Heat treatment increases the bioactivity of C-terminally PEGylated staphylokinase

PEGylation can effectively improve the therapeutic potential of staphylokinase (SAK), a thrombolysis agent for therapy of myocardial infarction. However, polyethylene glycol (PEG) can sterically shield SAK and drastically decrease its bioactivity. In the present study, N-terminally PEGylated SAKs (5 and 20 kDa PEG), C-terminally PEGylated SAKs with phenyl linker and the ones with amyl linker (5 and 20 kDa PEG) were prepared. The effects of the PEG length, the PEGylation site and linker chemistry on the bioactivity of the heat-treated PEGylated SAK were investigated. Heat treatment at 70 °C for 2 h can improve the bioactivity of the C-terminally PEGylated SAKs, where the one with amyl linker and 20 kDa PEG showed the highest increase extent (27%) in the bioactivity. Thus, our study can advance the development of long-acting pharmaceutical protein with high bioactivity.     

Kinetic and stoichiometric analysis of the modification process for N-terminal PEGylation of staphylokinase

Staphylokinase (SAK) is a therapeutic protein with promise for thrombolytic therapy of acute myocardial infarction. In this study, polyethylene glycol (PEG) aldehyde was used for N-terminal PEGylation of SAK to improve the pharmacological profiles of SAK. Due to the presence of the competitive PEGylation between the N terminus and the Lys residues, kinetic and stoichiometric analysis was carried out to investigate the process for the N-terminal PEGylation of SAK. To achieve this objective, size exclusion chromatography and tryptic peptide mapping were used to measure the PEGylation extent of SAK molecule and its specific amino acid residues, respectively.     

Molecular Insight into the Steric Shielding Effect of PEG on the Conjugated Staphylokinase: Biochemical Characterization and Molecular Dynamics Simulation

PEGylation is a successful approach to improve potency of a therapeutic protein. The improved therapeutic potency is mainly due to the steric shielding effect of PEG. However, the underlying mechanism of this effect on the protein is not well understood, especially on the protein interaction with its high molecular weight substrate or receptor. Here, experimental study and molecular dynamics simulation were used to provide molecular insight into the interaction between the PEGylated protein and its receptor. Staphylokinase (Sak), a therapeutic protein for coronary thrombolysis, was used as a model protein. Four PEGylated Saks were prepared by site-specific conjugation of 5 kDa/20 kDa PEG to N-terminus and C-terminus of Sak, respectively. Experimental study suggests that the native conformation of Sak is essentially not altered by PEGylation. In contrast, the bioactivity, the hydrodynamic volume and the molecular symmetric shape of the PEGylated Sak are altered and dependent on the PEG chain length and the PEGylation site. Molecular modeling of the PEGylated Saks suggests that the PEG chain remains highly flexible and can form a distinctive hydrated layer, thereby resulting in the steric shielding effect of PEG. Docking analyses indicate that the binding affinity of Sak to its receptor is dependent on the PEG chain length and the PEGylation site. Computational simulation results explain experimental data well. Our present study clarifies molecular details of PEG chain on protein surface and may be essential to the rational design, fabrication and clinical application of PEGylated proteins.     

Protein carboxyl amidation increases the potential extent of protein polyethylene glycol conjugation

