Extravasation of macromolecules

Polymeric nanoparticles for photodynamic therapy

Photodynamic therapy is a relatively new clinical therapeutic modality that is based on three key components: photosensitizer, light, and molecular oxygen. Nanoparticles, especially targeted ones, have recently emerged as an efficient carrier of drugs or contrast agents, or multiple kinds of them, with many advantages over molecular drugs or contrast agents, especially for cancer detection and treatment. This paper describes the current status of PDT, including basic mechanisms, applications, and challenging issues in the optimization and adoption of PDT; as well as recent developments of nanoparticle-based PDT agents, their advantages, designs and examples of in vitro and in vivo applications, and demonstrations of their capability of enhancing PDT efficacy over existing molecular drug-based PDT.

Biocompatible blood pool MRI contrast agents based on hyaluronan

Biocompatible gadolinium blood pool contrast agents based on a biopolymer, hyaluronan, were investigated for magnetic resonance angiography application. Hyaluronan, a non-sulfated linear glucosaminoglycan composed of 2000-25,000 repeating disaccharide subunits of D-glucuronic acid and N-acetylglucosamine with molecular weight up to 20 MDa, is a major component of the extracellular matrix. Two gadolinium contrast agents based on 16 and 74 kDa hyaluronan were synthesized, both with R(1) relaxivity around 5 mM(-1)  s(-1) per gadolinium at 9.4 T at 25°C. These two hyaluronan based agents show significant enhancement of the vasculature for an extended period of time. Initial excretion was primarily through the renal system. Later uptake was observed in the stomach and lower gastrointestinal tract. Macromolecular hyaluronan-based gadolinium agents have a high clinical translation potential as hyaluronan is already approved by FDA for a variety of medical applications.

Acidic pH-responsive nanogels as smart cargo systems for the simultaneous loading and release of short oligonucleotides and magnetic nanoparticles

Smart materials able to sense environmental stimuli can be exploited as intelligent carrier systems. Acidic pH-responsive polymers, for instance, exhibit a variation in the ionization state upon lowering the pH, which leads to their swelling. The different permeability of these polymers as a function of the pH could be exploited for the incorporation and subsequent release of previously trapped payload molecules/nanoparticles. We provide here a proof of concept of a novel use of pH-responsive polymer nanostructures based on 2-vinylpyridine and divinylbenzene, having an overall size below 200 nm, as cargo system for magnetic nanoparticles, for oligonucleotide sequences, as well as for their simultaneous loading and controlled release mediated by the pH.

The neovessel occlusion efficacy of 15-hydroxypurpurin-7-lactone dimethyl ester induced with photodynamic therapy

In this study, the photodynamic therapy (PDT) induced efficacy of a semi-synthesized analogue 15(1)-hydroxypurpurin-7-lactone dimethyl ester or G2, in terms of chick chorioallantoic membrane blood vessel occlusion was evaluated in reference to verteporfin. Early formulation studies showed that G2 prepared in a system of cremophor EL 2.5% and ethanol 2.5% in saline was biocompatible up to 20 microL volume of injection. Following injection, G2 accumulation peaked within the first minute and its extravasation from intra- to extra-vascular occurred somewhat slower as compared with verteporfin. In the PDT study, closure of capillaries and small neovessels was observed with 4 microg per embryo of G2 and a light dose of 20 J cm(-2) at a fluence rate of 40 mW cm(-2) filtered at 400-440 nm-a result that may be considered optimum for the treatment of age-related macular degeneration (AMD). Also, partial occlusion of the large vessels was observed using the same dose of G2 and light-an effect which is desirable for cancer treatment. From this study, we conclude that G2 has the potential to be developed as a therapeutic agent for photodynamic treatment for AMD and cancer.

Screening of nanoparticulate delivery systems for the photodetection of cancer in a simple and cost-effective model

AIMS

In urology, fluorescence-based imaging methods have been proven to significantly improve the detection of small, barely visible tumors and reduce the recurrence rate. Under ethical and economical pressure, new effective screening systems have to be developed to exploit and assess novel strategies for fluorescence photodetection in other areas. For this purpose, the chorioallantoic membrane (CAM) of the developing chick embryo is an attractive alternative model to the mammalian models.

MATERIALS & METHODS

Hypericin encapsulated into nanoparticles for the photodetection of ovarian metastases was evaluated in the CAM model with respect to vascular extravazation and tumor targeting and compared with free drug following intravenous administration.

RESULTS

To validate the CAM model as a valuable screening system for photodetection of cancer, we drew a comparison with results obtained on a conventional rodent model.

