Reduction-sensitive nanoparticles (RSNP) consist of nanocarriers that are chemically responsive to reduction. Drug delivery systems using RSNP can be loaded with different drugs that are designed to be released within a concentrated reducing environment, such as the tumor-targeted microenvironment. Reduction-Sensitive Nanoparticles provide an efficient method of targeted drug delivery for the improved controlled release of medication within localized areas of the body.
Nanoparticles are small in size with maximized surface area and have an enhanced level of solubility; these elements result in an improved bioavailability. [1] [2] Reduction-Sensitive Nanoparticles are nanoparticles that are responsive to reduction signaling environments. Redox-Sensitive Nanoparticles can be responsive to signaling through a reduction activation or an oxidative activation. Therefore, degradation of chemical bonds can be either activated through oxidants or reductants in the localized area. The cleavage/degradation of chemical bonds will enable the drugs loaded within the nanoparticle to be released into the body. Depending on the activation mechanism, Redox-Sensitive Nanoparticles can be associated with Reduction-Sensitive Nanoparticles if the chemical activation method is through reduction.
Nanoparticle Drug Loading is dependent on the mass ratio of the drug being loaded and the drug-loaded nanoparticle. [3] Variations necessary to consider are the pore volume size, the surface, shape, and charge of the nanoparticle. The mode of drug loading will depend on the type of drug being administered, which will vary depending on the illness that is treated.
One of the limitations of nanoparticles for drug delivery is the insufficient or slow release of drugs. The rate of release is a critical element to identify how slowed drug release could limit the proper concentration of treatment. If the drug is not administered in concentrations high enough it could result in undertreatment of tumor cells with little to no effect. Concentration thresholds must be met to initiate cell death amongst tumor cells. However, the uncontrolled release of treatment could also permit adverse side effects. RSNPs have improved rates of drug release which improves the medication concentrations that can be administered to a specific area.
RSNPs consist of reduction or redox-sensitive bonds. After administration in the body, the RSNP will eventually come into contact with the tumor microenvironment (TME). Nanoparticles can be synthesized to activate when exposed to selective characteristics of the tumor microenvironments. TMEs depict unique characteristics that create a differing microenvironment in comparison to healthy tissue. Thus, nanoparticles can be designed to react to the unique elements of TMEs such as the formation of a reducing environment. The reducing abilities of the TMEs are due to the expression of reducing agents. [4] RSNPs are formulated to express reduction-sensitive bonds that are cleaved when exposed to reducing agents. After the reduction occurs the degradation of the nanoparticles commences and the loaded drugs begin to release.
The physicochemical characteristics of nanoparticles are inclusive of the size, shape, chemical composition, stability, topography, surface charge, and surface area. [5] Deviations of these characteristics can be impacted by the classification of the nanoparticle. For example, the RSNP can be classified as a polymeric, micelle, or lipid-polymeric hybrid. [6] [7] The reduction sensitivity of nanoparticles is reliant on the reduction-responsive chemical structures infused into the nanoparticle. Reduction occurs when the number of electrons increases in a chemical species. Reduction sensitive nanoparticles depict high plasma stability and quick responsiveness/activation. [2] The reducing environment of tumor cells is greatly impacted by the oxidation and reduction states of NADPH/NADP+ and Glutathione. [8] [9]
For the effective application of RSNPs, the physicochemical characteristics of the tumor microenvironment must also be considered. The characteristics depicted by the TME are tumor hypoxia, angiogenesis, metabolism, acidosis, reactive oxygen species (ROS), etc. [4] The elements of the tumor microenvironment can affect the reduction-inducing environment. Tumor cells abnormally regulate redox homeostasis leading to differences in the redox balance and increases in ROS levels. Research trends have shown that increased levels of ROS are correlated with high levels of antioxidant activity, such as intracellular GSH. [4]
