Nanopartículas para la terapia del cáncer:avances y desafíos actuales

Nanopartículas en la terapia contra el cáncer

Las NP que se utilizan ampliamente en los sistemas de administración de fármacos incluyen NP orgánicas, NP inorgánicas y NP híbridas (Fig. 5).

Varios tipos de nanomateriales utilizados en la terapia del cáncer

Nanopartículas orgánicas

Nanopartículas poliméricas

Las nanopartículas poliméricas (PNP) están bien definidas como "macromoléculas coloidales" con una arquitectura estructural específica formada por diferentes monómeros [54]. El fármaco está atrapado o adherido al exterior de las NP, creando una nanoesfera o una nanocápsula para lograr la liberación regulada del fármaco en el objetivo [55]. Inicialmente, los PNP estaban compuestos por polímeros no biodegradables como poliacrilamida, polimetilmetacrilato (PMMA) y poliestireno [56]. Sin embargo, la acumulación de estos provocó toxicidad debido a la dificultad para eliminarlos del sistema. Actualmente se utilizan polímeros biodegradables como el ácido poliláctico, poli (aminoácidos), quitosano, alginato y albúmina y se sabe que reducen la toxicidad y mejoran la liberación y biocompatibilidad del fármaco [57]. La investigación comprobada ha reflejado que al recubrir los PNP con polisorbatos y al usar polisorbatos con efecto tensioactivo. El revestimiento exterior mejora las interacciones de las NP con la membrana celular endotelial de la barrera hematoencefálica (BBB) ​​[58].

Un estudio mostró que las nanocápsulas cargadas con indometacina implicaron una disminución sustancial del tamaño del tumor y una mejor supervivencia en un modelo de glioma de xenoinjerto en ratas [59]. Este es un campo en crecimiento con más de diez NP poliméricos que contienen medicamentos contra el cáncer que se encuentran en desarrollo clínico. Algunos ejemplos incluyen paclitaxel poliglumex (Xyotax), PEG-camptotecina (Prothecan), dextrano-camptotecina modificada (DE 310), copolímero de HPMA-DACH-platinato (AP5346), copolímero de HPMA-platinato (AP 5280), copolímero de HPMA-paclitaxel ( PNU166945) y copolímero de HPMA-doxorrubicina galactosamina (PK2) [60].

Dendrímeros

Los dendrímeros son macromoléculas poliméricas esféricas con una arquitectura hiperramificada definida. Las estructuras muy ramificadas son el rasgo característico de los dendrímeros. Normalmente, la síntesis de dendrímeros se inicia haciendo reaccionar un núcleo de amoniaco con ácido acrílico. Esta reacción da como resultado la formación de una molécula de "tri-ácido" que reacciona más con etilendiamina para producir "tri-amina", un producto de GO. Este producto reacciona además con el ácido acrílico para dar lugar al hexaácido, que además produce el producto “hexa-amina” (Generación 1) y así sucesivamente [61]. Normalmente, el tamaño de los dendrímeros varía entre 1 y 10 nm. Sin embargo, el tamaño puede alcanzar hasta 15 nm [62]. Dada su estructura específica como peso molecular definido, ramas ajustables, biodisponibilidad y carga, estos se utilizan para apuntar a ácidos nucleicos. Algunos dendrímeros que se utilizan ampliamente son poliamidoamina (PAMAM), PEG (poli (etilenglicol)), PPI (polipropilenimina) y TEA (trietanolamina) [63].

Inicialmente, se diseñó un dendrímero PAMAM para lograr la gestión de MDR. Los dendrímeros PAMAM ensamblados con ADN se han descrito extensamente. En comparación con los animales tratados con quimioterapia de agente único, los dendrímeros sintetizados retrasaron significativamente el crecimiento de xenoinjertos de cáncer epitelial [64].

Nanopartículas de mAb

Los anticuerpos monoclonales se utilizan ampliamente en el tratamiento del cáncer por sus capacidades específicas de focalización [65]. Estos mAb ahora se combinan con NP para formar conjugados anticuerpo-fármaco (ADC). Se ha demostrado que estos son muy específicos y convincentes que los fármacos citotóxicos o los mAb solos. Por ejemplo, un NP de anticuerpo-fármaco compuesto por un núcleo de paclitaxel y una superficie modificada con trastuzumab presentó una mejor eficacia antitumoral y una menor toxicidad que el paclitaxel como agente único o el trastuzumab solo en el control de células epiteliales mamarias positivas para HER2 [66].

