Lipid NPs have been introduced with the objective to not only increase delivery load but also decrease expulsion of drug (Joshi et al., 2019). Liposomes are biodegradable and biocompatible, with high loading capacity, and are able to cross the BBB. The injected liposomes were allowed to leak into the brain parenchyma, and then these liposomes gradually accumulated in the ischemic region during the acute phase of cerebral ischemia, due to the enhanced permeability and retention effect. Liposomes act as carriers of the neuroprotective agents FK506 (Fukuta et al., 2015) and Cyclosporin A (Partoazar et al., 2017), enabling them to pass through the blood-brain barrier more easily and function more efficiently. After the infusion of these two protective agents, infarct size was reduced, leukocyte infiltration was inhibited and the expression of TNF-α was reduced. Xenon (Xe), a noble gas, was reported to have promising neuroprotective properties with no adverse side effects (Altschuler, 2001). The Xe delivered by echogenic liposomes was proved to reduced bleeding, reduced apoptotic neuronal death, and neutrophil infiltration (Miao et al., 2018), as shown in Table 1.

Polymeric NPs, in particular, are a promising choice as drug delivery platform for central nervous system targeting, due to their tunable architecture (10 to 1000 nm), non-toxicity, biocompatibility, and controllable drug release (Kreuter et al., 2002). Polymers used to synthesis NPs include both synthetic polymers and natural polymers. The common synthetic polymers include poly-n-butylcyanoacrylate (PBCA), polyesters lactic acid (PLA), and poly lactic-co-glycolic acid (PLGA), while the natural polymers contains polysaccharides, amino acids, and proteins (Saraiva et al., 2016). Synthetic NPs can be reproducibly synthesized with specific functional groups, and their size, stability, shape is controlled (Khan et al., 2018), as show in Figure 3. A copolymer of polyethylene glycol (PEG)-PLA could effectively deliver C3-siRNA and curcumin into ischemic penumbra across the blood-brain barrier (BBB) and significantly decrease C3 expression and inhibit M1-microglial activation (Wang et al., 2018). NP-curcumin can also reduce the infarct size and improved function recovery (Wang et al., 2019). RNA therapy has broad prospects and can prevent or treat complex diseases in many fields. miR-195 was proved to possess the potential to become a new drug to treat acute ischemic stroke for it can anti-inflammation by directly blocking the NF-kB pathway in both cell and animal models. The reduction of injured brain volume by treatment of miR-195 could be up to 45% in stroke rats through multiple mechanisms including antiapoptotic and anti-inflammatory pathways (Cheng H. Y. et al., 2019).

Inorganic materials, such as silica and iron oxide, are employed to produce inorganic NPs, with both diagnostic and therapeutic functions, for their ability to be tracked by magnetic resonance imaging (MRI) (Saraiva et al., 2016). Previous research has confirmed Se take part in modulation of neurogenesis and Se administration contributes to mitochondrial dynamics after focal cerebral ischemia (Nissen-Druey, 1989). Anti-transferrin receptor antibody was synthesized with Se-NPs. The therapeutic effect of Se NPs was significantly effective in a murine stroke model because the activity of jak2/stat3 signaling pathways was inhibit and the transcription level of inflammation-related factor Adamts1 was reduced (Amani et al., 2019). Functionalized magnetic Fe₃O₄-NPs synthesized with L-carnosine peptide was demonstrated as efficient drug delivery platform for treatment of ischemic stroke, which can cross through the BBB (Lu et al., 2021). The drug loading rate of the NPs was stable at 95.6 ± 0.2% and releasing efficiency of the NPs was controlled and sustainable. Furthermore, the cytotoxicity and the biocompatibility test results were found to be satisfactory. However, inorganic NPs have low biocompatibility due to their inherent toxicity and may not be easily eliminated from the body. What’s worse is that they may cause chronic immune reactions (Yang et al., 2019).

Carbon-based NPs (CBNs) are becoming attractive NPs, containing renowned allotropic phases such as amorphous carbon, graphite and diamonds, as well as newly discovered auspicious carbon nanotubes (CNTs), graphene oxide (GO), graphene quantum dots (GQDs) and fullerene (Maiti et al., 2018). Each member of the carbon family exhibits inimitable features and has been widely applied in diverse aspects including drug delivery, imaging, diagnosis, and disease therapy (Bhattacharya et al., 2016). However, for CBNs still contain toxicity, more powerful studies are needed to be focused on the toxicity and pharmacokinetics of CBNs (Maiti et al., 2018). Fuller enol (OH-F) are radical scavengers acting as neuroprotective agents. MRI imaging revealed a significant reduction of infarct volume in ischemic rats treated with OH-F, and these derivatives mitigate the cellular damage and inflammation after stroke. Another study (Sarami Foroshani et al., 2018) has proved that OH-F NPs can reduce the transcription of IL-6 and MMP-9 to protect BBB integrity and relieve brain edema after cerebral ischemia-reperfusion injury.

