| Growth hormone releasing peptide-6(GHRP-6) is a six-amino-acid peptide.GHRP-6can increase animal growth, improve animal production, and so on.Furthermore, GHRP-6can improve the quality of the skin in fur-bearing animals.However, GHRP-6has a short half-life and is hydrolyzed quickly by enzyme in vivo,which decreased the application of GHRP-6in livestock production. Therefore, it isnecessary to develop a long lasting GHRP-6sustained-released formulation. In thisstudy, we have synthesized triblock copolymer PLGA-PEG-PLGA and then preparedthe GHRP-6sustained-release hydrogels. In vitro, the releasing mechanism ofGHRP-6in copolymer was investigated. In addition, the interaction between GHRP-6and the copolymer was characterized by Fourier transform infrared spectroscopy andX-ray diffraction. In vivo, rex rabbits were used as animal model to investigate theeffects of GHRP-6–copolymer hydrogel on the growth and skin quality of rex rabbits.In this study, the copolymer was synthesized with ring-opening copolymerizationand characterized by nuclear magnetic resonance, and gel permeation chromatography.The results showed that the structure of the copolymer was consistent with the targetproduct. The gel temperature of copolymer solution was detected via the test tubeinversion approach. The results demonstrated that the gel temperature of differentconcentrations of copolymer solution was agreed with the range of physiologicaltemperature. The phase-transition temperature of the aqueous solution was decreasedfrom a sol to a gel and increased from a gel to a sol with the increased solutionconcentration. The results of urgent toxicity and maximal tolerance dose experimentdemonstrated that the mice have not displayed abnormal or death after injectedcopolymer for14days. The maximal tolerance dose of copolymer in subcutaneousand intramuscular injection was1250folds and250folds of human respectively, which indicated that the copolymer was safe to animals.We have prepared GHRP-6sustained-release gel formulation carried byPLGA-PEG-PLGA copolymer. The release of GHRP-6from the copolymer in vitrowas detected. The effects of different mass fraction copolymer solution, drug loading,and molecular weight of proteins on the release rate of GHRP-6were investigated.The results showed that the copolymer exhibited well release effectivity and couldmaintain18days. The release rate was decreased with the increase of copolymer massfraction and protein molecular weight, while the release rate was increased with theincrease of drug dose. The release data of GHRP-6was fitted to theKorsmeyer-Peppas equation. The release mechanism of GHRP-6from the copolymerwas drug diffusion and gel degradation. Hydrogen bonds formed between thecopolymer and GHRP-6detected by X-ray diffraction and Fourier transform infraredspectroscopy.Rex rabbits were randomly divided into four groups. The rabbits were treatedwith blank hydrogel, saline, GHRP-6, and GHRP-6hydrogel respectively, toinvestigate the effects of GHRP-6on the growth of rex rabbits. The serum levels ofinsulin-like growth factor-1(IGF-1) were exceeded15.85%,15.1%,16.81%and7.60%in the GHRP-6copolymer hydrogel group compared with the blank hydrogel groupon days7,14,21and28, respectively. The average daily gain of the rex rabbits wereexceeded68.35%,85.97%and68.34%in the GHRP-6copolymer hydrogel groupcompared with the blank hydrogel group on days7,14and21, respectively. Theaverage cumulative weight gain of the rex rabbits in the GHRP-6hydrogel group wassignificantly higher than that of the other three groups on days28. The skin area wassignificantly bigger in the GHRP-6copolymer hydrogel group than in the blankhydrogel group. The follicle density was markedly higher in the GHRP-6copolymerhydrogel group than in the blank hydrogel and GHRP-6solution groups. Takentogether, these results indicate that the hydrogel controlled the release of GHRP-6,which increased the levels of IGF-1, thereby improving the average daily weight gain，average cumulative weight gain and hair coat quality of the rex rabbits. |