Vesículas extracelulares

Las vesículas extracelulares (VE) son vesículas de fósforo-lípidos de doble capa que varían entre 50 y 1000 nm n de tamaño [67]. Los EV son secretados continuamente por diferentes tipos de células y varían en origen, tamaño y composición. Los EV se dividen en tres clases:1) exosomas, 2) microvesículas y 3) cuerpos apoptóticos [68]. Las NP combinadas con exosomas se utilizan ampliamente ya que tienen lípidos y moléculas muy similares a las células de origen. Además, escapan a la vigilancia inmunológica y se internalizan muy rápidamente dentro de las células cancerosas. Actúan como vehículos naturales al administrar fármacos citotóxicos y otros fármacos antitumorales a los sitios de destino. Los exosomas cargados con doxorrubicina (exoDOX) son el mejor ejemplo. exoDOX se utiliza para tratar el cáncer de mama y ha mostrado excelentes resultados en comparación con el tratamiento conservador con doxorrubicina al mejorar la citotoxicidad y evitar la cardiotoxicidad [69]. Los NP de exosomas tienen características de biocompatibilidad intrínsecas, estabilidad química avanzada y comunicaciones intracelulares en comparación con los NP sintéticos. Sin embargo, los inconvenientes como la deficiencia de las condiciones estándar para el aislamiento y la purificación exosomal son cruciales y deben abordarse [70, 71].

Liposomas

Se trata de vesículas esféricas que comprenden fosfolípidos que pueden ser unilamelares o multilamelares para encapsular moléculas de fármaco [72]. Los liposomas son únicos por tener características tales como baja toxicidad intrínseca, inmunogenicidad débil e inercia biológica [73]. Los liposomas son el primer fármaco a nanoescala que se aprobó en 1965 [74]. Una estructura de liposoma típica se compone de un "núcleo hidrófilo" y una "bicapa de fosfolípidos hidrófobos". Esta arquitectura única les permite atrapar fármacos tanto hidrófilos como hidrófobos para proteger eficazmente el fármaco atrapado de la degradación ambiental en la circulación [75].

Los liposomas proporcionan una plataforma excelente para la administración de fármacos como doxorrubicina, paclitaxel y ácido nucleico, además de demostrar una mayor eficacia antitumoral y una mayor biodisponibilidad [76]. Doxil® y Myocet® son formulaciones aprobadas de daunorrubicina a base de liposomas que se utilizan para tratar el CMM [77, 78]. Sin embargo, debido a deficiencias como la disminución de la eficacia de la encapsulación, la eliminación rápida por MP, la adsorción celular y la corta vida útil, la aplicación de NP a base de liposomas es limitada.

Nanopartículas de lípidos sólidos (SLN)

Son nanoportadores coloidales (1-100 nm) compuestos por una monocapa de fosfolípidos, emulsionante y agua [79]. Estos se conocen como nanomateriales de dimensión cero. El componente lipídico puede ser triglicéridos, ácidos grasos, ceras, esteroides y lípidos PEGilados [80]. Unlike conventional liposomes, SLNs have a “micelle-like structure” within which the drug is entrapped in a non-aqueous core. Examples include mitoxantrone-loaded SLN, which has shown reduced toxicity and enhanced bioavailability [81]. The incorporation of doxorubicin and idarubicin by SLN in “P388/ADR leukemia cells” and the “murine leukemia mouse model” has shown positive results [82].

Nanoemulsions

Nanoemulsions are colloidal NPs with heterogeneous mixtures of an oil droplet in aqueous media ranging from 10–1000 nm [83]. Three representative types of nanoemulsions can be made in:1) oil-in-water system, 2) water-in-oil system, and 3) bi-continuous nanoemulsions. Membrane-modified nanoemulsions have been extensively studied. For instance, nanoemulsions loaded with spirulina and paclitaxel showed an improved anti-tumor effect by regulating immunity through TLR4/NF-kB signaling pathways [84]. Nanoemulsion consisting of rapamycin, bevacizumab, and temozolomide is known to treat advanced melanoma [85]. Nanoemulsions are different from liposomes and certainly have enhanced characteristics than others, such as optical clarity, stability, and biodegradability [86]. However, there are challenges to clinical applications of these nanoemulsions as these involve high temperature and pressure and instruments such as homogenizers and microfluidizers that are expensive.

Cyclodextrin Nanosponges

Cyclodextrins are usually used as stabilizers to increase the drug loading capacity of NPs [87]. Nanosponges are tiny, mesh-like structures [88]. Β-cyclodextrin nanosponges loaded with paclitaxel have shown sound cytotoxic effects in MCF-7 cell line culture [89]. Similarly, camptothecin has shown improved solubility and stability when formulated with cyclodextrin-based nanosponges [90].

