Neurodegenerative Disease Treatment Drug PBT2 Breaks Intrinsic Polymyxin Resistance in Gram-Positive Bacteria

Gram-positive bacteria do not produce lipopolysaccharide as a cell wall component. As such, the polymyxin class of antibiotics, which exert bactericidal activity against Gram-negative pathogens, are ineffective against Gram-positive bacteria. The safe-for-human-use hydroxyquinoline analog ionophore PBT2 has been previously shown to break polymyxin resistance in Gram-negative bacteria, independent of the lipopolysaccharide modification pathways that confer polymyxin resistance. Here, in combination with zinc, PBT2 was shown to break intrinsic polymyxin resistance in Streptococcus pyogenes (Group A Streptococcus; GAS), Staphylococcus aureus (including methicillin-resistant S. aureus), and vancomycin-resistant Enterococcus faecium. Using the globally disseminated M1T1 GAS strain 5448 as a proof of principle model, colistin in the presence of PBT2 + zinc was shown to be bactericidal in activity. Any resistance that did arise imposed a substantial fitness cost. PBT2 + zinc dysregulated GAS metal ion homeostasis, notably decreasing the cellular manganese content. Using a murine model of wound infection, PBT2 in combination with zinc and colistin proved an efficacious treatment against streptococcal skin infection. These findings provide a foundation from which to investigate the utility of PBT2 and next-generation polymyxin antibiotics for the treatment of Gram-positive bacterial infections.

Discovery of Cofactor Competitive Inhibitors against the Human Methyltransferase Fibrillarin

Fibrillarin (FBL) is an essential and evolutionarily highly conserved S-adenosyl methionine (SAM) dependent methyltransferase. It is the catalytic component of a multiprotein complex that facilitates 2'-O-methylation of ribosomal RNAs (rRNAs), a modification essential for accurate and efficient protein synthesis in eukaryotic cells. It was recently established that human FBL (hFBL) is critical for Nipah, Hendra, and respiratory syncytial virus infections. In addition, overexpression of hFBL contributes towards tumorgenesis and is associated with poor survival in patients with breast cancer, suggesting that hFBL is a potential target for the development of both antiviral and anticancer drugs. An attractive strategy to target cofactor-dependent enzymes is the selective inhibition of cofactor binding, which has been successful for the development of inhibitors against several protein methyltransferases including PRMT5, DOT1L, and EZH2. In this work, we solved crystal structures of the methyltransferase domain of hFBL in apo form and in complex with the cofactor SAM. Screening of a fluorinated fragment library, via X-ray crystallography and 19F NMR spectroscopy, yielded seven hit compounds that competed with cofactor binding, two of which resulted in co-crystal structures. One of these structures revealed unexpected conformational variability in the cofactor binding site, which allows it to accommodate a compound significantly different from SAM. Our structural data provide critical information for the design of selective cofactor competitive inhibitors targeting hFBL, and preliminary elaboration of hit compounds has led to additional cofactor site binders.

A drug candidate for Alzheimer's and Huntington's disease, PBT2, can be repurposed to render Neisseria gonorrhoeae susceptible to natural cationic antimicrobial peptides

BACKGROUND

Neisseria gonorrhoeae is a Gram-negative bacterial pathogen that causes gonorrhoea. No vaccine is available to prevent gonorrhoea and the emergence of MDR N. gonorrhoeae strains represents an immediate public health threat.

OBJECTIVES

To evaluate whether PBT2/zinc may sensitize MDR N. gonorrhoeae to natural cationic antimicrobial peptides.

METHODS

MDR strains that contain differing resistance mechanisms against numerous antibiotics were tested in MIC assays. MIC assays were performed using the broth microdilution method according to CLSI guidelines in a microtitre plate. Serially diluted LL-37 or PG-1 was tested in combination with a sub-inhibitory concentration of PBT2/zinc. Serially diluted tetracycline was also tested with sub-inhibitory concentrations of PBT2/zinc and LL-37. SWATH-MS proteomic analysis of N. gonorrhoeae treated with PBT2/zinc, LL-37 and/or tetracycline was performed to determine the mechanism(s) of N. gonorrhoeae susceptibility to antibiotics and peptides.

RESULTS

Sub-inhibitory concentrations of LL-37 and PBT2/zinc synergized to render strain WHO-Z susceptible to tetracycline, whereas the killing effect of PG-1 and PBT2/zinc was additive. SWATH-MS proteomic analysis suggested that PBT2/zinc most likely leads to a loss of membrane integrity and increased protein misfolding and, in turn, results in bacterial death.

