The conjugation of proteins with polyethylene glycol and other polymers

Effect of lyophilization on the structure and phase changes of PEGylated-bovine serum albumin

Poly (ethylene glycol) (PEG) conjugation masks the protein's surface and increases the molecular size of the polypeptide, thus preventing the approach of antibodies or antigen processing cells and reducing the degradation by proteolytic enzymes. Proteins are readily denatured by numerous stresses arising in solution (e.g., heating, agitation, freezing and pH changes) or by chemical reactions (e.g., hydrolysis and deamidation), many of which are mediated by water. Lyophilization is most commonly used to prepare dehydrated proteins, which, theoretically, should have the desired long-term stability at ambient temperatures. Through Raman spectroscopy, differential scanning calorimetry (DSC) associated with the determination of water content by Karl Fisher titration, it was observed that after the modification of BSA-PEG in a ratio of 1:0.25 showed lower degree of structural alterations and consequently lower variation on the physical-chemical characteristics when it was compared to BSA-PEG (1:0.5). Moreover, the BSA-PEG (1:0.25) optimizes the conditions during the lyophilization process and storage of the protein.

Biopharmaceuticals: recent approvals and likely directions

Some 160 biopharmaceuticals have now gained medical approval and several hundred are in the pipeline. Most are protein-based, although two nucleic acid-based products are now on the US/European market. An increasing proportion of approvals are engineered in some way and advances in alternative production systems and delivery methods will also likely impact upon the approvals profile over the remainder of this decade.

Modulation of cell membrane disruption by pH-responsive pseudo-peptides through grafting with hydrophilic side chains

The effect of grafting an amphiphilic pseudo-peptide, poly (L-lysine iso-phthalamide), with poly (ethylene glycol) or a hydrophilic poly (ethylene glycol) analogue, Jeffamine M-1000, on the pH-dependent erythrolytic activity and in vitro cytotoxicity have been studied together with the concentration-dependent haemolysis of the polymers with different degrees of grafting. PEGylated polymers showed pH-dependent membrane-disruptive ability similar to the parent poly (L-lysine iso-phthalamide). The polymers showed a better ability to haemolyse the erythrocyte membrane at mildly acidic pHs with increasing degree of PEGylation (up to 17.0 wt.%). Further increasing the degree of PEGylation resulted in a decrease in haemolytic ability. Grafting poly (L-lysine iso-phthalamide) with the lower molecular weight Jeffamine M-1000 had little effect on the haemolytic ability. Finally, the in vitro cytotoxicity of the grafted polymers was assessed by MTT assay, LDH assay and viable cell counts. At pH 7.4, these polymers were well tolerated by a range of mammalian cell lines and grafting reduced the cytotoxicity of polymers. However, at pH 5.5, relative to poly (L-lysine iso-phthalamide), the grafted polymers displayed a better ability to rupture the outer membranes of these cells.

Reproducible preparation and effective separation of PEGylated recombinant human granulocyte colony-stimulating factor with novel “PEG-pellet” PEGylation mode and ion-exchange chromatography

A novel preparation for polyethylene glycol (PEG) derivatives and chromatographic separation procedure of the PEGylated recombinant human granulocyte colony-stimulating factor (rhG-CSF) were designed to evaluate the reproducibility and scalability at large laboratory-scale level. The new "PEG-pellet" PEGylation mode was successfully applied to control the pH fluctuation during the conjugation reaction, a general problem in traditional liquid-phase conjugation mode. Moreover, two consecutive ion-exchange chromatography steps were successfully used to separate and purify the PEGylated rhG-CSF. Cation-exchange chromatography was firstly applied to separate PEGylated rhG-CSF from intact rhG-CSF, followed by anion-exchange chromatography to obtain individual PEG-rhG-CSF species (mono-, di- and tri-PEGylated rhG-CSF) and remove the excess free PEG. Furthermore, the molecular weight of individual PEGylated rhG-CSF was identified by matrix-assisted laser desorption/ionization-time of flight (MALDI-TOF) mass spectrometry and SDS-PAGE, and cell proliferation activity in vitro was detected by MTT assay using NFS-60 cell.

Reversible PEGylation of peptide YY3-36prolongs its inhibition of food intake in mice

Administration of peptide YY(3-36) (PYY(3-36)) to fasting humans or mice shortly before re-feeding effectively reduced their food intake, but PYY(3-36) exhibited a functional half-life of only approximately 3 h. Attachment of poly(ethylene glycol) to proteins and peptides (PEGylation) prolongs their half-life in vivo, but completely inactivated PYY(3-36). We developed a reversibly PEGylated PYY(3-36) derivative by coupling it to a 40 kDa PEG through a spontaneously cleavable linker. The resulting conjugate (PEG(40)-FMS-PYY(3-36)) gradually released unmodified PYY(3-36) in vivo, exhibiting an eightfold increase in its functional half-life, to approximately 24h. This long-acting PYY(3-36) pro-drug may serve as an effective means for controlling food intake in humans.

C-terminal site-specific PEGylation of a truncated thrombomodulin mutant with retention of full bioactivity

Addition of poly(ethylene glycol) to bioactive proteins (PEGylation) improves their plasma half-life, enhances stability against proteolytic cleavage, and may also decrease protein immunogenicity. Characteristically, PEGylation usually involves a reaction to available lysine amino groups, some of which may be within or near a bioactive site. Thus, most protocols are nonspecific and result in a loss of protein activity. We report herein a strategy for site-specific PEGylation of a thrombomodulin (TM) derivative at the C terminus. A truncated TM mutant consisting of epidermal growth factor (EGF)-like domains 4-6 was expressed in Escherichia coli with a C-terminal azido-methionine. The TM mutant was site-specifically conjugated to a methyl-PEG-triarylphosphine compound via the Staudinger reaction. Enzymatic activity of the TM construct before and after PEGylation was unchanged, which confirms the utility of this site-specific PEGylation scheme.

