Targeting CD73 with AB680 (Quemliclustat), a Novel and Potent Small-Molecule CD73 Inhibitor, Restores Immune Functionality and Facilitates Antitumor Immunity

T cells play a critical role in the control of cancer. The development of immune checkpoint blockers (ICB) aimed at enhancing antitumor T-cell responses has revolutionized cancer treatment. However, durable clinical benefit is observed in only a subset of patients, prompting research efforts to focus on strategies that target multiple inhibitory signals within the tumor microenvironment (TME) to limit tumor evasion and improve patient outcomes. Adenosine has emerged as a potent immune suppressant within the TME, and CD73 is the major enzyme responsible for its extracellular production. CD73 can be co-opted within the TME to impair T-cell-mediated antitumor immunity and promote tumor growth. To target this pathway and block the formation of adenosine, we designed a novel, selective, and potent class of small-molecule inhibitors of CD73, including AB680 (quemliclustat), which is currently being tested in patients with cancer. AB680 effectively restored T-cell proliferation, cytokine secretion, and cytotoxicity that were dampened by the formation of immunosuppressive adenosine by CD73. Furthermore, in an allogeneic mixed lymphocyte reaction where CD73-derived adenosine had a dominant suppressive effect in the presence of PD-1 blockade, AB680 restored T-cell activation and function. Finally, in a preclinical mouse model of melanoma, AB680 inhibited CD73 in the TME and increased the antitumor activity of PD-1 blockade. Collectively, these data provide a rationale for the inhibition of CD73 with AB680 in combination with ICB, such as anti-PD-1, to improve cancer patient outcomes.

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.

Abstract 710: CD73 inhibitors (CD73i) reverse the AMP/adenosine-mediated impairment of immune effector cell activation by immune checkpoint inhibitors (ICI)

INTRODUCTION: CD73 catalyzes the extracellular generation of adenosine (ADO) from adenosine monophosphate (AMP). ADO suppresses immune responses, including those of T cells, NK cells and dendritic cells through activation of A2aR and A2bR receptors. Exhausted T cells and NK cells express high levels of several immune checkpoint proteins, including PD-1 and TIGIT. We present here preclinical data on the ability of CD73i to reverse effector cell suppression from exposure to ADO even in the presence of ICI.

METHODS: CD73i effects in a monotherapeutic setting were assessed by CD3/CD28/CD2 T cell stimulation and cytolytic assays. Combinatorial settings were assessed using mixed lymphocyte reactions (MLRs). In vivo effects of CD73i + ICI were determined using syngeneic tumor models.

RESULTS: CD73 is expressed across a wide range of tumor types, including those with limited response to anti-PD-1 therapy. CD73i completely rescued AMP-mediated inhibition of T cell proliferation and effector function as well as NK cell cytolytic function. AMP abrogated the enhanced allogeneic CD4+ T cell activation and IFN-γ production mediated by blocking PD-1/PD-L1 and TIGIT, an effect that was reversed by CD73i. Mechanistically, addition of AMP in MLRs repressed expression of activation markers and immune checkpoint proteins. Thus, activation of the adenosinergic pathway may limit the efficacy of ICI. TCGA data from anti-PD-1-treated melanoma patients identified CD73 expression as a negative prognostic factor. Finally, co-administration of a CD73i with an anti-PD-1 mAb resulted in significant reduction of tumor volume associated with increases in immune cell infiltration.

CONCLUSIONS: CD73 inhibition, alone or in combination with anti-PD-1 and anti-TIGIT antibodies, translates into potent enhancement of immune cell activation in a variety of studies. These data provide a rationale for CD73i + ICI combinations.

Citation Format: Annette Becker, Nell Narasappa, Fangfang Yin, Kristen Zhang, Daniel DiRenzo, Timothy Park, Jaroslaw Kalisiak, Ken Lawson, Jenna Jeffrey, Jay P. Powers, Ulrike Schindler, Matthew J. Walters, Joanne B. Tan. CD73 inhibitors (CD73i) reverse the AMP/adenosine-mediated impairment of immune effector cell activation by immune checkpoint inhibitors (ICI) [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 710.

