An Azaindole-Based MAP4K1 Inhibitor for the Enhancement of T-Cell Immunity against Cancer

Mitogen-activated protein kinase kinase kinase kinase 1 (MAP4K1) is a serine/threonine kinase that acts as an immune checkpoint downstream of T-cell receptor stimulation. MAP4K1 activity is enhanced by prostaglandin E2 (PGE2) and transforming growth factor beta (TGFβ), immune modulators commonly present in the tumor microenvironment. Therefore, its pharmacological inhibition is an attractive immuno-oncology concept for inducing therapeutic T-cell responses in cancer patients. Here, we describe the systematic optimization of azaindole-based lead compound 1, resulting in the discovery of potent and selective MAP4K1 inhibitor 38 (BAY-405) that displays nanomolar potency in biochemical and cellular assays as well as in vivo exposure after oral dosing. BAY-405 enhances T-cell immunity and overcomes the suppressive effect of PGE2 and TGFβ. Treatment of tumor-bearing mice shows T-cell-dependent antitumor efficacy. MAP4K1 inhibition in conjunction with PD-L1 blockade results in a superior antitumor impact, illustrating the complementarity of the single agent treatments.

Discovery of an Exceptionally Orally Bioavailable and Potent HPK1 PROTAC with Enhancement of Antitumor Efficacy of Anti-PD-L1 Therapy

HPK1, a well-known negative regulator of T cell receptors, can cause T cell dysfunction when abnormally activated. In this study, a PROTAC C3 was designed and synthesized by optimizing the physicochemical properties of the warhead, linker, and CRBN ligand. C3 demonstrated significant HPK1 degradation with a DC50 of 21.26 nM, excellent oral absorption with a Cmax of 10,899.92 ng/mL, and a bioavailability (F %) of 81.7%. C3 also showed degradation selectivity and potent immune activation effects. Proteomic and WB analyses revealed that immune-activating effect of C3 is attributed to the inhibition of SLP76 and NF-κB signaling pathways, as well as the enhancement of MAPK signaling pathway transduction. In vivo efficacy study demonstrated that oral administration of C3 in combination with anti-PDL1 antibody significantly inhibited tumor growth (tumor growth inhibition = 65.58%). These findings suggest that C3, a novel HPK1 PROTAC, holds promise as a therapeutic agent for tumor immunotherapy.

Novel hematopoietic progenitor kinase 1 inhibitor KHK-6 enhances T-cell activation

Inhibiting the functional role of negative regulators in immune cells is an effective approach for developing immunotherapies. The serine/threonine kinase hematopoietic progenitor kinase 1 (HPK1) involved in the T-cell receptor signaling pathway attenuates T-cell activation by inducing the degradation of SLP-76 through its phosphorylation at Ser-376, reducing the immune response. Interestingly, several studies have shown that the genetic ablation or pharmacological inhibition of HPK1 kinase activity improves the immune response to cancers by enhancing T-cell activation and cytokine production; therefore, HPK1 could be a promising druggable target for T-cell-based cancer immunotherapy. To increase the immune response against cancer cells, we designed and synthesized KHK-6 and evaluated its cellular activity to inhibit HPK1 and enhance T-cell activation. KHK-6 inhibited HPK1 kinase activity with an IC50 value of 20 nM and CD3/CD28-induced phosphorylation of SLP-76 at Ser-376 Moreover, KHK-6 significantly enhanced CD3/CD28-induced production of cytokines; proportion of CD4+ and CD8+ T cells that expressed CD69, CD25, and HLA-DR markers; and T-cell-mediated killing activity of SKOV3 and A549 cells. In conclusion, KHK-6 is a novel ATP-competitive HPK1 inhibitor that blocks the phosphorylation of HPK1 downstream of SLP-76, enhancing the functional activation of T cells. In summary, our study showed the usefulness of KHK-6 in the drug discovery for the HPK1-inhibiting immunotherapy.

HPK1 citron homology domain regulates phosphorylation of SLP76 and modulates kinase domain interaction dynamics

Hematopoietic progenitor kinase 1 (HPK1) is a negative regulator of T-cell receptor signaling and as such is an attractive target for cancer immunotherapy. Although the role of the HPK1 kinase domain (KD) has been extensively characterized, the function of its citron homology domain (CHD) remains elusive. Through a combination of structural, biochemical, and mechanistic studies, we characterize the structure-function of CHD in relationship to KD. Crystallography and hydrogen-deuterium exchange mass spectrometry reveal that CHD adopts a seven-bladed β-propellor fold that binds to KD. Mutagenesis associated with binding and functional studies show a direct correlation between domain-domain interaction and negative regulation of kinase activity. We further demonstrate that the CHD provides stability to HPK1 protein in cells as well as contributes to the docking of its substrate SLP76. Altogether, this study highlights the importance of the CHD in the direct and indirect regulation of HPK1 function.

Current strategies for targeting HPK1 in cancer and the barriers to preclinical progress

INTRODUCTION

Hematopoietic progenitor kinase 1 (HPK1), a 97-kDa serine/threonine Ste20-related protein kinase, functions as an intracellular negative regulator, primarily in hematopoietic lineage cells, where it regulates T cells, B cells, dendritic cells, and other immune cells. Loss of HPK1 kinase activity results in exacerbated cytokine secretion, enhanced T cell signaling, improved viral clearance, and thus increased restraint of tumor growth. These findings highlight HPK1 as a promising target for immuno-oncology treatments, culminating in the advancement of candidate compounds targeting HPK1 to clinical trials by several biotech enterprises.

