Advantages of nanocarriers for basic research in the field of traumatic brain injury

Abstract

A major challenge for the efficient treatment of traumatic brain injury is the need for therapeutic molecules to cross the blood-brain barrier to enter and accumulate in brain tissue. To overcome this problem, researchers have begun to focus on nanocarriers and other brain-targeting drug delivery systems. In this review, we summarize the epidemiology, basic pathophysiology, current clinical treatment, the establishment of models, and the evaluation indicators that are commonly used for traumatic brain injury. We also report the current status of traumatic brain injury when treated with nanocarriers such as liposomes and vesicles. Nanocarriers can overcome a variety of key biological barriers, improve drug bioavailability, increase intracellular penetration and retention time, achieve drug enrichment, control drug release, and achieve brain-targeting drug delivery. However, the application of nanocarriers remains in the basic research stage and has yet to be fully translated to the clinic.

Introduction

With better awareness of safety and the emergence of appropriate medical facilities, the incidence of traumatic brain injury (TBI) has decreased over recent years. However, the mortality and paralysis rates of TBIs have not been significantly reduced (Silverberg et al., 2020). There is no doubt that the treatment of TBI remains a significant challenge (Galgano et al., 2017) due to a lack of effective treatments and therapeutic drugs, the high costs of treatment, and low recovery rates.

TBIs are caused by the direct impacts of local primary injuries or more extensive diffuse injuries before the onset of secondary injuries, thus resulting in long-term consequences to a wide range of body functions. TBIs have become a significant focus of research attention over recent years; consequently, data is emerging in relation to treatment plans, effective drugs, and drug delivery systems. Different degrees of TBIs cannot be separated by drug treatment (Polich et al., 2019). One of the first considerations relating to medications for TBIs is how to enable a drug to penetrate the blood-brain barrier (BBB). The second consideration is how we can reduce the systemic toxicity of effective drugs and reduce damage to normal tissues and cells. A suitable drug delivery system can help drugs to deliver better therapeutic effects and minimize adverse effects of the drug. Targeted agents can enable the efficient delivery of drugs to the site of injury and reduce systemic adverse effects. In this review, we summarize the most promising applications in TBI treatment, including brain-targeting nano-drug delivery systems and associated modifications. The nanocarriers such as liposomes and vesicles for the treatment of TBI has been applied just in animal models. Other potential carriers, such as magnetic nanoparticles and extracellular vesicles, have shown excellent properties in the treatment of brain diseases, but have not been used in TBI. The promising nanocarriers for the efficient treatment of TBI are summarized and compared.

Search Strategy

The articles considered in this narrative review related to TBI and nanocarriers were electronically retrieved from the PubMed database using the following search terms: pathophysiology, clinical status, animal models and measurement indicators for brain injuries; we considered articles that were published from 2009 to 2023. The search words related to brain targeting and nanodelivery systems included brain injuries (MeSH Terms); traumatic (MeSH Terms); brain injuries (MeSH Terms) AND clinical (MeSH Terms); brain injuries (MeSH Terms) AND indicators (MeSH Terms); Brain injuries (MeSH Terms) AND drug carriers (MeSH Terms). Identified articles were screened by title and abstract and articles that were inconsistent with our inclusion criteria were excluded from further analysis.

Epidemiology of Traumatic Brain Injuries

TBIs are a major cause of unnatural death and disability. The global incidence of TBIs stands at approximately 50 million cases per year (Capizzi et al., 2020), and on average, one in two persons will be threatened by TBIs in their lifetime (Jiang et al., 2019). TBIs remain a significant challenge for physicians (Hartings et al., 2011). Severe TBIs are associated with a mortality rate of 30–40%; these injuries are difficult to treat, have a low curation rate and a poor prognosis (Mollayeva et al., 2018). Many of the patients with TBI are vulnerable to sequelae and disability which can exert adverse effects on their quality-of-life and living standards. Existing data shows that sports injuries, falls, traffic accidents, being hit by objects, and explosion impact, can all lead to TBIs (Coronado et al., 2012). TBIs are also likely during military activities, which can exert significant impact on the combat efficiency of soldiers. The groups of patients with the highest incidence rates of TBI are children who are 0 to 4 years of age, young people who are 15 to 24 years of age, and the elderly over 65 years of age (Khellaf et al., 2019). Owing to an enhanced awareness of self-protection, and improved management and treatment measures, the number of deaths related to TBI has decreased over recent years; however, the overall incidence of TBI deserves attention.

Pathophysiology of Traumatic Brain Injuries

‘TBI’ is the disease varied widely in a manner that depends on the injury severity, such as mild symptoms manifested as concussion and the severe case manifested as coma. A mild concussion only manifests as a transient loss of consciousness and retrograde amnesia but recovery is fast and relatively efficient (Sussman et al., 2018). Brain injuries with a relatively small amount of bleeding can result in the formation of a hematoma in the brain, double submembrane hemorrhages, a minor fracture, and a small amount of epidural or subdural hematoma; these effects manifest as dizziness and headache (Pearn et al., 2017). If the disease does not progress, it will recover after a period of conservative treatment. On the other hand, the disease can rapidly deteriorate, causing brain thrombosis and cardiac apnea, thus requiring immediate rescue (Cash and Theus, 2020). Injury to the frontal lobe may lead to personality changes, occipital lobe injury may lead to visual impairment, and temporal lobe injury may induce epilepsy. A brain stem injury or diffuse axonal injury may lead to coma and even make the patients into a vegetative state. Damage to the cerebellum will cause dizziness, nausea, and vomiting. Syndromes and manifestations after brain injury are known to vary widely (Paterno et al., 2017); thus, ‘TBI’ represents a comprehensive disease that involves multiple symptoms, rather than a single disease symptom.

TBIs are divided into primary injuries and secondary injuries (Figure 1). Primary injuries disrupt the BBB and damage neuronal and glial tissue (Karve et al., 2016), thereby causing local inflammation (Postolache et al., 2020) and secondary neurodegeneration while secondary injuries are characterized by the persistent up-regulation of proinflammatory cytokines, a reduction in the number of oligodendrocytes and a reduction in glial reactivity caused by cerebral ischemia (Dixon, 2017). The lack of glucose and oxygen in brain tissue leads to glutamate excitability toxicity and the excessive influx of calcium; this places tissue cells in a state of stress and promotes the formation of free radicals (Figure 2; Sulhan et al., 2020). Inflammation will cause further damage to the brain (Khatri et al., 2018), including cognitive impairment, memory impairment, movement disorders, loss of hearing and vision, and psychological problems.

Figure 1.

Primary and secondary injury processes associated with traumatic brain injuries.

Mild brain injury mostly manifests as concussion, and the transient loss of consciousness may occur. Severe brain injury can seriously affect the central nervous system, resulting in primary brain damage, including brain edema and damage to the blood-brain barrier. If left uncontrolled, this damage will lead to neuroinflammation, neuronal loss, nerve tissue damage and other forms of secondary damage, eventually leading to cognitive impairment. Created with PowerPoint (version 2019).

