Emerging Frontiers in Antioxidant Therapeutics: From Nanotechnology-Enabled Delivery to Mitochondria-Targeted Interventions

2.1. What the body makes versus what we eat

Every time a redox reaction happens, it is accompanied by the continuous formation of free radicals and reactive oxygen species. The organism has created defence mechanisms to keep the level of these aggressive species under control and thus avoid the risk of losing the cells to oxidative damage. The natural antioxidant system of the body, composed mainly of enzymes, is efficient. The superoxide dismutase (SOD) enzyme turns superoxide radicals into hydrogen peroxide, while catalase (CAT) converts the peroxide to water and oxygen. GPx, on the other hand, removes both hydrogen peroxide and lipid peroxides from the system. Enzymatic antioxidants function alongside low molecular weight antioxidants with glutathione (GSH) being the most important intracellular antioxidant; coenzyme Q10 that safeguards mitochondrial membranes; melatonin that neutralizes hydroxyl radicals; and metabolic waste products such as uric acid and bilirubin that are reactive oxygen species (ROS) scavengers. The body continuously modulates these defences, with production during stress being higher and during homeostasis being lower.^[8,9] Exogenous antioxidants are mainly provided by food, although dietary supplements and pharmaceuticals are becoming increasingly important sources. The water-soluble vitamin C acts in the prevention of oxidative damage in water-soluble environments, whereas vitamin E, which is fat-soluble, performs the same function for lipids found in cell membranes. The consumption of fruits and vegetables provides the body with polyphenols derived from plants, such as quercetin and resveratrol, and carotenoids, like lycopene. Essential minerals are crucial: selenium is a requirement for GPx activity, zinc supports SOD, and copper is needed for SOD catalysis; however, free copper ions can also cause oxidative damage.

2.2. Enzymes versus small molecules

Enzymatic antioxidants act as catalysts and speed up reactions without being used up. A single enzyme molecule can neutralize thousands of reactive species. Three types of SOD function in different parts of the cell: Cu/Zn-SOD (SOD1) is in the cytoplasm, Mn-SOD (SOD2) is in mitochondria, and extracellular SOD (SOD3) is in the extracellular space. All of them convert superoxide to hydrogen peroxide, but their different locations provide protection across various cellular areas. Catalase is one of the fastest known enzymes and is highly concentrated in peroxisomes. It breaks down hydrogen peroxide at rates over one million molecules per second. GPx works more slowly but is more versatile; it breaks down both hydrogen peroxide and lipid peroxides that form during oxidative damage to membranes. GPx needs selenium as a cofactor and uses glutathione as a substrate. Other enzymatic antioxidants include thioredoxin reductase, peroxiredoxins, and heme oxygenase-1, each targeting various substrates or cellular compartments.^[10,11] Non-enzymatic antioxidants work by sacrificing themselves. Vitamin C donates electrons to neutralize water-soluble radicals, becoming oxidised in the process, though cellular reducing systems can regenerate it. Vitamin E is found in membranes where it stops lipid peroxidation chain reactions. Glutathione is the main intracellular redox buffer, maintaining reducing conditions and serving as a substrate for protective enzymes. Plant polyphenols, which come in many forms, scavenge radicals, bind pro-oxidant metals, and influence cellular signalling pathways.^[12,13]