Synthesis of bis- and tris-branched COOH-terminal pegylating reagents: conjugation to NH-terminal peptides

In previous studies we reported an orthogonal protection scheme that was developed for the solution-phase synthesis of a family of bis- and tris-pegylating reagents which contain a free NH(2)-terminus. These pegylating reagents were coupled to the COOH-terminus of a model peptide. In the present study we report on the solution synthesis of a novel family of bis- and tris-pegylating reagents which contain a free COOH-terminus. To illustrate their general utility, conditions were developed for the coupling of these novel pegylating reagents to the NH(2)-function of a model pentapeptide. Taken together, our studies demonstrate that these pegylating reagents are well suited for conjugation to peptides and proteins that contain either free COOH- or NH(2)-functions. These reagents may have general utility in therapeutic development as branched pegylation has been shown to provide more effective protection of proteins from proteolysis by shielding the protein surface from approaching macromolecules.

Recent Advances in Chemistry and Applications of Substituted Poly(ethylene glycol)s

Poly(ethylene glycol)s are well defined and easily accessible macromolecules with suitable properties for applications in chemistry, biotechnologies and medicine. The terminal hydroxy groups of poly(ethylene glycol)s can easily be converted into reactive functional groups by a number of routine reactions of organic chemistry. The chemical reagents or catalysts anchored to poly(ethylene glycol) chains were used in a number of syntheses including the enantioselective ones. Poly(ethylene glycol)s serve as carriers in combinatorial syntheses in the liquid phase. Coupling of poly(ethylene glycol)s with other polymers was used to prepare series of block copolymers having numerous applications. From the point of view of medical applications it is significant that substituted poly(ethylene glycol)s are non-toxic and resistant to recognition by the immunity system. That is why they are often used as carriers of many low-molecular-weight as well as high-molecular-weight medical drugs (drug delivery systems). In the conjugates with drugs their biological activity increases and their toxicity decreases. A review with 41 references.

Immune reactions of lymphocytes and macrophages against PEG-grafted pancreatic islets

Graft rejection is the major limiting factor in islet transplantation and is closely related with the recruitment and activation of T cells and macrophages against the graft. To reduce the immunogenicity of islets, we have grafted biocompatible polyethylene glycol (PEG) onto the collagen capsule of islets without changing the morphology and function of islets. In this study, we evaluated whether the grafted PEG molecules on the collagen capsule of islet could prevent the activation of immune cells, and investigated factors that are mainly related to the immune reaction in vitro. During the co-culture with lymphocytes, the morphology and viability of PEG-grafted islets were not damaged, and the amounts of IL-2 and TNF-alpha secreted from lymphocytes co-cultured with PEG-grafted islets were significantly lower than that of free islets. However, when both kinds of islets were cultured with macrophages, there were no significant differences in morphology, viability and the secreted amounts of cytokines and nitric oxide. In conclusion, the grafted PEG could inhibit activation of lymphocytes, which are essential in initiating the graft rejection process. However, the grafted PEG molecules could not completely prevent the infiltration of cytotoxic molecules into the islets.

Prolonging the Action of Protein and Peptide Drugs by a Novel Approach of Reversible Polyethylene Glycol Modification

Polyethylene glycol (PEG)-conjugated therapeutic peptides/proteins have been shown to exhibit clinical properties superior to those of their corresponding unmodified parent molecules. However, the desirable pharmacological features gained by protein PEGylation become irrelevant if conjugates are inactivated or cannot reach their target tissues. Here we describe the design and synthesis of MAL-FMS-OSU. This bifunctional agent enables PEG chains to be linked to peptides and proteins through a slowly hydrolysable chemical bond. PEG-FMS-peptide/protein conjugates thus formed undergo spontaneous hydrolysis at a slow rate upon incubation at pH 8.5, 37 degrees C with a t(1/2) value of 8-14 +/- 2 h, generating the unmodified parent molecule. The validity of this approach was studied with exendin-4 and human growth hormone. A single subcutaneous administration of PEG(40,000)-FMS-exendin-4 facilitated a prolonged and stable reduction in glucose levels in mice (t(1/2) = 30 +/- 2 h) and exceeded the effect obtained by the same dose of the native hormone by 7-8 times.

Separation of polyethylene glycol-modified proteins by open tubular capillary electrochromatography

This study involves the characterization of six polyethylene glycol-modified proteins by open tubular capillary electrochromatography, a high-resolution, versatile and reproducible technique for the analysis of biomolecules and pharmaceuticals. Optimized conditions were obtained with respect to type of capillary modification (cholesterol and octadecyl), applied voltage (+20 and -20 kV), buffer pH (2.14-8.14) and addition of methanol modifier to the mobile phase. Electrochromatograms were obtained with both cathodic and anodic applied electric fields. In the case of one PEG-protein, superoxide dismutase, a comparison was made to a previous study. Reproducibility and column lifetime were also evaluated in assessing the usefulness of the method.

Molecularly designed water soluble, intelligent, nanosize polymeric carriers

Intelligent polymers, also referred as "stimuli-responsive polymers" undergo strong property changes (in shape, surface characteristics, solubility, etc.) when only small changes in their environment (changes in temperature, pH, ionic strength light, electrical and magnetic field, etc.). They have been used in several novel applications, drug delivery systems, tissue engineering scaffolds, bioseparation, biomimetic actuators, etc. The most popular member of these type of polymers is poly(N-isopropylacrylamide) (poly(NIPA)) which exhibits temperature-sensitive character, in which the polymer chains change from water-soluble coils to water-insoluble globules in aqueous solution as temperature increases above the lower critical solution temperature (LCST) of the polymer. Copolymerization of NIPA with acrylic acid (AAc) allows the synthesis of both pH and temperature-responsive copolymers. This paper summarizes some of our related studies in which NIPA and its copolymers were synthesized and used as intelligent carriers in diverse applications.

