Challenges and Opportunities of Nanomedicine: Novel Comprehensive Approaches for Brain Metastasis

Brain metastasis is the leading cause of death in most cancer patients; thus, anti‐brain metastasis is a crucial step in cancer treatment. The unique microenvironment and pathophysiological characteristics of brain metastases hamper the development of effective treatment methods, while the advances in nanomedicine demonstrate its immense potential for addressing this challenge. In recent years, due to breakthroughs in nanotechnology, functional and complex nanomedicine is prepared and it has been extensively explored to improve the treatment of brain metastasis. Compared to traditional treatment methods, nanomedicine can combine multiple traditional treatment methods into one single medicinal format and target unique characteristics of brain metastases to enhance the treatment efficacy. This review summarizes the main obstacles in the field of brain metastasis treatment and clinical/preclinical studies and surveys the use of a variety of nanomedicine candidates as antibrain metastasis strategies for different types of brain metastases.


Introduction
Metastasis is one of the biggest challenges in cancer treatment, as 90% of cancer-related deaths are caused by metastasis.[3] The brain tissue is the most common site for brain metastases, followed by the leptomeningeal space. [4,5][8] Over 40% of malignant tumor patients develop metastases; thus, the number of patients with brain metastases is also increasing. [9,10]The growing concern is the underestimated incidence of brain metastases.Autopsy studies of cancer patients confirm that 10-30% of adults with cancer have already developed brain metastases. [1,7,11] considerable number of cancer patients who have developed brain metastases are asymptomatic for a long time; alternatively, they miss the timely detection of brain metastases due to lack of specialized and sensitive diagnostic techniques when they have typical but not severe symptoms. [12,13]Even in patients with severe cancer symptoms at the primary site, failure in identifying the symptoms of brain metastases can lead to delayed diagnosis and treatment.Without timely diagnosis and treatment during the clinical period, patients with brain metastases ultimately become dead.Thus, brain metastases occur at a rate more than four times higher than primary brain tumors, and they have become the most common types of tumors in the adult CNS. [12,14,15]he increasing incidence of brain metastases could be ascribed to a few factors including an aging population, the extended survival period of cancer patients due to advances in medical technologies, and improvements in neuroimaging technologies that enable detection of more cases of brain metastases. [12,16]In an aging society, the number of people who develop cancer and subsequently brain metastases is gradually increasing each year.Effective systemic cancer treatment methods could significantly extend the survival period of cancer patients with the advancement of medical technologies.Unfortunately, a longer survival period of these patients is accompanied with a higher risk of developing cancer metastases into the brain.[19] Thus, early, effective, and targeted intervention of them could remarkably retard the growth of metastatic brain tumors and eventually eradicate these tumors in patients.
However, the treatment of brain metastasis is currently challenged by unique characteristics of the disease.The challenges include eradication of tumor cells at the primary lesion, high variabilities in their pathological features at the primary site and at the brain site, the aggressive nature of metastatic tumor cells during progression, the presence of the blood-brain barrier (BBB) and blood-tumor barrier (BTB), and the complex metastatic tumor microenvironment and immune microenvironment at the brain site. [1,3,20]][23][24] Fortunately, the emergence of nanomedicine in recent years opens a new avenue to treating brain metastasis.This review will introduce common types of brain metastases, elaborate the obstacles for treating them, and detail the development and application of a variety of nanomedicinal products to treat brain metastases.Interest and attention will be drawn to formulate more efficient strategies for developing nanomedicine to treat brain metastases.

The Origins and Treatment Challenges of Brain Metastasis
Understanding the occurrence and development of brain metastases is essential for developing therapeutic methods.Brain metastasis most commonly originates from three primary cancers, including lung cancer, breast cancer, and melanoma.[27][28] Tumor cells from the primary site must undergo adaptations to become metastatic.Generally, cancer cells are adapted to various stresses to invade blood vessels and extend their survival during circulation. [3,29]Once cancer cells endure a series of stress and reach the brain tissue, they may choose to enter a proliferative or dormant state based on the magnitude of stress in the brain tissue environment.Colonized tumor cells modify their genome expression and epigenetic inheritance so that they are adapted to a vastly different microenvironment within the brain. [1,30]Through the adaptive process, they gain immune privileges by interacting with astrocytes in the brain and eventually start proliferation to form brain metastases. [3,31]he unique microenvironment of the brain challenges the treatment methods for brain metastases which are distinct from those in other locations.Table 1 summarizes major obstacles and potential solutions for current treatment methods for brain metastases.

Prevalent Primary Tumors
Lung cancer is the most common primary cancer that metastasizes to the brain.Among patients initially clinically diagnosed with brain metastases, the primary tumor site cannot be determined for up to 15% of patients even with multiple diagnostic methods. [19,32,33]However, most of these patients are eventually found to have lung cancer during follow-up examinations.][36] Among various types of lung cancer, non-small cell lung cancer (NSCLC) accounts for the largest proportion of brain metastases. [35,37]40][41][42][43] Adenocarcinoma is more prone to metastasizing to the brain than squamous cell carcinoma. [16]Small-cell lung cancer (SCLC) accounts for only 15% of lung cancer.However, the risk of developing brain metastases in SCLC patients is quite high as over 50% of SCLC patients eventually develop brain metastases. [44]Therefore, it is crucial to focus on treating brain metastases from both NSCLC and SCLC, but most current treatment methods are focused on NSCLC.
Breast cancer is the second type of cancer that progresses to brain metastasis, accounting for 15% of brain metastases.][47] Due to a great population of breast cancer patients, the actual number of cases for breast cancer brain metastases is astonishingly high.[50] Therefore, the actual incidence of breast cancer brain metastases is much higher than that clinically observed, indicating that both diagnostic and treatment methods should be developed for brain metastases from breast cancer in order to achieve better clinical outcomes for breast cancer patients.[53] Therefore, special attention should be paid to the treatment methods for brain metastases derived from TNBC.
Compared to a high proportion of brain metastases originating from lung and breast cancer, the percentage of brain metastases from melanoma is lower, accounting for 5-10% of the cases of brain metastases. [20,54,55]However, the incidence of brain metastases from melanoma is very high, and over 50% of melanoma patients develop brain metastases, which is comparable to that of lung and breast cancer. [54,56]Despite a high incidence of brain metastases from melanoma, the treatment methods for brain metastases from melanoma are frustratingly ineffective.The median survival period for melanoma patients with brain metastases is about 4 months after standard clinical treatment. [57,58]More dishearteningly, the prognosis for melanoma patients with multiple brain metastases is worse.After aggressive treatment methods for these patients, such as whole-brain radiation therapy, their median survival period is less than 6 months. [59]he proportion of brain metastases originating from other primary tumors is relatively low but the clinical significance is still very high.For example, patients with urogenital and colorectal cancers still stand a relatively high chance of developing brain metastases. [33]Although the proportion of small cell or neuroendocrine carcinoma among prostate cancer patients with urogenital cancer is less than 1%, they account for more than one-fourth of the proportion of brain metastases developed from prostate cancer. [23,60]Regardless of a high or low incidence of brain metastases from different types of primary tumors with specific cancers or molecular subtypes, the mechanisms for developing brain metastases should be revealed to reduce or mitigate their development from primary tumors.Various methods for treating brain metastases should be explored and special strategies may be required for those from some of primary tumors.