CONCLUSIONS

Here we show that PBT2, a candidate Alzheimer's and Huntington's disease drug, can be repurposed to render MDR N. gonorrhoeae more susceptible to the endogenous antimicrobial peptides LL-37 and PG-1. In the presence of LL-37, PBT2/zinc can synergize with tetracycline to restore tetracycline susceptibility to gonococci resistant to this antibiotic.

Repurposing a neurodegenerative disease drug to treat Gram-negative antibiotic-resistant bacterial sepsis

The emergence of polymyxin resistance in carbapenem-resistant and extended-spectrum β-lactamase (ESBL)-producing bacteria is a critical threat to human health, and alternative treatment strategies are urgently required. We investigated the ability of the hydroxyquinoline analog ionophore PBT2 to restore antibiotic sensitivity in polymyxin-resistant, ESBL-producing, carbapenem-resistant Gram-negative human pathogens. PBT2 resensitized Klebsiella pneumoniae, Escherichia coli, Acinetobacter baumannii, and Pseudomonas aeruginosa to last-resort polymyxin class antibiotics, including the less toxic next-generation polymyxin derivative FADDI-287, in vitro. We were unable to select for mutants resistant to PBT2 + FADDI-287 in polymyxin-resistant E. coli containing a plasmid-borne mcr-1 gene or K. pneumoniae carrying a chromosomal mgrB mutation. Using a highly invasive K. pneumoniae strain engineered for polymyxin resistance through mgrB mutation, we successfully demonstrated the efficacy of PBT2 + polymyxin (colistin or FADDI-287) for the treatment of Gram-negative sepsis in immunocompetent mice. In comparison to polymyxin alone, the combination of PBT2 + polymyxin improved survival and reduced bacterial dissemination to the lungs and spleen of infected mice. These data present a treatment modality to break antibiotic resistance in high-priority polymyxin-resistant Gram-negative pathogens.

Abstract 6558: Thymoquinone and epicatechin ameliorate the anticancer properties of tafuramycin-A against naïve and resistant breast cancer cells

Tafuramycin-A (TAF) is naturally occurring duocarmycin-SA derivative with known DNA alkylating and/or intercalating potential. On the other hand, TAF possesses excessive and non-specific toxic properties. Epicatechin (EPI) and thymoquinone (TQ) are naturally occurring compounds with a wide range of biological activities, such as anticancer and chemomodulatory potentials. Herein, we temporally assessed the anti-breast cancer properties of TAF alone and in combination with EPI or TQ against naïve (MCF-7, MDA-MB-231 and T47D cells) and resistant breast cancer cells (MCF-7Adr). TAF alone showed very potent cell-killing properties against both naïve and resistant breast cancer cell lines in a time-dependent manner with IC50's ranging from 17 - 190 nM, 2 - 19 nM and 1 - 2 nM after 24 h, 48 h, and 72 h exposures, respectively. To a lesser extent, TQ alone showed moderate cytotoxic properties against all cell lines in a time-dependent manner with IC50's ranging from 4.4 - 18.9 µM, 2.8 - 16.5 µM and 2.1 - 22.7 µM after 24 h, 48 h, and 72 h exposures, respectively. EPI was the weakest in comparison to the previous two agents with IC50's above 100 µM in all cell lines in all durations of exposure. Except 24 h exposure of MDA-MB-231 cells, equitoxic combinations of TAF with TQ showed antagonistic interaction in all cells under investigation with combination indices ranging from 2 - 9.3, 1.3 - 3.7, 1.4 - 3.2 and 0.6 - 3.7 in MCF-7, MCF-7Adr, T47D, and MDA-MB-231 cells, respectively. Combination of TAF with 10 µM EPI did not induce any prominent enhancement in TAF cytotoxic properties. Cell cycle analysis using DNA content flow cytometry showed moderate S-phase and G2/M-phase partial arrest in response to treatment with TAF, TQ and their combinations. While treatment with EPI induced significant arrest in G0/G1-phase which is similar to its reported antiproliferative activity. Further analysis for the differential apoptosis/necrosis cell death using annexin-V/FITC with PI counterstain and coupled with flow cytometric analysis showed significant necrosis induction of TAF alone against breast cancer cells under investigation. Yet, a combination of TAF with TQ or EPI decreased the percentage of necrosis induced by TAF alone; however, it induced significant apoptosis cell death. Yet, the explanation for shifting breast cancer cell death from necrosis to apoptosis due to a combination of TAF with TQ or EPI is currently under molecular investigation and might constitute a high potential in utilizing TAF for the treatment of breast cancer.