AREAS COVERED

Through searching PubMed, Espacenet-patent search, and clinicaltrials.gov, this review provides a comprehensive analysis of HPK1, encompassing its structure and roles in various downstream signaling pathways, the consequences of constitutive activation of HPK1, and potential therapeutic strategies to treat HPK1-driven malignancies. Moreover, the review outlines the patents issued for small molecule inhibitors and clinical investigations of HPK1.

EXPERT OPINION

To enhance the success of tumor immunotherapy in clinical trials, it is important to develop protein degraders, allosteric inhibitors, and antibody-drug conjugates based on the crystal structure of HPK1, and to explore combination therapy approaches. Although several challenges remain, the development of HPK1 inhibitors display promising in preclinical and clinical studies.

Discovery of novel, potent, selective and orally bioavailable HPK1 inhibitor for enhancing the efficacy of anti-PD-L1 antibody

Hematopoietic progenitor kinase 1 (HPK1), a serine/threonine kinase in the MAP4K family, is expressed predominantly in immune cells, and has been identified as a negative regulator of immune signaling. Accumulating evidences demonstrated that loss of HPK1 kinase function effectively enhances anti-tumor responses. In this study, we disclose the medicinal chemistry campaigns to discovery potent, selective, and orally active HPK1 inhibitors, starting from our previous work based on rigidification strategy. Systematically structure-activity relationship (SAR) exploration led to the identification of F03 (HMC-B17). The representative compound, HMC-B17, showed the potent HPK1 inhibition with an IC50 value of 1.39 nM and favorable selectivity against TCR-related kinases. In addition, the HMC-B17 effectively enhanced the IL-2 secretion in Jurkat cells (EC50 = 11.56 nM). Strikingly, immune-reverse effects and improved immune response in vivo were observed after HMC-B17 treatment. Furthermore, HMC-B17 combined with anti-PD-L1 antibody demonstrated a synergistic antitumor efficacy with TGI% value of 71.24 % in CT26 model. Collectively, our findings suggest that HMC-B17 could be a valuable lead compound to develop a safe and potent HPK1 inhibitor for further cancer immunotherapy.

Discovery of highly efficient CRBN-recruiting HPK1-PROTAC as a potential chemical tool for investigation of scaffolding roles in TCR signaling

Hematopoietic progenitor kinase 1 (HPK1, MAP4K1) is a promising target for immune-oncology therapy. It has been recently demonstrated that loss of HPK1 kinase activity can enhance T cell receptor (TCR) signaling. However, many essential functions mediated by the HPK1 scaffolding role are still beyond the reach of any kinase inhibitor. Proteolysis targeting chimera (PROTAC) has emerged as a promising strategy for pathogenic proteins degradation with the characteristics of rapid, reversible, and low-cost versus RNA interference or DNA knock-out technology. Herein we first disclosed the design, synthesis, and evaluation of a series of thalidomide-based PROTAC molecules and identified B1 as a highly efficient HPK1 degrader with DC50 value of 1.8 nM. Further mechanism investigation demonstrated that compound B1 inhibits phosphorylation of the SLP76 protein with IC50 value of 496.1 nM, and confirmed that B1 is a bona fide HPK1-PROTAC degrader. Thus, this study provides a basis for HPK1 degraders development and the candidate could be used as a potential chemical tool for further investigation of the kinase-independent signaling of HPK1 in TCR.

Discovery of potent and selective HPK1 inhibitors based on the 2,4-disubstituted pyrimidine scaffold with immune modulatory properties for ameliorating T cell exhaustion

Hematopoietic progenitor kinase 1 (HPK1), a member of the mitogen-activated protein kinase (MAP4K) family, is a serine/threonine (SER/THR) kinase and has been demonstrated as a negative regulator of T cell receptor signaling. Targeting HPK1 has been considered as an attractive therapeutic strategy for immune-oncology. Here, we describe the discovery and structure-activity relationship (SAR) of potent HPK1 inhibitors based on the 2,4-disubstituted pyrimidine scaffold. Systematically SAR exploration afforded the desired compound HMC-H8 (F1) with potent HPK1 inhibition (IC50 = 1.11 nM) and highly selectivity profile. Compound HMC-H8 also exhibited robust inhibition of p-SLP 76 (IC50 = 283.0 nM) and promotion IL-2 release (EC50 = 157.08 nM), and INF-γ production in a dose-dependent manner in vitro assays. Strikingly, HMC-H8 shown effective immune reversal response in immunesuppressive condition. Moreover, Compound HMC-H8 displayed acceptable metabolic stability (T1/2 = 56.87 min), along with low CYP450 inhibition in human liver microsomes and good oral bioavailability (F = 15.05%) in rat. Furthermore, HMC-H8 was found to modulate the expression of c-Myc in Western blotting experiments. Taken together, this study provides new potent HPK1 inhibitors for further anticancer drug discovery based on immuno-oncology.