Special Pathophysiology Characteristics
Metastasis is a highly selective and synchronous phenomenon.The development of brain metastases is highly impacted by the types of tumors.Cancer cells that undergo metastasis and reach the CNS may not survive at the brain site, and only a subset of them can successfully adapt themselves to the specialized microenvironment required for CNS colonization. [1,7,61]There are also differences in the propensity for metastasis between individual cells within a single tumor, which is controlled by a complex interplay of genetic and epigenetic factors. [1,53]Therefore, brain metastases and their original primary tumors appear similar at a pathological level, and the histological appearance of brain metastases usually resembles that of primary tumors, however, their biological and genetic characteristics can be significantly different. [13,28,62]hen metastatic tumor cells reach the brain, they encounter a complex microenvironment that is drastically different from the ecological environment at the primary site.Various types of cells, including neurons, astrocytes, and microglia, constitute the special microenvironment of the brain, which are not present in the extracranial organs from which primary tumors originate. [3,63]etastatic cells survive and proliferate after conforming to this unique environment.The adaptation process involves a series of changes in gene expression and epigenetic modifications, which induce pronounced alterations in the characteristics of metastatic cells.These metastatic cells could acquire normal neuronal cell traits to help them adapt to the brain microenvironment. [31,64]At the site of brain metastases, immune cells from the circulatory system infiltrate into the brain metastatic tissue.The changes in the immune properties are manifested at the metastatic site as these immune cells have to conform to the unique immune microenvironment.For example, brain metastatic cells may exhibit immune evasion. [65]These cells can also regulate the immune infiltrating cells by interacting with reactive astrocytes around the lesion, in turn, an immune microenvironment for promoting metastatic tumor growth could be developed. [66]rain metastases typically occur in areas of the brain with a slower blood flow rate, and the gray-white matter junction is a common site for brain metastases. [67,68]Melanoma-derived brain metastases are more likely to invade the gray matter of the brain compared to those from other primary cancers.The majority of cancer metastases to the brain occurs through hematogenous dissemination, as cancer cells become trapped in the slow-moving terminal arteries, eventually developing into brain metastases. [69]Therefore, the distribution of brain metastases within the brain is generally positively correlated with the size and blood flow of the brain, and the majority of brain metastases are found in the cerebral hemispheres and approximately onefifth in the cerebellum and brainstem. [70]Brain metastases from different primary cancers may invade specific regions of the brain as they have unique characteristics during their evolution.For example, pelvic tumors may form brain metastases in the cerebellum.Brain metastases from renal cell carcinoma are usually solitary, while those from melanoma are more likely to be multiple. [12,71]

BBB and BTB
In the CNS, the presence of BBB physically and functionally prevents most exogenous drugs from freely entering or exiting the CNS, [72,73] while the selectively permeable properties of the BBB are critical to maintain the normal structure and function of brain tissues.The unique vascular structure of the BBB, distinct from the vascular system outside the CNS, plays a crucial role in restricting the transportation activity. [74]Thus, failure to deliver drugs through the BBB to the CNS remains the biggest challenge to successfully treat CNS disorders.Therapeutic drugs, including nearly 99% of small molecules and all large molecules, are unable to cross the BBB.Emerging therapeutic formulations based on genes, peptides, or oligonucleotides encounter the same challenge of crossing the BBB from the vascular circulation. [75,76]he tight junctions between the endothelial cells of the capillaries in the brain play a crucial role in the formation of the BBB. [77]They are specialized fissures around the endothelial cells of the arachnoid membrane and the perivascular cells of the choroid plexus.They are combined with the end feet of astrocytes and the basal lamina of the capillaries to form physical filters that hamper the passage of large molecules and/or polar molecules.In the process of forming the physical filters, two functions are important: the gatekeeper function that inhibits the diffusion of large molecules around or inside cells and the barrier function that distinguishes basal and apical fragments on the plasma membrane. [78]In addition, astrocytes are essential for maintaining the CNS homeostasis, and outer membranes play an important role in the maintenance of the BBB, particularly its integrity. [79]hen cancer metastasizes to the brain, the integrity of the BBB is disturbed and the permeability of its vascular structure changes, resulting in the BTB. [80]The characteristics of the BTB are typically described as an increased permeability, a decreased blood flow, and up or down regulation of transporter expression. [81]Although the vascular permeability of the BTB is generally enhanced compared to the BBB, the change in the permeability is not uniform.In fact, the tumor vascular permeability significantly varies within the lesion.It also has a distinguished difference among different types of metastatic tumors even in the same lesion.It has been shown that there is a nearly 30-fold difference in the permeability between two experimental models from breast cancer. [76,80]he changes in the permeability of the BTB induced by brain metastases can be described at the cellular and molecular levels.At the cellular level, the presence of a large number of tumor cells leads to formation of an increasing number of aberrant blood vessels.There are inadequate tight junctions for these newly formed blood vessels.Improper contact with astrocytes in the normal BBB structure leads to the formation of high-density aberrant blood vessels. [28,82]At the molecular level, brain metastases downregulate the expression of tight-junction proteins, including anchoring protein ZO-1, resulting in the loss of tight junctions and an increase in the permeability of blood vessels. [72]Due to a hypoxic environment caused by brain injury, the secretion of vascular endothelial growth factor increases, leading to the formation of ineffective and tortuous blood vessels that are highly dense but lack biological functions. [83]However, such an increase in the permeability of the BBB or BTB does not help the delivery of chemotherapeutic drugs.The BTB, similar to the BBB, remains a huge obstacle for drug delivery.The main characteristics of the BTB and its potential therapeutic targets and current therapeutic approaches to targeting the BTB in clinical practice are shown in Figure 1. [1]

