3.2.1. Internal Stimuli-Responsive Drug Release from MSNs pH-Responsive Release

Cancer is well-known for its acidic tumor microenvironment with a lower pH than healthy cells/tissues. Consequently, pH-sensitive release is one of the approaches used in cancer nanomedicines. The most investigated pH-responsive delivery systems for anticancer therapeutic drugs have been inspired by applying diverse techniques and vary according to the loading strategies. In this section, we focus on some examples of recent studies published for natural anticancer prodrugs with pH-sensitive release. Nasab et al. [222] fabricated MSNs (MCM-41) capped with chitosan polymer and subsequently loaded with curcumin. This pH-responsive design depends on the degradation of chitosan, allowing high curcumin release at a low pH of 5.5 and resulting in low release at normal physiological pH (7.4). This is favorable for killing U87MG glioblastoma cancer cells. Mishra et al. [223] synthesized MSNs (SBA-15), followed by folic acid functionalization and further loading with quercetin and acid-labile magnetic nanoparticles (Figure 15). The system was investigated in vitro and in vivo in HCT-116 human colorectal carcinoma cells. The results showed that quercetin release was a pH-dependent effect, increasing with decreasing pH. Eventually, the system exhibits promising chemo-theranostic effects for managing colon carcinoma. In this context, Rashidi et al. [224] reported that the release of gallic acid (GA) from MSNs strongly depends on the pH levels of the release media. Furthermore, a pH-sensitive delivery system for ursolic acid prodrug was synthesized by incorporating an acid-sensitive linkage between the drug and MSNs [200]. This sustained release of ursolic acid enhances the anticancer effects against hepatocellular carcinoma cancer. A pH-responsive release of evodiamine and berberine was also achieved by loading them into lipid-coated MSNs [225]. In another strategy using Fe3O4 nanoparticles as gatekeepers, artemisinin is initially loaded into the inner space of hollow MSNs and Fe3O4 capped onto the pore outlets through acid-labile acetal linkers. The results proved that the system is stable under neutral conditions at pH 7.4 (no release), but it releases the prodrug upon exposure to the acidic lysosomal compartment (pH 3.8–5.0). The acetal linkers can be hydrolyzed under acidic conditions. This delivery system has an efficient and desirable anticancer action [226].

**Figure 15.** (**A**) Schematic representation of the delivery design for quercetin "FA-FE-SBA15QN". (**B**) The release kinetics of quercetin from FA-FE-SBA15QN at different pH (7.4 and 5.5). The values are represented as the mean ± SEM. Reproduced with permission from [223], The Royal Society of Chemistry, 2020. This article is licensed under a Creative Commons Attribution-Non-commercial 3.0 Unported License.

#### Redox-Responsive Release

The delivery systems that consider redox-sensitive release are popular in cancertargeted therapy. They take advantage of intracellular conditions and rely specifically on the presence of glutathione (GSH) with a high level of expression in cancer cells compared to normal cells [227]. For example, Lin et al. [228] prepared pH and redox dual-stage responsive release of curcumin with Dox through specific cleavable PEGylation and hydrogel coating (crosslinked by disulfide bonds). The used MSNs were loaded with Dox, whereas

the curcumin was encapsulated in a hydrogel coating. The results indicated that dualresponsive release by means of GSH and pH allows efficient and specific cancer targeting (Figure 16). In another example, Xu et al. [229] developed a stimuli-responsive delivery for curcumin gatekeepers based on MSNs characterized by large pores (named LP). In this design, curcumin is anchored to the surface of LP using thiol-ene as the click chemistry approach, followed by a coating of the pluronic polymer (F127) on the surface by means of self-assembly. The release studies proved that curcumin exhibits a redox-responsive release depending on the absence or presence of GSH at different pH levels.

**Figure 16.** (**A**) Illustration of the dual-response release of p-Cur and Dox co-delivery. (**B**) In vitro release profiles of Cur from MSN/SP/bPEG at 37 ◦C. Error bars indicate standard deviation. Reproduced with permission from [228], Elsevier B.V, 2019.

#### Enzyme-Responsive Release

In the human body, many chemicals and enzymes are inherently expressed during pathological conditions, including cancers, which are explored to trigger drug release from numerous MSN types [10]. A delivery system tailored for anticancer treatment with enzyme-responsive release, in which matrix metalloproteinase (MMP) substrate peptide containing PLGLAR, which is sensitive to MMPs, is immobilized onto amine-modified MSNs and further capped with bovine serum albumin by covalent bonding. The results revealed that the nanoplatform delivery exhibits enzyme-triggered release of drug and efficiently inhibits tumor growth in vivo. MMP enzyme-trigger release of cisplatin-based MSNs was reported by Vaghasiya [230]. The system constructed by coating collagen on the surface of drug-loaded MSNs eventually results in sensitive enzyme release.

#### 3.2.2. External Stimuli-Responsive Drug Release from MSNs

Responsive Release Using Magnetic Fields

This approach is largely employed for responsive release due to the magnetic guidance by a permanent magnetic field and locally increases the internal temperature by changing the magnetic field potential [32]. The delivery systems concerning this method widely use magnetic nanoparticles (superparamagnetic iron oxide-SPIONs) 5–10 nm in size as a core and mesoporous silica shell permitting drug loading and release [231]. Regarding natural prodrugs, the nano platform developed by Janus MSNs consists of magnetic nanoparticles to achieve magnetic targeting and delivery of berberine. This system produces a sustained release and exerts extraordinarily site-specific internalization into hepatocellular carcinoma cells, facilitating a high antitumor effect against liver cancer due to an external magnetic field [232]. Another very recent example is Asgari et al. [233] developing a novel in situ encapsulation delivery for curcumin consisting of magnetite-silica core-shell nanocomposites. The system could be effective for clinical application by means of magnetic hyperthermia therapy. In addition, nanoparticles of DNA-capped magnetic mesoporous silica composite exhibit temperature-dependent release of Dox and magnetic hyperthermia effects against cancer [234].