Figure 2.

Mechanisms underlying TBI secondary injury.

Primary injury destroys the BBB and activates microglia and white blood cells, thus causing astrocytes to react and secrete various cellular inflammatory factors, chemokines, and ROS; these factors can threaten nerve cells. The lack of glucose and oxygen in nerve cells leads to glutamate excitotoxicity and excessive calcium influx, thus promoting the formation of ROS, the activation of Caspase-3 protease, and destruction of the nucleus, thus leading to apoptosis. BBB: Blood-brain barrier; IL: interleukin; NF-κB: nuclear factor-κB; ROS: reactive oxygen species; TBI: traumatic brain injury; TNF-α: tumor necrosis factor-α. Created with PowerPoint (version 2019).

Clinical Status of Traumatic Brain Injuries

TBIs are one of the most difficult neurological diseases to treat. There is no officially approved systematic treatment or specific treatment other than surgical treatment and non-surgical conservative treatment. For severe primary brain injuries, surgical treatment, the evacuation of hematoma, decompression of a bone flap, the relief of hematoma compression, and other methods, are generally used for emergency treatment (Iaccarino et al., 2021). The commonly used interventional drugs for TBIs include psychostimulants, anti-depressants, anti-Parkinson drugs, and anti-convulsants. Non-drug treatments include hyperthermia therapy, hyperbaric oxygen therapy, and antioxidant therapy. Each treatment is intended to stabilize the patient’s condition, prevent secondary brain injury, and reduce complications and sequelae (Reddi et al., 2022). Clinical diagnosis is mainly based on the Glasgow Coma Scale: severe (3–8 points), moderate (9–12 points) and mild (13–15 points), and whether computed tomography (CT) of the head is abnormal (Chesnut et al., 2018). The discovery of sensitive biological indicators for the evaluation of TBIs in biological fluids will facilitate the diagnosis of patients with mild TBI and prevent additional damage caused by imaging diagnosis. The occurrence of TBIs is usually accompanied by a variety of other injuries, including acute craniocerebral trauma, shock, airway obstruction and asphyxia, cardiopulmonary failure, severe pulmonary infection, acute respiratory distress syndrome, gastrointestinal bleeding, intracranial infection, electrolyte disorders (Gao et al., 2020). Given the vast range of injuries, clinical priorities should be given to life-threatening injuries in order to reduce the series of fatal complications caused by TBIs and improve the survival rate of TBI patients. TBIs can also have long-term effects on patients, including epilepsy, dementia, neurodegenerative diseases such as Alzheimer’s disease, and secondary headache disorders (Ashina et al., 2021). The clinical management of TBIs should not only focus on short-term injury, but also work towards the long-term survival of patients.

Therapeutic Targets for Traumatic Brain Injuries

Despite ongoing research on TBIs, the clinical diagnosis and treatment of TBIs are extremely limited. Currently, there are few officially approved and effective drugs for the treatment of TBIs. Moreover, symptomatic treatment strategies are mostly used in clinical practice to delay brain lesions and improve brain function. There is an urgent need to identify more effective therapeutic drugs and treatment methods based on precise therapeutic targets. In this study, we summarize the characteristics and treatment strategies of each of the known therapeutic targets described in the existing literature (Table 1). Primary and secondary injury causes by TBI can result in the expression of microRNAs (miRNAs) (Paul et al., 2020). miRNAs can pass through the BBB (Zhang et al., 2022) and the inhibition or up-regulation of specific miRNAs may contribute to the treatment of TBIs. Calcium/calmodulin-dependent protein kinase type 2 delta (CAMK2D) is expressed predominantly in the pyramidal neurons of the hippocampal CA3 region (Rijkers et al., 2010); the expression levels of CAMK2D are known to be reduced in the hippocampus after brain injury. A previous study showed that the cognitive function of mice was improved after the stereotaxic injection of CAMK2D protein into the CA3 region of the brain (Figueiredo et al., 2020). The inhibition of poly ADP-ribose polymerase over-activation in brain neurons has been shown to reduce neuronal death caused by the excessive release of inflammatory factors (Sun et al., 2022). Poly ADP-ribose polymerase inhibitors have shown good clinical therapeutic potential as a therapeutic target for TBI.

Table 1.

Summary of therapeutic targets for traumatic brain injuries

Therapeutic target	Characteristic	Therapeutic strategy	Reference
MicroRNA (miRNA)	Low number of nucleotides, conservation among species	miRNA inhibitors and miRNA mimics	Paul et al., 2020
CAMK2D	CAMK2D affects the expression of brain-derived trophic factor BDNF	Stereotactic injection of CAMK2D-overexpressing virus into the brain	Figueiredo et al., 2020
PARP	Small toxic side effects, clear effect, mitochondrial targeted delivery	Inhibitor of PARP	Chen et al., 2016; Sun et al., 2022
Microglia	Produce neuroprotective factors, clear cell debris, coordinate nerve repair, but after dysregulation produce pro-inflammatory factors and toxic mediators	Minocycline, atorvastatin, phosphodiesterase 4B, soluble epoxide hydrolase	Clausen et al., 2009; Liu et al., 2023
Ferroptosis	Iron dependence, non-apoptotic forms of programmed cell death	Inhibitor of Janus kinases 1 and 2	Xie et al., 2019
NMDAR	Neuroplasticity, nerve regeneration	D-cycloserine	Chamoun et al., 2010

BDNF: Brain-derived neurotrophic factor; CAMK2D: calcium/calmodulin-dependent protein kinase type 2 delta; NMDAR: N-methyl-D-aspartate receptor; PARP: poly ADP-ribose polymerase.

Many anti-inflammatory strategies specifically target the microglia. Microglia of the M2 phenotype are more likely to remove cellular debris in the early stages of TBI while microglia of the M1 phenotype may be associated with chronic neuroinflammation. Following TBI, the dominant phenotype of activated microglia may change from an acutely activated M2 to a chronically activated M1 phenotype. However, an imbalance in microglial phenotype can produce pro-inflammatory factors, cause cytotoxicity, and lead to neuronal damage. Therefore, regulating the ratio of M1 to M2 microglia could help microglia to play a positive therapeutic role in TBI; thus, the polarization of microglia may represent a new target for pharmacological intervention (Clausen et al., 2009).

Free irons play a key role in reactive oxygen species (ROS) and lipid peroxidation. The autophagy of ferritin leads to the occurrence of ferroptosis; this results in an increase of free iron, thereby leading to secondary TBI injury. It is believed that an increasing number of therapeutic targets for TBI will be elucidated and deliver more benefits to TBI patients.