Preclinical Models Restriction
Brain metastasis is a devastating complication of cancer, and its development involves a series of complex and interconnected events.These events include the growth of cancer cells within the primary tumor, the entry of cancer cells into the bloodstream, survival and transit during circulation, penetration through the BBB, invasion into the brain parenchyma, and the generation of blood vessels within the brain.Each of these steps presents a challenge to reveal the underlying mechanisms of brain metastasis and develop effective treatment methods. [3,30]reclinical models are crucial in advancing our understanding of brain metastasis.However, the current preclinical model cannot fully replicate the entire process of brain metastasis.Currently in the field of nanomedicine for brain metastasis treatment, brain metastasis models are often created through intracranial (IC) inoculation, intracardiac (ICD) injection, and intravenous (IV) injection.However, these construction methods do not consider the interaction between primary tumors and brain metastases, and these models can only achieve partial simulation of the development process of brain metastases in clinical patients.Instead, the models are developed to simulate specific aspects of the cascade of reactions during the metastasis process.For instance, stereotactic injection of cancer cells into a mouse model was demonstrated to be an excellent tool for monitoring the growth of metastatic cells within the brain parenchyma.In this model, fluorescently labeled cells were tracked via a bioluminescent imaging system to provide insights into the growth patterns of metastatic cells. [63,84,85]njection of cancer cells through the carotid artery and heart is another commonly used preclinical model for studying brain metastasis.This approach simulates the entry of cancer cells into the bloodstream and allows revealing penetration of the BBB, colonization, and growth of metastatic cells within the brain. [69,86,87]owever, this model cannot simulate all steps during the metastasis. [85]Mouse genetic engineering models (GEMMs) have been developed to recapitulate the cascade of metastatic events.Specific cancer-related mutations are generated in the genetically modified mice, and they are more susceptible to developing tumors.The GEMMs could help unveiling the underlying mechanisms of brain metastasis, including the genetic background of human brain metastases. [63,88]Despite the benefits of the GEMMs, they have limitations.The current GEMMs exhibit varying frequencies of extracranial and IC metastases, and they may not accurately represent the genetic background of human brain metastases.Furthermore, it is challenging to develop GEMMs that precisely replicate the complete process of brain metastasis from the primary tumors to the final settlement and growth in the brain. [89,90]ince the development of brain metastasis involves a series of cascading reactions of cancer cells, the currently developed preclinical models cannot map the complete process of brain metastasis.While these models have contributed significantly to our understanding of the brain metastasis process, more accurate and comprehensive preclinical models that can replicate the entire process of brain metastasis are actively pursued, ultimately leading to effective treatment options for brain metastasis.

Nanomedicine for Brain Metastases
In the fight against cancer, it is crucial for patients to eliminate both the primary tumor and its metastases.Due to the unique characteristics of brain metastasis, it is often detected in patients at the end stage of the disease. [91,92]Therefore, the best strategy would be to prevent brain metastasis from occurring in the first place.If brain metastasis is identified in the patient, it should be immediately tackled to maximize the benefits for the patient.[95] In this context, nanomedicine could offer versatile roles for the diagnosis/treatment of brain metastases.The current landscape of emerging nanomedicine in the treatment/diagnosis of brain metastasis is summarized and its roles are elaborated.

Different Nanoplatforms for Brain Metastases
The rapid development of material and biomedicine science has led to remarkable advances in the application of nanotechnology in the field of biomedicine, especially in cancer treatment.Nanoparticles (NPs) can be designed and prepared with a variety of physiochemical properties that can be employed for conjugation with various therapeutic agents, endowing them with therapeutic roles. [96,97]The permeability and retention (EPR) effect is the most widely utilized principle for capturing and accumulating therapeutic nanoparticles at tumor sites. [98,99]his effect is enhanced by the presence of incomplete and underdeveloped vasculatures in and around metastatic sites.The passive targeting of nanoparticles could be improved via modification with tumor-sensitive ligands. [100,101]Specific responsive agents can be conjugated to nanoparticles to act as a switch that is activated by unique stimuli such as hypoxia, [102,103] pH, [104][105][106] enzymes, [107,108] temperature, [109] magnetic fields, [110] ultrasound, [111] and their combinations, thus improving the accuracy and efficacy of nanomedicine for targeting treatment of tumors.114] Although the nanomedicine approach for treating cancer metastasis has attracted great attention in recent decades, obstacles such as a weaker EPR effect, the BBB, and the BTB, especially at the early stage of brain metastases, remain to be overcome. [3,5,115]It is challenging to treat metastatic tumors by nanomedicine through either passive or active targeting.However, novel nanomedicinal approaches for treating antimetastatic tumors have been recently developed, and their treatment efficacies and effects are quite promising and encouraging.Nanotechniques and nanoparticles for capture of circulating tumor cells and chemotherapy have been well detailed in a recent review article, and some of the results may be useful in the clinical setting. [3,116]Although a couple of nanomedicines have been developed for the treatment of metastatic tumors and they have advanced into clinical trial phases, [116b,c] very few nanomedicines developed for diagnosing and treating brain metastases have progressed to the clinical stages.AGuIX® ultrasmall nanoparticles, nanoparticles with a hydrodynamic diameter <10 nm for imaging-guided therapy of brain metastases, are one example of the nanomedicince in the clinical phase I/II. [116d,e,f] Currently, due to distinctive pathological and physiological characteristics of brain metastases, the majority of nanomedicine candidates to address these challenges is still in their early stages of development.Table 2 summarizes the representative promising NPs and preclinical models for brain metastases.
To optimize the efficacy of treating brain metastases and reduce the side effects of clinical drug entities, drug delivery systems based on nanoplatforms with tumor targeting abilities for diagnosis and therapy have been developed.To create such a drug delivery nanosystem, the following issues should be considered.First, the circulation time in the vasculature should be long enough so that the nanomedicine could reach the targeting site; premature release of drug entities from the nanomedicine should be avoided; and biocompatibility and biodegradability of the nanoplatforms should be comprehensively assessed.Second, the nanosystem should be empowered with strong and accurate targeting abilities for the tumor site and resilient penetration abilities to cross physical barriers such as tumor stroma, the BTB, and the BBB.Finally, the nanosystem should effectively accumulate inside or around specific tumor cells, and the therapeutic entities should be efficiently released from the nanosystem. [117,118]The drug delivery nanosystems against both primary and metastatic cancers have been widely explored and they become more appealing due to their improved targetability and enhanced antitumor efficacies.[121] Different types of nanoplatforms have been chosen for developing the drug delivery nanosystem because they possess distinct structures and properties.To treat brain metastases, these nanoplatforms are often tuned to specific characteristics of brain metastases to enhance therapeutic effects of the therapeutic agents carried by the nanoplatforms.Currently, the nanoplatforms primarily used in brain metastases treatment are shown in Figure 2.
Polymeric nanoparticles are one of the most extensively studied types of nanoplatforms for treating brain metastases.As a drug carrier, they can encapsulate or attach therapeutic drugs to the surface to form nanocapsules or nanospheres, delivering the drugs to the targeted cancerous sites and achieving sustained release. [122,123]Over the years, different polymers have been used, for example, polylactic acid poly(lactic-co-glycolic acid) and poly(ε-caprolactone). [124] Polymeric NPs protect the drugs from degradation in the circulation system of the body, helping reduce the toxicity of the drugs to normal tissues. [125]For example, polymeric NPs were prepared with a crosslinked zwitterionic polymer layer to create functional nanocapsules for monoclonal antibodies, and the resulting nanomedicine was demonstrated to exert remarkable therapeutic effects against human lymphoma brain metastases through continuous release of monoclonal antibodies. [126]Modification of polymersomes with transferrin was found to have a distinctive efficacy in treating breast cancer brain metastases. [126]In addition, natural polymers, such as ferritin and albumin, are essential components of the nanomedicine for the treatment of brain metastases due to their excellent biocompatibility.The therapeutic effects have been significantly enhanced against brain metastases from breast and lung cancer after using albumin nanoparticles.Therefore, polymeric nanoparticles are the most commonly utilized material for delivering drugs for brain metastases. [127,128]ipid-based nanoparticles, particularly liposomes, are characterized by their excellent biocompatibility and biodegradability due to their bilayer membranes composed of phospholipids. [129,130]They can load both hydrophilic and hydrophobic drugs simultaneously through a controllable system size. [131]Notably, a new nanomedicine, PEGylated liposomes loaded with doxorubicin (DOX), has been approved by the U.S. Food and Drug Administration (FDA) for breast cancer metastasis treatment due to its safety and efficacy. [132]This nanomedicine has been shown to possess great penetration ability through the BBB and BTB in a human breast cancer brain metastasis model and significantly inhibit the growth of breast cancer brain metastasis. [133]Additionally, liposomes after specific modifications could be equipped with active targeting abilities.For example, BRBP1 peptide-modified liposomes were demonstrated to have distinct aggregation and antitumor effects in breast cancer brain metastases. [134]Solid lipid nanomedicine also plays an important role in the treatment of brain metastases.Modification of the surface of solid lipids could significantly enhance their penetration and boost the inhibition effect on breast cancer brain metastases in a mouse xenograft model. [135,136]icelles constructed from amphiphilic polymers have been widely used in the development of nanomedicine for cancer therapy. [137,138]It has been demonstrated that drug-loaded micellar nanoparticles have great potential to treat metastases. [139,140]n the process of targeting metastatic tumors, it has been discovered that the morphology of micelles plays an important role in their passive targeting, and rod-like micellar nanostructures are better at homing into cancer sites compared to spherical ones. [141]The pH responsiveness of micelles can also be considered as an active targeting strategy to some extent, because it allows drug release from the micellar nanomedicine at specific tumor sites. [142]Moreover, the use of nucleic acids carried by micelle-based drug delivery systems to resist cancer, particularly metastases, in addition to or in combination with chemotherapies, is emerging as an efficacious treatment method.Modification of micelles with Angiopep-2 significantly enhanced the penetration ability through the BBB and BTB, and the resulting nanomedicine was demonstrated with an impressive antibrain metastasis effect in a breast cancer brain metastasis model. [143]Additionally, modification of micelles with D-type cyclic peptides was found to significantly enhance the BBB and BTB penetration and greatly improve the targeting ability for breast cancer brain metastases. [144][147] The use of tumor cell membranes to load drug entities resulted in a high response rate for breast cancer brain metastases after IV injection of the nanodrug in xenograft MDA-MB-231 metastatic mice models. [148,149]dditionally, cell membrane-coated nanomedicine has great potential in the safe and effective delivery of therapeutic genes.For instance, PEGylated mesenchymal stem cell membranes were constructed to form a 200 nm nanostructure, which retained the natural ability of effective tumor targeting; thus, the nanostructure was coined as a nanoghost.When the nanoghost was loaded with naked plasmid cDNA, the resulting nanomedicine demonstrated excellent lethality against PC-3 cells, and the metastases of NSCLC was pronouncedly inhibited in a xenograft orthotopic NSCLC mice model.149b] Inorganic nanoparticles have also been utilized in recent years as drug delivery carriers, and many encouraging examples for treating cancers have been reported. [150]By modifying the surface of gold nanoparticles with short peptides, the nanomedicine was empowered with excellent targeting capability as well as therapeutic potency for breast cancer brain metastases. [151]dditionally, silica-based nanoparticles loaded with photosensitizers could suppress the growth of breast cancer brain metastases through photodynamic therapy (PDT). [152]

