The mitochondria are energy-producing organelles that are present in aerobic organisms while being the primary source of reactive oxygen and nitrogen species (RONS), and possessing the ability to produce adenosine triphosphate (ATP) from the conserved redox energy of nutrients [1]. The mitochondria are usually termed the “powerhouse of the cell” mainly due to its production of ATP, and their importance for cell proliferation and survival, as well as their support to anabolism features through the generation of tricarboxylic acid (TCA) metabolites. Two of the generated metabolites are oxaloacetate and citrate, which in turn generate nucleotides and lipids, respectively [2]. The mitochondria are involved in both cell death and survival, as well as the progression of numerous diseases that include: neurodegeneration, genetic disorders, cancer, and diabetes. This is due to their crucial role in the function, regulation, and influencing the redox status, as well as signalling ion homeostasis [3]. Furthermore, it has been reported that the mitochondria are responsible for generating 90% of the intracellular reactive oxygen species (ROS) and high levels of H₂O₂, making them surrounded in high ROS environments. Yet, the high abnormal biochemical factors with excess ROS production affect several body moieties, causing irreversible oxidative damage to DNA, lipids, and proteins [4]. In addition, the mitochondria significantly influence cell ageing, as their functional decline becomes evident with age. This decline is linked to several factors, including mitochondrial DNA mutations, increased oxidative stress, diminished energy conversion efficiency, and the formation of danger-associated molecular patterns (DAMPs), which play a critical role in triggering inflammatory and immune responses, contributing further to cellular ageing and degeneration [5].

The mitochondria play a crucial role in energy production [6], cellular metabolism, and cell death regulation [7], making them ideal targets to enhance drug efficacy, particularly in cancer treatment [8]. In tumour tissues, the mitochondria can alter their metabolic phenotypes to adapt to the high demands for energy and macromolecular synthesis [9]. Additionally, they can interact with the tumour microenvironment, receiving signals from cancer-associated fibroblasts that influence mitochondrial function. Cancer cells may also adopt a hybrid metabolic phenotype, enabling them to utilise both glycolysis and oxidative phosphorylation [10]. By targetting the mitochondria, therapies can disrupt cancer cell metabolism by depriving cells of the necessary metabolites for macromolecule synthesis and producing oncometabolites [11], induce apoptosis via the mitochondrial outer membrane permeabilisation that is a critical event in the process of regulated cell death [12], and improve therapeutic outcomes while minimising side effects [13]. Further, the mitochondria can affect bone remodelling through influencing protein function and affecting stem cell mobilisation, proliferation, differentiation, and inflammation [14]. Therefore, developing novel drug carriers that can reduce ROS enhances the treatment of oxidative stress-related diseases and tissue regeneration [15]. Over recent decades, significant advances in mitochondrial biology have paved the way for the development of mitochondria-targeted therapies. This approach offers a promising strategy for improving drug efficiency, especially for diseases where mitochondrial dysfunction is a key factor, providing more precise and effective treatments for patients [16, 17].

Multiple candidates have been researched and utilised as moieties for targeting the mitochondria. One example is MitoQ, an antioxidant capable of crossing the blood-brain barrier, which influences mitochondrial respiration by creating a pseudo-mitochondrial membrane potential [18]. Another example is SkQ1, another antioxidant that can penetrate membranes when linked to positively charged cations such as Triphenylphosphonium (TPP+), allowing it to accumulate up to a thousand folds within the inner layer of the inner mitochondrial membrane [19]. Other drugs were reported including Elamipretide (SS-31), a small peptide that targets the inner mitochondrial membrane and is designed to enhance mitochondrial function in aging. It has been shown to be well-tolerated and beneficial for patients with heart failure [20, 21]. The mitochondrial targeting effect is based on Elamipretide binding to cardiolipin on the inner mitochondrial membrane through electrostatic and hydrophobic interactions. This binding stabilises cardiolipin and prevents it from converting cytochrome c into a peroxidase, preserving its electron transport role and enhancing mitochondrial function. By supporting cardiolipin, SS-31 protects mitochondrial cristae structure, promotes oxidative phosphorylation, and reduces oxidative stress, which helps maintain energy production and cellular health, especially in age-related mitochondrial decline [22, 23].

