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

Controlling rates and reversibilities of elimination reactions of hydroxybenzylammoniums by tuning dearomatization energies

Hydroxybenzylammonium compounds can undergo a reversible 1,4- or 1,6-elimination to afford quinone methide intermediates after release of the amine. These molecules are useful for the reversible conjugation of payloads to amines. We hypothesized that aromaticity could be used to alter the rate of reversibility as a distinct thermodynamic driving force. We describe the use of density functional theory (DFT) calculations to determine the effect of aromaticity on the rate of release of the amine from hydroxybenzylammonium compounds. Namely, the aromatic scaffold affects the dearomatization reaction to reduce the kinetic barrier and prevent the reversibility of the amine elimination. We consequently synthesized a small library of polycyclic hydroxybenzylammoniums, which resulted in a range of release half-lives from 18 minutes to 350 hours. The novel mechanistic insight provided herein significantly expands the range of release rates amenable to hydroxybenzylammonium-containing compounds. This work provides another way to affect the rate of payload release in hydroxybenzylammoniums.

Rational Control of Protein-Protein Interactions with Protein-ATRP-Generated Protease-Sensitive Polymer Cages

Protease-protease interactions lie at the heart of the biological cascades that provide rapid molecular responses to living systems. Blood clotting cascades, apoptosis signaling networks, bacterial infection, and virus trafficking have all evolved to be activated and sustained by protease-protease interactions. Biomimetic strategies designed to target drugs to specific locations have generated proprotein drugs that can be activated by proteolytic cleavage to release native protein. We have previously demonstrated that the modification of enzymes with a custom-designed comb-shaped polymer nanoarmor can shield the enzyme surface and eliminate almost all protein-protein interactions. We now describe the synthesis and characterization of protease-sensitive comb-shaped nanoarmor cages using poly(ethylene glycol) [Sundy, J. S. Arthritis Rheum. 2008, 58(9), 2882-2891]methacrylate macromonomers where the PEG tines of the comb are connected to the backbone of the growing polymer chain by peptide linkers. Protease-induced cleavage of the tines of the comb releases a polymer-modified protein that can once again participate in protein-protein interactions. Atom transfer radical polymerization (ATRP) was used to copolymerize the macromonomer and carboxybetaine methacrylate from initiator-labeled chymotrypsin and trypsin enzymes, yielding proprotease conjugates that retained activity toward small peptide substrates but prevented activity against proteins. Native proteases triggered the release of the PEG side chains from the polymer backbone within 20 min, thereby increasing the activity of the conjugate toward larger protein substrates by 100%. Biomimetic cascade initiation of nanoarmored protease-sensitive protein-polymer conjugates may open the door to a new class of responsive targeted therapies.

Self-Immolative Hydroxybenzylamine Linkers for Traceless Protein Modification

Traceless self-immolative linkers are widely used for the reversible modification of proteins and peptides. This article describes a new class of traceless linkers based on ortho- or para-hydroxybenzylamines. The introduction of electron-donating substituents on the aromatic core stabilizes the quinone methide intermediate, thus providing a platform for payload release that can be modulated. To determine the extent to which the electronics affect the rate of release, we prepared a small library of hydroxybenzylamine linkers with varied electronics in the aromatic core, resulting in half-lives ranging from 20 to 144 h. Optimization of the linker design was carried out with mechanistic insights from density functional theory (DFT) and the in silico design of an intramolecular trapping agent through the use of DFT and intramolecular distortion energy calculations. This resulted in the development of a faster self-immolative linker with a half-life of 4.6 h. To demonstrate their effectiveness as traceless linkers for bioconjugation, reversible protein-polyethylene glycol conjugates with a model protein lysozyme were prepared, which had reduced protein activity but recovered ≥94% activity upon traceless release of the polymer. This new class of linkers with tunable release rates expands the traceless linkers toolbox for a variety of bioconjugation applications.