Comprehensive Strategies of Nanomedicine
Currently, the field of nanomedicine has made substantial advancements in enhancing the effectiveness of treating brain metastases through various approaches.We categorize these strategies of using nanomedicine for augmenting treatment effectiveness into nanomedicine-enhanced targeted therapy, immunotherapy, gene therapy, phototherapy, and combination therapy.Each of these strategies will be thoroughly discussed in the following sections.

Nanomedicine with Enhanced Targeted Therapy
The treatment for brain metastases remains a great challenge in clinical oncology due to the presence of the BBB and BTB, which prevent the entry of most chemotherapeutic agents into the brain.The therapeutic effects of most current antitumor drugs are primarily diminished due to the heterogeneity of brain metastases and the blockage of the BBB and BTB during their routine use.Despite the development of various therapies, the survival rate for patients with brain metastases remains poor.Therefore, there is a critical need for developing new strategies to improve the delivery efficiency of drugs to the sites of brain metastasis.
Nanomedicine is a powerful therapeutic modality for overcoming numerous obstacles for drug delivery and achieving remarkable antibrain metastasis effects.By constructing unique structures and implementing specific modifications of nanoplatforms, nanomedicine can precisely deliver its incorporated drug(s) to the target sites in the body and enhance the therapeutic effect.One of the most important obstacles in the treatment of brain metastasis, the BBB and the BTB, could be overcome by enhancing the passive and active targeting of nanomedicine, thereby increasing their penetration into the BBB/BTB to effectively deliver therapeutic agents to the site of brain metastasis. [153,154]assive targeting, a property of nanoparticles, enables them to accumulate at the site of a tumor based on their physical and chemical properties.The enhanced EPR effect is a typical mechanism of passive targeting of nanomedicine toward brain metastases.Smaller nanoparticles can relatively easily penetrate the BBB/BTB and accumulate at the site of brain metastases. [117]owever, nanoparticles are typically designed to achieve a long circulation time in the blood, which increases the chances of accumulating at the site of brain metastases through passive targeting.Unfortunately, the passive targeting of nanoparticles is very weak for brain metastases because the BBB/BTB prevents the majority of therapeutic agents from reaching the tumor site, and the drug concentration is often below the expected dose at the tumor site. [76]Therefore, active targeting is usually pursued on the nanoplatform as an alternative to its passive targeting.Active targeting is realized through specific surface modifications of the nanoplatform.After modification, the nanoplatform could recognize specific molecules expressed on the surface of specific tumor cells and elicit specific reactions to these molecules, thus actively targeting these tumor cells.Therefore, the nanoplatform is often modified with ligands that recognize and bind to specific receptors or antigens expressed on the surface of tumor cells or BBB/BTB, and it could cross the BBB/BTB and accumulate specifically at the tumor site.This approach can enhance the accumulation of the resulting nanomedicine at the tumor site compared to passive targeting and improve its therapeutic efficacy.Meanwhile, active targeting can significantly reduce the accumulation of therapeutic agents in healthy tissues, thereby reducing side effects of these drugs.
A BRBP1 peptide, [134] an anti-PD-L1 nanobody, and an transferrin receptor (TfR)-binding peptide T12 [155] have been explored as a ligand for liposomes, and they can recognize and bind to BRBP1, PD-L1, and TfR, respectively, on the surface of breast cancer brain metastatic cells and NSCLC brain metastatic cells.After modification of liposomes with these ligands, the liposomederived nanomedicine has an excellent brain metastasis targeting ability.
Ligands for the BBB/BTB have been discovered for micelles and inorganic nanoplatforms.Angiopep-2, [143] a D-type cyclic peptide(Figure 3a), [144] and a TAT peptide [151] are able to bind to IGF-1R, MMP1 or LRP1 on the BBB and BTB, respectively.The use of these ligands on the surface of micelles or gold nanoparticles allows penetration of the resulting nanomedicine through the BBB and BTB and the nanomedicine shows excellent therapeutic effects against breast cancer brain metastases.
Another popular ligand is a CXCL13 ligand for chemokine receptor CXCR5 on the surface of cancer cells.For example, the therapeutic effect of a monoclonal antibody in the nanomedicine against lymphoma brain metastases was significantly enhanced after introducing a CXCL13 ligand onto the surface of the nanomedicine.Rituximab was encapsulated within a crosslinked polycationic polymer layer, and the polymer surface was modified with a CXCL13 ligand.The monoclonal antibody was gradually released from the nanomedicine to exert its therapeutic effect through the hydrolysis of crosslinkers after the nanomedicine reached the tumor site via blood circulation.The nanomedicine-assisted delivery of Rituximab significantly increased the degree of its accumulation in the CNS compared to direct administration of Rituximab, and its concentration was improved by ten times.The polymeric surface was further functionalized by CXCR5 before introducing the CXCL13 ligands (Figure 3b).The resulting nanomedicine demonstrated a remarkable inhibitory effect on the growth of transplantable cancer in non-Hodgkin's lymphoma mice after a single use.The antibrain metastasis effect of this nanomedicine was further validated in a mouse model of humanized bone marrowliver-thymus transplantation-induced brain metastases, and the treatment with the nanomedicine significantly inhibited the growth of lymphoma brain metastases and prolonged the survival time of the mice. [126]