CONCLUSION

Rodent and CAM models led to the same conclusion regarding the benefits of nanoencapsulation to improve selective accumulation of drug in ovarian micrometastases.

Benefits of nanoencapsulation for the hypercin-mediated photodetection of ovarian micrometastases

The high recurrence and lethality of ovarian cancer at advanced stages is problematic, especially due to the development of numerous micrometastases scattered throughout the abdominal cavity. Fluorescence photodetection (PD) used in combination with surgical resection of malignant tissues has been suggested to improve recovery. Based on promising in vivo results for the detection of bladder cancer, hypericin (Hy), a natural photosensitizer (PS), stands as a good candidate for the photodetection of ovarian cancer. However, due to its hydrophobicity, systemic administration of Hy is problematic. Polymeric nanoparticles (NPs) help to overcome these delivery and stability problems and enable intravenous administration of Hy. In this study, Hy-loaded NPs of polylactic acid were produced with the following properties: (i) mean size of 268 nm, (ii) negative zeta potential, (iii) low residual surfactant and (iv) drug loading of 3.7 % (w/w). The potential of hypericin-loaded nanoparticles for the fluorescence photodetection of ovarian metastases in Fischer 344 rats bearing ovarian tumours was compared to free drug. The selectivity of Hy administered with both formulations was assessed first by fluorescence endoscopy, and then quantified after tissue extraction. The results showed an improved selective accumulation of Hy in ovarian micrometastases when NPs were used.

Nanoparticles as vehicles for delivery of photodynamic therapy agents

Photodynamic therapy (PDT) in cancer treatment involves the uptake of a photosensitizer by cancer tissue followed by photoirradiation. The use of nanoparticles as carriers of photosensitizers is a very promising approach because these nanomaterials can satisfy all the requirements for an ideal PDT agent. This review describes and compares the different individual types of nanoparticles that are currently in use for PDT applications. Recent advances in the use of nanoparticles, including inorganic oxide-, metallic-, ceramic-, and biodegradable polymer-based nanomaterials as carriers of photosensitizing agents, are highlighted. We describe the nanoparticles in terms of stability, photocytotoxic efficiency, biodistribution and therapeutic efficiency. Finally, we summarize exciting new results concerning the improvement of the photophysical properties of nanoparticles by means of biphotonic absorption and upconversion.

Targeted delivery systems of small interfering RNA by systemic administration

RNA interference (RNAi) is induced by 21-25 nucleotide, double-stranded small interfering RNA (siRNA), which is incorporated into the RNAi-induced silencing complex (RISC) and is a guide for cleavage of the complementary target mRNA in the cytoplasm. There are many obstacles to in vivo delivery of siRNAs, such as degradation by enzymes in blood, interaction with blood components and non-specific uptake by the cells, which govern biodistribution in the body. In order to achieve the knockdown by siRNAs in vivo, many delivery systems of siRNAs based on physical and pharmaceutical approaches have been proposed. In addition, the immune responses of siRNA must be taken into account when considering the application of siRNAs to in vivo therapy. This review focuses on recent reports about delivery systems and immune responses of siRNAs.

Nanoparticles for drug delivery: the need for precision in reporting particle size parameters

Polymeric drug-loaded nanoparticles have been extensively studied in the field of drug delivery. Biodistribution depends on the physicochemical properties of particles, especially size. The global message from the literature is that small particles have an enhanced ability to reach their target. The present review highlights the difficulties in validating the data from biodistribution studies without accurate particle size determination.

Inhibition of tumour metastasis by targeted delivery of antioxidant enzymes

Metastasis is one of the most harmful aspects of malignant neoplasm. Interaction of tumour cells with normal cells such as tissue macrophages may generate reactive oxygen species, which would affect various aspects of tumour metastasis. Reactive oxygen species cause damage to both tumour and normal cells and some of them, especially hydrogen peroxide, can also act as intracellular second messengers at sublethal concentrations to increase the transcription of various genes, which can then accelerate the proliferation of tumour cells in metastatic colonies. Therefore, eliminating hydrogen peroxide is one approach to inhibiting tumour metastasis. In this article, the roles of reactive oxygen species in tumour metastasis are reviewed, and the strategies to inhibit tumour metastasis by the targeted delivery of catalase, an enzyme that detoxifies hydrogen peroxide, are discussed.

