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Importance of theranostics in rare brain-eating amoebae infections Ayaz Anwar, Ruqaiyyah Siddiqui, and Naveed Ahmed Khan ACS Chem. Neurosci., Just Accepted Manuscript • DOI: 10.1021/acschemneuro.8b00321 • Publication Date (Web): 27 Aug 2018 Downloaded from http://pubs.acs.org on August 29, 2018
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ACS Chemical Neuroscience
Importance of theranostics in rare brain-eating amoebae infections
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Ayaz Anwar, Ruqaiyyah Siddiqui, Naveed Ahmed Khan*
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Department of Biological Sciences, School of Science and Technology, Sunway University,
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Malaysia.
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Short title: Theranostics against brain-eating amoebae
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*Corresponding address: Department of Biological Sciences, School of Science and Technology,
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Sunway University, Selangor, 47500, Malaysia. Tel: 60-(0)3-7491-8622. Ext: 7169. Fax: 60-
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(0)3-5635-8630. E-mail:
[email protected] 14
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Abstract Pathogenic free-living amoebae including Acanthamoeba spp., Balamuthia mandrillaris,
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and Naegleria fowleri cause infections to the central nervous system (CNS) which almost always
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prove fatal. The mortality rate is high with the CNS infections caused by these microbes despite
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modern developments in healthcare and antimicrobial chemotherapy. The low awareness,
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delayed diagnosis and lack of effective drugs are major hurdles to overcome these challenges.
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Nanomaterials have emerged as vital tools for concurrent diagnosis and therapy which are
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commonly referred to as theranostics. Nanomaterials offer highly sensitive diagnostic systems
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and viable therapeutic effects as a single modality. There has been good progress to develop
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nanomaterials based efficient theranostic systems against numerous kinds of tumors but this field
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is yet immature in the context of infectious diseases, particularly parasitic infections. Herein, we
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describe the potential value of theranostic applications of nanomaterials against brain infections
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due to pathogenic amoebae.
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Keywords: Theranostics; Nanomaterials; Infectious diseases; Parasites; Brain-eating amoeba.
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Theranostics’ are defined as a strategy consisting of therapeutic and diagnostic methods
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combined in one platform. Theranostic tools have acquired significant attention in the field of
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nanomedicine due to associated advantages such as the development of patient-specific
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personalized medicine producing improved outcomes at reduced costs, and fewer side effects.
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The basis of theranostic nanomedicine is nanotechnology that impacts medical research in both
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diagnosis and treatment. 1 It is anticipated that theranostic tools are on the verge of overcoming
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conventional diagnostic and therapeutic limitations. 2 There has been massive advancement in
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the progress of theranostic agents in oncology and neurodegenerative diseases, however there are
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limited reports against infectious diseases and even fewer in parasitology e.g., against
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leishmaniasis 3 and Malaria. 4, 5 Here, we propose that theranostic knowledge obtained from
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brain disorders can be extracted and applied against CNS infections due to pathogenic amoebae.
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The proposed theranostic strategies should be of potential value in reducing the disease burden
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associated with brain-eating amoebae by timely and sensitive diagnosis augmented with effective
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therapeutic capabilities (Fig. 1).
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Limitations in current clinical diagnostic tools
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Encephalitis due to pathogenic free-living amoebae either primary amoebic
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meningoencephalitis (PAM) or granulomatous amoebic encephalitis (GAE) are rare but deadly
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brain infections. 6, 7 These diseases are difficult to diagnose due to unawareness, limitations of
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diagnostic tools, and are challenging to treat due to the lack of effective drugs. Current diagnosis
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of brain-eating amoebae is based on the following: Real time PCR; Culture on monkey kidney;
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Agar plates culturing, however all of these techniques require a post-surgical sample from brain
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which is both painful and time taking. Notwithstanding the high fatality, pharmaceutical and
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healthcare industries have been failed to produce any drug against infections due to brain-eating 3 ACS Paragon Plus Environment
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amoebae. 8 One of the main challenges in the successful therapy of amoebal brain infection is the
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inefficacy of available antiamoebic drugs to cross the blood-brain barrier to reach the infection
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site and effectively target the residing parasite. The rationale behind nanotheranostics is to
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develop potential nanomaterials which can be used as biosensors for molecular image tracking in
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biological systems that can be further combined with controlled and site-specific drug release at
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the target tissues by locally inducing parameters such as pH, temperature, and enzymes to allow
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efficient killing of cells. There are various nanotheranostic agents based on their composition and
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mode of action. A few theranostic nanocarriers in the context of infectious and non-infectious
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diseases of brain are given below and summarized in Fig. 1.
