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

Design, Synthesis, and biological evaluation of 7H-Pyrrolo[2,3-d]pyrimidines as potent HPK1 kinase inhibitors

Hematopoietic progenitor kinase 1 (HPK1) has emerged as a promising target for cancer immunotherapy due to its critical role as a negative regulator of T cell receptor (TCR) signaling. Despite this potential, no HPK1 inhibitors have been approved for cancer treatment, underscoring the need for structurally novel inhibitors. Herein, we describe the design, synthesis and biological evaluation of a series of potent HPK1 inhibitors based on our previously identified hit 9. Among them, compound 24 demonstrated strong HPK1 inhibition (IC50 of 10.1 nM) and effectively suppressed phosphorylation of the downstream protein SLP76. Notably, compound 24 exhibited enhanced potency in promoting IL-2 secretion in Jurkat T cells, reduced cellular toxicity, and improved liver microsomal stability compared to hit 9. Overall, this study provides a promising lead compound for further optimization as a candidate for cancer immunotherapy.

Discovery of 1,2,4-benzotriazine derivatives as new hematopoietic progenitor kinase 1 (HPK1) inhibitors

Hematopoietic progenitor kinase 1 (HPK1), which negatively regulates immune signaling, has emerged as an attractive small-molecule drug target for tumor immunotherapy. Herein, we report the discovery of the 1,2,4-benzotriazine derivatives as new potent HPK1 inhibitors. Notably, compound A29 exhibited improved HPK1 inhibitory activity relative to compound 1 in the ADP-Glo kinase assay (IC50 = 2.70 and 13.6 nM, respectively). The pronounced inhibitory activity of A29 against downstream p-SLP76 in Jurkat T cells (IC50 = 8.1 nM) as well as the ability to induce the production of interleukin 2 (IL-2) in human peripheral blood mononuclear cells (PBMCs) confirmed its cellular target engagement and immune stimulatory effect. Consistently, this lead compound significantly enhanced T-cell killing ability against murine colon cancer cells CT26 or MC38 in a co-culture system. Furthermore, A29 was efficacious in a CT26 xenograft mouse model alone, and significantly enhanced the antitumor efficacy of an anti-PD-1 antibody. This work provides a promising lead for the development of effective HPK1 inhibitors for tumor immunotherapy.

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.

Design, synthesis, and biological evaluation of novel HPK1 inhibitors possessing 3-cyano-quinoline moiety

Hematopoietic progenitor kinase 1 (HPK1), a negative regulator of T cell receptor signaling, plays a crucial role in multiple cellular immune responses. Emerging researches have demonstrated that inhibiting HPK1 kinase function enhances T cells' ability to recognize tumor antigens and boosts anti-tumor immune responses. As a result, HPK1 has become a promising target for tumor immunotherapy. Herein, we report the design, synthesis, and biological evaluation of a series of novel HPK1 inhibitors featuring a 3-cyano-quinoline scaffold. Among these, compound 3a was identified as the most potent HPK1 inhibitor (HPK1 IC50 = 48 nM). It effectively inhibited SLP76 phosphorylation, enhanced IL-2 cytokine secretion, and reversed PGE2-induced immunosuppression in Jurkat cells. In addition, compound 3a exhibited favorable metabolic stability in mouse liver microsomes and plasma. Overall, this work provides a structurally novel lead compound for the development of HPK1 inhibitors.

Small-molecule in cancer immunotherapy: Revolutionizing cancer treatment with transformative, game-changing breakthroughs

Immunotherapy has revolutionized cancer management, with antibody-based treatments leading the charge due to their superior pharmacodynamics, including enhanced effectiveness and specificity. However, these therapies are hampered by limitations such as prolonged half-lives, poor tissue and tumor penetration, and minimal oral bioavailability. Additionally, their immunogenic nature can cause adverse effects. Consequently, the focus is shifting towards small-molecule-based immunotherapies, which potentially overcome these drawbacks. Emerging as a promising alternative, small molecules offer the benefits of therapeutic antibodies and immunomodulators, often yielding synergistic effects when combined. Recent advancements in small-molecule cancer immunotherapy are notable, featuring inhibitors, agonists, and degraders that act as immunomodulators. This article delves into the current landscape of small-molecule immunotherapy in cancer treatment, highlighting novel agents targeting key pathways such as Toll-like receptors (TLR), PD-1/PD-L1, chemokine receptors, and stimulators of interferon genes (STING). The review emphasizes newly discovered molecular entities and their modulatory roles in tumorigenesis, many of which have progressed to clinical trials, that aims to provide a comprehensive snapshot of the evolving frontier in cancer treatment, driven by small-molecule immunomodulators.