Abstract B46: Small-molecule inhibitors of ecto-nucleotidase CD73 promote activation of human CD8+ T cells and have profound effects on tumor growth and immune parameters in experimental tumor models

Introduction: The intra-tumoral generation of adenosine (ADO), a potent inhibitor of T-cell activation, depends on the coordinated and sequential cleavage of extracellular adenosine triphosphate (ATP) by the ecto-nucleotidases CD39 (which produces adenosine monophosphate, AMP) and CD73 (which hydrolyzes AMP to form ADO). For this reason, a number of anti-CD73 antibodies are being advanced into clinical trials; however, to date there have been few reports of potent, selective, small-molecule CD73 inhibitors, such as those described here.

Methods: Ecto-nucleotidase activity was calculated using the Malachite green assay after 50-min incubation with 25µM AMP, in the presence of varying concentrations of test compound(s). The following systems were used. Endogenous expression: hCD73/SKOV-3 cells; hCD73/CD8 T cells. Stable over-expression: hCD73/CHO. Transient expression: mCD73/CHO; NTPDase2/CHO; NTPDase3/CHO; NTPDase8/CHO. Human CD8 T cells enriched from buffy coats or leukopaks were pre-treated with varying concentrations of CD73 inhibitors prior to addition of 50 μM AMP + 10 μM EHNA and activated with T cell Activation/Expansion kit (Miltenyi). In some experiments, exogenous recombinant human IL12p70 (1-10 ng/mL) was added to the culture. Activation (CD25) and effector functions (Granzyme B and IFNγ) were measured by flow cytometry. CT26 cells were implanted into the shaved right flank of 7-8 week old Balb/c mice and measured three times a week starting at 7 days. Mice were enrolled into 4 cohorts and dosed according to the following conditions when tumour volume reached ~100 mm3.

Group 1: 1% HPMC (sc/QD) + 2A3 (10 mg/kg; IP/Q3D)

Group 2: A000830 (30 mg/kg; sc/QD) + 2A3 (10 mg/kg; IP/Q3D)

Group 3: 1% HPMC (sc/QD) + RMP1-14 (10 mg/kg; IP/Q3D)

Group 4: A000830 (30 mg/kg; sc/QD) + RMP1-14 (10 mg/kg; IP/Q3D)

For interim analysis, single cell suspension was generated from tissues, blocked (clone 2.4G2), and stained with antibodies. For intracellular FOXP3 staining, samples were fixed and stained using FOXP3/Transcription Factor Staining Buffer Set. Non-specific blocking was performed with 20% normal rat serum prior to addition of anti-mouse FOXP3 antibody.

Results: We have designed a series of potent and specific small-molecule inhibitors of human and mouse CD73, represented by A000830 and A001190 with the following IC50 values in overexpression systems:

A000830: Mouse IC50 (1 nM); Human IC50 (3 nM)

A001190: Mouse IC50 (n.d.); Human IC50 (0.03 nM)

A000830 and A001190 also blocked AMP hydrolysis by freshly isolated human CD8 T cells at potencies comparable to the over-expression systems. In in vitro models of AMP/ADO-driven inhibition of human CD8+ T-cell activation, A000830 and A001190 showed robust rescue of CD25 expression and granzyme B production. Complete rescue of IFNγ; production was achieved by adding exogenous IL12. In vivo, A000830 was well-tolerated in mice, resulting in sustained plasma concentrations above IC90. Therapeutic dosing of A000830 to these mice in combination with an α-PD1 antibody resulted in robust CT26 tumor growth inhibition, greater than either treatment alone.

Conclusions: Cumulatively, these data provide the initial characterization of a novel class of potent and selective small-molecule CD73 inhibitors that effectively block the generation of ADO from extracellular ATP, reverse the ADO-driven inhibition of human T-cell activation, and display promising anti-tumor activity when dosed in combination with PD-1 blockade.

Citation Format: Joanne BL Tan, Ada Chen, Manmohan Leleti, Annette Becker, Erick Lindsey, Jaroslaw Kalisiak, Jay P. Powers, Steve Young, Ulrike Schindler, Juan C. Jaen. Small-molecule inhibitors of ecto-nucleotidase CD73 promote activation of human CD8+ T cells and have profound effects on tumor growth and immune parameters in experimental tumor models. [abstract]. In: Proceedings of the AACR Special Conference on Tumor Immunology and Immunotherapy; 2016 Oct 20-23; Boston, MA. Philadelphia (PA): AACR; Cancer Immunol Res 2017;5(3 Suppl):Abstract nr B46.