Synthesis of symmetrically and asymmetrically branched pegylating reagents

A solution-phase procedure using an orthogonal protection scheme was developed for the synthesis of a novel family of multi-pegylating reagents. The procedure was exemplified by the synthesis of bis- and tris-pegylating reagents prepared by stepwise insertion of the poly(ethylene glycol) units thereby enabling the preparation of both symmetrical and asymmetrical pegylating reagents. Asymmetrical pegylation and tris-pegylation of peptides and proteins introduces new variables for use in the optimization of pegylated peptides and proteins. These reagents are ideally suited for conjugation to peptides and proteins as they possess a required functional group and will be useful intermediates for the synthesis of a new generation of pegylated products. Tris-pegylation can also provide more effective protection from proteolysis by shielding the protein surface from approaching macromolecules. To illustrate this potential, conditions were developed for the successful coupling of the tris-pegylating reagent to a model pentapeptide.

Highly Effective Poly(Ethylene Glycol) Architectures for Specific Inhibition of Immune Receptor Activation†

Architectural features of synthetic ligands were systematically varied to optimize inhibition of mast cell degranulation initiated by multivalent crossing of IgE-receptor complexes. A series of ligands were generated by end-capping poly(ethylene glycol) (PEG) polymers and amine-based dendrimers with the hapten 2,4-dinitrophenyl (DNP). These were used to explore the influence of polymeric backbone length, valency, and hapten presentation on binding to anti-DNP IgE and inhibition of stimulated activation of RBL cells. Monovalent MPEG(5000)-DNP (IC(50) = 50 nM), bivalent DNP-PEG(3350)-DNP (IC(50) = 8 nM), bismonovalent MPEG(5000)-DNP(2) (IC(50) = 20 nM), bisbivalent DNP(2)-PEG(3350)-DNP(2) (IC(50) = 3nM) and DNP(4)-dendrimer ligands (IC(50) = 50 nM) all effectively inhibit cellular activation caused by multivalent antigen, DNP-bovine serum albumin. For different DNP ligands, we provide evidence for more effective inhibition due to (i) preferential formation of intra-IgE cross-links by bivalent ligands of sufficient length, (ii) self-association of monovalent ligands with longer tails, and (iii) higher probability of binding for bisvalent ligands. We also show that larger DNP(16)-dendrimers of higher valency trigger degranulation by cross-linking IgE-receptor complexes, whereas smaller DNP-dendrimers are inhibitory. Thus, features of synthetic ligands can be manipulated to control receptor occupation, aggregation, and inhibition of the cellular response.

Multivalent Poly(ethylene glycol)-Containing Conjugates for In Vivo Antibody Suppression

Poly(ethylene glycol) (PEG) was incorporated into multivalent conjugates of the N-terminal domain of beta(2)GPI (domain 1). PEG was incorporated to reduce the rate of elimination of the conjugates from plasma and to putatively improve their efficacy as toleragens for the suppression of anti-beta(2)GPI antibodies and the treatment of antiphospholipid syndrome (APS). Three structurally distinct types of multivalent platforms were constructed by incorporating PEG into the platform structures in different ways. The amount of PEG incorporated ranged from about 5000 g per mole to about 30000 g per mole. The platforms were functionalized with either four or eight aminooxy groups. The conjugates were prepared by forming oxime linkages between the aminooxy groups and N-terminally glyoxylated domain 1 polypeptide. The plasma half-life of each conjugate, labeled with (125)I, was measured in both mice and rats. The half-lives of the conjugates ranged from less than 10 min to about 1 h in mice, and from less than 3 h to about 19 h in rats. The ability of five tetravalent conjugates to suppress anti-domain 1 antibodies in immunized rats was also measured. Incorporation of PEG in the conjugates significantly reduced the doses required for suppression, and the amount of reduction correlated with the amount of PEG incorporated.

The sustained granulopoietic effect of progenipoietin encapsulated in multivesicular liposomes

Progenipoietin (ProGP), a dual receptor agonist of fetal liver tyrosine kinase-3 (flt3) and granulocyte colony-stimulating factor (G-CSF) receptors, has been shown to significantly enhance production of both polymorphonuclear leukocytes and dendritic cells (DCs) in the peripheral blood and spleen of mice, when administered as daily s.c. injections for about 10 days. Here, we have successfully designed a sustained-delivery formulation for this novel chimeric protein using multivesicular liposomes (DepoFoam), and studied the effects of changing both the triglyceride and phospholipid composition of the lipid matrix to modulate its delivery profile. Encapsulation of ProGP in these particles led to retention of its structural integrity, and maintenance of its biological activity in vivo. Administration of a single s.c. dose of 1mg/kg of an optimized DepoProGP formulation in rats, led to significant elevation of absolute neutrophil counts (ANC) that were maintained at levels >10,000 microliter(-1) for 9-11 days, in a reproducible manner. In contrast, administration of the unencapsulated ProGP at the same dose, resulted in elevation of neutrophils by day 1, followed by a quick decline to base line levels by day 3. These data suggest the possibility of administering a single dose of DepoFoam-encapsulated ProGP to improve hematopoietic recovery time after chemotherapy, and for other indications that require multiple daily doses of ProGP.