Nanomedicine with Immunotherapy
In recent years, immunotherapy for cancers, including primary and metastatic cancers, has garnered increasing attention due to promising experimental and clinical responses.158] Tumor vaccines, a novel type of vaccine, are designed to inhibit the growth of cancer cells or induce their death by enhancing the host immune system through stimulation.Elegant design and precise application of the vaccines could prevent the occurrence of primary and metastatic cancers to some extent. [159,160]With the development of various nanomaterials and nanotechnologies, an increasing number of nanoparticles are found to be excellent adjuvants for tumor vaccines, as they are strong in boosting the immunogenicity of the vaccines, enhancing the antigen transmission efficiency, increasing the cellular uptake rate, and modulating the activity of immune cells. [161,162]Essential characteristics of the nanoparticles as an excellent tumor vaccine adjuvant have been identified, including their surface chemistry and particle size, the antigen release effect, the immune activation route, and their targeting and modulation efficiency.These characteristics must be taken into account during engineering design and production processes of these nanoparticles to overcome barriers, such as immune cell interaction rates, lymph node targeting rates, mucosal tissue penetration rates, and other obstacles that may hinder the effectiveness of tumor vaccines. [161,163]ell-based immunotherapeutic nanomedicine strategies involve using advanced nanoplatforms as adjuvants to enhance their tumor targeting ability and boost the subsequent antitumor response of immune effector cells, including lymphocytes, macrophages, natural killer cells, cytotoxic T lymphocytes, which essentially induce the tumor vaccine effect. [164,165]The great potential of the use of newly developed nanoplatforms has been demonstrated in the field of cell-based immunotherapy, especially in the imaging-guided cell therapy.Various types of NPs, such as magnetic NPs, upconversion NPs, and NIR quantum dot NPs, have been applied to track specific immune cells, including dendritic cells, artificial antigen-presenting cells, natural killer cells, and monitor the subsequent immune response to a certain degree. [166,167]For example, poor immunogenicity of tumor vaccines for lung cancer was significantly boosted through the use of poly-L-glutamic acid NPs modified with cell-penetrating peptides, into which granulocyte macrophage colony-stimulating factor and interleukin-2 (IL-2) were loaded.This approach not only significantly enhanced the immune tolerance of the body, but also induced a significant inhibitory effect on tumor growth and metastasis. [168] biomimetic nanoplatform was developed for the treatment of brain metastasis from lung cancer by mediating the innate immune response of immune cells.The growth of lung cancer brain metastases in the mouse model is shown in Figure 4a. [127]he nanomedicine was derived from T12-peptide-modified albumin nanoparticles loaded with regorafenib and disulfiram/copper ion chelate.By reshaping the tumor immune microenvironment, this nanomedicine enhanced the body immune response to tumors (Figure 4b-d).When administered intravenously, this nanomedicine significantly suppressed brain metastases in the mouse model of lung cancer and improved the survival rate of the mice (Figure 4e).

Nanomedicine with Gene Therapy
Gene therapy has been a rapidly evolving field in the biopharmaceutic industry, with the potential to revolutionize the way we treat diseases. [169,170]Novel approaches to modifying genes at the cellular level have been explored to address a wide range of medical conditions, including genetic disorders and various types of cancer. [171,172]The gene therapeutic approach, especially This peptide endowed the nanomedicine with a remarkable capability for traversing the BBB and specifically targeting brain metastatic cells of TNBC.Reproduced with permission. [144]Copyright 2022, Elsevier.b) By enriching monomers and crosslinkers around the Rituximab (RTX) molecules, a thin shell was formed to encapsulate RTX.CXCL13 was released from the nanomedicine to achieve targeted therapy against non-Hodgkin lymphoma (NHL) brain metastases. [126]ith the aid of advanced nanoplatforms, has shown promising results for the treatment of brain metastases.
The use of nanoplatforms to deliver small interference RNA (siRNA) to the site of brain metastases is a promising approach.siRNA can be introduced to cancer cells to precisely silence specific genes that control the growth, development, metastasis, and drug resistance of tumors, leading to a reduction/suppression effect on tumor growth.For instance, specific siRNA can inhibit the expression of drug resistance genes, thus mitigating multidrug resistance of cancer cells. [173,174]The nanoplatforms could be functionalized with specific ligands for cancer cells in the brain so they can transport the siRNA molecules to the cancer cells, achieving targeted gene therapy.After regulating the expression of specific genes in the cancer cells via the delivered siRNA, the growth of cancer cells can be retarded or completely suppressed, ultimately leading to better prognosis for patients. [175]While gene therapy for brain metastases is still in its early stage, it has enormous potential to become a groundbreaking intervention for brain metastases.
siRNA can be combined with other treatment modalities, such as chemotherapy, to enhance its antitumor effectiveness, suppress tumor metastasis, and bring additional benefits to patients. [169,175,176]Tang et al. combined the delivery of two transcription factors-associated siRNA with a chemotherapy drug paclitaxel (PTX).Snail siRNA and Twist siRNA were codelivered to knock down the expression of multidrug resistance-associated protein 1 (MRP1) and B-cell lymphoma 2 (BCL2), respectively, in order to strengthen the antimetastases effect of PTX.The cooperative delivery strategy synergistically reduced the growth rate, restricted the cell motility, and promoted extracellular matrix degradation, ultimately preventing cancer metastasis. [177] nanoplatform was developed to selectively deliver siRNA and a chemotherapy drug to breast cancer brain metastases to enhance the efficacy of the siRNA for targeting brain metastasis in breast cancer.The nanomedicine was constructed from liposomes and modified with a BRBP1 peptide to enhance its penetrating ability through the BBB and the BTB.The experiments via a mouse model of breast cancer brain metastasis confirmed the enhancement penetration ability of the nanomedicine through the BBB and the BTB.The expression level of the TWF1 gene in breast cancer brain metastases was significantly intervened via the delivered siRNA in the nanomedicine.The antitumor effect of the co-administered chemotherapy drug PTX was successfully sensitized.Therefore, the survival time of the mice was significantly prolonged.[134] Another siRNA nanomedicine, designed specifically for TNBC, was developed for targeting brain metastatic tumors.This nanomedicine, Tf@TBP-CPs-siPLK1, was primarily composed of a polymer nanoplatform modified with transferrin that carried siRNA (Figure 5a).The nanomedicine had a nearly neutral zeta potential and could effectively target endothelial cells of the BBB and the BTB as well as breast cancer brain metastatic tumor cells, with the help of transferrin modification.Meanwhile, the internalization of the nanomedicine by these cells was remarkably enhanced.Eventually, the nanomedicine passed through endothelial cells and released siRNA to silence The uptake of the nanomedicine by brain metastatic tumor cells in lung cancer.c) The antitumor effect of this nanomedicine on lung cancer brain metastatic tumor cells in the presence of M1 and M2 macrophages.d) Analysis of the changes in the macrophage subpopulations under different treatment conditions.e) Survival status of the mice with lung cancer brain metastasis after treatment with the nanomedicine and the control group.Reproduced with permission.[127] Copyright 2020, Elsevier.
polo-like kinase 1 (PLK1) within the tumor cells to exert its antibreast cancer brain metastasis effect (Figure 5b).Treatment experimental results on a mouse model with breast cancer brain metastasis supported that after IV injection of this nanomedicine, the growth of brain metastatic tumors in the mice was significantly inhibited (Figure 5c), and their survival and quality of life were also improved (Figure 5d,e). [178]