Chemical coupling of polyethylene glycol (PEG) to therapeutic proteins reduces their immunogenicity and prolongs their circulating half-life. The limitation of this approach is the number and distribution of sites on proteins available for PEGylation (the N terminus and the -amino group of lysines). To increase the extent of PEGylation, we have developed a method to increase the number of PEGylation sites in a model protein, recombinant methionine alpha,gamma-lyase (recombinant methioninase; rMETase), an enzyme cancer therapeutic cloned from Pseudomonas putida. rMETase was first PEGylated with methoxypolyethylene glycol succinimidyl glutarate-5000 with a molar ratio of PEG:rMETase of 15:1. The carboxyl groups of the initially PEGylated protein were then conjugated with diaminobutane, resulting in carboxyl amidation. This reaction was catalyzed by 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide, a water-soluble carbodiimide. The steric hindrance provided by the PEG chains already coupled to the protein prevented cross-linking between rMETase molecules during the carboxyl amidation reaction. The carboxyl-amidated PEGylated rMETase was hyper-PEGylated at a molar ratio of PEG to PEG-rMETase of 60:1. Biochemical analysis indicated that 13 PEG chains were coupled to each subunit of rMETase after hyper-PEGylation compared with 6-8 PEG chains attached to the non-carboxyl-amidated PEG-rMETase. Approximately 15-20% of the non-PEGylated rMETase activity was retained in the hyper-PEGylated molecule. Immunogenicity of the hyper-PEG-rMETase was significantly reduced relative to PEG-rMETase and rMETase. Initial results suggest that hyper-PEGylation may become a new strategy for PEGylation of protein biologics.     

Mono-PEGylated dimeric exendin-4 as high receptor binding and long-acting conjugates for type 2 anti-diabetes therapeutics

Dimerization is viewed as the most effective means of increasing receptor binding affinity, and both dimerization and PEGylation effectively prolong the life spans of short-lived peptides and proteins in vivo by delaying excretion via the renal route. Here, we describe the high binding affinities of two long-acting exendin-4 (Ex4) conjugates, dimerized Ex4 (Di-Ex4) and PEGylated Di-Ex-4 (PEG-Di-Ex4). Di-Ex4 and PEG-Di-Ex4 were prepared using cysteine and amine residue specific coupling reactions using Ex4-Cys, bisMal-NH(2), and activated PEG. The Ex4 conjugates produced were of high purity (>98.5%), as determined by size-exclusion chromatography and MALDI-TOF mass spectrometry. The receptor binding affinity of Di-Ex4 on RIN-m5F cells was 3.5-fold higher than that of Ex4, and the in vivo antihyperglycemic efficacy of Di-Ex4 was also greater than that of native Ex4 in type 2 diabetic db/db mice. Furthermore, Di-Ex4 and PEG-Di-Ex4 were found to have greater blood circulating t(1/2) and AUC(inf) values than native Ex4 by 2.7- and 13.7-fold, and by 4.0- and 17.3-fold, respectively. Accordingly, hypoglycemic durations were greatly increased to 15.0 and 40.1 h, respectively, at a dose of 25 nmol/kg (native Ex4 7.3 h). The results of this study show that combined dimerization and PEGylation are effective when applied to Ex4, and suggest that PEG-Di-Ex4 has considerable potential as a type 2 anti-diabetic agent.     

GlycoPEGylation of recombinant therapeutic proteins produced in Escherichia coli

Covalent attachment of polyethylene glycol, PEGylation, has been shown to prolong the half-life and enhance the pharmacodynamics of therapeutic proteins. Current methods for PEGylation, which rely on chemical conjugation through reactive groups on amino acids, often generate isoforms in which PEG is attached at sites that interfere with bioactivity. Here, we present a novel strategy for site-directed PEGylation using glycosyltransferases to attach PEG to O-glycans. The process involves enzymatic GalNAc glycosylation at specific serine and threonine residues in proteins expressed without glycosylation in Escherichia coli, followed by enzymatic transfer of sialic acid conjugated with PEG to the introduced GalNAc residues. The strategy was applied to three therapeutic polypeptides, granulocyte colony stimulating factor (G-CSF), interferon-alpha2b (IFN-alpha2b), and granulocyte/macrophage colony stimulating factor (GM-CSF), which are currently in clinical use.     

