The ubiquitin system regulates a wide variety of cellular functions. Not surprisingly, dysregulation of the ubiquitin system is associated with various disorders. Therefore, drugs that can modulate the functions of the ubiquitin system have been actively developed to treat these disorders. Chemical knockdown of pathogenic proteins using the ubiquitin-proteasome system is also a promising approach. The ubiquitin system regulates the assemble and disassemble of primary cilia through balanced control over the ubiquitination and deubiquitination of ciliary proteins. Primary cilia are antenna-like structures present in many vertebrate cells that sense and transduce extracellular cues to control cellular processes such as proliferation and differentiation. Impairment of primary cilia is associated with many diseases, including cancer and ciliopathy, a group of multisystem developmental disorders. In this review, we focus on the role of the ubiquitin system on cilia-related disorders and discuss the possibility of the ubiquitin system as therapeutic targets for these diseases through regulation of primary cilia formation.

We review recent advances of Ubiquitin-Proteasome System (UPS)-based research and development with increased focus as drug discovery approaches and introduce applications of chimera-type small molecule compounds (SNIPER/PROTAC) that selectively promote degradation of a drug target protein. UPS makes the point (polyubiquitin chain) of targeting protein as a substrate and has a property that degrade the target protein with proteasome. Protein knockout technologies degrade the drug target protein by apply this protein degrading system. In current technologies, polyubiquitin chains are artificially added to the drug target proteins through small molecules and introduce degradation of the target proteins. The approaches are divided into 2 types, one of which is E3 modulator-based technology represented by thalidomide, the other one is chimera compound-based technology represented by SNIPER/PROTAC. Furthermore, novel technologies are practically used to identify small molecule E3 binders as well as E3-targeting protein binders. These new approaches are expected to contribute to the efficient UPS-based drug discovery.

Recently, it has been reported that dysfunction of astrocytes is involved vulnerability of neuronal cells in several neurological disorders. Glutathione (GSH) is the most abundant intrinsic antioxidant in the central nervous system, and its substrate cysteine is readily becomes the oxidized dimeric cystine. Since neurons lack a cystine transport system, neuronal GSH synthesis depends on cystine uptake via the cystine/glutamate exchange transporter (xCT), GSH synthesis and release in/from surrounding astrocytes. The expression and release of the zinc-binding protein metallothionein (MT) in astrocytes, which is a strong antioxidant, is induced and exerts neuroprotective in the case of dopaminergic neuronal damage. In addition, the transcription factor Nrf2 induces expression of MT-1 and GSH related molecules. We previously revealed that several antiepileptic drugs, serotonin 5-HT1A receptor agonists, plant-derived chemicals (phytochemicals) increased xCT expression, Nrf2 activation, GSH or MT expression and release in/from astrocytes, and exerted a neuroprotective effect against dopaminergic neurodegeneration in Parkinson’s disease model. Our serial studies on neuroprotection via antioxidant defense mechanism of astrocytes have found three target molecular systems of astrocytes for neuroprotection: (1) xCT-GSH synthetic system, (2) Nrf2 system and (3) 5-HT1A receptor-Nrf2-MT system, 5-HT1A-S100β system. In this article, possible neuroprotective strategy for Parkinson’s disease has been reviewed targeting antioxidative molecules in astrocytes.

Ischemic stroke is one of the most prevalent brain disorders and the major cause of long-term disability. In particularly, hippocampal injury after ischemia-reperfusion is a serious problem as it contributes to vascular dementia. Many researches have revealed that ischemia-reperfusion causes increase in reactive oxygen species production and disruption of neuronal Zn²⁺ homeostasis in the hippocampus, which induces hippocampal neuron death. Glutathione (GSH) is present in all mammalian cells and plays a crucial role in neuronal cell defense against oxidative stress. On the other hand, thiol group of GSH chemically chelates Zn²⁺ and functions as a regulator of neuronal Zn²⁺ homeostasis. These evidences suggest that neuronal GSH levels could be an important factor affecting neuronal surviving. The synthesis of GSH is largely influenced by intracellular cysteine availability. In neurons, excitatory amino acid carrier type 1 (EAAC1) acts as a cysteine transporter and provides cysteine substrate for GSH synthesis. Recently, several animal studies have revealed that promotion of neuronal GSH synthesis through EAAC1 reduces ischemia-induced hippocampal neuron death. This review aims to describe neuroprotective role of GSH against hippocampal injury following ischemia-reperfusion, focusing on EAAC1.

