Progress in Remotely Triggered Hybrid Nanostructures for Next

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Review

Progress in Remotely Triggered Hybrid Nanostructures for Next Generation Brain Cancer Theranostics Nanasaheb D. Thorat, Helen Townley, Grace Brennan, Abdul Kareem Parchur, Christophe Silien, Joanna Bauer, and Syed A. M. Tofail ACS Biomater. Sci. Eng., Just Accepted Manuscript • Publication Date (Web): 14 May 2019 Downloaded from http://pubs.acs.org on May 14, 2019

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ACS Biomaterials Science & Engineering

Progress in Remotely Triggered Hybrid Nanostructures for Next Generation Brain Cancer Theranostics Nanasaheb D. Thorat1,2, Helen Townely3, Grace Brennan1, Abdul K Parchur4, Christophe Silien1, Joanna Bauer2, and Syed A.M.Tofail1

1 Modelling

Simulation and Innovative Characterisation (MOSAIC), Department of Physics and Bernal Institute, University of Limerick, , Limerick, V94 T9PX, Ireland 2 Department of Biomedical Engineering, Faculty of Fundamental Problems of Technology Wroclaw University of Science and Technology, wybrzeże Stanisława Wyspiańskiego 27, 50-370 Wrocław, Poland 3 Nuffield Department of Obstetrics and Gynaecology, Medical Science Division, John Radcliffe Hospital University of Oxford, OX3 9DU Oxford, UK 4 Department of Radiology, Medical College of Wisconsin, 9200 W Wisconsin Ave, Milwaukee, WI 53226, USA

Corresponding Author Dr. Nanasaheb Thorat Department of Physics, Bernal Institute, University of Limerick, Limerick, V94 T9PX, Ireland, Ireland Department of Biomedical Engineering, Wroclaw University of Science and Technology, wybrzeże Stanisława Wyspiańskiego 27, 50-370, Wroclaw, Poland Email: [email protected], [email protected] [email protected]

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Abstract: Progresses in nanomedicine have enabled the development of smart hybrid nanostructures (HNS) for brain cancer theranostics - a novel platform that can diagnose brain while concurrently beginning primary treatment, initiate secondary treatments where necessary and monitor the therapy response. These HNS can release guest molecules/cargoes directly to brain tumours in response to external physical stimuli. Such physical stimulation is generally referred to as remote stimuli which can be externally applied examples include alternating magnetic field, visible or near-infrared light, ultrasound radiation, X-ray, and radiofrequency. The release of therapeutic cargoes in response to physical stimuli can be performed along with photodynamic

therapy,

photothermal

therapy,

photo-triggered

chemotherapeutics,

sonodynamic therapy, electrothermal therapy, and magnetothermal therapy. Herein, we review different HNS currently used as remotely triggered modalities in brain cancer, such as organicinorganic HNS, polymer micelles, and liposomes HNS. We also summarise underlying mechanisms of remote triggering brain cancer therapeutics including single- and two-photon triggering, thermoresponsive HNS, photoresponsive HNS, magnetoresponsive HNS, and electrically, and ultrasound stimulated HNS. In addition to a brief synopsis of ongoing research progress on “smart” HNS-based platforms of novel brain cancer therapeutics, the review offers an up-to-date development in this field for neuro-oncologists, material/nanoscientists, and radiologists so that a rapid clinical impact can be achieved through a convergence of multidisciplinary expertise. Keywords: hybrid nanostructures; nanomedicine; magnetic materials; brain cancer therapy; photothermal therapy


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Review Outline: 1. Introduction 2. Current Approaches vs Hybrid Nanotheranostics for Brain Cancer 2.1 Remotely Triggered Hybrid Nanostructures 2.2 Remotely Triggered Hybrid Nanostructures: Brain Cancer Theranostics 2.2.1 Why remotely triggered therapies? 2.2.2 Remote Triggering Modalities 3. Remote Triggering Modalities in Brain Cancer Theranostics 3.1 Light Triggered Hybrid Nanostructures 3.1.1 Inorganic Hybrid Nanostructures 3.1.2 Polymers and Liposomes 3.1.3 Strengths and challenges 3.2. Two-Photon Triggered Hybrid Nanostructures 3.3 Magnetically Triggered Hybrid Nanostructures 3.3.1 Organic-Inorganic HNS 3.3.2 Strengths and challenges 3.4 Ultrasound Triggered Hybrid Nanostructures 3.4.1 Liposomes 3.4.2 Organic-Inorganic HNS 3.4.3 Strengths and challenges 3.5 Electro-Magnetically Triggered Hybrid Nanostructures 3.5.1 Organic-Inorganic HNS 3.5.2 Strengths and challenges 4. Challenges, Conclusions and Future Outlooks 4.1 Challenges 4.1.1 HNS targeted delivery 4.1.2 HNS physico-chemical properties 4.1.3 Other challenges 4.2 Conclusions 4.3 Future Outlooks



1. Introduction In 2012, there was 256,213 incidences of primary brain malignancy (3.4 people per 100,000. For central nervous system (CNS)1 malignancy, which has a higher rate of incidence (5.1 per 100,000) in developed countries than developing countries (3.0 per 100,000)2. This is also the cancer that is most commonly seen in children under the age of 14, accounting for 25% of all tumours in this age range3. Glioblastoma (GBM) is considered to be the most unexceptional aggressive form of primary brain malignancy. The median survival of GBM patients after initial diagnosis and maximal therapy is less than two years4. The reappearance of the tumour at the same location where the primary tumour was initially located, is the leading cause of death by brain cancer5. Nearly half of the patients having brain cancer will have metastasis all through the course of their condition. Metastases are particularly challenging to treat because brain cancer patients are often fragile, malnourished, and have an abundance of co-morbidities. Even with the best available treatments, median survival time in metastatic brain cancer patients reduces to less than half of typical survival time (~10 months)6. Molecular changes in cancer cells drive the expansion and progression of brain cancer, the treatment of which becomes highly difficult due to the complexity of the brain tissues, and the difficulty in passing therapeutic agents. The human brain is an outstandingly complex organ with complicated anatomical structure and physiological functions. Its functions include perception, learning, memory storage, processing information, arousal, motor control, and motivation. It is the master controller and central processing unit of the human body. The high molecular and genetics heterogeneity in solid brain tumours at all levels leads to extraordinary histopathological variability of tumour tissues. This single feature of the tumour is foremost for low predictability of the tumour response to any treatment. Additionally, brain tumours have aggressive progression and high revival and proliferation. These molecular features of brain tumour tissues, supplement extra challenges for brain tumour surgery. Surgical procedures in brain cancer therapy therefore demand an extremely careful and delicate removal of all tumorous tissues without damaging the healthy environment of nerves systems7. This is indeed very challenging to achieve in practice because of tumour heterogeneity and leaves open the possibility of re-occurrence due to fragmentary exclusion. Brain cancer diagnosis depends on the tumour extent, position and cancerous cells spreading rate. Common symptoms in patients with a primary brain cancer consist of nausea, vomiting, cognitive changes, personality changes, hemiparesis, aphasia, urinary incontinence, 4 ACS Paragon Plus Environment

