Ultrafiltration characteristics of pegylated proteins

Membrane-Based Hybrid Method for Purifying PEGylated Proteins

PEGylated proteins are usually purified using chromatographic methods, which are limited in terms of both speed and scalability. In this paper, we describe a microfiltration membrane-based hybrid method for purifying PEGylated proteins. Polyethylene glycol (or PEG) is a lower critical solution temperature polymer which undergoes phase transition in the presence of a lyotropic salt and forms micelle-like structures which are several microns in size. In the proposed hybrid method, the PEGylated proteins are first converted to their micellar form by the addition of a lyotropic salt (1.65 M ammonium sulfate). While the micelles are retained using a microfiltration membrane, soluble impurities such as the unmodified protein are washed out through the membrane. The PEGylated proteins thus retained by the membrane are recovered by solubilizing them by removing the lyotropic salt. Further, by precisely controlling the salt removal, the different PEGylated forms of the protein, i.e., mono-PEGylated and di-PEGylated forms, are fractionated from each other. Hybrid separation using two different types of microfiltration membrane devices, i.e., a stirred cell and a tangential flow filtration device, are examined in this paper. The membrane-based hybrid method for purifying PEGylated proteins is both fast and scalable.

Moving Protein PEGylation from an Art to a Data Science

PEGylation is a well-established and clinically proven half-life extension strategy for protein delivery. Protein modification with amine-reactive poly(ethylene glycol) (PEG) generates heterogeneous and complex bioconjugate mixtures, often composed of several PEG positional isomers with varied therapeutic efficacy. Laborious and costly experiments for reaction optimization and purification are needed to generate a therapeutically useful PEG conjugate. Kinetic models which accurately predict the outcome of so-called “random” PEGylation reactions provide an opportunity to bypass extensive wet lab experimentation and streamline the bioconjugation process. In this study, we propose a protein tertiary structure-dependent reactivity model that describes the rate of protein-amine PEGylation and introduces “PEG chain coverage” as a tangible metric to assess the shielding effect of PEG chains. This structure-dependent reactivity model was implemented into three models (linear, structure-based, and machine-learned) to gain insight into how protein-specific molecular descriptors (exposed surface areas, pK_a, and surface charge) impacted amine reactivity at each site. Linear and machine-learned models demonstrated over 75% prediction accuracy with butylcholinesterase. Model validation with Somavert, PEGASYS, and phenylalanine ammonia lyase showed good correlation between predicted and experimentally determined degrees of modification. Our structure-dependent reactivity model was also able to simulate PEGylation progress curves and estimate “PEGmer” distribution with accurate predictions across different proteins, PEG linker chemistry, and PEG molecular weights. Moreover, in-depth analysis of these simulated reaction curves highlighted possible PEG conformational transitions (from dumbbell to brush) on the surface of lysozyme, as a function of PEG molecular weight.

Purification of Modified Therapeutic Proteins Available on the Market: An Analysis of Chromatography-Based Strategies

Proteins, which have inherent biorecognition properties, have long been used as therapeutic agents for the treatment of a wide variety of clinical indications. Protein modification through covalent attachment to different moieties improves the therapeutic's pharmacokinetic properties, affinity, stability, confers protection against proteolytic degradation, and increases circulation half-life. Nowadays, several modified therapeutic proteins, including PEGylated, Fc-fused, lipidated, albumin-fused, and glycosylated proteins have obtained regulatory approval for commercialization. During its manufacturing, the purification steps of the therapeutic agent are decisive to ensure the quality, effectiveness, potency, and safety of the final product. Due to the robustness, selectivity, and high resolution of chromatographic methods, these are recognized as the gold standard in the downstream processing of therapeutic proteins. Moreover, depending on the modification strategy, the protein will suffer different physicochemical changes, which must be considered to define a purification approach. This review aims to deeply analyze the purification methods employed for modified therapeutic proteins that are currently available on the market, to understand why the selected strategies were successful. Emphasis is placed on chromatographic methods since they govern the purification processes within the pharmaceutical industry. Furthermore, to discuss how the modification type strongly influences the purification strategy, the purification processes of three different modified versions of coagulation factor IX are contrasted.