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Metallic nanoparticles
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Nanoparticles lie in the range of 1-100 nm and consist of metals, semiconductors,
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polymeric, carbon-based, lipid-based. 9 Metallic nanoparticles are the most commonly used for
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widespread applications in medical and healthcare sciences. 10 The most common metallic
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nanoparticles are based on gold, silver and iron oxide. There are more than 35 FDA-approved
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nanoparticles currently used for imaging and therapy. 11-13 Most of these nanoparticles are used
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as drug delivery systems, while a few examples of their uses as MRI contrast agents are also
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reported.
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Gold nanoparticles (GNPs) are most studied nanomaterials in biomedicine due to their
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biocompatibility and tunable properties. GNPs are capable of conjugating and delivering
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biomolecules to specific targets whilst providing a theranostic approach simultaneously. 14 GNPs
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hybridized with other metals have also been utilized in theranostic-based nanomedicine. 15 The
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inert vehicles activated with surface functionalities provide them extensive applications in cancer
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and infectious disease. 16, 17 GNP-based molecular amplification and microassays have been 4 ACS Paragon Plus Environment
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developed to augment accurate and sensitive diagnosis of infectious diseases. 18 GNPs have high
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coefficient of X-ray absorption which is an essential requirement for a material to be used for
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computed tomography (CT) contrast agents, and radiotherapy sensitizers. 19 Various lateral flow
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assays (LFAs) have been developed based on the antibody-conjugated GNPs as detection probes.
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20
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genome sequence, GNPs probes functionalized with oligonucleotide have been used with high
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efficiency and sensitivity. 21
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For the identification of pathogens containing unique nucleic acid markers as part of their
However, in the context of brain eating amoebae, not a single report can be found for
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theranostic applications of any of the aforementioned material. Promising theranostic application
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of multifunctional nanoparticles has been proposed for chronic neurodegenerative disorders such
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as Alzheimer’s, Parkinson’s disease and strokes. 22, 23 Liu et al. 24 reported nanoparticles-based
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iron chelation therapy against Alzheimer’s disease. The nanoparticles were found to be capable
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of crossing the blood-brain barrier. Cheng et al. 25 reported GNPs modified with transactivator of
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transcription (TAT) peptide, which are also demonstrated to be crossing the blood-brain barrier
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and efficiently delivering anticancer drug doxorubicin and contrast agent Gd3+ for enhanced
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malignant glioma. As these probes have been utilized in CNS disorders previously and have
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overcome the blood-brain barrier, there is a strong rationale to use these in the treatment of
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infections due to brain-eating amoebae. Research carried out by our group reports the potential
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of nanoparticles against infectious diseases against MDR bacteria and brain eating amoebae. For
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example, Chlorhexidine coated gold nanoparticles were shown to inhibit Acanthamoeba
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castellanii at much lower concentration as compared to the drug alone, moreover host cell
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cytotoxicity was significantly reduced. 26 These nanoparticles also inhibit Klebsiella pneumoniae
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biofilms as well as disrupted pre-formed biofilms. 27 More recently, the size of gold
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nanoparticles has shown antibiofilm potential due to Gram positive bacteria. 28 In another report,
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synthetic ligand dimethylamino pyridine (DMAP) propylthioacetate conjugated gold
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nanoparticles were shown to enhance the antibacterial activity of Pefloxacin against E. coli. 29
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Silver nanoparticles decorated with drugs such as Amphotericin B, Nystatin, and Fluconazole
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have shown to exhibit antiamoebic efficacy against brain-eating amoebae, A. castellanii and N.
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fowleri. 30, 31
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These reports show that metallic nanoparticles show potential as therapy for infections
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caused by brain-eating amoebae. We propose that due to the tunable physicochemical properties
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of metallic nanoparticles these should be conjugated with both targeting/imaging materials with
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effective drugs to produce theranostic effects. For example, anti-amoebal antibodies, florescence
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dyes such as Fluorescein isothiocyanate (FITC) or mannose to target mannose-binding protein
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adhesion of amoebae along with effective drugs such as Chlorhexidine, Pentamidine,
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Amphotericin B, azoles and other antiamoebic drugs.
