**Background:** Traditional drug administration often suffers from low therapeutic efficacy due to poor stability, bioavailability, or toxicity. Drug delivery systems, particularly polymeric nanoparticles, can overcome these drawbacks. Among biodegradable polymers, polypeptides (poly(amino acids)) offer distinct advantages: biodegradability, biocompatibility, diverse side-chain functionality, and the ability to form stable secondary structures (α-helices, β-sheets). This review provides a comprehensive overview of polypeptide-based systems, covering synthesis, nanoparticle types, preparation methods, properties, and applications in drug delivery.
**Methods:** The review synthesizes literature on the ring-opening polymerization (ROP) of N-carboxyanhydrides (NCAs) of α-amino acids, the primary method for synthesizing polypeptides. It discusses various initiators (primary amines, silazanes, transition metal complexes) and mechanisms (normal amine mechanism, activated monomer mechanism) to control molecular weight and dispersity. Methods for preparing copolypeptides (random, block, graft, star-shaped, dendrimeric) are described, including sequential polymerization, macroinitiator approaches, and click chemistry. Nanoparticle formation methods include nanoprecipitation, dialysis, direct dissolution, film rehydration, emulsification, electrospraying, complexation, polymerization-induced self-assembly (PISA), and miniemulsion polymerization. The review also covers hydrogel preparation via physical gelation (secondary structure formation, hydrophobic interactions) and chemical crosslinking (disulfide bonds, Schiff bases, click chemistry).
**Key Results:** Polypeptide-based nanoparticles can form various morphologies: micelles (hydrodynamic diameters 20–670 nm, CMC ~10⁻⁶ M), polymersomes (110–1210 nm, CAC ~10⁻⁶ M), vesicles, nanogels (72–256 nm), nanospheres (120–350 nm), and polyplexes (60–350 nm). Key properties include: (1) Colloidal stability: many systems show high storage stability (e.g., PTMC-b-PGlu polymersomes show negligible DOX release at 4°C for ≥6 months). Crosslinking (disulfide, ketal, hydrazone) enhances stability and enables stimuli-responsive release. (2) Degradability: Polypeptides are cleaved by proteolytic enzymes (e.g., cathepsin B, papain). Degradation rate depends on composition, chirality, and secondary structure. For example, PGlu degrades 13- and 17-fold faster than PAsp and PDGlu in liver lysosomal extracts. (3) Cellular uptake and cytotoxicity: Positively charged NPs (e.g., P(Lys-co-Phe), ζ=36 mV) show 3- to 4-fold higher uptake in A549 cells than anionic or neutral NPs, but also higher cytotoxicity. Negatively charged and neutral NPs are often non-toxic up to 1000 µg/mL. PEGylation reduces cytotoxicity and macrophage uptake. (4) Hemolysis: Negatively charged PGlu-b-PLeu polymersomes cause only 0.4–0.6% hemolysis at 2–125 µg/mL. (5) Immunogenicity: Cationic PLys NPs can be immunogenic, while anionic PGlu systems show low immunogenicity. The review details applications in cancer treatment (single and dual drug delivery, gene delivery), antimicrobial therapy, anti-inflammatory/antioxidant therapy, and protein/peptide delivery. For example, PEG-b-PGlu/Ca²⁺ DOX-loaded NPs showed ~80% tumor suppression in osteosarcoma mice. Co-delivery systems (e.g., docetaxel + siRNA) demonstrated synergistic antitumor effects.
**Clinical Implications:** Polypeptide-based systems are versatile platforms for delivering a wide range of therapeutics (small molecules, nucleic acids, proteins). Their biodegradability, biocompatibility, and tunable properties (pH-, redox-, enzyme-responsiveness) make them promising for targeted and controlled release. However, limitations include high cost of monomers, complex synthesis, and potential immunogenicity. Systematic studies of structure-property relationships and in vivo toxicity/immunogenicity are needed to translate these systems into clinical practice. The development of injectable hydrogels and combination therapies (e.g., chemo-immunotherapy) represents a significant step forward.