Stimuli-Responsive Nanocarriers for Targeted Drug Delivery: Formulation Design, Therapeutic Applications and Translational Challenges

Abstract

Stimuli-responsive nanocarriers are designed to hold a drug securely during transport and to release it, or to change shape, surface charge or permeability, when they meet a defined biological cue or an externally applied form of energy. This review examines such systems from a pharmaceutics perspective. Endogenous triggers (pH, glutathione, reactive oxygen species, enzymes, hypoxia and glucose) and exogenous triggers (light, ultrasound, heat, magnetic fields and ionising radiation) are discussed together with the lipidic, polymeric, inorganic, biomimetic and hybrid carriers built around them. Attention is given to trigger chemistry, payload compatibility, colloidal stability, biodistribution and release kinetics, and to applications in cancer, inflammatory bowel disease, infection, brain disease, diabetes and nucleic-acid delivery. Clinical-stage products, including thermosensitive liposomal doxorubicin, hafnium oxide nanoparticles, magnetic thermotherapy and implantable ultrasound devices, show that externally triggered approaches have reached patients, whereas purely biochemical triggers have yet to yield an approved product. The evidence indicates that responsiveness adds value only when the trigger shows a reproducible difference between diseased and normal tissue, the carrier stays intact until activated, and the whole drug–carrier–device system can be manufactured and used in routine care. A quality-by-design framework for defining critical material attributes, process parameters and release specifications is outlined, followed by recommendations for evaluation, reporting and trial design. Future progress is most likely to come from simpler platforms matched to a defined clinical need and supported by mechanistic pharmacokinetic and trigger-performance data.