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Magnetic nanoparticles
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Magnetic nanoparticles (MNPs) have a metallic core whose outer shell is generally
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conjugated with chemical functionalities. 32 MNPs are smaller than 100 nm, those can be
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composed of any material having some characteristics of magnetism, but mostly the metallic
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core is made up of iron oxide compounds. 33 There is a growing interest in the applications of
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MNPs for a wide variety of biomedical applications including the theranostics of cancer and
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infectious diseases. 34 Superparamagnetic iron oxide (SPIO) nanoparticles have been
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successfully utilized for staging and monitoring of treatment in a variety of cancers with MRI.
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MNPs have been widely studied against neurodegenerative diseases such as Alzheimer due to
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their tremendous potential to act as contrast agents in MRI. 35 6 ACS Paragon Plus Environment
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MNPs are known to produce magnetic hysteresis as a response to alternating current
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(AC) magnetic fields which results in hyperthermia (a localized thermal shock) ultimately
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leading to cell death. 36 Dai et al. 37 reported a multifunctional platform based on magnetic core
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plasmonic shell nanoparticles coated with methylene blue aptamer and a specific antibody for
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Salmonella DT104. This nanotheranostic system is shown to be useful for in vivo imaging of
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Salmonella DT104 followed by selective separation and photo-destruction of these MDR
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bacteria. 37 Park et al. 38 showed that MNPs could be used to eradicate Pseudomonas aeruginosa
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PA01 biofilms. More recently, Singh et al. 39 showed that up to 99 % reduction in the viability of
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Escherichia coli was observed as a result of magnetic hyperthermia using iron-based MNPs. In
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another report, Thomas et al. 40 showed iron-based MNPs stabilized by tiopronin, could diminish
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the 107 colonies of Staphylococcus aureus within 4 minutes of in vitro magnetic hyperthermia
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treatment. Magnetite-zinc oxide MNPs are also shown to exhibit an intrinsic bactericidal effect.
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pathogens via hyperthermia. However, the invasive nature of hyperthmal treatment is in some
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cases can also be non-selective to host cells therefore, careful design in development of these
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materials is of prime importance. Furthermore, proteins or nucleic acid conjugated MNPs can be
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used as specific probes for separation of pathogens to give theranostic benefits. Based on the
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above, it is evident that magnetic nanoparticles have been widely studied against CNS disorders
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and pathogenic bacteria and could be of value against brain-eating amoebae. We propose that
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combining the approaches of CNS targeting magnetic theranostics with effective antiparasitic
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agents would be an imperative step in the development of nanotheranostics against brain-eating
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amoebae infections.
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Quantum dots
Hence, it is anticipated that MNPs have potential in identifying, isolating and killing infectious
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Quantum dots (QDs) are semi-conducting materials with high luminescent properties of
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crystallinity. 42 They are made up from the II, III, IV, V, and VI group elements from the
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periodic table. Their quantum yield for emission depends on the electronic environment within
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the crystal, as well as on their size and shape. The semi-conducting inorganic core dictates the
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optical properties of the crystals, whereas the outer shell is commonly functionalized by a layer
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of organic molecules. 43
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Their high quantum yield and long luminescence lifetime are fundamental properties
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which have revolutionized bioanalytical measurements. QDs have been studied extensively for
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bio-imaging purposes since their discovery in the early 1990s. 44 QDs have also been utilized
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against CNS diseases due to their potential to cross the blood-brain barrier for example in
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encephalopathy associated with HIV. 45 Various QDs based bacterial detection systems,
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bioimaging platforms and cidal agents via photodynamic therapy have been reported. For
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example, Thakur et al. 46 developed ciprofloxacin conjugated carbon dots stabilized by gum
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arabica as a tool for the bioimaging of bacteria with high antimicrobial activity against a variety
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of Gram-positive and Gram-negative bacteria. Cheng et al. 47 reported antibodies conjugated
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QDs as fluorescence markers for simultaneous detection of three bacteria by using laboratory-
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based system. In another report, photoexcited QDs were used to kill a wide range of MDR
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bacteria. 48 The killing effect is shown to be controlled by the redox potential of photogenerated
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charge carriers that alters the redox states at cellular levels which ultimately leads to bacterial
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killing. 48
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Despite the toxic nature of metals generally used in the synthesis of QDs, the surface
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modifications and smart administration of QDs have paved their way into cancer theranostics. 49
