Structural Stability of Recombinant Human Growth Hormone (r-hGH) as a Function of Polymer Surface Properties

PLGA and PEG based porous microparticles as vehicles for pulmonary somatropin delivery

Breakthrough advances in protein therapeutics and sustained release systems continue to fuel innovation in novel, non-invasive polymeric platforms for delivery of biologicals. Despite the bench potential and proof-of-concept work, market analysis still shows biologicals to be predominantly injections. Characterized by insufficient secretion of growth hormone by the pituitary gland, growth hormone deficiency (GHD) is a rare disorder. Currently, chronic somatropin (r-hGH) replacement therapy is only available as subcutaneous injections administered several times a week. We aim to prepare large, porous, biodegradable and aerodynamically light microparticles as tunable carriers for pulmonary r-hGH delivery. We developed a range of microparticles using PLGA 5050 1Awith sizes between 5 μm and 13 μm, densities lower than 0.4 g/cc and aerodynamic diameters lower than 6 μm. Polyethylene glycol's multitude of advantages - plasticizing PLGA, improving the biocompatibility of the system and preventing protein burst release - have been extensively studied, making it our excipient (pore-former) of choice. Drug loading was characterized at pH 4.0 (acidic), 5.3 (pI) and pH 7.2 (neutral) and was a result of an interplay of electrostatic and hydrophobic interactions between the polymer and somatropin. Considering the physicochemical interactions, we observed some pH dependent protein unfolding characterized by reduction in intrinsic fluorescence of the Tryptophan 86 residue at 331 nm. The secondary ⍺-helix structure characterized by 2 negative minima at 209 nm and 222 nm in the circular dichroism spectra, was intact at all pH values. R-hGH was released over a period of seven days, and the release profile was a function of the microparticle porosity.

Design and synthesis of polyacrylic acid/deoxycholic acid-modified chitosan copolymer and a close inspection of human growth hormone-copolymer interactions: An experimental and computational study

Despite efforts to achieve a long-acting formulation for human growth hormone (hGH), daily injections are still prescribed for children with growth hormone deficiency. To grapple with the issue, acquiring a deep knowledge of the protein and understanding its interaction mechanism with the carrier can be beneficial. Herein, we designed and synthesized a novel chitosan-based copolymer and investigated its interaction with hGH using a combination of experimental and computational strategies. To construct the amphiphilic triblock copolymers (CDP), we grafted deoxycholic acid (DCA) and polyacrylic acid (PAA) onto the chitosan chains, and Fourier-transform infrared (FTIR) analysis confirmed the proper formation of CDP. Circular dichroism (CD) demonstrated the preservation of the secondary structure of hGH interacting with CDP, and, further, fluorescence spectroscopy proved the stability of the tertiary structure of the protein. Applying molecular dynamics simulation (MD), we examined the dynamics and integrity of hGH in the presence of the copolymer and compared its behavior with the protein in aquatic environments. Additionally, energy and contact analysis illustrated that the residues involved in the interaction were located predominantly in the connecting loops, and van der Waals (vdW) and electrostatic interactions were the main driving forces of the polymer-protein complex formation. This research's main aim was to trace the protein-polymer interaction's mechanism. We anticipate that the utility of the copolymer can address the challenges of fabricating a new sustained-release delivery platform for therapeutic proteins.

Insight into the Microcosm of the Human Growth Hormone and Its Interactions with Polymers and Copolymers: A Molecular Dynamics Perspective

