Pyrimidine depletion enhances targeted and immune therapy combinations in acute myeloid leukemia

Acute myeloid leukemia (AML) is a fatal disease characterized by the accumulation of undifferentiated myeloblasts, and agents that promote differentiation have been effective in this disease but are not curative. Dihydroorotate dehydrogenase inhibitors (DHODHi) have the ability to promote AML differentiation and target aberrant malignant myelopoiesis. We introduce HOSU-53, a DHODHi with significant monotherapy activity, which is further enhanced when combined with other standard-of-care therapeutics. We further discovered that DHODHi modulated surface expression of CD38 and CD47, prompting the evaluation of HOSU-53 combined with anti-CD38 and anti-CD47 therapies, where we identified a compelling curative potential in an aggressive AML model with CD47 targeting. Finally, we explored using plasma dihydroorotate (DHO) levels to monitor HOSU-53 safety and found that the level of DHO accumulation could predict HOSU-53 intolerability, suggesting the clinical use of plasma DHO to determine safe DHODHi doses. Collectively, our data support the clinical translation of HOSU-53 in AML, particularly to augment immune therapies. Potent DHODHi to date have been limited by their therapeutic index; however, we introduce pharmacodynamic monitoring to predict tolerability while preserving antitumor activity. We additionally suggest that DHODHi is effective at lower doses with select immune therapies, widening the therapeutic index.

Discovery of Nanomolar Inhibitors for Human Dihydroorotate Dehydrogenase Using Structure-Based Drug Discovery Methods

We used a structure-based drug discovery approach to identify novel inhibitors of human dihydroorotate dehydrogenase (DHODH), which is a therapeutic target for treating cancer and autoimmune and inflammatory diseases. In the case of acute myeloid leukemia, no previously discovered DHODH inhibitors have yet succeeded in this clinical application. Thus, there remains a strong need for new inhibitors that could be used as alternatives to the current standard-of-care. Our goal was to identify novel inhibitors of DHODH. We implemented prefiltering steps to omit PAINS and Lipinski violators at the earliest stages of this project. This enriched compounds in the data set that had a higher potential of favorable oral druggability. Guided by Glide SP docking scores, we found 20 structurally unique compounds from the ChemBridge EXPRESS-pick library that inhibited DHODH with IC50, DHODH values between 91 nM and 2.7 μM. Ten of these compounds reduced MOLM-13 cell viability with IC50, MOLM-13 values between 2.3 and 50.6 μM. Compound 16 (IC50, DHODH = 91 nM) inhibited DHODH more potently than the known DHODH inhibitor, teriflunomide (IC50, DHODH = 130 nM), during biochemical characterizations and presented a promising scaffold for future hit-to-lead optimization efforts. Compound 17 (IC50, MOLM-13 = 2.3 μM) was most successful at reducing survival in MOLM-13 cell lines compared with our other hits. The discovered compounds represent excellent starting points for the development and optimization of novel DHODH inhibitors.

Antibacterial activity of novel bacterial topoisomerase inhibitors against key veterinary pathogens

Multidrug-resistant bacteria infect companion animals and livestock in addition to their devastating impact on human health. Novel Bacterial Topoisomerase Inhibitors (NBTIs) with excellent activity against Gram-positive bacteria have previously been identified as promising new antibacterial agents. Herein, we evaluate the antibacterial activity of these NBTIs against a variety of important veterinary pathogens and demonstrate outstanding in vitro activity, especially against staphylococci.