Design, construction, and in vitro analyses of multivalent antibodies

Some Abs are more efficacious after being cross-linked to form dimers or multimers, presumably as a result of binding to and clustering more surface target to either amplify or diversify cellular signaling. To improve the therapeutic potency of these types of Abs, we designed and generated Abs that express tandem Fab repeats with the aim of mimicking cross-linked Abs. The versatile design of the system enables the creation of a series of multivalent human IgG Ab forms including tetravalent IgG1, tetravalent F(ab')2, and linear Fab multimers with either three or four consecutively linked Fabs. The multimerized Abs target the cell surface receptors HER2, death receptor 5, and CD20, and are more efficacious than their parent mAbs in triggering antitumor cellular responses, indicating they could be useful both as reagents for study as well as novel therapeutics.

Effect of pegylation on pharmaceuticals

Protein and peptide drugs hold great promise as therapeutic agents. However, many are degraded by proteolytic enzymes, can be rapidly cleared by the kidneys, generate neutralizing antibodies and have a short circulating half-life. Pegylation, the process by which polyethylene glycol chains are attached to protein and peptide drugs, can overcome these and other shortcomings. By increasing the molecular mass of proteins and peptides and shielding them from proteolytic enzymes, pegylation improves pharmacokinetics. This article will review how PEGylation can result in drugs that are often more effective and safer, and which show improved patient convenience and compliance.

Effective drug delivery by PEGylated drug conjugates

The current review presents an update of drug delivery using poly(ethylene glycol) (PEG), that focuses on recent developments in both protein and organic drugs. Certainly the past 10 years has resulted in a renaissance of the field of PEG drug conjugates, initiated by the use of higher molecular weight PEGs (M(w)>20,000), especially 40,000 which is estimated to have a plasma circulating t(1/2) of approximately 10 h in mice. This recent resuscitation of small organic molecule delivery by high molecular weight PEG conjugates was founded on meaningful in vivo testing using established tumor models, and has led to a clinical candidate, PEG-camptothecin (PROTHECAN), an ester based prodrug currently in phase II trials. Additional applications of high molecular weight PEG prodrug strategies to amino containing drugs are presented: similar tripartate systems based on lower M(w) PEG and their use with proteins is expounded on. The modification of a benzyl elimination tripartate prodrug specific for mercaptans is presented, and its successful application to 6-mercaptopurine giving a water soluble formulation is discussed. Recent novel PEG oligonucleotides and immunoconjugates are also covered. Clinical results of FDA approved PEGylated proteins are also presented.

Poly(ethylene glycol)-avidin bioconjugates: suitable candidates for tumor pretargeting

Avidin-poly(ethylene glycol) (PEG) conjugates were obtained by derivatization of about 10% of the protein amino groups (four amino groups per protein molecule) with linear 5 kDa PEG or branched 10 or 20 kDa PEGs. Circular dichroism analysis showed that the polymer conjugation neither altered the protein structure nor the environment of the aromatic amino acids which are present at the level of the biotin binding site. Spectroscopic studies were carried out to evaluate the biotin recognition activity of the conjugates either in terms of number of biotin binding sites or avidin/biotin affinity. Avidin-PEG 5 kDa and avidin-PEG 10 kDa displayed over 90% of the native protein biological activity while a reduction in the recognition of biotinylated antibodies of about 25% was found with PEG 20 kDa. In vivo studies demonstrated that the protein immunogenicity was in the order: wild type avidin>avidin-PEG 5 kDa>avidin-PEG 10 kDa>avidin-PEG 20 kDa. By intravenous injection into mice bearing a solid tumor, all conjugates displayed prolonged permanence in the circulation with respect to the native protein. The area under the curve values of avidin-PEG 5 kDa, avidin-PEG 10 kDa and avidin-PEG 20 kDa were about 3-, 7- and 30-times higher than the wild type avidin with reduced accumulation in kidneys and liver. Interestingly, all conjugates accumulated significantly in the tumor mass. In particular, in the case of avidin-PEG 20 kDa, 8% of the injected dose (ID)/g of tissue accumulated in the tumor after 5 h from the administration and over 6% of the ID/g was maintained throughout 72 h.

Novel poly(ethylene glycol) derivatives for preparation of ribosome-inactivating protein conjugates

This study describes the synthesis, characterization, and reactivity of new methoxypoly(ethylene glycol) (mPEG) derivatives containing a thioimidoester reactive group. These activated polymers are able to react with the lysyl epsilon-amino groups of suitable proteins, generating an amidinated linkage and thereby preserving the protein's positive charge. mPEG derivatives of molecular weight 2000 and 5000 Da were used, and two spacer arms were prepared, introducing chains of different lengths between the hydroxyl group of the polymer and the thioimidate group. These mPEG derivatives were used to modify gelonin, a cytotoxic single-chain glycoprotein widely used in preparation of antitumoral conjugates, whose biological activity is strongly influenced by charge modification. The reactivity of mPEG thioimidates toward lysil epsilon-amino groups of gelonin was evaluated, and the results showed an increased degree of derivatization in proportion to the molar excesses of the polymer used and to the length of the alkyl spacer. Further studies showed that the thioimidate reactive is able to maintain gelonin's significant biological activity and immunogenicity. On the contrary, modification of the protein with N-hydroxysuccinimide derivative of mPEG strongly reduces the protein's cytotoxic activity. Evaluation of the pharmacokinetic behavior of native and PEG-grafted gelonin showed a marked increase in plasma half-life after protein PEGylation; in particular, the circulating life of the conjugates increased with increased molecular weight of the polymer used. The biodistribution test showed lower organ uptake after PEGylation, in particular by the liver and spleen.