Perspectives, Tendencies, and Guidelines in Affinity-Based Strategies for the Recovery and Purification of PEGylated Proteins

In recent years, the effective purification of PEGylated therapeutic proteins from reaction media has received particular attention. Although several techniques have been used, affinity-based strategies have been scarcely explored despite the fact that, after PEGylation, marked changes in the molecular affinity parameters of the modified molecules are observed. With this in mind, future contributions in the bioseparation of these polymer-protein conjugates are expected to exploit affinity in chromatographic and nonchromatographic techniques which will surely derive in the integration of different operations. However, this will only occur as novel ligands which are simultaneously found. As it will be mentioned, these novel ligands may be screened or designed. In both cases, computer-aided tools will support their identification or development. Additionally, ligand discovery by high-throughput screening (HTS) is believed to become a fast, economic, and informative technology that will aid in the mass production of ligands along with genetic engineering and related technologies. Therefore, besides analyzing the state of the art in affinity separation strategies for PEGylated molecules, this review proposes a basic guideline for the selection of adequate ligands to provide information and prospective on the future of affinity operations in solving this particular bioengineering problem.

PEGylation within a confined hydrophobic cavity of a protein

The conjugation of polyethylene glycol (PEG) to proteins, known as PEGylation, has increasingly been employed to expand the efficacy of therapeutic drugs. Recently, research has emphasized the effect of the conjugation site on protein-polymer interactions. In this study, we performed atomistic molecular dynamics (MD) simulations of lysine 116 PEGylated bovine serum albumin (BSA) to illustrate how conjugation near a hydrophobic pocket affects the conjugate's dynamics and observed altered low mode vibrations in the protein. MD simulations were performed for a total of 1.5 μs for each PEG chain molecular mass from 2 to 20 kDa. Analysis of preferential PEG-BSA interactions showed that polymer behavior was also affected as proximity to the attractive protein surface patches promoted interactions in small (2 kDa) PEG chains, while the confined environment of the conjugation site reduced the expected BSA surface coverage when the polymer molecular mass increased to 10 kDa. This thorough analysis of PEG-BSA interactions and polymer dynamics increases the molecular understanding of site-specific PEGylation and enhances the use of protein-polymer conjugates as therapeutics.

Characterization of a Polyethylene Glycol-Amphotericin B Conjugate Loaded with Free AMB for Improved Antifungal Efficacy

Amphotericin B (AMB) is a highly hydrophobic antifungal, whose use is limited by its toxicity and poor solubility. To improve its solubility, AMB was reacted with a functionalized polyethylene glycol (PEG), yielding soluble complex AmB-PEG formulations that theoretically comprise of chemically conjugated AMB-PEG and free AMB that is physically associated with the conjugate. Reverse-phase chromatography and size exclusion chromatography methods using HPLC were developed to separate conjugated AMB-PEG and free AmB, enabling the further characterization of these formulations. Using HPLC and dynamic light scattering analyses, it was observed that the AMB-PEG 2 formulation, having a higher molar ratio of 2 AMB: 1 PEG, possesses more free AMB and has relatively larger particle diameters compared to the AMB-PEG 1 formulation, that consists of 1 AMB: 1 PEG. The identity of the conjugate was also verified using mass spectrometry. AMB-PEG 2 demonstrates improved antifungal efficacy relative to AMB-PEG 1, without a concurrent increase in in vitro toxicity to mammalian cells, implying that the additional loading of free AMB in the AMB-PEG formulation can potentially increase its therapeutic index. Compared to unconjugated AMB, AMB-PEG formulations are less toxic to mammalian cells in vitro, even though their MIC50 values are comparatively higher in a variety of fungal strains tested. Our in vitro results suggest that AMB-PEG 2 formulations are two times less toxic than unconjugated AMB with antifungal efficacy on Candida albicans and Cryptococcus neoformans.

Pharmacokinetics, metabolism and distribution of PEGs and PEGylated proteins: quo vadis?

The pharmacokinetics (PK), metabolism and biodistribution of polyethylene glycol (PEG) in PEGylated proteins are important to understand the increased cellular vacuolation reported in various tissues in animals. The tissue distribution profile of PEGylated proteins and 'metabolic' PEG is guided largely by absolute PEG load, PEG molecular weight and, where applicable, receptor-mediated uptake via the protein moiety. High molecular weight PEGs show slow renal clearance, and consequently have a greater potential to accumulate within cells. The intracellular nonbiodegradable PEG can accumulate within the lysosome ultimately causing distension and vacuolation observed by standard histological examinations. Improved bioanalytical methodologies will contribute to the identification of specific PK parameters including distribution behavior to support development of PEGylated proteins as therapeutics.