Abstract 1060: Introducing a novel DHODH inhibitor with superior in vivo activity as monotherapy or in novel combination regimen with immunotherapy for hematological malignancies

Acute myeloid leukemia (AML) is characterized by the uncontrolled expansion of un-differentiated hematopoietic progenitor myeloblasts. AML treatment is very challenging owing to its complex heterogeneity resulting in a dismal 5-year overall survival rate particularly in elderly patients unfit for standard induction chemotherapy. The expansion of AML requires the availability of sufficient nucleotides supporting the anabolic processes required for AML growth thus, targeting nucleotide biosynthesis can halt AML progression. Indeed, targeting dihydroorotate dehydrogenase (DHODH), a critical rate-limiting step in the de novo pyrimidine synthesis pathway not only induced cytotoxicity but has been shown to promote blast differentiation in a HOXA9/MEIS1 over-expressing model. We sought to develop a DHODH inhibitor that had superior properties to those reported for AML therapy. Compound 41 (cmpd 41) demonstrates sub-nanomolar 50% inhibitory concentration for DHODH biochemical activity and potent in vitro activity across several AML cell lines and primary AML cells independent of mutational subtype, including mutated TP53. Cmpd 41 also demonstrated superior in vivo anti-leukemic activity in multiple AML xenograft models as monotherapy and demonstrated synergy with a hypomethylating agent, decitabine in TP53 mutated AML. Given the heterogeneity of AML and frequent emergence of resistant clones, we aimed to investigate ways to enhance response to DHODH inhibitors through combination. After in vitro treatment of AML cell lines and primary patient samples with DHODH inhibitors, we observed an increase in CD38 surface expression suggesting synergy with CD38 targeting monoclonal antibody (mAb) immunotherapies. Indeed, we are the first to report synergy between DHODH inhibitors and anti-CD38 mAb in AML which emphasizes the synergy between this promising novel class of agents with immunotherapies via recruiting innate immunity. Consequently, given the relevance of CD38 mAb therapy to multiple myeloma (MM), we extended these studies to MM and remarkably found that cmpd 41 was highly efficacious as a monotherapy and in combination with CD38 mAb, resulted in complete tumor regression in a subcutaneous MM xenograft model. In summary, we introduce a best in class DHODH inhibitor with a data-driven combination strategy for both AML and MM. Our studies suggest a highly synergistic combination strategy involving immunotherapy for AML and other hematologic malignancies.

Citation Format: Ola A. Elgamal, Sandip Vibhute, Sydney Fobare, Abeera Mehmood, Mariah L. Johnson, Jean Truxall, Emily Stahl, Bridget Carmichael, Shelley J. Orwick, Ramasamy Santhanam, Kasey Hill, Susheela Tridandapani, Christopher C. Coss, Alice S. Mims, Karilyn T. Larkin, Mitch A. Phelps, Sharyn D. Baker, Alex Sparreboom, Thomas E. Goodwin, Gerard Hilinski, Chad E. Bennett, Erin Hertlein, John C. Byrd. Introducing a novel DHODH inhibitor with superior in vivo activity as monotherapy or in novel combination regimen with immunotherapy for hematological malignancies [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 1060.

1,3-Dioxane-Linked Novel Bacterial Topoisomerase Inhibitors: Expanding Structural Diversity and the Antibacterial Spectrum

Antibacterial resistance continues its devastation of available therapies. Novel bacterial topoisomerase inhibitors (NBTIs) offer one solution to this critical issue. Two series of amine NBTIs bearing tricyclic DNA-binding moieties as well as amide NBTIs with a bicyclic DNA-binding moiety were synthesized and evaluated against methicillin-resistant Staphylococcus aureus (MRSA). Additionally, these compounds and a series of bicyclic amine analogues displayed high activity against susceptible and drug-resistant Neisseria gonorrhoeae, expanding the spectrum of these dioxane-linked NBTIs.

Optimization of TopoIV Potency, ADMET Properties, and hERG Inhibition of 5-Amino-1,3-dioxane-Linked Novel Bacterial Topoisomerase Inhibitors: Identification of a Lead with In Vivo Efficacy against MRSA

Novel bacterial topoisomerase inhibitors (NBTIs) are among the most promising new antibiotics in preclinical/clinical development. We previously reported dioxane-linked NBTIs with potent antistaphylococcal activity and reduced hERG inhibition, a key safety liability. Herein, polarity-focused optimization enabled the delineation of clear structure-property relationships for both microsomal metabolic stability and hERG inhibition, resulting in the identification of lead compound 79. This molecule demonstrates potent antibacterial activity against diverse Gram-positive pathogens, inhibition of both DNA gyrase and topoisomerase IV, a low frequency of resistance, a favorable in vitro cardiovascular safety profile, and in vivo efficacy in a murine model of methicillin-resistant Staphylococcus aureus infection.