Glutathione (GSH) or γ-glutamyl-cysteinyl-glycine is a critical biological reducing agent for drug delivery applications; it creates an effective reducing environment in the cytosol and nucleus of a cell. [8] [9] Glutathione is an antioxidant that is naturally produced in the liver and takes part in tissue building, tissue repair, immune responses, chemical production, and protein production. GSH is also a significant signaler of cell differentiation, proliferation, apoptosis, and ferroptosis. [9] Furthermore, the glutathione concentration in the tumor microenvironment is reportedly at least four times higher compared to regular tissue. This is due to the high metabolic needs of tumor cells; for example, the rapid proliferation rates of tumor cells. [8]
The over-expression of nicotinamide adenine dinucleotide phosphate NADPH can lead to higher ROS levels. NADPH has a lower concentration than GSH in the reducing environment. [9] NADPH is an electron donor that exists among all organisms; additionally, the NADPH is used as a source of reduction to drive anabolic reactions and redox balances. The reduction and oxidation states of NADPH/NADP+ will influence the reduced responsiveness of the environment. Cancer cells express a unique NADPH homeostasis due to the adaptive alterations of signaling pathways and metabolic enzymes. [10]
Redox-Sensitive Nanoparticles with Disulfide bonds are commonly observed in medical research. RSNP can consist of disulfide bonds that are cleaved and introduced to a reduction condition. Additionally, the reduction of glutathione results in the formation of sulfhydryl groups. [8] [9] In large concentrations of GSH, the disulfide bonds are capable of being cleaved. Following the activation process, the degradation of the drug carrier results in the drug release. These linkages are commonly used between hydrophilic and hydrophobic segments in copolymers. Moreover, RSNP's hydrophilic shells will degrade in response to the reducing environment. The disulfide bonds are used as linkers and cross-linking agents. Disulfide bonds can be expressed attached to the side chains, the backbone, on the surface, and as linkages between moieties. [9] [11]
Disulfide bonds can also act as cross-linking agents in micelles nanoparticles. Micelles lack the structural stability as a nanocarrier for drug delivery. The lack of stability can result in the loss of drugs after administration and before reaching the infected area. This occurrence can potentially cause adverse side effects from the improper release of medication. Disulfide bonds can be used as crosslinked structures to increase the structural stability of micelle nanocarriers. In general, these crosslinks are located in the shell or the core of micelles nanoparticles. [9] [12] [13]
Redox-Sensitive Nanoparticles with Diselenide bonds share comparable reduction responsiveness to disulfide bonds. Diselenide consists of two selenium atoms along with an additional element or radical. Diselenide bonds are dynamic covalent bonds that can be exchange between molecules. Diselenide bonds have an estimated bond energy of 172 kJ/mol, and disulfide bonds have estimated bond energy of 268 kJ/mol; the lower bond energy holds a higher potential to design an increased sensitive redox-responsive delivery. [9] Diselenide bonds have been observed to be attached to hydrophobic parts of amphiphilic triblocks or hyperbranched copolymers to create micelles. [8]
Succinimide-thioether linkages express sensitivity to reducing environments and can be cleaved as a result. Succinimide-thioether bonds show slower rates of release in comparison to disulfide bonds; however, succinimide-thioether nanoparticles are still sensitive to the reducing environment and are cleaved by GSH for fast intracellular release. [9]
Nanoparticles with Trimethyl Benzoquinone have demonstrated responsiveness to reduced environments. The experiments that have been conducted testing TMBQ are limited in observing the full scope of TMBQ nanoparticles in delivery systems. [9]
The synthesis of reduction sensitive nanoparticles is dependent on the mechanism subtype of the nanoparticle. Additionally, the synthesis can vary within subtype classes depending on how the different reduction sensitive bonds are expressed. The deviations of RSNPs can range from attachments to the backbone, side chains, on the surface, etc. Research has been conducted with reduction sensitivity mechanisms using polymeric, lipid-polymer hybrids, and micelles nanoparticles. The production methods would be dependent on the delivery method design for the nanoparticle. Polymeric nanoparticle synthesis occurs from the addition of electrolyte-saturated or a nonelectrolyte-saturated solution with a water-miscible solvent; additionally, the mixture should be constantly stirred. Lipid micelles are formed by amphiphilic molecules through self-assembly. Lipid-polymer hybrids have multiple synthesis methods which consist of the single-step method, the two-step method, nanoprecipitation, emulsification-solvent evaporation, and a non-conventional two-step method. [6] [7] [14] [15]
