Introduction
Stabilising a Growth Hormone Protein API within poly(lactic-co-glycolic acid) (PLGA) microspheres requires the control of several degradation mechanisms, including microclimate acidification, nucleophilic acylation, and interfacial denaturation, to achieve a consistent 30-day therapeutic drug release profile. A combination of zinc salt complexation, hydrophobic antacid incorporation, and carefully selected polymer matrix characteristics can help preserve protein integrity while minimizing the initial burst release. Recombinant human growth hormone (rhGH) is a 22 kDa single-chain polypeptide composed of 191 amino acid residues and stabilized by two intrachain disulfide bonds. Chronic clinical conditions that require somatotropin replacement, including pediatric growth hormone deficiency, adult growth hormone deficiency, and Turner syndrome, commonly require daily subcutaneous injections, which can negatively affect long-term patient compliance.
A once-monthly sustained-release depot formulation can substantially improve treatment adherence and potentially enhance therapeutic outcomes. Poly(lactic-co-glycolic acid) (PLGA) is considered an effective biodegradable polymer matrix because of its biocompatibility and predictable hydrolytic degradation into lactic acid (LA) and glycolic acid (GA), which can subsequently be cleared through physiological metabolic pathways, including the Krebs cycle. Nevertheless, incorporating rhGH into a sustained-release 1-month parenteral depot introduces significant biophysical and chemical challenges. ResolveMass Laboratories Inc. investigated formulation approaches designed to address these degradation mechanisms and established controlled process parameters intended to preserve tertiary protein structure while supporting a controlled, zero-order release profile over 30 days.
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Quick Summary:
- Goal: Develop a once-monthly PLGA microsphere depot that delivers bioactive recombinant human growth hormone (rhGH) consistently over 30 days, reducing the burden of daily injections.
- Key degradation risks: PLGA breakdown can cause microclimate acidification, nucleophilic acylation, and interfacial denaturation, leading to protein modification, aggregation, and loss of potency.
- Zinc complexation: Forming a Zn²⁺–rhGH complex helps immobilize the protein structure, reduce solvent-induced unfolding, and suppress initial burst release.
- Microclimate control: Hydrophobic rosin acid–modified magnesium hydroxide (RA-MH) neutralizes acidic degradation products and helps maintain a more favorable internal pH.
- Polymer optimization: Ester-capped PLGA 50:50, controlled molecular weight, and optional PLGA-PEG-PLGA blending help balance degradation, reduce acylation, and support 30-day release.
- Process engineering: Solid-in-Oil-in-Water (S/O/W) and Aqueous-Aqueous (A/A) encapsulation reduce protein–solvent contact, improving structural protection and potentially achieving >90% encapsulation efficiency.
- Performance: Optimized microspheres can achieve **
Mechanistic Drivers of Degradation in Stabilising a Growth Hormone Protein API
Degradation of recombinant human growth hormone (rhGH) within PLGA matrices is predominantly associated with microclimate acidification (μpH < 2.8), nucleophilic acylation of lysine residues, and conformational unfolding at water/organic solvent interfaces. A clear understanding of these physical and chemical degradation mechanisms is essential for selecting appropriate stabilization additives and defining suitable processing conditions. If these degradation pathways are not adequately controlled during the 30-day dissolution period, they can result in protein aggregation, chemical modification, structural instability, and loss of biological activity.
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Microclimate Acidification and Deamidation Kinetics in Stabilising a Growth Hormone Protein API
Microclimate acidification develops as hydrophobic PLGA undergoes bulk hydrolytic ester cleavage, producing lactic and glycolic acid monomers that can reduce the pH within internal pores to approximately 1.5–2.8. Such a highly acidic localized environment can accelerate deamidation of asparagine residues through the formation of a cyclic succinimide intermediate mediated by undissociated glycolic acid.
Confocal microclimate pH (μpH) mapping using fluorescent dextran probes has demonstrated that water-filled pores within degrading PLGA 50/50 microspheres can undergo rapid acidification within approximately 2 to 4 weeks after hydration. A lower polymer molecular weight and increased glycolic acid content can accelerate hydrolytic cleavage, resulting in greater localized concentrations of carboxylic acid end-groups. Undissociated glycolic acid (GA) can function as a proton-transfer catalyst and facilitate intramolecular nucleophilic attack involving the backbone amide nitrogen and the side-chain carbonyl carbon of asparagine (Asn) residues. The resulting cyclic succinimide intermediate can subsequently hydrolyze to form Aspartate (Asp) or iso-Aspartate (β-Asp) residues, thereby compromising the biological activity and structural integrity of the growth hormone API.
