Recent Trends in Radiopharmaceuticals: Focused on Drug Delivery to the Targeted Tissues

Over the past decade, the development of radiopharmaceuticals for diagnostic and therapeutic purposes has been accelerating due to the advancement of scientific technologies, rising medical needs for precision medicines, and the success of radiopharmaceuticals in the market. The therapeutic effect of radiopharmaceuticals results primarily from the radiation emissions of radionuclides. Radiopharmaceuticals emitting α- or β--particles, known for their high cytotoxicity, serve as therapeutic agents. Conversely, those leveraging the high tissue penetration properties of γ rays (including from positron annihilation) are employed as diagnostic agents. Therapeutic effects and diagnostic capabilities can be achieved by selecting appropriate radionuclides, giving rise to the concept of theranostics. Target tissue selectively remains a significant challenge for diagnostic and therapeutic radiopharmaceuticals. When the high tissue selectivity of radiopharmaceuticals can be achieved, it leads to good contrast, enabling clear diagnosis and high therapeutic efficacy. Additionally, radiation exposure to normal tissues can be minimized, making tissue selectivity important from both efficacy and safety perspectives. Some radionuclides have tissue preference and are used as radiopharmaceuticals because of this property. On the other hand, this property makes it difficult to target other tissues with radionuclides themselves. To overcome this limitation, radionuclides are combined with targeting ligands to specific tissues. This Perspective highlights recent trends in radiopharmaceutical development and provides a comprehensive classification of radionuclides and ligands with favorable biodistribution properties.