2.3. Three mechanisms of protection

Three major approaches by which antioxidants may protect are depicted in Figure 2. One approach (a simplistic one) is directly radical scavenging. It refers to the giving of an electron or a proton to a very reactive species. This makes the radical more stable and ends further destruction. Vitamins C and E are two such examples. In the same way, polyphenols and flavonoids generally generate stable radicals that do not give rise to chain reactions. The latter purpose is solved by metal chelation. One such reaction is the ability of free transition metals, such as iron and copper, to transform hydrogen peroxide into hydroxyl radicals, one of the most damaging reactive species in biological systems. This form of chemistry, known as Fenton or Haber-Weiss chemistry, takes place normally in living organisms in the presence of both metals and peroxides. Chelators chelate these metals, creating unreactive complexes that prevent radical generation at its origin.^[13] The most cumbersome way to enhance the antioxidative potential of the cells is by transcriptional activation. Instead of scavenging individual radicals, some compounds can activate specific genetic programs to increase the overall capacity of the particular cell to mount an antioxidant defence. The Nrf2-Keap1-ARE pathway governs this response process. In times of normalcy, the Keap1 protein binds to a transcription factor called Nrf2 and targets it for degradation; this interaction is disrupted when oxidative stress or specific chemicals modify Keap1. After being launched into the nucleus, Nrf2 binds to DNA at sites called antioxidant response elements and activates dozens of defence genes. Sulforaphane from broccoli sprouts, curcumin, resveratrol, and synthetic agents such as bardoxolone are examples of activators that are produced via this pathway.^[14,15] In this way, the cells learn to better defend themselves through increased production of SOD, GPx, glutathione synthesis enzymes, and detoxification systems.

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Figure 2:

The three fundamental mechanisms of antioxidant action: direct radical scavenging through electron donation, metal chelation to inhibit radical formation, and transcriptional activation of endogenous antioxidant genes. ROS: Reactive oxygen species.

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3.1. Nanoformulations enhancing solubility and bioavailability

Usually, a natural antioxidant like curcumin, resveratrol, or quercetin would have tremendous redox-modulating ability; however, these compounds have little water solubility and low absorption when taken orally. Nanometre delivery systems have come up as one solution to this problem. Liposomes are among the early systems. They are composed of one or more biocompatible phospholipid layers encapsulating compounds that can be either water-soluble or fat-soluble, protecting them from degradation and sometimes allowing longer circulation in the body by PEGylation. Animal studies have revealed that liposomal formulations of curcumin show up to 9 times higher bioavailability for neuroinflammation and memory decline than free curcumin.^[16,17] SLNs (Solid Lipid Nanoparticles) and NLCs (Nanostructured Lipid Carriers) help antioxidants overcome the real-world challenges of oxidation and enzymatic degradation. SLNs carry a solid lipid core just supported by surfactants, allowing for a sustained release. The presence of liquefied lipid in NLCs increases opportunities for drug loading and shelf life. These systems provide a successful delivery for vitamin E and coenzyme Q10 to treat skin aging and heart disorders.^[18,19] Among polymeric nanoparticles, those made from poly (lactic-co-glycolic acid) (PLGA) allow very precise tuning of the rate of drug release and attachment of targeting ligands. Quercetin-loaded PLGA nanoparticles demonstrate better neuroprotective effects in stroke and brain injury models. Nanoemulsions are considerable stability mixes of small oil droplets and help in solubilizing poorly water-soluble antioxidants like carotenoids. Self-nanoemulsifying drug delivery system (SNEDDS) create nanoemulsions at the gastrointestinal tract, increasing absorption and plasma levels.^[20,21] Listed in Table 2 are some major nanocarrier systems, including liposomes, SLNs, PLGA-based polymeric nanoparticles, and nanoemulsions, while playing roles in improving antioxidant solubility, stability, bioavailability, and targeted delivery. These nanotechnologies successfully enhance antioxidative treatments in various disease models overcoming the shortcomings found in traditional formulations.