Common Animal Models of Traumatic Brain Injury

Research involving TBI animal models could help us to identify the mechanisms responsible for the development of TBIs. Currently, most of the animal models used for TBI research are experimental rats, most commonly male Sprague-Dawley and Wistar rats weighing 230 to 300 g. However, mouse models have also been used; mostly involving C57BL/6 and BALB/C strains and animals aged 6 to 8 weeks of either sex. The severity and location of brain injury are very important for the success of model establishment, and reproducibility is critical. Several mature modeling methods have been established by continuous practice and testing. Several animal models are widely used in current research, including the hydraulic impact model, free fall model, controlled cortical injury (CCI) model, penetrating ballistic brain injury model, and simulated blast injury model (Ackermans et al., 2021).

Fluid percussion injury models

The fluid percussion injury model is established by placing anesthetized animals on an encephaloscope before opening the scalp and drilling a hole with a diameter of 4.8 mm between the anterior fontanelle and the sagittal suture to keep the dura mater intact (Dai et al., 2018). A fluid pressure pulse is then applied to the bone window through the impact of a pendulum, thus causing transient displacement and deformation of the brain (Brady et al., 2019). The degree of injury depends on the height of the pendulum and the pressure created by fluid ejection and this model is suitable for generating mild brain injury and diffuse axonal injury (Ma et al., 2019; Moro et al., 2021). The model is highly reproducible and can easily control the degree of injury; however, surgical aspects are complicated, the equipment is expensive, and the mortality rate is higher than for other models.

Weight-drop injury models

Several weight-drop injury models have been developed, including the Feeney model, Marmarou model, and Shohami model; these models are suitable for focal injury, diffuse injury, and mixed injury, respectively (Petersen et al., 2021). In these models, the skull and dura of experimental animals are exposed to a guided free fall; the degree of injury is determined by adjusting the mass of the weight and the height of the fall. The free-fall model is simple, inexpensive, easy to operate, controllable, and can replicate graded brain injury (Estrada-Rojo et al., 2018). However, these models are associated with a high mortality rate and poor stability and reproducibility.

CCI models

The CCI model is suitable for mild, moderate, and severe focal brain injury (Petersen et al., 2021). This model uses a controlled pneumatic or electromagnetic device. First, a bone window is created; then, a metal head, controlled by pneumatic or electromagnetic means, is used to directly hit the dura mater into the brain tissue. The degree of damage is determined by changing the impact depth, impact speed, and tip size; these parameters are all controlled by a computer (Ma et al., 2019). This model is one of the most accurate models available and has good stability and clinical relevance (Padmakumar et al., 2022). Although the CCI model can be used to manipulate the parameters of damage to control the severity of injury, there is no laboratory standardization for mild, moderate, or severe damage. In addition, the device is expensive and requires frequent maintenance due to the need for high experimental accuracy.

Penetrating ballistic models

Penetrating ballistic models are usually caused by gunshot or bomb fragments, which create temporary cavities in the brain (Li et al., 2021b). Penetrating ballistic injuries are common in wars and are associated with high levels of morbidity and mortality. Rats, cats, and non-primates are commonly used in experimental studies involving high-energy warheads and shock waves. A temporary cavity is formed in the animal model’s brain, and the volume of the cavity created is many times larger than the size of the warhead itself. This model is suitable for moderate to severe focal injury. However, the major disadvantage of this model is that the temporary cavity formed by the injury can cause extensive intracranial hemorrhage in the primary area of the lesion; furthermore, the mortality rate associated with this experimental model is high. In addition, the degree of injury is difficult to evaluate (Bailey et al., 2019; Ma et al., 2019).

Blast-induced models of TBI

Blast injuries are common in combat situations. The blast-induced TBI model is often used to study the mechanism of TBI injury and to identify treatment methods to reduce casualties during wartime. Experiments have been performed to simulate blast waves similar to those produced by explosives by placing animals in detonation tubes and exposing them to blast waves caused by air pressure or explosions (Nonaka et al., 2021). However, research related to the laboratory simulation of blast injury models has progressed slowly and breakthroughs are not possible without multidisciplinary collaboration (such as mechanics, mathematics and computer technology) (Wermer et al., 2020; Matsuura et al., 2021). However, the application of advanced techniques such as in vivo imaging and high-resolution imaging to simulate blast injury may help elucidate the mechanisms of blast injury and identify potential solutions.

Evaluation Indicators for Traumatic Brain Injuries

Many mechanisms are associated with secondary injury caused by TBI and a range of different mechanisms can cause various pathological changes in the model, thus leading to different alterations in indicators. Therefore, it is of profound significance to identify accurate and reliable evaluation indicators to clarify the mechanism of TBI damage and evaluate the effect of drug treatment. Common evaluation indicators include neurological deficit scores, behavioral evaluation indicators, oxidative stress-related indicators involved in the disease process, inflammatory factor indicators and calcium overload (Table 2). Each evaluation index has its own advantages, and several indexes are often selected for comprehensive evaluation.

Table 2.

Evaluation indicators for traumatic brain injuries

Category	Evaluation indicator	Reference
Behavioral evaluation indicators	mNSS, pen field test, water maze test, elevated plus maze test, novel object recognition test, line grasping test, bilateral forepaw muscle grasping test	Toshkezi et al., 2018; Ma et al., 2020; Friedman-Levi et al., 2021; Sun et al., 2021
Oxidative stress	Nrf2, HO-1, NQO-1, SOD, MDA	Khatri et al., 2018; Wang et al., 2020; Mei et al., 2021; Ponomarenko et al., 2021; Zhang et al., 2021
Inflammation	TNF-α, IL-1, IL-1β, IL-6, IL-8, IL-10, IL-12, IL-18, ICAM-1, VCAM-1, NF-κB, NLRP3, Caspase-1	Sara et al., 2020; Kempuraj et al., 2021; Ponomarenko et al., 2021; Wang et al., 2021; Yuan et al., 2021
Apoptosis	Caspase-3, TfR1, Fpn, FTH, FTL, RIPK1, MLKL	Chen et al., 2021; Lorente et al., 2021; Rui et al., 2021; Wehn et al., 2021; Yu et al., 2021
Calcium overload	Glutamate, ROS, Calpain	Ng and Lee, 2019; Han et al., 2022; Zong et al., 2022

Fpn: Ferroportin; FTH: ferritin heavy chain; FTL: ferritin light chain; HO-1: heme oxygenase-1; IL: interleukin; ICAM-1: intercellular cell adhesion molecule-1; mNSS: modified Neurological Severity score; MDA: malondialdehyde; MLKL: mixed lineage kinase domain-like protein; NF-κB: nuclear factor-κB; NLRP3: NOD-like receptor family pyrin domain containing 3; Nrf2: nuclear factor E2 related factor 2; NQO-1: quinone oxidoreductase 1; RIPK1: receptor-interacting protein kinase 1; ROS: reactive oxygen species; TfR1: transferrin receptor 1; TNF-α: tumor necrosis factor-α; VCAM-1: vascular cell adhesion molecule-1.