Negative intracellular regulators of T-cell receptor (TCR) signaling as potential antitumor immunotherapy targets

Immunotherapy strategies aim to mobilize immune defenses against tumor cells by targeting mainly T cells. Co-inhibitory receptors or immune checkpoints (ICPs) (such as PD-1 and CTLA4) can limit T cell receptor (TCR) signal propagation in T cells. Antibody-based blocking of immune checkpoints (immune checkpoint inhibitors, ICIs) enable escape from ICP inhibition of TCR signaling. ICI therapies have significantly impacted the prognosis and survival of patients with cancer. However, many patients remain refractory to these treatments. Thus, alternative approaches for cancer immunotherapy are needed. In addition to membrane-associated inhibitory molecules, a growing number of intracellular molecules may also serve to downregulate signaling cascades triggered by TCR engagement. These molecules are known as intracellular immune checkpoints (iICPs). Blocking the expression or the activity of these intracellular negative signaling molecules is a novel field of action to boost T cell-mediated antitumor responses. This area is rapidly expanding. Indeed, more than 30 different potential iICPs have been identified. Over the past 5 years, several phase I/II clinical trials targeting iICPs in T cells have been registered. In this study, we summarize recent preclinical and clinical data demonstrating that immunotherapies targeting T cell iICPs can mediate regression of solid tumors including (membrane associated) immune-checkpoint inhibitor refractory cancers. Finally, we discuss how these iICPs are targeted and controlled. Thereby, iICP inhibition is a promising strategy opening new avenues for future cancer immunotherapy treatments.

Design and Synthesis of Functionally Active 5-Amino-6-Aryl Pyrrolopyrimidine Inhibitors of Hematopoietic Progenitor Kinase 1

Immune activating agents represent a valuable class of therapeutics for the treatment of cancer. An area of active research is expanding the types of these therapeutics that are available to patients via targeting new biological mechanisms. Hematopoietic progenitor kinase 1 (HPK1) is a negative regulator of immune signaling and a target of high interest for the treatment of cancer. Herein, we present the discovery and optimization of novel amino-6-aryl pyrrolopyrimidine inhibitors of HPK1 starting from hits identified via virtual screening. Key components of this discovery effort were structure-based drug design aided by analyses of normalized B-factors and optimization of lipophilic efficiency.

PROTACs: Novel tools for improving immunotherapy in cancer

Posttranslational modifications (PTMs), such as phosphorylation, methylation, ubiquitination, and acetylation, are important in governing protein expression levels. Proteolysis targeting chimeras (PROTACs) are novel structures designed to target a protein of interest (POI) for ubiquitination and degradation, leading to the selective reduction in the expression levels of the POI. PROTACs have exhibited great promise due to their ability to target undruggable proteins, including several transcription factors. Recently, PROTACs have been characterized to improve anticancer immunotherapy via the regulation of specific proteins. In this review, we describe how the PROTACs target several molecules, including HDAC6, IDO1, EGFR, FoxM1, PD-L1, SHP2, HPK1, BCL-xL, BET proteins, NAMPT, and COX-1/2, to regulate immunotherapy in human cancers. PROTACs may provide potential treatment benefits by enhancing immunotherapy in cancer patients.

Theoretical Studies on Selectivity of HPK1/JAK1 Inhibitors by Molecular Dynamics Simulations and Free Energy Calculations

Hematopoietic progenitor kinase 1 (HPK1) is a negative regulator of T cell receptor, which has been regarded as a potential target for immunotherapy. Yu et al. observed the off-target effect of the high-throughput screening HPK1 kinase inhibitor hits on JAK1 kinase. The off-target effect is usually due to the lack of specificity of the drug, resulting in toxic side effects. Therefore, exploring the mechanisms to selectively inhibit HPK1 is critical for developing effective and safe inhibitors. In this study, two indazole compounds as HPK1 inhibitors with different selectivity towards JAK1 were used to investigate the selectivity mechanism using multiple computational methods, including conventional molecular dynamics simulations, binding free energy calculations and umbrella sampling simulations. The results indicate that the salt bridge between the inhibitor and residue Asp101 of HPK1 favors their selectivity towards HPK1 over JAK1. Information obtained from this study can be used to discover and design more potent and selective HPK1 inhibitors for immunotherapy.

Discovery of Pyrazolopyridine Derivatives as HPK1 Inhibitors

In spite of the great success of immune checkpoint inhibitors in immune-oncology therapy, an urgent need still exists to identify alternative approaches to broaden the scope of therapeutic coverage. Hematopoietic progenitor kinase 1 (HPK1), also known as MAP4K1, functions as a negative regulator of activation signals generated by the T cell antigen receptor. Herein we report the discovery of novel pyrazolopyridine derivatives as selective inhibitors of HPK1. The structure-activity relationship campaign led to the discovery of compound 16, which has shown promising enzymatic and cellular potency with encouraging kinome selectivity. The outstanding pharmacokinetic profiles of 16 in rats and monkeys supported further evaluations of its efficacy and safety in preclinical models.