Bulk Erosion vs. Surface Erosion in PLGA Understand Bulk Erosion vs Surface Erosion in PLGA
Covalent Nucleophilic Acylation at Lysine Residues
Covalent acylation occurs when primary amines, particularly the N-terminus and nine ε-amino lysine groups present on rhGH, undergo nucleophilic attack on reactive ester carbonyl groups within the PLGA backbone or glycolic/lactic oligomers. The resulting covalent adducts can permanently modify the molecular weight, charge characteristics, and binding affinity of the growth hormone protein.
Acylation reactions become more pronounced under acidic microclimate conditions and can be further influenced by the concentration of terminal carboxyl groups present in un-capped PLGA polymers. Lower molecular weight PLGA fractions generally contain a greater density of carboxyl group end-groups, which can increase autocatalytic degradation and promote the formation of covalent peptide-polymer adducts during hydration. These glycolyl and lactyl adducts constitute irreversible chemical modifications that can reduce or deactivate biological activity and may increase the potential for immunogenicity.
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Interfacial Denaturation and Irreversible Aggregation
Exposure of native rhGH to high-shear mixing conditions and organic solvents such as dichloromethane during emulsification can promote conformational unfolding and expose normally buried hydrophobic domains. These partially unfolded protein intermediates can subsequently associate into non-covalent complexes or form covalent dimers, decreasing bioactivity and contributing to increased initial burst release.
During conventional double-emulsion (W1/O/W2) processing, soluble rhGH molecules can accumulate at the water/organic solvent interface. Dynamic shear forces and direct exposure to dichloromethane (DCM) can cause hydrophobic amino acid side chains located within the protein interior to become exposed toward the organic phase. As the solvent is removed, hydrophobic interactions can promote the association of partially unfolded monomers into insoluble non-covalent aggregates and covalent dimers. In addition, osmotic pressure gradients across the oil phase during curing can generate porous surface shells. This structural characteristic can contribute to an initial burst release, during which peak plasma concentrations (Cmax) may exceed therapeutic thresholds by as much as 80-fold when appropriate mitigation strategies are not implemented.
Surfactants and Emulsifiers in PLGA Microsphere Fabrication Discover Surfactants and Emulsifiers in PLGA Microsphere Fabrication
| Degradation Pathway | Direct Cause / Mechanism | Impact on rhGH API | Advanced Stabilization Strategy |
|---|---|---|---|
| Microclimate Acidification | Accumulation of LA/GA hydrolytic monomers (μpH < 2.8) | Acid-catalyzed deamidation, succinimide formation, cleavage | Surface-modified basic additives (RA-Mg(OH)2), PEG hydrogel blending |
| Nucleophilic Acylation | Lysine ε-amine attack on PLGA ester carbonyls | Covalent lactyl/glycolyl adduct formation, loss of potency | Ester-capped PLGA selection, solid-state basic buffering |
| Interfacial Denaturation | Shear forces and organic solvent contact (W/O phase) | Structural unfolding, insoluble hydrophobic aggregation | Zn2+-rhGH salt complexation, aqueous-aqueous phase separation |
| Initial Burst Release | Rapid surface pore dissolution and osmotic swelling | Hyper-acute peak concentrations (Cmax), systemic toxicity | Solid-in-oil (S/O) encapsulation, insoluble salt formation |
Formulatory Strategies for Stabilising a Growth Hormone Protein API
Stabilising a Growth Hormone Protein API within PLGA formulations requires an integrated formulation strategy involving divalent metal salt complexation, solid-state basic buffering, and optimization of the polymer matrix. When used together, these approaches can help protect the native tertiary structure of the protein and regulate the internal pore environment throughout the 30-day degradation period.
Zinc Ion Salt Complexation for Structural Immobilization
Complexation of rhGH with divalent zinc ions (Zn2+), such as zinc acetate at a 50:1 molar ratio, can induce reversible precipitation and formation of dense, insoluble protein-zinc aggregates before encapsulation. Converting the protein into this physical state immobilizes its native tertiary structure, reduces its solubility in dichloromethane, and helps prevent interfacial unfolding during emulsification.