Citation Format: Ohoud Y. Alshehri, Hanan A. Henidi, Fahad A. Alabbasi, Ibrahim M. El-Deeb, Majed A. Halwani, Ahmed M. Al-Abd. Thymoquinone and epicatechin ameliorate the anticancer properties of tafuramycin-A against naïve and resistant breast cancer cells [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 6558.

Multiple Bactericidal Mechanisms of the Zinc Ionophore PBT2

Globally, more antimicrobials are used in food-producing animals than in humans, and the extensive use of medically important human antimicrobials poses a significant public health threat in the face of rising antimicrobial resistance (AMR). The development of novel ionophores, a class of antimicrobials used exclusively in animals, holds promise as a strategy to replace or reduce essential human antimicrobials in veterinary practice. PBT2 is a zinc ionophore with recently demonstrated antibacterial activity against several Gram-positive pathogens, although the underlying mechanism of action is unknown. Here, we investigated the bactericidal mechanism of PBT2 in the bovine mastitis-causing pathogen, Streptococcus uberis In this work, we show that PBT2 functions as a Zn2+/H+ ionophore, exchanging extracellular zinc for intracellular protons in an electroneutral process that leads to cellular zinc accumulation. Zinc accumulation occurs concomitantly with manganese depletion and the production of reactive oxygen species (ROS). PBT2 inhibits the activity of the manganese-dependent superoxide dismutase, SodA, thereby impairing oxidative stress protection. We propose that PBT2-mediated intracellular zinc toxicity in S. uberis leads to lethality through multiple bactericidal mechanisms: the production of toxic ROS and the impairment of manganese-dependent antioxidant functions. Collectively, these data show that PBT2 represents a new class of antibacterial ionophores capable of targeting bacterial metal ion homeostasis and cellular redox balance. We propose that this novel and multitarget mechanism of PBT2 makes the development of cross-resistance to medically important antimicrobials unlikely.IMPORTANCE More antimicrobials are used in food-producing animals than in humans, and the extensive use of medically important human antimicrobials poses a significant public health threat in the face of rising antimicrobial resistance. Therefore, the elimination of antimicrobial crossover between human and veterinary medicine is of great interest. Unfortunately, the development of new antimicrobials is an expensive high-risk process fraught with difficulties. The repurposing of chemical agents provides a solution to this problem, and while many have not been originally developed as antimicrobials, they have been proven safe in clinical trials. PBT2, a zinc ionophore, is an experimental therapeutic that met safety criteria but failed efficacy checkpoints against both Alzheimer's and Huntington's diseases. It was recently found that PBT2 possessed potent antimicrobial activity, although the mechanism of bacterial cell death is unresolved. In this body of work, we show that PBT2 has multiple mechanisms of antimicrobial action, making the development of PBT2 resistance unlikely.

Neisseria gonorrhoeae Becomes Susceptible to Polymyxin B and Colistin in the Presence of PBT2

Neisseria gonorrhoeae (N. gonorrhoeae) causes the sexually transmitted disease gonorrhea, which has a global incidence of 106 million cases per year. No vaccine is available to prevent the disease, and the emergence of multidrug resistant (MDR) strains makes N. gonorrhoeae an immediate public health threat. Here, we show that an ionophore, PBT2, can reverse the intrinsic resistance of N. gonorrhoeae to polymyxin B and colistin. These antibiotics administered in combination with PBT2 may be an effective path to treat MDR gonococcal infections.

Design and synthesis of novel phenylaminopyrimidines with antiproliferative activity against colorectal cancer

New phenylaminopyrimidine (PAP) derivatives have been designed and synthesised as potential tyrosine kinase inhibitors for the treatment of cancer. The synthesized compounds share a general structure and vary in the substitution pattern at position-2 of the pyridine ring. Several derivatives have demonstrated potent anticancer activities against HCT-116, HT-29 and LS-174T colorectal cancer cells. Furthermore, a number of hits showed good selectivity to Src-kinase. The cytotoxic mechanisms of these compounds were also investigated by studying their effects on cell-cycle distribution. Among all the compounds examined, compound 8b (with a terminal pyridin-3-yl moiety at the pyridine ring) showed the highest inhibitory selectivity towards src-kinase, which was coupled with cell cycle arrest, and apoptotic and autophagic interference, in colorectal cancer cells. This report introduces a novel category of PAP derivatives with promising kinase inhibitory and anticancer effects against colon cancer.