Therapeutic proteins nowadays have increasingly been applied for their considerable potential in treating a wide variety of diseases. The effectiveness and potency of native therapeutic proteins are limited by various factors (e.g., stability, blood circulation time, specificity). Over the past years, a great deal of effort has been devoted to developing safe and efficient protein delivery systems. Entrapment of protein into polymeric and copolymeric matrices is common among the different types of protein-based drug formulation. However, despite the massive efforts toward developing therapeutic protein delivery in experimental studies and industrial applications, there is relatively little data on the influence of polymers and copolymers on therapeutic proteins at the atomic and molecular levels. Herein, molecular dynamics (MD) simulations are used to study the effects of biocompatible synthetic polymers including methoxy poly(ethylene glycol) (MPEG), poly(lactic acid) (PLA), and poly(lactic acid) copolymers (poly(lactic-co-glycolic acid)) PLGA and MPEG-PLA(PELA)) on the structure and dynamics of the human growth hormone (hGH), and the results are compared with previous experimental findings. Our results indicate that the hGH conformation remains stable both in pure water and in the presence of polymers, and these results are in good agreement with previous experimental data. It is shown that the MPEG chains are self-assembled and folded into a semicrystalline structure; therefore, only a small portion of the protein interacts with the polymer. The other three polymers, however, interact well with the protein and partially cover its surface. Our findings suggest that the use of these polymers for protein encapsulation has the advantage of reducing protein aggregation and thus increasing drug serum half-life. Eventually, we anticipate that the research results will expand the current knowledge about encapsulation mechanisms and the molecular interactions between hGH and the polymers.

Effect of the Interaction Between an Ionic Surfactant and Polymer on the Dissolution of a Poorly Soluble Drug

Surfactants are commonly incorporated in conventional and enabled formulations to enhance the rate and extent of dissolution of drugs exhibiting poor aqueous solubility. Generally the interactions between the drug and excipients are systematically evaluated, however, limited attention is paid towards understanding the effect of interaction between functional excipients and its impact on the performance of the product. In the current study, the effect of potential interaction between a nonionic polymer binder, povidone, and anionic surfactant docusate sodium on the rate and extent of dissolution of a drug exhibiting poor aqueous solubility was evaluated by varying the proportions of the binder and the surfactant in the formulation. Potential complexation or aggregation between the excipients was investigated by fluorescence spectroscopy and zeta potential measurements of the aqueous solutions of docusate sodium, povidone, and sodium lauryl sulfate (SLS). The rate and extent of drug release was found to decrease with an increase in the proportion of docusate sodium and povidone in the formulations. Difference in magnitude of surface charge (zeta potential) of docusate sodium in presence of povidone indicated potential surfactant-polymer aggregation during dissolution which was corroborated by CAC/CMC values derived from fluorescence spectroscopic measurements. The decrease in the rate of drug release was attributed to an increase in the viscosity of the microenvironment of dissolving particles due to the interaction of docusate sodium and povidone in the aqueous media during dissolution. These findings highlight the importance of systematic evaluation of the interaction of ionic surfactants with the polymeric components within the formulation to ensure the appropriate selection of the type of surfactant as well as its proportion in the formulation.

Thermodynamic analysis of r-hGH-polymer surface Interaction using isothermal titration calorimetry

Delivery of protein drugs would be an effective approach if mechanistic understanding of protein adsorption at solid/liquid surface is obtained and nonspecific adsorption can be controlled. This study involves evaluation of the thermodynamic parameters for interaction between recombinant human growth hormone (r-hGH) and nanoparticles of poly (lactic- co- glycolic) acid polymer of different molecular weight. Energy changes related to protein-nanoparticle interactions are usually very small and isothermal titration calorimetry (ITC) is the only technique that determines the binding constant (Ka), enthalpy, (ΔH) and stoichiometry, (n) in a single experiment. Therefore, we used ITC to study the energetics of the binding of r-hGH with PLGA nanoparticles. The largely negative ∆H and ∆S suggested that binding was driven by non-classical hydrophobic effect for interaction with PLGA 50501A and PLGA 8515E nanoparticles at pH 7.2 where higher surface coverage was noted for the latter. Endothermic, entropically driven reaction was observed upon interaction of r-hGH with PLGA 5050 5E nanoparticles at pH 7.2, PLGA 5050 1A nanoparticles at pH 5.3 and pH 4.0. Moreover, negative value of ΔCp for PLGA 50501A NPs at pH 7.2 indicated cooperative disorder of hydrogen-bonding networks and no evidence of hydrophobic elements. ITC proved to be very efficient method in studying the thermodynamics of the protein polymer interaction. In agreement with results from previous studies using fluorescence spectroscopy, circular dichroism and dynamic light scattering in our laboratory, this study demonstrated that adsorption can be controlled by selecting the polymer of low to moderate hydrophobicity depending on the pH of media.