Inorganic Nanoparticles

Carbon Nanoparticles

Carbon NPs as the name suggests are based on the element carbon. They have been widely utilized in medical arenas because of their optical, mechanical, and electronic properties combined with biocompatibility [91]. Due to their inherent hydrophobic nature, carbon NPs can encapsulate drugs through π-π stacking [92]. Carbon NPs are further categorized into graphene, carbon nanotubes, fullerenes, carbon nanohorns, and graphyne. Although all these are carbon-based, they vary in their structure, morphology, and properties.

“Graphene” is 2D crystal with sp2-hybridized carbon sheet that holds extraordinary mechanical, electrochemical, and high drug loading properties. Further, based on composition, properties, and composition, graphene can be divided as follows:1) single-layer graphene, 2) graphene oxide (GO), 3) reduced graphene oxide (rGO), and 4) multi-layer graphene [93]. GO and rGOs are widely used due to their ability to target hypoxia [94] and irregular angiogenesis in TME [95]. Studies have shown that GO-doxorubicin exhibits higher anticancer activities in cellular models of breast cancer [96].

Fullerenes are large carbon-cage molecules composed of carbon allotrope with different conformation types such as sphere, ellipsoid, or tube. They are the most widely studied nanocarriers as they have typical structural, physical, chemical, and electrical properties [97]. These are used in photodynamic therapy as they have triple yield and generate oxygen species due to the presence of extended π-conjugation and the ability to absorb light [98]. PEG-modified fullerenes showed promising photodynamic effects on tumor cells [99].

Carbon nanotubes (CNTs) are cylindrical tubes, most often considered as rolls of graphene, were discovered in the late 1980s. They are classified into two groups:1) single-walled CNTs and 2) multi-walled CNTs. As they are carbon-based, they can bring upon immune response by interacting with immune cells, thereby suppressing the tumor growth. Traditionally, they have been used as DNA delivery vectors and for thermal ablation therapy. For instance, a fluorescent single-walled CNT with mAb encapsulating doxorubicin is used to target colon cancer cells. Such CNTs form a complex which is effectively engulfed by the cancer cells leading to the intracellular release of doxorubicin, whereas the CNTs are retained in the cytoplasm [100].

Quantum Dots

Quantum dots are typically nanometer-scale semiconductors with a broad spectrum of absorption, narrow emission bands, and high photostability, allowing them to be widely used in biological imaging [101]. Based on carbon, these are divided into:1) graphene quantum dots, 2) nanodiamond quantum dots, and 3) carbon quantum dots. Besides biological imaging, quantum dots are being actively investigated in cancer treatment. The most commonly used quantum dots is graphene quantum dots due to their inherent biocompatibility and rapid excretion. For example, quantum dots aptamer—doxorubicin conjugate targets prostate cancer cells [102]. However, the deficiency of optimized process in producing quantum dots is the major obstacle.

Metallic Nanoparticles

Metallic nanoparticles are commonly explored in “biological imaging” and targeted DDS due to their remarkable optical, magnetic, and photothermal properties. Some of the most commonly used metallic NPs are gold NPs, silver NPs, iron-based NPs, and copper NPs. Gold NPs are used as intracellular targeting drug carriers because the size and surface properties are easily controlled [103]. Moreover, their visible light extinction behavior makes it possible to track NP trajectories in the cells. “Anti-HER2 functionalized gold-on-silica nanoshells” have been shown to aim HER2 positive breast cancer cells [104]. Combidex®, an iron oxide NP formulation, is presently in the late-stage clinical testing phase to detect nodal metastases [105]. Feraheme®, a ferumoxytol containing iron oxide NP formulation, is used to treat iron-deficiency anemia. This is also used to treat nodal metastases in prostate and testicular cancer and was approved by FDA in June 2009 [106, 107].

Magnetic Nanoparticles

Magnetic NPs are generally used in MRI imaging, and drug delivery contains metal or metal oxides. These are usually covered with organic substances like polymers and fatty acids to enhance stability and biocompatibility [108]. LHRH-conjugated superparamagnetic iron oxide NPs are effective in targeting and imaging of breast cancer [109]. Moreover, magnetic NPs are used in magnetic hyperthermia for thermal ablation of cancer cells [110, 111]. Some of the magnetic NPs that are in the market or in the clinical trial phase are Feridex® and Resovist® for liver metastasis and colon cancer [112].

Calcium Phosphate Nanoparticles

“Calcium phosphate NPs” is biologically compatible, biodegradable, and do not cause any harsh adverse reactions. Hence, they are used as a delivery agent for insulin, growth factors, antibiotics, and contraceptives [113]. They are also used in the delivery of oligonucleotides and plasmid DNA [114]. Calcium phosphate NPs combined with either viral or non-viral vector has been positively used as delivery vectors in cellular gene transfer. A “liposomal nanolipoplex formulation” of calcium and glycerol has shown decreased toxicity and enhanced transfection features [115, 116].

