Human growth hormone (hGH) or growth hormone is an endogenous, non-glycosylated protein synthesized, stored and secreted by the pituitary gland [1]. It contains 191 amino acids, a molecular weight of approximately 22 kDa, and consists of four groups α-helical bundle with two disulfide bonds [2]. This protein has a wide range of roles in promoting longitudinal bone growth and regulating protein, lipid and carbohydrate metabolism [3]. RhGH has been approved for clinical use since 1985 [4]. With the further research, rhGH application includes not only childhood dwarfism (GHD) caused by growth hormone deficiency, but also growth disorders caused by chronic renal failure and Turner syndrome and adult GHD [5]. It also plays an important role in the treatment of obesity, wound healing and burns [6]. whereas, due to the short half-life in the systemic blood circulation, rhGH is administered for a period of several years by injecting subcutaneously daily or three times a week [7]. The patient's compliance was poor due to the long and inconvenient administration and the pain caused by subcutaneous injection. In order to improve patient compliance, people have been looking for easier and more convenient ways to administer drugs. On the other hand, reducing the sensitivity of rhGH to physical and chemical degradation is also a challenge [8]. Various environmental factors, including temperature, light, oxidants, pH, freezing, shaking, and shear stress, affect rhGH structure during the production, transportation, and storage of protein drugs [9]. The need for low temperatures makes it difficult for protein-based drugs to escape the cold chain. Therefore, how to transport and store drugs at room temperature is also a topic of great interest.
Various preparations for delivery of rhGH have been tried, including crystalline formulation [10], polymeric microspheres [11, 12], injectable hydrogels [13–15] and oral and transdermal deliveries [16, 17]. Among them, oral administration is widely favored because of its flexible administration time, less need for medical personnel and frequency of hospital or clinic visits [18]. However, the harsh environment of the gastrointestinal tract (pH, protease, etc.) and the complex and diverse biological barriers (epithelial cells, mucus layer, tight intercellular connections) seriously hinder the oral administration of protein drugs [19], resulting in the oral bioavailability of most protein drugs being about 2% [20]. At present, there have been a large number of studies on oral insulin delivery, but there are few studies on how to effectively deliver rhGH. Several methods have reportedly been used in studies of oral rhGH [21–23], but their clinical value is not high. Therefore, an efficient drug delivery system for rhGH is worth studying.
As a ligand of bile acid transport protein, bile acid has been used in many studies as a preparation of nanoparticles or microcapsules, which has excellent properties [24]. After entering the body, bile acids can enter the intestinal epithelial cells through the apical sodium-dependent bile acid transporter (ASBT), and further bile acid recycling [25], with recycling efficiency of more than 90%. Previously, combining PLGA with bile acids allowed the drug to survive the harsh environment of the stomach [26]. At the same time, choline-based ionic liquids (IL) have attracted extensive attention because of their obvious biocompatibility and biodegradability [27]. IL are liquid molten salts at room temperature and have the characteristics of low volatility, high ionic conductivity, and high electrochemical stability [28]. As a green solvent, it has been deeply researched in electrochemistry, biocatalysis, materials science, etc [29]. In the field of drug delivery, it is used in biofilm infections [30], skin disease [31] and other diseases. For protein drugs, it improves drug stability by keeping protein drugs stable at room temperature for 2 months and refrigerated for at least 4 months [32], and improving the catalytic activity of cytochrome C [28]. IL have good biocompatibility, are cheap and easy to obtain, and have a good prospect of engineering.
Hence, we reported an IL and deoxycholic acid (DCA)-based multifunctional nanoparticle. As a protein protector, IL was used to improve the stability of rhGH at room temperature. PLGA coats the IL in which the drug is dissolved, and releases the drug slowly after entering the blood circulation. DCA is modified on the surface of PLGA to ensure the integrity of the nanoparticles in the gastrointestinal tract. After entering the intestinal tract, the nanoparticles enter intestinal epithelial cells through ASBT receptor, escape through lysosome, intracellular transport mediated by IBABP, and release into the blood circulation by the lateral basement membrane. Various materials of the nanoparticle, such as arginine, IL, PLGA, have a protective effect on rhGH, so it can be guessed that the rhGH coated with the nanoparticle will have higher stability.