Cathepsin Family Proteases in Prodrug Design: Recent Advances.

Prodrug strategies are widely used to improve the efficacy, selectivity, and safety of therapeutic agents. Among stimulus-responsive approaches, enzyme-triggered activation has emerged as a particularly attractive method because it can couple drug release to disease-associated biochemical activity. Cathepsins, a family of lysosomal proteases including 15 types, implicated in cancer, inflammation, and other pathological processes, have become important enzymatic triggers in this area. Most cathepsin-responsive systems focused on cathepsin B, whose elevated activity in tumors and favorable intracellular localization have supported the development of peptide-cleavable prodrugs, from small-molecule prodrugs to various drug conjugates. More recent studies have extended this concept beyond cathepsin B to other cathepsins such as L, E, and S, highlighting a shift toward substrate-selective design and improved biological precision. Meanwhile, comparative studies and unnatural amino acid engineering have emphasized that successful cathepsin targeting depends not only on enzyme overexpression but also on substrate specificity, circulation stability, intracellular trafficking, and productive payload release. This review summarizes the recent advances in cathepsin-activated prodrugs with a focus on the novel therapeutic applications and major challenges limiting translation, providing insight for the future development of cathepsin-activated prodrugs.