Structure‐Based Site‐Specific PEGylation of Fibroblast Growth Factor 2 Facilitates Rational Selection of Conjugate Sites

Polyethylene glycol modification (PEGylation) can enhance the pharmacokinetic properties of therapeutic proteins by the attachment of polyethylene glycol (PEG) to the surface of a protein to shield the protein surface from proteolytic degradation and limit aggregation. However, current PEGylation strategies often reduce biological activity, potentially as a result of steric hindrance of PEG. Overall, there are no structure‐based guidelines for selection of conjugate sites that retain optimal biological activity with improved pharmacokinetic properties. In this study, site‐specific PEGylation based on the FGF2‐FGFR1‐heparin complex structure is performed. The effects of the conjugate sites on protein function are investigated by measuring the receptor/heparin binding affinities of the modified proteins and performing assays to measure cell‐based bio‐activity and in vivo stability. Comprehensive analysis of these data demonstrates that PEGylation of FGF2 that avoids the binding sites for fibroblast growth factor receptor 1 (FGFR1) and heparin provides optimal pharmacokinetic enhancement with minimal losses to biological activity. Animal experiments demonstrate that PEGylated FGF2 exhibits greater efficacy in protecting against traumatic brain injury‐induced brain damage and neurological functions than the non‐modified FGF2. This rational structure‐based PEGylation strategy for protein modification is expected to have a major impact in the area of protein‐based therapeutics.     

Enhanced plasminogen activation by staphylokinase in the presence of streptokinase beta/betagamma domains: plasminogen kringles play a role

Presence of isolated beta or betagamma domains of streptokinase (SK) increased the catalytic activity of staphylokinase (SAK)-plasmin (Pm) complex up to 60%. In contrast, fusion of SK beta or betagamma domains with the C-terminal end of SAK drastically reduced the catalytic activity of the activator complex. The enhancement effect mediated by beta or betagamma domain on Pg activator activity of SAK-Pm complex was reduced greatly (45%) in the presence of isolated kringles of Pg, whereas, kringles did not change cofactor activity of SAK fusion proteins (carrying beta or betagamma domains) significantly. When catalytic activity of SAK-microPm (catalytic domain of Pm lacking kringle domains) complex was examined in the presence of isolated beta and betagamma domains, no enhancement effect on Pg activation was observed, whereas, enzyme complex formed between microplasmin and SAK fusion proteins (SAKbeta and SAKbetagamma) displayed 50-70% reduction in their catalytic activity. The present study, thus, suggests that the exogenously present beta and betagamma interact with Pg/Pm via kringle domains and elevate catalytic activity of SAK-Pm activator complex resulting in enhanced substrate Pg activation. Fusion of beta or betagamma domains with SAK might alter these intermolecular interactions resulting in attenuated functional activity of SAK.     

微生物来源的谷氨酰胺转氨酶(mTG)介导的单一定点修饰的PEG IFN-α2a(英文)

PEG修饰被认为是改善重组蛋白药物特性的最有效手段,包括增加蛋白质药物在体内的血浆半衰期,降低免疫原性和抗原性。目前典型的PEG修饰手段为将PEG连接至蛋白质的游离氨基,包括赖氨酸和N-末端,但这种连接缺乏选择性,产物为混合物,活性及工艺稳定性差,难以控制。酶法PEG化修饰能有效克服上述缺点,其中谷氨酰胺转氨酶(TGase)可以作为PEG化定点修饰用酶。文中选择重组人干扰素α2a(IFNα2a)进行酶法修饰反应,通过计算机模拟预测IFNα2a可以在第101位Gln特异性定点修饰。将IFNα2a与40 kDa的Y型PEG在微生物来源的谷氨酰胺转氨酶(mTG)催化下进行定点PEG化修饰。结果显示,mTG可以介导IFNα2a特异性位点Gln的单一定点PEG修饰,产生分子量为58 495.6 Da的PEG-Gln101-IFNα2a分子。圆二色谱结果显示,PEG-Gln101-IFNα2a与未修饰的IFNα2a具有相同的二级结构。SD大鼠药代结果显示,与IFNα2a相比,PEG-Gln101-IFNα2a能有效提高药代动力学参数,强于已上市PEGIFNα2a-PEGASYS?。     

A long-acting, highly potent interferon alpha-2 conjugate created using site-specific PEGylation