Modified neurological severity score

The modified neurological severity score (mNSS) is a comprehensive method used to evaluate motor ability, vision, touch and proprioception, and the normality of various reflex activities in rats (Xiong et al., 2009). The mNSS is often used to evaluate TBI models in rats. The international standard currently defines normal SD rats as 0 points; mNSS scores range between 1 and 6 points for mild injury, 7 to 12 points for moderate injury, and 13 to 18 points for severe injury. Higher scores indicate more a severe neurological impairment. The neurological symptom scoring method is intuitive and simple and can evaluate the degree of nerve injury from an overall perspective without the need for other special equipment; furthermore, this test is commonly used in behavioral tests (Zhang et al., 2017). However, compared with other testing methods, this method is subjective and crude, capable of rough observations and estimations only, and unlikely to accurately describe nerve injury and recovery. Therefore, this score is often used in combination with other experiments to evaluate the sensorimotor function of ischemic animal models rather than being used alone.

Behavioral evaluation indicators

The behavioral evaluation of animal experiments includes the open field test, rotor-rod test, water maze test, elevated plus maze test, novel object recognition test, line grasping test, and the bilateral forepaw muscle grasping test as nerve function evaluation methods to evaluate the balance motor function of animals after brain injury, and to assess the degree of nerve injury and recovery of the model (Figure 3).

Figure 3.

Commonly used behavioral tools that can evaluate traumatic brain injuries.

(A) The open field test is used to evaluate spontaneous activity, exploratory behavior, anxiety, and depression in animal models. (B) The elevated plus maze can be used to examine the state of anxiety. (C) The water maze can be used to assess memory ability for spatial location and orientation. (D) The rotarod test provides a convenient method for detecting motor function in rodents, and can be used to evaluate fatigue, skeletal muscle relaxation, and central nervous system inhibition. (E) The novel objects recognition test is a learning and memory test based on the principle that animals have an innate tendency to explore new objects. Created with PowerPoint (version 2019).

Several animal experiments have been used to verify the ability of animals to determine the prognosis of TBI. Open field experiments have been used to investigate spontaneous exploration ability, performance ability and the anxiety of animals (Friedman-Levi et al., 2021). The rotarod test is used to study the exercise tolerance of an animal model (Toshkezi et al., 2018). The Morris water maze is commonly used to study the spatial learning and memory ability of an animal model (Ma et al., 2020). The fear and anxiety of novel objects can be investigated by performing elevated plus maze experiments. Novel object recognition experiments are intended to study whether animals have exploratory learning ability to identify new objects or not. The time spent on the rotarod can be used to measure the motor ability of mice to evaluate motor ability or mental and behavioral disorders (Sun et al., 2021).

Indicators related to oxidative stress

ROS plays a vital role in regulating cell signaling and tissue homeostasis pathways; maintaining balanced ROS levels is critical in a physiological state (Khatri et al., 2018). TBI causes mitochondrial damage and can lead to the accumulation of oxidative stress products and metabolic abnormalities. The activation of inflammatory pathways also leads to the production of ROS (Wang et al., 2020; Mei et al., 2021). Furthermore, ROS products mainly consist of malondialdehyde and superoxide dismutase; there is a negative correlation between superoxide dismutase and malondialdehyde. Superoxide dismutase can catalyze the decomposition of superoxide into oxygen and hydrogen peroxide, thus leading to the occurrence of oxidative stress. Malondialdehyde is a lipid peroxidation product; excessive accumulation of this product can cause cellular damage (Ponomarenko et al., 2021; Zhang et al., 2021). At the molecular level, the expression of nuclear factor E2 related factor 2, heme oxygenase-1, and quinone oxidoreductase 1, in the cortex can also reflect the progression of oxidative stress; therefore, these factors can be used as indices to evaluate drug efficacy against oxidative stress and provide evidence for new TBI treatments.

Indicators of inflammation

In the inflammatory response mechanism generated by TBI animal experiments, tumor necrosis factor-α (TNF-α) can activate macrophages and microglia to produce inflammatory metabolites which maintain and aggravate the inflammatory response, thus leading to the aggravation of TBI secondary injury. Interleukin (IL)-1, IL-1β, IL-6, IL-8, IL-10, IL-12, IL-18, and other proinflammatory factors play an essential role in the sterile immune response after TBI and participate in the inflammatory cascade (Wang et al., 2021). Proinflammatory factors and TNF-α stimulate the upregulation of chemokines and adhesion molecules, cell adhesion molecules such as intercellular adhesion molecule-1 and vascular adhesion molecule-1, and continuously mobilize immune cells and glial cells in a parallel and synergistic manner (Kempuraj et al., 2021). Nuclear factor-κB is a transcription factor critical to the regulation of the inflammatory response. Nuclear factor-κB can be activated after TBI, and the inhibition of its activity can improve the prognosis of brain injury (Sara et al., 2020). Nucleotide-binding oligomerization domain-like receptor thermal protein domain associated protein 3 (NLRP3) inflammasome is an important response factor of pyroptosis. Caspase-1 and its dependent inflammatory factors IL-1β, IL-6 and IL-18, are the indicator molecules of NLRP3 inflammasome activation, and the classical pyroptosis pathway mediated by NLRP3 is involved in the pathological process of TBI (Ponomarenko et al., 2021; Yuan et al., 2021). TNF-α, proinflammatory factors, intercellular adhesion molecule-1, vascular endothelial adhesion molecule-1, nuclear factor-κB, NLRP3 and Caspase-1, can be used as indicators to investigate the therapeutic effect of TBIs by measuring changes in their levels.

Apoptosis-related indicators

During apoptosis and autophagic cell death, the activation of Caspase-3 plays a key role and is one of the key proteases to execute the apoptosis program; changes in the levels of this enzyme can reflect the level of cellular apoptosis (Lorente et al., 2021). By performing western blotting, immunofluorescence, and other techniques, the levels of Caspase-3 in the brain tissue around the injury can be used as an evaluation index for TBI injury and recovery. Iron metabolism is dysregulated after TBI and can lead to ferroptosis. Proteins of iron metabolism pathways, such as transferrin receptor 1, ferroportin, ferritin heavy chain, and ferritin light chain, are expressed in the injured cortex in a sequential manner; hence the need to regulate the concentrations of Fe³⁺ and Fe²⁺ to maintain iron homeostasis. Transferrin receptor 1 and ferroportin can reach peak levels 6–12 hours after TBI, while levels of ferritin heavy chain and ferritin light chain can reach peaks 3–7 days after TBI (Chen et al., 2021; Rui et al., 2021).Ferritin heavy chain resulted in more severe iron deposition, neuronal degeneration, and increased levels of toxic substances such as 4-hydroxynonenal in injured cortical areas (Rui et al., 2021). Necroptosis is a type of apoptosis that can follow TBI and can be caused by caused by Toll-like receptor-3 and -4 agonists, TNF-α, and T cell receptors. Necroptosis signaling is modulated by receptor-interacting protein kinase (RIPK) 1 when the activity of Caspase-8 becomes compromised. Activated death receptors cause the activation of RIPK1 and the formation of a RIPK1-RIPK3-mixed lineage kinase domain-like protein in a manner that is dependent on RIPK1 kinase activity (Yu et al., 2021). RIPK3 phosphorylates RIPK1-RIPK3-mixed lineage kinase domain-like protein, ultimately leading to necrosis via plasma membrane disruption and cell lysis (Wehn et al., 2021; Yu et al., 2021).