Abstract 3067: Exploring strategies to modulate mitochondrial metabolism to alleviate tumor hypoxia

Radiation therapy is a standard type of treatment modality used as a part of curative or palliative treatment in more than 50% of all cancer patients. However, tumor hypoxia reduces the effectiveness of radiation therapy by limiting the biologically effective dose. Here, we explore and compare strategies to alleviate hypoxia by modifying the oxygen supply or demand within mouse tumor models. We used pODD-Luc, a dynamic in vivo luciferase reporter system that monitors relative levels of tumor hypoxia in real time. We show that in mouse tumor models, contrary to the reported results of human clinical trials, acute manipulation of systemic oxygen delivery significantly reduces the level of hypoxia. We then examined pharmacological manipulation of oxygen demand, which is an emerging trend in therapeutic efforts to sensitize tumors to anti-cancer therapy. Mitochondrial oxygen consumption (OCR) constitutes more than 90% of cellular oxygen demand by coupling carbon source oxidation with ATP generation in a series of redox reactions transferring electrons from reduced cofactors to molecular oxygen. Mechanistically, a decrease in mitochondrial OCR would lead to increased oxygen availability within the tumor, allowing initially hypoxic areas to become re-oxygenated. We found that strategies that directly stimulate mitochondrial OCR exacerbate levels of hypoxia while OCR suppression in turn promotes tumor re-oxygenation. We have previously shown that papaverine, an FDA-approved phosphodiesterase 10A (PDE10A) inhibitor effectively reduces hypoxic tumor fractions in vivo by an off-target effect that inhibits mitochondrial complex I. Here we show that its novel derivative SMV-32 that acts as complex I inhibitor with reduced PDE10A effect mediates superior OCR inhibition in both orthotopic and heterotopic mouse tumor models. These findings further establish the direct impact of OCR manipulation on tumor hypoxia and support the role of mitochondrial metabolism as an attractive target to alleviate tumor hypoxia and overcome treatment resistance.

Citation Format: Martin Benej, Jinghai Wu, McKenzie Kreamer, Sandip Vibhute, Ioanna Papandreou, Nicholas C. Denko. Exploring strategies to modulate mitochondrial metabolism to alleviate tumor hypoxia [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 3067.

Structure-Activity Relationship of para-Carborane Selective Estrogen Receptor β Agonists

Selective agonism of the estrogen receptor (ER) subtypes, ERα and ERβ, has historically been difficult to achieve due to the high degree of ligand-binding domain structural similarity. Multiple efforts have focused on the use of classical organic scaffolds to model 17β-estradiol geometry in the design of ERβ selective agonists, with several proceeding to various stages of clinical development. Carborane scaffolds offer many unique advantages including the potential for novel ligand/receptor interactions but remain relatively unexplored. We synthesized a series of para-carborane estrogen receptor agonists revealing an ERβ selective structure-activity relationship. We report ERβ agonists with low nanomolar potency, greater than 200-fold selectivity for ERβ over ERα, limited off-target activity against other nuclear receptors, and only sparse CYP450 inhibition at very high micromolar concentrations. The pharmacological properties of our para-carborane ERβ selective agonists measure favorably against clinically developed ERβ agonists and support further evaluation of carborane-based selective estrogen receptor modulators.