Glutathione (GSH) is a tripeptide consisting of glutamate, cysteine, and glycine that acts as an important neuroprotective molecule in the central nervous system. In neurodegenerative diseases such as Alzheimer’s disease and Parkinson’s disease, GSH levels in the brain would be decreased before the onset, and GSH dysregulation is considered to be involved in the development of these neurodegenerative diseases. Cysteine uptake into neurons is the rate-limiting step for GSH synthesis. Excitatory amino acid carrier 1 (EAAC1), which is a glutamate/cysteine cotransporter, is responsible for the neuronal cysteine uptake, and EAAC1 dysfunction reduces GSH levels in the brain and has a significant influence on the process of neurodegeneration. Since miR-96-5p, which is one of microRNAs, suppresses EAAC1 expression, it is conceivable that miR-96-5p inhibitor suppresses the onset or slows the progression of neurodegenerative diseases by increasing EAAC1 levels leading to promoting neuronal GSH production.

In recent years, the success rate of drug development has declined, and along with it, R&D costs have continued to rise. The rate of discontinuation of drug development due to safety reasons remains unchanged from 20 years ago. Therefore, it is important to check the safety of candidate compounds early in drug discovery in order to improve drug discovery efficiency. Under such circumstances, each company is focusing on establishing a low-cost, high-precision, and high-throughput safety screening system. The zebrafish is expected as a new experimental animal that serves as a bridge between in vitro and in vivo, and the progress of research in the last 15 years has been remarkable. At present, zebrafish are becoming a major experimental animal in Japan. At the same time, the gap between ideal and reality began to be seen, and it was time to once again understand the characteristics of zebrafish and think about its usage. This paper summarizes the points to be noted in the screening using zebrafish and introduces the use for actual safety evaluation.

Gilteritinib fumarate (Xospata^® tablets 40 mg) is a novel, highly selective, oral FMS-like tyrosine kinase 3 (FLT3) inhibitor used for the treatment of patients with relapsed or refractory FLT3-mutated acute myeloid leukemia (AML), and it was approved in Japan in September 2018. Preclinical studies demonstrated that gilteritinib inhibited FLT3 and showed antiproliferative activity against Ba/F3 cells expressing mutated FLT3. In addition, gilteritinib inhibited tumor growth, induced tumor regression, and prolonged survival in mice xenografted with MV4-11 cells endogenously expressing FLT3-internal tandem duplication. In clinical trials conducted in the United States, Europe, and Japan, plasma concentrations after administration of gilteritinib 20 to 450 mg/day were generally dose proportional, and gilteritinib was well tolerated. Multiple clinical trials, including a global Phase III study, in patients with relapsed or refractory FLT3-mutated AML treated with gilteritinib demonstrated higher response rates of complete remission or complete remission with partial hematologic recovery and longer overall survival compared with patients treated with salvage chemotherapy. Some clinical trials are ongoing in patients with FLT3-mutated AML at various treatment stages, such as induction therapy, maintenance therapy, and treatment after hematopoietic stem cell transplantation. In conclusion, in vitro, in vivo, and clinical data indicate that gilteritinib fumarate is an effective treatment option in adult patients with relapsed or refractory FLT3-mutated AML in Japan.

Antibody-drug conjugates (ADCs) combine the specific antibody and cytotoxic agent by a linker and represent a promising drug class with a wider therapeutic window than conventional chemotherapeutic agents by substantiating efficient and specific drug delivery to antigen-expressing tumor cells. However, there are rooms for improvement in terms of efficacy, safety, physicochemical property; therefore, the development of promising ADC drugs across multiple indications are eagerly awaited. In 2015, Daiichi Sankyo initiated the first-in-human study of HER2 ADC, trastuzumab deruxtecan (T-DXd, ENHERTU^®) which possesses DNA topoisomerase I inhibitor, exatecan derivative and proprietary linker, in Japan. Based on the provocative results in phase 1 study, the global development program has been accelerated to show the high and durable efficacy in patients with HER2 positive breast cancer pretreated with trastuzumab emtansine. As a result, T-DXd was approved based on single arm phase 2 study in the US (Dec 2019) and Japan (March 2020) by leveraging the breakthrough designation and conditional early approval system, respectively, at the first time for the HER2 positive breast cancer. In addition, T-DXd was recently approved in gastric cancer through Sakigake designation in Japan based on a randomized phase 2 study. T-DXd is also being developed in the earlier lines or other indications where no anti-HER2 therapies were approved to date. Combination studies with other agents, such as immune checkpoint inhibitors are underway. In the near future, we hope that more patients worldwide can enjoy the therapeutic benefits of T-DXd through our continuous efforts to expand its indications.