Targeting to Brain Tumor: Nanocarrier-Based Drug Delivery Platforms, Opportunities, and Challenges.

Cancer is a class of disorder characterized by anomalous growth of cells escalating in an uncontrolled way. Among all the cancers, treatment of cancerous brain tumors has been a tough challenge for the research scientists. Moreover, the absence of early-stage symptoms delays its diagnosis, consequently worsening its severity. Conventional treatments such as surgery, radiation, and chemotherapy are still linked with several limitations. The therapeutic effect of most of the anticancer drugs is highly restricted by their inability to pass the blood-brain barrier, low solubility, limited therapeutic window, and so on. Alarming incidences of brain cases associated with low survival rate across the globe coupled with the inefficiency of current treatment strategies have forced the formulation scientists to investigate nanotechnology-based advanced therapeutic approaches to tackle the disease. Various nanoplatforms such as polymeric nanoparticles (NPs), nanoliposomes, dendrimers, carbon nanotubes, and magnetic NPs have been reported in the past years to improve the drug administration into brain tumor cells and to minimize their off-target distribution for lesser side effects and better treatment outcomes. The review presents updated information on the nanocarrier-based drug delivery systems reported in the past few years for the treatment of brain tumor along with new advancements in this field. It also throws some light on the recent challenges faced in the practical field for the successful clinical translation of such nanodrug carriers along with a discussion on the future prospects. Copyright:

INTRODUCTION

Successful delivery of therapeutic agents to brain remains a herculean task for formulation scientists till now. The main culprit that hinders the process is the blood–brain barrier (BBB).[1] Almost 98% of drugs fail to cross BBB due to the presence of tight endothelial junctions. BBB allows the passage of very small molecules, essential nutrients (glucose), gases, etc., but strictly restricts other molecules of larger size or hydrophilic nature. Only the highly lipophilic or ultrasmall size molecules can get entry through BBB. Owing to the most delicate and complex ecosystem of brain, it has been extremely difficult to treat brain-related ailments including brain tumor. Thus, there is an ever demanding thrust for the development of efficient methods to transport therapeutic agents to the brain tissue for improved treatment outcomes.

Today, an estimated of 700,000 people in the US are living with a primary brain tumor and 87,240 more cases are diagnosed in 2020 with a survival rate of nearly 35%. According to estimation, nearly 16,830 people have died from brain tumor in 2019 all over the world.[2] For the medication of most cancers, surgical treatment, radiation, and chemotherapy have been the mainstream treatment strategies till now. However, all these treatment strategies have been found to be insufficient since the incidence of brain tumor is increasing day by day all over the world and poses a serious question mark on our present health-care system.

In this context, nanocarrier-based drug delivery platforms including polymeric nanoparticles (PNPs), liposomes, dendrimers, nanoshells, carbon nanotubes (CNTs), and magnetic nanoparticles (MNPs) have raised some hope in providing greater therapeutic response in the treatment of brain tumor.[3] Because of many desirable properties such as tuneable size, biocompatibility, biodegradability, high surface-to-volume ratio, higher permeation into tumor cells, ease of surface modification, and in vivo stability, nanodrug carrier systems have drawn the attention of research scientists. They offer the advantages of protection of drug against degradation, less interaction of drugs with healthy cells, which made them preferable for cancer therapy.[4]

In this review, we summarize the recently investigated nanodrug carrier platforms for brain tumor therapy. We also gathered data on the recent advancement in the field of nanomedicines. Side by side, a brief discussion on the current challenges encountered on the practical field for the successful industrial production and clinical translation of nanocarrier platforms has been provided along with future prospects.

Blood–brain barrier

The brain is a very sensitive organ, covered by protective membranes called meninges. Brain has also another protective element is called as BBB. This is a tight boundary between the blood vessels, cells, and other substances in the brain that make up brain tissue.[5] The BBB stands as a tough obstacle toward disease-inflicting pathogens and pollutants found in blood.[6] The key structure of the BBB is the presence of endothelial tight junction, which permits only small particles, fats dissolvable molecules, and many gases to pass into the brain tissue. However, for large size drug molecules, it is difficult to cross the BBB or to maintain an optimum therapeutic concentration.[67] The BBB and its essential parts are shown in Figure 1.

Figure 1

(a) LS of Blood Brain Barrier. (b) Sectional view of Blood Brain Barrier

Brain targeting strategies

Targeting to brain tissue by the nanocarrier-based platforms can be discussed under two headings such as passive targeting and active targeting.