Potent and Selective Biaryl Amide Inhibitors of Hematopoietic Progenitor Kinase 1 (HPK1)

Herein we report the discovery of a novel biaryl amide series as selective inhibitors of hematopoietic protein kinase 1 (HPK1). Structure-activity relationship development, aided by molecular modeling, identified indazole 5b as a core for further exploration because of its outstanding enzymatic and cellular potency coupled with encouraging kinome selectivity. Late-stage manipulation of the right-hand aryl and amine moieties surmounted issues of selectivity over TRKA, MAP4K2, and STK4 as well as generating compounds with balanced in vitro ADME profiles and promising pharmacokinetics.

Improving cancer immunotherapy by rationally combining oncolytic virus with modulators targeting key signaling pathways

Oncolytic viruses (OVs) represent a new class of multi-modal immunotherapies for cancer, with OV-elicited antitumor immunity being key to their overall therapeutic efficacy. Currently, the clinical effectiveness of OV as monotherapy remains limited, and thus investigators have been exploring various combinations with other anti-cancer agents and demonstrated improved therapeutic efficacy. As cancer cells have evolved to alter key signaling pathways for enhanced cell proliferation, cancer progression and metastasis, these cellular and molecular changes offer promising targets for rational cancer therapy design. In this regard, key molecules in relevant signaling pathways for cancer cells or/and immune cells, such as EGFR-KRAS (e.g., KRAS^G12C), PI3K-AKT-mTOR, ERK-MEK, JAK-STAT, p53, PD-1-PD-L1, and epigenetic, or immune pathways (e.g., histone deacetylases, cGAS-STING) are currently under investigation and have the potential to synergize with OV to modulate the immune milieu of the tumor microenvironment (TME), thereby improving and sustaining antitumor immunity. As many small molecule modulators of these signaling pathways have been developed and have shown strong therapeutic potential, here we review key findings related to both OV-mediated immunotherapy and the utility of small molecule modulators of signaling pathways in immuno-oncology. Then, we focus on discussion of the rationales and potential strategies for combining OV with selected modulators targeting key cellular signaling pathways in cancer or/and immune cells to modulate the TME and enhance antitumor immunity and therapeutic efficacy. Finally, we provide perspectives and viewpoints on the application of novel experimental systems and technologies that can propel this exciting branch of medicine into a bright future.

A novel lipidic peptide with potential to promote balanced effector-regulatory T cell responses

T cell-dendritic cell (DC) interactions contribute to reciprocal stimulation leading to DC maturation that results in production of interleukin-12 (IL-12) and interferon-gamma (IFN-γ). Both cytokines have been implicated in autoimmune diseases while being necessary for effective immune responses against foreign antigens. We describe a lipidic peptide, designated IK14004, that modifies crosstalk between T cells and DCs resulting in suppression of IL-12p40/IFN-γ production. T cell production of interleukin-2 (IL-2) and IFN-γ is uncoupled and IL-12p70 production is enhanced. IK14004 induces expression of activating co-receptors in CD8+ T cells and increases the proportion of Foxp3-expressing CD4+ T regulatory cells. The potential for IK14004 to impact on signalling pathways required to achieve a balanced immune response upon stimulation of DCs and T cells is highlighted. This novel compound provides an opportunity to gain further insights into the complexity of T cell-DC interactions relevant to autoimmunity associated with malignancies and may have therapeutic benefit.

Hematopoietic Progenitor Kinase 1 in Tumor Immunology: A Medicinal Chemistry Perspective

Hematopoietic progenitor kinase 1 (HPK1), a hematopoietic cell-restricted member of the serine/threonine Ste20-related protein kinases, is a negative regulator of the T cell receptor, B cell receptor, and dendritic cells. Loss of HPK1 kinase function increases cytokine secretion and enhances T cell signaling, virus clearance, and tumor growth inhibition. Therefore, HPK1 is considered a promising target for tumor immunotherapy. Several HPK1 inhibitors have been reported to regulate T cell function. In addition, HPK1-targeting PROTACs, which can induce the degradation of HPK1, have also been developed. Here, we provide an overview of research concerning HPK1 protein structure, function, and inhibitors and propose perspectives and insights for the future development of agents targeting HPK1.

The HPK1 Inhibitor A-745 Verifies the Potential of Modulating T Cell Kinase Signaling for Immunotherapy

Hematopoietic progenitor kinase 1 (HPK1) is an MAP4K family member within the Ste20-like serine/threonine branch of the kinome. HPK1 expression is limited to hematopoietic cells and has a predominant role as a negative regulator of T cell function. Because of the central/dominant role in negatively regulating T cell function, HPK1 has long been in the center of interest as a potential pharmacological target for immune therapy. The development of a small molecule HPK1 inhibitor remains challenging because of the need for high specificity relative to other kinases, including additional MAP4K family members, that are required for efficient immune cell activation. Here, we report the identification of the selective and potent HPK1 chemical probe, A-745. In unbiased cellular kinase-binding assays, A-745 demonstrates an excellent cellular selectivity binding profile within pharmacologically relevant concentrations. This HPK1 selectivity translates to an in vitro immune cell activation phenotype reminiscent of Hpk1-deficient and Hpk1-kinase-dead T cells, including augmented proliferation and cytokine production. The results from this work give a path forward for further developmental efforts to generate additional selective and potent small molecule HPK1 inhibitors with the pharmacological properties for immunotherapy.