Chemical Synergy between Ionophore PBT2 and Zinc Reverses Antibiotic Resistance

The World Health Organization reports that antibiotic-resistant pathogens represent an imminent global health disaster for the 21st century. Gram-positive superbugs threaten to breach last-line antibiotic treatment, and the pharmaceutical industry antibiotic development pipeline is waning. Here we report the synergy between ionophore-induced physiological stress in Gram-positive bacteria and antibiotic treatment. PBT2 is a safe-for-human-use zinc ionophore that has progressed to phase 2 clinical trials for Alzheimer's and Huntington's disease treatment. In combination with zinc, PBT2 exhibits antibacterial activity and disrupts cellular homeostasis in erythromycin-resistant group A Streptococcus (GAS), methicillin-resistant Staphylococcus aureus (MRSA), and vancomycin-resistant Enterococcus (VRE). We were unable to select for mutants resistant to PBT2-zinc treatment. While ineffective alone against resistant bacteria, several clinically relevant antibiotics act synergistically with PBT2-zinc to enhance killing of these Gram-positive pathogens. These data represent a new paradigm whereby disruption of bacterial metal homeostasis reverses antibiotic-resistant phenotypes in a number of priority human bacterial pathogens.IMPORTANCE The rise of bacterial antibiotic resistance coupled with a reduction in new antibiotic development has placed significant burdens on global health care. Resistant bacterial pathogens such as methicillin-resistant Staphylococcus aureus and vancomycin-resistant Enterococcus are leading causes of community- and hospital-acquired infection and present a significant clinical challenge. These pathogens have acquired resistance to broad classes of antimicrobials. Furthermore, Streptococcus pyogenes, a significant disease agent among Indigenous Australians, has now acquired resistance to several antibiotic classes. With a rise in antibiotic resistance and reduction in new antibiotic discovery, it is imperative to investigate alternative therapeutic regimens that complement the use of current antibiotic treatment strategies. As stated by the WHO Director-General, "On current trends, common diseases may become untreatable. Doctors facing patients will have to say, Sorry, there is nothing I can do for you."

Curcumin⁻Copper Complex Nanoparticles for the Management of Triple-Negative Breast Cancer

Breast cancer is the most common cancer diagnosed among females worldwide. Although breast cancer survival has largely improved in the past 30 years, it remains highly heterogeneous in its response to treatment. Triple-negative breast cancer (TNBC) is a subtype of breast cancer that lacks the expression of the estrogen receptor (ER), progesterone receptor (PR) and epidermal growth factor receptor-2 (Her2). While TNBC may initially be responsive to chemotherapy, recurrence and subsequent high mortality rates are frequently reported. Studies have shown curcumin and its derivatives to be effective against TNBC cell lines in vitro. To improve its anti-cancer effects, we have synthesized Fe³⁺⁻curcumin (Fe⁻Cur₃) and Cu²⁺⁻curcumin (CD) complexes and investigated them experimentally. Further, CD was encapsulated into a poly(styrene)-co-maleic acid (SMA) micelle to enhance its stability. We assessed the cytotoxicity of these formulations both in vitro and in vivo. SMA⁻CD demonstrated dose-dependent cytotoxicity and abolished TNBC tumor growth in vivo. The encapsulation of the curcumin⁻copper complex improved its anti-cancer activity without overt adverse effects in a murine model of TNBC. These results provide evidence and insights into the value of nanoformulations in enhancing drug-delivery and increasing the potential therapeutic efficacy of curcumin derivatives.

The impact of the butterfly effect on human parainfluenza virus haemagglutinin-neuraminidase inhibitor design

Human parainfluenza viruses represent a leading cause of lower respiratory tract disease in children, with currently no available approved drug or vaccine. The viral surface glycoprotein haemagglutinin-neuraminidase (HN) represents an ideal antiviral target. Herein, we describe the first structure-based study on the rearrangement of key active site amino acid residues by an induced opening of the 216-loop, through the accommodation of appropriately functionalised neuraminic acid-based inhibitors. We discovered that the rearrangement is influenced by the degree of loop opening and is controlled by the neuraminic acid’s C-4 substituent’s size (large or small). In this study, we found that these rearrangements induce a butterfly effect of paramount importance in HN inhibitor design and define criteria for the ideal substituent size in two different categories of HN inhibitors and provide novel structural insight into the druggable viral HN protein.