Passive targeting

This utilizes unique tumor properties such as punctured vasculature, leaky along with low lymphatic drainage systems, which help the nanocarriers loaded with anticancer drug(s) to be available at the intratumoral region for a reasonable period of time. For crossing the BBB, small size and high lipophilicity are the two prime requisites. Since the BBB is a highly lipophilic barrier, nanocarriers with required lipophilicity are expected to cross BBB effectively to accumulate inside the brain. Nanocarriers preferably of size below 100 nm have been found as most effective systems to bypass the macrophages and to remain in the blood circulation for a sufficient time, which boost them to permeate through the BBB.[8]

Active targeting

The main aim of active targeting is to expand the selectivity of the medication at a particular site of action and thus to get an improved therapeutic efficacy.[9] The techniques for active targeting of tumors include targeting the surface membranes protein/receptor that is overexpressed in cancer cells.[10] Targeting ligands include aptamers, antibodies, small molecules, peptides, and fragments of DNA. The nanocarriers combined with these surface ligands can be directed to the desired tissue, which expresses specific receptor or antigens to recognize the ligands. This ensures site-specific delivery of the loaded cargo along with the minimization of serious toxic effects usually associated with conventional chemodrugs.[11]

NANOCARRIER PLATFORMS FOR BRAIN DELIVERY

There are various forms of nanocarriers for delivery of the drugs which include PNPs, gold nanoparticles (Gold NPs), liposomes, dendrimers, magnetic NPs, CNTs, etc., which are shown in Figure 2.

Figure 2

Different types of nanodrug delivery systems. (a) Polymeric NPs (b) Polymeric micelles (c) Dendrimers (d) Liposomes (e) Gold NPs (f) Carbon Nanotubes

Polymeric nanoparticles

Biocompatible and biodegradable polymers are employed to form PNPs. They are basically categorized into two types based on the method of preparation such as nanocapsules (where the drug becomes constricted a cavity enclosed by some kinds of polymer) and nanospheres (where the matrix is used to spread the drug physically and chemically).[12] The polymers used for the formulation of nanocarriers should be essentially biocompatible or nontoxic to healthy tissues and also biodegradable in nature. Among all the synthetic polymers, poly (D, L-lactide-co-glycolide), PLGA is only Food and Drug Administration (FDA) approved polymer for human application.

Wang et al., 2015, delivered PNPs which were produced by double emulsion method and allowed them on acidic pH for triggered drug release. For active targeting, they conjugated hyaluronic acid, pluronicF127, and PLGA, which resulted in higher accumulation of experimental PNP in the brain tumor as compared to free drug.[13]

An another study by Ramesh and Kumar, 2019, constructed noscapine-loaded polycaprolactone (Nos-PCL) for the treatment of GBM by double emulsion solvent evaporation method.

Polymeric micelles

Polymeric micelles (PMs) are the center shell type NPs framed through the self-accumulation of block or copolymers in the specific solvents. Average PMs have a round shape and a size in the range of 10–100 nm. The hydrophobic shell acts as a reservoir of hydrophobic drugs which enables the encapsulation for anticancer drugs, while the hydrophilic shell is essential for stabilizing the hydrophobic core that makes the polymer water soluble.[14]

Miura et al. 2013 composed a platinum-based anticancer drug consolidating PM with cyclic Arg-Gly-Asp ligand particles, and it was communicated in U87MG tumors. Intravital confocal laser scanning microscopy reported that the cRGD-connected PMs (cRGD/m) deposited rapidly and had excess penetrability from vessels into the tumor parenchyma.[15]

Nanoliposomes

Nanoliposomes (NLs) are simple microscopic vesicles with an aqueous volume enclosed entirely by phospholipid bilayer molecules. Owing to their structural uniqueness, NLs possess the ability to carry both hydrophilic drugs in their structure.[16]

Belhadj 2017 integrated a versatile liposomal glioma-focused drug transport method (c (RGDyK)/pHA-LS) modified with cyclic RGD (c (RGDyK)) with p-hydroxybenzoic corrosive (pHA), wherein c (RGDyK) can target integrin αvβ3 overexposure in the glioma cells and pHA may select dopamine receptors on the BBB. Concurrently, c (RGDyK)/pHA-LS can likewise extend the cytotoxicity of DOX encapsulated in liposomes on glioblastoma cells and had the alternative to permeate within the glioma spheroids.[17]

Temozolomide-encapsulating nanoliposomes were reported by Sun et al., 2019. They took biomolecular corona fingerprints (BCFs) around cationic liposomes for targeting purpose. The BCFs have capacity in binding receptors overexpressed at BBB. The formulations exhibited the highest level of targeting and entered the glioblastoma cell lines and finally result in enhanced antitumor efficacy.[18]