Hydrolytically Stable Maleimide-End-Functionalized Polymers for Site-Specific Protein Conjugation

Site-specific conjugation to cysteines of proteins often uses ester groups to link maleimide or alkene groups to polymers. However, the ester group is susceptible to hydrolysis, potentially losing the benefits gained through bioconjugation. Here, we present a simple conjugation strategy that utilizes the amide bond stability of traditional 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide/N-hydroxysuccinimide coupling while introducing site specificity. Hydrolytically stable maleimide-end-functionalized polymers for site-specific conjugation to free cysteines of proteins were synthesized using reversible addition-fragmentation chain-transfer (RAFT) polymerization. The alpha terminus of the polymers was amidated with a furan-protected aminoethyl maleimide using carbodiimide-based chemistry. Finally, the maleimide was exposed by a retro Diels-Alder reaction to yield the maleimide group, allowing for thiol-maleimide click chemistry for bioconjugation. A thermophilic cellulase from Fervidobacterium nodosum (FnCel5a) was conjugated using various strategies, including random 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC)/N-hydroxysuccinimide (NHS) coupling, site-specific hydroxyethyl maleimide (HEMI) end-functionalized coupling, hydroxyethyl acrylate (HEA) end-functionalized coupling, and amidoethyl maleimide (AEMI) end-functionalized coupling. Only the polymers conjugated by EDC and AEMI remained conjugated a week after attachment. This indicates that hydrolytically stable amide-based maleimides are an important bioconjugation strategy for conjugates that require long-term stability, while esters are better suited for systems that require debonding of polymers over time.

Predictive Platforms of Bond Cleavage and Drug Release Kinetics for Macromolecule–Drug Conjugates

Macromolecule–drug conjugates (MDCs) occupy a critical niche in modern pharmaceuticals that deals with the assembly and combination of a macromolecular carrier, a drug cargo, and a linker toward the creation of effective therapeutics. Macromolecular carriers such as synthetic biocompatible polymers and proteins are often exploited for their inherent ability to improve drug circulation, prevent off-target drug cytotoxicity, and widen the therapeutic index of drugs. One of the most significant challenges in MDC design involves tuning their drug release kinetics to achieve high spatiotemporal precision. This level of control requires a thorough qualitative and quantitative understanding of the bond cleavage event. In this review, we highlight specific research findings that emphasize the importance of establishing a precise structure–function relationship for MDCs that can be used to predict their bond cleavage and drug release kinetic parameters.

The Influence of a Polyethylene Glycol Linker on the Metabolism and Pharmacokinetics of a 89Zr-Radiolabeled Antibody

Most experimental work in the space of bioconjugation chemistry focuses on using new methods to construct covalent bonds between a cargo molecule and a protein of interest such as a monoclonal antibody (mAb). Bond formation is important for generating new diagnostic tools, yet when these compounds advance to preclinical in vitro and in vivo studies, and later for translation to the clinic, understanding the fate of potential metabolites that arise from chemical or enzymatic degradation of the construct is important to obtain a full picture of the pharmacokinetic performance of a new compound. In the context of designing new bioconjugate methods for labeling antibodies with the positron-emitting radionuclide ⁸⁹Zr, we previously developed a photochemical process for making ⁸⁹Zr-mAbs. Experimental studies on [⁸⁹Zr]ZrDFO-PEG₃-azepin-mAb constructs revealed that incorporation of the tris-polyethylene glycol (PEG₃) linker improved the aqueous phase solubility and radiochemical conversion. However, the use of a PEG₃ linker also has an impact on the whole-body residence time of the construct, leading to a more rapid excretion of the ⁸⁹Zr activity when compared with radiotracers that lack the PEG₃ chain. In this work, we investigated the metabolic fate of eight possible metabolites that arise from the logical disconnection of [⁸⁹Zr]ZrDFO-PEG₃-azepin-mAb at bonds which are susceptible to chemical or enzymatic cleavage. Synthesis combined with ⁸⁹Zr-radiolabeling, small-animal positron emission tomography imaging at multiple time points from 0 to 20 h, and measurements of the effective half-life for whole-body excretion are reported. The conclusions are that the use of a PEG₃ linker is non-innocent in terms of its impact on enhancing the metabolism of [⁸⁹Zr]ZrDFO-PEG₃-azepin-mAbs. In most cases, degradation can produce metabolites that are rapidly eliminated from the body, thereby enhancing image contrast by reducing nonspecific accumulation and retention of ⁸⁹Zr in background organs such as the liver, spleen, kidney, and bone.

