In this chapter, we discuss antibody engineering-based ADC optimization strategies and their future directions. Development of antibody drug conjugates（ADC） started from the optimization of the payload and linker. Today, antibody engineering has emerged as another option in optimization strategy. Bispecific antibodies have shown success in promoting cell internalization of ADC which leads to efficient payload release. Biparatopic antibodies binding two different epitopes of an antigen also promote cell internalization by the formation of bridged structures on the cell membrane, and several biparatopic ADCs are under clinical trials. While studying effective design of biparatopic antibodies, we designed a platform which leads to the development of novel two-component types of ADC. In addition to these bispecific approaches, control of antigen binding ability is also of interest. These engineering approaches will expand the potential of ADCs for variety of diseases.

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Bispecific antibodies, which possess two distinct specificities, are primarily produced through genetic engineering. They are often used in cancer therapy, and strategies for cross-linking cancer cells and T cells have been studied extensively for a long time. Bispecific antibodies are categorized as either natural IgG-type or non-natural, and various technologies have contributed to the uniform production of both types. Small bispecific antibodies are expected to be produced using a cost-effective bacterial expression system and to effectively permeate solid tumors. Here, we first outline the development history and commercialized examples of natural IgG-type and non-natural bispecific antibodies as pharmaceuticals. Then, we will introduce the development of small bispecific antibodies and the multifaceted progress we have made in enhancing their functionality.

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Monoclonal antibody therapeutics possess high antigen specificity and structural stability, and they have been widely used in the treatment of cancer and autoimmune diseases. In recent years, antibody–drug conjugates（ADCs） have attracted attention as a novel modality with high therapeutic efficacy; however, their complex structures raise concerns regarding decreased stability. For the physicochemical evaluation of ADCs, Raman spectroscopy and dynamic light scattering（DLS） are effective techniques. Raman spectroscopy enables non-destructive analysis of structural changes and stability in highly concentrated formulations. DLS allows rapid evaluation of particle size changes during ADC production processes, making it possible to monitor reaction progress and aggregation states. These analytical methods contribute to quality control of ADCs and the optimization of manufacturing processes.

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The development of next-generation antibody–drug conjugates（ADCs） requires the exploration of payloads with novel mechanisms of action and the rational design of structurally diverse molecules. Through structure-activity relationship studies of cysteine derivatives with KSP inhibitory activity, the authors have discovered their potential as payloads for cancer-selective prodrug and ADC. Furthermore, enzyme-cleavable linkers were introduced to generate drug–linker constructs, and cathepsin B-mediated cleavage and structure-dependent reactivity were verified. This review outlines the design strategy and therapeutic potential of novel drug–linker systems incorporating KSP inhibitors as payloads.

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Nuclear medicine has evolved from early diagnostic use of radioisotopes to an integrated field combining molecular imaging and therapy. The concept of theranostics, using the same molecular carrier for both diagnosis and treatment, has placed antibodies at the center due to their high specificity and established clinical utility. Within this framework, Targeted Alpha Therapy（TAT） has emerged as a promising approach. Alpha particles deliver extremely high linear energy transfer（LET） over short ranges, inducing irreparable double-strand DNA breaks with only a few hits. This unique property makes TAT effective against radioresistant and hypoxic tumors but also imposes stringent requirements on carrier localization and pharmacokinetics. From a drug delivery system（DDS） perspective, optimal therapeutic outcomes rely on harmonizing isotope half-life with antibody trafficking, ensuring precise spatiotemporal delivery. Different alpha emitters pose distinct design challenges: ²²⁵Ac offers high dose density but risks daughter nuclide redistribution, whereas ²¹¹At, with a half-life of 7.2 hours and stable chemistry, aligns well with antibody kinetics and clinical logistics. Japan has pioneered ²¹¹At research, establishing GMP-compliant production and conducting the world’s first physician-initiated trial（Alpha-T1） in thyroid cancer, demonstrating safety and partial remission. In parallel, imaging-guided theranostics using ⁸⁹Zr for PET and ²¹¹At for therapy exemplify precision medicine, enabling patient-tailored dosing. Recent innovations include single-domain antibodies（VHH） and gold nanoparticles for improved tumor penetration and isotope stabilization, as well as dual immunological effects where α-irradiation enhances antitumor immunity and synergizes with checkpoint inhibitors. Despite progress, challenges remain: securing stable isotope supply, regulatory harmonization, logistics for short-lived isotopes, and balancing antibody size with tumor penetration and systemic toxicity. DDS approaches—including antibody fragment engineering, linker and chelator chemistry, and quantitative PET imaging—are key to overcoming these hurdles. The expansion of antibody-based therapeutics such as bispecific antibodies and ADCs further strengthens the foundation for TAT. Ultimately, integrating supply chains, regulatory frameworks, molecular design, and immunological strategies will transform antibody-based TAT from laboratory innovation into clinical routine, offering a new paradigm in oncology.