Recombinant interferon alpha-2 (IFN-alpha2) is used clinically to treat a variety of viral diseases and cancers. IFN-alpha2 has a short circulating half-life, which necessitates frequent administration to patients. Previous studies showed that it is possible to extend the circulating half-life of IFN-alpha2 by modifying lysine residues of the protein with amine-reactive poly(ethylene glycol) (PEG) reagents. However, amine-PEGylated IFN-alpha2 comprises a heterogeneous product mixture with low specific activity due to the large number and critical locations of lysine residues in IFN-alpha2. In an effort to overcome these problems we determined the feasibility of creating site-specific, mono-PEGylated IFN-alpha2 analogues by introducing a free (unpaired) cysteine residue into the protein, followed by modification of the added cysteine residue with a maleimide-PEG reagent. IFN-alpha2 cysteine analogues were expressed in Escherichia coli and purified, and their in vitro bioactivities were measured in the human Daudi cell line growth inhibition assay. Several cysteine analogues were identified that do not significantly affect in vitro biological activity of IFN-alpha2. Certain of the cysteine analogues, but not wild-type IFN-alpha2, reacted with maleimide-PEG to produce mono-PEGylated proteins. The PEG-Q5C analogue retained high in vitro bioactivity (within 3- to 4-fold of wild-type IFN-alpha2) even when modified with 20- and 40-kDa PEGs. Pharmacokinetic experiments indicated that the 20-kDa PEG-Q5C and 40-kDa PEG-Q5C proteins have 20-fold and 40-fold longer half-lives, respectively, than IFN-alpha2 following subcutaneous administration to rats. These studies demonstrate the feasibility of using site-specific PEGylation technology to create a long-acting, mono-PEGylated IFN-alpha2 protein with high specific activity.     

A solid-phase adsorption method for PEGylation of human serum albumin and staphylokinase: preparation,purification and biochemical characterization

A solid-phase adsorption method was developed to circumvent the disadvantage of the conventional liquid-phase PEGylation, i.e. the heterogeneity of the PEGylated products. The model proteins, human serum albumin (HSA) and staphylokinase (SAK), were adsorbed on the ion exchange chromatography media, followed by PEGylation with succinimidyl carbonate (SC)-mPEG5K and salt elution. Since PEGylation with SC-PEG5K alters the positive charge of the proteins, Q-Sepharose Big Beads and DEAE Sepharose Fast Flow were used for adsorption of HSA and SAK, respectively. Size exclusion chromatography and SDS-PAGE studies demonstrated that solid-phase PEGylation of proteins generate monoPEGylated proteins with the yield of 35–47%. Circular dichroism and intrinsic fluorescence studies showed that solid-phase PEGylation led to little conformational change of the proteins. Solid-phase PEGylation resulted in 35% loss in the biological activity of SAK, which is lower than the liquid-phase PEGylation (70%).     

PEGylation of native disulfide bonds in proteins

PEGylation has turned proteins into important new biopharmaceuticals. The fundamental problems with the existing approaches to PEGylation are inefficient conjugation and the formation of heterogeneous mixtures. This is because poly(ethylene glycol) (PEG) is usually conjugated to nucleophilic amine residues. Our PEGylation protocol solves these problems by exploiting the chemical reactivity of both of the sulfur atoms in the disulfide bond of many biologically relevant proteins. An accessible disulfide bond is mildly reduced to liberate the two cysteine sulfur atoms without disturbing the protein's tertiary structure. Site-specific PEGylation is achieved with a bis-thiol alkylating PEG reagent that sequentially undergoes conjugation to form a three-carbon bridge. The two sulfur atoms are re-linked with PEG selectively conjugated to the bridge. PEGylation of a protein can be completed in 24 h and purification of the PEG-protein conjugate in another 3 h. We have successfully applied this approach to PEGylation of cytokines, enzymes, antibody fragments and peptides, without destroying their tertiary structure or abolishing their biological activity.     