Calcium overload

Ca²⁺ homeostasis is integral to cellular physiology and pathology; furthermore, research has shown that Ca²⁺ is involved in the secondary injury caused by TBIs (Kant et al., 2021). Following TBI, the activity of Ca²⁺-Mg²⁺-ATPase is reduced, Ca²⁺ influx is increased, intracellular osmotic pressure is increased, and water molecules are infiltrated into the intracellular environment, thus leading to brain edema (Belov Kirdajova et al., 2020). Glutamate acts on the N-methyl-D-aspartic acid receptor on the cell membrane, opening the receptor-dependent Ca²⁺ channel and causing significant Ca²⁺ influx. On the other hand, glutamate can increase the membrane’s permeability to Ca²⁺ and Na⁺, increase the concentration of intracellular Ca²⁺, and cause neuronal damage (Zong et al., 2022). A excessive amount of Ca²⁺ influx will overload the mitochondria, open mitochondrial permeability transition pores, release Ca²⁺ and other substances into the cytoplasm, and further aggravate Ca²⁺ imbalance (Zhou et al., 2021b). The excessive deposition of Ca²⁺ in the mitochondria could cause the decoupling of mitochondrial oxidative phosphorylation, the inhibition of cellular respiration, and lead to irreversible nerve cell death (Zhang et al., 2019). Mitochondrial dysfunction can also cause nitric oxide production in cells and the formation of intracellular free radical reactive oxygen substances, which destroy key components of cells through peroxidation (Han and Jiang, 2021). In addition, the excessive Ca²⁺ influx after TBI activates calpain via N-methyl-D-aspartic acid receptors; activated calpain are known to cleave structural proteins in the cytoplasm and cytoskeleton (Ng and Lee, 2019). After TBIs, Ca²⁺, as the initial factor, can mediate the injury mechanisms described above and further aggravate the secondary injury of TBIs.

Nano-Delivery Vehicles for the Treatment of Traumatic Brain Injury

TBIs can trigger central and peripheral inflammatory responses (Zinger et al., 2021). However, due to the existence of the BBB, most drugs cannot be rapidly and efficiently delivered to lesion sites in the brain for therapeutic purposes, thus hindering the treatment of TBIs (Umlauf and Shusta, 2019). Over recent years, nanocarriers have gradually gained significant importance. Studies have found that nanoparticles, liposomes, extracellular vesicles (EVs), micelles, and some polymers, have large surface areas and excellent electronic, steric, optical, biological and other properties. Furthermore, these structures can cross various physiological barriers, penetrate the BBB, and exhibit specific targeting properties (Cho and Borgens, 2012). The targeting ability of a nanoparticle delivery system relies on the adsorption of nanoparticles onto the capillary wall, thus resulting in a high drug concentration (including the solubilization of surfactants, an increase in bypass transport, and the endocytosis of endothelial cells). Consequently, nanocarriers have become an efficient drug delivery tool for many diseases and have achieved remarkable results.

Nanocarriers can be synthesized by different methods based on their specific applications and the types of drugs that need to be delivered. Of the currently available nanocarriers, nanoparticles, liposomes, EVs, micelles, and polymers are commonly used (Figure 4 and Table 3). The use of various nanocrystals has led to subtle changes in drug formulation and delivery (Han and Jiang, 2021) which not only improves the efficiency of drug delivery but also reduces the risk of toxicity to normal tissues and organs in patients.

Figure 4.

Schematic structures of several nanocarriers.

Liposomes are small and closed vesicles with a phospholipid bilayer encapsulated aqueous phase structure. Polymer nanoparticles are made of polymer materials or other materials with particle sizes ranging from 1 to 1000 nm. Their surface can be modified to perform special functions. Micelles are mainly composed of a hydrophilic shell and a lipophilic core; the materials incorporated are mostly hydrophilic and hydrophobic block copolymers. Dendrimers are linear polymers formed by the polymerization of thousands of dendrimer motifs. Extracellular vesicles are natural nanoparticles that are similar to liposomes but with a more complicated bilayer structure. Created with PowerPoint (version 2019).

Table 3.

Summary of nanoparticle characteristics and applications for traumatic brain injury

Nanocarrier	Characteristic	Application	Reference
Nanoparticles	Improved drug bioavailability, targeting and slow release	Cancer diagnosis and treatment, polypeptide and gene fragment delivery vector	Florence, 2012; Saraiva et al., 2016
Liposomes	Amphiphilic, sustained-release drugs, immune properties	Tumor targeted therapy, vaccine vector	Fan et al., 2021
Extracellular vesicles	Homing effect and tissue specificity	Cancer diagnosis and treatment, stem cell therapy	Li et al., 2021a
Micelles	Increased solubility, long circulation, targeting heat sensitivity, pH sensitivity through a variety of physiological barriers	Cancer treatment, delivery of anti-inflammatory drugs, delivery of antifungal drugs	Smiley et al., 2021
Dendrimer	Targeting, slow-release, reducing drug toxicity, through various physiological barriers	Cancer drug delivery, nuclear medicine imaging, radiation therapy	Nance et al., 2016; Xiao et al., 2020

Nanoparticles

Polymer nanoparticles

Polymer nanoparticles can penetrate the BBB and accumulate in target endothelial cells in the area of TBI. Furthermore, these particles can deliver drugs to endothelial cells, thereby mitigating nervous system injury during the course of treatment (Onyeje and Lavik, 2021). Polylactide and polylactic acid-glycolic acid (PLGA) are the most common polymeric nanoparticles and are often used for cancer therapy. PLGA was previously modified by polysorbate 80 (Tahara et al., 2011); the prepared polysorbate 80-PLGA nanoparticles had a higher proportion of brain distribution than unmodified blank PLGA, and could be used as a drug carried targeted to the central nervous system. Monosialoganglioside 1 is a brain ganglioside and has been used clinically to reduce brain injury and improve cognitive impairment; however, it also has poor targeting and significant side effects. LysoGM1, as its hydrolysate, has the same efficacy as GM1. PLGA can be functionalized with LysoGM1; furthermore, the nanofiber structure PLGA-LysoGM1 can be obtained by electrospinning (Tang et al., 2020c) to generate an effective nerve tissue scaffold that can promote the viability of neurons, up-regulate the expression levels of anti-apoptotic genes, and reduce scar formation at the lesion site. PLGA-loaded nanoparticles prepared with cerebrolysin were shown to exhibit better neuroprotective effects than cerebrolysin alone, thus reducing the formation of brain edema and destruction of the BBB (Ruozi et al., 2015). Brain-derived neurotrophic factor (BDNF) can provide neuronal protection and repair but has a short half-life and poor BBB permeability. PLGA modified with poloxam188 has been used to deliver BDN and increase the blood concentration of BDNF in the bilateral cerebral hemispheres of the weight-drop injury TBI mouse model. The mNSS score of mice treated with PLGA loaded with BDNF modified by poloxamer 188 was significantly lower than the scores of mice treated with BDNF alone; behavioral evaluation of the animal model further showed that the cognitive deficits of TBI mice were significantly improved (Khalin et al., 2016).