Silica Nanoparticles

Silica being a significant component of many natural materials was only studied concerning biology recently. Silica NPs are commonly used to deliver genes by functionalizing the NP surface with amino-silicanes [117]. N-(6–aminohexyl)–3–aminopropyl–trimethoxysilane functionalized silica NPs have shown excellent efficiency in the transfection of Cos-1 cells with minimal toxicity and is now commercially available [118]. Mesoporous silica NPs are considered one of the best drug carriers due to their better pharmacokinetic properties. They have been extensively used in immunotherapy. According to a study, colorectal cancer cells have shown successful uptake of camptothecin-loaded mesoporous silica NPs.

Mechanism of NPs in Overcoming Drug Resistance

Drug resistance is one of the chief problems in cancer therapy and management. It prevails across all types of cancer and all possible treatment modalities. Drug resistance is a phenomenon that results when diseases become tolerant to pharmaceutical treatments. Drug resistance can be classified into two types:1) innate and 2) acquired [119]. Innate resistance usually results from pre-existing mutations in the genes that are involved in cell growth or apoptosis. Acquired resistance is defined as the type of resistance that is developed after a particular anti-tumor treatment, which may result from the development of new mutations or from alterations in the TME during treatment. Nanoparticles, due to their extraordinary ability to co-encapsulate multiple therapeutic agents, can also be used to overcome cancer-related drug resistance.

Targeting Efflux Transporters

Efflux transporters are classified under the family of “ATP-binding cassette (ABC) transporters.” These have a significant role in MDR. The primary function of these transporters is to pump out drugs out of the cell and reduce the concentration. “P-glycoprotein (P-gp)” is one such efflux transporter that is overexpressed by drug-resistant cancer cells [120, 121].

Overexpression of P-gp has been linked with inadequate treatment response, especially in breast cancer [122] and ovarian cancer [123]. NPs can be used to tackle efflux pumps. As NPs internalize the cell via “endocytosis” instead of diffusion and release the drug at the “perinuclear site,” which is distant from active efflux pumps, NPs can bypass the efflux pumps [124]. Besides, by modifying the control of drug releases, such as by utilizing low pH levels and redox as triggers, NPs can effectively bypass efflux pumps [125, 126].

Combination therapy is yet another method to overcome MDR. NPs can be loaded with multiple drugs within a single drug carrier [127]. Inhibiting efflux transporter expression instead of just dodging them would be another viable option. This can be achieved by building NPs in such a way that it can entrap both efflux pump inhibitors and chemotherapy agents [128]. A recent study positively reflected upon reversing MDR in breast cancer cells by using NPs that co-deliver COX-2 inhibitors and doxorubicin [129]. Similarly, using silica NP that encapsulates miRNA-495 and doxorubicin has proved effective in overcoming drug resistance in lung cancer cells [130]. Another interesting study found out that using NPs in the tumor neo-vasculature targeting KDR receptors is a more effective anti-tumor function than P-gp inhibitor combination therapy. Yet, another way of overcoming drug resistance is by depleting the source of ATP, which is essential for the functioning of ABC transporters. This can be done by targeting mitochondria which leads to a decrease in ATP production.

Targeting an Apoptotic Pathway

Cancer cells proliferate due to faulty apoptotic machinery and upsurge their survival adding to drug resistance [131]. The faulty apoptotic pathway gets activated by “deregulation of Bcl-2” and “nuclear factor kappa B (NF-κB).” These are the most widely investigated anti-apoptotic proteins and can be potentially used as the target for reversing drug resistance. Using a classic process of co-delivery of “Bcl-2 siRNA and chemotherapeutics” by NPs is a way to overcome MDR [132]. NF-κB inhibitors have been used in combination with “pyrrolidine dithiocarbamate (PDTC)” [133] and curcumin [134]. Besides suppressing anti-apoptotic factors, triggering pro-apoptotic factors is another to fight “apoptotic pathway-mediated drug resistance.” For instance, a combination of ceramide and paclitaxel is a good example [135]. Ceramide restores the expression of a chief tumor suppressor, p53 protein, by regulating alternative pre-mRNA splicing. Delivering ceramide via NPs is an excellent way to correct the p53 missense mutation [136]. Owing to its potential, a combination of ceramide and paclitaxel has shown significant therapeutic efficacy in cancer drug resistance models. Transfecting the p53 gene by cationic SLNs has been reported in lung cancer cases [137]. Similarly, transfecting the p53 gene by PLGA has been carried out in breast cancer cells models that have shown potent induction of apoptosis and inhibition of tumor growth [138].