3.2. Tissue- and organelle-targeted nanodelivery

Stimuli-responsive nanocarriers constitute a major advancement in targeted antioxidant delivery, as they enable site-specific release in diseased or inflamed tissues while minimizing exposure to healthy cells. These systems are designed to respond to pathological microenvironmental cues, most notably pH, ROS, and temperature. pH-responsive platforms take advantage of the acidic milieu characteristic of tumours and endosomes (pH 4.5–6.8) through acid-cleavable linkers or pH-sensitive polymers that release antioxidants upon reaching these sites.^[22-25] ROS-responsive nanocarriers incorporate redox-cleavable structures such as thioketal, boronic ester, or peroxalate moieties that degrade under elevated oxidative stress, providing a smart feedback mechanism in which increased ROS levels trigger drug release; thioketal nanoparticles loaded with curcumin are a representative example showing enhanced efficacy in ROS-rich tumour environments.^[26] Thermo-responsive carriers, often based on polymers such as poly(N-isopropylacrylamide) (PNIPAM), undergo conformational transitions at defined temperatures, enabling controlled release during hyperthermia and allowing synergistic use with focused ultrasound or external heating modalities.^[27] Emerging multi-stimuli systems integrate combinations of ROS, pH, and thermal triggers to achieve even finer control of antioxidant deployment, offering a promising direction for precision-targeted redox therapy. Figure 3 is a schematic representation of major nanocarrier platforms used for site-specific antioxidant delivery. The figure illustrates (i) nanoemulsions designed for improved solubility and controlled release of lipophilic and hydrophilic antioxidants; (ii) liposomal structures showing multilayered phospholipid organisation enabling encapsulation of diverse antioxidant agents; and (iii) nanoparticles with either inherent antioxidant properties (e.g., melanin-, metal oxide-, lignin-, or carbon-based systems) or functionalised surfaces for targeted delivery, magnetic guidance, and encapsulation of synthetic or natural antioxidants. Together, these systems enhance stability, bioavailability, and microenvironment-responsive release, supporting precision tissue- and organelle-targeted antioxidant therapy.

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Figure 3:

Stimuli-responsive nanocarrier systems for targeted antioxidant delivery.

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3.3. Limitations and challenges of nanocarrier-based antioxidant delivery

Despite significant progress in nano-enabled antioxidant delivery, several limitations continue to impede clinical translation. A primary concern involves the potential toxicity of nanomaterials, as their small size and high surface reactivity may lead to unintended interactions with immune cells, off-target tissue accumulation, oxidative imbalance, or long-term retention in organs such as the liver and spleen. Manufacturing scalability also poses major challenges; many nanocarrier systems require complex synthesis steps, specialised equipment, and meticulous quality control, making it difficult to achieve reproducible, cost-effective production at industrial scale. Stability issues—including nanoparticle aggregation, premature drug leakage, and degradation during storage—may further compromise therapeutic consistency. In addition, regulatory frameworks for nanomedicine remain underdeveloped, with limited consensus on safety testing requirements, biodistribution evaluation, and long-term toxicity assessment. These scientific, technical, and regulatory barriers highlight the need for cautious interpretation of preclinical results and underscore the importance of rigorous standardisation before nanoformulated antioxidants can be widely adopted in clinical practice.

4.1. Modern perspectives on plant-derived antioxidants

It is Curcumin, the main bioactive compound in turmeric, and phytochemicals that have garnered interest from researchers due to its vast biological properties. Free radicals are scavenged by it, along with many transcription regulators such as Nrf2, NF-κB, and AP-1, enzymes such as iNOS and COX-2, and intracellular enzymes such as mitogen-activated protein kinases (MAPKs) and Akt.^[28] In its full capacity of anti-inflammatory and neuroprotective effects, it has very little clinical use because its oral bioavailability is less than 1% and it is rapidly eliminated from the body. Multiple nanocarrier, phospholipid complex, and analog formulations have been designed to enhance the pharmacokinetics of curcumin. On these reformulations, clinical trials have indicated that such disorders as neuroinflammation, mood disorders, and systemic inflammatory syndromes respond.^[28,29] Resveratrol brings a style of suspicion on longevity with the presence of stilbene in grapes and red wine. This molecule activates silent mating type information regulation 2 homolog 1 (SIRT1), key to mitochondrial integrity, stress resistance, and metabolic homeostasis, as well as enhancing the Nrf2-dependent expression of antioxidants genes.^[29] Possibly, its biological action is hormetic, where the action is observed at low concentrations by stimulation of adaptive stress responses. But, it is rapidly metabolized and exhibits poor bioavailability, thereby hampering its clinical application; this has spurred interest in the development of resveratrol analogs and delivery vehicles.^[29] Quercetin shows antioxidant activity and preferentially induces apoptosis of senescent cells that sustain a microenvironment for chronic inflammation and tissue aging through mechanisms such as metal ion chelation, free radical scavenging, and activating the Nrf2 pathway. Preliminary and preclinical aging models suggest that quercetin, in combination with dasatinib, significantly ameliorates physical function and proinflammatory cytokine expression.^[30,31] Other important phytochemicals are:

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  Epigallocatechin gallate (EGCG) from green tea gives protection to neurons in metabolic regulation.

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  Sulforaphane is an intense activator of the Nrf2 pathway present in crucifers.

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  Pterostilbene: higher bioavailability than resveratrol

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  Anthocyanins from berries are considered cardiovascular and cognitive health agents.

Natural source overlooked research could still reveal new antioxidant compounds.^[32,33]

4.2. Synergy and combinatorial effects among polyphenols

Studies exhibit combinations of polyphenols exert greater effects in comparison to singular compounds because of redox-based cooperation, enzyme activities, and bioavailability. Possible mechanisms of synergy include:

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  Recycling of oxidised antioxidants

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  Activation of complementary signalling pathways

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  Inhibiting metabolic enzymes that otherwise would limit absorption

This suggests that the Mediterranean diet, with its polyphenols complexly mixed in wine, olive oil, fruits, and vegetables, spells out how natural food products tend to offer higher biological effects when consumed together rather than alone. Several synergistic mixtures illustrate this point. For instance, quercetin combined with EGCG exhibits potent anticancer synergy by promoting cell-cycle arrest and apoptosis. Similarly, resveratrol combined with pterostilbene offers superior benefits for cognitive and metabolic health. Another notable example is curcumin with piperine, where piperine inhibits glucuronidation in the liver, thereby increasing curcumin’s bioavailability by an estimated 2000%.

Generally, whole food extracts display higher potency than phenolics or any other isolated compounds; thus, the so-called matrix effect is said to exist. The set-up provides further rationale for standardised botanical formulations that retain native-type interactions amongst themselves. Researchers have begun to employ metabolomics and network pharmacology in attempts to shed light onto the interaction networks that govern polyphenol synergy and that may then allow for rational bioactive combination designs.^[34-36]

4.3. Innovative extraction technologies for phytochemical antioxidants

The actions of phytochemical antioxidants depend on the natural activity of the compounds and on their extraction methods. These methods account for extract yield, purity, and stability. Traditional or old-school means-at-day extraction by using solvents mainly at high temperature-may destroy the heat-sensitive antioxidants or sometimes leave solvent residues. Some of these methods are considered greener and better choices: Supercritical fluid extraction (SFE) uses CO₂ above its critical point (31°C, 74 bar). In some respects, this acts as a gas in diffusion and as a liquid in solvation. It is an extraction technique in the absence of oxygen and organic solvents, suitable for lipophilic compounds such as tocopherols and carotenoids.^[37,38] These co-solvents may be helpful in broadening the variety of phytochemicals extracted: ethanol being one. Ultrasound-assisted extraction (UAE) uses acoustic cavitation to degrade plant matrices and assist mass transfer. UAE extraction takes place at a lower temperature, better preventing thermal degradation and promoting a higher extraction rate for polyphenols and flavonoids.^[39] Microwave-assisted extraction (MAE), on the other hand, selectively heats polar plant tissues to speed up extraction while maintaining bioactivity. Rapid processing time and good application with green solvents are also offered. Enzyme-assisted extraction (EAE) gives polyphenols the free run by using pectinases and cellulases to degrade the cell walls of plants.^[40]