Responsive Release of Drugs Using Light

As a non-invasive and spatiotemporal strategy, different wavelengths of ultraviolet, visible, or near-infrared light can be employed to trigger and control drugs from MSNs. The main advantages are easy application by the clinician and focalization to the target tissue [235–237].

Kuang et al. [238] developed a curcumin delivery system by means of photodynamic therapy, achieving PEGylated MSNs loaded with curcumin (Figure 17). The results demonstrated that the developed system, "MSN-PEG@Cur", exhibits efficient endocytosis into cells and the release of curcumin. As a photodynamic therapy, it promptly generates ROS upon irradiation, allowing effective treatment for cancer. In another example, Li et al. [239] preloaded berberine into folic acid-modified Janus gold MSNs. The in vitro and in vivo results demonstrated that the delivery system verifies sustained release dependent on light and an efficient anti-tumor effect with good biosafety for normal tissue. Feng et al. [225] fabricated a dual delivery platform for evodiamine and berberine loaded into lipid-coated MSNs with thermo-sensitive release. Their results suggest that the temperature-responsive release is promising for both hydrophobic and hydrophilic drugs. Using an important natural prodrug of capsaicin, the main ingredient in red or hot chili pepper, Yu et al. [240] reported a novel design of NIR-triggered plasmonic nanodot-capped MSNs for inhibiting metastasis of human papillary thyroid carcinoma. The nanoplatform consisting of gold nanodot-capped MSNs loaded the prodrug. The results depicted that the delivery of capsaicin by the developed nanoformulation exhibited strong cytotoxicity against the FTC-133 and B-CPAP cell lines compared to free capsaicin.

**Figure 17.** The preparation process for MSN-PEG@Cur and schematic representation of the intracellular photodynamic therapy (PDT) process after endocytosis of MSN-PEG@Cur. Reproduced with permission from [238], The Royal Society of Chemistry, 2020. This article is licensed under a Creative Commons Attribution-Non-commercial 3.0 Unported License.

#### Responsive Release of Drugs by Ultrasound

Ultrasound is considered an interesting and efficient approach to trigger the release of drugs from MSNs. The main advantages include deep penetration of living tissues without causing damage, and it is non-invasive and can be concentrated to the desired tissue [32,241]. In this approach, drugs can be released from pores of MSNs due to the thermal effect of ultrasound radiation on chemical bonds and thermosensitive polymers while closing in the absence of a radiation effect [242–244]. An example is MSNs modified with amine groups covered by sodium alginate polymer and subsequently loaded with a model cargo (rhodamine B) [245]. The results indicated that rhodamine B releases based on changing the ultrasound potential (ultrasound on–off responsiveness).

#### **4. Selective Targeting Strategies for Cancer**

One of the hottest areas in delivery systems is the delivery of drugs or therapeutic agents directly to specific tissues where the desired therapy is required. The main goal of nanomedicine application for cancers is avoiding the expected side effects from drugs and damaging the healthy cells surrounding the tumor site [21,246]. Two routes have been used depending on nano-particulate delivery for cancers, passive and active selective targeting.

Passive targeting was first postulated by Matsumura and Maeda in 1986 [247]. Nanoparticles can accumulate in tumor tissue by the enhanced permeability and retention (EPR) effect. They hypothesized that the localization of macromolecules and particles of certain sizes differ, which is attributed to the tumor microenvironment, the relatively slow elimination rate, and poor lymphatic drainage. Particle size, surface charge, or hydrophobicity can be mediated by the so-called EPR effect, or passive targeting [248,249] (Figure 18). Passive targeting is due to abnormalities in tumor blood vessels, which have wide interendothelial junctions with pores (700 nm). Injected nanoparticles travel through the bloodstream and accumulate in the tumor interstitium because of this characteristic of tumor vessels [247,249]. The nanoparticles already located in the tumor would remain there because of the ineffective lymphatic drainage with the fast growth of the tumor tissue [221]. However, the EPR effect is often not efficient enough to selectively deliver and reduce the side effects of anticancer drugs [250].

Active targeting is used to enhance the ability of a nanoparticle delivery platform carrying drugs to be taken up and bind to cancer cells via specific receptors on their surfaces compared to normal cells [251]. It is well known that some tumor cells overexpress certain receptors on their surface. Thus, nano-delivery systems functionalized with various ligands permit a high affinity for receptors facilitating specific retention and uptake by cancer cells. Thus, the role of targeting ligands is to allow the nanocarriers to selectively enter the cancerous cells, but not normal cells. This not only reduces the administration dosage, but also diminishes toxic side effects of drugs during circulation [252]. Many ligands have been investigated to functionalize/decorate nano-delivery systems based on MSNs for selectively targeting cancers (Figure 18). These include antibodies, proteins, peptides, aptamers, small molecules, and saccharides [221]. For example, transferrin [237], folic acid [42], epidermal growth factor (EGF) [253], methotrexate [254], RGD-type peptide [255], anti-HER2/neu [256], hyaluronic acid [257], and mannose [258].

**Figure 18.** Schematic representation of the enhanced permeability and retention (EPR) effect (lift side). (**A**) Normal blood vessels (no fenestrations), showing that MSNs remain in the bloodstream. (**B**) Tumor tissues (defective blood vessels present) showing that MSNs leak out through the endothelial gap–gap and eventually accumulate in the tumor. On the right is a schematic depiction of active targeting with a variety of possibilities depending on the MSNs. Reproduced from [32,259], MDPI, 2020.