Some NP-based DDS act by impeding efflux pumps and encouraging apoptosis [139]. A pioneering study conducted to prove both pump- and non-pump-mediated drug resistance used an “amphiphilic cationic NP” entrapping paclitaxel and Bcl-2 converter gene in drug-resistant liver cancer models. NP complex diminished P-gp-induced drug efflux and the apoptosis activation. Similarly, co-delivery of “doxorubicin and resveratrol encapsulated in NPs” has shown noteworthy cellular toxicity on doxorubicin resistance breast cancer cells by downregulating the expression of Bcl-2 and NF-κB, thereby initiating apoptosis as well as through the inhibition of efflux transporter expression [140]. A similar study was done on multi-drug resistant prostate cancer cells by using folic acid-conjugated planetary ball milled NPs encapsulated with resveratrol and docetaxel. This worked by downregulating anti-apoptotic gene expression while inhibiting ABC transporter markers [141].

Targeting Hypoxia

Hypoxia is yet an additional aspect that backs MDR [142]. Due to abnormal blood vessels in the vicinity of the tumor and due to the increasing demand of oxygen by the rapidly growing tumor, some tumor cells are repeatedly in a hypoxic condition. The part of the tumor that is in hypoxic condition often escapes from the chemotherapy drugs. Hypoxia creates an oxygen ramp inside the tumor that intensifies tumor heterogeneity, encouraging a more aggressive phenotype. Moreover, the hypoxia condition has been established to facilitate the overexpression of efflux proteins [143]. The major protein, “hypoxia-inducible factor 1α (HIF-1α)” acts an important role. Hence targeting HIF-1α or silencing HIF-1α gene is a way to overcome drug resistance. NPs containing HIF-1α siRNA can be used to reduce hypoxia-mediated drug resistance [144]. Instead of directly targeting HIF-1α, indirect inhibition of HIF-1α signaling can be used. For example, the “PI3K/Akt/mTOR pathway” is known to control the expression of HIF-1α. Inhibition of this pathway effectively downregulates the expression of HIF-1α, which enhances the sensitivity of MDR cells to cancer treatment [145]. NPs like PLGA-PEG and PEGylated and non-PEGylated liposomes can be used effectively. In addition, “heat shock protein 90 (HSP90)” is needed for transcriptional activity of HIF-1 and inhibition of HSP90, which downregulates the expression of HIF-1α [146]. The HSP90 inhibitor in “17AAG loaded NPs” has dramatically improved MDR in bladder cancer treatment [147].

Nanoparticles and Proteomics

When NPs are subjected to the biological system, they are surrounded by cellular and serum proteins which form a structure known as protein corona (PC) [148]. Based on the degree of interaction of these proteins with the NPs, there are classified into the hard corona and soft corona. “Hard corona” is formed when these proteins have a high binding affinity towards the NPs. “Soft corona” is produced when these proteins are loosely bound to the NPS. It has been established that the most protein forming a PC first will be eventually substituted by proteins with higher affinities. This is known as Vroman effect [149]. Hence developing the technology that can manufacture NPs with desired properties is essential. Several proteomic approaches such as MS, LC–MS, SDS-PAGE, isothermal microcalorimetry (ITC), etc. [150], are being used. PC affects the crosstalk of NP with the biological setting and thereby governs the application and usage of the same in the medical field.

Cancer proteomics studies the number of proteins in cancer cells and serum, which supports hunting proteins and biomarkers that aids in diagnosis, treatment, and prognosis [151]. It also helps in understanding cancer pathogenesis and drug resistance mechanism. Post-translational modifications (PTMs) play an indispensable part in occurrence, recurrence, and metastasis. Besides using chemotherapy and kinase inhibitors, novel agents like siRNA, mRNA, and gene editing are central therapeutics used with NPs.

Nanotechnology for Small Interfering RNA (siRNA) Delivery

siRNAs are small ds RNA molecules (around 21 nucleotides long) that suppress the expression of genes in the target. This process is known as “RNA interference.” A few siRNA-based NPs that are currently under clinical investigations are ALN-TTR01 that is used to target the transthyretin gene to treat transthyretin-mediated amyloidosis, and Atu027, which is a liposomal siRNA that targets protein kinase N3 and TKM-ApoB that knock downs the expression of ApoB [152, 153].