Reduction Sensitive Nanoparticles provide a mode of localized drug delivery by targeting elements of the tumor microenvironment. RSNP has the advantages of high stability when adhering to hydraulic degradation, fast responsiveness to the intracellular reducing environment, and drug release occurs in the cytosol and cell nucleus. Furthermore, drug release in the cytosol and cell nucleus has shown the potential to effectively administer treatment of more potent and poorly soluble anticancer drugs. [16] The quick-release of RSNPs has the potential to offer an effective treatment for multidrug-resistant tumors. This addresses an important limitation of nanoparticles. Nanoparticle drug delivery often exhibits slow drug release. The slow release can lead the nanomedicine to be released at low concentrations; moreover, these limited concentrations inhibit the cell death of the tumor cells. Polymeric RSNPs have shown improved solubility, stability, biocompatibility, and decreased drug toxicity; for example, carbohydrate polymers. [17]
The effectiveness of reduction-sensitive nanoparticles is dependent on the responsiveness of the RSNP throughout the body. The microtumor and inflammatory environments contain higher concentrations of reducing agents in contrast to healthy cells; however, healthy cells still express GSH and NADPH. RSNPs are designed to be receptive to higher concentrations of reducing agents for the ability to distinguish between cancer cells and healthy cells. Furthermore, the other limitations are dependent on other characterizations, such as the type of nanoparticle; For example, micelles nanoparticles' lower levels of physical stability which can lead to drug loss and release in unwanted locations. Additionally, polymeric nanoparticles cannot effectively target the tumor and often undergo drug release too early. [8]
Reduction Sensitive Nanoparticles are used as nanomedicines for drug delivery. As nanocarriers, RSNP can be loaded with drugs for disease therapeutics. [18] This is commonly observed in the use of tumor and cancer treatments. Cancer cells create reducing environments that are used for RSNP activation. RSNPS can also increase the penetration of cancer treatment to the cancer cells. Specific applications include, but are not limited to Breast Cancer, Liver Cancer (hepatoma), Melanoma, Lung Cancer, Malignant Glioma, Ovarian Cancer, Cervical Cancer, Subcutaneous EAT, Pancreatic Cancer, Colon Cancer, Prostate Cancer, etc. [2] [8] [19] [20] [21]
The development of RSNP for inflammatory diseases has been explored to a lesser extent. Regardless, in more recent years reduction-sensitive and redox-sensitive nanoparticles have gained more momentum in the realm of inflammatory diseases. Further advances have demonstrated Research has been conducted to evaluate the potential of RSNP as a therapeutic for inflammatory bowel disease. The activation mechanism consisted of pH and redox sensitivity. The outcomes of the experiment demonstrated higher selectivity to the reducing potential; therefore establishing the promising potential of RSNPs for the treatment of inflammatory bowel disease. [22] Other studies have demonstrated potential applications as activatable magnetic resonance contrast agents. These proposed agents would help detect and monitor the treatment of inflammatory diseases by applying redox dysregulation. [23] [24] [25]
Nanomedicine is the medical application of nanotechnology. Nanomedicine ranges from the medical applications of nanomaterials and biological devices, to nanoelectronic biosensors, and even possible future applications of molecular nanotechnology such as biological machines. Current problems for nanomedicine involve understanding the issues related to toxicity and environmental impact of nanoscale materials.
Thioredoxin reductases are enzymes that reduce thioredoxin (Trx). Two classes of thioredoxin reductase have been identified: one class in bacteria and some eukaryotes and one in animals. In bacteria TrxR also catalyzes the reduction of glutaredoxin like proteins known as NrdH. Both classes are flavoproteins which function as homodimers. Each monomer contains a FAD prosthetic group, a NADPH binding domain, and an active site containing a redox-active disulfide bond.
Dendrimers are highly ordered, branched polymeric molecules. Synonymous terms for dendrimer include arborols and cascade molecules. Typically, dendrimers are symmetric about the core, and often adopt a spherical three-dimensional morphology. The word dendron is also encountered frequently. A dendron usually contains a single chemically addressable group called the focal point or core. The difference between dendrons and dendrimers is illustrated in the top figure, but the terms are typically encountered interchangeably.