Synthesis of a New Tri-Branched PEG-IFNα2 and Its Impact on Anti Viral Bioactivity

Efficacy of proteins can be enhanced by using polyethylene glycol (PEG) conjugation (PEGylation) to the protein molecules. Mobile non-toxic PEG chains conjugated to bio-therapeutics increase their hydrodynamic volume and in turn can prolong their plasma retention time and increase their solubility. An important aspect of PEGylation is the selection of PEG molecule with suitable structure and molecular weight. In this study, conceiving the idea that branched PEG-conjugates show superior efficacy over the linear PEG-conjugates, a tri-branched PEG-interferon (mPEG₃L₂-IFN) was synthesized by reacting a 30 KDa tri-branched mPEG₃L₂-NHS reagent with IFN to improve its pharmacokinetic properties and reduce the loss of in vitro bioactivity (which is generally exhibited by PEGylation) of the conjugated protein to some extent. The PEGylation procedure was optimized in terms of concentration and molar ratios of reactants, reaction time, temperature and pH conditions of the reaction mix. The conjugate was purified by cation exchange chromatography and characterized by SDS-PAGE and SE-HPLC. Trypsin digestion of mPEG₃L₂-IFN indicated a single site specificity of PEGylation. Anti viral bioactivity of mPEG₃L₂-IFN was found to be 2.38 × 10⁷ IU/mg which is approximately 9.52% of native IFNα2 (2.5 × 10⁸ IU/mg) and better than PEGasys from Roche Pharma. Therefore, it is reported that the tri-branched mPEG₃L₂-NHS reagent has the potential to be used to conjugate proteins for the promising therapeutic results.     

Reducing the immunogenicity and improving the in vivo activity of trichosanthin by site-directed pegylation.

PEG modification (PEGylation) has been shown to reduce immunogenicity and prolong circulating half-life of proteins. In the present study, site-directed PEGylation was used to reduce immunogenicity and prolong plasma half-life of trichosanthin (TCS). Four TCS mutants, i.e. S7C, Q219C, K173C and [K173C,Q219C] (KQ), were constructed by site-directed mutagenesis. PEG modifications were done by reacting PEG5k-maleimide or PEG20k-maleimide reagent with the newly introduced cysteine residue of the mutants. The plasma clearance rate of PEGylated TCS mutants decreased up to 100-fold and the decrease was inversely proportional to the effective molecular size. The in vitro activities such as ribosome-inactivating activity and cytotoxicity were also decreased. However, the in vivo abortifacient activity was, slightly decreased, unchanged, or even enhanced in some preparations. PEG5k modification had little effect on immunogenicity. However, PEG20k modification significantly reduced immunogenicity. All PEG20k modified TCS mutants induced lower level IgG and IgE antibodies. In particular, PEG20k-KQ and PEG20k-K173C induced weaker systemic anaphylaxis reaction in guinea pigs. In conclusion, the present results suggest that PEG20k is better than PEG5k for reducing immunogenicity and prolonging plasma half-life. The conjugate can become a better therapeutic agent.     

Model‐Based Investigation on the Mass Transfer and Adsorption Mechanisms of Mono‐Pegylated Lysozyme in Ion‐Exchange Chromatography

Recent studies highlighted the potential of PEGylated proteins to improve stabilities and pharmacokinetics of protein drugs. Ion‐exchange chromatography (IEX) is among the most frequently used purification methods for PEGylated proteins. However, the underlying physical mechanisms allowing for a separation of different PEGamers (proteins with a varying number of attached PEG molecules) are not yet fully understood. In this work, mechanistic chromatography modeling is applied to gain a deeper understanding of the mass transfer and adsorption/desorption mechanisms of mono‐PEGylated proteins in IEX. Using a combination of the general rate model (GRM) and the steric mass action (SMA) isotherm, simulation results in good agreement with the experimental data are achieved. During linear gradient elution of proteins attached with PEG of different molecular weight, similar peak heights, and peak shapes at constant gradient length are observed. A superimposed effect of increased desorption rate and reduced diffusion rate as a function of the hydrodynamic radius of PEGylated proteins is identified to be the reason of this anomaly. That is why the concept of the diffusion‐desorption‐compensation effect is proposed. In addition to the altered elution orders, PEGylation results in a considerable decrease of maximum binding capacity. By using the SMA model in a kinetic formulation, the adsorption behavior of PEGylated proteins in the highly concentrated state is described mechanistically. An exponential increase in the steric hindrance effect with increasing PEG molecular weight is observed. This suggests the formation of multiple PEG layers in the interstitial space between bound proteins and an associated shielding of ligands on the adsorber surface to be the cause of the reduced maximum binding capacity. The presented in silico approach thus complements the hitherto proposed theories on the binding mechanisms of PEGylated proteins in IEX.     