As an example, Kundu et al. [203] designed targeted delivery for umbelliferone prodrug, with the system consisting of umbelliferone loaded in MSNs and capped with a pH-sensitive poly acrylic acid and further grafted with folic acid on the surface. The delivery with folic acid conjugation increases the anticancer potential of umbelliferone against breast cancer cells. In another example, Yinxue et al. [199] investigated myricetin prodrug (Myr)-loaded MSNs combined with multidrug resistance protein (MRP-1) siRNA and the surface modified with folic acid to treat non-small cell lung cancer (NSCLC). In vivo fluorescence demonstrated that folic acid-conjugated MSNs with Myr and MRP-1 nanoparticles could specifically accumulate at tumor sites. Compared to free Myr and MSNs combined with MRP-1/Myr nanoparticles, folic acid-conjugated MSNs loaded with Myr and MRP-1 nanoparticles could more effectively suppress tumor growth with few side effects. Overall, a folic acid-conjugated nanoparticle system could provide a novel and effective platform for the treatment of NSCLC. We also reported a targeted delivery system consisting of folic acid conjugated to amine-modified MSNs (KCC-1 and MCM-41) and subsequently loaded with various prodrugs (curcumin, colchicine, and quercetin) [42]. The nanoformulation containing curcumin exhibited the highest anticancer activity against liver cancer cells through apoptosis via caspase-3, H2O2, c-MET, and MCL-1 (Figure 19). Table 7 lists some other examples of targeted delivery systems for anticancer natural prodrugs.

**Figure 19.** Schematic representation of the preparation, internalization, and anticancer mechanism of action of the prepared nanosystem in human liver carcinoma (HepG2) cells. This schematic shows the prodrug release into cancer cells and the main anticancer action for inducing apoptosis via activation of caspase-3 for killing HepG2 cancer cells proposed by assistance from important signaling pathways (c-MET, MCL-1, and H2O2). Reproduced from [42], Impact Journals, 2018.

**Table 7.** Some examples of targeted delivery systems for anticancer natural prodrugs using MSNs.



#### **Table 7.** *Cont.*

FA = folic acid, HA = hyaluronic acid.

#### **5. Motivation towards Natural Anticancer Agents**

Nature is a great source of thousands of chemical substances/compounds generally considered natural products, as well as natural prodrugs if they are used for treating diseases [269,270]. Natural products (of natural origin) and herbal medicines have been used in traditional and modern medicine to treat cancer, and account for nearly 60% of pharmaceutical drugs [271–279]. Natural prodrugs provide medical effects against cancers as either chemotherapeutics or chemopreventive drugs. Regarding chemotherapeutics, anticancer natural prodrugs have been utilized for various cancer treatments and are becoming rising stars in the field of drug discovery for their contributions [280]. Some available drugs used in clinical applications for cancer patients diagnosed with different cancers are derived from plants, including vincristine, vinblastine, topotecan, and taxol [281]. There are also some examples of anticancer drugs originating from microbes, including Dox, daunorubicin, and bleomycin. Regarding cancer prevention, there are numerous natural substances (e.g., in fruits and vegetables) that have also been applied in cancer prevention along with human health enhancement with no detectable side effects [282]. To achieve cancer prevention goals, by completely preventing or delaying cancer, the main strategies that can be used, such as maintenance (healthy lifestyle), avoidance (exposure to toxicants/carcinogens), and dietary consumption (chemopreventive substances to drugs) [283]. There is no doubt that prevention leads to better management and treatment of tumor growth and the risk for developing metastases, secondary tumors, and recurrence [283]. Eliminating cancer, decreasing metastasis, reducing reappearance, and improving patient survival are key to curing cancers [284].

Among the main natural sources, plants are a considerable domain for supplying a variety of natural products with diverse chemical structures with a wide range of health benefits. The natural products are the main secondary metabolites produced by plants and can be classified into four major classes: phenolics and polyphenolics, terpenes, nitrogencontaining alkaloids, and sulfur-containing compounds (Figure 20) [285–287].

**Figure 20.** Chemical structures of various classes of natural compounds (prodrugs). Reproduced with permission from [285], Elsevier Ltd., 2019.

In recent years, attention has been focused on solving the problems associated with natural prodrug substances to increase their use in cancers and other pathological disorders. As an advanced strategy, nanotechnology application in medicine, called nanomedicine, is a promising approach being developed to overcome the limitations of natural prodrugs and improve their efficiency in cancer therapy. The advent of nanomedicine for cancer therapy occurred recently, and the rate of its progress and transformation in cancer treatments has also been rapid [285]. This technology can solve the major drawbacks of natural anticancer prodrugs, including low aqueous solubility, low bioavailability, multidrug resistance, and non-specific targeting. The developed nanoformulations for delivery of natural anticancer prodrugs are intentionally being explored with several classes of prodrugs based on various organic and inorganic nanocarriers [285,288–298]. By reviewing in vitro and in vivo cancer models in the literature, it seems that nanoplatforms for delivering anticancer natural prodrugs have potentially improved the therapeutic activity, specific targeting, solubility, and bioavailability, and reduced side effects. The better patient response and survival are accompanied by possible enhancement of the pharmacological impacts and clinical outcome. Below, we discuss the delivery systems that have been established for select anticancer natural prodrugs employing MSNs.

#### *5.1. Curcumin*

Curcumin (1,7-bis(4-hydroxy-3-methoxyphenyl)-1,6-heptadiene-3,5-dione) is a natural hydrophobic polyphenol compound, and is the major constituent derived from turmeric rhizome (*Curcuma longa* L.). Turmeric is a well-known spice in the kitchen and has a long history in traditional medicine for a wide range of diseases. Curcumin has numerous pharmacological activities, including anticancer, antiviral, antioxidant, anti-inflammatory, wound healing, and antimicrobial, among others [299–315].