Nanotechnology for Tumor microRNA Profiling and Delivery

MicroRNAs are a class of endogenous “single-stranded non-coding RNA” molecules that control post-transcription gene expression by blocking translation of the target mRNA or repressing protein production by destabilizing mRNA [154]. These are emerging as vital biomarkers that are a significant target for cancer diagnosis, therapy, and treatment. The base priming nature of nucleic acid forms the very foundation for nanotechnology used miRNA profiling techniques. Several profiling techniques use biosensors or surface plasmon resonance imaging techniques in combination with molecular biology enzymatic reactions. Nanotechnology can be used for the delivery of MicroRNAs. For example, biodegradable polycationic prodrugs showed promising results in the regulation of polyamine metabolism [155]. MicroRNA-loaded polycation-hyaluronic acid NPs of single-chain antibody fragments have shown progressive downregulation of “survivin expression” in high metastatic cancer load in the lung of murine B16F10 melanoma.

DNA Nanotechnology for Cancer Therapy

DNA-based nanostructures have been synthesized for DNA sensors to detect nucleic acid, DNA-coated gold NPs for lead sensing by hybridizing Pb-activated DNAzyme to the linking DNA, scaffolds to organize organics, inorganic, and biomolecules into distinct morphology molecular transporters, and drug delivery (Table 1).

Advantages of Nanoparticles in Cancer Therapy

The utilization of nanotechnology in the diagnosis, treatment, and management of cancer has led to a whole new era. NPs, either by active or passive targeting, augment the intracellular concentration of drugs while avoiding toxicity in the healthy tissue. The targeted NPs can be designed and altered as either pH-sensitive or temperature-sensitive to establish and regulate the drug release. The pH-sensitive drug delivery system can deliver drugs within the acidic TME. Similarly, the temperature-sensitive NPs release the drugs in the target site due to changes in temperature brought in by sources like magnetic fields and ultrasound waves. In addition, the “physicochemical characteristics” of NPs, such as shape, size, molecular mass, and surface chemistry, have a significant part in the targeted drug delivery system. Further, NPs can be modified according to the target and used to target a particular moiety.

Conventional chemotherapy and radiation therapy have several disadvantages concerning efficacy and side effects because of uneven dispersal and cytotoxicity. Therefore, cautious dosing is required that effectively kills cancer cells without any significant toxicity. To reach the target site, the drug has to pass several fortifications. Drug metabolism is a very complex process. In physiological conditions, the drug needs to pass TME, RES, BBB, and kidney infiltration. RES or macrophage system is made up of “blood monocytes, macrophages, and other immune cells” [160]. MPS in the liver, spleen, or lungs react with the drugs and activate “macrophages or leukocytes” that rapidly remove the drug. This leads to a short half-life of the drug [161]. To overcome this, NPs with “surface modification,” such as PEG, bypass this mechanism and increase the “drug half-life.” Besides, kidney infiltration is a crucial function in the human body. Proper kidney infiltration thus minimizes the toxicity caused by NPs.

The brain-blood barrier (BBB) is a specialized protection structure offered to protect the CNS from harmful and toxic agents. “Brain capillary endothelial cells” are arranged in the form of a wall that provides essential nutrients to the brain. Since the primary function of BBB is to block toxic agents to reach the brain, currently available chemotherapy agents for brain cancer are highly limited to intraventricular or intracerebral infusions [162]. However, NPs are known to cross BBB. Now, several approaches such as EPR effect, focused ultrasound, peptide-modified endocytosis, and transcytosis are used to deliver NPs. Glutathione PEGylated liposome encapsulated with methotrexate showed improved methotrexate uptake in rats [163]. Au-NPs are often used as they have proven to help transport drugs to induce apoptosis [164].

NPs being carriers also increase the drug stability by preventing the degradation of the encapsulated cargo. Additionally, a large volume of drugs can be encapsulated without any chemical reaction. Dry solid dosage forms are more stable than nanoliquid products [165]. Stabilizers can be used to enhance stability. Yet another way to increase stability is to use porous NPs.

Tumor has unique pathophysiology features such as extensive angiogenesis, flawed vascular architecture and defective lymphatic drainage. The NPs use these features to target tumor tissue. Due to reduced venous return in tumor tissue and meager lymphatic clearance, NPs are effectively retained. This phenomenon is known as EPR. Similarly, by targeting the adjacent tissues, tumor-targeting can be accomplished [166].

NPs can be administered through several routes like oral, nasal, parenteral, intra-ocular etc. NPs have a high surface-to-volume ratio and intracellular uptake. Studies have reported that NPs are more effective than microparticles as drug carriers [167].