Lipid-based nanoparticles

Currently, efficient liposome preparation methods include the Bangham method, the washing dialysis method, and the reverse phase evaporation method (Niu et al., 2010), although there are more advanced technologies, including supercritical fluid technology, antisolvent and supercritical phase evaporation. Modern smart liposomes include pegylated liposomes, radiolabeled liposomes (Antimisiaris, 2023), and therapeutic liposomes (containing therapeutic and imaging agents). Smart liposomes can be loaded with antibodies, carbohydrates, protein fragments, vitamins, and peptides (Allen and Cullis, 2013), and are generally very sensitive to external stimuli that can deliver cargo efficiently into target cells.

The opening of the BBB tight junctions after TBI allows the passage of large drug carriers such as liposomes. The selective influx of liposomes occurs 0–8 hours after TBI, while BBB closure occurs 8–24 hours after injury, thus providing suitable conditions for the delivery of liposomes (Boyd et al., 2015). The intranasal delivery of IL-4 encapsulated by liposomes has been shown to promote the differentiation of oligodendrocyte progenitor cells into mature oligodendrocytes and significantly improve the repair of the sensorimotor nervous system in a mouse model of CCI TBI (Pu et al., 2021). Injection of the CCI TBI mouse model with liposome-encapsulated chlorophosphate was shown to reduce the number of monocytes within 24 hours of TBI when compared with a model group, reduce the infiltration of neutrophils, and reduce brain edema after TBI (Makinde et al., 2018). Liposomes carrying vascular endothelial adhesion molecule-1 and mRNA were also shown to significantly alleviate TNF-α induced cerebral vascular edema and exhibit the potential to improve TBI (Marcos-Contreras et al., 2020).

EVs

EVs are biologically active membrane vesicles that are secreted by most cells of the body and are nano-sized lipid membrane-encapsulated particles. According to our current understanding of the size and biogenesis of EVs, EVs can be divided into exosomes, microvesicles, and apoptotic bodies (Tang et al., 2020b). EVs contain a variety of biologically active substances such as nucleic acids and proteins, which can exchange and transmit information between cells (Neupane et al., 2021), thereby affecting the progression of various diseases. EVs not only regulate physiological changes in the brain after TBI, they can also regulate synaptic plasticity and neuronal regeneration. EVs can carry relevant therapeutic drugs and molecular information through the peripheral circulation and the BBB, thus providing a new option for treating TBI (Beard et al., 2020; Gao et al., 2022). In addition, EVs have a set of unique advantages, including low immunogenicity, biological barrier permeability, and inherent cell targeting.

Micelles

Micelles are amphiphilic block copolymers that self-assemble into hydrophilic and hydrophobic blocks in water. Micelles are unstable entities formed by the non-covalent aggregation of surfactant monomers and can be spherical, cylindrical, or planar (disk or bilayer) (Shin et al., 2020). Micelles exhibit enhanced drug solubility, good biodegradability, small particle size, high stability, and strong functionality. Micelles can also ensure that drug release can be controlled by adjusting specific parameters, including pH, temperature, radiation, or enzymes, thus rendering these copolymers a very promising nanocarrier for brain-targeting (Bhatia et al., 2021).

A previous study generated micellar nanoparticles composed of hydrophilic polyethylene glycol and a hydrophobic polylactide which were injected intravenously four hours after TBI; these nanoparticles penetrated the damaged BBB, adjusted the action potential of the compound after injury, and improved the function of axons in the corpus callosum (Ping et al., 2014). Magnetic micelles modified by chitosan and polyethyleneimine were previously shown to pass through the BBB and enter the brain under the action of a magnetic field after nasal delivery; this method could be used as an effective carrier for the treatment of fluid percussion injury TBI (Das et al., 2014).

Dendrimers

The best-known dendritic nanoparticle is polyamindoamine (PAMAM). Dendritic polymers are highly branched and can generate unique nanoscale systems which have nanoscale size, low viscosity, multi-functional terminal groups, high solubility, and good biocompatibility (Fox et al., 2020). In nanoscale drug delivery systems, branched dendritic polymers exhibit more advantages than linear polymers. The multi-branched structure of dendritic macromolecules can be used to encapsulate or couple multiple therapeutic molecules, targeting agents and imaging probes (Nussbaumer et al., 2016). Dendritic polymers can exhibit chemical intelligence due to their dendritic structure and the synergistic effects of special grafted groups. These polymers can be sensitive to the environmental regulation and can enter cells rapidly, thus avoiding absorption by macrophages; furthermore, they can easily cross biological barriers and achieve targeting (Calderón et al., 2010). Dendritic polymers are expected to improve patient medication adherence and have already played a role in the treatment of TBI.

N-acetyl cysteine is an antioxidant and anti-inflammatory agent used in targeted therapy in the clinic. In a previous study, N-acetyl cysteine and triphenylphosphine-modified PAMAM were prepared as nano-targeting agents. Triphenylphosphine, as a mitochondrial targeting ligand, targeted N-acetyl cysteine on microglia with mitochondrial damage and showed excellent anti-oxidative stress effects in a rabbit model of CCI TBI (Sharma et al., 2018). Furthermore, sinomenine was shown to bind to PAMAM dendritic molecules to target and activate microglia in the rabbit model of CCI TBI, thus overcoming the short biological half-life of sinomenine and the adverse reactions caused by large doses of sinomenine when administered intravenously. Furthermore, the levels of TNF-α, IL-1β and IL-6 when sinomenine bound to the PAMAM group were significantly lower than those of the TBI group, thus reducing the inflammatory response created by TBI (Sharma et al., 2020).

Other research showed that the administration of pentobarbital during the acute treatment phase of TBI increased the uptake of PAMAM by microglia. BV2 microglia were treated with fluorescently labeled PAMAM. At 2 and 6 hours after treatment, the uptake of PAMAM by microglia treated with pentobarbital was about twice that of the control group without pentobarbital, thus indicating that pentobarbital could activate microglia (Kannan et al., 2017). PAMAM, which targets microglia, has significant potential for the early recovery of acute TBI; however, further research is required to determine how this strategy could be used safely and effectively in clinical practice.