Despite these potential pharmacological activities, the pharmacokinetics of curcumin show inherently poor solubility and bioavailability because of the limited absorption, rapid metabolism, and quick systematic elimination [316–319]. To take advantage of the medical actions of curcumin and improve the inherent limitations, versatile nanoplatform delivery systems have been constructed and studied, including MSNs. Concerning MSNs for curcumin delivery contribution, MSN-based nanosystems show great promise for combating cancers and will be seen soon in clinical stages.

Ma'mani and co-workers [196] fabricated guanidine-functionalized PEGylated KIT-6 MSNs 60–70 nm in size for delivery of curcumin to breast cancer cells. The system exhib-

ited pH-sustained release of curcumin with long-term anticancer efficacy in human breast cancer cells (MCF-7 human breast adenocarcinoma cells, 4T1 mouse breast cancer cells, and MCF10A human mammary epithelial cells). A similar trend was observed for MSNs, namely MSU-2 and MCM-41 loaded with curcumin showing significant anticancer effects against different cancer cells (A549 human lung carcinoma cells, MCF-7 human breast cancer cells, and B16F10 mouse melanoma cells) compared to pure curcumin [320]. In further investigations, they found that the plausible mechanism contributing to anticancer effects is the generation of intracellular ROS and the induction of apoptosis. Lin et al. [228] tailored a co-delivery system of Dox loaded into MSNs as the core and curcumin loaded into the polymeric coating shell. The results indicate the long duration of blood circulation due to the PEG shell, GSH-sensitive release effect for drugs, and high cellular uptake resulting in synergistic anticancer effects through enhanced apoptosis of Hela cells. As an interesting nanoplatform, the fabricated lipid bilayer-coated curcumin-based MSNs unveiled a controllable and highly biocompatible theranostic nanosystem for cancer delivery [321]. Another recent strategy for building a delivery system for curcumin is by loading the prodrug into amino-MSNs using APTES silanes (KIL-2 and KIT-6), then coated by polyelectrolyte polymer complex by means of the layer-by-layer technique [197]. Based on the comparative data from this study, the nanoformulation exerts an anticancer effect on human cell lines, namely HL-60, EJ, and HEK-293, compared to free curcumin, demonstrating the promising delivery of prodrug with a sustained release effect. Considering active cancer-targeting designs, our group constructed selective targeted anticancer delivery of curcumin using MSNs (KCC-1-NH2-FA-CUR and MCM-41-NH2-FA-CUR) showing selective targeting of liver cancer cells (HepG2). The killing mechanism was found to be apoptosis [42]. The aspartic acid-functionalized PEGylated MSN-graphene oxide loaded with curcumin exhibited pH-sensitive release and excellent killing of breast cancer cells (MCF-7) [322]. With the occurrence of drug resistance in come cancers, silver-decorated SBA-15 (as metal-doped nanocomposites) coated with melanin-like polydopamine was used to deliver curcumin [323]. They found that the utilization of a nanoplatform containing curcumin enhances anticancer efficiency against select cancer cells (HeLa and taxol-resistant NSCLC (A549/TAX) compared to free curcumin.

To verify the antitumor action against breast cancer in vivo, Gao et al. investigated PE-Gylated lipid bilayer-coated MSNs for a dual-delivery of PTX and curcumin with prolonged release to determine their pharmacokinetic properties, uptake, subcellular localization, biodistribution and tumor site targeting, and effectiveness [324]. The delivery system could significantly increase the anti-tumor effect either by intravenous or intratumoral administration compared to free drug. The nanoplatform effectively led to the accumulation of the nanoformulation carrying drugs in the tumor site, resulting in highly efficient therapeutic effects in breast cancer. As such evidence of utilization of curcumin for co-delivery systems is important for further improvements and reducing side effects and drug resistance in cancers, which is the main issue for conventional cancer therapy. Sun et al. [325] conducted a study of cancer targeting by means of folic acid and PEI-modified-MSNs for curcumin; they concluded that the system exhibits sustained release (pH-sensitive delivery), which is suitable for antineoplastic drugs. Several studies have reported the delivery of curcumin in different cancers in vitro or in vivo (Table 8).


**Table 8.** Delivery designs for curcumin in cancer (in vitro/in vivo studies) based on mesoporous silica nanoparticles (MSNs).

NA = Not applicable, CUR = curcumin.

#### *5.2. Quercetin*

Quercetin is a dietary flavonoid compound derived from plants (e.g., medicinal plants, vegetable, fruits). It is a 3,30 ,40 ,5,7-pentahydroxyflvanone named by the International Union of Pure and Applied Chemistry (IUPAC) [337]. Quercetin has unique biological properties that play an important role in mental/physical performance, as well as reducing infection risk [338]. It has shown numerous pharmacological actions, including anti-oxidant, antimicrobial, anti-diabetic, anti-inflammatory, anti-cancer, anti-Alzheimer, psychostimulant, mitochondrial biogenesis stimulant, lipid peroxidation inhibitor, platelet aggregation inhibitor, and capillary permeability inhibitor, among others [339–348]. The dietary intake of quercetin varies in many countries. The estimated intake dosage of flavonoid (quercetin accounts for nearly 75%) ranges from 50–800 mg/day according to the consumption of fruits, vegetables, tea, and herbals [349]. In addition, quercetin is safe with a single dose of up to 4000 mg orally and up to 100 mg via intravenous administration [350]. Quercetin is an excellent free radical scavenging antioxidant [344] and is considered one of the most effective antioxidants [351]. Consequently, quercetin exhibits promising effects against cancer [339,352] in vitro and in vivo [353–360]. Nevertheless, its potential impacts in clinical applications are drastically limited due to its poor solubility, low bioavailability, and instability [361]. According to the pharmacokinetics of quercetin in humans, only ~2% is bioavailable (from single dose) with an absorption rate of 3 to 17% (from 100 mg applied in individual healthy persons) [337]. The factors affecting oral bioavailability are low absorption, extensive metabolism, and/or rapid elimination, in addition to low solubility and non-selective targeting of cancers. Several nanoplatform delivery systems focus on overcoming these challenges to introduce quercetin into clinical applications soon for cancer [362–368].