Divalent zinc ions can coordinate with histidine imidazole residues located on the rhGH surface and promote dense molecular packing. The resulting micronized Zn2+-rhGH complex remains in a solid state during organic-phase processing, thereby providing a physical barrier against phase-boundary denaturation. Because the insoluble zinc complex does not readily dissolve in the organic phase, migration of protein particles into the external aqueous phase is restricted, which can help suppress the initial burst release. Following subcutaneous administration, endogenous physiological sodium and phosphate ions can progressively displace Zn2+, allowing the complex to solubilize and release native, bioactive rhGH gradually throughout the 30-day delivery period.
Microclimate Neutralization via Hydrophobically Modified Antacids
The incorporation of poorly soluble basic antacids, including rosin acid-modified magnesium hydroxide (RA-MH), can neutralize free hydronium ions and acidic oligomers generated within degrading PLGA pores. Hydrophobic surface modification can enhance antacid dispersion within organic solvents and improve encapsulation efficiency by more than fourfold compared with unmodified magnesium hydroxide.
Although inorganic bases such as Mg(OH)2, MgCO3, and ZnCO3 can neutralize acidity within the polymer matrix, pristine salts often demonstrate poor encapsulation efficiency because they can rapidly separate from organic polymer solutions. Modification of Mg(OH)2 with rosin acid (RA) through coordination bonding produces hydrophobically modified nanoparticles capable of dispersing more uniformly within dichloromethane. As PLGA hydrolysis produces acidic oligomers, the incorporated RA-MH can neutralize these species within the microsphere pores and help maintain μpH > 5.0 during a 4-week dissolution period. By controlling the acidic microenvironment, this approach can suppress acid-catalyzed deamidation, decrease nucleophilic acylation of lysine residues, and reduce inflammatory foreign-body responses at the subcutaneous injection site.
Polymer Matrix Optimization: LA/GA Ratios, Molecular Weight, and End-Group Capping
Selection of ester-capped PLGA with a 50:50 lactic-to-glycolic acid (LA/GA) ratio and an appropriately controlled molecular weight can provide a balance between continuous hydrolytic erosion and minimized accumulation of autocatalytic acidic products. Removing terminal carboxyl groups can substantially decrease the potential for nucleophilic acylation involving lysine residues.
Lactide-rich copolymers, including PLGA 75:25 and PLA 100/0, contain hydrophobic methyl side chains that can restrict water penetration and extend polymer degradation beyond the intended 30-day period. In contrast, PLGA 50:50 can provide favorable mass-loss and erosion kinetics for a 1-month delivery system. The use of ester-capped end-groups, including lauryl or methyl esters, instead of un-capped carboxyl end-groups can reduce initial hydrolytic autocatalysis and minimize the availability of free carboxylic acid groups that can react with primary amines on rhGH. Additionally, incorporating 10–20% hydrophilic triblock copolymers, such as PLGA-PEG-PLGA, can increase matrix hydration and promote outward diffusion of acidic monomeric products, thereby helping prevent the development of a highly acidic microclimate.
Difference Between PLGA 50:50 and PLGA 75:25 Compare the Difference Between PLGA 50:50 and PLGA 75:25
Process Engineering and Emulsification Optimization for Stabilising a Growth Hormone Protein API
Process engineering for long-acting depot formulations can employ Solid-in-Oil-in-Water (S/O/W) processing or organic-free Aqueous-Aqueous (A/A) phase separation to reduce direct high-shear contact between liquid water and organic solvent. These advanced processing approaches can support encapsulation efficiencies above 90% while helping limit initial burst release to below 15–20%.
In Solid-in-Oil-in-Water (S/O/W) processing, a pre-complexed, spray-freeze-dried Zn2+-rhGH solid powder is directly dispersed into an organic polymer phase containing dichloromethane. Maintaining the protein in a solid state during suspension reduces liquid-phase protein unfolding at the organic solvent interface during primary homogenization. As an alternative, aqueous-aqueous (A/A) emulsion systems can preferentially partition rhGH into a dextran phase suspended within a polyethylene glycol (PEG) continuous phase, thereby eliminating the need for organic solvents. Subsequent lyophilization produces stable dextran-rhGH micro-cores, which can then be incorporated into PLGA microspheres through self-regulated sedimentation and membrane-regulated collection. This approach can achieve encapsulation efficiencies exceeding 95%.
