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

Synthesis and photodynamic activity of new 5-[(E)-2-(3-alkoxy-1-phenyl-1H-pyrazol-4-yl)ethenyl]-2-phenyl-3H-indoles

A series of new indole-pyrazole hybrids 8a-m were synthesized through the palladium-catalyzed ligandless Heck coupling reaction from easily accessible unsubstituted, methoxy- or fluoro-substituted 4-ethenyl-1H-pyrazoles and 5-bromo-3H-indoles. These compounds exerted cytotoxicity to melanoma G361 cells when irradiated with blue light (414 nm) and no cytotoxicity in the dark at concentrations up to 10 µM, prompting us to explore their photodynamic effects. The photodynamic properties of the example compound 8d were further investigated in breast cancer MCF-7 cells. Evaluation revealed comparable anticancer activities of 8d in both breast and melanoma cancer cell lines within the submicromolar range. The treatment induced a massive generation of reactive oxygen species, leading to different types of cell death depending on the compound concentration and the irradiation intensity.

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.

Early Stage Preclinical Formulation Strategies to Alter the Pharmacokinetic Profile of Two Small Molecule Therapeutics

Early stage chemical development presents numerous challenges, and achieving a functional balance is a major hurdle, with many early compounds not meeting the clinical requirements for advancement benchmarks due to issues like poor oral bioavailability. There is a need to develop strategies for achieving the desired systemic concentration for these compounds. This will enable further evaluation of the biological response upon a compound-target interaction, providing deeper insight into the postulated biological pathways. Our study elucidates alternative drug delivery paradigms by comparing formulation strategies across oral (PO), intraperitoneal (IP), subcutaneous (SC), and intravenous (IV) routes. While each modality boasts its own set of merits and constraints, it is the drug's formulation that crucially influences its pharmacokinetic (PK) trajectory and the maintenance of its therapeutic levels. Our examination of model compounds G7883 and G6893 highlighted their distinct physio-chemical attributes. By harnessing varied formulation methods, we sought to fine-tune their PK profiles. PK studies showcased G7883's extended half-life using an SC oil formulation, resulting in a 4.5-fold and 2.5-fold enhancement compared with the IP and PO routes, respectively. In contrast, with G6893, we achieved a prolonged systemic coverage time above the desired target concentration through a different approach using an IV infusion pump. These outcomes underscore the need for tailored formulation strategies, which are dictated by the compound's innate properties, to reach the optimal in vivo systemic concentrations. Prioritizing formulation and delivery optimization early on is pivotal for effective systemic uptake, thereby facilitating a deeper understanding of biological pathways and expediting the overall clinical drug development timeline.

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.

Design and synthesis of 1H-pyrazolo[3,4-d]pyrimidine derivatives as hematopoietic progenitor kinase 1 (HPK1) inhibitors

Despite immune checkpoint inhibitors' tremendous success in the treatment of tumors, the moderate response rate limits their widespread use. Hematopoietic progenitor kinase 1 (HPK1) is served as an essential negative regulator of T-cell receptor, which has been identified as a promising target for enhancing antitumor immunity. However, the development of a selective HPK1 inhibitor is still challenging. Herein, we reported a novel series of 1H-pyrazolo[3,4-d]pyrimidine derivatives as HPK1 inhibitors by structure-based rational design. The optimal compound 10n significantly inhibited HPK1 with an IC50 value of 29.0 nM and the phosphorylation of SLP76 at a concentration as low as 0.1 μM. Furthermore, compound 10n exhibited good selectivity over a panel of 25 kinases, including GLK from the same MAP4K family. Together, the current study provided a novel, potent, and selective HPK1 inhibitor, acting as a lead compound for the future development of cancer immunotherapy.

Modular, automated synthesis of spirocyclic tetrahydronaphthyridines from primary alkylamines

Spirocyclic tetrahydronaphthyridines (THNs) are valuable scaffolds for drug discovery campaigns, but access to this 3D chemical space is hampered by a lack of modular and scalable synthetic methods. We hereby report an automated, continuous flow synthesis of α-alkylated and spirocyclic 1,2,3,4-tetrahydro-1,8-naphthyridines ("1,8-THNs"), in addition to their regioisomeric 1,6-THN analogues, from abundant primary amine feedstocks. An annulative disconnection approach based on photoredox-catalysed hydroaminoalkylation (HAA) of halogenated vinylpyridines is sequenced in combination with intramolecular SNAr N-arylation. To access the remaining 1,7- and 1,5-THN isomers, a photoredox-catalysed HAA step is telescoped with a palladium-catalysed C-N bond formation. Altogether, this provides a highly modular access to four isomeric THN cores from a common set of unprotected primary amine starting materials, using the same bond disconnections. The simplifying power of the methodology is illustrated by a concise synthesis of the spirocyclic THN core of Pfizer's MC4R antagonist PF-07258669.