Several enzymes produced by the human body such as superoxide dismutase (SOD), glutathione peroxidase and catalase; and non-enzymatic natural compounds such as ascorbate, vitamin E, glutathione, thymoquinone and melanin show favorable biocompatibility and robust anti-oxidative properties (Panzella et al., 2013; Couto et al., 2016; Li et al., 2017).

Therapeutic effect based on the native form of SOD are limited because of its short half-life in vivo and poor permeability across the BBB (Kinouchi et al., 1991). An approach is based on incorporation of SOD into nano-sized polyion complexes with cationic block copolymers (“nanozymes”). Nanozymes are core-shell structured NPs with the polyion complex core consisting of charge-neutralized polycation chains and protein globules, and the shell consisting of PEG chains. Primary amine groups in the core were cross-linked (cl) using low molecular mass chemical cross-linkers to form cl-nanozyme and further purified to improve sample homogeneity by removing non-cl-nanozymes (Manickam et al., 2012). A rat ischemia-reperfusion injury model treated with cl-nanozymes-SOD demonstrated a 65% reduction in infarct volume. cl-nanozymes-SOD localized in the endothelium of the cerebral vasculature demonstrated a significant reduction in the ROS activity, and protected neurons from undergoing apoptosis (Jiang et al., 2015). Besides, SOD-NPs maintained BBB integrity, thereby relieving cerebral edema. Melanin nanoparticles (MeNPs) is known to function as a potential radical scavenger for more potent and safer antioxidative therapy (Liu et al., 2017). PEG-MeNPs show powerful antioxidative function, and antioxidant targets contains multiple toxic RONS such as O2⋅−, H2O2, ⋅OH, ONOO−, and ⋅NO. In addition to antioxidant function, MeNPs can also inhibit the inflammatory caused by RONS through the suppression of inflammatory mediators and cytokines with negligible side effects.

Another interesting antioxidant compound is Thymoquinone (TQ), which has been proved to possess antioxidant and anti-inflammatory activity. The encapsulation of TQ in mesoporous silica nanocarriers (MSNs) enhanced its delivery to some brain areas (cortex, thalamus, hypothalamus, and midbrain). Furthermore, the activities of SOD and catalase and glutathione level were increased, while the malondialdehyde level was decreased in the brain of MSNs-TQ treated SHRSP rats, indicating its antioxidant (Guan et al., 2018; Fahmy et al., 2019), as shown in Table 2.

Common antioxidant drugs contain Tempol (Hosoo et al., 2017), edaravone (EDV) (Jin et al., 2017), succinobucol (He et al., 2021b), and curcumin (Wang et al., 2019), which were extracted naturally from plants or artificially synthesized. NPs as a feasible strategy extend the half-life of these drugs and improve their ability to cross the BBB, so improve the therapeutic effect of these drugs of ischemic stroke.

In H9c2 cells pre-treated with hypoxia conditions, Tempol could protect H9c2 cells from hypoxia-induced injury through suppressing ROS generation and lipid peroxidation, as well as enhancing antioxidant enzyme activity. Furthermore, the increased expression of Bcl-2 and decreased expression of Bax and caspase-3 in Tempol pre-treatment reduced apoptosis (Jing et al., 2017). During cerebral ischemia-reperfusion injury, pharmacologically active oligosaccharide material prepared by covalently conjugating a radical-scavenging compound (Tempol) and a hydrogen-peroxide-eliminating moiety of phenylboronic acid pinacol ester (PBAP) on β-cyclodextrin (TPCD) preserved the inter-endothelial tight junctions and significantly suppressed neuronal apoptosis, O^2– production, thence reduced BBB damage and infarction volume. In general, it can provide neurovascular unit protection (Hosoo et al., 2017; Yuan et al., 2021).

Encapsulated is a clinically approved neuroprotective drug, removing over-produced ROS with an unlimited therapeutic time-window. However, the shortcomings of EDV biological treatment are short circulation half-life and inadequate cerebral uptake. To overcome these weak points, an EDV-encapsulated agonistic micelle (EDV-AM) which adjusted BBB initiatively was developed to specifically deliver EDV into brain ischemia (Jin et al., 2017). The sustainable EDV release of EDV-AM in cells was confirmed by persistent intracellular ROS elimination, providing a basis for EDV-AM to become a promising neuroprotective treatment, especially for ischemic stroke patients who miss the narrow time window of thrombolytic therapy (Hou et al., 2019).