5.1. Mitochondria as sources and sites of ROS damage

Mitochondria perform a dual function in cellular life and death-they are a cellular power generator on the one side and a major reactive oxygen species producing centre on the other. During oxidative phosphorylation, electrons occasionally escape from the electron transport chain (mainly complexes I and III) and reduce oxygen prematurely to superoxide radicals. Normally, mitochondrial antioxidants (MnSOD and glutathione peroxidase) scavenge these ROS. From pathological states (aging, metabolic syndrome, neurodegeneration), maybe the excessive production of ROS generally goes beyond their scavenging mechanisms, leading to damage to mtDNA, lipid peroxidation (especially cardiolipin), protein dysfunction, and impaired adenosine triphosphate (ATP) production.^[41]

5.2. Novel mitochondria-targeted antioxidants

The classic antioxidants like vitamins C and E had very poor accumulation in mitochondria. Therefore, mitochondria-targeted novel compounds were developed to localise in this organelle and scavenge ROS at its source. Figure 4 of the attached image from the document shows how the agents get accumulated in mitochondria to hamper ROS-induced cellular damage.^[41] Table 3 lists some important mitochondrial antioxidants such as MitoQ, SkQ1, SS-31, and MitoTEMPO along with their targeted strategies, status of development, and clinical or preclinical applications. These compounds are consigned to accumulate within mitochondria with the aim of removing oxidative damage and increasing bioenergetics to counteract diseases linked to mitochondrial dysfunction.

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Figure 4:

Mitochondria-targeted antioxidants and their mechanisms of action. ROS: Reactive oxygen species; MitoQ: Mitoquinone; SkQ1: Plastoquinonyl decyltriphenylphosphonium.

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As seen in Figure 4, MitoQ, SkQ₁, etc., utilise triphenylphosphonium (TPP) moieties for translocation across mitochondrial membranes. From there, they go on to scavenge lipid ROS and prevent cardiolipin oxidation-an event that finally initiates mitochondrial apoptosis. Unlike these TPP conjugates, SS-31 binds to cardiolipin on the inner membrane and maintains the structural organisation, thus improving bioenergetics. XJB-5-131, an emerging peptide-mimetic, scavenges matrix-localised ROS by virtue of its peculiar peptide backbone and antioxidant groups.^[42,43]

5.3. Therapeutic impact in age-related and metabolic diseases

Mitochondrial antioxidants therapeutics intervene at an upstream level in the pathological cascade. By protecting mitochondrial function, they avert downstream effects such as inflammation, insulin resistance, neurodegeneration, and tissue aging. Major Therapeutic Advantages include:^[44-47]

• Increasing Insulin Sensitivity: MitoQ and SkQ₁ are believed to reduce mitochondrial ROS production in adipocytes and skeletal muscles, which in turn accounts for improvements in insulin signalling and glucose uptake.

• Protecting Muscle from Age-Related Decline: Both SS-31 and its congeners inhibit mitochondrial dysfunction and muscle wasting in aged animals, thus enabling muscle function to be maintained.

• Abating Neurodegeneration: A considerable potential for applications in Parkinson’s models is afforded since MitoQ and XJB-5-131 protect dopaminergic neurons against oxidative stress.

• Cardioprotective Action: Shown in clinical trials, SS-31 prevents ischemia reperfusion injury by preservation of mitochondrial membrane potential and avoidance of calcium overload.

Note that Table 4 displays major antioxidants in many physiological and pathological conditions with their respective modes of action and clinical evidence. While some antioxidants seem to show promise, such as CoQ10 in early Parkinson’s disease and curcumin in rheumatoid arthritis, the rest show rather inconsistent and weak evidence against their claim, depending on formulation and dosage, and at times even the stage of the disease. The table arranges antioxidants based on their disease causality, facilitating a broader perspective on their redox-modulatory functions and therapeutic relevance.^[48-51]