Discovery of Novel HPK1 Inhibitors Through Structure-Based Virtual Screening

Hematopoietic progenitor kinase (HPK1) is a negative regulator of T-cell receptor and B-cell signaling, which has been recognized as a novel antitumor target for immunotherapy. In this work, Glide docking-based virtual screening and kinase inhibition assay were performed to identify novel HPK1 inhibitors. The kinase inhibition assay results demonstrated five compounds with IC50 values below 20 μM, and the most potent one (compound M074-2865) had an IC50 value of 2.93 ± 0.09 μM. Molecular dynamics (MD) simulations were performed to delve into the interaction of sunitinib and the identified compound M074-2865 with the kinase domain of HPK1. The five compounds identified in this work could be considered promising hit compounds for further development of HPK1 inhibitors for immunotherapy.

Discovery of Spiro-azaindoline Inhibitors of Hematopoietic Progenitor Kinase 1 (HPK1)

Hematopoietic progenitor kinase 1 (HPK1) is implicated as a negative regulator of T-cell receptor-induced T-cell activation. Studies using HPK1 kinase-dead knock-in animals have demonstrated the loss of HPK1 kinase activity resulted in an increase in T-cell function and tumor growth inhibition in glioma models. Herein, we describe the discovery of a series of small molecule inhibitors of HPK1. Using a structure-based drug design approach, the kinase selectivity of the molecules was significantly improved by inducing and stabilizing an unusual P-loop folded binding mode. The metabolic liabilities of the initial 7-azaindole high-throughput screening hit were mitigated by addressing a key metabolic soft spot along with physicochemical property-based optimization. The resulting spiro-azaindoline HPK1 inhibitors demonstrated improved in vitro ADME properties and the ability to induce cytokine production in primary human T-cells.

Discovery of an Allosteric, Inactive Conformation-Selective Inhibitor of Full-Length HPK1 Utilizing a Kinase Cascade Assay

Achieving selectivity across the human kinome is a major hurdle in kinase inhibitor drug discovery. Assays using active, phosphorylated protein kinases bias hits toward poorly selective inhibitors that bind within the highly conserved adenosine triphosphate (ATP) pocket. Targeting inactive (vs active) kinase conformations offers advantages in achieving selectivity because of their more diversified structures. Kinase cascade assays are typically initiated with target kinases in their unphosphorylated inactive forms, which are activated during the assays. Therefore, these assays are capable of identifying inhibitors that preferentially bind to the unphosphorylated form of the enzyme in addition to those that bind to the active form. We applied this cascade assay to the emerging cancer immunotherapy target hematopoietic progenitor kinase 1 (HPK1), a serine/threonine kinase that negatively regulates T cell receptor signaling. Using this approach, we discovered an allosteric, inactive conformation-selective triazolopyrimidinone HPK1 inhibitor, compound 1. Compound 1 binds to unphosphorylated HPK1 >24-fold more potently than active HPK1, is not competitive with ATP, and is highly selective against kinases critical for T cell signaling. Furthermore, compound 1 does not bind to the isolated HPK1 kinase domain alone but requires other domains. Together, these data indicate that 1 is an allosteric HPK1 inhibitor that attenuates kinase autophosphorylation by binding to a pocket consisting of residues within and outside of the kinase domain. Our study demonstrates that cascade assays can lead to the discovery of highly selective kinase inhibitors. The triazolopyrimidinone described in this study may represent a privileged chemical scaffold for further development of potent and selective HPK1 inhibitors.

Inhibitors of immuno-oncology target HPK1 - a patent review (2016 to 2020)

Introduction: Hematopoietic progenitor kinase (HPK1), a serine/threonine kinase, which is primarily expressed in hematopoietic cells is a negative regulator of T-cell receptor and B cell signaling. Studies using genetic disruption of HPK1 function show enhanced T-cell signaling, cytokine production, and in vivo tumor growth inhibition. This profile of enhanced immune response highlights small molecule inhibition of HPK1 as an attractive approach for the immunotherapy of cancer.Areas covered: This article summarizes the biological rationale for the inhibition of HPK1 as a potential adjunct to the current immuno-oncology (IO) therapies. The article primarily discloses the current state of development of HPK1 inhibitors.Expert Opinion: The rapid increase in the identification of small molecule inhibitors of HPK1 should translate into a fuller understanding of the role of HPK1 inhibition in the IO setting. This understanding will be of huge importance in determining whether HPK1 inhibition alone will be sufficient for tumor growth inhibition or if combination with current IO therapies will be required.

Identification of Potent Reverse Indazole Inhibitors for HPK1

Hematopoietic progenitor kinase (HPK1), a negative regulator of TCR-mediated T-cell activation, has been recognized as a novel antitumor immunotherapy target. Structural optimization of kinase inhibitor 4 through a systematic two-dimensional diversity screen of pyrazolopyridines led to the identification of potent and selective compounds. Crystallographic studies with HPK1 revealed a favorable water-mediated interaction with Asp155 and a salt bridge to Asp101 with optimized heterocyclic solvent fronts that were critical for enhanced potency and selectivity. Computational studies of model systems revealed differences in torsional profiles that allowed for these beneficial protein-ligand interactions. Further optimization of molecular properties led to identification of potent and selective reverse indazole inhibitor 36 that inhibited phosphorylation of adaptor protein SLP76 in human PBMC and exhibited low clearance with notable bioavailability in in vivo rat studies.

