Abstract 518: AB801 is a highly potent and selective AXL kinase inhibitor that demonstrates significant anti-tumor activity

Background: AXL receptor tyrosine kinase (AXL) is overexpressed in a variety of tumors and correlates with poor prognosis in cancer patients. AXL is expressed in cancer, stromal, and select immune cells, and has been implicated in the development of resistance to chemotherapy, targeted therapies & immunotherapies. Activation of AXL can be mediated by its ligand, growth arrest specific protein 6 (GAS6), or via ligand-independent homo/hetero-dimerization, both of which facilitate AXL phosphorylation, initiation of signaling cascades that promote cancer cell proliferation, survival, and an immunosuppressive microenvironment. Here we present the discovery and characterization of a novel, highly potent and selective AXL inhibitor, AB801.

Materials and Methods: The potency and selectivity of AB801 against AXL and other kinases were determined using a panel of HTRF KinEASE-TK assays and via a competition binding assay utilizing DNA-tagged kinases. The effects of AB801 were further assessed by a cell-based phospho-AXL ELISA. The pharmacokinetic (PK) profile of the molecule was evaluated in preclinical species. AB801 was characterized in routine in vitro safety assays, including hERG inhibition. Downstream signaling of AXL was evaluated by phospho-array, Western blot, and qPCR. Pharmacodynamics (PD) and anti-tumor efficacy in combination with standard of care (SOC) therapies were assessed in murine cancer models.

Results: The novel AXL inhibitor AB801 is potent, reversible, and selective. AB801 exhibits cellular activity at low nanomolar concentrations and retains significant activity in 100% human serum. Excellent selectivity was observed against MERTK (860x), TYRO3 (1,400x), and the overall kinome. Importantly, AB801 does not show significant CYP450 or hERG inhibition. Favorable preclinical PK is consistent with projected once-a-day oral administration in humans. AB801 increases sensitivity to SOC therapeutics such as chemotherapy, and results in increased DNA damage. Moreover, AB801 treatment sensitizes tumors to checkpoint blockade by increasing immune cell activation. Significant anti-tumor efficacy is observed in combination SOC therapies in multiple in vivo models.

Conclusions: AXL inhibition is a promising therapeutic mechanism for impairing the growth of tumors resistant to SOC therapeutics. AB801 exhibits improved potency, selectivity, and safety profiles compared to other AXL inhibitors currently advancing into clinical development.

Citation Format: Susan Lee Paprcka, Armon Goshayeshi, Suan Liu, Ruben Flores, Lauren Rocha, Jhansi L. Leslie, Dillon H. Miles, Corinne N. Foley, Shiwei Qu, Manjunath Lamani, Srinivas Paladugu, Hsin-Ting Huang, Nidhi Tribewal, Ada Chen, Joseph Kulusich, Stefan Garrido-Shaqfeh, Patricia Fabila, Salema Jafri, Anuja Devarajan, Ester Fernandez-Salas. AB801 is a highly potent and selective AXL kinase inhibitor that demonstrates significant anti-tumor activity [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 518.

Design, Synthesis, and Structure-Activity Relationship Optimization of Pyrazolopyrimidine Amide Inhibitors of Phosphoinositide 3-Kinase γ (PI3Kγ)

Phosphoinositide-3-kinase γ (PI3Kγ) is highly expressed in immune cells and promotes the production and migration of inflammatory mediators. The inhibition of PI3Kγ has been shown to repolarize the tumor immune microenvironment to a more inflammatory phenotype, thereby controlling immune suppression in cancer. Herein, we report the structure-based optimization of an early lead series of pyrazolopyrimidine isoindolinones, which culminated in the discovery of highly potent and isoform-selective PI3Kγ inhibitors with favorable drug-like properties. X-ray cocrystal structure analysis, molecular docking studies, and detailed structure-activity relationship investigations resulted in the identification of the optimal amide and isoindolinone substituents to achieve a desirable combination of potency, selectivity, and metabolic stability. Preliminary in vitro studies indicate that inhibition of PI3Kγ with compound 56 results in a significant immune response by increasing pro-inflammatory cytokine gene expression in M1 macrophages.

Discovery of Potent and Selective Methylenephosphonic Acid CD73 Inhibitors

Solid tumors are often associated with high levels of extracellular ATP. Ectonucleotidases catalyze the sequential hydrolysis of ATP to adenosine, which potently suppresses T-cell and NK-cell functions via the adenosine receptors (A_2a and A_2b). The ectonucleotidase CD73 catalyzes the conversion of AMP to adenosine. Thus, increased CD73 enzymatic activity in the tumor microenvironment is a potential mechanism for tumor immune evasion and has been associated with poor prognosis in the clinic. CD73 inhibition is anticipated to restore immune function by skirting this major mechanism of adenosine generation. We have developed a series of potent and selective methylenephosphonic acid CD73 inhibitors via a structure-based design. Key binding interactions of the known inhibitor adenosine-5'-(α,β-methylene)diphosphate (AMPCP) with hCD73 provided the foundation for our early designs. The structure-activity relationship study guided by this structure-based design led to the discovery of 4a, which exhibits excellent potency against CD73, exquisite selectivity against related ectonucleotidases, and a favorable pharmacokinetic profile.