Poly(ethylene glycol) block copolymers

The ubiquitous use of poly(ethylene glycol) in the biomaterials field has also boosted the research activity in the chemical derivatization of this polymer. We focused our interest on the preparation of tailor-made poly(ethylene glycol)-based structures and on the study of structure-activity relationships for its functionalization, as preliminary steps for the preparation of smart functional materials. More specifically, amphiphilic and cationic block copolymers were prepared for prospective use in the preparation of self-assembled carriers, and Michael-type addition of thiols onto acrylates was studied as a model for end-group reaction leading to hydrogel formation.

Pegylation: a novel process for modifying pharmacokinetics

The use of liposomal carriers and the modification of therapeutic molecules through the attachment of poly(ethylene glycol) [PEG] moieties ('pegylation') are the most common approaches for enhancing the delivery of parenteral agents. Although 'classical' liposomes (i.e. phospholipid bilayer vehicles) have been effective in decreasing the clearance of encapsulated agents and in passively targeting specific tissues, they are associated with considerable limitations. Pegylation may be an effective method of delivering therapeutic proteins and modifying their pharmacokinetic properties, in turn modifying pharmacodynamics, via a mechanism dependent on altered binding properties of the native protein. Pegylation reduces renal clearance and, for some products, results in a more sustained absorption after subcutaneous administration as well as restricted distribution. These pharmacokinetic changes may result in more constant and sustained plasma concentrations, which can lead to increases in clinical effectiveness when the desired effects are concentration-dependent. Maintaining drug concentrations at or near a target concentration for an extended period of time is often clinically advantageous, and is particularly useful in antiviral therapy, since constant antiviral pressure should prevent replication and may thereby suppress the emergence of resistant variants. Additionally, PEG modification may decrease adverse effects caused by the large variations in peak-to-trough plasma drug concentrations associated with frequent administration and by the immunogenicity of unmodified proteins. Pegylated proteins may have reduced immunogenicity because PEG-induced steric hindrance can prevent immune recognition. Two PEG-modified proteins are currently approved by the US Food and Drug Administration; several others, including cytokines such as interferon-alpha (IFNalpha), growth factors and free radical scavengers, are under development. Careful assessment of various pegylated IFNalpha products suggests that pegylated molecules can be differentiated on the basis of their pharmacokinetic properties and related changes in pharmacodynamics. Because the size, geometry and attachment site of the PEG moiety play a crucial role in determining these properties, therapeutically optimised agents must be designed on a protein-by-protein basis.

Stimuli-responsive properties of conjugates of N-isopropylacrylamide-co-acrylic acid oligomers with alanine, glycine and serine mono-, di- and tri-peptides

A random oligomer of N-isopropylacrylamide (NIPAAm) and acrylic acid (AAc) with a AAc content of 3.1+/-0.19 mmol carboxylic acid groups per gram of the oligomer and with a number average molecular weight of 1400 was synthesised by a free radical polymerisation using AIBN in DMF. Then, mono-, di-, and tri-peptide conjugates of this oligomer were prepared by using carboxyl-ends-protected (with methyl ester hydrochloride) forms of alanine, glycine and serine, with a water-soluble carbodiimide. 95, 93, and 31% of the carboxylic acids were conjugated (loaded) at the first step (mono-peptides) with glycine, alanine and serine, respectively. At the second step, percentage of the conjugation of carboxylic acid groups with glycine, alanine and serine were between 99 and 80, 68 and 100, and 21 and 58%, respectively, while the third amino acids were attached to only 21-64% of the carboxylic acids available on the conjugate chains. A decrease was observed in the lower critical solution temperatures (LCSTs) of the amino acid conjugates at pH 4.0 compared with the unconjugated oligomer, which has LCST at 37.7 degrees C at the same pH. LCSTs of di- and tri-peptide conjugates at pH 4.0 were in the range of 38.4-43.3 degrees C, and 42.6-50.8 degrees C, respectively. At pH 7.4, LCSTs of the mono- and di-peptide conjugates were observed in the range of 41.6-43.9 degrees C, and 46.2-60.2 degrees C, respectively, while the co-oligomer at pH 7.4 did not show a LCST up to 60 degrees C. Tri-peptide conjugates did not display LCST at pH 7.4, except the one with glycine-alanine-serine sequence.

Peptide and protein PEGylation: a review of problems and solutions

The paper discusses general problems in using PEG for conjugation to high or low molecular weight molecules. Methods of binding PEG to different functional groups in macromolecules is reported together with their eventual limitations. Problems encountered in conjugation, such as the evaluation of the number of PEG chains bound, the localisation of the site of conjugation in polypeptides and the procedure to direct PEGylation to the desired site in the molecule are discussed. Finally, the paper reports on more specific methods regarding reversible PEGylation, cross-linking reagents with PEG arms, PEG for enzyme solubilization in organic solvent and new polymers as alternative to PEG.

Drug delivery systems employing 1,6-elimination: releasable poly(ethylene glycol) conjugates of proteins

Using lysozyme as a representative protein substrate that loses its activity when PEGylation takes place on the epsilon-amino group of lysine residues, various amounts of a novel releasable PEG linker (rPEG) were conjugated to the protein. rPEG-lysozyme conjugates were relatively stable in pH 7.4 buffer for over 24 h. However, regeneration of native protein from the rPEG conjugates occurred in a predictable manner during incubation in high pH buffer or rat plasma, as demonstrated by enzymatic activity and structural characterization. The rates of regeneration were also correlated with PEG number: native lysozyme was released more rapidly from the monosubstituted conjugate than from the disubstituted conjugate, suggesting possible steric hindrance to the approach of cleaving enzymes. Recovery of normal activity and structure for the regenerated native lysozyme was shown by a variety of assays.