Enhanced antitumor immunity by a novel small molecule HPK1 inhibitor

Background

Hematopoietic progenitor kinase 1 (HPK1 or MAP4K1) has been demonstrated as a negative intracellular immune checkpoint in mediating antitumor immunity in studies with HPK1 knockout and kinase dead mice. Pharmacological inhibition of HPK1 is desirable to investigate the role of HPK1 in human immune cells with therapeutic implications. However, a significant challenge remains to identify a small molecule inhibitor of HPK1 with sufficient potency, selectivity, and other drug-like properties suitable for proof-of-concept studies. In this report, we identified a novel, potent, and selective HPK1 small molecule kinase inhibitor, compound K (CompK). A series of studies were conducted to investigate the mechanism of action of CompK, aiming to understand its potential application in cancer immunotherapy.

Methods

Human primary T cells and dendritic cells (DCs) were investigated with CompK treatment under conditions relevant to tumor microenvironment (TME). Syngeneic tumor models were used to assess the in vivo pharmacology of CompK followed by human tumor interrogation ex vivo.

Results

CompK treatment demonstrated markedly enhanced human T-cell immune responses under immunosuppressive conditions relevant to the TME and an increased avidity of the T-cell receptor (TCR) to recognize viral and tumor-associated antigens (TAAs) in significant synergy with anti-PD1. Animal model studies, including 1956 sarcoma and MC38 syngeneic models, revealed improved immune responses and superb antitumor efficacy in combination of CompK with anti-PD-1. An elevated immune response induced by CompK was observed with fresh tumor samples from multiple patients with colorectal carcinoma, suggesting a mechanistic translation from mouse model to human disease.

Conclusion

CompK treatment significantly improved human T-cell functions, with enhanced TCR avidity to recognize TAAs and tumor cytolytic activity by CD8+ T cells. Additional benefits include DC maturation and priming facilitation in tumor draining lymph node. CompK represents a novel pharmacological agent to address cancer treatment resistance.

Pharmacological inhibition of hematopoietic progenitor kinase 1 positively regulates T-cell function

Hematopoietic progenitor kinase 1 (HPK1), a hematopoietic cell-specific Ste20-related serine/threonine kinase, is a negative regulator of signal transduction in immune cells, including T cells, B cells, and dendritic cells (DCs). In mice, HPK1 deficiency subverts inhibition of the anti-tumor immune response and is associated with functional augmentation of anti-tumor T cells. We have used a potent, small molecule HPK1 inhibitor, Compound 1, to investigate the effects of pharmacological intervention of HPK1 kinase activity in immune cells. Compound 1 enhanced Th1 cytokine production in T cells and fully reverted immune suppression imposed by the prostaglandin E2 (PGE2) and adenosine pathways in human T cells. Moreover, the combination of Compound 1 with pembrolizumab, a humanized monoclonal antibody against the programmed cell death protein 1 (PD-1), demonstrated a synergistic effect, resulting in enhanced interferon (IFN)-γ production. Collectively, our results suggest that blocking HPK1 kinase activity with small molecule inhibitors alone or in combination with checkpoint blockade may be an attractive approach for the immunotherapy of cancer.

A perspective on HPK1 as a novel immuno-oncology drug target

In this perspective review, the role Hematopoietic Progenitor Kinase 1 (HPK1) in tumor immunity will be reviewed, with special emphasis on how T cells are negatively-regulated at different junctures of cancer-immunity cycle by this regulatory kinase. The review will highlight the strengths and weaknesses of HPK1 as a candidate target for novel immuno-oncology (IO) drug development that is centered on the use of small molecule kinase inhibitor to modulate the immune response against cancer. Such a therapeutic approach, if proven successful, could supplement the cancer cell-centric standard of care therapies in order to fully meet the therapeutic needs of cancer patients.

Development of High-Throughput Assays for Evaluation of Hematopoietic Progenitor Kinase 1 Inhibitors

Hematopoietic progenitor kinase 1 (HPK1), also referred to as mitogen-activated protein kinase kinase kinase kinase 1 (MAP4K1), is a serine/threonine kinase that negatively regulates T-cell signaling by phosphorylating Ser376 of Src homology 2 (SH2) domain-containing leukocyte protein of 76 kDa (SLP-76), a critical mediator of T-cell receptor activation. HPK1 loss of function mouse models demonstrated enhanced immune cell activation and beneficial antitumor activity. To enable discovery and functional characterization of high-affinity small-molecule HPK1 inhibitors, we have established high-throughput biochemical, cell-based, and novel pharmacodynamic (PD) assays. Kinase activity-based time-resolved fluorescence energy transfer (TR-FRET) assays were established as the primary biochemical approach to screen for potent inhibitors and assess selectivity against members of MAP4K and other closely related kinases. A proximal target engagement (TE) assay quantifying pSLP-76 levels as a readout and a distal assay measuring IL-2 secretion as a functional response were established using human peripheral blood mononuclear cells (PBMCs) from two healthy donors. Significant correlations between biochemical and cellular assays as well as excellent correlation between the two donors for the cellular assays were observed. pSLP-76 levels were further used as a PD marker in the preclinical murine model. This effort required the development of a novel ultrasensitive single-molecule array (SiMoA) assay to monitor pSLP-76 changes in mouse spleen.