Respiratory burst is the rapid release of the reactive oxygen species (ROS), superoxide anion and hydrogen peroxide, from different cell types.
A drug carrier or drug vehicle is a substrate used in the process of drug delivery which serves to improve the selectivity, effectiveness, and/or safety of drug administration. Drug carriers are primarily used to control the release of drugs into systemic circulation. This can be accomplished either by slow release of a particular drug over a long period of time or by triggered release at the drug's target by some stimulus, such as changes in pH, application of heat, and activation by light. Drug carriers are also used to improve the pharmacokinetic properties, specifically the bioavailability, of many drugs with poor water solubility and/or membrane permeability.
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A nanogel is a polymer-based, crosslinked hydrogel particle on the sub-micron scale. These complex networks of polymers present a unique opportunity in the field of drug delivery at the intersection of nanoparticles and hydrogel synthesis. Nanogels can be natural, synthetic, or a combination of the two and have a high degree of tunability in terms of their size, shape, surface functionalization, and degradation mechanisms. Given these inherent characteristics in addition to their biocompatibility and capacity to encapsulate small drugs and molecules, nanogels are a promising strategy to treat disease and dysfunction by serving as delivery vehicles capable of navigating across challenging physiological barriers within the body.
Sonodynamic therapy (SDT) is a noninvasive treatment, often used for tumor irradiation, that utilizes a sonosensitizer and the deep penetration of ultrasound to treat lesions of varying depths by reducing target cell number and preventing future tumor growth. Many existing cancer treatment strategies cause systemic toxicity or cannot penetrate tissue deep enough to reach the entire tumor; however, emerging ultrasound stimulated therapies could offer an alternative to these treatments with their increased efficiency, greater penetration depth, and reduced side effects. Sonodynamic therapy could be used to treat cancers and other diseases, such as atherosclerosis, and diminish the risk associated with other treatment strategies since it induces cytotoxic effects only when externally stimulated by ultrasound and only at the cancerous region, as opposed to the systemic administration of chemotherapy drugs.
A nanocarrier is nanomaterial being used as a transport module for another substance, such as a drug. Commonly used nanocarriers include micelles, polymers, carbon-based materials, liposomes and other substances. Nanocarriers are currently being studied for their use in drug delivery and their unique characteristics demonstrate potential use in chemotherapy. This class of materials was first reported by a team of researchers of University of Évora, Alentejo in early 1960's, and grew exponentially in relevance since then.
Gold nanoparticles in chemotherapy and radiotherapy is the use of colloidal gold in therapeutic treatments, often for cancer or arthritis. Gold nanoparticle technology shows promise in the advancement of cancer treatments. Some of the properties that gold nanoparticles possess, such as small size, non-toxicity and non-immunogenicity make these molecules useful candidates for targeted drug delivery systems. With tumor-targeting delivery vectors becoming smaller, the ability to by-pass the natural barriers and obstacles of the body becomes more probable. To increase specificity and likelihood of drug delivery, tumor specific ligands may be grafted onto the particles along with the chemotherapeutic drug molecules, to allow these molecules to circulate throughout the tumor without being redistributed into the body.
Reductive stress (RS) is defined as an abnormal accumulation of reducing equivalents despite being in the presence of intact oxidation and reduction systems. A redox reaction involves the transfer of electrons from reducing agents (reductants) to oxidizing agents (oxidants) and redox couples are accountable for the majority of the cellular electron flow. RS is a state where there are more reducing equivalents compared to reductive oxygen species (ROS) in the form of known biological redox couples such as GSH/GSSG, NADP+/NADPH, and NAD+/NADH. Reductive stress is the counterpart to oxidative stress, where electron acceptors are expected to be mostly reduced. Reductive stress is likely derived from intrinsic signals that allow for the cellular defense against pro-oxidative conditions. There is a feedback regulation balance between reductive and oxidative stress where chronic RS induce oxidative species (OS), resulting in an increase in production of RS, again.
Nanoparticle drug delivery systems are engineered technologies that use nanoparticles for the targeted delivery and controlled release of therapeutic agents. The modern form of a drug delivery system should minimize side-effects and reduce both dosage and dosage frequency. Recently, nanoparticles have aroused attention due to their potential application for effective drug delivery.