Nanoparticles in Immunotherapy

The immune system sets an important part in the establishment and development of cancer cells. The advancement of immunotherapy has revolutionized cancer therapy. It is found that NPs not only help in target delivery of chemotherapy but can also be used in combination with immunotherapy. There are several approaches in immunotherapy aimed at activating the immune system against cancer cells [168] by “immune checkpoint blockade therapy,” “cancer vaccine therapy,” “chimeric antigen receptor (CAR)-T cell therapy,” and “immune system modulator therapy” [169,170,171]. NP-based immunotherapy includes “nanovaccines,” “aAPCs (artificial antigen-presenting cells),” and “immunosuppressed TME targeting.”

Nanovaccines specialize in delivering “tumor-associated antigens” and “adjuvants” to antigen-presenting cells, such as dendritic cells (DCs) [172]. Moreover, these can also be employed as adjuvants to enhance “APC antigen presentation” and promote DC maturation that leads to the stimulation of cytotoxic T cells that have anti-tumor function [173, 174]. Liposomes, PLGA NPs, gold NPs are found to have the ability to deliver TAAs into DCs in the cytoplasm [175]. Mesoporous silica, the most used inorganic NP, has exhibited an adjuvant role, leading to immune response stimulation [176]. Artificial APCs interact with MHC-antigen complexes directly which binds to T cells. They also bind to co-stimulatory molecules that bind to co-stimulatory receptors leading to T cell activation [177]. Targeting the immunosuppressed TME is yet another method of using NPs in immunotherapies. This is done by targeting essential cell types in TME such as “tumor-associated macrophages (TAMs),” regulatory T cells, and “myeloid-derived suppressor cells (MDSCs).”

Besides, the combination of chemoimmunotherapy has been demonstrated to be a capable approach in cancer therapy. For instance, a study has shown that co-loading Nutlin-3a, which is a chemotherapeutic agent and cytokine GM-CSF, in “spermine-modified acetylated dextran (AcDEX) NPs” improved cytotoxic CD8( +) T cells proliferation and activated an immune response [178].

“Programmed cell death protein 1 (PD-1)” and “programmed cell death ligand 1 (PD-L1)” are some of the essential immune checkpoints [179]. Hence immune checkpoint inhibitors are used to target these using NPs. According to a study, conventional immune checkpoint inhibitors of PD-L1/PD-1 displayed inconsistent responses. To enhance the chances and bonding of immune checkpoint inhibitors and immune checkpoints, multivalent poly (amidoamine) dendrimers were used. Usage of these dendrimers not only showed enhanced PD-L1 blockade but also showed improved drug accumulation at the tumor site [180].

Nanoparticles in Cryosurgery

Cryosurgery is an advanced practice of freeze-destroying cancer tissue. Although this is less invasive and causes intraoperative bleeding and postoperative complications, certain drawbacks like inadequate freezing capacity and damage to adjacent cells need to be addressed [181]. The rise of nanotechnology has enabled the use of NPs in cryosurgery.

The primary working of nanocryosurgery is introducing NPs with particular properties into the cancer cells and causing freezing [182]. During this process, ice is formed within the cells, which causes damage to it. This is an important process and can be carried out effectively using NPs. The thermal conductivity property of NPs can be exploited, which significantly freeze the tumor tissue and cause tumor damage [183]. Besides, they cool down rapidly, and it is feasible to regulate the “growth direction” and “direction of the ice ball” (Fig. 6).

Diagrammatic representation of NPs in cryosurgery

When the location of the tumor makes it not feasible for cryosurgery or if other adjacent organs are at risk, there are high chances that the freezing can damage healthy tissue. Recently, phase change materials (PMs) made up of NPs are used to protect the adjacent normal healthy tissue during cryosurgery [184]. For instance, liposome-based microencapsulated phase change NPs have shown incredible results in protecting surrounding healthy tissue [185]. These NPs are deemed to possess large latent heat and low thermal conductivity, making them perfect for cryosurgery.

Significant Challenges in the Clinical Application of Nanoparticles

At present, as nanotechnology has bloomed, the amount of knowledge and research put into nanoparticles has steeply raised. But only a few of them actually make it up to clinical trials. Most of them only halt at in vivo and in vitro stages. Each individual nanoformulation has particular challenges in their clinical translation, but most NPs face similar challenges that can be divided into biological, technological, and study-design related.

Biological challenges include lack of routes of administration, tempering biodistribution, the channel of NPs across the biological barriers, their degradation, and toxicity [186]. NPs are usually injected via intravenous injections directly into the blood, which takes away NPs, making it challenging to stay and interact with the target site. As a result, a high concentration drug is used, which might not provide desired therapeutic effects [187]. However, magnetic NPs can be used to overcome this as many in vivo and in vitro studies have proved the usage of 3D magnetic fields to control the movement of NPs against blood flow. But, the effect of magnetic fields on the human body, crosstalk between magnetic fields, and a large number of NPs has to be researched upon.