The use of MSNs to develop new delivery systems for quercetin against cancers has attracted many research groups. Liu et al. [369] fabricated a system for dual delivery of PTX and quercetin into MSNs to overcome multidrug resistance in breast cancer. The nanosystem exhibited CD44 receptor-mediated active targeting for MCF-7/ADR cells. At the same time, the addition of quercetin with PTX significantly improves the sensitivity of MCF-7/ADR cells to PTX, providing a solution to multidrug resistance in breast cancer. Huang et al. [370] designed a novel nanoformulation consisting of quercetin-loaded MSNs coating cancer cell membranes for enhanced tumor targeting and radiotherapy. In vitro and in vivo investigations revealed that the system has many advantages, including excellent tumor targeting ability and efficient inhibition of tumor growth. The platform fulfills innovative ideas for targeting cancer and improving therapy. In another attempt, polydopaminecoated hollow MSNs combining Dox hydrochloride with quercetin efficiently overcame multidrug resistance in taxol and Dox double-resistant human colorectal cancer cells (HCT-8/TAX cells) [371]. Fang et al. [262] developed a hyaluronic acid-modified MSNs that co-deliver quercetin and Dox to enhance the efficacy of chemotherapy for gastric carcinoma. They found that the system enables stability, sustained release, and selective killing effects. An in vivo study disclosed that the co-delivery significantly enhances the anticancer efficacy compared to a single drug, showing the importance of quercetin in clinical application. In this context, Murugan et al. [264] loaded topotecan into the pores of MSNs, followed by poly(acrylic acid)-chitosan as an outer layer to further conjugate quercetin, and then grafting with arginine-glycine-aspartic acid (cRGD) peptide on the surface as targeting ligands for cancers. The system released the drugs as a function of pH and uptake occurred through integrin receptor-mediated endocytosis, enabling efficient anti-tumor effects in multidrug resistant breast cancer cells and animal studies. As far as active targeting and bioavailability are concerned, MSNs conjugated with folic acid and loaded with quercetin exhibit higher cellular uptake and more quercetin bioavailability in breast cancer cells, as well as an enhanced antitumor effect through apoptosis [265]. These studies demonstrate the prospective application of quercetin in cancers by means of single or co-delivery, facilitating efficient targeting and antitumor effects, creating new possibilities for clinical applications.

#### *5.3. Resveratrol*

Resveratrol (RSV, 3,5,40 -trihydroxy-trans-stilbene) is a natural polystilbene and nonflavonoid polyphenol. As a phytoestrogen compound, RSV is present in a wide range of plants and is abundant in extracts from the grape skin and other fruits and vegetables. RSV has been reported to exert multiple pharmacological effects, including anti-inflammatory, anti-viral, anti-microbial, anti-Alzheimer, anticancer, cardioprotective, neuroprotective, and immunomodulatory actions [372–386]. Concerning the anticancer effects on the preclinical level, RSV has also been reported to possess important antitumor actions in several preclinical animal models [387–398]. The clinical prospective of RSV has also been evaluated in a few clinical trials. The first clinical trial by Nguyen et al. [399] indicated that the freeze-dried grape powder (containing RSV) effectively inhibits colon cancer in patients. In addition, Patel et al. [374] showed that a daily dose of RSV at 0.5 or 1.0 g produces sufficient anticarcinogenic effects in colorectal cancer. Furthermore, Howells et al. [400] demonstrated that RSV given at micronized formulation with 5.0 g daily for 14 days in patients with colorectal cancer and hepatic metastases prevented malignancies by increasing apoptosis.

Despite promising preclinical (in vitro and in vivo) and prospective clinical results as an anticancer agent, RSV still has many challenges due to the pharmacokinetics, metabolism, bioavailability, and toxicity in cancer patients [374,401]. These associated properties prevent translation into more clinical trials and human benefits. In addition, RSV has shown poor bioavailability due to its quick extensive metabolism, and large doses (up to 5 g/day) should be applied to provide anticancer therapeutic activity [402]. Such high doses result in adverse effects (e.g., diarrhea, nausea, and abdominal pain with >1 g/day) [402]. As the poor bioavailability limits the RSV activity, there are various approaches for overwhelming the bioavailability, including co-delivery with piperine prodrug [403], micronized powders [403], and nanoplatform delivery [404–407]. Application of nanomedicine can improve the stability and bioavailability, and minimize side effects of RSV, which is making RSV a prospective candidate for treating many diseases, including cancers.

Few investigations in recent years have used MSNs for the delivery of RSV. Chaudhary et al. [408] loaded RSV into MSN-modified phosphonate or MSN-modified amine to improve the anti-proliferative activity and sensitization of drug-resistant prostate cancer. The RSV is released as a function of pH, and the phosphonate-modified nanoparticles effectively kill cancer cells better than amine-modified nanoparticles. Hu et al. [267] constructed a dual delivery system for anti-miR21 and RSV using MSNs conjugated with hyaluronic acid to target gastric carcinoma through overexpression of the CD44 receptor on cell membranes. They found that this nanoformulation has a superior anticancer effect due to synergistic effects specifically delivered by combining anti-miR21 and RSV in gastric cancer cells. Furthermore, Summerlin et al. [409] encapsulated RSV in colloidal MCM-48 and found that the nanoformulation enhances saturated solubility (∼95%) and release effect compared to pure RSV. The nanoformulation also possesses a higher killing ability for HT-29 and LS147T colon cancer cells compared to pure RSV by mediating the PARP and cIAP1 pathways.

