Research Progress of Glucose Sensitive Insulin Smart Drug Delivery System

Diabetes is one of the major social health problems in the world. Subcutaneous injection of insulin is the traditional administration in the diabetes treatment, which is inconvenient and painful. Moreover, subcutaneous injection of insulin might result in hypoglycemia. In addition to oral insulin, controlled drug delivery systems are able to improve efficiency and safety of therapy by optimizing the duration and kinetics of release, and it also becomes the current hotspot. The smart insulin delivery system with glucose-sensitive characteristics attract considerable attention. The smart insulin delivery system stimulates the endogenous pancreatic β cells and releases an appropriate amount of insulin at the right time according to the patient's blood glucose level. According to the glucose sensitive unit, the smart insulin delivery system can be divided into three categories, glucose oxidase (GOx), concanavalin A (ConA) and phenylboronic acid (PBA). When the blood glucose concentration changes, the carrier structure changes or the competition of binding site between glucose and insulin triggers drug release. 

1. Drug delivery system based on glucose oxidase 

Glucose reacts with oxygen to form gluconolactone and H2O2 under the catalysis of glucose oxidase (GOx), and then glucolactone is rapidly hydrolyzed to gluconic acid in an aqueous environment. Therefore, insulin delivery system based on GOx can be divided into pH trigger and H2O2 trigger types. 

The pH-triggered insulin release system utilizes the gluconic acid produced when GOx oxidizes glucose to reduce the local pH, which leads to the change of carrier structure (dissociation, swelling or collapse), and ultimately leads to drug release. Peptide hydrogel is an excellent carrier for insulin delivery, but its application is limited by the inconvenient use of cross-linking agent and implantation. The self-assembling peptide hydrogel avoids the use of cross-linking agents. It has good biocompatibility properties, and the advantage of injectable administration. An injectable pH-sensitive peptide hydrogel containing GOx, catalase (CAT) and insulin has been developed by some researchers. The oxidation of glucose by GOx causes a decrease in local pH, which leads to the rejection of the adjacent alkaline amino acid side chains in the peptide hydrogel, the opening of the hairpin structure and the decomposition of the peptide hydrogel to release insulin. It is the most common and simple way to physically coat GOx in nanoparticles, but the loss of GOx during the drug release process will cause the glucose sensitivity of the system to decrease. In addition, microspheres could provide more durable drug delivery than nanoparticles. Microneedle is a minimally invasive drug delivery system that can be administered by patients themselves. It can be temporarily inserted into the stratum corneum to penetrate the epidermis and prevent damage to deeper tissues. 

The insulin release system is triggered by H2O2 to use H2O2 generated when GOx oxidizes glucose to trigger insulin release. For example, a multilayer film of shikimic acid modified poly allylamine hydrochloride (SA-PAH) and phenylboronic acid modified poly allylamine hydrochloride (PBA-PAH) is constructed by alternately depositing on calcium carbonate microspheres. The H2O2 produced by the oxidation of glucose by GOx causes the carbon boron bond of PBA-PAH to break and decompose. The drug-loaded particles remain stable at a glucose concentration of 0.9mg·ml﹣1, and insulin is released only at a glucose concentration of more than 1.8mg·ml﹣1. A core-shell microneedle patch is composed of cross-linked polyvinyl alcohol (PVA) gel. The core of the microneedle is 4-nitrophenyl 4-(4,4,5,5-tetramethyl-1,3,2-dioxaborocine-2-yl) benzyl carbonate immobilizes insulin on the PVA matrix, and then produces H2O2 to trigger the release. CAT is added to the shell to reduce the risk of inflammation caused by H2O2. When glucose reacts with GOx to produce H2O2, the core gel is triggered to dissociate and release insulin. The patch can effectively reduce the blood glucose level in mice with diabetes. The blood glucose level is reduced to 1mg·ml-1 within 2 hours, and maintained at 2mg·ml-1 for 6 hours. In addition, colloidal particle integration microneedle is another strategy to realize the intellectualization of microneedles. 

2. Drug delivery system based on concanavalin A 

Concanavalin A (Con A) has four binding sites with reversible affinity for D-glucose, D-mannose and polysaccharides. In the 1970s, glycosylated insulin derivatives were synthesized for the first time and combined with Con A to prepare a glucose-responsive insulin delivery system. Insulin derivatives are released by competitive replacement at high glucose concentrations. For example, the intraperitoneal glucose sensitive peptide hydrogel composed of carboxylation amylopectin and Con A can achieve controlled release of insulin from hydrogels through the combination with Con A-glucose. The nanoparticles for carrying insulin consist of Con A and amylopectin. The entrapment efficiency of insulin is about 69.73%. The drug loading capacity of insulin is as high as 17%. The release rate of nanoparticles in 3mg·ml-1 glucose solution is 2.23 times higher than that in a solution free of glucose. Some researchers have developed a resistant starch-glycoprotein complex bioadhesive carrier and a colon-targeted bioadhesive oral microparticle system. Con A combines with resistant starch acetate to form the resistant starch-glycoprotein compounds. The microparticles can control the blood glucose level of mice with diabetes within the normal range for 44-52 hours. 