Considering the importance of mitochondria throughout the system’s biology, targeting it has been considered an approach for enhancing drug delivery and treatment. Targeting the mitochondria faces numerous barriers related to the physiology of the body, such as insufficient cellular uptake, lysosomal degradation, and mitochondrial membranes, which highly affect target efficiency. In addition, the targeted organs could impose a barrier to drug delivery, considering that they contain a barrier since these tissue microenvironments possess different physiological conditions, such as elevated pH values and higher enzymatic activity [24, 25].

The extracellular barriers include the reticuloendothelial system (RES) which contains cellular and non-cellular components including the phagocytes, working towards clearing particles that are larger than 200 nm, and eliminating the circulating nanoparticles after systematic administration [26, 27]. Additionally, the tissue stroma, extracellular matrix and connective tissue cells present additional barriers to the remaining nanoparticles that escaped RES and are directed towards the mitochondria [28]. As for the intracellular barriers, they include the endosomes that entrap the nanoparticles after internalisation. Further, the mitochondrial membranes create additional barriers as the mitochondria contain the outer mitochondrial membrane (OMM) and inner mitochondrial membrane (IMM). The OMM is porous in structure and surrounds the organelles, while on the other hand, the IMM is compact and hydrophobic, folds and enhances the mitochondrial surface area. Furthermore, both membranes control access, as molecules below 5,000 Da can diffuse, while CO₂, oxygen and water can pass through the OMM and IMM, respectively [2, 24].

According to Pegoraro et al., the design of a successful targeting drug delivery system (DDS) should focus on four aspects: the size, shape, and rigidity of the drug carrier, as they dictate cellular uptake, surface area, and diffusion. The last and most crucial factor for mitochondria is the charge, considering the latter’s negative charge, in which positively charged moieties will favour better interaction [25].

As mentioned above, the mitochondria play crucial roles in system biology and affect numerous processes such as energy production and cell ageing. Additionally, it has been reported that the mitochondria regulate the stemness of non-malignant stem cells, and that altering the mitochondrial function dictates the acquisition and maintenance of stemness of non-cancerous cells [29]. In general, cationic moieties with lipophilic properties are considered an effective approach to target the mitochondria based on the electrostatic interactions between them (the cationic moieties) and the anionic inner membrane of the mitochondria. Yet, the rapid clearance and the non-specific tissue distribution of the cationic moieties restrict their In vivo applications [30, 31].

There are several approaches to target the mitochondria, which can be classified as molecule- and nanocarrier-based targeting. The two approaches are highlighted in Figure 1 and explained in the following sections.

Mitochondrial genome editing is a novel proposed technique that has been discovered in the past two decades, where specific DNA sequences in the mitochondria could be targeted and modified using imported gene-editing proteins. In animal models and heteroplasmic cells, this can occur through specifically cleaving and eliminating mutant mitochondrial DNA via mitochondria-targeted nucleases [32]. One of the reported techniques of genomic editing is the use of clustered regularly interspaced short palindromic repeats-associated protein-9 nuclease (CRISPR-Cas9), which could have the ability to perform mitochondrial DNA gene editing [33]. However, this approach is still in its early stages and requires further development due to several challenges associated with CRISPR-Cas9, such as the potential for unintended genetic modifications and disruption of regulatory elements [34]. Another possible technique is the use of mitochondrial targeting sequences (MTS), possessing little sequence similarity to the mitochondrial proteins’ sequence since they have an amphiphilic α-helic, but consisting of 15–55 amino acids. These sequences could target the OMM, IMM, or the intermembrane space (IMS) to deliver proteins to the matrix, making them possible candidates as mitochondria-targeting DDSs [35–37].

Mitochondrial penetrating peptides (MPPs) are modified cell-penetrating peptides (CPPs), which have the ability to penetrate the mitochondrial membranes. They could be positively charged, which enhances their interaction with negatively charged mitochondria. Also, they could include lipophilic amino acids to enhance the penetration and interaction with the IMM, considering the latter’s higher lipophilicity [38]. Szeto–Schiller peptides (SSPs) are another variation of penetrating short peptides where they are composed of a small number of amino acids [4–10], while possessing a D amino acid and alternating cationic and aromatic amino acids [39].