#### *5.4. Berberine*

Berberine is an isoquinoline alkaloid found in a handful of plants widely used in botanical medical practice, such as *Hydrastis canadensis* (Goldenseal), *Berberis aquifolium* (Oregon grape), *Berberis vulgaris* (Barberry), and *Coptis chinensis* (Chinese Goldthread) [410,411]. Versatile pharmacological activities have been reported for berberine, including anti-viral, anti-microbial, anticancer, anti-diabetic, anti-diarrhea, and anti-inflammatory, and treatment for congestive heart failure, cardiac arrhythmia, and hypertension. Recently, berberine extract or pure compound has gained much attention in the newly published research and is among the top pharmaceutical supplements on shelves [412]. The preclinical evidence from huge studies reveals the capability of berberine to treat many diseases [411,413–419]. Thus, berberine is clinically studied for many diseases, such as diabetes [410,420–423]. Particular attention has been given to berberine in cancers, so it is expected to be one of the

most common natural compounds under the scope of extensive clinical investigations of cancers [424]. The main challenges in translating berberine to the clinical application are low solubility, poor aqueous solubility, slight absorption, and low bioavailability. There are some strategies to deal with these limitations, such as producing berberine hydrochloride to increase its solubility. Another approach is encapsulating berberine into nanocarriers for nanoplatform delivery [425–427].

Berberine loaded into folic acid-conjugated gold-MSNs shows superb anticancer effects, good biosafety, and protection of normal tissue in vitro and in vivo for chemoradiotherapy of liver cancer [239]. Another conformation obtained by Feng et al. [225] showed that MSNs based on dual delivery of hydrophobic prodrugs with berberine and evodiamine through thermo/pH-responsiveness improves antitumor effects in vitro and in vivo. Other results propose that the berberine-loaded Janus nanocarriers (MSNs containing iron oxide) driven by a magnetic field provide an effective and safe approach against hepatocellular carcinoma [232]. As with other drugs, berberine can be released depending on different conditions; by disulfide bond linking, berberine releases from MSNs under glutathione conditions upon breakage of the disulfide bond, promoting the anticancer action against liver cancer [428].

#### *5.5. Thymoquinone*

Thymoquinone (TQ, 2-methyl-5-isopropyl-1,4-benzoquinone), a monoterpene diketone compound, is the main active component in essential oil (volatile oil) of *Nigella sativa* L. (known as black seed or black cumin). TQ was isolated for the first time in 1963 [429] and exhibits various pharmacological activities in vitro and in preclinical investigations. The most reported activities are anticancer, antioxidant, anti-microbial, neuroprotective, anti-inflammatory, anti-microbial, and anti-diabetic [430–440]. A considerable amount of available data from preclinical studies encourage the translation of TQ into clinical settings. There is no doubt of the promising anticancer effects of TQ, but the lack of bioavailability and pharmacokinetic parameters delay the use of TQ in clinical applications. The main issues are low bioavailability, solubility, biodistribution in the body, rapid metabolism, and excretion [441–443]. In recent years, several strategies have been investigated to improve these limitations, such as the development of novel analogs [444], use of different routes (e.g., oral, intraperitoneal, intravenous), and nano-delivery systems [296,445,446].

Few delivery systems have been designed for TQ based on MSNs. The TQ-loaded MSNs produce more anticancer effects against MCF-7 and HeLa cells than pure TQ [447]. In addition, both TQ-loaded MSNs and pure TQ exert their anticancer activity by means of ROS-mediated apoptosis. To enhance the targeting ability towards glioma cells, we fabricated core–shell nanoformulations [44], with the core consisting of MSNs loaded with TQ and the shell consisting of whey protein–Arabic gum or chitosan–stearic acid complex. Interestingly, TQ releases as a function of pH and induces selective killing of cancer cells compared to normal cells. Furthermore, the core–shell nanoformulations significantly kill glioma cancer cells via apoptosis-mediated pathways due to caspase-3 activation, cytochrome c triggers, and cell cycle arrest at G2/M signaling. In this sense, the efficient anticancer effects against brain cancers can be attributed to the distribution of TQ-loaded MSNs [448]. The study showed that encapsulating TQ in MSNs improves delivery to some brain areas, including the cortex, thalamus, hypothalamus, and midbrain, but reduces its delivery to the cerebellum compared to pure TQ. The results also indicated that neither free TQ nor MSN-TQ reaches the hippocampus. Thus, MSNs potentially target TQ to certain brain areas.

#### *5.6. Gallic Acid*

GA (3,4,5-trihydroxybenzoic acid) is one of the most abundant phenolic acids present in plants (e.g., fruits and medicinal plants. GA can be isolated from different plants of *Quercus* spp. and has extensive applications in the food and pharmaceutical industries [449]. The therapeutic uses include antimicrobial [450], anticancer, gastrointestinal

disease, cardiovascular disease, metabolic disease, neuropsychological disease, and other miscellaneous diseases [449,451–455]. GA has a potential antioxidant action modulated by various signaling pathways (e.g., inflammatory cytokines, and enzymatic and nonenzymatic antioxidants) that lead to its therapeutic effects [453]. However, as with many prodrugs, limitations still exist for clinical use of GA and to confirm its therapeutic outcomes. Several nanostructures have been used to fabricate delivery systems to solve these limitations and achieve effectiveness to translate GA into clinical investigations [456–460].