Use of ion-exchange chromatography and hydrophobic interaction chromatography in the preparation and recovery of polyethylene glycol-linked proteins

Cation- and anion-exchange chromatography can be used to purify a polyethylene glycol-linked protein dimer (PEG dimer) made with M, 20 000 PEG bis-vinylsulfone, even when there are no net charge differences between the components that are being separated. The retention time on ion-exchange generally is inversely proportional to the PEG:protein ratio (on a mass basis). One of the biggest challenges in developing the process for making this PEG dimer was the quality of the PEG linker. Reversed-phase HPLC can be used to determine both size heterogeneity and the degree of end-group activation of Mr 20 000 PEG bis-vinylsulfone. In addition, we have found that hydrophobic interaction chromatography can be used make more size homogeneous preparations of Mr 20000 PEG bis-vinylsulfone, which significantly increased the recovery of the PEG dimer.

A biodegradable multiblock co-polymer derived from an alpha, omega-bis(methylamino)peptide and an alpha, omega-bis(oxiranylmethyl)poly(ethylene glycol)

A novel peptide containing the lysosomally degradable sequence GlyPheLeuGly and a sequence-inverting unit has been prepared. This peptide presents the methylamino groups of sarcosine (N-methylglycine) at both termini for co-polymerisation with alpha, omega-bis(oxiranylmethoxy)poly(ethylene glycol) of mean M(w) 1650. Gel permeation chromatographic analysis showed the presence of a mixture of oligomers. Preliminary degradation studies showed that these oligomers are cleaved by cathepsin B, an important lysosomal enzyme.

Synthesis and characterisation of polyamine-poly(ethylene glycol) constructs for DNA binding and gene delivery

Improved non-viral vector systems are needed for efficient delivery of DNA to target cell nuclei in gene therapy. A series of linear polyamine poly(ethylene glycol) (PEG) constructs has been synthesised by reaction of appropriately Boc-protected thermine derivatives with omega-methoxyPEG oxiranylmethyl ethers. Constructs carrying 1-3 MeOPEG units and 0, 2 or 4 N-methyl groups have been prepared by this method. H2N(CH2)3NBoc(CH2)3NBoc(CH2)3NHBoc was prepared efficiently by mono-trifluoroacetylation of thermine, attachment of Boc and removal of the trifluoroacetyl group in one pot. A similar process gave H2N(CH2)3NBoc(CH2)3NBoc(CH2)3NH2. BocMeN(CH2)3NHMe was alkylated by 1,3-dibromopropane to give BocMeN(CH2)3NMe(CH2)3NMe(CH2)3NMeBoc. A cyanoethylation/reduction sequence extended H2N(CH2)3NBoc(CH2)3NBoc(CH2)3NH2 to give H2N(CH2)3NBoc(CH2)3NBoc(CH2)3NBoc(CH2)3NBoc(CH2) 3NH2, which was converted to its mono- and di-MeOPEG550 derivatives. Deprotection gave the linear polyamine MeOPEG constructs. A branched triamine-poly(ethylene glycol) construct was prepared by acylation of (BocHN(CH2)3)2N(CH2)3NH2 with omega-methoxyPEG 550 chloroformate, followed by deprotection. A cyanoethylation/reduction/protection sequence from (H2N(CH2)3)2 N(CH2)3NHBoc gave a protected pentamine. Alkylation with Br(CH2)5CONH(CH2)2NHBoc, deprotection, acylation with MeOPEG chloroformate and deprotection gave a pentamine MeOPEG construct in which the MeOPEG is attached through a linker to the central amine. The linear hexamine construct carrying MeOPEG550 at only one terminus was the most effective DNA-interactive member of the two series in an ethidium displacement assay and was effective in delivering a reporter gene to RIF-1 tumours.

B-Domain deleted recombinant coagulation factor VIII modified with monomethoxy polyethylene glycol

Recombinant coagulation factor VIII (r-VIII SQ) was chemically modified with monomethoxy poly(ethylene glycol) (mPEG). Three mPEG derivatives were used for coupling to the r-VIII SQ lysines, a mixed anhydride of monomethoxy poly(ethylene glycol) succinic acid (mPEG-SAH), monomethoxy poly(ethylene glycol) succinimidyl succinate (mPEG-SS), and monomethoxy poly(ethylene glycol) tresylate (mPEG-TRES). A consequence of the modification with all derivatives was a substantial reduction in coagulant activity, even at very low degrees of modification. A method was developed with the purpose of avoiding conjugation at certain important biological sites on the factor VIII and thereby producing conjugates with better retained activity. This was achieved by immobilizing the protein onto a solid matrix during the modification reaction. Characterization of conjugates by SDS-PAGE, western blots, interaction with von Willebrand factor (vWf), and thrombin activation/inactivation analyses was undertaken. The SDS-PAGE and western blots revealed coupling heterogeneity regarding degree of modification. The amount of factor VIII able to bind to vWf decreased with the conjugation. Thrombin activated the modified factor VIII to essentially the same extent as the reference preparation of r-VIII SQ. Inactivation of the modified factor VIII was, however, slower than inactivation of the unmodified protein. Finally, an in vitro study was performed to evaluate the influence of the mPEG modification on the protein stability in extract of porcine tissue. Despite that conjugates with low degrees of modification were included in the study, the coagulant activity was preserved to a significantly higher extent in all incubation mixtures containing conjugates compared to that with unmodified protein.