Triphenylphosphonium (TPP+) is one of the most common mitochondria-targeting moieties due to several characteristics: large ionic radius, high lipophilicity, and the diffusion of the positive charge on three phenyl rings. These properties provide TPP+ with distinctive features that allow it to move freely across biological membranes without transporters and accumulate inside the mitochondria. This accumulation is due to the membrane potential and the difference in charge, which could reach 100–500 folds of higher accumulation for every 60 mV of membrane potential [40–42].

Mito-porters are liposome-based drug carriers that carry the drug cargo and deliver it to the mitochondria through a membrane fusion mechanism. The surface can be functionalised to enhance the interaction since the modified drug carriers are internalised via micropinocytosis compared to the clathrin-mediated endocytosis for cationic liposomes [43]. The drug delivery process using these carriers follows several consecutive steps: the internalisation followed by effective nedosomal escape, the electrostatic interaction with the mitochondrial membrane, and the membrane fusion to deliver and release the loaded drug cargo [44].

Dendrimers are polymeric-based nanoparticles that possess a very distinctive feature: being highly branched in three dimensions while preserving symmetry. They are considered good candidates as DDSs for targeting due to their monodispersity, controlled size, and abundant functional groups on the surface, making them suitable for encapsulation [45]. Amine-functionalised cationic dendrimers could condense the genetic materials like plasmid DNA into nanoparticles while protecting them against degradation. Additionally, they can be functionalised with targeting ligands such as TPP⁺, presenting them with enhanced targeting properties [46].

Liposomes are spherically shaped nanoparticles made from two layers of phospholipids, where they enfold an aqueous core while the hydrophobic region is in the space between the two lipid bilayers [47]. Lipsosomes have great potential for targeting the mitochondria if they are functionalised with numerous ligands, proteins, and moieties that have an affinity to the mitochondria and, thus, enhance targeting capabilities. They can be used to manage breast cancer and overcome multi-drug resistance by loading the active compound (paclitaxel), which will accumulate inside the cancerous cells and facilitate mitochondrial-based targeting. Further, they can be modified with TPP⁺, which enhances the interaction due to its cationic features [48, 49].

Micelles are amphiphilic copolymers consisting of an external hydrophilic shell and internal lipophilic core suitable for encapsulating poorly soluble drugs to provide an easy functionalisation for targeting purposes [50]. Micelles have been used to design pH/ROS-responsive DDS of doxorubicin (DOX) to target the mitochondria, while PEG and TPP⁺ have been used to enhance targeting. Another approach utilised chondroitin sulfate to shield functionalised micelles, which included poly(d,l-lactide) (PLA) in addition to the TPP⁺-PEG [51, 52].

The use of mesoporous silica carriers is the main aim of this review and will be discussed in the following sections.

Generally, conventional particles for drug delivery are spherically shaped and possess a variety of morphologies. However, porous carriers, conversely, can also be spherical but have several holes within the structure that provide the loading carriers with an initial burst release of the drug [53]. The pores within these carriers can either have an ordered pore arrangement and uniform size or have a broad size distribution with a disordered pore size [54, 55]. According to the International Union of Pure and Applied Chemistry (IUPAC), porous materials are classified into three categories depending on the pore size: microporous where the pore size is less than 2 nm, mesoporous where the pore size is in the range of 2–50 nm, and macroporous where the pore size exceeds 50 nm [56].

The mesoporous carriers have gained significant attention in research, and they are categorised into silicon-based and non-siliceous mesoporous materials. The first are inorganic materials and the most studied as they are synthesised from the condensation of either silicon alkoxides or sodium silicates around a template, which is an ordered surfactant [57]. On the other hand, the synthesis of non-siliceous mesoporous materials can be achieved from polymers, carbon nitrides, carbons, and metals [58]. The different classifications of porous carriers are identified in Figure 2.