Controlling the biological fate of NPs is very hard and needs a lot of focus. Even though NPs are made up of biosafety materials and are modulated accordingly to increase the retention time and half-life, there runs a risk of lung, liver and kidney damage. Some factors that govern toxicity are surface area, particle size and shape, solubility, and agglomeration [188]. NPs have shown greater deposition in the lung with inflammatory, oxidative and cytotoxic effects [189]. Studies reveal that healthy cells often suffer from free radicals generated by NPs [190]. Fabricating NPs with more biocompatible substances like chitosan and materials that disintegrate after near infrared light irradiation may be potential solutions.

Another tricky challenge is avoiding the “mononuclear phagocytic system (MPS).” In biological fluids, NPs adsorb proteins to produce PC, which attacks MPS to uptake NPs. To escape this, NPs have been coated with materials that prevent the formation of the protein corona. However, they have not shown any significant results. Designing NPs that target “macrophages” and using those as new drug vehicles can be pitched to overcome this problem. Currently, preventing macrophage recruitment, depleting and reprograming TAMs, and obstructing “CD47-SIRPα pathways” are commonly used strategies [191].

Technological challenges of NPs include scale-up synthesis, equal optimization, and performance predictions. These are very crucial in safeguarding the clinical success of NPs. Most of the NPs that are used in vivo and in vitro studies are usually produced in minor batches, and scale-up for huge quantities is not constantly feasible given instrumentation and other reasons. The lead clinical candidates that prove to be the best in animal models are not systematically designed optimized. To overcome this, we can use certain methods that can test numerous nanoformulation and by selective iterations selecting a single optimized formulation [192,193,194]. However, such hits shouldn’t be introduced directly in human testing. Predicting nanoparticle efficacy and performance is hard and replicating the in vivo results in human trials is a herculean task. Computational or theoretical modeling along with experimental results can be designed to imitate physiological tissue and surrounding. For instance, organs-on-chips are being actively studied and can improve NP predictions of efficacy and performance.

Study-design challenges like study size, intent, and timing of NP therapies during the therapy impact significantly during clinical studies. Most of the studies revolve around “cell and animal models” that may not provide comprehensible results in human trials. Therefore, the usage of a single model is tough to imitate natural reactions in the human body. In addition, “models of cancer metastasis” should be actively researched as metastasis is one of the significant properties of cancer. Moreover, N  = 1 clinical studies will be required if we focus on personalized medicine. This needs to count in many factors such as genetic, environmental, and past medical history. [195, 196]. Another major challenge is that NPs are never used as first-line therapies. Although we have effectively approved nanoformulations, they are usually saved for further treatment if disease progression is found in the clinical trial scenario. Most of the patients have either had progressed on multiple lines of therapies or have gained drug resistance. These situations often skew the clinical trial results and lessen the chance of NP treatment to benefit those who are likely still treatable.

Conclusion and Future Perspective

Nanotechnology has shown a promising new era of cancer treatment by delivering small molecules for cancer detection, diagnosis, and therapy. Cancer therapies based on the exceptional features of NPs are being vastly used in the clinical setting of several cancer types. NP-based DDS is linked with enhanced pharmacokinetics, biocompatibility, tumor targeting, and stability compared to conventional drugs. Moreover, NPs provide an excellent platform for combination therapy which helps in overcoming MDR. With increasing research, several types of NPs, such as polymeric NPs, metallic NPs, and hybrid NPs, have shown improved efficacy of drug delivery. Researchers must be well attentive to the features of the nominated nanoplatforms and the properties of therapeutic agents. However, there are certain limitations like deficiency of in vitro models that precisely replicate in vivo stage, immunotoxicity, the long-term toxicity, and neurotoxicity. Although “nanovaccines” and “artificial APCs” have proved improved efficacy compared to conventional immunotherapy, the clinical efficacy is substandard. The safety and tolerance of these new modalities should to be inspected. Additionally, developing “immunomodulatory factor-loaded NPs” may advance the efficiency of vaccines for immunotherapy.

This is an emerging area, and it is anticipated that with growth in proteomics research on the “mechanism of cancer origin, MDR, occurrence,” more NP-based drugs can be exploited. Compared to the mammoth amount of investigations, only a few NP-based drugs are actually in use, a few others in clinical trials, and most in the exploratory stage. For rational nanotechnology design, more efforts must be reserved in “understanding toxicity, cellular and physiological factors that regulate NP-based drug delivery, EPR, and PC mechanism” in the human body. Based on the evidence cited above, we presuppose that the revolution in clinical translation for NP-based cancer therapy will be attained with nanotechnology and cancer therapy development.

Availability of Data and Materials

Not applicable.

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