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Antibody-drug conjugates （ADCs） combine cytotoxic payloads with monoclonal antibodies via chemical linkers. While the number of ADCs approved by the FDA continues to increase, the conventional Val-Cit linker still presents several challenges, including increased aggregation due to hydrophobicity, limitations in drug-to-antibody ratio （DAR）, and instability of the linker moiety during blood circulation. In this study, we incorporated hydrophilic amino acid residues into the cathepsin-cleavable Val-Cit linker and introduced the cleavable linker at the “Exo” position—i.e., the branch position of the linear connection between the antibody and payload—to enhance the masking effect by shortening the distance between the antibody and payload. This design addresses the limitations of the Val-Cit platform and improves the ADC in vivo profiles. In vitro and in vivo evaluations showed that Exo-linker ADCs reduced premature payload release, increased DAR, and avoided significant aggregation, even with hydrophobic payloads. Furthermore, the payload remained stably attached to the ADC even in the presence of enzymes like carboxylesterases and human neutrophil elastase, indicating the potential for a favorable safety profile.

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Lipid nanoparticles（LNPs） have long been employed as a delivery platform for functional nucleic acids. However, recent advances in chemical modification technologies have drawn increasing attention to conjugate-based delivery systems, in which ligands are directly conjugated to nucleic acids. With the exception of N-acetylgalactosamine（GalNAc）, however, few ligands have demonstrated broad utility, and the number of successful reports remains limited. In particular, there is a strong unmet need for ligands capable of mediating delivery to organs other than the liver. This review focuses on “Antibody–Nucleic acid Conjugate,” which utilize antibodies as ligands that possess high specificity and stability and potentially enable targeted delivery to a variety of tissues. We summarize the progress of research efforts aimed at achieving nucleic acid delivery to antibody-accumulating tissues.

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Difelikefalin acetate （product name：KORSUVA^Ⓡ IV Injection Syringe for Dialysis 17.5μg, 25.0μg, and 35.0μg） is a novel intravenously administered κ-opioid receptor agonist developed by Cara Therapeutics, Inc. in the United States. In Japan, Maruishi Pharmaceutical Co., Ltd. initiated its domestic development in April 2013, and subsequently, clinical trials were conducted jointly with Kissei Pharmaceutical Co., Ltd. starting in March 2017. Based on the confirmed efficacy and safety demonstrated in a domestic Phase III clinical trial targeting hemodialysis patients with pruritus refractory to existing treatments, a marketing authorization application was submitted. In September 2023, approval was granted for the indication “improvement of pruritus in hemodialysis patients （limited to cases where existing treatments are insufficiently effective）,” followed by inclusion in the National Health Insurance drug price list in November and commercial launch in December of the same year. Since the drug is a prefilled syringe formulation that is administered via the venous line of the dialysis circuit at the time of blood return upon completion of dialysis, its administration is not affected by fluid intake restrictions or impaired swallowing function, and can be performed under the supervision of a physician.

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