Only a few studies have been reported on MSN nanosystems for GA. MSNs functionalized with amino acid and coated with chitosan exhibit a high loading capacity of ~20–38%, leading to better killing potency of MCF-7cells than pure GA [195]. GA is an unstable molecule under specific pH; by encapsulating it in MSNs, the release of GA can be controlled by media with different pH and released in the presence of higher antioxidant activity [224]. With respect to the anticancer effect, incorporation of GA into MSNs by means of covalent bonding increases its activity against HeLa and KB cells, with a killing efficiency of up to 67% [461]. Thus, GA-loaded MSNs easily internalize into Caco-2 cells, releasing GA to enhance cytotoxic effects against colon cancer [462].

#### *5.7. Essential Oils*

Among the plant natural prodrugs, the essential oils (also known as volatile oils) have particular importance in many sectors (e.g., pharmaceutical, cosmetic, agricultural, and food) [463,464]. With a long history in many cultures, essential oils can be used for different purposes [465]. For example, Ancient Egyptians used essential oils as early as 4500 BC for cosmetics and ointments [466]. They made a mixture of many herbals containing essential oils (e.g., aniseed, cedar, onion, myrrh, grapes, etc.) as preparations in perfume or medicine. In recent years, the most important use of essential oils has been aromatherapy due to their curative effects [467]. Essential oils are complex mixtures of volatile compounds found especially in aromatic plants, such as clary sage (*Salvia sclarea* L.), eucalyptus (*Eucalyptus globulus* Labill.), geranium (*Pelargonium graveolens* L.), lavender (*Lavandula officinalis* Chaix), lemon (*Citrus limon* L.), peppermint (*Mentha piperita* L.), roman chamomile (*Anthemis nobilis* L.), rosemary (*Rosmarinus officinalis* L.), basil (*Ocimum basilicum*), rosemary (*Rosmarinus officinalis*), and ginger (*Zingiber officinale*). The essential oils are obtained from plant sources by several methods, such as hydrodistillation, steam distillation, cold pressing, solvent extraction, microwave-assisted processing, and carbon dioxide extraction. Concerning their chemical composition, essential oils were originally characterized as monoterpene and sesquiterpene hydrocarbons together with their oxygenated derivatives, besides the aliphatic aldehydes, alcohols, and ester structures [466]. Due to the chemical compositions of essential oils with versatile compounds that possess many roles and modes of action in various pharmacological entities and therapeutics, including anticancer, cardiovascular disease treatment, anti-bacterial, anti-viral, anti-oxidants, analgesics, and antidiabetics [468,469]. The main applications are enhanced transdermal drug delivery due to their skin penetration, and aroma and massage therapy [470]. Essential oil compounds have been reported to have potential anticancer effects in vitro and in animal models [471–477]. However, essential oils generally have low stability, high volatility, and a high risk of deterioration by exposure to direct heat, humidity, light, or oxygen [478]. Nanoformulations are a recent strategy being developed for essential oils and their constituents to solve these problems [463,479–482].

To the best of our knowledge, no anticancer nanoformulations have been designed for essential oils and their constituents. MSNs are efficient particles for the high loading of essential oil substances. Melendez-Rodriguez et al. demonstrated that eugenol, an important component in various essential oils of herbs, is efficiently encapsulated in pores of MSNs up to 50 wt.%. by means of vapor adsorption [483]. Ebadollahi et al. [484] reported that the loading of essential oils of thymus species into MCM-41 increases their stability and persistence up to 20 days. Furthermore, Janatova et al. [485] demonstrated that different encapsulated essential oil components in MCM-41 provide long-term effects through

controlled release compared to the same pure substances. In addition, Jobdeedamrong et al. [486] showed that the release of essential oils (peppermint, thyme, cinnamon, and clove oil) is controlled by loading them into MSN-functionalized particles and grafting them with hyaluronic acid. Confirmation of delayed volatilization was reported for lavender oil loaded into MSNs [487]. Jin et al. [488] showed that MCM-41 modified nanoparticles enable high loading of pepper fragrant along with bactericidal activities against different microbes. Thus, the incorporation of essential oils from different herbs could be used effectively for infectious diseases [489,490] and treating biofilm [491].

#### *5.8. Other Natural Products*

Artemisinin is a sesquiterpene lactone derived from *Artemisia annua*. It is used as an antimalarial for treating multi-drug-resistant strains of falciparum malaria. It has also shown promising anticancer effects [492]. Artemisinin loaded into pores of hollow MSNs and capped with Fe3O4 nanoparticles act as gatekeepers [226]. The system shows a pH-dependent release effect, with stable release achieved at pH 7.4 and higher artemisinin release at low pH (3.8–5.0). This system exhibits excellent anticancer efficacy. Another multifunctional nanocarrier, Fe3O4@C/Ag@mSiO2 loaded with a high amount of artemisinin, allows pH-stimuli release and more killing of HeLa cancer cells compared to free artemisinin [493].

Some natural prodrugs are toxic compounds, and this toxicity prevents them from being used to treat cancers. An important example is colchicine, a natural alkaloid derived mainly from *Colchicum automnale*. It has long been used clinically to treat gout and familial Mediterranean fever. Colchicine is an important antimitotic prodrug and efficiently kills cancer cells [494], but the major challenge to its use is its toxicity. Earlier, Cauda et al. reported a one-step fabrication of colchicine-loaded in lipid bilayer-coated MSNs, making the system more stable and leading to effective microtubule depolymerization upon cell uptake [495]. We also loaded colchicine into folic acid-conjugated MCM-41 and KCC-1 for anticancer and antioxidant effects, obtaining higher anticancer effects than with free colchicine [42]. Very recently, we developed a novel DDS for colchicine. The system consisted of KCC-1-functionalized with phosphonate groups and loaded with colchicine, and subsequently coated with folic acid chitosan–glycine complex (MSNsPCOL/CG-FA) [43]. This nanoformulation revealed enhanced selective killing towards cancer cells compared to free colchicine in this order: colon cancer (HCT116) > liver cancer (HepG2) > prostate cancer (PC3). As its cytotoxicity is a major concern, the system is also promising because it exhibits low cytotoxicity (4%) compared to free colchicine (~60%) in normal BJ1 cells. The main mechanism of action was studied in detail for HCT116 cells, indicating primarily intrinsic apoptosis as a result of enhanced antimitotic effects with a contribution of genetic regulation by MALAT 1 and mir-205 and immunotherapy effects by Ang-2 protein and PD-1.

