Functional block copolymer assemblies responsive to tumor and intracellular microenvironments for site-specific drug delivery and enhanced imaging performance
Self-assembled nanostructures of amphiphilic and double hydrophilic block copolymers have been increasingly utilized as potent polymeric nanocarriers of therapeutic drugs, genes, bioactive molecules, and imaging/contrast agents due to improved water solubility, bioavailability, and extended blood circulation duration. Though passive and active targeted drug delivery strategies have long been proposed to promote desirable drug accumulation specifically at the disease sites, the introduction of stimuli-responsiveness into self-assembled block copolymer nanocarriers can additionally lead to controlled/triggered release of therapeutic/imaging agents into target pathological tissues and cells, with concomitant advantages of enhanced delivery efficiency and therapeutic efficacy. Appropriately designed stimuli-responsive block copolymer assemblies can exhibit chemical structure transformation, microstructural rearrangement and inversion, or even disassembly into unimers or smaller ones under external stimuli such as pH, temperature, ion strength, redox potential, light, electric, and magnetic fields, and specific bioactive molecules and metabolites. Compared to normal tissues, pathological sites such as tumor tissues typically exhibit vascular abnormalities, weak acidity (∼pH 6.8), abnormal temperatures, over-expressed proteins and enzymes, hypoxia, high levels of metabolites and reactive small molecule species, etc. Moreover, upon cellular uptake, drug-loaded polymeric nanocarriers will be subjected to intracellular pH gradients (pH 5.9–6.2 in early endosomes and pH 5.0–5.5 in late endosomes and lysosomes) and redox and H2O2 gradients within different cell organelles and the cytosol. Thus, block copolymer nanocarriers responsive to the above described bio-relevant stimuli or biochemical signals characteristic of pathologic tissues and cells will provide an alternative type of “active targeting” strategy, which can be utilized to further boost therapeutic efficacy and imaging sensitivity via disease site-specific delivery and controlled release. A variety of extracellular or intracellular stimuli innate to disease sites, such as mildly acidic pH, temperature, enzymes (matrix metalloproteinase, β-glucuronidase, and phosphatase), oxidative/reductive microenvironments, and abnormal levels of bioactive molecules or metabolites, have been utilized for this purpose. In this review, we summarize recent advances in stimuli-responsive block copolymer assemblies which are responsive to tumor and intracellular microenvironments and their applications in anticancer drug delivery and enhanced imaging sensitivity.