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QD–aptamer–doxorubicin conjugate, have been used for simultaneous cancer imaging and 8 ACS Paragon Plus Environment
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traceable drug delivery. 50 In a recent report, QD-based micelles coated with anti-epidermal
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growth factor receptor (EGFR) nanobody, further loaded with aminoflavone for drug delivery,
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has been developed for triple negative breast cancer theranostics. 51 Ho et al. 52 highlighted
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aspects of imaging correlated with theranostics application of QDs. The major limitation in
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advancement of QDs to clinical applications is the use of toxic elements such as cadmium,
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arsenide or lead in their core. Therefore, not only the transmigration of therapeutic agents, but
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their safer administration strategies are required to develop effective theranostic tools. 53 The
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application of quantum dots in fluorescence mapping of sentinel lymph node could be promising
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due to less challenging local administration of QDs. 54 The ability of sensitive imaging of QDs
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due to high fluorescence is a tremendous opportunity to detect brain-eating amoebae at an early
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stage. Moreover, their applications against neurodegenerative diseases as mentioned above
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makes them ideal target for development of nanotheranostic tools against brain-eating amoebae
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and other infectious CNS pathogens. Hence, to make it possible QDs should be conjugated with
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specific antibodies against brain-eating amoebae and/or mannose due to its affinity towards
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mannose-binding proteins expressed on pathogenic Acanthamoeba, which further augmented
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with effective drugs for examples biguanides, amidines, and azoles and/or a mixture of these
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should produce excellent theranostic platforms against CNS infections caused by brain-eating
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amoebae. Furthermore, the luminescence ability of QDs may provide additional advantages for
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live imaging during diagnosis and therapy.
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Photo-triggered theranostic nanocarriers
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Theranostic probes that utilize photo-based techniques are of interest due to minimally
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invasive modalities. 55 A few examples of photo-triggered killing of MDR bacteria has been
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presented in previous sections. 48 Metal oxide nanoparticles such as TiO2, Fe2O3, ZnO etc. are
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frequently used against MDR microbes due to their photoactive nature and sensitivity to generate
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reactive oxygen species (ROS) while irradiated with UV light. Hence these nanoparticles can
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ideally be used as antimicrobial agents via photochemotherapy. Imran et al. 56 used Zinc doped
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TiO2 nanoparticles against Acanthamoeba via photodynamic therapy. Photo-triggered theranostic
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strategy include the advantage to customize the localized treatment. However, the limitations
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include high intensity of thermal dose in case of deeper tissues and prior information of the exact
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location of disease is needed to initiate treatment. 10
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Halas and West have pioneered photodynamic nanotheranostics against cancer. 57-60
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These systems work on the principle of photosensitization. For example, Khlebtsovet al. 61
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synthesized nanocage core of gold-silver alloy and shell made up of silica containing Yb–2,4-
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dimethoxyhematoporphyrin as near infrared (NIR) photosensitizer and used this multifunctional
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probe as tumor monitoring as well as simultaneous photodynamic and plasmonic heating
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therapy. In another report, NIR fluorescent heptamethine cyanine dye and Si-naphthalocyanine
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heating dyes containing non-plasmonic silica NPs have been shown for NIR fluorescence
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imaging and increased light absorption. 62 By direct tumor injection into mice, high fluorescence
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intensity and about 95% decrease in the viability of tumor was observed after irradiation with
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NIR laser. 62 Another novel nanoformulation for dual-imaging was prepared by Huang et al. they
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synthesized silica-modified gold nanorods conjugated with folic acid which can be
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simultaneously imaged in vivo using X ray or computed tomography (CT). 63 Photo-responsive
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materials such as Porphyrins have shown usefulness against infections caused by brain-eating
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amoebae. 64 Hence, Porphyrin and/or other photosensitizers based nanomaterials which can be
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further conjugated with specific antibodies, targeting agents and antiparasitic drugs can be of
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high value as theranostic tools against brain-eating amoebae. 10 ACS Paragon Plus Environment
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Liposomes Liposomes are clinically established nanomaterials that are used to deliver drugs, genes,
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vaccines, imaging agents, etc. 65 There are numerous examples of liposome-nanoparticles
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systems with varied characteristics that incorporate therapeutics and imaging capabilities. 66
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Similarly, liposome-QDs hybrid systems have also shown great potential in theranostics. 67
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Recently, Seleci et al. 68 reported liposome-QDs nanotheranostic agent to deliver topotecan.