Polymersomes

Polymersomes are similar to liposomes in structure in that they both contain hydrophobic and hydrophilic regions. Polymer vesicles are essential substances that mimic lipid bilayers; however, these vesicles utilize polymers instead of lipids, thus generating significant chemical flexibility. The basic advantages of polymeric vesicles are similar to those of ordinary polymeric nanoparticles; however, they contain hydrophilic polymers with a stronger loading capacity and can encapsulate more drugs. Furthermore, the functional groups on the polymer chain can be modified to respond quickly to various external stimuli; thus, these groups can respond to an intelligent signal to improve the efficiency of drug delivery. Multifunctional polymeric vesicles are considered to represent promising drug carriers for the treatment of TBI and other biomedical applications, including gene therapy and magnetic resonance imaging (Wang et al., 2018).

Brain-Targeted Strategies for the Treatment of Traumatic Brain Injury

Due to the existence of BBBs, the efficiency of drugs for the treatment of secondary injury caused by TBI is low following administration. Nanocarriers can overcome the low BBB permeability of drugs for the treatment of TBI, at least to some extent. In addition, the drugs delivered to the site of brain injury by some targeting strategies play a particular retention role to achieve brain-targeted delivery, thus improving the therapeutic effect and reducing adverse effects for the whole body. An ideal drug delivery system would deliver drugs to lesions without affecting the surrounding tissues. Therefore, brain-targeted delivery has attracted significant attention over recent years.

The existing brain-targeted drug delivery routes include the intranasal route, cerebrospinal fluid injection, and the intracranial pathway. Despite the important role these drug delivery methods can play in brain targeting, patient compliance is poor; this represents a major limitation of these techniques. By modifying the form and dose of a drug, a targeted drug delivery system could improve drug compliance, improve acceptance by patients, and improve the drug targeting to ensure therapeutic effect.

A targeted drug delivery system could achieve non-invasive drug delivery to the brain and overcome the BBB. The most widely studied new TBI brain targeting strategies include magnetic nanoparticles (MNPs) that move directionally under the action of a magnetic field, peptides as targeting agents of targeting molecules, cell-mediated targeted therapies and targeted drug delivery.

MNPs

MNPs can be used effectively as drug carriers. The targeted drug delivery of MNPs can be achieved based on two basic elements: a magnetic field source and a magnetically responsive drug carrier particle (Ghosal et al., 2022). The general structure of MNPs includes a magnetic core with a metal coating and a polymer that can be functionalized (Maier-Hauff et al., 2011; Pohland et al., 2022). By implanting a magnet at the target site or creating a magnetic field close to the target site in vitro, drug-loaded MNPs can be attracted and targeted drug delivery can be achieved. MNPs can remain in the blood or serum for a long period of time, thus increasing drug exposure time, improving the rate of contact between drugs and receptors, and help to improve the efficacy of targeted drug delivery (Zahn et al., 2020). By virtue of their small size (Bhattacharya et al., 2022), MNPs face fewer spatial obstacles and can penetrate the BBB effectively; targeted drug delivery was previously demonstrated by combining Fe₃O₄ superparamagnetic iron oxide nanocarriers with drugs.

Magnetite (γ-Fe₂O₃ and Fe₃O₄) are the most commonly-used superparamagnetic molecules due to their high resistance to corrosion (Chanana et al., 2009). Currently, the commonly used methods for synthesizing magnetic nanocarriers include chemical co-precipitation, ultrasonic irradiation, hydrothermal synthesis, solvothermal synthesis, microemulsion, and thermal decomposition. Nanocarriers can successfully pass through the BBB and accumulate under magnetic control. Magnetosome are magnetic nanoparticles synthesized by magnetotactic bacteria, whose main elements are iron, calcium, phosphorus, oxygen, and magnesium. All of these can be attached to drugs for active propulsion without external force. This approach can be applied for the treatment of hard-to-reach lesions, such as brain tissue after TBI.

Erythropoietin plays an important role in neuroprotection, nerve regeneration and erythropoiesis. Erythropoietin has been used successfully as a therapeutic method to treat injuries to the central nervous system; however, this requires that erythropoietin reaches the lesion site as soon as possible (within 6–8 hours of injury) to improve the delivery efficiency of erythropoietin. Therefore, erythropoietin needs to be prepared into rapidly targeted drug delivery agents such as nanocarriers. When loaded onto magnetic nanocarriers, erythropoietin can reach injury sites in the central nervous system more rapidly and accurately under the action of a magnetic field; this process can improve enrichment at the targeted site through mutual magnetic force to play a retention role and improve therapeutic effect.

Modifications with peptides as targeting molecules

The brain has a very complex structure and contains a variety of brain cells with different functions. For example, the brain contains neurons which are electrically excited cells that transmit information, endothelial cells that are tightly packed to form brain microvessels, astrocytes that provide support for neuronal function (Raha et al., 2011), and microglia, a form of macrophage that can facilitate immune surveillance in the brain.

The inner and outer surface of the BBB is highly dynamic and contains endothelial cells, astrocytes, pericytes, and various neuronal cells that control the permeability of molecules. Special proteins in endothelial cells create channels for certain metabolites such as glucose; these channels can control the entry and exit of molecules into and out of cells (Waggoner et al., 2021). Due to the dense BBB, macromolecular substances are rarely transported through cells (David, 2017). Drugs and some essential molecules enter the brain primarily by passive diffusion, carrier-mediated transport, receptor-mediated endocytosis, adsorption-mediated endocytosis, or cell-mediated endocytosis. Endocytosis is a process by which vesicles transfer molecules from one side of the cell through the interior of the cell to the other side by binding to the membrane (Mann et al., 2016; Figure 5). Cerebral endothelial cells have a lower rate of vesicle transport due to peripheral endothelial cells. The barrier prevents most molecules and drugs from entering the brain. A promising non-invasive drug delivery strategy is to utilize BBB penetrating peptides or BBB penetrating enzymes as a carrier to deliver cargo inside and outside of the brain; these combine with special proteins on BBB membranes to mediate drug transport and to increase transport efficiency through the BBB. Brain permeability peptide-drug conjugates (Figure 6) are composed of therapeutic drugs and penetrating peptides bonded by connecting agents. BBB penetrating peptides are mainly derived from neurotropic endogenous proteins, neurotoxins, some viruses, and endogenous peptides or peptides identified by biological screening and modified by bacteriophages (Zhou et al., 2021a). Peptide-drug conjugates bind to receptors on the BBB and use receptor-mediated endocytosis to cross the BBB while transporting drugs. Penetrating peptides can carry drugs to target tissues and organs and have become a promising means to deliver drugs to the central nervous system. BBB penetrating peptides or BBB penetrating enzymes have become a significant research hotspot due to their ease of synthesis and modification, low immunogenicity, and low cost.