Nanomedicine with Phototherapy
Phototherapy includes photothermal therapy (PTT) and PDT. [179,180]PTT is a recently developed strategy for treating metastatic cancers, and the method, especially the imaging-guided PTT, has received increasing attention. [181,182]After uptake of nontoxic NIR absorbing agents by cancer cells, these cells could be killed by converting optical energy to heat via the internalized agents upon irradiation with a laser, leading to an excellent treatment efficacy and negligible side effects.Various types of NIR-absorbing agents have been developed in recent decades, including nanoscale agents with organic or inorganic characteristics. [183]t was reported that nanoshell-assisted thermal ablation could be effective in treating brain metastases in an orthotopic mouse model with transmissible venereal tumors by using NIRabsorbing nanoshells at a specific dose of 808 nm laser.Tuning the process parameters allowed accumulation of the drug entity at metastasis sites in the acceptable range and induction of minimal thermal damage to normal brain tissues. [184]Imageguided PTT may have advantages against brain tumor metastasis by targeting lymphatic metastases, even though lymph node metastasis to the brain is not as common as hematogenous metastasis.Liang et al. reported the use of a nanotheranostic agent consisting of PEG-coated single-walled carbon nanotubes and an NIR-II fluorescent probe to treat breast tumors.The conjugated therapeutic agent exhibited excellent fluorescence imaging with a high level of tissue penetration, spatial distribution, Figure 5. Representative nanomedicine for gene therapy against brain metastases.a) Tf@TBP-CPs-siPLK1 was constructed from transferrin-modified polymers to deliver siPLK1.b) After IV administration, this nanomedicine enhanced the uptake of siPLK1 in brain metastatic tumor cells through transferrin-enhanced endocytosis and released siPLK1 in the tumor cells at the site of breast cancer brain metastasis in a mouse model.c) The growth of brain metastasis in the mouse model with breast cancer brain metastasis was assessed after administration of different formulations.d) The weight changes of the mice with breast cancer brain metastasis were monitored after administration of different formulations.e) The survival of the mice with breast cancer brain metastasis was observed after administration of different formulations.Reproduced with permission. [178]Copyright 2021, Elsevier.and accumulation in lymph nodes.After photothermal ablation, the tumor growth at both the origin site and sentinel lymph nodes was significantly inhibited, leading to significantly improved overall survival in an animal model.187] PDT, also known as photochemotherapy, is an emerging treatment strategy against primary and metastatic cancers.The therapy is achieved by combining photosensitizing drugs uptaken by tumor cells, molecular dissolved oxygen in the tumor tissue, and an external laser at specific wavelengths to induce phototoxicity for tumor cells. [180,188,189]By incorporating imaging agents, PDT can also be guided to target metastases.The majority of PDT studies focus on treating malignant neoplasms in the lung, prostate, bladder, and especially the skin. [190]Conventional PDT faces challenges in treating cancer metastasis, such as hypoxia at the tumor site and poor light penetration into deep tumor tissues.To overcome these barriers, a novel nanostructure that was assembled from a photosensitizer and mitoxantrone was developed.The nanostructure possessed multiple characteristics, such as activatable fluorescence emission and a concomitant chemotherapeutic effect.The nanostructure could target cancer tissues and accumulate at the cancer site, resulting in excellent outcomes in treating primary and metastatic cancers. [191]nother nanoplatform derived from silicon dioxide for delivery of a second-generation photosensitizer, temoporfin, was developed to enhance traditional PDT.By subjecting this silicon dioxide-based nanomedicine to surface chemistry modifications, significant tumor suppression effects on both mouse-derived and human-derived breast cancer brain metastasis models were achieved when activated by irradiation.This study suggests that PDT treatment for breast cancer brain metastasis could be distinctly improved with the help of a nanoplatform. [152]2.5.Nanomedicine with Combination Therapy A single therapy may not achieve satisfactory inhibitory effects for brain metastases.Therefore, nanomedicine that could selectively load multiple therapeutic agents is often used to achieve targeted combination therapy for brain metastases.This combination therapy could be the combination of different therapeutic modalities including chemotherapy, immunotherapy, gene therapy, and phototherapy or different therapeutic agents for the same therapeutic modality.[167,192,193] One of the most common approaches is to incorporate more than one chemotherapeutic drug in the nanomedicine.Meanwhile, the nanomedicine could be functionalized to regulate the brain metastatic microenvironment.Such a combination therapy scheme could hold great promise in significantly improving the treatment outcome of breast cancer brain metastases.[194,195] For example, a solid lipid-based nanomedicine targeting breast cancer brain metastases was developed.The surface of the nanomedicine was modified with internalizing arginineglycine-aspartic acid (iRGD) and polysorbate 80, and it simultaneously delivered both DOX and mitomycin C (MMC) to achieve combination therapy and enhance the therapeutic effect against  [135] Copyright 2019, Wiley-VCH GmbH.
breast cancer metastases (Figure 6a).Modification with polysorbate 80 could help recruiting ApoE to bind to the low-density lipoprotein receptors (LDLRs), achieving great penetration through the BBB and the BTB as well as regulating the tumor immune microenvironment by targeting tumor-associated macrophages (TAMs) to enhance the uptake of the nanomedicine by metastatic tumor cells.iRGD could specifically bind to overexpressed αv integrins by breast cancer brain metastatic cells and the microvascular endothelium of brain tumors to enhance the uptake of nanomedicine by the metastatic cells and promote the penetration of the BBB/BTB (Figure 6a).There was a pronounced enhancement in the accumulation of DOX and MMC with iRGD modification of the nanomedicine at the site of breast cancer brain metastases in the model mice compared to direct administration of both drugs without iRGD modification (Figure 6b).The therapeutic effect against breast cancer brain metastases was significantly improved, and the survival rate of the model mice was also substantially increased (Figure 6c) compared to administration of free DOX and MMC drugs. [135]argeted treatment of NSCLC brain metastases was improved by combining chemotherapy and targeted therapy.In this study, a nanoplatform loaded with both osimertinib (AZD9291) and DOX was constructed, which significantly improved the treatment outcome of brain metastases in an animal model.The nanoplatform surface was decorated with a T7 peptide, which could target the transferrin receptor.Both chemotherapeutic and targeted therapeutic drugs were introduced onto the nanoplatform via glutathione (GSH)-responsive groups which could be cleaved by highly expressed GSH in the tumor microenvironment.As a result, the constructed nanomedicine had a significantly enhanced level of penetration through the BBB/BTB at the site of brain metastases due to transferrin receptor-mediated transcytosis.Finally, the intervention by the nanomedicine resulted in a strong antibrain metastasis effect in a mouse model of lung cancer brain metastases and significant extension of the survival time of the model mice. [196]he therapeutic efficacy of the nanomedicine treatment against brain metastases of breast cancer could be enhanced by combining chemotherapy with tumor microenvironment modulation.For instance, a chemotherapy drug DOX and statins were coloaded onto a nanoplatform and the nanoplatform was modified with Angiopep-2.This nanomedicine selectively interacted with highly expressed LRP1 on the microvascular endothelial cells and brain metastatic tumor cells at the brain metastases site through Angiopep-2, facilitating penetration of the BBB/BTB and enhancing internalization of DOX by the metastatic tumor cells, thus improving the chemotherapeutic efficacy against brain metastases.The carried statins in the nanomedicine upregulated the expression level of LRP1 in the brain metastatic tumor cells and the BTB microvascular endothelial cells, thus enhancing the targeting ability of this nanomedicine.Finally, this nanomedicine showed a significant improvement in the therapeutic effect of DOX in a mouse model of breast cancer brain metastases and significantly prolonged the survival time of the mice. [197]nother nanomedicine that combined targeted therapy with immunotherapy showed promising results in treating brain metastases from NSCLC.A biomimic nanoplatform was built to deliver ubiquitin-specific protease 7 (USP7), an enzyme inhibitor, to the brain metastatic site in a murine model (Figure 7a,c).
A PD-L1 immune checkpoint inhibitor was also combined with the targeted therapeutic agent to inhibit the tumor growth.The nanoplatform created from microparticles that were produced from gene-edited lung cancer cells upon radiation possessed three important characteristics.First, they were designed to target and reprogram M2 macrophages, which play a critical role in promoting tumor growth.Second, a genetically expressed SR-B1targeting peptide was used to facilitate the penetration of the nanomedicine through the BBB/BTB.Finally, the USP7 inhibitor incorporated in the microparticles targeted tumor cells and reprogrammed M2 macrophages (Figure 7b).Experimental data supported that these microparticles were successful in crossing the BBB/BTB and targeting M2 macrophages both in vitro and in vivo (Figure 7d), resulting in reprogramming of the macrophages and extension of the survival time in a murine brain metastasis model.Furthermore, when the targeted therapy was combined with the immune checkpoint blockade, the therapeutic effects were further enhanced (Figure 7e). [198]