Self-Immolative RAFT-Polymer End Group Modification

Reversible modifications of reversible addition-fragmentation chain transfer (RAFT)-polymerization derived end groups are usually limited to reductive degradable disulfide conjugates. However, self-immolative linkers can promote ligation and traceless release of primary and secondary amines as well as alcohols via carbonates or carbamates in β-position to disulfides. In this study, these two strategies are combined and the concept of self-immolative RAFT-polymer end group modifications is introduced: As model compounds, benzylamine, dibenzylamine, and benzyl alcohol are first attached as carbamates or carbonates to a symmetrical disulfide, and in a straightforward one-pot reaction these groups are reversibly attached to aminolyzed trithiocarbonate end groups of RAFT-polymerized poly(N,N-dimethylacrylamide). Quantitative end group modification is confirmed by ¹ H NMR spectroscopy, size exclusion chromatography, and mass spectrometry, while reversible release of attached compounds under physiological reductive conditions is successfully monitored by diffusion ordered NMR spectroscopy and thin layer chromatography. Additionally, this concept is further expanded to protein-reactive, self-immolative carbonate species that enable reversible bioconjugation of lysozyme and α-macrophage mannose receptor (MMR) nanobodies as model proteins. Altogether, self-immolative RAFT end group modifications can form the new basis for reversible introduction of various functionalities to polymer chain ends including protein bioconjugates and, thus, opening novel opportunities for stimuli-responsive polymer hybrids.

Pd-Catalyzed Functionalization of Aryl Amines on a Soluble Polymer Support

Herein we report the use of a soluble polymer support PEG-2000 on Pd-catalyzed reactions to improve the functionalization of aromatic amines and the synthesis of N-heterocycles. Compatibility of metal-catalyzed reactions for assembling privileged structures such as functionalized anilines were studied. PEG-supported anilines were found to be suitable substrates for Pd-catalyzed N-arylation, Sonogashira and Heck reactions. PEGylated substrates were prepared in yields up to 94%. This work consists on a proof of concept on the use of PEGylated anilines on Pd-catalyzed cross-coupling reactions. Indole core was attained in 82% and 62% yields, via two different routes.

Designing hydrogels for controlled drug delivery

Hydrogel delivery systems can leverage therapeutically beneficial outcomes of drug delivery and have found clinical use. Hydrogels can provide spatial and temporal control over the release of various therapeutic agents, including small-molecule drugs, macromolecular drugs and cells. Owing to their tunable physical properties, controllable degradability and capability to protect labile drugs from degradation, hydrogels serve as a platform in which various physiochemical interactions with the encapsulated drugs control their release. In this Review, we cover multiscale mechanisms underlying the design of hydrogel drug delivery systems, focusing on physical and chemical properties of the hydrogel network and the hydrogel-drug interactions across the network, mesh, and molecular (or atomistic) scales. We discuss how different mechanisms interact and can be integrated to exert fine control in time and space over the drug presentation. We also collect experimental release data from the literature, review clinical translation to date of these systems, and present quantitative comparisons between different systems to provide guidelines for the rational design of hydrogel delivery systems.

Polymerization of Ethylene Oxide, Propylene Oxide, and Other Alkylene Oxides: Synthesis, Novel Polymer Architectures, and Bioconjugation