Discovery of Potent and Selective 7-Azaindole Isoindolinone-Based PI3Kγ Inhibitors

The successful application of immunotherapy in the treatment of cancer relies on effective engagement of immune cells in the tumor microenvironment. Phosphoinositide 3-kinase γ (PI3Kγ) is highly expressed in tumor-associated macrophages, and its expression levels are associated with tumor immunosuppression and growth. Selective inhibition of PI3Kγ offers a promising strategy in immuno-oncology, which has led to the development of numerous potent PI3Kγ inhibitors with variable selectivity profiles. To facilitate further investigation of the therapeutic potential of PI3Kγ inhibition, we required a potent and PI3Kγ-selective tool compound with sufficient metabolic stability for use in future in vivo studies. Herein, we describe some of our efforts to realize this goal through the systematic study of SARs within a series of 7-azaindole-based PI3Kγ inhibitors. The large volume of data generated from this study helped guide our subsequent lead optimization efforts and will inform further development of PI3Kγ-selective inhibitors for use in immunomodulation.

Discovery of Potent and Selective PI3Kγ Inhibitors

The selective inhibition of the lipid signaling enzyme PI3Kγ constitutes an opportunity to mediate immunosuppression and inflammation within the tumor microenvironment but is difficult to achieve due to the high sequence homology across the class I PI3K isoforms. Here, we describe the design of a novel series of potent PI3Kγ inhibitors that attain high isoform selectivity through the divergent projection of substituents into both the "selectivity" and "alkyl-induced" pockets within the adenosine triphosphate (ATP) binding site of PI3Kγ. These efforts have culminated in the discovery of 5-[2-amino-3-(1-methyl-1H-pyrazol-4-yl)pyrazolo[1,5-a]pyrimidin-5-yl]-2-[(1S)-1-cyclopropylethyl]-7-(trifluoromethyl)-2,3-dihydro-1H-isoindol-1-one (4, IC₅₀ = 0.064 μM, THP-1 cells), which displays >600-fold selectivity for PI3Kγ over the other class I isoforms and is a promising step toward the identification of a clinical development candidate. The structure-activity relationships identified throughout this campaign demonstrate that greater γ-selectivity can be achieved by inhibitors that occupy an "alkyl-induced" pocket and possess bicyclic hinge-binding motifs capable of forming more than one hydrogen bond to the hinge region of PI3Kγ.

Enantioselective Synthesis of N,S-Acetals by an Oxidative Pummerer-Type Transformation using Phase-Transfer Catalysis

Reported is the first enantioselective oxidative Pummerer-type transformation using phase-transfer catalysis to deliver enantioenriched sulfur-bearing heterocycles. This reaction includes the direct oxidation of sulfides to a thionium intermediate, followed by an asymmetric intramolecular nucleophilic addition to form chiral cyclic N,S-acetals with moderate to high enantioselectivites. Deuterium-labelling experiments were performed to identify the stereodiscrimination step of this process. Further analysis of the reaction transition states, by means of multidimensional correlations and DFT calculations, highlight the existence of a set of weak noncovalent interactions between the catalyst and substrate that govern the enantioselectivity of the reaction.

A Nucleophilic Strategy for Enantioselective Intermolecular α-Amination: Access to Enantioenriched α-Arylamino Ketones

The enantioselective addition of anilines to azoalkenes was accomplished through the use of a chiral phosphoric acid catalyst. The resulting α-arylamino hydrazones were obtained in good yields and excellent enantioselectivities and provide access to enantioenriched α-arylamino ketones. A serendipitous kinetic resolution of racemic α-arylamino hydrazones is also described.

