3.1.2. Polymeric Nanoparticles as Delivery Platforms

Biodegradable polymeric NPs have also been developed as carriers for drug delivery to treat neurological diseases because they are not toxic and biocompatible, and they have the sustained-release feature [73]. Liu and co-workers developed tanshinone IIA PEGylated nanoparticles (CBSA-PEG-TIIA-NPs) conjugated with cationic bovine serum albumin (CBSA) to target negatively-charged lumens of brain microvessels [54]. The study demonstrated that NPs delivered tanshinone IIA into the rat brain in the cerebral I/R injury model, significantly suppressing the inflammation response. In addition, NPs reduced the infarcted volume and prevented the neuronal apoptosis. In another study, the same group investigated the mechanism of therapeutic effects using CBSA-PEG-TIIA-NPs. They found that NPs decreased neutrophil infiltration and prevented microglial activation. Furthermore, it is shown that prevention of neuronal apoptosis and suppression of inflammatory mediators (such as MMP-9 and COX-2) were regulated by mitogen-activated protein kinases (MAPK) signal pathways [55].

Nanoparticles were also bioengineered with ROS-responsive features that can control drug release in ischemic brain. In a study [56], Lv et al. designed nanoparticles to deliver NR2B9C (a neuroprotectant agent) to treat ischemic stroke. Nanoparticles (named SHp-RBC-NPs) composed of a red blood cell (RBC) membrane as a shell and a polymer nanoparticle with ROS-responsive boronic ester as a core. To target the ischemic brain, a peptide, SHp (CLEVSRKNC), was conjugated on the surface of nanoparticles. Triggered by high levels of ROS in ischemic region, the nanoparticles can control the release of NR2B9C in ischemic brain tissues. Ex vivo brain fluorescence imaging showed that SHp-RBC-NP can target the ischemic brain to significantly prevent neurological damage and reduce the brain infarction size.

3-n-Butylphthalide (dl-NBP) [74] has demonstrated the therapeutic value to treat ischemic stroke in clinic, but it is challenging to deliver it to the ischemic brain. To solve this problem, PEGylated-lipid nanoparticles (PLNs) conjugated with Fas ligand was developed to target the ischemic region of the brain and deliver 3-n-Butylphthalide (dl-NBP). The results are promising as the nanoparticle formulation alleviates the brain neurological injury [57].

Guo et al. designed AMD3100 (a targeting ligand)-conjugated and size-shrinkable nanoparticles (ASNPs) with the features of protease-responsiveness and brain-targeting. The nanoparticles specifically targeted the CXCR4 (C-X-C chemokine receptor type 4)-enriched ischemic brain tissue and penetrated

the ischemic brain. Glyburide, a promising anti-stroke drug, was delivered by the nanoparticles, and this system enhanced the therapeutic outcomes in the ischemic stroke model [58].

Peptide inhibitors for caspases possess neuroprotection effects, but they also cannot cross the BBB. Based on this problem, Karatas et al. developed chitosan nanospheres loaded with N-benzyloxycarbonyl-Asp(OMe)-Glu(OMe)-Val-Asp(OMe)-fluoromethyl ketone (Z-DEVD-FMK), a caspase-3 inhibitor. Anti-mouse transferrin receptor monoclonal antibody (TfRMAb) was conjugated to polyethylene glycol-coated nanospheres because the antibody can selectively bind to TfR type 1 expressed on cerebral vasculature. Nanospheres suppressed the caspase-3 activity and improved neurological repair after ischemic stroke [59].

#### 3.1.3. Biomimetic Nanoparticles as Delivery Platforms

Cell-derived biomimetic carriers have provided new options for drug delivery due to their advanced targeting abilities and better biosafety than artificial carriers. Shi et al. reported a T7 peptide (a brain targeting peptide)-linked erythrocyte membrane nanovesicles loaded with Mn3O4 nanoparticles (Mn3O4@nanoerythrocyte-T7, MNET), and this complex formulation may scavenge free radicals and change hypoxia environments in the ischemic brain. They found that MNET had a long circulation time and were capable of crossing BBB. In subsequent studies, MNET scavenged free radical and oxygen supply to rescue neurons during ischemic phase, and possibly MNET helped to remove oxygen generated by reperfusion [60].

Platelet membrane-derived nanovesicles were loaded with L-arginine, a nitric oxide (NO) donor, and γ-Fe2O3 magnetic nanoparticles (PAMNs) to achieve dual therapeutic and diagnostic purposes. The PAMNs were able to adhere to the damaged cerebral vessel induced by thrombosis and deliver L-arginine when the external magnetic field was applied. Furthermore, the release of L-arginine at ischemic lesions promoted vasodilation and the thrombosis was disrupted to restore the blood flow. This study showed that PAMNs may be a useful tool for both MRI imaging and targeted therapy for ischemic stroke [61].