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hemineglect (or spatial neglect), visual field deficiency, and apprehension. Headaches are present in about 50% of individuals at the time of diagnosis, but frequently with a nonspecific pain pattern thus making diagnosis very difficult. Among these symptoms, headaches that worsen with time or become more frequent, unilateral headaches that gives a constant throbbing sensation on one side of the head in patients older than 50 years are often taken as signs to be a tumour-associated pain rather than just a standard headache8. Brain tumour diagnosis typically starts with radiological diagnosis approach; this includes magnetic resonance imaging (MRI) and computed tomography (CT) imaging. These radio imaging techniques are currently used in oncology to confirm and locate the anatomical tumour region. MRI provides a superior temporal and spatial resolution along with anatomical details. As such, MRI has mostly replaced CT imaging in brain cancer diagnosis. Molecular imaging techniques such as positron emission tomography (PET) is also comonally used in brain cancer diagnosis. PET is primarily used to investigate tumour metabolic processes such as receptor binding, enzyme activity, oxygen metabolism, blood flow consumption, deoxyribonucleic acid (DNA) synthesis. This technique is beneficial to identify metastasis sites in the brain. However, calculating the authentic tumour dimensions and identification of precise tumour margins remains difficult even with using these state of the art diagnostic imaging techniques9. Contrast agents are used to overcome these limitations. Many magnetic and gold based hybrid nanostructures (HNS) are currently used as excellent contrast agents in MRI, CT, and PET imaging (Figure 1). There are challenges associated with treating brain cancer using systematic delivery and release of drugs to the brain tumour site due to the brain protection mechanism called the Blood Brain Barrier (BBB), which presents between endothelial cells tight junctions possessing high electrical resistance (1500-2000 Ω cm-2) 10. Efficient transport and release of therapeutics cargoes into the brain beyond the BBB requires strategies such as chemical alterations of chemotherapeutic drug and prodrugs, temporary disruptions of the tight electrical junctions using external physical forces, local delivery of therapeutics into the brain by neurosurgery, and nanoparticle-mediated drug delivery11. In recent years, remote triggering HNS is proposed to overcome the BBB for brain theranostics using, for example, light, ultrasound or magnetic triggers. Generally, these physical triggering techniques are noninvasive and allow for good selectivity and localisation of targeting, fewer side effects, and multiple operations. Such approaches provide a tremendous advantage when compared to the current approaches in opening the BBB, e.g. using hyperosmotic glucose 12.



Figure 1. Different types of hybrid nanostructures (HNS) used as remotely triggered modalities in brain cancer theranostics. Chakroun et al., provides a recent review of various modalities, advantages, and limitations of current brain cancer therapies such as surgery, radiotherapy, systemic therapy, localised gene therapy, and immunotherapy13. The use of light-triggering by near-infrared (NIR) radiation of nanostructures to deliver brain cancer therapy has been reviewed 14,15.

The approach of remote triggering is, however, more diverse and is not limited to NIR triggering only. As we discussed earlier in this Review, remote triggering modalities in brain cancer theranostics can be achieved in HNS, polymers, and liposomes by a variety of stimuli such as light, magnetic field, ultrasound, electrical and electromagnetic field. This prompts us to emphasise a systematic review of the up to date advancement in the field of remotely triggered brain cancer theranostics based on multifunctional HNS with high efficiency and safety by taking advantage of nanoscale surface functionalities and synergetic therapeutic effect. Thus, the current review outlines latest advances in detail by the ground of different physical stimulus, focusing on the inherent physical phenomenon of the material,


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multifunctionality, as well as the mechanism of brain tumour cells elimination and killing with maximum therapeutic ability. 2. Current Approaches vs Hybrid Nanotheranostics for Brain Cancer In addition to the current brain cancer treatments such as surgical excision, radiation therapy (e.g. whole brain radiation therapy; WBRT), and chemotherapy, new treatment methods have also been studied, for example, angiogenesis inhibitors therapy16, immunotherapy17, photodynamic therapy18, hyperthermia therapy19 and gene therapy20. Nanomedicine a theranostic branch of nanotechnology has advanced the state of the art brain tumour diagnosis at the same enables primary treatment procedure, initiate secondary treatments where necessary and monitor therapy response21 and is termed as ‘‘Theranostic Nanomedicine.’’ Recently, various mono-component and HNS, e.g. polymer micelles, inorganic-organic core-shell nanostructures, mesoporous silica, and liposomes have been exploited in brain cancer therapy as well as overall nanomedicine based cancer theranostics. HNS have advantages over mono-component nanostructures, as they can possess a synergistic combination of different functions. HNS can also carry diagnostic and therapeutic payloads to the tumour and release it by remote triggering. The advantage of the broad field of nanomedicine in brain cancer therapy over conventional cancer treatments has been reviewed in recent literature7. In the rest of the current review, we are covering progress in hybrid nanostructure-based brain cancer therapies and how they are controlled under external stimuli such as external magnetic fields, near-infrared (NIR) light, ultrasound for advancing brain cancer theranostics (Figure 1). 2.1 Remotely Triggered Hybrid Nanostructures Remotely triggered nanosystems are “smart” nanosystems that carry and release drugs or cargos in response to an external stimulus to induce a therapeutic effect to the area surrounding the nanosystem. These systems promise superior clinical results over commonly used techniques that rely on passive targeting or internal stimuli to release their cargo.21 Over decades, various nanocargoes have been developed either as imaging contrasts in MRI and CT, ultrasound imaging, targeted drug delivery systems or as therapeutic stimulants such as photothermal, photodynamic, magnetic hyperthermia. Nanostructures fabricated with more than one or different types of nanoassembly generally termed as “hybrid nanostructures”22,23. Organic-Inorganic HNS are promising materials demonstrating unique physiochemical and 7 ACS Paragon Plus Environment