Discovery of quinazoline HPK1 inhibitors with high cellular potency

Hematopoietic progenitor kinase 1 (HPK1) is regarded as a highly validated target in pre-clinical immune oncology. HPK1 has been described as regulating multiple critical signaling pathway in both adaptive and innate cells. In support of this role, HPK1 KO T cells show enhanced sensitivity to TCR activation and HPK1 KO mice display enhanced anti-tumor activity. Taken together, inhibition of HPK1 has the potential to induce enhanced anti-tumor immune response. Herein, we described the discovery of highly potent HPK1 inhibitors starting form a weak HTS hit. Using a structure-based drug design, HPK1 inhibitors exhibiting excellent cellular single-digit nanomolar potency in both proximal (pSLP76) and distal (IL-2) biomarkers along with sustained elevation of IL-2 cytokine secretion were discovered.

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.

Design, synthesis, and biological evaluation of novel spirocyclic compounds as potential anti-glioblastoma agents

Glioblastoma (GBM) is an aggressive brain tumor with extremely limited clinical treatment options. Because of the blood-brain barrier (BBB), it is difficult for anti-GBM drug candidates to enter the brain to exert their therapeutic effects. The spirocyclic skeleton structure exhibits good lipophilicity and permeability, enabling small-molecule compounds to cross the BBB. Herein, we designed and synthesized novel 3-oxetanone-derived spirocyclic compounds containing a spiro[3.4]octane ring and determined their structure-activity relationship for antiproliferation in GBM cells. Among these, the chalcone-spirocycle hybrid 10m/ZS44 exhibited high antiproliferative activity in U251 cells and permeability in vitro. Furthermore, 10m/ZS44 activated the SIRT1/p53-mediated apoptosis pathway to inhibit proliferation in U251 cells, whereas it minimally impaired other cell-death pathways, such as pyroptosis or necroptosis. In a mouse xenograft model, 10m/ZS44 exhibited a substantial inhibitory effect on GBM tumor growth without showing obvious toxicity. Overall, 10m/ZS44 represents a promising spirocyclic compound for the treatment of GBM.

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.

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.

Discovery of Macrocycle-Based HPK1 Inhibitors for T-Cell-Based Immunotherapy

Hematopoietic progenitor kinase 1 (HPK1) is a negative regulator of T-cell activation, and targeting HPK1 is considered a promising strategy for improving responses to antitumor immune therapies. The biggest challenge of HPK1 inhibitor design is to achieve a higher selectivity to GLK, an HPK1 homology protein as a positive regulator of T-cell activation. Herein, we report the design of a series of macrocycle-based HPK1 inhibitors via a conformational constraint strategy. The identified candidate compound 5i exhibited HPK1 inhibition with an IC₅₀ value of 0.8 nM and 101.3-fold selectivity against GLK. Compound 5i also displayed good oral bioavailability (F = 27-49%) in mice and beagles and favorable metabolic stability (T_1/2 > 186.4 min) in human liver microsomes. More importantly, compound 5i demonstrated a clear synergistic effect with anti-PD-1 in both MC38 (MSI) and CT26 (MSS) syngeneic tumor mouse models. These results showed that compound 5i has a great potential in immunotherapy.

Multicomponent Direct Assembly of N-Heterospirocycles Facilitated by Visible-Light-Driven Photocatalysis

N-heterospirocycles are interesting structural units found in both natural products and medicinal compounds but have relatively few reliable methods for their synthesis. Here, we enlist the photocatalytic generation of N-centered radicals to construct β-spirocyclic pyrrolidines from N-allylsulfonamides and alkenes. A variety of β-spirocyclic pyrrolidines have been constructed, including drug derivatives, in moderate to very good yields. Further derivatization of the products has also been demonstrated as has a viable scale-up procedure, making use of flow chemistry techniques.

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.