Dendrimers

Dendrimers are nano-sized, completely symmetric, well-characterized, homogeneous, and monodisperse molecules with a regularly symmetric nucleus, an inward shell, and an outer shell. These can be categorized as polymers, hyperbranched polymers, or brush polymers and also be graded as low or excessive molecular.[19]

In the treatment of cancer, the most commonly used dendrimers are polypropylene imines (PPI), polyamidoamine (PAMAM), poly l-lysine (PLL) dendrimers, polyesters (PGLSA-OH), poly (2,2-bis (hydroxymethyl) propionic acid scaffold dendrimers (bis-MPA), and aminobis (methylene phosphoric acid) scaffold dendrimers. From the above the PPI, PAMAM, and PLL dendrimers are mostly used in the treatment of brain tumor.[20]

The PEGylated PAMAM dendrimers were used as a drug carrier with improved penetration ability for effective drug delivery.

Magnetic nanoparticles

The MNPs are the type of carrier which can be controlled under the influence magnetic field. These may be polymer coated; however, they also can be encapsulated inside the liposomes, consequently forming is known as magneto liposomes.[21] MNPs can be administered into the bloodstream systematically and directed to a target with the application of an external magnetic field.

The MNPs size is smaller than 100 nm, whereas ultrasmall superparamagnetic NPs (USPIONS) size is <50 nm. The USPIONS have increasingly been as they can be imagined as a hypointense signal in T1-weighted magnetic resonance imaging sequences. The USPIONS are used by malignant tumor cells in addition to using phagocytic microglia.[22]

In a study, the researchers concluded that even mild hyperthermia at a temperature of 45°C was sufficient to cause tumor cells to undergo apoptosis effects. For example, cancer is an important target for magnetic hyperthermia, as cancer cells are more sensitive to heat than normal ones.[23]

Ganipineni et al. delivered an composition PTX and superparamagnetic iron oxide (SPIO)-loaded nanoparticles (NPs; PTX/SPIO-NPs) based on PEGylated poly (lactic-co-glycolic acid) that serve as an efficient nanocarrier device for magnetic targeting. The BBB was interrupted in the GBM region because of the magnetic resonance imaging. An ex vivo bio-distribution study reveals increased accumulation of NPs in GBM-bearing brains.[24]

Carbon nanotubes

CNTs are carbon allotropes, made of graphite, and constructed in cylindrical tubes with diameter nanometer and lengths of several millimeters.[25]

The cylindrical shape of CNTs has a great advantage in transmembrane penetration which also facilitates intracellular CNTs to effectively penetrate the BBB. CNTs have a property of electrical conductor and have the excellent photothermal characteristics.[26]

Hopkins et al. prepared single-walled CNTs with PEG and tetrahydrofurfuryl-terminated PEG solute is added on culture media makes the cell less round. It reduces cell proliferation along with an increase in cell death rates in human glioma cells.[27]

Harsha et al. synthesized a CNT with mangiferin that was connected to PEG. They performed cytotoxicity tests and flow cytometry studies on U87 cell lines. The cytotoxicity studies have led to a decrease of 1.28 folds in the IC50 value which proved effective anticancer activity of the selected formulation. Flow cytometry studies also proved apoptosis of U-87 cells with minimum necrosis.[28]

In this paper, we have provided examples of few FDA approved therapeutic agents and their clinical status [Table 1].

Table 1

List of food and drug administration approved therapeutic agents at various stages of clinical trials for treatment of brain tumor

NCT number	Composition	Status
NCT02478164	Ponatinib with bevacizumab	Phase 2 completed[29]
NCT03139916	Bavituximab with radiation and temozolomide	Phase 2 completed[30]
NCT03714334	DNX-2440 oncolytic adenovirus	Phase 1 completed[31]
NCT03231501	Epitinib succinate (HMPL-813)	Phase 1 completed[32]
NCT01349036	Pexidartinib (PLX3397)	Phase 2 completed[33]
NCT02336165	Durvalumab (MEDI4736)	Phase 2 completed[34]
NCT03151772	Disulfiram and metformin	Early phase 1[35]
NCT01124240	Temozolomide with procarbazine with cilengitide	Phase 2[36]

NCT: National clinical trial

CHALLENGES

Past decades have witnessed several important developments in clinical oncology. However, till today, it is an accepted fact that all such advanced strategies have not been able to bring any significant change neither in the mortality rate of brain cancer cases nor to improve the quality of life of patients. Nanocarrier-based drug delivery systems though have brought a ray of hope; however, many issues still exist which need to be addressed in time.