biochemical aspects that make them superior in cancer diagnosis and therapy24,25. HNS can be designed for delivery in pristine form. Alternatively, to yield high efficiency and new functionalities, it may be integrated into other biocompatible materials. A uniquely synthesised new class of organic HNS can be designed from a variety of organic ingredients such as polymers, lipids, nucleic acids, and proteins in brain cancer therapy. Inorganic HNS perhaps, made by bringing together various features of other specific inorganic counterparts such as silica, iron oxide, carbon, graphene, gold or silver. Cellular interactions such as biocompatibility of both type of HNS can be altered by adding additional surface multifunctionality to facilitate or promote potential bio-applications25. Exciting properties demonstrated by both types of HNS have motivated scientists to integrate them and to form new classes of functional HNS for new generation brain cancer theranostics. After the activation by a remotely controlled physical stimuli, e.g. NIR light, alternating external magnetic field, electric field and ultrasonic radiation, the release of conjugated targeting molecules and chemotherapeutic drugs can be stimulated in a controlled manner into the cellular environment26,27. 2.2 Remotely Triggered Hybrid Nanostructures: Brain Cancer Theranostics 2.2.1 Why remotely triggered therapies? The brain tumour treatments are exceptionally challenging to neuro-oncologists, because of the location which in most cases is an intracranial tumor28. The blood-brain barrier (BBB) hampers the administration of drugs or clinical foreign agents, shielding intracranial tumours. The BBB is a tight junction of epithelial cells provides a biophysical and metabolic barrier to foreign agents inside the brain microenvironment29. Thus, the significant contribution to the failure of brain cancer therapy is the inadequate passage of therapeutic drugs through the BBB. Therefore, any drug delivery system targeting brain tumours primarily needs to be capable of carefully delivering an adequate supply of chemotherapeutic drugs to the brain tumour localised site, ideally with negligible or zero early release in the physiological environment of the body to minimise damage to healthy tissues. This latter point can be achieved by the incorporation of specific stimuli for activation30. The stimulus may be the in vivo tumour cell microenvironment (e.g. change of pH in a tumour cell), or a remote stimulus such as light, magnetic field, or ultrasound. The use of in vivo stimuli can result in an uncontrolled release of the chemotherapeutic payload. It is anticipated that these drawbacks could potentially be overcome by using HNS in combination with remote stimuli/triggering (Figure 2). Fortunately, these remote triggers have fewer side-effects, are widely accessible and can be 8 ACS Paragon Plus Environment

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easily manipulated for optimal controllability, making them favourable for brain cancer theranostics 31. 2.2.2 Remote Triggering Modalities Two of the most commonly used remote triggering modalities are NIR light and magnetism. The other non-invasive and controlled modality uses a magnetic field with, for example, superparamagnetic nanoparticles which without an external magnetic field have negligible magnetism but are highly sensitive to external magnetic fields if present. Thus, superparamagnetic nanoparticles can be used as drug carriers under the magnetic field and can be triggered under externally applied magnetic fields (both alternating current and direct current). High-frequency ultrasound is another effective modality for triggering thermosensitive lipids conjugated to nano-sized liposomes. Upon irradiation from highfrequency ultrasound, these liposomes change phase. The currently used physical remotely triggered modalities based on HNS in brain cancer theranostics are summarised in Figure 1 and 2.

Figure 2. (1) schematics of novel organic-inorganic HNS projected for remotely triggered cancer theranostics; (a) liposomes, (b) polymer nanogels or micelles, (c) organic-inorganic core–shell HNS, (d) surface engineered silica NPs with mesoporous structures, and (e) host– guest nanostructures. (2) passive or active delivery of HNS to the target site. (3) different remote stimulation modalities to trigger the release of chemotherapeutic cargoes; (4) tumour cell killing mechanism mainly apoptosis because of the synergistic chemo-thermal effect



under physical triggering modalities. Reproduced with permission from ref32. Copyright ©2015 The Royal Society of Chemistry.

3. Remote Triggering Modalities in Brain Cancer Theranostics 3.1 Light-Triggered Hybrid Nanostructures Light is a widely utilised external stimulus. Almost all living organisms interact with light in some way, and therefore, simulation with light is not a new phenomenon. In the human body, the pigment melanin which resides in the epidermal layer of the skin results in skin darkening and thus, inhibits the ability of the body to produce vitamin D from light from the sun 33. Plants can interact with sunlight by small pigments named chlorophyll that captures light energy and converts this energy into chemical energy, which is subsequently transferred, stored until required, used, and lastly, dissipated 34. In the field of theranostics, more applications use NIR radiation than UV/visible as NIR can penetrate to a greater depth in tissues without much scattering or autofluorescence. Many HNS interact with NIR light and can be used as agents for remote triggering to exert antitumor activity through light irradiation. The advantages of light-activated cancer theranostics have been reviewed in recent literature by Kim et al.35 3.1.1 Inorganic Hybrid Nanostructures Light interaction with inorganic noble metal-based HNS can be either radiating its energy as light through the strong surface plasmon oscillation/resonance phenomenon or by dissipating this light energy as heat to the local area. This interesting property allowing the application of these HNS in photothermal therapy (PTT), photodynamic therapy (PDT), and optical imaging of brain tumours 36. Gold based HNS with strong NIR absorption are ideally suited for PTT for tumours in the brain. Gold NPs exhibit a surface plasmon resonance (SPR) in the optical region, however with particular geometry of gold nanostructures for example nanorods, nanostars, etc. have SPR in the NIR domain, and are therefore highly responsive to NIR light37. A variety of gold HNS have been investigated as the remotely triggered PTT mediator in brain cancer theranostics38. Hybrid Gold nanoparticles conjugated with various polymers, biomolecules, silica, and other inorganic moieties are widely studied for biosensing and molecular diagnostic applications39 as well as remotely triggered brain cancer theranostics. A recent review by Hirschberg et al. summarised all hybrid gold nanostructures for brain theranostics40. Gao et al. developed hybrid gold nanoprobes that can penetrate the BBB to reach the brain tumour site41. These nanoprobes show high-performance surface‐enhanced Raman spectroscopy (SERS) and 10 ACS Paragon Plus Environment

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magnetic resonance (MR) signals, which further improve the surgical outcome in brain surgery by providing high specificity, improved safety, quantification and aids in the reduction of image distortions resulting from inevitable brain shifts during the surgery. Meyers et al., developed Au nanoparticles (NPs) loaded with PDT drug Silicon phthalocyanine (Pc 4) as a light-triggered HNC for brain tumor42. By utilising Epidermal Growth Factor (EGF) peptidemodified gold nanoparticles and the PDT drug in combination, they were able to effectively target and deliver therapy to subcutaneous brain tumor-bearing mice. Dixit et al., synthesised gold nanoparticles conjugated to a transferrin peptide (Tfpep) and Pc4 (Tfpep-Au-Pc4) and successfully using them in photo-triggered orthotopic brain tumours.43 Another very novel application uses silica-shelled gold nanorods which imitate the rabies virus (RVG-PEGAuNRs@SiO2)44. The hybrid gold nanorods (AuNRs) was functionalised with RVG29, which facilitated their transport across the BBB and targeting the brain by way of the CNS. Also, the RVG-PEG-AuNRs@SiO2 when irradiated externally using a NIR laser (808 nm), this HNS effectively eliminated brain tumour cells in clinical experiments performed on brain tumourbearing mice (Figure 3). In addition to gold, rare earth elements have been used as a promising candidate in light-triggered brain cancer theranostics such as Tb3+- Au nanostars45. Yuan et al. developed highly plasmon-active gold nanostars for a pulsed laser-modulated plasmon-enhanced therapeutic system for targeting brain tumours. This system is a truly organic-inorganic hybrid, which combines liposomes and magnetic plasmonic NPs for a trimodal image-guided drug delivery, which is successful for brain cancer theranostics 46. This system can overcome the drawbacks of a single gold-magnetic or organic hybrid system and can target payloads effectively to the brain under light and magnetic field guidance. Additionally, hybrid silicagold nanoshells are effective in treating brain tumours under laser light.47 In a recent report, Fe@Fe3O4 NPs have been modified with