Wnt inhibition correlates with human embryonic stem cell cardiomyogenesis: a structure-activity relationship study based on inhibitors for the Wnt response

Human embryonic stem cell-based high-content screening of 550 known signal transduction modulators showed that one "lead" (1, a recently described inhibitor of the proteolytic degradation of Axin) stimulated cardiomyogenesis. Because Axin controls canonical Wnt signaling, we conducted an investigation to determine whether the cardiogenic activity of 1 is Wnt-dependent, and we developed a structure-activity relationship to optimize the cardiogenic properties of 1. We prepared analogues with a range of potencies (low nanomolar to inactive) for Wnt/β-catenin inhibition and for cardiogenic induction. Both functional activities correlated positively (r(2) = 0.72). The optimal compounds induced cardiogenesis 1.5-fold greater than 1 at 30-fold lower concentrations. In contrast, no correlation was observed for cardiogenesis and modulation of transforming growth factor β (TGFβ)/Smad signaling that prominently influences cardiogenesis. Taken together, these data show that Wnt signaling inhibition is essential for cardiogenic activity and that the pathway can be targeted for the design of druglike cardiogenic molecules.

Oxime-assisted acetylcholinesterase catalytic scavengers of organophosphates that resist aging

The cholinesterases, acetylcholinesterase (AChE) and butyrylcholinesterase, are primary targets of organophosphates (OPs). Exposure to OPs can lead to serious cardiovascular complications, respiratory compromise, and death. Current therapy to combat OP poisoning involves an oxime reactivator (2-PAM, obidoxime, TMB4, or HI-6) combined with atropine and on occasion an anticonvulsant. Butyrylcholinesterase, administered in the plasma compartment as a bio-scavenger, has also shown efficacy but is limited by its strict stoichiometric scavenging, slow reactivation, and a propensity for aging. Here, we characterize 10 human (h) AChE mutants that, when coupled with an oxime, give rise to catalytic reactivation and aging resistance of the soman conjugate. With the most efficient human AChE mutant Y337A/F338A, we show enhanced reactivation rates for several OP-hAChE conjugates compared with wild-type hAChE when reactivated with HI-6 (1-(2'-hydroxyiminomethyl-1'-pyridinium)-3-(4'-carbamoyl-1-pyridinium)). In addition, we interrogated an 840-member novel oxime library for reactivation of Y337A/F338A hAChE-OP conjugates to delineate the most efficient oxime-mutant enzyme pairs for catalytic bio-scavenging. Combining the increased accessibility of the Y337A mutation to oximes within the space-impacted active center gorge with the aging resistance of the F338A mutation provides increased substrate diversity in scavenging potential for aging-prone alkyl phosphate inhibitors.

Interaction kinetics of oximes with native, phosphylated and aged human acetylcholinesterase

Oximes are commonly used nucleophilic reactivators of alkyl phosphorylated and alkyl methylphosphonylated acetylcholinesterase (AChE) and butyrylcholinesterase. Covalent inhibition of these enzymes by organophosphate (OP) pesticides results typically in phosphorylated enzymes, while covalent inhibition by nerve agent OPs results in methyl phosphonylated cholinesterases. In this study we determined kinetic constants for interaction of three triazole containing oximes with native human AChE, enzyme diethylphosphorylated by paraoxon, enzyme phosphonylated by VX and cyclosarin as well as enzyme aged upon phosphonylation by soman. Stopped-flow kinetics of oxime interaction was monitored using quenching of intrinsic tryptophan fluorescence of AChE as an indicator of oxime binding. Triazole oximes were efficiently synthesized using copper catalyzed cycloaddition between azide and alkyne building blocks ("Click chemistry"). Equilibrium dissociation constants determined for both native enzymes were in low micromolar range for all three oximes, while dissociation constants for phosphylated (phosphorylated and phosphonylated) enzymes were typically one to two orders of magnitude larger. Dissociation constants for interaction with aged enzymes were similar or smaller than those determined for native enzymes. Similar results were obtained with reference oximes, 2PAM and HI6. Association rate constants for formation of oxime complexes were similar for both native, phosphylated and aged enzymes. In summary our data suggest that modification of active site gorge in AChEs by phosphylation of the active serine compromises oxime binding. Dealkylation of phosphonylated enzyme, however opens space in the gorge allowing oximes to bind tighter.