The review summarizes current trends and developments in the polymerization of alkylene oxides in the last two decades since 1995, with a particular focus on the most important epoxide monomers ethylene oxide (EO), propylene oxide (PO), and butylene oxide (BO). Classical synthetic pathways, i.e., anionic polymerization, coordination polymerization, and cationic polymerization of epoxides (oxiranes), are briefly reviewed. The main focus of the review lies on more recent and in some cases metal-free methods for epoxide polymerization, i.e., the activated monomer strategy, the use of organocatalysts, such as N-heterocyclic carbenes (NHCs) and N-heterocyclic olefins (NHOs) as well as phosphazene bases. In addition, the commercially relevant double-metal cyanide (DMC) catalyst systems are discussed. Besides the synthetic progress, new types of multifunctional linear PEG (mf-PEG) and PPO structures accessible by copolymerization of EO or PO with functional epoxide comonomers are presented as well as complex branched, hyperbranched, and dendrimer like polyethers. Amphiphilic block copolymers based on PEO and PPO (Poloxamers and Pluronics) and advances in the area of PEGylation as the most important bioconjugation strategy are also summarized. With the ever growing toolbox for epoxide polymerization, a "polyether universe" may be envisaged that in its structural diversity parallels the immense variety of structural options available for polymers based on vinyl monomers with a purely carbon-based backbone.

Cleavable carbamate linkers for controlled protein delivery from hydrogels

The reversible attachment of proteins to polymers is one potential strategy to control protein release from hydrogels. In this study, we report the reversible attachment of lysozyme to poly(ethylene glycol) (PEG) by degradable carbamate linkers. Phenyl groups with different substituents were used to control the rate of carbamate hydrolysis and the resulting protein release. Sodium dodecyl sulfate polyacrylamide gel electrophoresis showed modification with 1-3 PEG chains per lysozyme molecule. Protein PEGylation and PEG chain elimination occurred without changes in secondary protein structure, as demonstrated by circular dichroism spectroscopy. The lytic activity of lysozyme was restored to 73.4±1.7%-92.5±1.2% during PEG chain elimination. Attached PEG chains were eliminated within 24h to 28days, depending on the used linker molecule. When formulated into hydrogels, a maximum of about 60% of the initial dose was released within 7days to 21days. Linker elimination occurs 'traceless', so that the protein is released in its native, unmodified form. Altogether, we believe that tethering proteins by degradable carbamate linkers is a promising strategy to control their release from hydrogels.

IL-1Ra and its delivery strategies: inserting the association in perspective

Interleukin-1 receptor antagonist (IL-1Ra) is a naturally occurring anti-inflammatory antagonist of interleukin-1 family of pro-inflammatory cytokines. The broad spectrum anti-inflammatory effects of IL-1Ra have been investigated against various auto-immune diseases such as diabetes mellitus, rheumatoid arthritis. Despite of its outstanding broad spectrum anti-inflammatory effects, IL-1Ra has short biological half-life (4-6 h) and to cope with this problem, up till now, many delivery strategies have been applied either to extend the half-life and/or prolong the steady-state sustained release of IL-1Ra from its target site. Here in our present paper, we have provided an overview of all approaches attempted to prolong the duration of therapeutic effects of IL-1Ra either by fusing IL-1Ra using fusion protein technology to extend the half-life and/or development of new dosage forms using various biodegradable polymers to prolong its steady-state sustained release at the site of administration. These approaches have been characterized by their intended impact on either in vitro release characteristics and/or pharmacokinetic and pharmacodynamic parameters of IL-1Ra. We have also compared these delivery strategies with each other on the basis of bioactivity of IL-1Ra after fusion with fusion protein partner and/or encapsulation with biodegradable polymer.

In vivo anti-tumor activity of polypeptide HM-3 modified by different polyethylene glycols (PEG)