125I-c(RGDyK)

peptide and PEGylated

125I-RGD-

PEG-MNPs to allow photo-triggered single-photon emission computed tomography (SPECT) and MRI for image-guided brain tumour theranostics48. This HNS (~40 nm size) exhibited good tumour targeting ability, low mononuclear phagocyte uptake and has achieved phototriggered cargo release along with the photothermal effect that can be monitored under MR imaging on the αvβ3-positive U87MG glioblastoma xenograft model. Doped LaF3 NPs in combination with a photosensitizer have also been developed for light-triggered brain cancer therapeutics49. This novel HNC combines light-triggered non-invasive photo-therapy under soft X-ray (180 kVp) and as a proof of concept activated the photosensitizer, mesotetra(4carboxyphenyl)porphyrin (MTCP). 11 ACS Paragon Plus Environment


Organic dye and/or polymer conjugated silica HNS are usually proposed to achieve the multifunctionality in brain cancer theranostics. These HNS are stable both physicochemically and biochemically, and are intrinsically immune to hydrolysis and enzymatic degradation50. High meso/nanoporosity and pore volumes allow these HNS to hold unusually large payloads of chemotherapeutic cargo to enhance drug treatment efficacy while evading side effects. Benachour et al., recently demonstrated light stimulated multimodal brain cancer therapy by using surface-localised tumour vasculature targeting NRP-1 grafted gadolinium chelate conjugated silica nanostructures51. These multifunctional HNS achieved efficient photodynamic activity on orthotopic human brain tumour (U87) along with MRI. Tang et al., proposed a new type of dynamic nanoplatform (DNP) based on methylene blue conjugated silica HNS, that only delivers 1O2 under photodynamic triggering for brain cancer therapy52. Since the discovery of helical microtubules of graphitic carbon called carbon nanotubes (CNTs) in 199153, CNTs have attracted increasing interest for their versatile applications in brain cancer theranostics54. The hollow tubular structure of CNT is advantageous to attach various biomolecules, drugs and targeting moieties. This feature of CNT is elaborated in a recent report where CNT’s were functionalised with an immune adjuvant CpG oligodeoxynucleotide (CpG), fluorescently labelled with Cy5.5. The combined nanosystem was then intratumorally injected into an intracranial GL261 glioma to induce antiglioma effect55. Eldridge et al., used a thick coating of phospholipid-poly(ethylene glycol) on multiwalled CNTs (MWCNTS) facilitating superior diffusion through brain phantoms56. These HNS were also exposed to NIR light as a novel therapy for non-resectable and drug-resistant brain tumours. The high loading ability of molecules through π–π stacking in nanoscaled graphene oxide (GO) can be advantageous for a multifunctional HNS in cancer therapy57. Porphyrin functionalized-GO has been used to facilitate photothermal ablation of glioblastoma cell line U87-MG under 808 nm irradiation and further validated by in vivo experiments where the tumour was completely eliminated58. In the recent review article from Cardano et al., photoresponsive graphene and carbon nanotubes for biological systems including brain cancer were reviewed59. Systemic, long-term, and dose-dependent toxicity of CNT’s and GO towards healthy brain cells still requires study to enable further clinical translation. The light stimulation performance of HNS into dense solid brain tumours is very weak as the penetration depth of ultraviolet or visible light is not more than 2 mm; this hinders lightHNS interactions in deep brain tumors60. Lanthanide-doped upconversion nanoparticles (UCNPs) have been employed to improve the photo-therapeutic outcome in intracranial tumours. In the pioneering work of Tsai et al., novel malignant GBM targeted light triggered 12 ACS Paragon Plus Environment

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PTT/PDT therapy is projected. Tsai’s group designed novel multifunctional UCNPs that carrying photosensitizers for combined synergetic photothermal/photodynamic treatment of GBM61. To achieve theranostic multimodality two-step process is used, in the first step UCNPs are functionalized with polymer PEG/angiopep-2, in the second step photothermal agent (IR780), and photosensitizer (5,10,15,20-tetrakis(3-hydroxyphenyl) chlorin (mTHPC)) is conjugated to enable GBM-specific co-delivery and external light-triggered targeted therapy. In another study Ni et al., developed multifunctional dual-targeting imaging nanoprobes for the penetration of the BBB and combined with the application of intracranial light, giving lightactivated glioblastoma theranostics62. PEGylated Gd-doped UCNPs were designed to achieve systematic in vitro and in vivo MR/NIR-to-NIR upconversion luminescence imaging of intracranial glioblastoma, which can potentially be further expanded into phototherapy. Very recently novel hybrid blue-emitting NaYF4:Yb/Tm@SiO2 UCNPs are developed for brain stimulation under NIR light. This novel HNS can upconvert NIR from outside the brain into the emission of blue light local to the nanoparticle, thus providing a new horizon in light triggered brain cancer therapy in the near future.



Figure 3. Schematics of recently used light-triggered HNS for brain cancer theranostics. (a) schematics of strategy used to deliver rabies virus-mimetic RVG-PEG-AuNRs@SiO2 into brain via neuronal pathway and PTT using 808 laser light. (b) synthesis mechanism of rabies virus-mimetic silica-coated gold nanorods. (c) TEM image of AuNRs (after transverse growth). Panels a-c reproduced with permission from ref 44. Copyright ©2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (d) Schematics of designing hybrid ANG-IMNPs to cross the BBB and combined PTT/PDT therapy in an orthotopic GBM tumor model. Reproduced from ref 61, under the terms of the Creative Commons Attribution (CC BY-NC) license. Copyright©2018 Ivyspring International Publisher.


