The incidence of Alzheimer’s disease (AD), the most representative neurodegenerative brain disease, is steadily increasing as the aging population increases. However, only drugs that can alleviate and delay symptoms are currently being used, and no specific treatment methods or therapeutic agents ^[1][19] have been developed yet. Over the past decades, several clinical trials have been conducted with various targets, focusing on these two clinical indications: amyloid beta plaques and neurofibrillary tau tangles ^[2][20]. However, all clinical trials have failed; only Aducanumab, which targets amyloid-β (Aβ) plaque removal, has shown a therapeutic effect, but it is controversial due to side effects ^[3][4][21,22]. Although the amyloid hypothesis remains controversial, since the accumulation of Aβ is a representative pathological hallmark of AD, numerous therapeutic studies targeting Aβ have been conducted.

The first preclinical study on FUS for AD aimed to deliver anti-Aβ antibodies targeting amyloid plaques into the brain by a BBB opening. Consequently, anti-Aβ antibodies bound to the Aβ plaques and rapidly reduced the plaque pathology ^[5][23]. Subsequently, research on delivering therapeutic agents through FUS-mediated BBB opening in patients with AD has gained attention ^{[6][7][8][9][10][11][12]}[24,25,26,27,28,29,30]. Interestingly, studies have reported that amyloid pathology ^{[6][13][14][15][16]}[24,31,32,33,34] and phosphorylated tau ^[17][18][35,36] are reduced only by FUS-induced BBB opening without specific drug delivery. Treatment delivery via FUS-mediated BBB opening also affected memory recovery in AD animal models ^{[14][19][20][21][22]}[32,37,38,39,40]. Research studies on various biological changes by FUS-mediated BBB opening are ongoing. However, for FUS to be a promising non-pharmacological treatment delivery method for AD, further research is needed on why amyloid is reduced and cognitive function is restored. FUS induces the activation of microglia and astrocytes, which may increase phagocytosis of the amyloid plaques ^[13][14][23][31,32,41]. Recently, a study confirming the therapeutic effect in an AD mouse model (5×FAD) by combining FUS and Aducanumab was reported ^[24][42]. Aducanumab, a monoclonal antibody targeting fibril forms and beta-amyloid oligomer, has been proven effective since receiving FDA approval in 2021. However, due to side effects, debate continues as to whether or not it should be used.

In conclusion, combined treatment with FUS and Aducanumab reduced amyloid plaque levels, increased hippocampal neurogenesis, and restored cognitive function. Here, FUS activated phagocytic microglia and increased the number of astrocytes associated with amyloid plaques. This suggests that FUS can induce a reduction in amyloid plaques through phagocytosis. In addition, an RNA sequencing analysis showed that the combined treatment with FUS and Aducanumab upregulated neuroinflammation signaling, phagosome formation, reelin signaling, and CREB signaling ^[24][42]. The immunomodulatory effect of FUS, such as the activation of various innate immune cells, plays a vital role in reducing amyloid plaques ^[19][37]. Regarding the recovery of cognitive function by FUS, the increase in hippocampal neurogenesis ^{[25][26][27][28]}[43,44,45,46] or synaptic plasticity ^[27][29][45,47] may play a role here, but further research is needed on this topic. The researchers summarized the most relevant preclinical studies on FUS-mediated BBB opening in AD (Table 1).

(Table 2).

Parkinson’s disease (PD) is a neurodegenerative brain disease accompanied by motor dysfunction due to the loss of dopaminergic neurons. PD is neuropathologically characterized by proteinaceous inclusions called Lewy bodies ^[38][56]. Notably, as many studies have reported that α-synuclein plays a direct role in disease development, PD is classified as α-synucleinopathies ^[39][57]. Currently, there are no clear treatments to slow or alleviate the progression of neurodegenerative diseases such as PD. Treatment with glial-derived neurotrophic factor (GDNF) is considered appropriate for PD due to its neuroprotective and neurotrophic effects ^[40][41][42][58,59,60]. The overexpression of neuroprotective genes that induce dopamine regeneration in activated neurons can delay disease progression ^[43][61]. The potential benefit of GDNF with regard to recovery and the functional improvement of dopaminergic neurons has been confirmed ^[43][44][45][61,62,63]; however, one study was discontinued due to safety concerns in clinical trials ^[46][64]. Animal studies of GDNF gene delivery by FUS began in PD and have highlighted the possibility of effective gene therapy ^[47][48][49][65,66,67].