Intermolecular interactions during ultrafiltration of pegylated proteins

Recent studies have demonstrated the feasibility of using membrane ultrafiltration for the purification of pegylated proteins; however, the separations have all been performed at relatively low protein concentrations where intermolecular interactions are unimportant. The objective of this study was to examine the behavior at higher PEG concentrations and to develop an appropriate theoretical framework to describe the effects of intermolecular interactions. Ultrafiltration experiments were performed using pegylated α-lactalbumin as a model protein with both neutral and charged composite regenerated cellulose membranes. The transmission of the pegylated α-lactalbumin, PEG, and α-lactalbumin all increase with increasing PEG concentration due to the increase in the solute partition coefficient arising from unfavorable intermolecular interactions in the bulk solution. The experimental results were in good agreement with a simple model that accounts for the change in Gibbs free energy associated with these intermolecular interactions, including the effects of concentration polarization on the local solute concentrations upstream of the membrane. These intermolecular interactions are shown to cause a greater than expected loss of pegylated product in a batch ultrafiltration system, and they alter the yield and purification factor that can be achieved during a diafiltration process to remove unreacted PEG.

Covalent conjugation of poly(ethylene glycol) to proteins and peptides: strategies and methods

PEGylation, the covalent linking of PEG chains, has become the leading drug delivery approach for proteins. This technique initiated its first steps almost 40 years ago, and since then, a variety of methods and strategies for protein-polymer coupling have been devised. PEGylation can give a number of relevant advantages to the conjugated protein, such as an important in vivo half-life prolongation, a reduction or an abolishment of immunogenicity, and a reduction of aggregation. Furthermore, the technique has demonstrated a great degree of versatility and efficacy--not only PEG-protein conjugates have reached the commercial marketplace (with nine types of derivatives), but a PEG-aptamer and PEGylated liposomes are now also available. Most of this success is due to the development of several PEGylation strategies and to the large selection of PEGylating agents presently at hand for researchers. Nevertheless, this technique still requires a certain level of familiarity and knowledge in order to achieve a positive outcome for a PEGylation project. To draw general guidelines for conducting PEGylation studies is not always easy or even possible because such experiments often require case-by-case optimization. On the other hand, several common methods can be used as starting examples for the development of tailor-made coupling conditions. Therefore, this chapter aims to provide a basic introduction to a wide range of PEGylation procedures for those researchers who may not be familiar with this field.

Purification of singly PEGylated α-lactalbumin using charged ultrafiltration membranes

One of the challenges in producing a PEGylated therapeutic protein is that the PEGylation reaction typically generates a mixture of both singly and multiply PEGylated species. The objective of this study was to examine the feasibility of using ultrafiltration for the purification of a singly PEGylated protein from the multiply PEGylated conjugates. Data were obtained with α-lactalbumin that was PEGylated with a 20 kDa activated PEG, with the ultrafiltration performed over a range of pH and ionic strength using both unmodified and negatively charged composite regenerated cellulose membranes. Purification of the singly PEGylated α-lactalbumin from the multiply PEGylated species was accomplished using a diafiltration process with a negatively charged membrane at pH 5 and an ionic strength of 0.4 mM, conditions that maximized the electrostatic exclusion of the multiply PEGylated species from the charged membrane. The diafiltration process provided more than 97% yield with greater than 20-fold purification between the singly and doubly PEGylated proteins and nearly complete removal of the more heavily PEGylated species. The singly PEGylated α-lactalbumin was recovered as a dilute filtrate solution, although this dilution could be eliminated using a cascade filtration or the final product could be re-concentrated in a second ultrafiltration as part of the final formulation. These results demonstrate the feasibility of using ultrafiltration for the purification of singly PEGylated protein therapeutics.

PEGylation: Posttranslational bioengineering of protein biotherapeutics

Polymer conjugation, especially by poly(ethylene glycol), has become a leading technology for the delivery of proteins. Nowadays, biotech drugs represent an increasing share of the new approved drugs, but their use is often prevented by drawbacks and safety concern. In particular, short in vivo half-life and immunogenicity are significant problems faced by the researchers dealing with the development of protein and peptide drugs. The chemical linking of a polymer to the protein surface has proved effective in prolonging protein blood circulation and reducing the immunogenicity by decreasing renal clearance and shielding immunogenic epitopes, respectively. So far, PEGylation has already led to nine marketed conjugates with great therapeutic success.:

Size comparison between proteins PEGylated with branched and linear poly(ethylene glycol) molecules