Abstract PO-033: Papaverine derivative smv-32 alleviates tumor hypoxia and radiosensitizes tumors by inhibiting mitochondrial metabolism

Radiation therapy is a standard type of treatment modality used to achieve local control in more than 50% of all cancer patients. However, tumor hypoxia reduces the effectiveness of radiation therapy by limiting the biologically effective dose. We identified that FDA-approved phosphodiesterase 10A (PDE10A) inhibitor papaverine (PPV) has an off-target effect inhibiting mitochondrial oxygen consumption (OCR) leading to acute reduction of tumor hypoxia and significant radiosensitization of model tumors. Our data suggests that PDE10A does not contribute to the radiosensitizing effect. Based on PPV’s structure, we used biomedicinal chemistry approach to engineer its novel derivative SMV-32 with reduced PDE10A inhibition. We used Seahorse metabolic flux analyzer to determine the effect of PPV and SMV-32 on mitochondrial OCR in vitro. Near-infrared optical spectroscopy and pODD-luciferase in vivo reporter system was used for pharmacodynamics analyses and dose response evaluation. Local radiation therapy was delivered using Small Animal Radiation Research Platform (SARRP). We compared intraperitoneal (IP) and intravenous (IV) delivery and optimized dosing using pODD luciferase reporter system. SMV-32 showed superior OCR inhibition in vitro and enhanced hypoxia reduction in vivo compared to PPV. A single clinically relevant dose of PPV or SMV-32 significantly reduced the hypoxic fractions of heterotopic and orthotopic tumors in mice and sensitized the tumors to radiation therapy. PPV and SMV-32 appear to be ideal candidates for clinical radiosensitization. PPV is already in phase I clinical trial for treatment of non-small cell lung cancer combined with stereotactic body radiation therapy.

Citation Format: Martin Benej, Jinghai Wu, McKenzie Kreamer, Sandip Vibhute, Ioanna Papandreou, Nicholas C. Denko. Papaverine derivative smv-32 alleviates tumor hypoxia and radiosensitizes tumors by inhibiting mitochondrial metabolism [abstract]. In: Proceedings of the AACR Virtual Special Conference on Radiation Science and Medicine; 2021 Mar 2-3. Philadelphia (PA): AACR; Clin Cancer Res 2021;27(8_Suppl):Abstract nr PO-033.

Dendrimeric calcium-sensitive MRI probes: the first low-field relaxometric study

In the present work the first investigation ever of calcium sensitive dendrimer relaxation mechanisms at low fields is reported.

Solid-Phase-Supported Approach for the Preparation of Bioresponsive and Multifunctional MRI Probes

The development of bifunctional imaging probes can often be challenging with difficult and time-consuming solution phase chemistry protocols and purification techniques. A solid phase synthetic protocol was therefore utilized to produce a functionalized derivative of a potent bismacrocyclic calcium-responsive contrast agent for magnetic resonance imaging. Through a convenient building block approach, the applicability of this methodology in the preparation and simple future development of multifunctional imaging probes was demonstrated.

1,3-Dioxane-Linked Bacterial Topoisomerase Inhibitors with Enhanced Antibacterial Activity and Reduced hERG Inhibition

The development of new therapies to treat methicillin-resistant Staphylococcus aureus (MRSA) is needed to counteract the significant threat that MRSA presents to human health. Novel inhibitors of DNA gyrase and topoisomerase IV (TopoIV) constitute one highly promising approach, but continued optimization is required to realize the full potential of this class of antibiotics. Herein, we report further studies on a series of dioxane-linked derivatives, demonstrating improved antistaphylococcal activity and reduced hERG inhibition. A subseries of analogues also possesses enhanced inhibition of the secondary target, TopoIV.