Since neurturin has been found to have neuroprotective and neuro-regenerative effects on dopaminergic neurons ^[50][68], the FUS-based delivery of neurturin has been studied to find an alternative to GDNF ^[51][52][69,70]. Recently, recombinant adeno-associated viral (rAAV) vectors have received much attention as a tool for gene delivery to the brain. The technology of delivering rAAV using FUS-mediated BBB permeability and expressing the delivered gene has already been examined ^[53][71]. Accordingly, recent studies on PD models using FUS mainly involve gene delivery using rAAV. While there are many studies on delaying disease symptoms by delivering various therapeutic agents using FUS, there is a lack of preclinical research studies on α-synuclein-based PD models. TWe summarized the most relevant preclinical studies on FUS-mediated BBB opening in PD were summarized

Glioblastoma is the most aggressive brain tumor with a high recurrence rate and poor prognosis despite treatments such as resection, radiotherapy, and chemotherapy ^[59][77]. The blood–tumor barrier (BTB) is created by the often heterogeneous disruption of the BBB within the tumor due to aberrant angiogenic signaling. As the delivery of anticancer drugs is limited despite the irregular leakiness of the BTB, quantitative drug delivery through FUS-mediated BBB opening is required ^[60][78]. Many previous studies on drug delivery by FUS have involved patients with brain tumors. Doxorubicin is a chemotherapeutic agent that inhibits cell growth and induces apoptosis in malignant glioma cells; however, it is not commonly used because it cannot cross the BBB. In 2007, Treat et al. delivered doxorubicin to a tumor in the brain via FUS-mediated BBB opening, indicating that this drug could be a viable treatment option ^[61][79]. Until now, various therapeutic agents have been used to treat glioblastomas, and FUS-mediated BBB opening technology is being developed. In the early days of FUS research, unencapsulated drugs such as the common anticancer drug temozolomide (TMZ) ^[62][63][80,81], carmustine (BCNU) ^[64][82], and immunostimulatory interleukin-12 (IL-12) ^[65][66][83,84] were mainly used.

Brain metastasis represents an important predictor of mortality for various non-brain cancers such as breast cancer. Like primary brain tumors, brain metastases do not have an intact BBB, but most therapeutics still have lower intra-tumoral bioavailability than non-brain tumors ^[67][85]. FUS studies have continued to treat metastatic brain tumors as well as primary brain tumors. In 2012, there was a study confirming the therapeutic effect by delivering Trastuzumab based on FUS-BBB opening in a breast cancer brain metastases model ^[68][86]. Additional research reported in 2016 demonstrated that the administration of trastuzumab and pertuzumab in a brain metastasis mouse model of breast cancer inhibited the growth of brain metastasis when used with FUS, compared to chemotherapy alone ^[69][87].

Whether it is a primary brain tumor or a metastatic brain tumor, the critical factor in the tumor microenvironment is to what extent the anticancer drugs could be delivered into the target region. It has been reported that the delivery of chemotherapeutic agents with small molecular weights to the brain tumor microenvironment is approximately 3.9-fold higher under FUS-mediated BBB opening conditions ^[70][88]. This enhanced delivery rate has been shown to increase median survival by approximately 30% compared to chemotherapy alone.

However, efflux transporters such as Pgp are overexpressed in cancer cells and prevent the uptake of anticancer drugs into the cells, resulting in resistance to them. FUS-mediated BBB opening temporarily inhibits Pgp expression, thereby preventing drug efflux and interfering with functional components of the BBB ^[71][89]. Additional research is needed on efflux transporter inhibitors targeting cancer cells. In addition to unencapsulated drugs, studies have reported that tumors (metastatic breast cancer) can be effectively controlled by delivering natural killer cells under BBB opening ^[72][90]. Furthermore, studies on suppressing brain tumors by delivering patient-specific antibodies or complexes loaded on short-hairpin RNA-liposomes have also been previously reported ^[73][91]. Since then, several studies have been conducted to enhance the safety and efficiency of tumor treatment by delivering encapsulated therapeutics through the conjugation of existing drugs or genes with improved MB, virus, and nanoparticles ^{[74][75][76][77]}[92,93,94,95]. As immunotherapy is a critical issue in neuro-oncology, additional research on immunotherapy using FUS-mediated BBB opening is expected to become more active in the future. TWe summarized the most relevant preclinical studies on FUS-mediated BBB opening in brain tumors  were summarized (Table 3).