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3.1.2 Polymers and Liposomes Organic polymeric nanostructures have been shown to be more biocompatible than inorganic HNS, as they do not contain heavy metals and hence attract attention for brain cancer theranostics. Recently, Jiang et al., synthesised a novel semiconducting polymer HNS for brain cancer photoacoustic (PA) imaging. This HNC absorbs light in the NIR-I and NIR-II regions, allowing comparative PA imaging between imaging at 750 nm and 1064 nm63. Multifunctional polymeric HNS have been developed, containing polylactide-co-glycolide (PLGA) NPs which can concurrently deliver the dye indocyanine green (ICG) and the chemotherapy agent docetaxel (DTX) to the brain. Through surface functionalisation with the brain-targeting peptide angiopep-2, they achieved combined chemo-phototherapy for glioma through NIR imaging 64. Along with NIR image-guided chemo-phototherapy, these HNS could cause cell death of U87MG cells in vitro, and additionally when studied in vivo, facilitated the prolongation of the lifespan of brain orthotopic U87MG glioma xenograft-bearing mice (Figure 4).

Figure 4. (a) Schematic of the hybrid ANG/PLGA/DTX/ICG NPs structure; (b) The process of irradiation treatment with ANG/PLGA/DTX/ICG, the surface temperature of U87MGglioma-bearing mice at different times (c: 0 s; d: 30 s; e: 60 s.) irradiated by 808 nm NIR laser irradiation. Reproduced with permission from ref 64. Copyright© 2015 WILEYVCH Verlag GmbH. Photofrin, a mixture of hydrophobic dimers and oligomers which are linked by ether bonds encapsulated into liposome carriers, have been tested against the U87 glioma cell line 65.

These photo-responsive hybrid liposomes effectively enhanced the sensitivity to PDT. In 15 ACS Paragon Plus Environment


another light triggered PDT study with photofrin, it was shown that there is an improved efficiency against a human glioma which had been implanted into a rat brain when compared to photofrin which was not encapsulated into a liposome 66. Recently Chen.et.al., has developed strong NIR activated H2O2-responsive hybrid liposomes loaded with HRP using the substrate 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS), (Lipo@HRP&ABTS) for tumour-targeted photothermal therapy in brain cancer theranostics 67. In the current scenario, liposomes are less studied for brain cancer theranostics, however, in the near future, it is likely that clinically light-activated brain tumour theranostics will involve using a local photosensitizer encapsulated within a liposome. 3.1.3 Strengths and Limitations For the light triggered HNS in brain cancer theranostics, the first advantage is the intensified exclusive therapy in the tumour location without extreme side effects and gentle to healthy brain and neural cells. HNS used in light-activated brain tumour therapies are typically responsive to the radiations of ultraviolet (UV), visible (Vis) and near-infrared (NIR) lights. NIR light (wavelength ranging from 700∼1000 nm), is capable of penetrating living tissue up to 10 cm in depth

68

and has no harm to normal brain tissues or neural cells and hence light

activated HNS or nanomedicine is a broad research field for brain cancer treatment. The low– energy of NIR light and high penetration depth are the majors of NIR light for its extensive exploration in brain cancer therapy. Meanwhile, this light activated remote stimulation is actively reliant on the materials properties and therapeutic molecules. The clinical neuro-oncology and initial clinical investigation determined that the therapeutic effect of single light activated monotherapy is impotent of eradicating the whole brain tumour tissues. The light-activated HNS are currently effective only on or just convenient for brain tumours diagnosed at the primary stage. Preventing cancer metastasis is currently the most challenging task in neuro-oncology. The diversity, complexity, and heterogeneity of brain tumour are limiting the application window of light triggering HNS theranostics and further self-limiting the proposed single therapy. Therefore, to improve the therapeutic outcome, the current trend in designing HNS and amplifying therapeutic potential by combining different physical modalities is necessary.


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3.2 Two-Photon Triggered Hybrid Nanostructures While there has been progress with using NIR light, the penetration depth of photons into tissue in this region or the visible region is not sufficiently deep, therefore making singlephoton HNS remote triggering less effective for brain cancer. Two-photon absorption (TPA)induced excitation of HNS (photosensitizers) is a more hopeful approach to increase the penetration of light into tissue. The development of TPA HNS can be achieved by the conjugation of dye molecules onto the nanostructure. The dye molecules concurrently absorb two photons of lower energy to an excited electronic state (Figure 5a). These excited molecules react with oxygen, then generate singlet oxygen which can be used to induce cell death. When two-photon dyes are excited in the near-infrared, light can penetrate deeper into tissue as between 800nm and 1100nm; there is minimal absorption and scattering of light. For an excellent review of the underlying physics and chemistry behind the light triggered TPA nanosystems, see Shen et al.69 De Gao et al. published the design of 5,10,15,20-tetrakis(1methyl 4-pyridino)porphyrin tetra(p-toluenesulfonate) (TMPyP), which was encapsulated in a polyacrylamide-based nanoparticle as a TPA modality for brain cancer70. Silica modified with organics (ORMOSIL) HNS containing either IR-820 NIR fluorophores or PpIX (protoporphyrin IX) photosensitizers were synthesised as a TPA mediator as well as a brain cancer mapping agent for brain cancer

71.

Alifu et al., synthesized the dye called 2-(4-

bromophenyl)-3-(4-(4-(diphenylamino)styryl)phenyl)fumaronitrile (TPABDFN), which had a broadband two-photon absorption cross-section and strong NIR light emission, the dye was then encapsulated in poly(styreneco-maleic anhydride) (PSMA), forming hybrid fluorescent nanostructures (Figure 5b). Subsequently, these HNS are injected into a tumor-bearing mouse by intravenous route to visualise mouse brain blood vessels, the three-dimensional (3D) anatomical architecture of the mouse brain could be visualised72. Furthermore, this assembly could also be used for two-photon triggered PTT or PDT for brain cancer therapy.



Figure 5. (a) Energy diagrams depicting one-photon absorption (OPA) and two-photon absorption (TPA). Panel A reproduced with permission from ref 73. Copyright 2017 IOP Publishing Ltd. (b) NIR emissive TPABDFN-PSMA-PEG nanoparticles were injected intravenously into a mouse, and the 3D structure of the blood vessels in the mouse brain could be reconstructed with two-photon fluorescence microscopy with a depth of ∼1200 μm. Reproduced with permission from ref 72. Copyright ©2017 Elsevier. The use of two-photon absorption of NIR light rather than a single visible region photon to sensitise HNS or photosensitizers could provide better spatial selectivity and deeper tissue penetration of the brain. Therefore, brain tumours, brain metastasis, and angiogenesis can be visualised superiorly than that of one photon excitation by visible light, thus it is a highly promising modality74. 3.3 Magnetically Triggered Hybrid Nanostructures Magnetic-responsive drug conjugated HNS or nanocarriers enable magnetic field stimulated, on-demand release of chemotherapeutics drugs in a brain tumour, which decreases the risk of overdosages associated with conventional brain cancer treatments. Magnetic triggering of HNS has many advantages over other physical stimuli. For example, the magnetic 18 ACS Paragon Plus Environment

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field (MF) enters deep into the tissue without damaging them, and then stimulate solid tumours by oscillations or heat (magnetic hyperthermia) without damaging healthy hypodermal tissues. The physical principles and the targeting mechanism is illustrated in Figure 6a and b. MF has a higher penetration depth compared to NIR light or ultrasound (US) as well as almost zero dangerous ionising consequence over X-ray radiations. Easy control on the hyperthermic temperature within brain tumour and drug release is possible by tuning the field strength (H) and frequency (f) of the alternating magnetic field75. This novel approach is recently explored as magneto-chemotherapy30 in oncology. HNS based magnetically triggered carriers have specific advantages due to their multimodality, and the particles’ magnetic controllability also allows active targeting to the brain tumour site, so that it may be possible to avoid the severe side effects of systemic chemotherapy76.