An improved method for detection and quantification of differential interactions between poliovirus internal ribosome entry site RNA and pyrimidine tract binding protein from primary cells

Attenuated and pathogenic viral variants are often extremely similar viruses with drastically different replication potentials. Despite precise knowledge of viral residues responsible for poliovirus attenuation/neurovirulence, molecular mechanisms mediating these effects remain poorly understood. Data from numerous sources suggest that a functional difference in translation initiation is one responsible factor. However, direct evidence, as well as a comprehensive model are lacking. Several difficulties, including lack of an assay system to quantify differential internal ribosome entry site/pyrimidine tract binding protein interaction in relevant systems, have precluded progress. A novel assay system that overcomes some difficulties is presented below. The assay uses streptavidin paramagnetic particles, biotinylated RNA and glutathione-S-transferase/pyrimidine tract binding protein fusion to detect nanogram levels of uncloned cellular pyrimidine tract binding protein species that interact with internal ribosome entry site RNA. Using this assay, it was shown that pyrimidine tract binding protein from primary human monocytes binds to internal ribosome entry site RNA from virulent poliovirus better than to that from attenuated virus, while pyrimidine tract binding protein from HeLa cells does not distinguish between the two internal ribosome entry sites. Since primary human monocytes reflect neurovirulence-related differential poliovirus replication, these results suggest that pyrimidine tract binding protein may contribute to differential poliovirus replication in vivo. This assay also has the potential to be applicable broadly to other nucleic acid/protein interactions.

Antithrombin activity of medicinal plants of the Azores

A chromogenic bioassay was utilized to determine the antithrombin activity of methylene chloride and methanol extracts prepared from 50 plants of Azores. Extracts of the six plants Hedychium gardneranum, Tropaeolum majus, Gunnera tinctoria, Hedera helix, Festuca jubata and Laurus azorica demonstrated activity of 78% or higher in this bioassay system.

Utilization of natural products for treatment of blood diseases

This chapter presents an introduction to several diseases of the blood including infectious mononucleosis, leukemia, thrombosis and coagulation, bone marrow disorders, malaria, and anemia. In addition a survey of the recent literature is presented relative to natural products that have been utilized for the treatment of these diseases. The natural products that are reported represent a wide range of structural types and present interesting mechanisms of action. Thus the possibility exists that new drugs may be developed from these natural products which are more effective than those currently on the market.

Agrochemical activity and isolation of N-(4'-bromophenyl)-2,2-diphenylacetanilide from the Thai plant Arundo donax

The CHCl3 extract of the whole plant of Arundo donax yielded N-(4'-bromophenyl)-2,2-diphenylacetanilide [1]. The structure was determined on the basis of chemical, spectroscopic, and X-ray crystallographic data. Compound 1 has not been previously reported as a natural product and showed inhibition of feeding for boll weevils.

Investigation of the antitumor activity of podocarpic acid derivatives

As a class, octahydrophenanthrene lactones, podolactones , and related podocarpic acid derivatives have been reported to possess a wide variety of biological activities, including antileukemic activity, inhibition of plant cell growth, and hormonal and anti-inflammatory properties. In the present study, a series of synthetic intermediates derived from podocarpic acid have been prepared and evaluated with respect to their ability to inhibit human epidermoid carcinoma of the nasopharynx in vitro. The significant cytotoxicity demonstrated by methyl 6 alpha-bromo-7-oxo-O- methylpodocarpate (50% inhibition of cells at 8.85 X 10(-9) M) was markedly higher than that of the other derivatives examined. Further evaluation against L1210 and P388 lymphoid leukemias in mice failed to demonstrate significant antitumor activity.

Toxic effects of juncusol, a marsh plant phenolic extract, on estuarine fish and shrimp

Juncusol, phenolic compound, was isolated from the black needlerush, Juncus roemerianus. Different concentrations of this extract were tested for toxicity against selected species of estuarine fish and grass shrimp. Juncusol was ten times more toxic to the various species of fish than to grass shrimp. At 2.0 ppm concentration all fishes tested were killed within 36 hr. One hundred per cent mortality in the grass shrimp occurred at 20 ppm during 95 hr exposure. The LC10, LC50, and LC90 concentration for juncusol were calculated for grass shrimp and for juvenile sailfin molly.

Constituents of Spartina cynosuroides: isolation and 13C-NMR analysis of tricin

Tricin was isoalted from the aerial parts of the marsh plant Spartina cynosuroides, which yielded a fraction enriched in antiluekemic activity. The 13C-NMR spectrum of tricin is discussed.

Tumor inhibitors II: constituents and antitumor activity of Sarracenia flava

The chloroform extracts of the roots and leaves of Sarracenia flava showed antitumor activity against lymphocytic leukemia P-388. Lupeol was identified as one constituent responsible for this activity. Beta-Sitosterol and alpha-amyrin were isolated from the hexane extract of the roots.

Hydrocarbons from males, females, and larvae of pecan weevil: Curculio caryae (Horn)

As part of a program to identify as many as possible of the components of the pecan weevil, Curculio caryae (Horn), the hydrocarbons from males, females, and larvae were isolated by solvent extraction and column chromatography and subjected to gas lipuid chromatography-mass spectrometry analysis. n-Alkanes from C14-C32 in the larvae and unsaturated and branched chain hydrocarbons from C20-C32 in males and females were found. There are no significant differences between the hydrocarbons of the male and female pecan weevils.