Design, synthesis, and biological evaluation of 2,4-diaminopyrimidine derivatives as potent Hematopoietic Progenitor Kinase 1 (HPK1) inhibitors

HPK1 also referred to as MAP4K1, belongs to the category of mammalian STE20-like protein serine/threonine kinases. Its physiological function involves the down-regulation of T cell signals, and it is regarded as a new immune checkpoint of tumor immunology. In this study, we commenced our investigation with the hit compounds, focusing the efforts on structural optimization and SAR exploration to identify a novel class of 2,4-diaminopyrimidine HPK1 inhibitors. Notably, compound 14g exhibited a remarkable inhibitory effect on HPK1 kinase (IC50 = 0.15 nM), significantly suppressed the phosphorylation of the downstream adaptor protein SLP76 (pSLP76 IC50 = 27.92 nM), and effectively stimulated the secretion of the T cell activation marker IL-2 (EC50 = 46.64 nM). In vitro microsomal stability assay, compound 14g showed moderate stability in HLMs with T1/2 = 38.2 min and CLint = 36.4 µL·min-1·mg-1 proteins. In vivo pharmacokinetic studies, compound 14g demonstrated heightened plasma exposure (AUC0-inf = 644 ng·h·mL-1), extended half-life (T1/2 = 9.98 h), and reduced plasma clearance (CL = 52.3 mL·min-1·kg-1) compared to the reference compound after a single intravenous dose of 2 mg/kg in rats. These results indicated that compound 14g emerged as a promising inhibitor of HPK1.

Discovery of 5-aminopyrido[2,3-d]pyrimidin-7(8H)-one derivatives as new hematopoietic progenitor kinase 1 (HPK1) inhibitors

Hematopoietic progenitor kinase 1 (HPK1) is a negative regulator of T-cell receptor signaling. While HPK1 is considered as a promising target for cancer immunotherapy, no small-molecule HPK1 inhibitors have been approved for cancer treatment. Herein, we report the discovery of a series of new HPK1 inhibitors with a 5-aminopyrido[2,3-d]pyrimidin-7(8H)-one scaffold. The most potent compound 9f inhibited HPK1 kinase activity with an IC50 of 0.32 nM in the time-resolved fluorescence resonance energy transfer (TR-FRET) assays, while displayed reasonable selectivity in a panel of 416 kinases. Cellular engagement of HPK1 by compound 9f was confirmed through the nano-bioluminescence resonance energy transfer (Nano-BRET) experiments. Compound 9f effectively reduced the phosphorylation of the downstream protein SLP-76 in primary peripheral blood mononuclear cells (PBMCs) and human T lymphocytic leukemia Jurkat cells. Compound 9f also enhanced the IL-2 and IFN-γ secretion in PBMCs. Furthermore, the binding mode of compound 9f with HPK1 was confirmed by the resolved cocrystal structure. Taken together, this study provides HPK1 inhibitors with a novel scaffold and clear binding mode for further development of HPK1-targeted therapeutic agents.

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.

Small-molecule agents for cancer immunotherapy

Cancer immunotherapy, exemplified by the remarkable clinical benefits of the immune checkpoint blockade and chimeric antigen receptor T-cell therapy, is revolutionizing cancer therapy. They induce long-term tumor regression and overall survival benefit in many types of cancer. With the advances in our knowledge about the tumor immune microenvironment, remarkable progress has been made in the development of small-molecule drugs for immunotherapy. Small molecules targeting PRR-associated pathways, immune checkpoints, oncogenic signaling, metabolic pathways, cytokine/chemokine signaling, and immune-related kinases have been extensively investigated. Monotherapy of small-molecule immunotherapeutic drugs and their combinations with other antitumor modalities are under active clinical investigations to overcome immune tolerance and circumvent immune checkpoint inhibitor resistance. Here, we review the latest development of small-molecule agents for cancer immunotherapy by targeting defined pathways and highlighting their progress in recent clinical investigations.

Design, Synthesis, and Pharmacological Evaluation of Isoindoline Analogues as New HPK1 Inhibitors

Hematopoietic progenitor kinase 1 (HPK1) is an important negative regulator in T-cell receptor signaling and as a promising key target for immunotherapy. Herein, based on the reported HPK1 inhibitor 2 featuring an isofuranone component, a structural optimization approach was conducted leading to several series of derivatives characterized by containing an isoindoline structural motif. Compound 49 was identified as a new potent HPK1 inhibitor with an IC50 value of 0.9 nM, more potent than compound 2 (5.5 nM). It also has an improved IV profile in rats and enhanced aqueous solubility. It effectively inhibited pSLP76 and reinvigorated T-cell receptor (TCR) signaling, promoting T-cell function and cytokine production both in naïve and antigen-specific T cells. Furthermore, compound 49 reversed the inhibition on T-cell activity mediated by classic immunosuppressive factors in the tumor microenvironment (TME). In the murine CT-26 tumor model, this compound reinvigorated the T cell and synergistically enhanced the antitumor efficacy of anti-PD1 at a well-tolerant dosage.