Revolutionizing the landscape of colorectal cancer treatment: The potential role of immune checkpoint inhibitors

Colorectal cancer (CRC) represents the third cause of cancer-related mortalities worldwide. The progression of CRC to the metastatic phase significantly compromises the overall survival rates. Despite the advances in the therapeutic protocols, CRC treatment is still challenging. Cancer immunotherapy joined the ranks of surgery, chemotherapy, radiotherapy and targeted therapy as the fifth pillar in the foundation of cancer therapeutics. Interruption of the immunosuppressive signals within the tumor microenvironment and reactivation of antitumor immunity via targeting the molecular immune checkpoints generated promising therapeutic outcomes in several types of hard-to-treat cancers. The year 2017 witnessed the first US Food and Drug Administration (FDA) approval of immune checkpoint inhibitor (ICI) immunotherapy for the management of CRC. The approval was granted to pembrolizumab (anti-PD-1) for the treatment of patients with advanced/metastatic solid malignancies with mismatch-repair deficiency including CRCs. Such natively immunogenic tumors constitute only a minor percentage of all CRCs. Therefore, it is imperative to utilize novel combinatorial regimens to enhance the response of a wider range of CRC tumors to cancer immunotherapy and help in extending the survival rates in patients with advanced/metastatic disease. This review highlights the novel approaches under clinical development to overcome the resistance of CRCs to immunotherapy and improve the therapeutic outcomes.

Identifying hub genes of clear cell renal cell carcinoma associated with the proportion of regulatory T cells by weighted gene co-expression network analysis

BACKGROUND

Numerous patients with clear cell renal cell carcinoma (ccRCC) experience drug resistance after immunotherapy. Regulatory T (Treg) cells may work as a suppressor for anti-tumor immune response.

PURPOSE

We performed bioinformatics analysis to better understand the role of Treg cells in ccRCC.

RESULTS

Module 10 revealed the most relevance with Treg cells. Functional annotation showed that biological processes and pathways were mainly related to activation of the immune system and the processes of immunoreaction. Four hub genes were selected: LCK, MAP4K1, SLAMF6, and RHOH. Further validation showed that the four hub genes well-distinguished tumor and normal tissues and were good prognostic biomarkers for ccRCC.

CONCLUSION

The identified hub genes facilitate our knowledge of the underlying molecular mechanism of how Treg cells affect ccRCC in anti-tumor immune therapy.

METHODS

The CIBERSORT algorithm was performed to evaluate tumor-infiltrating immune cells based on the Cancer Genome Atlas cohort. Weighted gene co-expression network analysis was conducted to explore the modules related to Treg cells. Gene Ontology analysis and pathway enrichment analysis were performed for functional annotation and a protein-protein interaction network was built. Samples from the International Cancer Genomics Consortium database was used as a validation set.

Multiple conformational states of the HPK1 kinase domain in complex with sunitinib reveal the structural changes accompanying HPK1 trans-regulation

Hematopoietic progenitor kinase 1 (HPK1 or MAP4K1) is a Ser/Thr kinase that operates via the c-Jun N-terminal kinase (JNK) and extracellular signal-regulated kinase (ERK) signaling pathways to dampen the T-cell response and antitumor immunity. Accordingly, selective HPK1 inhibition is considered a means to enhance antitumor immunity. Sunitinib, a multi-receptor tyrosine kinase (RTK) inhibitor approved for the management of gastrointestinal stromal tumors (GISTs), renal cell carcinoma (RCC), and pancreatic cancer, has been reported to inhibit HPK1 in vitro In this report, we describe the crystal structures of the native HPK1 kinase domain in both nonphosphorylated and doubly phosphorylated states, in addition to a double phosphomimetic mutant (T165E,S171E), each complexed with sunitinib at 2.17-3.00-Å resolutions. The native nonphosphorylated cocrystal structure revealed an inactive dimer in which the activation loop of each monomer partially occupies the ATP- and substrate-binding sites of the partner monomer. In contrast, the structure of the protein with a doubly phosphorylated activation loop exhibited an active kinase conformation with a greatly reduced monomer-monomer interface. Conversely, the phosphomimetic mutant cocrystal structure disclosed an alternative arrangement in which the activation loops are in an extended domain-swapped configuration. These structural results indicate that HPK1 is a highly dynamic kinase that undergoes trans-regulation via dimer formation and extensive intramolecular and intermolecular remodeling of the activation segment.