HM-3, designed by our laboratory, is a polypeptide composed of 18 amino acids. Pharmacodynamic studies in vivo and in vitro indicated that HM-3 could inhibit endothelial cell migration and angiogenesis, thereby inhibiting tumor growth. However, the half-life of HM-3 is short. In this study, we modified HM-3 with different polyethylene glycols (PEG) in order to reduce the plasma clearance rate, extend the half-life in the body, maintain a high concentration of HM-3 in the blood and increase the therapeutic efficiency. HM-3 was modified with four different types of PEG with different molecular weights (ALD-mPEG_5k, ALD-mPEG_10k, SC-mPEG_10k and SC-mPEG_20k), resulting in four modified products (ALD-mPEG_5k-HM-3, ALD-mPEG_10k-HM-3, SC-mPEG_10k-HM-3 and SC-mPEG_20k-HM-3, respectively). Anti-tumor activity of these four modified HM-3 was determined in BALB/c mice with Taxol as a positive control and normal saline as a negative control. Tumor weight inhibition rates of mice treated with Taxol, HM-3, ALD-mPEG_5k-HM-3, ALD-mPEG_10k-HM-3, SC-mPEG_10k-HM-3 and SC-mPEG_20k-HM-3 were 44.50%, 43.92%, 37.95%, 31.64%, 20.27% and 50.23%, respectively. Tumor inhibition rates in the Taxol, HM-3 and SC-mPEG_20k-HM-3 groups were significantly higher than that in the negative control group. The efficiency of tumor inhibition in the SC-mPEG_20k-HM-3 group (drug treatment frequency: once per two days) was better than that in the HM-3 group (drug treatment frequency: twice per day). In addition, tumor inhibition rate in the SC-mPEG_20k-HM-3 group was higher than that in the taxol group. We conclude that SC-mPEG_20k-HM-3 had a low plasma clearance rate and long half-life, resulting in high anti-tumor therapeutic efficacy in vivo. Therefore, SC-mPEG_20k-HM-3 could be potentially developed as new anti-tumor drugs.

In situ growth of side-chain PEG polymers from functionalized human growth hormone-a new technique for preparation of enhanced protein-polymer conjugates

The application of atom transfer radical polymerization (ATRP) for preparation of a novel class of protein-polymer bioconjugates is described, exemplified by the synthesis of a recombinant human growth hormone (rh-GH) poly(ethylene glycol) methyl ether methacrylate (PEGMA) hybrid. The rh-GH protein was activated via a bromo-ester functionalized linker and used as a macroinitiator to polymerize the hydrophilic monomer PEGMA under solely aqueous conditions at 4 degrees C. ATRP conditions resulted in controlled polymer growth from rh-GH with low-polydispersity polyPEGMA chains. The rh-GH PEGMA product exhibited properties consistent with the presence of attached hydrophilic polymer chains, namely, high stability to denaturation and proteolysis. The polymerization conditions and conjugation proceeded with retention of the biological activity of the hormone. The rh-GH PEGMA was administered subcutaneously to rats and the activity compared to native rh-GH. The rh-GH PEGMA exhibited similar activity as the native rh-GH in vivo when a daily dose of 40 microg was administered. However, when a higher dose of 120 microg was administered with 3 days between injections the bioavailability of the rh-GH PEGMA was significantly better than that of the native. The results therefore demonstrate that ATRP can be successfully used as a general alternative approach to direct polymer conjugation, namely, PEGylation, to produce PEG-like protein conjugates. This technique can be exploited to design and synthesize protein-polymer derivatives with tailored therapeutic properties.

Batch purification of high-purity lysozyme from egg white and characterization of the enzyme modified by PEGylation

PEGylation is one of the most promising and extensively studied strategies for improving the pharmacological properties of proteins as well as their physical and thermal stability. Purified lysozyme obtained from hen egg white by batch mode was modified by PEGylation with methoxypolyethyleneglycol succinimidyl succinato (mPEG-SS, MW 5000). The conjugates produced retained full enzyme activity with the substrate glycol chitosan, independent of degree of enzyme modification, although lysozyme activity with the substrate Micrococcus lysodeikticus was altered according to the degree of modification. The conjugate with a low degree of modification by mPEG-SS retained 67% of its enzyme activity with the M. lysodeikticus substrate. The mPEG-SS was also shown to be a highly reactive polymer. The effects of pH and temperature on PEGylated lysozymes indicated that the conjugate was active over a wide pH range and was stable up to 50 degrees C. This conjugate also showed resistance to proteolytic degradation, remained stable in human serum, and displayed greater antimicrobial activity than native lysozyme against Gram-negative bacteria.