Mesoporous silica were first developed in Japan by Yanagisawa et al. in the 1990s and was industrially produced by Mobil Corporation Laboratories [59]. There are several families of mesoporous silica, and amongst the most famous is the Molecular 41 Sieves (M41S) family, which contains several members, including MCM-41, MCM-48, and MCM-50, as the MCM stands for Mobil Crystalline Material [60]. Many types have been subsequently developed, and among the most reported in the literature is Santa Barbara 15 (SBA 15), a nanoparticle form of mesoporous silica that was invented by a research group in the United States [61, 62]. Mesoporous silica were originally designed for chemistry applications, notably in analytical and electrochemical fields. Its high surface area, consistent pore size, and adaptable structure made it ideal for catalysis, adsorption, and sensing tasks [63]. The chemical stability of SiO₂ matrices makes the abundant pores in mesoporous silica ideal for accommodating a wide range of molecules. Additionally, the exceptional biocompatibility of SiO₂ enhances its potential for various applications, that include drug delivery systems, bioimaging, and biosensing, where safe and effective interaction with biological systems is crucial [64]. However, the first use of mesoporous silica as carriers for drug delivery was reported in 2001, when they were explored as potential drug carriers for Ibuprofen to study the drug’s release behaviour from these porous structures. This marked the beginning of mesoporous silica’s application in controlled drug delivery [65].

Mesoporous silica nanoparticles (MSNs) have many characteristics that make them sought after as drug delivery carriers. For instance, the particle and pore sizes can be fine-tuned to be between 2 and 50 nm, which allows the carrier to contain molecules of varying sizes within the porous structure [66]. Also, MSNs possess a high surface area that could range between 600 and 1,000 m²/g, which makes the carrier a great candidate for loading purposes. Furthermore, this high surface area provides the opportunity for functionalisation, where the carrier’s surface is linked with different moieties that enhance targeting, enable controlled release, and determine interactions with the loaded molecules, as well as cells and tissues [67]. In addition, MSNs can be functionalised with gatekeepers (capping materials or molecular gates) that allow the delivery of the loaded drug to be triggered by applying a stimulus. The inclusion of the gatekeepers assists in retaining the entrapped molecules within the porous structure, which are to be delivered in the presence of an external stimulus. The use of this approach provides an alternative for liposomes or polymers in controlled release, as the previous two release their drug cargo based on diffusion or the carrier’s decomposition [68]. The release of the loaded drug cargo, as mentioned above, is related to stimuli, where it can be either internal or external, according to Mekaru et al. [69]. The internal or intracellular stimuli could be the activation by low pH, redox, and biomolecules or enzymes. On the other hand, external stimulants could be light or magnetic fields. Following the synthesis of MSNs in the 1990s and the first medical application for drug delivery in 2001 for the delivery of ibuprofen, they have attracted the attention of scientists for different uses and applications [65].

Compared to other carriers, and especially liposomes, MSNs offer several advantages. They exhibit stability in non-aqueous solutions, enabling the loading of hydrophobic drugs in organic solvents. Additionally, their large, easily functionalised surface area allows hydrophobic drugs to be loaded even in aqueous environments, as the surface serves as an effective reservoir for the drug [70]. Additionally, MSNs offer superior stability in biological environments compared to other nanoparticles, providing robust protection for encapsulated drugs. The rigid silica framework serves as a barrier against enzymatic degradation, which is particularly critical for sensitive therapeutics such as proteins and peptides that are vulnerable to breakdown in complex biological fluids. Furthermore, mesoporous carriers can be effectively used to load proteins for oral drug delivery, shielding them from enzymatic degradation, protecting them from pH variations in the gastrointestinal tract, and ultimately enhancing their bioavailability. This dual protection ensures that the therapeutic agents remain intact until they reach their target site, improving treatment efficacy [71, 72].

There are different approaches for using MSNs as DDSs to target the mitochondria. This could be passive, depending on the characteristics of diseased tissues, or active, which focuses on functionalising the carrier and the use of different ligands, and magnetic-based that includes implementing an external magnetic field to draw the nanoparticles (NPs) to inflamed tissues or disease sites [73]. Additionally, stimuli-responsive-based targeting can be included in this category as the MSNs will release their loaded drug cargo in response to certain stimuli, which can be divided into internal and external stimuli-responsive action according to Colilla and Vallet-Regi [74].