Although certain progress has been made in Con A insulin delivery system, there is still some limitations, including its toxicity, water solubility, stability and long response time. The solubility, stability and glucose sensitivity of Con A can be improved by the modification of hydrophilic polymers, but the immunogenicity of Con A is a key factor hindering its further application, and it remains to be resolved. 

3. Drug delivery system based on phenylboronic acid 

Compared with the system that uses GOx or Con A to achieve glucose triggering, the insulin delivery system based on phenylboronic acid (PBA) has the advantages of lower cost, no immunogenicity, strong versatility and good stability in vivo. PBA reacts with saccharides containing cis-diol structure to form a reversible ester bond. In this process, the change in the charge state of PBA and the conversion of hydrophilicity and hydrophobicity or competition with glycol-containing compounds are used to achieve the release of insulin. However, the pKa of PBA (approximately 9.0) is higher than the physiological pH, causing PBA to accumulate in the body, and the interaction between PBA and glucose is weakened, thereby limiting its glucose sensitivity. At present, two strategies are mainly used to solve this problem. One is to develop a polymer that combines PBA and other monomers to better interact with glucose under physiological conditions. The second is to use polymers containing carboxyl polymer that can coordinate with boron atoms to modify PBA. According to the mechanism of triggering drug release, the PBA insulin delivery system can be divided into three types: swelling, degradation and competition. 

The PBA insulin delivery system based on the swelling mechanism usually loads insulin onto the peptide hydrogel network or wraps it inside the peptide hydrogel. When the glucose concentration increases in the environment, the gel expands and the size of the gel screen increases, thereby releasing the drug. PBA insulin delivery system based on dissociation mechanism releases drugs by the collapse of carrier structure caused by the reaction between glucose and phenylboronic acid. Compared with the swelling-based system, the carrier-dissociated drug delivery system is more sensitive to glucose and more easily excreted from the body. The principle of the competition-based PBA smart insulin delivery system is that when the concentration of glucose in the environment increases, glucose competes with the drug for the same binding site or produces a substance that can occupy the drug binding site to release the drug. 

In addition, GOx and PBA are combined in some studies in order to enhance the glucose sensitivity of the PBA system. The H2O2 produced when GOx oxidizes glucose will accelerate the degradation of PBA in the body. In addition, the introduction of GOx will also enhance the glucose sensitivity of the system. For example, some recearchers have developed poly(acrylamide phenylboronic acid)/sodium alginate nanoparticles. Cyclic borate can react with glucose, causing it to be decomposed and form a hydrophilic phenylboronic acid ester-glucose compound. The H2O2 produced by the oxidation of glucose further oxidizes the cyclic borate to phenol and breaks the carbon-boron bond. In addition, in the process of GOx catalyzing the conversion of glucose to gluconic acid, the local pH decreases and the phenylboronic acid ester bond is further decomposed. These three damaging effects cause the nanoparticles to trigger the drug release behavior. After subcutaneously injecting the nanoparticles into the mice with diabetes, the blood glucose level dropped to a normal level within 0.5 hour and could be maintained for at least 14 hours. 

A safe and effective insulin delivery system is expected to enhance the therapeutic effect of drug treatment and improve the quality of life of patients. The glucose-sensitive drug delivery system can provide insulin that matches with the patient's metabolic needs, effectively treat hyperglycemia and prevent the occurrence of hypoglycemia symptoms. Certain progress has been made in the research of glucose-sensitive insulin delivery system based on GOx, PBA or Con A, providing the hope of smart blood glucose control without increasing the risk of hypoglycemia and reducing the burden on patients. However, there are still many challenges for the glucose-sensitive insulin delivery systems. First, there is potential biocompatibility. While GOx, Con A and PBA bring glucose responsiveness to the drug delivery system, their biocompatibility has also become a huge challenge, especially for preparations administered by injection. Secondly, there is the issue of controlled drug release. At present, all glucose-sensitive drug delivery systems can achieve effective drug release under simulated hyperglycemia conditions, but most systems still have 5%-80% insulin release under normal blood glucose concentrations. Attention still shall be paid to how to further reduce the amount of drug released during hypoglycemic blood glucose concentration, especially for the insulin delivery system based on the carrier dissociation, which is irreversible. The last one is the speed of glucose response . The development of smart insulin delivery system is limited by the response speed of glucose. The research of smart drug delivery system combining multiple stimulus responses still needs to be strengthened. 

References 

[1] Liu Hui, Li Qinwen, Xu Shuang, Ma Sijing, He Jun, Cai Xin, Yi Fang. Research Progress in Glucose-sensitive Insulin Delivery System [J]. China Pharmacist, 2021, 24(07): 353-360. 

[2] Xu Jiafu. Research on Smart Drug Delivery System of Insulin [J]. Qilu Pharmaceutical Affairs, 2009, 28(07): 419-421. 

About the author: Xiao Nisha, a food science and technology worker working in a large R&D company of drugs in China, engages in the R&D of nutritious food and functional food.