Carbohydrate-specifically polyethylene glycol-modified ricin A-chain with improved therapeutic potential

Ricin A-chain, which exhibits excellent cytotoxicity to tumor cells, has been widely used as an immunotoxin source. However, it has the fatal shortcoming of poor pharmacokinetics due to the tremendous liver uptake via carbohydrate-mediated recognition. Modification of proteins with polyethylene glycol, PEGylation, has the advantages of shielding the specific sites and prolonging the biological half-life. In this study, the carbohydrate-specific PEGylation of ricin A-chain was considered to be a novel approach to overcome this limitation. The carbohydrate group of ricin A-chain was oxidized by sodium m-periodate and further specifically conjugated with hydrazide-derivatized PEG. For a comparative study, the PEGylated ricin A-chain at amino groups was prepared using the hydroxysuccinimide ester-derivatized PEG. The carbohydrate-specifically PEGylated ricin A-chain showed a markedly lower liver uptake and systemic clearance compared with the amine-directly PEGylated ricin A-chain as well as the unmodified ricin A-chain. Furthermore, carbohydrate-specifically PEGylated ricin A-chain showed a significantly higher in vitro ribosome-inactivating activity than the amine-directly PEGylated ricin A-chain. These findings clearly demonstrate that the carbohydrate-specificity as well as PEGylation plays an important role in improving the in vivo pharmacokinetic properties and in vitro bioactivity. Therefore, these results suggest that the therapeutic use of immunotoxins constructed using this carbohydrate-specifically PEGylated ricin A-chain has potential as a cancer therapy.     

Site-specific PEGylation of engineered cysteine analogues of recombinant human granulocyte-macrophage colony-stimulating factor

Granulocyte macrophage colony-stimulating factor (GM-CSF) stimulates proliferation of hematopoietic cells of the macrophage and granulocyte lineages and is used clinically to treat neutropenia and other myeloid disorders. Because of its short circulating half-life, GM-CSF is administered to patients by daily injection. We describe here the engineering of highly potent, long-acting human GM-CSF proteins through site-specific modification of GM-CSF cysteine analogues with a cysteine-reactive poly(ethylene glycol) (PEG) reagent. Thirteen cysteine analogues of GM-CSF were constructed, primarily in nonhelical regions of the protein believed to lie away from the major receptor binding sites. The GM-CSF cysteine analogues were properly processed but insoluble following secretion into the Escherichia coli periplasm. The proteins were refolded and purified by column chromatography. Ten of the cysteine analogues could be modified with a 5-kDa maleimide PEG, and seven of the mono-PEGylated proteins were purified by ion-exchange column chromatography. Biological activities of the 13 cysteine analogues and 7 PEGylated cysteine analogues were comparable to that of wild-type GM-CSF in an in vitro cell proliferation assay using human TF-1 cells. One cysteine analogue was modified with larger 10-, 20-, and 40-kDa PEGs, with only minimal loss of in vitro bioactivity. Pharmacokinetic experiments in rats demonstrated that the PEGylated proteins had up to 47-fold longer circulating half-lives than wild-type GM-CSF. These data demonstrate the utility of site-specific PEGylation for creating highly potent, long-acting GM-CSF analogues and provide further evidence that the nonhelical regions of human GM-CSF examined are largely nonessential for biological activity of the protein.     

