Advances in Payload and Linker Designs for ADCs

Antibody-drug conjugates (ADCs) are a therapeutic approach that embodies the "magic bullet" concept envisioned by Paul Ehrlich. Since their initial clinical potential was recognized in the early 1980s, ADCs have undergone several generations of development to enhance their therapeutic index (Poon KA, et. al. 273(2):298-313, 2013). Recent advancements, particularly in the last third and the beginning of the fourth generation, have primarily focused on discovering new drug modalities and more potent cancer treatments, while also developing new linkers and improving site-specific and conjugation chemistries. The effectiveness and toxicity of ADCs are largely influenced by their overall physicochemical properties, especially hydrophobicity, plasma stability, resistance to payload release, and the efficient release of the payload at the target site (Saber H, et. al. 71(3):444-52, 2015), (Saber H, et. al. 107:104429, 2019). These properties are heavily dictated by the site of conjugation, conjugation chemistry, linkers, and payloads. Most payloads are hydrophobic and cannot undergo significant structural or functional changes without compromising their cytotoxicity. In contrast, linkers can be extensively modified to enhance the physicochemical characteristics of ADCs, improve stability, increase drug-to-antibody ratio (DAR), and reduce aggregation. This review will focus on the latest advancements in potent payloads with novel mechanisms of action, including immune-stimulating agents, protein degraders, radionuclides, light-activated photosensitizers for photoimmunotherapy, and dual-payload ADCs. Additionally, we will explore emerging linkers that offer varying stimuli for trigger payload release and improved release control.