Critical role of kinase activity of hematopoietic progenitor kinase 1 in anti-tumor immune surveillance

Immunotherapy has fundamentally changed the landscape of cancer treatment. Despite the encouraging results with the checkpoint modulators, response rates vary widely across tumor types, with a majority of patients exhibiting either primary resistance without a significant initial response to treatment or acquired resistance with subsequent disease progression. Hematopoietic progenitor kinase 1 (HPK1) is predominantly expressed in hematopoietic cell linages and serves as a negative regulator in T cells and dendritic cells (DC). While HPK1 gene knockout (KO) studies suggest its role in anti-tumor immune responses, the involvement of kinase activity and thereof its therapeutic potential remain unknown. To investigate the potential of pharmacological intervention using inhibitors of HPK1, we generated HPK1 kinase dead (KD) mice which carry a single loss-of-function point mutation in the kinase domain and interrogated the role of kinase activity in immune cells in the context of suppressive factors or the tumor microenvironment (TME). Our data provide novel findings that HKP1 kinase activity is critical in conferring suppressive functions of HPK1 in a wide range of immune cells including CD4+, CD8+, DC, NK to Tregs, and inactivation of kinase domain was sufficient to elicit robust anti-tumor immune responses. These data support the concept that an HPK1 small molecule kinase inhibitor could serve as a novel agent to provide additional benefit in combination with existing immunotherapies, particularly to overcome resistance to current treatment regimens.

Patent Highlights August-September 2018

A snapshot of noteworthy recent developments in the patent literature of relevance to pharmaceutical and medical research and development.

Hematopoietic Progenitor Kinase-1 Structure in a Domain-Swapped Dimer

Enhancement of antigen-specific T cell immunity has shown significant therapeutic benefit in infectious diseases and cancer. Hematopoietic progenitor kinase-1 (HPK1) is a negative-feedback regulator of T cell receptor signaling, which dampens T cell proliferation and effector function. A recent report showed that a catalytic dead mutant of HPK1 phenocopies augmented T cell responses observed in HPK1-knockout mice, indicating that kinase activity is critical for function. We evaluated active and inactive mutants and determined crystal structures of HPK1 kinase domain (HPK1-KD) in apo and ligand bound forms. In all structures HPK1-KD displays a rare domain-swapped dimer, in which the activation segment comprises a well-conserved dimer interface. Biophysical measurements show formation of dimer in solution. The activation segment adopts an α-helical structure which exhibits distinct orientations in active and inactive states. This face-to-face configuration suggests that the domain-swapped dimer may possess alternative selectivity for certain substrates of HPK1 under relevant cellular context.

Small molecule immuno-oncology therapeutic agents

Treatment of cancer by activation of an antitumor immune response is now a widely practiced and well-accepted approach to therapy. However, despite dramatic responses in some patients, the high proportion of unresponsive patients points to a considerable unmet medical need. Although antibody therapies have led the way, small molecule immuno-oncology agents are close behind. This perspective provides an overview of some of the many small molecule approaches being explored. It encompasses small molecule modulators of validated targets such as programed cell death 1 (PD-1) as well as novel approaches still to be proven clinically.

HPK1 positive expression associated with longer overall survival in patients with estrogen receptor-positive invasive ductal carcinoma‑not otherwise specified

Hematopoietic progenitor kinase 1 (HPK1) belongs to the mitogen activated protein kinase kinase kinase kinase (MAP4K) family of serine/threonine kinases, which have been associated with the incidence and progression of a variety of gastrointestinal malignant tumors in humans. However, the potential association between HPK1 expression and breast cancer, particularly invasive ductal carcinoma-not otherwise specified (IDC-NOS) development, has not yet been examined. To address this gap, the present study aimed to evaluate HPK1 expression in IDC-NOS samples and to determine a relationship with clinical prognostic indicators, such as the expression levels of estrogen receptor (ER), progesterone receptor (PR) and human epidermal growth factor receptor 2 (HER2), as well as overall survival of the patients with IDC-NOS. HPK1 mRNA and protein expression in samples from 148 patients with IDC-NOS were detected using immunohistochemistry, western blotting and reverse transcription-quantitative polymerase chain reaction. A total of 54 out of 148 (36.5%) samples were HPK1-positive, and 100 out of 148 (67.6%) were ER-positive. Of the latter, 28% (28/100) were HPK1-positive, and a significant negative association of HPK1 expression with ER positivity was observed (P=0.002; r=−0.254). In addition, 43.2% (64/148) and 32.4% (48/100) of IDC-NOS tissues were PR- or HER2-positive, respectively; however, neither indicator correlated with HPK1 (P=0.109 and P=0.558, respectively). HPK1 expression, axillary lymph node metastasis and tumor-node-metastasis (TNM) stage were identified as independent factors of overall survival (OS) in the ER-positive group (P<0.05), and HPK1 positivity was associated with increased OS (P=0.048). HPK1 mRNA levels did not differ between IDC-NOS and normal adjacent breast tissues, whereas HPK1 protein levels were lower in IDC-NOS (P<0.05). These results suggested that HPK1 protein may be a potentially effective IDC-NOS therapeutic target.

Targeting cancer with kinase inhibitors

Kinase inhibitors have played an increasingly prominent role in the treatment of cancer and other diseases. Currently, more than 25 oncology drugs that target kinases have been approved, and numerous additional therapeutics are in various stages of clinical evaluation. In this Review, we provide an in-depth analysis of activation mechanisms for kinases in cancer, highlight recent successes in drug discovery, and demonstrate the clinical impact of selective kinase inhibitors. We also describe the substantial progress that has been made in designing next-generation inhibitors to circumvent on-target resistance mechanisms, as well as ongoing strategies for combining kinase inhibitors in the clinic. Last, there are numerous prospects for the discovery of novel kinase targets, and we explore cancer immunotherapy as a new and promising research area for studying kinase biology.