Polymer-protein hybrids are a class of nanostructure composed of protein-polymer conjugates. The protein component generally gives the advantages of biocompatibility and biodegradability, as many proteins are produced naturally by the body and are therefore well tolerated and metabolized. Although proteins are used as targeted therapy drugs, the main limitations—the lack of stability and insufficient circulation times still remain. Therefore, protein-polymer conjugates have been investigated to further enhance pharmacologic behavior and stability. By adjusting the chemical structure of the protein-polymer conjugates, polymer-protein particles with unique structures and functions, such as stimulus responsiveness, enrichment in specific tissue types, and enzyme activity, can be synthesized. Polymer-protein particles have been the focus of much research recently because they possess potential uses including bioseparations, imaging, biosensing, gene and drug delivery.
Hamid Ghandehari is an Iranian-American drug delivery research scientist, and a professor in the Departments of Pharmaceutics and Pharmaceutical Chemistry and Biomedical Engineering at the University of Utah. His research is focused in recombinant polymers for drug and gene delivery, nanotoxicology of dendritic and inorganic constructs, water-soluble polymers for targeted delivery and poly(amidoamine) dendrimers for oral delivery.
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Conventional drug delivery is limited by the inability to control dosing, target specific sites, and achieve targeted permeability. Traditional methods of delivering therapeutics to the body experience challenges in achieving and maintaining maximum therapeutic effect while avoiding the effects of drug toxicity. Many drugs that are delivered orally or parenterally do not include mechanisms for sustained release, and as a result they require higher and more frequent dosing to achieve any therapeutic effect for the patient. As a result, the field of drug delivery systems developed into a large focus area for pharmaceutical research to address these limitations and improve quality of care for patients. Within the broad field of drug delivery, the development of stimuli-responsive drug delivery systems has created the ability to tune drug delivery systems to achieve more controlled dosing and targeted specificity based on material response to exogenous and endogenous stimuli.
Gated drug delivery systems are a method of controlled drug release that center around the use of physical molecules that cover the pores of drug carriers until triggered for removal by an external stimulus. Gated drug delivery systems are a recent innovation in the field of drug delivery and pose as a promising candidate for future drug delivery systems that are effective at targeting certain sites without having leakages or off target effects in normal tissues. This new technology has the potential to be used in a variety of tissues over a wide range of disease states and has the added benefit of protecting healthy tissues and reducing systemic side effects.
Pullulan bioconjugates are systems that use pullulan as a scaffold to attach biological materials to, such as drugs. These systems can be used to enhance the delivery of drugs to specific environments or the mechanism of delivery. These systems can be used in order to deliver drugs in response to stimuli, create a more controlled and sustained release, and provide a more targeted delivery of certain drugs.
pH-responsive tumor-targeted drug delivery is a specialized form of targeted drug delivery that utilizes nanoparticles to deliver therapeutic drugs directly to cancerous tumor tissue while minimizing its interaction with healthy tissue. Scientists have used drug delivery as a way to modify the pharmacokinetics and targeted action of a drug by combining it with various excipients, drug carriers, and medical devices. These drug delivery systems have been created to react to the pH environment of diseased or cancerous tissues, triggering structural and chemical changes within the drug delivery system. This form of targeted drug delivery is to localize drug delivery, prolongs the drug's effect, and protect the drug from being broken down or eliminated by the body before it reaches the tumor.
A ligand-targeted liposome (LTL) is a nanocarrier with specific ligands attached to its surface to enhance localization for targeted drug delivery. The targeting ability of LTLs enhances cellular localization and uptake of these liposomes for therapeutic or diagnostic purposes. LTLs have the potential to enhance drug delivery by decreasing peripheral systemic toxicity, increasing in vivo drug stability, enhancing cellular uptake, and increasing efficiency for chemotherapeutics and other applications. Liposomes are beneficial in therapeutic manufacturing because of low batch-to-batch variability, easy synthesis, favorable scalability, and strong biocompatibility. Ligand-targeting technology enhances liposomes by adding targeting properties for directed drug delivery.