One of such vital issues is the toxicity concern of nanodrug formulations. Due to extremely small size of the carriers, there are chances of tissue accumulation. In particular, metallic NPs, composed of iron, cobalt, zinc, gold, or other such heavy metals may have the chances of accumulation in liver, lungs, or brain such as vital organs of the body and can lead to chronic toxic effects. Metallic NPs can generate free oxygen species, which can lead to toxicity. Further, due to nonbiodegradable nature, such NPs can remain in the environment for a longer period of time, leading to constant human exposure with unknown consequences to the ecosystem. Reports say that CNTs can be responsible for the formation of reactive oxygen species, which may lead to lipid peroxidation and consequently mitochondrial dysfunction or cellular damage. Practically, there can be many types of toxic effects of nanodrug carriers, which we are not even aware. Of course, the toxic effects depend on various factors such as size, shape, chemical composition, biocompatibility, route of administration, and degradation mechanism, which need to be vividly studied. After administration, it is still not properly clear whether the properties of nanomaterials will change in brain microenvironments, or how will they affect complement activation, blood coagulation, or overall human immunity. Thus, many such important factors related to the in vivo behavior of nanodrug carriers and their effect on other healthy brain cells are need to be investigated thoroughly.

For anticancer drug-loaded nanocarriers, dose ranges should be correctly determined. Both blood and brain pharmacokinetics of nanomaterials should be investigated in vivo experiments to get idea regarding their ADME behaviors. However, there is still serious lack of ample preclinical data of nanodrug carriers on brain delivery, along with scarcity of data related to the in vitro-in vivo correlation, for which it is actually very difficult to draw any conclusive outcome regarding the efficacy of such nanodrug carriers for the treatment of brain tumor.

Even though there was a large number of reports and studies associated to nanodrug formulations, merely, a handful of such nanosystems (<10%) have actually cleared regulatory approval to get into the market. One of the main challenges lies in the in vivo effect of various nanocarriers, which is often very dissimilar from their in vitro actions. For nanodrug carriers, a serious gap in the in vitro-in vivo correlation always remains, which makes their regulatory clearance a tough job. Further, aspects such as cellular interactions, tissue transportation, diffusion, and biocompatibility are the other crucial reasons, which yet to be systematically investigated using appropriate animal models.

In case of in vivo experiments, concerns are also being raised by some medical experts on the practicality or rationality of experiments on laboratory animals due to the remarkable anatomical and physiological differences between laboratory animals and humans. It is a fact that animals (rodents) do not suffer from brain tumor or any such cancers so frequently as humans. Furthermore, laboratory animals and human subjects react and metabolize substances differently, i.e., the material, which nontoxic to animals may be toxic to humans or vice versa. Further, due to lack of sufficient knowledge on the exact mechanism of development and progression of brain tumor, along with the types of biochemical factors or specific antigens/proteins involved in the process, they are difficult to be established in experimental animal models. Thus, many scientists now argue that whether the use of such genetically modified animal models could serve the purpose of testing the in vivo efficacy of nanodrug carriers or whether those data generated out of the animal models can be rely up on.

Coming to the problems faced in practical field for large scale manufacturing of nanodrug carriers, many critical issues are still remain unaddressed. In spite of all advantages, pharmaceutical companies are hesitant to invest more in nanocarrier-based drug delivery platforms. As a result, maximum research outcomes are confined in academic or small-scale research laboratories and cannot able to reach from bench to bed side. Out of several factors, one of the major factors is the cost of manufacturing. Usually, the overall manufacturing cost of almost all nanocarrier-based formulations intended for human applications are quite higher than conventional formulations. Along with these, there are other practical problems at the production stage such as batch-to-batch variation, low amount of drug loading, and stability issues, for which most of the pharmaceutical companies are not willing to take the risk.

CONCLUSIONS AND FUTURE PROSPECTS

Advancements in the molecular neurosciences have actually revolutionized the implementation of nanotechnology-based strategies for improved treatment of metastatic brain tumors. It is expected that nanocarrier-based drug delivery platforms would reach new heights in coming years and would bring some significant changes in oncology research. As the 5-year survival rate of brain cancer patients still remains poor, the need for successful clinical translation of nanocarriers is the need of the hour. The nanodrug delivery systems for brain have to be well-characterized, biocompatible, biodegradable, and of course to be smart to act effectively inside in vivo environments. Nevertheless, it has to be target specific, i.e., it should be concentrated in brain tissue surpassing BBB, avoiding the off-target tissues in order to exert maximum therapeutic effect with lesser side effects. The future focus of the study should be directed to develop multifunctional targeted nanocarriers using existing drugs with standardized formulation parameters for smooth technology transfer. The ultimate outcome of all research investigations related to nanodrug carriers would be tangential, only when they would be translated into industrial scale.

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Conflicts of interest

There are no conflicts of interest.