Figure 6 (a) Schematic of MF targeting brain tumours following the systemic administration of MNPs. Panel A reproduced with permission from ref 77. Copyright ©2007 Elsevier. (b) Clinically proposed MNPs based localized malignant brain tumor hyperthermia treatment (i) patient undergoes magnetic hyperthermia, (ii) magnetic fluid inside tumour, (iii) targeted localization of MNPs adjacent to the brain tumour for strong therapeutic effect, (iv) Brain tumour cells are more susceptible to localized hyperthermia achieved via larger amount of MNP’s uptake. Panel B reproduced with permission from ref 78 under the terms of the creative commons attribution (CC BY-NC) license. Copyright ©2014 Mahmoudi and Hadjipanayis. (c) Schematics showing AMF directed delivery of HNS (MCNP-ATAP) into brain and breast cancer tissues. (d) the basic 19 MFH treatment based on MCNP’s. Panels mechanism of cancer cell death mechanism under 79 Copyright © 2014 American Chemical c and d reproduced with permission from Plus ref Environment ACS Paragon Society.


3.3.1 Organic-Inorganic HNS Drug-loaded organic-inorganic magnetic HNS have been shown to be activated in the brain after the application of an external magnetic field80. HNS comprising of an iron oxide core with a starch coating (designated G100), for example, was developed to penetrate the BBB and deliver magnetic particles to the brain81. The particles were administered intravenously, and magnetic targeting gave a staggering 5X rise in the total glioma exposure to MNPs when compared to tumours which were not targeted and additionally, a 3.6X improvement in the targeting ability of NPs in glioma tissue compared to healthy brain parenchyma. Another example of magnetically triggered brain therapy used core-shell nanocapsules stabilised with a polymer polyvinyl alcohol (PVA) /Polyacrylic acid (PAA) shell and Iron Oxide NPs as the core82. This facilitated the simultaneous encapsulation of hydrophilic DOX and hydrophobic Curcumin with great efficiency and resulted in a high amount of local accumulation and cell uptake of the particles both in vitro and in vivo under an external magnetic field. In addition to localisation using a magnetic field, magnetically triggered hyperthermia therapy can have direct therapeutic effects on brain cancer cells, enhance drug delivery and can enable controlled heating of brain tumour tissue in a single modality83. Magnetic iron oxide nanoparticles (IONPs) have also been PEGylated and used for remote brain cancer cell killing through magnetic hyperthermia84. This study emphasises the further efficiency of multicore (nanoflowers) in comparison to monocore (nanospheres) IONPs for remotely triggered drug delivery and magnetic hyperthermia, resulting in 80% brain cancer cell death in medically translatable conditions. Jordan et al. have recently translated the HNS based magnetically triggered hyperthermia to the clinic85. The group developed intra-tumoral thermotherapy using IONPs combined with external beam radiotherapy. The primary clinical investigation succeeded following the diagnosis of the first brain tumour recurrence (OS-2), whereas the secondary endpoint investigation achieved overall survival after the diagnosis of the primary tumour (OS-1). Other HNS composing superparamagnetic iron oxide core NPs have been created with a multifunctional shell consisting of PEG/PEI/polysorbate80 (Ps 80)86. These multifunctional HNS were used to encapsulate DOX, and it was shown that the cellular uptake of these DOX@Ps 80-SPIONs (superparamagnetic iron oxide nanoparticles) by glioma cells while under a magnetic field was considerably improved over that of free DOX in solution. Iron oxide nanoparticles doped with zinc (ZnFe2O4) have also been conjugated with microRNA (miRNA) and used as seed mediators for magnetic hyperthermia therapy (Figure 6c&d), simultaneously 20 ACS Paragon Plus Environment

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targeting the brain cancer cells and knocking out downstream heat shock proteins (HSP) that would otherwise promote cell survival and inhibit apoptosis following treatment87. The same research group further developed hybrid magnetic core-shell NPs for the delivery of a proapoptotic amphipathic tail-anchoring peptide (ATAP) which target mitochondria for both malignant brain and metastatic breast cancer cells79. The development of magnetically-triggered, non-invasive, tumour-selective delivery system has also been applied to the problem of passing through the BBB[66,67]. The model protein -Galactosidase was cationized and tested in orthotopic-glioma-bearing rats. Through using MRI-guided magnetic targeting methodology, physiological arterial hydrodynamics could be preserved during the administration of the nanocarrier and embolisation-free and tumour-targeted delivery of protein embedded nanocarriers to brain tumour lesions. Magnetically triggered delivery of neural stem cells (NSCs) has also been shown to be a hopeful strategy for the delivery of therapeutics to malignant glioma

90.

Spinning disk (SD)

MNPs were internalised into NSCs and an AMF was applied to trigger the drug release from HNS, allowing the HNS to be uptaken by cancerous tissues. This method resulted in 50% glioma cell death, and novel nanoplatform has shown benefit in drug release, HNS does not have functionalized biomolecules on the surface. This could reduce the consequences of ‘protein corona’ formation, via direct electrostatic interactions of blood proteins with HNS surface. Combination organic-inorganic HNS has been developed comprising of magnetic fluid-loaded

liposomes

(MFLs)91.