DepoFoam technology: a vehicle for controlled delivery of protein and peptide drugs

A major challenge in the development of sustained-release formulations for protein and peptide drugs is to achieve high drug loading sufficient for prolonged therapeutic effect coupled with a high recovery of the protein/peptide. This challenge has been successfully met in the formulation of several peptide and protein drugs using the DepoFoam, multivesicular lipid-based drug delivery system. DepoFoam technology consists of novel multivesicular liposomes characterized by their unique structure of multiple non-concentric aqueous chambers surrounded by a network of lipid membranes. The objective of this paper is to demonstrate that DepoFoam technology can be used to develop sustained-release formulations of therapeutic proteins and peptides with high loading. DepoFoam formulations of a protein such as insulin, and peptides such as leuprolide, enkephalin and octreotide have been developed and characterized. The data show that these formulations have high drug loading, high encapsulation efficiency, low content of free drug in the suspension, little chemical change in the drug caused by the formulation process, narrow particle size distribution, and spherical particle morphology. Drug release assays conducted in vitro in biological suspending media such as human plasma indicate that these formulations provide sustained release of encapsulated drug over a period from a few days to several weeks, and that the rate of release can be modulated. In vivo pharmacodynamic studies in rats also show a sustained therapeutic effect over a prolonged period. These results demonstrate that the DepoFoam system is capable of efficiently encapsulating therapeutic proteins and peptides and effectively providing controlled delivery of these biologically active macromolecules.

Bioconjugation in pharmaceutical chemistry

Polymer conjugation is of increasing interest in pharmaceutical chemistry for delivering drugs of simple structure or complex compounds such peptides, enzymes and oligonucleotides. For long time drugs, mainly with antitumoral activity, have been coupled to natural or synthetic polymers with the purpose of increasing their blood permanence time, taking advantage of the increased mass that reduces kidney ultrafiltration. However only recently complex constructs were devised that exploit the 'enhanced permeability and retention' (EPR) effect for an efficient tumor targeting, the high molecular weight for adsorption or receptor mediated endocytosis and finally a lysosomotropic targeting, taking advantage of acid labile bonds or cathepsin susceptible polypeptide spacers between polymer and drug. New original, very active conjugates of this type, as those based on poly(hydroxyacrylate) polymers, are already in advanced state of development. Labile oligonucleotides, including antisense drugs, were also successfully coupled to polymers in view of an increased cell penetration and stabilization towards nucleases. However, the most active research activity resides in the field of polypeptides and proteins delivery, mainly for the two following reasons: first of all because a great number of therapeutically interesting compounds are now being produced by genetic engineering in large quantity and, secondly, because these products are difficult to administer to patients for several inherent drawbacks. Proteins are in fact easily digested by many endo- and exo-peptidases present in blood or in other body districts; most of them are immunogenic to some extent and, finally, they are rapidly excreted by kidney ultrafiltration. Covalent polymer conjugation at protein surface was demonstrated to reduce or eliminate these problems, since the bound polymer behaves like a shield hindering the approach of proteolytic enzymes, antibodies, or antigen processing cell. Furthermore, the increase of the molecular weight of the conjugate allows to overcome the kidney elimination threshold. Many successful results were already obtained in peptides and proteins, conjugated mainly to water soluble or amphiphilic polymers like poly(ethylene glycol) (PEG), dextrans, or styrenemaleic acid anhydride. Among the most successful are the conjugates of asparaginase, interleukin-2 or -6 and neocarcinostatin, to remind some antitumor agents, adenosine deaminase employed in a genetic desease treatment, superoxide dismutase as scavenger of toxic radicals, hemoglobin as oxygen carrier and urokinase and streptokinase as proteins with antithrombotic activity. In pharmaceutical chemistry the conjugation with polymers is also of great importance for synthetic applications since many enzymes without loss of catalytic activity become soluble in organic solvents where many drug precursors are. The various and often difficult chemical problems encountered in conjugation of so many different products prompted the development of many synthetic procedures, all characterized by high specificity and mild condition of reaction, now known as 'bioconjugation chemistry'. Bioconjugation developed also the design of new tailor-made polymers with the wanted molecular weight, shape, structure and with the functional groups needed for coupling at the wanted positions in the chain.

Instability, stabilization, and formulation of liquid protein pharmaceuticals

One of the most challenging tasks in the development of protein pharmaceuticals is to deal with physical and chemical instabilities of proteins. Protein instability is one of the major reasons why protein pharmaceuticals are administered traditionally through injection rather than taken orally like most small chemical drugs. Protein pharmaceuticals usually have to be stored under cold conditions or freeze-dried to achieve an acceptable shelf life. To understand and maximize the stability of protein pharmaceuticals or any other usable proteins such as catalytic enzymes, many studies have been conducted, especially in the past two decades. These studies have covered many areas such as protein folding and unfolding/denaturation, mechanisms of chemical and physical instabilities of proteins, and various means of stabilizing proteins in aqueous or solid state and under various processing conditions such as freeze-thawing and drying. This article reviews these investigations and achievements in recent years and discusses the basic behavior of proteins, their instabilities, and stabilization in aqueous state in relation to the development of liquid protein pharmaceuticals.

Immunogenic and tolerogenic properties of monomethoxypoly(ethylene glycol) conjugated proteins

An immunogenic and tolerogenic characterisation of monomethoxypoly(ethylene glycol) conjugated proteins was carried out using, as immunogen models, an anti-malaria chimera monoclonal antibody (PfChMab) and a macrophage colony stimulating factor (M-CSF). Two conjugates of PfChMab were prepared by polymer derivatisation of 19 and 33% protein amino groups and one conjugate of M-CSF was obtained by modification of 24% amino groups. In mice M-CSF was found to elicit rapidly high IgG and IgM levels whereas the monomethoxypoly(ethylene glycol) derivatised M-CSF stimulated a significantly lower immunoresponse. Native PfChMab was found to induce a delayed immunoresponse with high IgM levels but low production of IgG. Furthermore, similar immunogenic profiles were obtained with the native and modified protein forms. The pre-administration of polymer conjugated M-CSF to mice subsequently treated with the native protein was found to suppress up to 75% of anti-native M-CSF IgG, while IgM production was not affected. On the other hand the pre-administration of monomethoxypoly(ethylene glycol) derivatised PfChMab was found to reduce significantly the generation of anti-native PfChMab IgM. Such suppression depended on the degree of modification: the conjugate with the higher number of polymer chains was more effective in suppressing the immunoresponse.