Therapeutic proteins conjugated with branched poly(ethylene glycol) (PEG) have extended in vivo circulation half-lives compared to linear PEG-proteins, thought to be due partly to a greater hydrodynamic volume of branched PEG-proteins, which reduces the glomerular sieving coefficient. In this paper, viscosity radii of PEGylated alpha-lactalbumin (M(r) = 14.2 kDa) and bovine serum albumin (M(r) = 67 kDa) prepared with linear and branched PEGs (with nominal molecular weights 5, 10, 20 and 40 kDa) were compared experimentally using size exclusion chromatography (SEC). PEG adduct:protein molecular weight ratios of the PEGylated proteins covered the range 1:12 to 6:1. Direct comparisons of experimentally measured viscosity radii were found to be misleading due to differences between actual and nominal molecular weights of the PEG reagents used. Comparison with predicted viscosity radii shows that there is no significant difference between the viscosity radii of branched and linear PEG-proteins having the same total molecular weight of PEG adducts. Therefore, longer in vivo circulation half-lives of branched PEG-proteins compared to linear PEG-proteins are not explained by size difference. It is also calculated that the molecular size cut-off for glomerular filtration, 60 A for a 30 kDa PEG, matches the 30-50 A size range for the pores of the glomerular basement membrane. Finally, it is confirmed that prediction of PEG-protein viscosity radii should be based upon conservation of the total PEG adduct surface area to volume ratio for both linear and branched PEG-proteins regardless of PEGylation extent.

Glomerular sieving of three neutral polysaccharides, polyethylene oxide and bikunin in rat. Effects of molecular size and conformation

AIM

Polysaccharides and many other non-protein polymers generally have a more open, flexible and asymmetrical structure compared with globular proteins. For a given molecular weight (MW), the Stokes-Einstein radius (a(e)) of the following polymers increases in the order: Ficoll < dextran <or= pullulan < polyethylene oxide (PEO). We have tested the hypothesis that such an increase in 'molecular extension' will increase the molecule's glomerular permeability. Thus, we investigated the glomerular sieving coefficients (theta) of the mentioned polymers and of the negatively charged and extended protein bikunin.

METHODS

In anaesthetized Wistar rats, glomerular sieving curves were generated for each FITC-labelled polymer from their respective concentration in urine and plasma, determined by size exclusion chromatography. The theta for bikunin was measured using a tissue uptake technique.

RESULTS

For a molecule of a(e) = 55 A (cf. IgG), theta increased in the order: Ficoll (0.00035 +/- 0.000013) < dextran (0.022 +/- 0.0029) < pullulan (0.033 +/- 0.0024) < PEO (0.12 +/- 0.0055). For a(e) = 36 A (cf. albumin) the order was: Ficoll (0.076 +/- 0.0061) < dextran (0.45 +/- 0.037) = pullulan (0.45 +/- 0.021) < PEO (0.65 +/- 0.0076). theta for bikunin (0.089 +/- 0.0045) was 150 times higher than that of albumin, having an equivalent a(e) and net negative charge.

CONCLUSION

From these results it is concluded that for flexible and asymmetric macromolecules, their degree of glomerular hyperpermeability is proportional to their degree of 'molecular extension'. Thus, compared with globular proteins, the polysaccharides investigated, including Ficoll, were found to be hyperpermeable across the glomerular filter in vivo.

Comparison of strong anion-exchangers for the purification of a PEGylated protein

We have studied the effect of protein PEGylation on ion-exchange adsorption using bovine serum albumin as a model system. The free sulfhydryl group of BSA, located on cysteine 34, was PEGylated using the maleimido-PEG chemistry. Several different BSA preparations were screened for extent of reaction using a 30 kDa PEG reagent. The highest yielding BSA preparation was PEGylated using linear 12 kDa and 30 kDa PEG reagents at the 1 liter scale. The PEGylated reaction mixture was purified by anion-exchange gradient elution chromatography to remove native protein and aggregates. Purity following anion-exchange chromatography was >90% as determined by analytical size exclusion chromatography. The elution salt concentration decreased with increasing PEG chain length. Breakthrough studies on six commercially available anion-exchange stationary phases with purified PEG-BSA conjugates confirm a very large decrease in dynamic binding capacity compared to the native protein. The decrease in dynamic binding capacity is likely due to modulation of electrostatic interactions caused by the neutral PEG chain and increased mass transfer resistance associated with the large size of the molecule. Of the stationary phases evaluated, the open porous structure of the agarose based ion-exchangers resulted in the highest dynamic binding capacities for the PEG-BSA conjugates. Frontal analysis experiments demonstrate use of this technique for purification of PEGylated proteins. A stationary phase that tended to exclude the large PEG-BSA conjugate was very efficient in removing native protein from a crude reaction mixture by frontal analysis.