Abstract 2927: Papaverine and its novel derivatives radiosensitize solid tumors by inhibiting mitochondrial metabolism

Radiation therapy is a standard type of treatment modality used to achieve local control in more than 50% of all cancer patients. However, tumor hypoxia reduces the effectiveness of radiation therapy by limiting the biologically effective dose. Limited distribution of oxygen is a direct consequence of abnormalities in vascular structure and angiogenesis that fails to provide sufficient oxygen to meet the demands of metabolically hyperactive cancer cells. An acute increase in tumor oxygenation prior to radiation treatment should therefore significantly improve the tumor cell kill after radiation. Nonetheless, therapeutic efforts to increase oxygen delivery to the tumor have not shown positive clinical results to this day. We have taken an alternative route by targeting the demand for oxygen rather than its supply. In the cell, mitochondrial respiration is the major oxygen-consuming process. We show that pharmacological inhibition of mitochondrial oxygen consumption (OCR) temporarily reduces the tumor cells’ demand for oxygen leading to increased tumor oxygenation and enhanced radiation response. We identified a previously unrecognized activity of the FDA-approved drug papaverine as an inhibitor of mitochondrial complex I. In vivo, a single clinically achievable dose of papaverine increased tumor, but not normal tissue oxygenation within 45 minutes and strikingly enhanced tumor response to radiation therapy in both ortho- and heterotopic rodent tumor models. Moreover, our GI tract studies show that this can be achieved without exacerbating normal tissue toxicity, as papaverine does not radiosensitize hypoxic normal tissues. Papaverine is an ergot alkaloid originally isolated from Papaver somniferum in 1848. It was used for decades as smooth muscle relaxant to treat vasospasms and erectile dysfunction. Its vascular effects were believed to be mediated by its ability to inhibit phosphodiesterase 10A (PDE10A). We provide genetic evidence that papaverine’s complex I inhibition, not its activity as a PDE10A inhibitor is directly responsible for increased oxygenation and enhanced radiation response. Furthermore, we describe novel derivatives of papaverine that have the potential to become a new generation of clinical radiosensitizers with potentially fewer side effects. Papaverine has a short half-life of 90-120 minutes, is cell-permeable, reversible and quickly cleared from the patient. In vitro, all 28 cancer and normal cell lines tested were sensitive to papaverine regardless of their oncogenic landscape, suggesting possible application for a broad spectrum of cancers that depends primarily on their level of hypoxia. In conclusion, PPV or one of its novel derivatives appear to be ideal candidates for clinical radiosensitization, applicable primarily in cancers where local control increases the overall survival.

Citation Format: Martin Benej, Xiangqian Hong, Sandip Vibhute, Sabina Scott, Jinghai Wu, Edward Graves, Quynh-Thu Le, Albert C. Koong, Amato J. Giaccia, Ching-Shih Chen, Bing Yu, Ioanna Papandreou, Nicholas C. Denko. Papaverine and its novel derivatives radiosensitize solid tumors by inhibiting mitochondrial metabolism [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 2927.

Coordination Properties of GdDO3A-Based Model Compounds of Bioresponsive MRI Contrast Agents

We report a detailed characterization of the thermodynamic stability and dissociation kinetics of Gd³⁺ complexes with DO3A derivatives containing a (methylethylcarbamoylmethylamino)acetic acid (L¹), (methylpropylcarbamoylmethylamino)acetic acid (L²), 2-dimethylamino- N-ethylacetamide (L³), or 2-dimethylamino- N-propylacetamide (L⁴) group attached to the fourth nitrogen atom of the macrocyclic unit. These ligands are model systems of Ca²⁺- and Zn²⁺-responsive contrast agents (CA) for application in magnetic resonance imaging (MRI). The results of the potentiometric studies ( I = 0.15 M NaCl) provide stability constants with log K_GdL values in the range 13.9-14.8. The complex speciation in solution was found to be quite complicated due to the formation of protonated species at low pH, hydroxido complexes at high pH, and stable dinuclear complexes in the case of L^1,2. At neutral pH significant fractions of the complexes are protonated at the amine group of the amide side chain (log K_GdL×H = 7.2-8.1). These ligands form rather weak complexes with Mg²⁺ and Ca²⁺ but very stable complexes with Cu²⁺ (log K_CuL = 20.4-22.3) and Zn²⁺ (log K_ZnL = 15.5-17.6). Structural studies using a combination of ¹H NMR and luminescence spectroscopy show that the amide group of the ligand is coordinated to the metal ion at pH ∼8.5, while protonation of the amine group provokes the decoordination of the amide O atom and a concomitant increase in the hydration number and proton relaxivity. The dissociation of the complexes occurs mainly through a rather efficient proton-assisted pathway, which results in kinetic inertness comparable to that of nonmacrocyclic ligands such as DTPA rather than DOTA-like complexes.