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Some of parasitic diseases such as Malaria, Leishmania, Trypanosomiasis, Amoebiasis,
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Acanthamoeba keratitis etc. have also been targeted by liposomes. Serrano-Luna et al. 69 studied
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the virulence of Entamoeba histolytica by adding phosphatidylcholine-cholesterol liposomes in
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their culture medium and assessed various biochemical and biological functions of the parasite.
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The addition of liposomes in the culture medium is shown to produce no change in the virulence
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of these parasites against hamster liver as the original infectious E. histolytica strain, even
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without passage through hamster liver for more than one year. 69 Liposomal complexations of
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pentamidine with ergosterol produced enhancement in its antiamoebic potency against keratitis
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causing A. castellani. 70 More recently, a formulation of nanoemulsion from the plant extract of
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Pterocaulon balansae rich in coumarin was used for in vitro treatment of ocular Acanthamoeba
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keratitis. 71 In another report, therapeutic siRNA sequence loaded liposomes were shown to
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inhibit the encystment of Acanthamoeba by targeting glycogen phosphorylase. 72 Linolenic acid
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loaded liposomes with phosphatidylcholine and cholesterol have proved to be a potent
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antibacterial formulation against MDR Helicobacter pylori infection. 73 Muppidi et al. 74
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investigated the pharmacokinetics and biodistribution of vancomycin loaded liposomes within a
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mouse model of MRSA infection. Gao et al. 75 described the usefulness of nanoparticle-
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stabilized liposomes with hydrogel technology which was shown to be more effective and
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sustainable topical drug delivery system. It was demonstrated that this hydrogel formulation 11 ACS Paragon Plus Environment
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effectively releases nanoparticle-stabilized liposomes to S. aureus culture, those then pH
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dependently fuse with bacterial membrane. Interestingly, the topical administration of these
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hydrogel formulations did not cause skin toxicity even after prolonged application. 75
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Some of the conventional drugs loaded liposomes have also been approved and
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commercialized. These include Amphotec, and Abelcet containing liposomal Amphotericin B
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formulation for the treatment of fungal infections, 76 and Ambisome contain Amphotericin B
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against leishmaniasis. Another example of liposomal marketed therapy is Inflexal V which is
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used against influenza and consists of inactivated hemagglutinin of the influenza virus. 77 Some
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oncological examples of liposomal approved drugs are as follows; Doxil, a liposomal injection of
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doxorubicin is being used to treat multiple myeloma and ovarian cancer. 78 Onivyde, a liposome
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encapsulated irinotecan formulation being used to treat metastatic pancreatic cancer. 79
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Commercially available Amphotericin B liposomal formlations may be further conjugated with
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flouresent dyes and can potentially be used as theranostic platforms against brain infections
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caused by free-living amoebae for imaging cum therapeutic applications. Although infections due to pathogenic amoebae are rare, but due to the high abundance
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and prevalence of free-living amoebae are of global concern. 80 Their diagnosis and treatment
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currently suffers from lack of advancements as compared to other infectious and CNS diseases.
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81
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infectious diseases caused by brain-eating amoebae to develop the missing link in timely and
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precise diagnosis merged with effective therapy for these devastating infections.
Here we highlighted some examples of nanotechnology driven theranostics and related them to
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Conclusion
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The theranostic approaches suggested here may play major role in reducing the
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devastating high mortality rate associated with rare infectious diseases caused by brain eating
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amoebae. There is an urgent need for development of smart materials based theranostics for
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improved laboratory and point-of-care testing. Nanomaterials have already shown promising
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theranostic applications in cancer, Alzheimers, Parkinson disease and bacteria imaging cum
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treatment. Moreover, they are anticipated to be a major breakthrough in parasitology especially
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against brain-eating amoebae. In our opinion, these suggested strategies have a strong rationale,
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and should yield a significant advancement in developing effective treatment of infections due to
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brain-eating amoebae as well as against other CNS pathogens.
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Acknowledgements: We are thankful to Sunway university for supporting this work.
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Funding: Not applicable.
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Authors’ contribution: NK proposed the concept. AA searched the literature and drafted the
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manuscript under the supervision of RS and NK. RS and NK corrected the manuscript. All
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authors read and approved the final version of the manuscript.
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Competing interests: None.
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References
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Figure legends
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Fig. 1. Schematic depiction of proposed theranostic approaches against infections caused by
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comprehensively discussed in the text. Nanomaterials due to their small size, unique
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physicochemical properties, and readiness to drug delivery applications are suitable theranostic
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platforms.
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