Changes in the functional properties of bovine hemoglobin induced by covalent modification with polyethylene glycol

Polyethylene glycol conjugation to proteins and peptides (PEGylation) has been shown to promote increased retention time in the circulation as well as to blunt immune or allergic reactions. PEGylated bovine hemoglobin (PEG-Hb) is being explored in human clinical trials as an oxygen delivering agent for the sensitization of solid tumors to radiation therapy. In this study the functional properties of PEG-Hb were compared to those of bovine hemoglobin (Hb), the mutant human hemoglobin Rothchild and bovine hemoglobin crosslinked between the beta chains. The rate of heme transfer from Hb to serum albumin at pH 9.0 was greatly increased by PEGylation, suggesting destabilization of the heme-globin linkage and of the bonds between alpha beta dimers. Measurement of oxygen binding equilibrium showed that the oxygen affinity of Hb became unusually dependent on temperature and Hb concentration after PEGylation. Evidence is presented to suggest that PEGylation of lysine beta-81 at the entrance to the central cavity of the Hb tetramer might be responsible for these observations. The alterations of the functional properties of Hb induced by PEGylation are consistent with the beneficial effects of PEG-Hb in exchange transfusion and radiation sensitization models of human conditions.

Preparation and characterization of polyethylene-glycol-modified salmon calcitonins

The conjugation of salmon calcitonin (sCT) by covalent linkage of polyethylene glycol (PEG) was attempted to overcome several disadvantages of sCT as a therapeutic drug, namely its rapid clearance from blood circulation and enzymatic degradation. The polymer employed was succinimidyl carbonate monomethoxypolyethylene glycol (12 kDa). Superose HR size-exclusion chromatography was applied to separate the PEGylated sCTs (mono-PEG-sCT and di-PEG-sCT) from the unmodified sCT. The PEGylation of sCT was verified by an electrophoresis gel stained with iodine and by MALDI-TOF mass spectrometry. The molecular weights of mono-PEG-sCT and di-PEG-sCT were determined to be 16,094 and 29,077 Da, respectively. PEGylated sCTs showed a substantially improved stability in rat liver homogenates as compared to the intact sCT, indicating that PEG molecules protected sCT from various degrading enzymes. These PEGylated sCTs exhibited similar biological activity to the intact sCT by adenosine cyclic 3',5'-phosphate (cAMP) assay. In clearance studies in the rat, PEGylated sCTs had significantly longer circulating half-lives than the intact sCT (11.2 min for mono-PEG-sCT and 54.0 min for di-PEG-sCT versus 4.7 min for intact sCT).

Multivesicular liposome (DepoFoam) technology for the sustained delivery of insulin-like growth factor-I (IGF-I)

Insulin-like Growth Factor I (IGF-I), a 7.65 kD protein which has a variety of metabolic functions, is being evaluated for its therapeutic benefit in several disease states. To sustain therapeutic blood levels in a number of these instances, IGF-I needs to be administered repeatedly. The objective of these studies was the development of a sustained-release depot delivery system for this protein which would replace repeated administration. Using a multivesicular liposome drug delivery system (DepoFoam), sustained delivery kinetics have been observed for IGF-I. IGF-I was successfully encapsulated in this system with good efficiency. The integrity of the encapsulated protein was maintained, as characterized by physiochemical (HPLC, SDS-PAGE), and by biological methods (mitogenic activity). The DepoIGF-I particles were also characterized by their morphology (particles were smooth, multivesicular, and there was no debris), particle size (ranged from 18 to 20 microm), and in vitro and in vivo release kinetics of IGF-I. The DepoIGF-I particles released the protein drug in a sustained manner both in vitro and in vivo without a rapid initial release, and the released protein maintained its structural integrity and biological activity. The in vitro studies in human plasma at 37 degreesC showed that the DepoIGF-I particles released IGF-I slowly over several days; 70-80% of the protein was released in 6-7 days. In a pharmacokinetic in vivo study, after subcutaneous injections in rats, IGF-I levels were sustained for 5-7 days with DepoIGF-I formulation, whereas IGF-I in the free form was cleared in 1 day. DepoFoam technology provides a pharmaceutically useful system of sustained delivery for proteins, which can be extended to other therapeutic macromolecules.

Extravasation of macromolecules

Macromolecules can extravasate across the normal endothelium by transcapillary pinocytosis as well as by passage through interendothelial cell junctions, gaps or fenestrae. The main biological factors that control extravasation of a solute include regional differences in the capillary structures, the disease state of the organ or tissue, and the rate of blood and lymph supply. Physicochemical properties that are of profound significance in the extravasation of macromolecules are molecular size, shape, charge and hydrophilic/lipophilic balance (HLB) characteristics. Extravasation of small drugs, proteins, oligonucleotides and genes can be controlled by conjugating or forming complexes with macromolecular carriers. This requires a thorough understanding of the relationship between the chemical structures, physicochemical properties and the pharmacokinetics of both carrier and active molecules. This review article discusses the extravasation of macromolecules from the view points of pharmacokinetics and drug delivery systems, with the main emphasis on the extravasation across the liver, kidney and tumor capillaries.

Scratching the (cell) surface: cytokine engineering for improved ligand/receptor trafficking dynamics

Cytokines can be engineered for greater potency in stimulating cellular functions. An obvious test criterion for an improved cytokine is receptor-binding affinity, but this does not always correlate with improved biological response. By combining protein-engineering techniques with studies of receptor trafficking and signaling, it might be possible to identify the ligand receptor-binding properties that should be sought.