Innovative Design of Ca-Sensitive Paramagnetic Liposomes Results in an Unprecedented Increase in Longitudinal Relaxivity

Bioresponsive MRI contrast agents sensitive to Ca(II) fluctuations may play a critical role in the development of functional molecular imaging methods to study brain physiology or abnormalities in muscle contraction. A great challenge in their chemistry is the preparation of probes capable of inducing a strong signal variation that could be detected in a robust way. To this end, the incorporation of small molecular weight bioresponsive agents into nanocarriers can improve the overall properties in a few ways: (i) the agent can be delivered into the tissue of interest, increasing the local concentration; (ii) its biokinetic properties and retention time will improve; (iii) the high molecular weight and size of the nanocarrier may cause additional changes in the MRI signal and raise the chances for their detection in functional experiments. In this work, we report the preparation of the new class of liposome-based, Ca-sensitive MRI agents. We synthesized a novel amphiphilic ligand which was incorporated into the liposome bilayer. A remarkable increase of ∼420% in longitudinal relaxivity r1, from 7.3 mM(-1) s(-1) to 38.1 mM(-1) s(-1) at 25 °C and 21.5 MHz in the absence and presence of Ca(II), respectively, was achieved by the most active liposomal formulation. To the best of our knowledge, this is the highest change in r1 observed for Ca-sensitive agents at physiological pH and can be explained by simultaneous Ca-triggered increase in hydration and reduction of local motion of Gd(III) complex, which can be followed at low magnetic fields.

Ultrasmall Nanoplatforms as Calcium-Responsive Contrast Agents for Magnetic Resonance Imaging

The preparation of ultrasmall and rigid platforms (USRPs) that are covalently coupled to macrocycle-based, calcium-responsive/smart contrast agents (SCAs), and the initial in vitro and in vivo validation of the resulting nanosized probes (SCA-USRPs) by means of magnetic resonance imaging (MRI) is reported. The synthetic procedure is robust, allowing preparation of the SCA-USRPs on a multigram scale. The resulting platforms display the desired MRI activity—i.e., longitudinal relaxivity increases almost twice at 7 T magnetic field strength upon saturation with Ca(2+). Cell viability is probed with the MTT assay using HEK-293 cells, which show good tolerance for lower contrast agent concentrations over longer periods of time. On intravenous administration of SCA-USRPs in living mice, MRI studies indicate their rapid accumulation in the renal pelvis and parenchyma. Importantly, the MRI signal increases in both kidney compartments when CaCl2 is also administrated. Laser-induced breakdown spectroscopy experiments confirm accumulation of SCA-USRPs in the renal cortex. To the best of our knowledge, these are the first studies which demonstrate calcium-sensitive MRI signal changes in vivo. Continuing contrast agent and MRI protocol optimizations should lead to wider application of these responsive probes and development of superior functional methods for monitoring calcium-dependent physiological and pathological processes in a dynamic manner.

Dendrimeric calcium-responsive MRI contrast agents with slow in vivo diffusion

We report a methodology which enables the preparation of dendrimeric contrast agents sensitive to Ca(2+) when starting from the monomeric analogue. The Ca-triggered longitudinal relaxivity response of these agents is not compromised by undertaking synthetic transformations, despite structural changes. The in vivo MRI studies in the rat cerebral cortex indicate that diffusion properties of dendrimeric contrast agents have great advantages as compared to their monomeric equivalents.