Loading glabridin, a prodrug compound obtained from the root extract of *Glycyrrhiza glabra,* on MSNs leads to remarkable improvement in its saturation solubility and dissolution velocity [496]. In this context, loading of breviscapine in MSNs significantly improves the solubility and bioavailability [17]. In addition, Ibrahim et al. [183] concluded that incorporating silymarin in MSNs within a lyophilized tablet remarkably increases the dissolution rate and saturation solubility. Similarly, loading of glabridin in MSNs improves the saturation solubility and dissolution velocity [496]. The biological activity, including anticancer activity, of polyphenols and flavonoids obtained from black chokeberry fruits is efficient compared to the free forms when loaded in MCM-41 and ZnO-MCM-41 [497] Co-delivery of topotecan and quercetin by MSNs results in pH-responsive release, subsequently increasing the intracellular release in cancer cells. Ultimately, it induces notable molecular activation (structural changes in tumor cell: endoplasmic reticulum, nucleus, and mitochondria) leading to cancer cell death [264]. Similar evidence has been obtained with targeted delivery of epigallocatechin-3-gallate-loaded MSNs for breast cancer treatment in vivo [266].

#### **6. Conclusions and Future Perspective**

Engineered MSNs with a variety of nanostructures are important inorganic nanocarriers for drug delivery in nanomedicine applications. MSNs have various unique physiochemical properties, including high pore volume, high specific surface area and porosity. In addition, various organic functional groups can be used for their surface modification by facile processes. MSNs are generally accepted to have good biocompatibility, being safe and showing no-significant side effects. The toxicity of MSNs as in the case of any drug or nanomaterial depends on dose/concentration, material properties, application routes. The degree of toxicity is low as indicated by several studies if the synthesis is performed in optimized conditions or overdose is avoided. Additionally, according to many animal studies, the toxicity of MSNs can diminish by optimizing the synthesis parameters and surface modification. Most in vivo studies generate data stipulating that the suggested average dose of 50 mg/kg is well tolerated in animals and safe without any toxicity or apparent abnormalities. This is considered as an adequate dose to be used, e.g., in cancer therapy. This dose can be increased for oral route administration compared to intraperitoneal or intravenous injection. Importantly, the use of MSN-drug-loaded nanoformulations can allow the use of an even higher dose (three or more times) [174]. As with other nanomaterials, for future translation to clinical applications, the safety aspects of MSNs should be considered carefully for each type because many nanostructures are reported. Recently, the first clinical trial in humans was conducted with oral delivery of fenofibrate formulation based on the ordered mesoporous silica [33].

MSNs can be used as multifunctional targeted anticancer delivery systems, delivering a variety of drugs, therapeutic proteins, and antibodies. Furthermore, due to their nanoporous structure, MSNs have a high loading capacity for therapeutic agents and are excellent nanocarriers for internal- and external-responsive release of drugs (e.g., pH, GSH, redox, light, magnetic direction, and ultrasound). The available data indicate that the use of MSNs as prodrug nanocarriers can overcome the present barriers in their application: poor water solubility, low bioavailability, and insufficient targeting. Therefore, the available literature suggests a high potential of MSNs as natural prodrug delivery vehicles. The present pre-clinical and clinical tests show that MSNs are promising drug delivery carriers from a biocompatibility/safety perspective, opening the door towards the clinical nanomedicine application for cancer therapy.

For future research directions, we suggest the importance of co-delivery systems in which two or more anticancer natural prodrugs are combined, as well as exploring thousands of natural prodrugs that have not been thoroughly investigated yet. Furthermore, scientists can investigate loading MSNs with crude extract from plant materials. This can also be explored in synergistic therapy with crude extract containing many prodrug components together. Particularly promising prodrug substances are essential oils applied using MSN-based delivery systems. Their traditional use is only limited to cosmetics and some pharmaceutical applications. The essential oil nanoformulations will add value to cancer therapy.

The core–shell nanoformulations containing a core of MSNs loaded with prodrugs and a shell of organic substances, such as chitosan, Arabic gum, or others, are highly recommended to establish prodrug delivery systems. As an important parameter, the stability and dispersibility of nanoformulations should be taken into consideration because they affect the biological performance and therapeutic actions. Additionally, we think that the large-scale production for each type of MSNs will lead to obtaining safe material by optimizing and stabilizing the material parameters. In our opinion, animal and reported clinical studies open the doors to develop MSNs-based nanoformulations to be translated into clinical evaluations for cancers soon.

**Author Contributions:** Conceptualization, K.A. and W.L.; literature review, K.A.; writing—original draft preparation, K.A.; writing-review and editing, K.A. and W.L.; visualization, K.A. and W.L.; supervision, W.L. Both authors have read and agreed to the published version of the manuscript.

**Funding:** This work was funded by the Institute of High Pressure Physics (IHPP), Polish Academy of Sciences (PAS), Poland and The APC was also funded by IHPP.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Acknowledgments:** We would like to thank the Institute of High Pressure Physics (IHPP), Polish Academy of Sciences (PAS), Poland, for the financial support.

**Conflicts of Interest:** The authors declare no conflict of interest regarding the publication of this paper.

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