Figure 5.

Several modes of transport of different molecules across the blood-brain barrier.

Different substances pass through the blood-brain barrier in different ways. The BBB controls the permeability of substances through a variety of cells such as endothelial cells, astrocytes, pericytes, and neuronal cells. Some water-soluble small molecules can be transported across the blood-brain barrier through cellular bypass, some lipophilic substances can be transported across the cell membrane, and substances that can specifically bind to membrane receptors can be wrapped in vesicles formed by the cell membrane through receptor-mediated endocytosis or adsorption-mediated endocytosis and pass through the BBB. All of these transport pathways require ATP consumption and require active transport. Created with PowerPoint (version 2019).

Figure 6.

Schematic diagram of brain permeability peptide-drug conjugates.

Brain permeability peptide-drug conjugates are formed by linking a penetrating peptide and a therapeutic drug with a linker. Penetrating peptides are mainly derived from neurotropic endogenous proteins, neurotoxins, some viruses, and endogenous peptides or peptides identified by phage biological screening and modification, and can facilitate the transport of drugs across the blood-brain barrier via receptor-mediated endocytosis and deliver drugs to the brain. Created with PowerPoint (version 2019).

Cell-mediated targeted therapy

Once brain tissue has been damaged, certain cells, including immune cells and stem cells, migrate specifically to the site of tissue damage (Tornero, 2022). The ability of these cells to penetrate the BBB and accumulate in brain tissue allows them to be potential carriers for the delivery of therapeutic drugs. Stem cells can differentiate, regenerate (Tang et al., 2020a) and migrate to the injury site and differentiate into corresponding target cells to complete their therapeutic function. TBIs destroy the BBB, thus leading to edema and cell infiltration; this damages neurons, activates astrocytes and microglia, trigger an inflammatory response, and cause further damage to neurons and other cells, thereby leading to cognitive deficits and other neurological injuries (Schepici et al., 2020). The intravenous transplantation of mesenchymal stem cells (MSCs) is a promising treatment for TBIs (Dewan et al., 2018). MSCs can cross the BBB and migrate to specific sites of injury in the brain (Kannan et al., 2017). In addition to secreting trophic factors, MSCs can also promote the secretion of trophic factors by adjacent brain tissues (Lee et al., 2012), limit the occurrence of further damage to brain tissues, promote the repair and regeneration of neurons, and play a neuroprotective role (Figure 7). Following transplantation, living MSCs may cause adverse effects in vivo, including immune responses, oncogenesis, microvascular embolism, seizures, and cellular condensation. The use of MSC-derived exosomes in transplantation therapy can help to avoid these adverse effects and improve therapeutic effect (Das et al., 2019).

Figure 7.

Stem cell therapy for post-traumatic brain injury.

Traumatic brain injury induces tissue edema and cell infiltration, injures neurons, activates astrocytes and microglia, and initiates inflammatory responses, thus leading to tissue damage and dysfunction in the nervous system. Mesenchymal stem cells can cross the blood-brain barrier and secrete trophic factors to the site of injury, promote the repair of damaged cells and tissues, and promote the recovery of structure and function in the nervous system. Created with PowerPoint (version 2019).

The brain undergoes an inflammatory response after TBIs, and pro-inflammatory factors can guide leukocytes to the pathological site; thus, leukocytes can be used to carry nanomedicine to specific target sites. Therapeutic drugs can be loaded inside or outside of the carrier cells (Cox et al., 2019). After injection, drug-loaded neutrophils can successfully transport drugs to the site of brain injury, release drugs or contrast agents, and play a role in treatment and monitoring (Luo et al., 2020). Based on the characteristics of cell differentiation, induction and homing, cell-mediated nanomedicine targeted therapy has attracted significant attention and will play an important role in future medicine.

Prospects

TBIs are unavoidable and pose a significant threat to human health and safety. Better awareness of safety and the emergence of appropriate protective and preventive measures have effectively reduced the incidence of TBI. However, therapeutic drugs need to be upgraded in an innovative manner and significant effort has been devoted to this area. Drugs play an important role in all stages of disease treatment. Research on TBI as a brain disease is increasingly focusing on nanocarriers and targeted drug delivery systems.

Nanocarrier drug delivery systems can overcome a variety of biological barriers, improve drug bioavailability, increase intracellular penetration and retention time, achieve drug enrichment, control drug release, and achieve targeted drug delivery to the brain. In the treatment of TBIs, the advantages of nanocarrier delivery systems are more prominent, and an ideal nanocarrier can facilitate the penetration of therapeutic drugs through the BBB. In addition, nanocarriers can be modified to achieve the targeting of therapeutic drugs, reduce the systemic toxicity caused by drugs, and reduce the adverse reactions of drugs. MNPs are being prepared to guide drug targeting to specific sites under the action of a magnetic field. Specific receptors can recognize the combination of nanoparticles and peptides to achieve endocytosis before the drug is absorbed by many specific sites containing the receptor to achieve targeted drug delivery. The homing effect, differentiation, regeneration, and the secretion of trophic factors secreted by stem cells can help to repair damaged tissues and improve inflammation. Drugs can be loaded inside or on the surface of neutrophils and other white blood cells that can spontaneously migrate to the site of inflammation to achieve targeted drug delivery. However, nanocarriers also have many shortcomings that need to be improved. For example, the development and application of lipid-based nanoparticles is limited due to their low drug loading, the inability to load macromolecular substances, and low biodistribution, thus leading to high uptake rates in the liver and spleen. MNPs are limited by low solubility and toxicity, especially in formulations using heavy metals; thus, their clinical use is often limited.

Nanoparticles and modified targeted drug delivery systems have broad prospects for the treatment of TBI according to their own characteristics. Despite unremitting efforts to innovate delivery systems, successful translation from basic research to clinical use remains a formidable challenge. In addition, the therapeutic targets and pathways of TBIs need to be studied more extensively to allow the discovery of TBI therapeutic drugs and delivery systems more traceable. With technological progression and the efforts to explore new drug delivery systems for TBI treatment, the clinical applicability of TBIs will eventually make headway, thus improving treatment compliance, and reducing adverse drug reactions, while enhancing therapeutic effect, curation rate and the prognosis of TBI. These effects will finally offer more protection for patient’s affected by TBIs.

Although this review summarizes the current status of TBI disease, mechanisms, animal models, nanocarriers and targeted delivery, how we connect nanocarriers and the targeted delivery based on the mechanisms of TBI more closely is a problem that we need to continue to explore. However, the nowadays research focuses more on basic study and has not been applied in clinics. The major obstacle lies on that the physical-chemical characteristics of drugs will determine the selection of nanocarriers, which means the universality is poor. Moreover, the production efficiency and reproducibility of nanocarriers are hard to be controlled.

Additional file: Open peer review reports 1 ^{(88.9KB, pdf)} and 2 ^{(94.1KB, pdf)} .