Biochemical and biophysical characterization of lysozyme modified by PEGylation

PEGylation is a strategy that has been used to improve the biochemical properties of proteins and their physical and thermal stabilities. In this study, hen egg-white lysozyme (EC 3.2.1.17; LZ) was modified with methoxypolyethylene glycol-p-nitrophenyl carbonate (mPEG-pNP, MW 5000). This PEGylation of LZ produced conjugates that retained full enzyme activity with glycol chitosan, independent of degree of enzyme modification; its biological activity with the substrate Micrococcus lysodeikticus was altered according to its degree of modification. The conjugate obtained with a low degree of mPEG-pNP/NH(2) modification was studied by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF), demonstrating a spectral peak at m/z 19,988 Da with 77% of its original enzymatic activity. Spectroscopic studies of Fourier transform infrared (FTIR) and circular dichroism (CD) did not show any relevant differences in protein structure between the native and conjugate LZ. Studies of the effects of pH and temperature on PEGylated LZ indicated that the conjugate was active over a broad pH range, stable at 50 degrees C, and demonstrated resistance to proteolytic degradation.

Modulating the pharmacokinetics of therapeutic antibodies

With the advent of antibody fragments and alternative binding scaffolds, that are devoid of Fc-regions, strategies to increase the half-life of small proteins are becoming increasingly important. Currently, the established method is chemical PEGylation, but more elaborate approaches are being described such as polysialylation, amino acid polymers and albumin-binding derivatives. This article reviews the main strategies for pharmacokinetic enhancement, primarily chemical conjugates and recombinant fusions that increase apparent molecular weight or hydrodynamic radius or interact with serum albumin which itself has a long plasma half-life. We highlight the key chemical linkage methods that preserve antibody function and retain stability and look forward to the next generation of technologies which promise to make better quality pharmaceuticals with lower side effects. Although restricted to antibodies, all of the approaches covered can be applied to other biotherapeutics.

Releasable PEGylation of proteins with customized linkers

Releasable PEGylation employs customized linkers that reversibly tether a therapeutic moiety with polyethylene glycol polymers. The choice of releasable PEG linkers may have numerous applications that are insufficiently addressed by stable polymer attachment. Releasable PEGylation provides regeneration of authentic and fully active drug and allows tailored design of critical pharmacological parameters such as the maximal drug concentration and total drug exposure. This provides a prodrug format that combines beneficial attributes of PEGylation with controlled release. The linker release mechanisms are shown to be kinetically controlled by the design of a hydrolytically labile center and side chains for the steric modulation of the intramolecular elimination reactions and linker self-immolation. Recent reports have described both aromatic and aliphatic based customized linkers that release the unaltered original drug under physiological conditions and at therapeutically useful release rates. These studies have examined bioconjugates of cytokines, peptide hormones, immunotoxins, enzymes, and reporter proteins.

Preparative purification of polyethylene glycol derivatives with polystyrene-divinylbenzene beads as chromatographic packing

A clear and powerful chromatographic approach to purify polyethylene glycol derivatives at a preparative scale was reported, which was based on the polystyrene-divinylbenzene beads with ethanol/water as eluants. The validity of this method was verified with the reaction mixture of mPEG-Glu and mPEG propionaldehyde diethylacetal (ALD-PEG) as the model. The target products were one-step achieved with the purity of >99% on the polymer resins column at gram scale. The method developed was free from such disadvantages as utility of toxic solvent and narrow application scope, which was combined with conventional approaches. The method developed provided an appealing and attractive alternative methods for purification of PEG derivatives at a preparative scale.

Investigation on PEGylation strategy of recombinant human interleukin-1 receptor antagonist

Although PEGylation is a potential approach to prolong the half-lives and reduce the dosing frequency of therapeutic proteins, conjugation behaviors of polymer have pivotal effects on the remaining bioactivities of the derivatives. In this study, the PEGylation strategy of recombinant human interleukin-1 receptor antagonist was investigated. The random conjugation of polyethylene glycol to amino groups on the protein resulted in a severe loss of activity and only retained 9.8% of the activity. In contrast, the PEGylation at the thiol groups had moderate effects on the bioactivity of protein and 40% of activity was conserved. The results suggested that the thiol-target PEGylation was more beneficial for IL-1ra.