Exploring inhibitor structural features required to engage the 216-loop of human parainfluenza virus type-3 hemagglutinin-neuraminidase

Human parainfluenza virus type-3 is a leading cause of acute respiratory infection in infants and children. There is currently neither vaccine nor clinically effective treatment for parainfluenza virus infection. Hemagglutinin-neuraminidase glycoprotein is a key protein in viral infection, and its inhibition has been a target for inhibitor development. In this study, we explore the structural features required for Neu2en derivatives to efficiently lock-open the 216-loop of the human parainfluenza virus type-3 hemagglutinin-neuraminidase protein.

A versatile synthesis of "tafuramycin A": a potent anticancer and parasite attenuating agent

An improved and versatile synthesis of tafuramycin A, a potent anticancer and parasite-attenuating agent, is reported. The three major improvements that optimized yield, simplified purification and allowed the synthesis of more versatile duocarmycin analogues are: a first-time reported regioselective bromination using DMAP as catalyst; the control of the aryl radical alkene cyclization step to prevent the dechlorination side reaction; and the design of a new protection/deprotection method to avoid furan double bond reduction during the classical O-benzyl deprotection in the final step. This alternative protection/deprotection strategy provides ready access to duocarmycin seco-analogues that carry labile functionalities under reducing reaction conditions. Tafuramycin A (3) was prepared in either 8 steps from intermediate 6 or 7 steps from intermediate 17 in 52% or 37% yield respectively. Our strategy provides a significant improvement on the original procedure (11% overall yield) and greater versatility for analogue development.

Exploring human parainfluenza virus type-1 hemagglutinin-neuraminidase as a target for inhibitor discovery

Human parainfluenza virus type 1 is the major cause of croup in infants and young children. There is currently neither vaccine nor clinically effective treatment for parainfluenza virus infection. Hemagglutinin-neuraminidase glycoprotein is a key protein in viral infection, and its inhibition has been a target for 2-deoxy-2,3-didehydro-d-N-acetylneuraminic acid (Neu5Ac2en)-based inhibitor development. In this study, we explore the effect of C-5 modifications on the potency of Neu5Ac2en derivatives that target the human parainfluenza type-1 hemagglutinin-neuraminidase protein. Our study demonstrates that the replacement of the Neu5Ac2en C-5 acetamido moiety with more hydrophobic alkane-based moieties improves the inhibitory potency for both hemagglutinin-neuraminidase functions. These findings shed light on the importance of C-5 substitution on Neu5Ac2en in the design of novel sialic acid-based inhibitors that target human parainfluenza type-1 hemagglutinin-neuraminidase.

Cross-species malaria immunity induced by chemically attenuated parasites

Vaccine development for the blood stages of malaria has focused on the induction of antibodies to parasite surface antigens, most of which are highly polymorphic. An alternate strategy has evolved from observations that low-density infections can induce antibody-independent immunity to different strains. To test this strategy, we treated parasitized red blood cells from the rodent parasite Plasmodium chabaudi with seco-cyclopropyl pyrrolo indole analogs. These drugs irreversibly alkylate parasite DNA, blocking their ability to replicate. After administration in mice, DNA from the vaccine could be detected in the blood for over 110 days and a single vaccination induced profound immunity to different malaria parasite species. Immunity was mediated by CD4+ T cells and was dependent on the red blood cell membrane remaining intact. The human parasite, Plasmodium falciparum, could also be attenuated by treatment with seco-cyclopropyl pyrrolo indole analogs. These data demonstrate that vaccination with chemically attenuated parasites induces protective immunity and provide a compelling rationale for testing a blood-stage parasite-based vaccine targeting human Plasmodium species.

Design, synthesis, screening, and molecular modeling study of a new series of ROS1 receptor tyrosine kinase inhibitors

A series of rationally designed ROS1 tyrosine kinase inhibitors was synthesized and screened. Compound 12b has showed good potency with IC50 value of 209 nM, which is comparable with that of the reference lead compound 1. Molecular modeling studies have been performed, that is, a homology model for ROS1 was built, and the screened inhibitors were docked into its major identified binding site. The docked poses along with the activity data have revealed a group of the essential features for activity. Overall, simplification of the lead compound 1 into compound 12b has maintained the activity, while facilitated the synthetic advantages. A molecular interaction model for ROS1 kinase and inhibitors has been proposed.