Investigating the structural influence of surface mutations on acetylcholinesterase inhibition by organophosphorus compounds and oxime reactivation

Organophosphates (OPs) exert their toxicity by inhibiting primarily acetylcholinesterase (AChE) and to a lesser extent butyrylcholinesterase (BChE). Binary mixtures of mammalian AChE and oximes of varying structure have been recently considered for treatment of OP poisoning as catalytic bioscavengers. In this study wild type human AChE and human AChE with residue mutations D134H, D134H_E202Q and D134H_F338A were characterized and investigated for inhibition by OPs and consequent oxime reactivation of phosphylated enzymes. The rationale for selecting these substitution positions was based on D134H being a naturally occurring single nucleotide polymorphism (SNP) in humans and that E202Q and F338A mutations slow aging of OP inhibited AChEs. Inhibition of D134H by paraoxon and analogues of cyclosarin was 2-8 times slower than inhibition of wild type (wt), while reactivation of the paraoxon inhibited enzyme by 2PAM was 6 times faster. Both inhibition and reactivation of D134H_E202Q and D134H_F338A double mutants were up to two orders of magnitude slower than the wt indicating that introduction of the active center substitutions abolished fully the effect of the peripherally located D134H. These results indicate that selected residues outside the active center influence inhibition, reactivation and catalysis rates through longer range interactions.

Synthesis and electrochemistry of 2-ethenyl and 2-ethanyl derivatives of 5-nitroimidazole and antimicrobial activity against Giardia lamblia

Infections with the diarrheagenic pathogen, Giardia lamblia, are commonly treated with the 5-nitroimidazole (5-NI) metronidazole (Mz), and yet treatment failures and Mz resistance occur. Using a panel of new 2-ethenyl and 2-ethanyl 5-NI derivatives, we found that compounds with a saturated bridge between the 5-NI core and a pendant ring system exhibited only modestly increased antigiardial activity and could not overcome Mz resistance. By contrast, olefins with a conjugated bridge connecting the core and a substituted phenyl or heterocyclic ring showed greatly increased antigiardial activity without toxicity, and several overcame Mz resistance and were more effective than Mz in a murine giardiasis model. Determination of the half-wave potential of the initial one-electron transfer by cyclic voltammetry revealed that easier redox activation correlated with greater antigiardial activity and capacity to overcome Mz resistance. These studies show the potential of combining systematic synthetic approaches with biological and electrochemical evaluations in developing improved 5-NI drugs.

Correlating the transcriptome, proteome, and metabolome in the environmental adaptation of a hyperthermophile

We have performed a comprehensive characterization of global molecular changes for a model organism Pyrococcus furiosus using transcriptomic (DNA microarray), proteomic, and metabolomic analysis as it undergoes a cold adaptation response from its optimal 95 to 72 degrees C. Metabolic profiling on the same set of samples shows the down-regulation of many metabolites. However, some metabolites are found to be strongly up-regulated. An approach using accurate mass, isotopic pattern, database searching, and retention time is used to putatively identify several metabolites of interest. Many of the up-regulated metabolites are part of an alternative polyamine biosynthesis pathway previously established in a thermophilic bacterium Thermus thermophilus. Arginine, agmatine, spermidine, and branched polyamines N4-aminopropylspermidine and N4-( N-acetylaminopropyl)spermidine were unambiguously identified based on their accurate mass, isotopic pattern, and matching of MS/MS data acquired under identical conditions for the natural metabolite and a high purity standard. Both DNA microarray and semiquantitative proteomic analysis using a label-free spectral counting approach indicate the down-regulation of a large majority of genes with diverse predicted functions related to growth such as transcription, amino acid biosynthesis, and translation. Some genes are, however, found to be up-regulated through the measurement of their relative mRNA and protein levels. The complimentary information obtained by the various "omics" techniques is used to catalogue and correlate the overall molecular changes.