For passive targeting, MSNs could accumulate inside targeted tissues, whether tumours or inflammatory areas, via two pathways. They include the enhanced permeability and retention (EPR) effect and the extravasation through leaky vasculature and the subsequent inflammatory cell-mediated sequestration (ELVIS) mechanisms in tumours and inflammatory tissues, respectively. As for active targeting, it involves the addition of targeting molecules to the surface of MSNs to selectively target intended cell types without harming other tissues. Amongst the most employed targeting moieties are aptamers, peptides, and proteins [75, 76].

Recently, the use of mitochondrion medicine has emerged as a biomedical direction for treatment, focusing on the mitochondria as the therapy target for numerous diseases, including cancer. Additionally, as mentioned above, the physiological role of the mitochondria in regard to programmed cell death and energy production is the critical concept for targeting it [77]. As reported earlier, MSNs can be used as targeting DDSs due to their small size and easily functionalised surface. Yet, an additional advantage is their accumulation within the targeted tissues, which enhances the precision of drug delivery and minimises side effects [78]. Interestingly, the size of MSNs provides an additional advantage as small-sized MSNs interact with cells without disorganising the membrane [79]. For this reason, research has focused on using MSNs as possible carriers for mitochondrial-based targeting. The following sections will focus on the different uses of MSNs as well as numerous functionalisation approaches, which include using targeting moieties like TPP⁺, surface modification, photodynamic therapy (PDT), and capping. These applications are summarised in Figure 3.

The use of TPP⁺ as a targeting moiety with MSNs has been investigated due to TPP⁺’s hydrophobic properties, positive charge, and the ability to accumulate inside the negatively charged mitochondria based on electrostatic interactions [44]. Interestingly, using mesoporous silica has been reported to construct a liposomal-based nano platform for mitochondrial targeting [80]. This work used MSNs as the core for the developed DDS, where TPP⁺ was attached on the surface, with subsequent modification in other moieties. The inclusion of MSNs has enhanced the mechanical stability of liposomes to withstand long circulation In vivo and avoid deformation as the modification of TPP⁺ can take many forms to enhance its therapeutic and targeting effects. Additionally, TPP⁺ surface functionalised-MSNs were loaded with DOX and β-Lapachone, which has the ability to respond to endogenous reactive oxygen species (ROS) in cells and increase the levels in cancerous cells in situ. Furthermore, this approach utilised chitosan coating for TPP⁺, which has a higher biocompatibility, biodegradability, and enhances the stability of the nanocarriers while maintaining mitochondrial targeting properties [81]. Another example explored the application of TPP⁺-functionalised MSNs for delivering α-Tocopheryl succinate (α-TOS), an effective therapeutical mitochondrial agent that induces apoptosis, which focused on using TPP⁺ as a targeting ligand as well as α-TOS to achieve good targeting efficiency [82]. The results presented high cellular uptake and low cytotoxicity against HeLa and HepG2 cells. In addition, the internalisation of MSNs via endocytosis in mammalian cells is a critical factor for the use of these carriers for targeting purposes. The inclusion of TPP⁺ as a targeting ligand has also been reported as a DDS for DOX, where TPP⁺ provided the targeting properties and DOX presented the anti-cancerous effects [83]. This approach resulted in an enhanced cancer cell-killing efficiency in HeLa cells, as well as a decrease in ATP production and a decreased mitochondria membrane potential. Another example of delivering DOX has been reported by Naz et al [84]. Their work focused on delivering DOX via hyaluronic acid (HA) and TPP⁺-modified MSNs. In this formulation, HA had a dual action in which it acted as a capping agent for the pores, and as a targeting ligand due to its degradation by hyaluronidase in the acidic environment of cancerous tissues. The novel carrier exhibited a selective uptake by CD44 receptor-mediated endocytosis and accumulated in the mitochondria while releasing DOX due to HA’s degradation.

An additional example of using TPP⁺ was reported by Cheng et al. in 2016. Their work focused on utilising MSNs to encapsulate DOX, while the surface of silica particles was functionalised using tumour-targeting cellular membrane-penetrating peptide (TCPP) and TPP⁺ to seal the pores via disulfide bonds. This platform presented a synergistic effect to target cancer cells and penetrate cell membranes to release the anti-cancer drug and mitochondria-targeting moiety in the cytoplasm [85].