Challenges in PLGA Microsphere Development Examine Challenges in PLGA Microsphere Development
| Encapsulation Process | Primary Phase Contact | Physical State of Protein API | Encapsulation Efficiency | Risk of Structural Aggregation / Acylation |
|---|---|---|---|---|
| Double Emulsion (W/O/W) | Direct Water/DCM organic interface | Aqueous Dissolved Solution | Moderate (60% – 80%) | High (Interface denaturation and acylation risks) |
| Solid-in-Oil-in-Water (S/O/W) | Solid/DCM organic boundary | Zn2+-Precipitated Micronized Solid | High (85% – 95%) | Low (Solid-state structural immobilization) |
| Aqueous-Aqueous (A/A) Emulsion | Organic-free polymer aqueous phase | Dextran Partitioned Solid Matrix | Very High (>95%) | Very Low (Eliminates solvent contact completely) |
In Vitro and In Vivo Bioactivity and Dissolution Performance
Optimized rhGH PLGA microspheres can provide continuous zero-order release over 30 days while maintaining secondary and tertiary biological potency, as assessed using RP-HPLC, SEC-HPLC, and cell-based proliferation assays. In vivo pharmacokinetic evaluations can further demonstrate sustained and therapeutically effective plasma concentrations without the toxic peak concentrations associated with uncontrolled burst release or significant subcutaneous tissue inflammation.
Dissolution studies performed in isotonic phosphate-buffered saline (PBS, pH 7.4, 37°C) demonstrate that the combined use of Zn2+ complexation and RA-MH antacid buffering can limit the initial 24-hour burst release to below 15%. Reversed-Phase HPLC (RP-HPLC) and Size-Exclusion HPLC (SEC-HPLC) analyses verify that chemical degradation, including deamidation and oxidation, remains below 2%, while soluble aggregate concentrations remain below 1.5% throughout the 30-day period. Circular Dichroism (CD) spectra confirm the preservation of native α-helical secondary structures following release. Subcutaneous administration in animal models demonstrates sustained plasma somatotropin concentrations for approximately 24 to 30 days, helping prevent rapid initial clearance and supporting sustained osteogenic growth responses.
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Conclusion
Successfully stabilising a Growth Hormone Protein API within PLGA microspheres for a 1-month depot formulation requires a comprehensive strategy that addresses microclimate acidity, nucleophilic acylation, and interfacial shear stress. The combined application of zinc salt precipitation, hydrophobically modified basic antacids such as RA-MH, and carefully selected ester-capped polymers can support stable, zero-order delivery of biological therapeutics.
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ResolveMass Laboratories Inc. provides advanced formulation development, particle engineering, and stability testing services for long-acting injectable biopharmaceutical pipelines. To consult with formulation scientists regarding Custom Microencapsulation and Depot Development Services, visit the ResolveMass Contact Page.
Frequently Asked Questions
Divalent zinc ions coordinate with surface histidine residues of rhGH and promote the formation of a dense, insoluble zinc-protein complex. This solid-state complex helps preserve the native tertiary structure of the protein during processing. It also limits exposure to organic solvent interfaces and can reduce the initial burst release after administration.
PLGA undergoes hydrolytic cleavage of its ester bonds during degradation, producing lactic acid and glycolic acid within the polymer matrix. When these acidic products accumulate faster than they diffuse into the surrounding dissolution medium, the internal pore environment becomes increasingly acidic. Carboxylic end-groups can further accelerate polymer degradation through autocatalysis, reducing the internal pH to approximately 1.5–2.8.
Protein acylation involves nucleophilic primary amines, including the N-terminus and nine ε-amino lysine groups of rhGH, attacking reactive ester carbonyls in PLGA or its lactic/glycolic oligomers. This reaction produces covalent lactyl or glycolyl adducts attached to the protein. Such irreversible modifications can alter protein structure and reduce its biological potency.
Magnesium hydroxide acts as an insoluble basic antacid that reacts with hydronium ions and acidic degradation products generated inside PLGA pores. By neutralizing these acidic species, Mg(OH)2 helps maintain a less acidic internal microenvironment. Maintaining the pore pH above 5.0 can reduce acid-catalyzed deamidation and limit nucleophilic acylation.
Rosin acid-modified magnesium hydroxide (RA-MH) consists of magnesium hydroxide nanoparticles whose surfaces are modified with rosin acid through coordination interactions. The modification increases hydrophobicity and promotes more uniform dispersion within dichloromethane. As a result, RA-MH can provide substantially higher encapsulation efficiency than unmodified Mg(OH)2.
