Cell-penetrating peptides for therapeutic applications: emerging design strategies and future directions

Cell-penetrating peptides (CPPs) are short amino acid sequences capable of traversing biological membranes and enabling intracellular delivery of diverse therapeutic cargos, thereby addressing core barriers in drug development, including poor cellular uptake, rapid systemic clearance, nonspecific distribution, and enzymatic instability. This review dissects the physicochemical design principles governing CPP-membrane interactions-charge density and distribution, amphipathicity, secondary structure, and environmental responsiveness-and explains how these features influence internalization pathways and cytosolic access. Therapeutic applications are organized by cargo class, including nucleic acids, proteins and antibodies, and small molecules and imaging agents. For each category, we analyze how cargo size, charge, and structural complexity constrain delivery efficiency and how strategies such as conjugation, nanocarrier functionalization, and chemical modification have been applied to circumvent these limitations. Effective CPP-mediated delivery is therefore determined not by peptide sequence alone but by deliberate alignment of peptide properties with cargo characteristics, biological context, and therapeutic objectives. By integrating mechanistic understanding with practical constraints in therapeutic development, we aim to provide practical guidance for the deliberate design of CPP systems with improved delivery efficiency and translational relevance.