Discovery of diaminotriazine carboxamides as potent inhibitors of hematopoetic progenitor kinase 1

Hematopoietic progenitor kinase 1 (HPK1), a member of mitogen-activated protein kinase kinase kinase kinase (MAP4K) family of Ste20 serine/threonine kinases, is a negative regulator of T-cell receptor (TCR) signaling. Inactivating HPK1 kinase has been reported to be sufficient to elicit antitumor immune response. Therefore, HPK1 has attracted much attention as a promising target for tumor immunotherapy. A few of HPK1 inhibitors have been reported, and none of them have been approved for clinical applications. Hence, more effective HPK1 inhibitors are needed. Herein, a series of structurally novel diaminotriazine carboxamides were rationally designed, synthesized and evaluated for their inhibitory activity against HPK1 kinase. Most of them exhibited potent inhibitory potency against HPK1 kinase. In particular, compound 15b showed more robust HPK1 inhibitory activity than that of 11d developed by Merck in kinase activity assay (IC50 = 3.1 and 8.2 nM, respectively). The significant inhibitory potency against SLP76 phosphorylation in Jurkat T cells further confirmed the efficacy of compound 15b. In human peripheral blood mononuclear cell (PBMC) functional assays, compound 15b more significantly induced the production of interleukin 2 (IL-2) and interferon γ (IFN-γ) relative to 11d. Furthermore, 15b alone or in combination with anti-PD-1 antibodies showed potent in vivo antitumor efficacy in MC38 tumor-bearing mice. Compound 15b represents a promising lead for the development of effective HPK1 small-molecule inhibitors.

Discovery of 7H-Pyrrolo[2,3-d]pyrimidine Derivatives as potent hematopoietic progenitor kinase 1 (HPK1) inhibitors

Hematopoietic progenitor kinase 1 (HPK1) is predominantly expressed in hematopoietic cells and is a negative regulator of T cell receptor (TCR) signaling. Recent studies have demonstrated that HPK1 is a promising therapeutic target for cancer immunotherapy. However, despite significant progress in the development of HPK1 inhibitors, none of them has been approved for cancer therapy. Development of HPK1 inhibitors with a structurally distinct scaffold is still needed. Herein, we describe the design and synthesis of a series of HPK1 inhibitors with a 7H-pyrrolo[2,3-d]pyrimidine scaffold, exemplified by 31. Compound 31 showed potent inhibitory activity against HPK1 with an IC₅₀ value of 3.5 nM and favorable selectivity within a panel of kinases. It also potently inhibited the phosphorylation level of SLP76, a substrate of HPK1, and enhanced the IL-2 secretion in Jurkat cells (human T cell leukemia). Our findings provide new clues for further optimization and development to generate HPK1 inhibitors for cancer immunotherapy.

Development of a series of quinazoline-2,5-diamine derivatives as potent hematopoietic progenitor kinase 1 (HPK1) inhibitors

Hematopoietic progenitor kinase 1 (HPK1) is a serine/threonine kinase that serves as the negative regulator of multiple immune signaling pathways. Genetic studies using HPK1 knockout and kinase-dead mice suggested that inhibiting HPK1 either alone or in combination with immune checkpoint blockade could be a promising strategy in cancer immunotherapy. Herein, we report the design, synthesis and structure-activity relationship (SAR) study of a series of potent HPK1 inhibitors bearing quinazoline-2,5-diamine scaffold. Three rounds of SAR exploration led to the identification of 9h, the most potent compound in this series which harbors a 2-methyl-1,2,3,4-tetrahydroisoquinolin-7-yl substituent. Further biological assessments using human immune cells demonstrated that 9h could strongly inhibit downstream phosphorylation, augment interleukin-2 (IL-2) production and reverse prostaglandin E2 (PGE2)-induced immune suppression. Overall, our study on these quinazoline-2,5-diamine derivatives provided not only a tool compound for the community to help with elucidating the HPK1 pharmacology, but also a reliable reference for subsequent development of HPK1 inhibitors.

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.

Discovery of Diaminopyrimidine Carboxamide HPK1 Inhibitors as Preclinical Immunotherapy Tool Compounds

Hematopoietic progenitor kinase 1 (HPK1), a serine/threonine kinase, is a negative immune regulator of T cell receptor (TCR) and B cell signaling that is primarily expressed in hematopoietic cells. Accordingly, it has been reported that HPK1 loss-of-function in HPK1 kinase-dead syngeneic mouse models shows enhanced T cell signaling and cytokine production as well as tumor growth inhibition in vivo, supporting its value as an immunotherapeutic target. Herein, we present the structurally enabled discovery of novel, potent, and selective diaminopyrimidine carboxamide HPK1 inhibitors. The key discovery of a carboxamide moiety was essential for enhanced enzyme inhibitory potency and kinome selectivity as well as sustained elevation of cellular IL-2 production across a titration range in human peripheral blood mononuclear cells. The elucidation of structure-activity relationships using various pendant amino ring systems allowed for the identification of several small molecule type-I inhibitors with promising in vitro profiles.

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.

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.