The

submicronic

magnetoliposomes

consist

of

superparamagnetic maghemite nanocrystals, which are PEGylated and functionalised with phospholipid vesicles, which are rhodamine labelled magnetically guided magnetoliposomes show selective brain tumour targeting and are traceable by MRI, and could, therefore, prevent damage to healthy brain tissue during treatment. Magnetosomes which are naturally synthesised by magnetotactic bacteria have been loaded with poly-l-lysine to yield a stable and non-pyrogenic HNS suspension for triggered brain cancer theranostics92. Magnetic Fe3O4 cores synthesised by magnetotactic bacteria and coated with the polymer poly[aniline-co-N-(1-onebutyric acid) aniline] (SPAnH) with a size distribution of 89.2 ± 8.5 nm was reported for use in magnetically triggered brain cancer theranostics. Weinberg et al. have proposed an innovative combined imaging/manipulation platform for successful translation of magnetic HNS in the brain cancer theranostics.93 The platform allowed MNPs to be driven and concentrated in the targeted area in the brain, reducing



chemotherapy side-effects. Alphandéry et al. have introduced a new type of hybrid magnetosomes chains, which have enhanced properties in comparison to commonly used chemically synthesised HNS94. AMF applications exhibited either disappearance of the full brain tumour in 40% of mice or no tumour regression using these novel hybrid magnetosomes chains. 3.3.2 Strengths and Challenges Combining magnetic triggering and chemotherapy along with MFH may control the anomalies associated with individual brain cancer therapies. These multimodal magnetically triggered nanomedicine approaches can cure brain cancer by producing different synergistic effects such as brain tumour tissues shrinking by magnetic hyperthermia which will further increase the intra and extracellular space to allow chemotherapeutic cargoes diffusion into the deep tumour and damage whole solid tumour. The other strength of the MF remote triggering of HNS is the proper utilisation of chemotherapeutic drug release into solid tumours, which avoids tumour escaping of drugs into the healthy surrounding tissue, thus shrinks toxic side effects. The HNS based magnetic hyperthermia combined with drug release can potentiate the effect of the anticancer activity within the tumour and enhancing the destruction of the entire tumour without affecting surrounding environment95. The primary challenge in magnetically triggered theranostics is controlling the thermal dose due to high amplitude ACMF. The unmanageable overheating of the brain at the time of magneto-chemotherapy may perhaps encourage necrosis of surrounding normal brain tissues. Self-regulated hybrid nanoheaters with the ability of magnetic phase change to regulate critical temperature threshold can resolve the issue. Further magnetically guided removal of the HNS from the brain or from the tumour after therapy should be addressed in the near future. 3.4 Ultrasound Triggered Hybrid Nanostructures Focused ultrasound sonication (FUS) in combination with microbubbles (MB) can effectively increase the BBB permeability hence facilitating brain tumour therapy96. The effect is transient, non-invasive, reversible, and does not damage neural cells. When subjected to FUS, the tumour vasculature has enhanced permeability to chemotherapeutic drugs, and there is a transient rupture of vascular barriers by FUS triggering. This is a result of the combined effect of oscillation in size and position of the MBs, the rupture of the MBs, microstreaming and radiation forces (Figure 7).


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3.4.1 Liposomes The first attempt at FUS-enhanced delivery for brain tumour involved 1,3-bis(2chloroethyl)-1- nitrosourea (BCNU) administered to rats with induced glioblastoma tumours 97.

FUS triggering significantly improved the BCNU penetration through the BBB in both the

normal (3.4X) and tumour implanted (2.02X) brains without causing haemorrhaging. This novel physical triggering supports the viability of this treatment in clinical applications by increasing the chemotherapeutic payload delivered to the targeted brain tumour. Ligandconjugated hybrid nanoliposomes triggered by FUS can facilitate increased uptake of chemotherapeutic agents in brain tumours along with improving the tumour-to contralateral brain ratio. Lipo-DOX in combination with ultrasound results in a unique pharmacokinetic profile. Yang et al. evaluated the pharmacokinetics of targeted and untargeted Lipo-DOX hybrid structures in GBM-bearing mice98. The targeted particles were modified with human atherosclerotic plaque-specific peptide-1 (AP-1). When administered to the animals in combination with FUS, there was an enhancement in the internalisation of the drug in the brain tumours. 3.4.2 Organic-Inorganic HNS Dual-modality particles can simultaneously open the BBB and release the drug payload upon exposure to FUS, and facilitate magnetic targeting to the brain tumour. To achieve this multimodality, novel HNS consisting of microbubbles with a superparamagnetic core loaded with doxorubicin (DOX-SPIO) have been developed99. These organic-inorganic HNS can serve as an effective drug carrier, burst under FUS exposure and release cargo into tumour as well as giving enhanced contrast for MR monitoring the delivery of the drug to the areas of the brain. The pairing of FUS and magnetic targeting delivers therapeutic HNS across the bloodbrain barrier to enter the brain tumour both passively and actively. Systemic administration of DOX in combination with using MRI-guided FUS with preformed microbubbles (Optison) has been shown to improve the DOX accumulation in glioma tissue100. Other groups have shown that modifications to the surface such as polyethylene glycol (PEG) coating on particles loaded with cisplatin as an MRI -guided FUS modality in brain cancer enabled them to cross both the BBB and blood-tumour barriers, deliver payload under ultrasound triggering and also be detectable under MRI101. This novel HNS MRI -guided FUS theranostics may provide an effective new approach for cancer therapy, predominantly for preventing the recurrence of brain tumours.



Poly(butyl cyanoacrylate)-based MBs, encapsulating ultrasmall SPIONs inside a shell, can be utilised to track BBB permeation by employing the FUS-MRI dual technique102. External FUS

Figure 7 (a) Schematic illustration of ultrasound triggered brain cancer therapy. (b) DOXSPIO-MBs are released in tumor microenvironment in controlled fashion when triggered by FUS. Clinical experimental set up (c) in vitro, (d) in vivo. (e) Time course of therapy monitoring by MRI. (f) T2 weighted MR images (confirms tumor location) and S weighted MR images (confirms NPs deposition), taken without and with FUS coupled with magnetic targeting for 40 min. Arrow: Location of SPIO deposition. Tumor areas are delineated by yellow dotted lines. Reproduced with permission from ref 99. Copyright©2013 Elsevier. triggering of HNS thus allows temporal and spatial control of the BBB opening; this is considered vital for individualising and refining drug delivery for brain tumours, Alzheimer’s disease, and Parkinson's disease. DOX was incorporated into liposomes making a perfect HNS 24 ACS Paragon Plus Environment