Prospects for Nanomedicine Development
Nanomedicine has been demonstrated to offer unparalleled specificity and flexibility by incorporating various functional moieties into a nanocarrier after loading therapeutic/imaging agents onto the nanocarrier.Therefore, nanomedicine could hold great promise in enhancing treatment effects on brain metastasis from a diversity of primary tumors and addressing the challenge of a high level of significant heterogeneity of brain metastases.
The ideal nanomedicine candidates should be preventive for primary tumor patients with a high risk of brain metastasis, thus completely preventing the occurrence of brain metastasis from its origin.Therefore, primary tumors and their metastasis should be considered when constructing these candidates.Prevention of ongoing brain metastasis may fall within the scope of these candidates.However, the most common clinical need is to treat already existing brain metastasis.To meet this need, nanomedicine should overcome various physiological/pathological barriers and target specific characteristics of brain metastases.Based on comprehensive analysis of current primary treatment modalities, the following perspectives are proposed for nanomedicine development: 1) Tailored nanoplatform and selective strategies: Identifying the type of primary tumors and profiling its metastatic characteristics are essential for designing an appropriate nanoplatform and formulating a treatment approach.For example, in the case of breast cancer, it is crucial to note that TNBC is prone to brain metastasis.Customized nanoplatforms, such as biomimetic nanoplatforms derived from tumor cells, in conjunction with immunotherapy, could be explored for this particular cancer type; 2) Targeted inhibition: Nanomedicine could be designed to specifically downregulate genes that could promote metastasis, such as targeted inhibition of the SOX2 gene to prevent the occurrence of metastasis; 3) Enhanced BBB/BTB penetration: Nanomedicine could be equipped with peptides to improve its penetration capabilities across the BBB and the BTB; 4) Integration with surgical intervention: Nanomedicine incorporated with imaging and/or therapeutic agents could be combined with surgical interventions, such as the administration of imaging agents for intraoperative navigation, to improve precision and effectiveness of surgical operations; 5) Enhanced radiation therapy: Radiosensitizers for radiation therapy could be formulated into nanomedicine to enhance their treatment efficacies while minimizing radiation-related side effects; 6) Multimodal approach: A multimodal approach could be employed by combining various therapeutic/imaging agents in one single nanomedicine.For instance, combining immunotherapy with chemotherapy could address the challenge of a high level of tumor heterogeneity in brain metastases.Furthermore, a synergistic combination of different immunotherapy approaches could greatly augment their efficacies; and 7) Exploration of aggressive strategies: More aggressive penetration strategies and greater-potency therapeutic ingredients could be explored by taking advantage of excellent characteristics of nanomedicine.

