Application of Protease-Responsive Nanocarriers In Near-Infrared Imaging

Cancer is one of the diseases that endanger human health. Successful diagnosis of early cancer is the key to treating and improving patient survival. Therefore, more and more research is focused on the development of dyes and drugs for cancer diagnosis and treatment. However, delivering these therapeutic (drug / dye) molecules to the site of a lesion in the body at an effective concentration is a challenge. With the research on nanotechnology, nanobiotechnology has played an important role in targeted drug application. This technology effectively improves the efficiency of therapeutic diagnostics by increasing the half-life of the molecule, maintaining effective concentrations at the target site, and reducing side effects. Recently, various nanoparticles such as metal nanoparticles, polymer nanoparticles, and liposomes have been used as drug carriers. However, most of these nanoparticles (NP) are limited due to poor biocompatibility, non-biodegradable properties, and long- and short-term tissue toxicity. Therefore, people have shown great interest in using biological macromolecules such as lipids, amino acids and proteins to make NPs for drugs and contrast agents.

The use of essential amino acid-based polymers as biocompatible and biodegradable nanocarriers is a new direction for cancer targeted imaging. Poly-L-lysine (PLL) is an important amino acid polymer that plays an important role in the body. The body uses it to promote bone development, protein synthesis, and collagen production. It also helps in the production of various hormones, antibodies and enzymes, as well as tissue repair. Existing studies have found that PLL is a cationic homopeptide, which is usually used for cell adhesion in cell culture. Recent research has found that PLLs can excel in the areas of gene therapy and surface coating of metal particles. Similarly, metal-based PLL composites have attracted interest as potential carriers for drug delivery. To overcome the limitations of therapeutic molecules, researchers have used the molecules encapsulated in PEG-PLL-PLLeu triblock copolymer micelles or platinum (II) -porphyrins of composite PLL. However, the composite materials of these PLLs are not completely biodegradable, and their manufacturing process requires the addition of organic solvents, which may have toxic effects. Therefore, researchers recently discovered a method based on two-step green chemistry for the preparation of biodegradable PLL nanocarriers in aqueous media.

In this study, the dye indocyanine green (ICG is a near-infrared (NIR) fluorescent dye approved by the US Food and Drug Administration (FDA) for various biomedical applications) was encapsulated in PLL NPs. Although ICG has many applications in previous research applications, the shortcomings of ICG as near-infrared active exogenous are limited by the shortcomings of free-form ICG in aqueous solution, such as off-site transfer, concentration-dependent aggregation, and poor light stability. To address these shortcomings, encapsulating free ICG in a nanocarrier is an effective method. In a recent study, researchers used ICG to load into a phase-locked loop using a two-step self-assembly method. Most importantly, the assembly of PLL NPs is done in an aqueous medium by mixing essential amino acids with peptides, ICG, and polyvalent anion salts, so all components used for manufacturing are non-toxic. In order to further ensure that imaging dyes are efficiently transferred within the cell, the phase-locked loops of these nanoparticles are designed to be protease-responsive. The design principle of protease-responsive NPs is that proteolytic enzymes exist in lysosomal compartments, and only by enzymatic hydrolysis in these compartments will these NPs be triggered to release the encapsulated cargo. The advantage of phase-locked loops is that the presence of proteolytic enzymes can regulate the release of cargo. Therefore, proteolytic enzymes can only degrade PLL NPs if they enter the cell through endocytosis and are transported to the lysosome. The degradation byproducts of these NPs are lysine, salt fragments and free-form ICG. These byproducts will be utilized by the cells without showing any toxic effects, and ICG will be used to stain the cells for near-infrared biological imaging. Compared with other traditional NPs, PLL NPs are completely biodegradable and biocompatible. The results of in vitro cell uptake studies show that compared with free-form ICG, cells incubated with PLL NPs have higher near-infrared fluorescence emission. In addition to ICG, these NPs can also be used to encapsulate various anticancer drug molecules, such as hydrophobic anticancer molecules, chemotherapeutic drugs, etc., for targeted drug delivery.