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for FUS triggered, and MRI monitored brain cancer therapy, they found significant FUSinduced enhancements and consistent drug release from HNS in the brain tumours over the time103. Ultrasound-assisted delivery and targeting of poly(2-ethyl-butyl cyanoacrylate) polymeric HNS in normal brain parenchyma of a metastatic brain tumour model was achieved by using a novel FUS system, combining 1.1 MHz MRI guided targeting of the treatment area104. The FUS triggering of HNS is a targeted, noninvasive, and reversible procedure with superior precision to a targeted area in the deep brain using MR image guidance. A hybrid magnetic-graphene oxide carrying Epirubicin (EPI) for magnetic guided and FUS triggered has been designed that can deliver the drug to the brain tumour while being monitored by MRI105. These HNS can easily reach the local tumour lesion and deliver chemotherapeutic cargo, showing 14.7 times more accumulation in the targeted area compared to the surrounding healthy tissue. The particles may then be used as heat-generating materials to achieve targeted hyperthermia by triggering local tumour heating upon stimulation by extremely low-power FUS (LFUS), with a 20-50 fold decrease from high-intensity FUS. Very recently, the use of low-intensity ultrasound therapy termed as sonodynamic therapy (SDT) began as an unusual method for brain and other cancerous tumors106. SDT uses a molecule sensitive to ultrasound known as sonosensitiser. The skin penetration power of the high-intensity ultrasound waves are limited and that is an inverse function of the ultrasound frequency, and hence its consequence is inadequate to the outermost layer of the skin (stratum corneum) at high frequencies107. Porphyrin-based dyes are proposed as sonosensitisers as glioma cells accumulate porphyrin derivatives including 5-aminolevulinic acid, protoporphyrin IX and talaporfin sodium108. The proposed sonosensitisers enhance the cytotoxicity to glioma cells under low-intensity sonication. The therapeutic effects can be further improved by enhancing the amount of cellular uptake of sonosensitiser into glioma cells. Nevertheless, these sonosensitisers are strongly hydrophobic and certainly agglomerates in biological fluids. The high aggregation of materials further moderates the SDT theranostic outcome. The application of newly introduced HNS in combination with SDT can address these issues109–112. Newly introduced cell targeting hybrid nano-liposomes are implemented to improve the therapeutic outcome of SDT for brain cancer113. This novel HNS consists of iRGD modified DVDMS (also called sinoporphyrin sodium) and is conjugated into the targeting liposome (iRGD-Lipo-DVDMS). This formulation has boosted the sonodynamic effect through reactive oxygen species generation in response to FUS exposure as well as showing in vivo biocompatibility and promising outcomes for fluorescence image-guided sonodynamic brain 25 ACS Paragon Plus Environment


cancer therapy. Researchers from Tokyo Women’s Medical University conducted safety trials of experimental SDT combined with nanobots on a 12-year-old dog (natural canine patient) with terminal-stage chondrosarcoma, this can be a revolutionary cancer therapy, and the predicated the 5-year survival rate for Grade II glioma tumours was 93 %114. There are still a few obstacles in translating the HNS-based SDT from fundamental nanoscience to a clinic. Firstly, the comprehensive in vivo cytotoxicity and biodistribution of many nanoformulations must be addressed. Next, the pharmacokinetics of the HNS when delivered into the brain tumour is an equally important issue for successful nano-SDT clinical translation. Recent scientific progress is directed towards assessing the clinical outcomes by performing valid in vitro and in vivo studies115,116 and combining PDT with SDT to enhance the therapeutic outcome.109 3.4.3 Strengths and Challenges FUS sonication techniques coupled with HNS in the presence of microbubbles promotes chemotherapeutic cargo release in the brain and improves the therapeutic potential. Compared to light and magnetic triggering FUS sonication creates a transient rupture of vascular barriers by radiation forces generated by oscillations. This physical phenomenon allows HNS to overcome the BBB and ensures improvement in the tumours vascular permeability. Tumour disruption by FUS is transient, recoverable, and gentle to surrounding neural cells. FUS triggering of HNS thus provides safer brain treatment over the light and remote magnetic activation and allows delivery of therapeutic or diagnostic agents to the brain tumour. The FUS triggering of HNS is recently proposed brain theranostics approach, and hence there is broad scope for researchers to develop this newer phenomenon in depth before clinical translation. The neuro-navigational guidance technique in neurosurgery or in neuro-oncology provided a new way of 3D images navigation of therapeutic cargoes. Designing an interface between FUS triggering of HNS and neuronavigator could allow better and more accurate brain tumour guidance and therapy. Neuronavigation-guided FUS triggering of HNS in the brain tumour has challenging but nanomedicine approach and has vast potential as open-ended medical technology inclusive of drug delivery across the BBB. However, more biomedical engineering development is required to predict the reliability of neuronavigation in leading FUS within the clinical limit, without such targeted HNS FUS triggering in the tumour is impracticable. 3.5 Electro-Magnetically Triggered Hybrid Nanostructures 26 ACS Paragon Plus Environment

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Electromagnetic triggering (EMT) has the potential to enable non-invasive controlled release of chemotherapeutic cargo and relies on using magneto-electric nanoparticles (MENPs). A brain tumour is surrounded by a complex neural network which may be negatively affected by external electric fields. EMT combined with MENPs can be utilised for brain stimulation or localised targeted brain tumour theranostics by using strong local electric charge oscillations induced by external fields that may interact with the sensitive neural network. This can allow for ‘on demand’ drug release in the tumour by changing the electrostatic interaction or binding force between the nanoparticle surface and the attached drugs. These interactive forces are restrained by the magnetoelectric (ME) effect generated by an external MF for remote control of the intrinsic local electric fields117. 3.5.1 Organic-Inorganic HNS For the first time, K Yue et al., predicted the artificial stimulation of neurons using MENPs deep inside the brain by using computational programming118. This study demonstrated the concept of coupling neuronal electric signals to the magnetic dipoles of MENPs in a noninvasive technology that could be used to treat various brain disorders and brain tumours. Other ‘on demand’ systems used tumour growth inhibiting synthetic peptides bound onto the surface of MENPs119. The system comprised a central MENPs (~30-nm in size) of CoFe2O4@BaTiO3, coated by growth hormone-releasing hormones antagonist of the MIA class (MIA690). The high-efficacy binding of MIA690 to MENPs is externally controlled. The peptide could be released on-demand by the application of external direct current (d.c.) and alternating current (a.c.) MFs. Intracellular chemotherapeutic cargo release was found to be 11-times greater in cells treated with MIA690-loaded MENPs coupled with EMT compared with treatment with MIA690 alone (p < 0.01). Drug-loaded liposomes and iron oxide hybrid nanoparticles have also been developed for EMT in brain tumours120. Liposomal membranes on the iron oxide nanoparticle were shown to be responsive to electro-mechanical oscillations produced by external low power radiofrequency field (amplitude B = 2 mT, frequency f = 10 kHz, RF power = 3–5 Watts). These novel low power RF based EMT could remotely trigger site-specific rapid drug release from the hybrid nanochains 121.



Figure 8. Schematics of the technology that couples electric field MENs together for mapping the deep regions of the brain. (A) MENs are targeted into selected regions of the brain by applying d.c. magnetic field, crossing the BBB, (B) d.c. magnetic field is used to distribute MENs homogeneously in the entire brain. (C) Noninvasive adequate selected area brain stimulation using focused and low strength (∼100 Oe) near-d.c. (∼

Progress in Remotely Triggered Hybrid Nanostructures for Next

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