Succinobucol (SCB) (Muldrew and Franks, 2009) is a derivative of probucol, which has lipid-lowering effects with anti-inflammatory and antioxidant properties. SCB has the capacity of preventing the mitochondrial dysfunction induced by tert-butyl hydroperoxide, which provides lines of evidence to clarify SCB as a potential neuroprotective agent (Colle et al., 2013). A SCB-loaded pH-sensitive polymeric nanovehicle with a 4T1 cell membrane (MPP/SCB) (He et al., 2021b), improves the ability to penetrate the BBB and increases the targeting effect of cerebral ischemic lesions. The fluorescence signals of ROS in PC12 cells treated with MPP/SCB was significantly weaker. Meanwhile, the proportion of apoptosis cells was largely reduced, showing remarkable neuroprotective effects in tMCAO rat model.

Another highly concerned anti-inflammatory and antioxidant traditional Chinese medicine component, curcumin (Li et al., 2020), has been delivered by different NPs to overcome its limited stability in circulation. For example, ischemia rats administrated with curcumin loaded solid lipid nanoparticles (C-SLNs) showed increased levels of superoxide dismutase, catalase, glutathione, and mitochondrial complex enzyme activities, while the lipid peroxidation, nitrite, and acetylcholinesterase levels were decreased (Sadegh Malvajerd et al., 2019).

Another line of neuroprotective strategies in cerebral ischemia stroke is the use of neuroprotective compounds to directly target ecotoxicity. Overstimulation of N-methyl-D-aspartate receptor (NMDAR) during ischemia-reperfusion stimulates the influx of Ca²⁺ (Wu and Tymianski, 2018). Calcium overload triggers a range of downstream pro-death signals including calpain activation, ROS overload, and mitochondrial damage (Dirnagl et al., 1999), resulting in nerve cell apoptosis. NMDAR channel blockers therapeutic approach may have fewer side effects and/or provide a wider therapeutic window for stroke.

The NR2B9c (Lv et al., 2018) is a well-known peptide confirmed to prevent NMDAR-mediated neurotoxicity without affecting its activity. Wheat germ agglutinin (WGA)-modified NPs carrying NR2B9c (NR2B9c-WGA-NPs) (Li et al., 2019) have a strong ability to cross the BBB for WGA has high affinity for abundant receptors on neuronal surface. NR2B9c-WGA-NPs effectively protected cultured primary cortical neurons from NMDA-induced excitotoxicity, relieved focal ischemic damage in the rat brain, as well as improved their neural function after stroke.

The disruption of the neuronal nitric oxide synthase -postsynaptic density protein-95 (nNOS-PSD-95) can indirectly inhibit the activity of NMDAR, thus preventing excitotoxicity induced by glutamate. ZL006 (Wang et al., 2017) had reformed cerebral ischemic damages and exerted neuroprotective activity in mice and rats subjected to middle cerebral artery occlusion through selectively uncoupling nNOS from PSD-95 induced by ischemia. An animal experiment (Zhao et al., 2016) confirmed that neuroprotectant ZL006 loaded by dual targeted lipid nanomaterials was capable of reducing infarct volume and relieving neurological impairment owing to the NPs could increase drug concentration of ischemic tissue. Neuroplasticity was improved by systemic administration of ZL006 after ischemia as it could reduce excessive neural excitability by dissociating nNOS-PSD-95 complex, and then increase dendrite spine density, thereby improving neuroprotection outcome (Lin et al., 2018).

Matrix metalloproteinases (MMPs) are enzymes capable of cleaving extracellular matrix, membrane, and secreted proteins. The importance of MMPs in ischemic stroke makes it a target for developing novel inhibitors which can serve as promising therapy in patients with stroke (Mashaqi et al., 2021). Among endogenous inhibitor of MMPs, tissue inhibitor of matrix metalloproteinases 1 (TIMP-1) has the greatest therapeutic potential for its strong affinity for MMPs (Sa et al., 2011; Zhong et al., 2019). Generally, the disadvantages of native TIMP-1 include disability to crossing the BBB, short half-life and low bioavailability. However, TIMP-1 wrapped by NPs could increase its bioavailability through enhancing delivery across BBB.

In a hippocampal slice culture model (Chaturvedi et al., 2012), results show that neurons are protected by TIMP-1 from excitotoxicity induced by excitatory amino acid. Further the protection effects are strengthened when TIMP-1 is transported in sustained manner by PLGA NPs. Ps80-coated NPs had an elevated systematic penetration rate, thus Ps80-coated TIMP-1 PLGA NPs may have longer circulation time vivo and better brain permeability compared to PLGA NPs without Ps80-coat. Besides, previous study also showed TIMP-1 was capable of suppressing MMP-9 activity in the brain through direct injection (Knapska et al., 2013).