Conclusion
Brain metastases could be tackled by regulating the metastatic microenvironment and the tumor immune microenvironment, suppressing tumor cell growth, and invigorating the antitumor immunity at the brain metastatic site.Although brain metastases occur in the brain, the treatment of brain metastases could be traced back to the primary tumor, its adaption under stress, and the spread pathway.This review elaborates a few key obstacles for the treatment of brain metastasis, such as tumor heterogeneity and the BBB/BTB integrity.Nanomedicine has shown great promise in treating brain metastases because the nanoplatform for the nanomedicine can be tuned with its composition, structure, morphology, hydrophilicity, magnetism, and immunogenicity.A variety of nanoplatforms have been explored to deliver chemotherapeutic drugs, peptides/proteins, small molecular inhibitors, immunostimulants, photothermal agents, and/or chemically kinetic agents for single-modality or combined multiple-modality therapy.The nanoplatform can be decorated with functional groups for promoting its penetration through the BBB/BTB, enhancing its internalization by tumor cells, improving its targetability towards tumor cells, endothelial cells of the BBB/BTB, or immune cells at the brain metastatic site.Copyright 2023, Wiley-VCH GmbH.
The combination of different treatment modalities has been demonstrated with great potential in overcome the obstacles to treat brain metastasis.Among them, the combination of targeted therapy and immunotherapy could outperform other combinations.With the development of multiomics technologies, the mechanisms of brain metastasis could be revealed and novel targeted pathways identified.Nanoplatforms for incorporating both targeted and immune-based therapeutic agents could be equipped with multifunctionalities to realize better therapeutic outcomes but diminish side effects of these agents.Currently, only a few nanomedicinal products have been used in clinical settings.Efforts should be devoted to reducing the toxicity of nanoplatforms and building a preclinical model that can mimic the in vivo environment.In summary, with the development of advanced nanotechnology and cancer therapeutic methods, clinical breakthroughs in the treatment of brain metastasis will be achieved, benefiting to the patients with brain metastasis.
Qiyong Gong is a professor and chief physician of clinical radiology.He is the president at West China Xiamen Hospital of Sichuan University and also serves as the vice president at West China Hospital of Sichuan University.He is the Changjiang Scholar Distinguished Professor, the former chairman of the Psychiatry and Psychology MRI Group of ISMRM, the editor-in-chief of Psychoradiology journal, and the associate editor of the American Journal of Psychiatry.His has made a large amount of systematic and innovative work in the field of radiological diagnosis of neurological and psychiatric diseases.

Figure 1 .
Figure 1.Distribution of cell types in the BTB region and potential therapeutic targets in the blood-tumor microenvironment.Specific treatment strategies include antiangiogenic agents, cancer cell-targeting peptides, and radiosensitizers.

Figure 3 .
Figure3.Nanomedicine for enhancing active targeting toward brain metastases.a) Through dialysis and electrostatic adsorption, the nanomedicine T-M/siRNA was anchored with a D-type CSKC cyclic short peptide.This peptide endowed the nanomedicine with a remarkable capability for traversing the BBB and specifically targeting brain metastatic cells of TNBC.Reproduced with permission.[144]Copyright 2022, Elsevier.b) By enriching monomers and crosslinkers around the Rituximab (RTX) molecules, a thin shell was formed to encapsulate RTX.CXCL13 was released from the nanomedicine to achieve targeted therapy against non-Hodgkin lymphoma (NHL) brain metastases.[126]

Figure 4 .
Figure 4. Representative immunotherapeutic nanomedicine for brain metastases.a) Histological sections of the brain tissue on the 5th and 9th days after implantation of lung cancer brain metastases.b)The uptake of the nanomedicine by brain metastatic tumor cells in lung cancer.c) The antitumor effect of this nanomedicine on lung cancer brain metastatic tumor cells in the presence of M1 and M2 macrophages.d) Analysis of the changes in the macrophage subpopulations under different treatment conditions.e) Survival status of the mice with lung cancer brain metastasis after treatment with the nanomedicine and the control group.Reproduced with permission.[127]Copyright 2020, Elsevier.

Figure 6 .
Figure 6.Representative nanomedicine for combination therapy against brain metastases.a) Self-assembly diagram of the iRGD-DMTPLN nanomedicine for the treatment of brain metastasis of breast cancer, which possesses abilities of targeting highly expressed αv integrins on the microvascular endothelium of brain tumors and breast cancer metastatic cells via iRGD on the surface of the nanomedicine, as well as targeting LDLR on the microvascular endothelium of brain tumors and TAMs through recruitment of ApoE decoration via polysorbate 80. Interaction between the nanomedicine and TAMs could regulate the immune microenvironment of brain metastasis, ultimately enhancing the therapeutic effect of both DOX and MMC.b) In vivo fluorescence images of the tumor sites in the model mice with brain metastasis from breast cancer after IV administration of DMTPLN with or without iRGD modification to differentiate the level of accumulation and the distribution patterns in the model mice.c) Survival analysis of the model mice for each group after drug administration.Reproduced with permission.[135]Copyright 2019, Wiley-VCH GmbH.

Figure 7 .
Figure 7. Representative nanomedicine for combined immunotherapy and targeted therapy against brain metastases.a) Nanomedicine produced by both gene-edition and irradiation interacted with tumor cell membranes to release its contents.b) This nanomedicine was constructed by irradiating gene-edited lung cancer cells to produce SR-B1-targeting EVs, which were loaded with a targeted drug, an USP7 inhibitor (P5091).The nanomedicine could easily cross the BBB/BTB at the site of brain metastasis.It could target and regulate M2 macrophages to modulate the immune microenvironment of the brain metastasis site to exert synergistic therapeutic effects of the targeted drug and the immune checkpoint inhibitor.c) Morphology analysis of the nanomedicine under transmission electron microscope (TEM), scale: 200 nm.d) Distribution of the administrated nanomedicine and controls in the brain tissue of a model mouse with lung cancer brain metastasis.e) Survival analysis of the model mice with lung cancer brain metastasis after administration of the nanomedicine and various controls.Reproduced with permission.[198]Copyright 2023, Wiley-VCH GmbH.

Kui
Luo is a professor and researcher at Department of Radiology, Huaxi MR Research Center (HMRRC) and Frontiers Science Center for Disease-Related Molecular Network, State Key Laboratory of Biotherapy in West China Hospital of Sichuan University.From September 2009 to October 2011, he worked as a postdoctoral fellow in the Department of Pharmacology and Pharmaceutical Chemistry at the University of Utah in the United States.He conducts research on intelligent macromolecules for tumor diagnosis and treatment.Yanhui Liu is a professor and chief physician of neurosurgery.He is serving as the deputy director at the Department of Neurosurgery at West China Hospital of Sichuan University.He is the academic leader of the Sichuan Provincial Health Commission and has served as a member of the expert group to participate in the compilation of consensus and guidelines related to gliomas.He is mainly engaged in basic and clinical research on central nervous system tumors, particularly precise and personalized treatment for gliomas, meningiomas, and deep brain lesions.

Table 1 .
Major obstacles in current brain metastasis treatment methods.
Side effects such as systemic toxicityPreclinical studiesLimitations of current preclinical models Conducting research using multiple brain metastasis models, and developing models that are more closely representative of human characteristics

Table 2 .
Representative NPs and preclinical models for brain metastases.