The LA/GA ratio influences water penetration, polymer hydrolysis, and the overall erosion rate of PLGA microspheres. A 50:50 ratio generally provides relatively rapid degradation suitable for approximately 30-day release, whereas higher lactide ratios such as 75:25 are more hydrophobic. The latter can slow hydration and polymer erosion, potentially extending drug release beyond 1 month.
Polyethylene glycol (PEG) and PLGA-PEG triblock hydrogels can increase water uptake within the polymer matrix and promote greater pore connectivity. Improved hydration can enhance the outward movement of lactic acid and glycolic acid generated during PLGA degradation. This helps reduce acid accumulation within the microspheres and limits formation of an extreme acidic microclimate.
S/O/W processing incorporates rhGH as a pre-precipitated solid-state zinc-protein complex into an organic polymer phase. In contrast, conventional W/O/W processing introduces rhGH as an aqueous solution that directly encounters the organic solvent interface. Maintaining the protein in a solid state can reduce interfacial unfolding and shear-associated structural damage during encapsulation.
RP-HPLC can be used to monitor chemical modifications such as deamidation and acylation, while SEC-HPLC evaluates soluble aggregates and dimers. Circular Dichroism (CD) provides information about the preservation of protein secondary structure. Cell-based biological assays are also important for confirming the retained biological activity and somatotropic receptor activity of released rhGH.
Reference:
- Kim, J.-K., Go, E.-J., Ko, K.-W., Oh, H.-J., Han, J., Han, D. K., & Park, W. (2021). PLGA microspheres containing hydrophobically modified magnesium hydroxide particles for acid neutralization-mediated anti-inflammation. Tissue Engineering and Regenerative Medicine, 18(4), 613–622. https://doi.org/10.1007/s13770-021-00338-z
- Diana, J. N., Tao, Y., Du, Q., Wang, M., Kumar, C. U., Wu, F., & Jin, T. (2020). PLGA microspheres of hGH of preserved native state prepared using a self-regulated process. Pharmaceutics, 12(7), 683. https://doi.org/10.3390/pharmaceutics12070683
- Cleland, J. L., Mac, A., Boyd, B., Yang, J., Duenas, E. T., Yeung, D., Brooks, D., Hsu, C., Chu, H., Mukku, V., & Jones, A. J. (1997). The stability of recombinant human growth hormone in poly(lactic-co-glycolic acid) (PLGA) microspheres. Pharmaceutical Research, 14(4), 420–425. https://doi.org/10.1023/A:1012031012367
- Jiang, W., Gao, X., Wang, Q., Chen, Y., Li, D., Zhang, X., & Yang, X. (2023). The modified exenatide microspheres: PLGA-PEG-PLGA gel and zinc-exenatide complex synergistically reduce burst release and shorten platform stage. AAPS PharmSciTech, 24(8), 251. https://doi.org/10.1208/s12249-023-02705-6
- Hua, Y., Su, Y., Zhang, H., Liu, N., Wang, Z., Gao, X., Gao, J., & Zheng, A. (2021). Poly(lactic-co-glycolic acid) microsphere production based on quality by design: A review. Drug Delivery, 28(1), 1342–1355. https://doi.org/10.1080/10717544.2021.1943056
- Liu, Y., Ghassemi, A. H., Hennink, W. E., & Schwendeman, S. P. (2012). The microclimate pH in poly(D,L-lactide-co-hydroxymethyl glycolide) microspheres during biodegradation. Biomaterials, 33(30), 7239–7248. https://doi.org/10.1016/j.biomaterials.2012.06.013
- Houchin, M. L., & Topp, E. M. (2008). Chemical degradation of peptides and proteins in PLGA: A review of reactions and mechanisms. Journal of Pharmaceutical Sciences, 97(7), 2395–2404. https://doi.org/10.1002/jps.21176
- Park, H., Ha, E.-S., Kim, J.-S., & Kim, M.-S. (2023). Injectable sustained-release poly(lactic-co-glycolic acid) (PLGA) microspheres of exenatide prepared by supercritical fluid extraction of emulsion process based on a design of experiment approach. Bioengineering & Translational Medicine, 8(3), e10485. https://doi.org/10.1002/btm2.10485
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