Design of cathepsin-sensitive linkers for tumor-selective bioconjugate drug delivery

Cysteine cathepsin, particularly cathepsin B, have emerged as pivotal enzymatic targets in the design of drug delivery systems owing to their overexpression in diverse pathological conditions, most notably cancer. This review provides a comprehensive overview of cathepsin B-cleavable linkers, emphasizing their role in current bioconjugate design and their application across multiple therapeutic platforms. It also provides a comparative overview of linker engineering guided by cathepsin B, ranging from simple dipeptides constructs to modified peptide linkers. These structural refinements are correlated with improvements in substrate discrimination, stability, and cleavage efficiency. Substantial attention is provided to three primary bioconjugate platforms: antibody-drug conjugates (ADCs), prodrug systems and nanoparticle conjugates. Each section enumerates the corresponding unique design, conjugation chemistry, payload distribution modalities, and progress in regulatory translation. The parallel evaluation supports that, while collectively successful, ADCs have yielded the most mature clinical outcomes, notwithstanding ongoing refinements that promise to widen the therapeutic index of prodrug and nanoparticle platforms. Key challenges include achieving a balance between linker stability in circulation and efficient cleavage at the target site, minimizing off-target activation, and accounting for variability in cathepsin expression among patients. Future direction focusses on both advancing linker technology through enhanced stability, refined pharmacokinetics, and multi-mechanism combination strategies and implementing patient stratification for clinical relevance.