Reversible protection of Cys-93(β) by PEG alters the structural and functional properties of the PEGylated hemoglobin

As a potential hemoglobin (Hb)-based oxygen carrier (HBOC), the PEGylated Hb has received much attention for its non-nephrotoxicity. However, PEGylation can adversely alter the structural and functional properties of Hb. The site of PEGylation is an important factor to determine the structure and function of the PEGylated Hb. Thus, protection of some sensitive residues of Hb from PEGylation is of great significance to develop the PEGylated Hb as HBOC. Here, Cys-93(β) of Hb was conjugated with 20 kDa polyethylene glycol (PEG20K) through hydrazone and disulfide bonds. Then, the conjugate was modified with PEG5K succinimidyl carbonate (PEG5K-SC) using acylation chemistry, followed by removal of PEG20K Hb with hydrazone hydrolysis and disulfide reduction. Reversible conjugation of PEG20K at Cys-93(β) can protect Lys-95(β), Val-1(α) and Lys-16(α) of Hb from PEGylation with PEG5K-SC. The autoxidation rate, oxygen affinity, structural perturbation and tetramer instability of the PEGylated Hb were significantly decreased upon protection with PEG20K. The present study is expected to improve the efficacy of the PEGylated Hb as an oxygen therapeutic.     

Effect of oriented or random PEGylation on bioactivity of a factor VIII inhibitor blocking peptide

Peptides as therapeutic substances are efficient agents in the treatment of several diseases. However, they often have to be chemically modified in order to be suited as therapeutic agent. Conjugation to large carrier molecules is often required. A critical step is the identification of available sites for chemical reaction, without influencing bioactivity. Peptide 238/S1 with the sequence NH(2)-PYWKWQYKYD-COOH previously selected from a combinatorial decapeptide library, has the ability to block inhibitory antibodies against blood clotting factor VIII (FVIII) and therefore, it constitutes a lead for developing a drug to treat patients suffering from development of such antibodies. The aims of this study were (i) to identify sites of the peptide, which are suited for modification without losing bioactivity and (ii) to find out the influence of molecular size of polyethylene glycol (PEG) for bioactivity of the peptide. The contribution of each amino acid residue to biological functionality was investigated by mutational analysis. This method confirmed that the N-terminus is crucial for activity, whereas both lysine residues could be exchanged by other L-amino acids. Using mutational analysis it was possible to identify peptides with higher reactivity compared to the wild type 238/S1. PEGylation experiments demonstrated that conjugation of the peptide to PEG 20,000 resulted in a loss of reactivity, while PEG 5,000 could maintain the bioactivity when conjugated in a site directed manner. The peptide lost its neutralization properties when PEG was coupled to the N-terminus, again indicating that this part of the peptide is important for functionality.     

Lysine-deficient lymphotoxin-α mutant for site-specific PEGylation

The cytokine lymphotoxin-α (LTα) is a promising anticancer agent; however, its instability currently limits its therapeutic potential. Modification of proteins with polyethylene glycol (PEGylation) can improve their in vivo stability, but PEGylation occurs randomly at lysine residues and the N-terminus. Therefore, PEGylated proteins are generally heterogeneous and have lower bioactivity than their non-PEGylated counterparts. Previously, we created phage libraries expressing mutant LTαs in which the lysine residues of wild-type LTα (wtLTα) were substituted for other amino acids. Here, we attempted to create a lysine-deficient mutant LTα with about the same bioactivity as wtLTα by using these libraries and site-specific PEGylation of the N-terminus. We isolated a lysine-deficient mutant LTα, LT-K0, with almost identical bioactivity to that of wtLTα against mouse LM cells. The bioactivity of wtLTα decreased to 10% following random PEGylation, whereas that of LT-K0 decreased to 50% following site-specific PEGylation; PEGylated LT-K0 retained five times the bioactivity of randomly PEGylated wtLTα. These results suggest that site-specific PEGylated LT-K0 may be useful in cancer therapy.