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.

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.

The dawning era of polymer therapeutics

As we enter the twenty-first century, research at the interface of polymer chemistry and the biomedical sciences has given rise to the first nano-sized (5-100 nm) polymer-based pharmaceuticals, the 'polymer therapeutics'. Polymer therapeutics include rationally designed macromolecular drugs, polymer-drug and polymer-protein conjugates, polymeric micelles containing covalently bound drug, and polyplexes for DNA delivery. The successful clinical application of polymer-protein conjugates, and promising clinical results arising from trials with polymer-anticancer-drug conjugates, bode well for the future design and development of the ever more sophisticated bio-nanotechnologies that are needed to realize the full potential of the post-genomic age.

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.

A phase I and pharmacokinetic study of pegylated camptothecin as a 1-hour infusion every 3 weeks in patients with advanced solid malignancies

PURPOSE

To assess the feasibility of administering camptothecin (CPT), the prototypic topoisomerase I inhibitor, as polyethylene glycol (PEG)-CPT, a macromolecule consisting of CPT conjugated to chemically modified PEG. The study also sought to determine the maximum-tolerated dose (MTD) of PEG-CPT, characterize its pharmacokinetic behavior, and seek preliminary evidence of anticancer activity.

PATIENTS AND METHODS

Patients with advanced solid malignancies were treated with escalating doses of PEG-CPT as a 1-hour intravenous (IV) infusion every 3 weeks. A modified continual reassessment method was used for dose-level assignment to determine the MTD, which was defined as the highest dose level at which the incidence of dose-limiting toxicity did not exceed 20%.

RESULTS

Thirty-seven patients were treated with 144 courses of PEG-CPT at seven dose levels ranging from 600 to 8,750 mg/m(2). Severe myelosuppression was consistently experienced by heavily pretreated (HP) and minimally pretreated (MP) patients at the highest dose level evaluated, 8,750 mg/m(2), whereas both HP and MP patients tolerated repetitive treatment at 7,000 mg/m(2). Cystitis, nausea, vomiting, and diarrhea were also observed but were rarely severe. A partial response was noted in a patient with platinum- and etoposide-resistant small-cell lung carcinoma, and minor responses were noted in one patient each with adenocarcinoma of unknown primary type and osteosarcoma. The pharmacokinetics of free CPT were dose proportional. Free CPT accumulated slowly in plasma, with maximal plasma concentrations achieved at 23 +/- 12.3 hours; the harmonic mean half-life (t(1/2)) of free CPT was long (t(1/2), 77.46 +/- 36.77 hours).

CONCLUSION

Clinically relevant doses of CPT can be delivered by administering PEG-CPT. The recommended dose for phase II studies in both MP and HP patients is 7,000 mg/m(2) as 1-hour IV every 3 weeks. The characteristics of the myelosuppressive effects of PEG-CPT, the paucity of severe nonhematologic toxicities with repetitive treatment, the preliminary antitumor activity noted, and the slow clearance of CPT enabling simulation of desirable pharmacokinetic parameters with a convenient single-dosing regimen warrant further disease-directed evaluations.

Chemistry for peptide and protein PEGylation

Poly(ethylene glycol) (PEG) is a highly investigated polymer for the covalent modification of biological macromolecules and surfaces for many pharmaceutical and biotechnical applications. In the modification of biological macromolecules, peptides and proteins are of extreme importance. Reasons for PEGylation (i.e. the covalent attachment of PEG) of peptides and proteins are numerous and include shielding of antigenic and immunogenic epitopes, shielding receptor-mediated uptake by the reticuloendothelial system (RES), and preventing recognition and degradation by proteolytic enzymes. PEG conjugation also increases the apparent size of the polypeptide, thus reducing the renal filtration and altering biodistribution. An important aspect of PEGylation is the incorporation of various PEG functional groups that are used to attach the PEG to the peptide or protein. In this paper, we review PEG chemistry and methods of preparation with a particular focus on new (second-generation) PEG derivatives, reversible conjugation and PEG structures.