Preclinical evaluation of a novel water-soluble chlorin E6 derivative (BLC 1010) as photosensitizer for the closure of the neovessels

In the present study, photodynamic activity of a novel photosensitizer (PS), Chlorin e(6)-2.5 N-methyl-d-glucamine (BLC 1010), was evaluated using the chorioallantoic membrane (CAM) as an in vivo model. After intravenous (i.v.) injection of BLC 1010 into the CAM vasculature, the applicability of this drug for photodynamic therapy (PDT) was assessed in terms of fluorescence pharmacokinetics, i.e. leakage from the CAM vessels, and photothrombic activity. The influence of different PDT parameters including drug and light doses on the photodynamic activity of BLC 1010 has been investigated. It was found that, irrespective of drug dose, an identical continuous decrease in fluorescence contrast between the drug inside and outside the blood vessels was observed. The optimal treatment conditions leading to desired vascular damage were obtained by varying drug and light doses. Indeed, observable damage was achieved when irradiation was performed at light doses up to 5 J/cm(2) 1 min after i.v. injection of drug doses up to 0.5 mg/kg body weight(b.w.). However, when irradiation with light doses of more than 10 J/cm(2) was performed 1 min after injection of drug doses up to 2 mg/kg body weight, this led to occlusion of large blood vessels. It has been demonstrated that it is possible to obtain the desired vascular occlusion and stasis with BLC 1010 for different combinations of drug and/or light doses.

Uptake ability of hepatic sinusoidal endothelial cells and enhancement by lipopolysaccharide

The liver is one of the major organs that remove exogenous substances and waste products from the blood circulation. Hepatic macrophages (Kupffer cells) and sinusoidal endothelial cells are responsible for the scavenger function of the liver. The sinusoidal endothelial cells, called scavenger endothelial cells, are believed to take up only soluble substances and nanometer-sized particles under normal conditions, while Kupffer cells can ingest larger particles and whole cells. However, the sinusoidal endothelial cells may have the potential to take up considerably large particles under special conditions. In this morphological study, we compared the uptake ability between sinusoidal endothelial cells and Kupffer cells after intravenous injections of latex beads (20 nm, 100 nm and 500 nm in diameter), bovine serum albumin (BSA) and dextran. Under normal conditions, the sinusoidal endothelial cells vigorously took up 100-nm-sized latex beads as well as 20-nm latex beads. BSA and dextran were ingested by the endothelial cells but not the Kupffer cells. The administration of lipopolysaccharide (LPS), which mimics inflammation, stimulated the uptake by endothelial cells. The uptake of latex beads by Kupffer cells was also elevated under LPS-stimulated conditions, but the uptake of BSA and dextran by them was not. These findings suggest that the sinusoidal endothelial cells can ingest not only soluble substances but also larger particles than those expected, and their uptake ability is strengthened under inflammatory conditions.

Synthesis of aminoacyl thiaolidones as potential antitumour agents

In the scope of a research program aiming to perform the synthesis and pharmacological evaluation of novel possible antitumour prototype compounds, we report in this paper the synthesis of new peptidyl derivatives from 4-thiazolidone nucleus. The synthesis reactions have been performed based in peptide synthesis as strategies. The characterisation of this new class of compounds was performed with IR and (1)H-NMR spectroscopy. In vivo, antitumour activity tests showed that some of these compounds were able to inhibit significantly sarcoma S-180 tumour growth in mice, revealing itself as a new potential class of drugs in cancer chemotherapy.

Plasmid delivery in vivo from porous tissue-engineering scaffolds: transgene expression and cellular transfection

Tissue engineering scaffolds capable of sustained plasmid release can promote gene transfer locally and stimulate new tissue formation. We have investigated the scaffold design parameters that influence the extent and duration of transgene expression and have characterized the distribution of transfected cells. Porous scaffolds with encapsulated plasmid were fabricated from poly(lactide-co-glycolide) with a gas foaming procedure, with wet granulation employed to mix the components homogeneously prior to foaming. Wet granulation enhanced plasmid incorporation relative to standard procedures and also enhanced in vivo transgene expression, possibly through the increased loading and maintenance of the scaffold pore structure. The plasmid loading regulated the quantity and duration of transgene expression, with expression for 105 days achieved at the highest dosage. Expression was localized to the implantation site, though the distribution of transfected cells varied with time. Transfected cells were initially observed at the scaffold periphery (day 3), then within the pores and adjacent to the polymer (day 17), and finally throughout the scaffold interior (day 126). Delivery of a plasmid encoding VEGF increased the blood vessel density relative to control. Correlating scaffold design with gene transfer efficiency and tissue formation will facilitate application of plasmid-releasing scaffolds to multiple tissues.

Tissue-Specific Characteristics of in Vivo Electric Gene: Transfer by Tissue and Intravenous Injection of Plasmid DNA

PURPOSE

To evaluate the tissue-specific characteristics of electric gene transfer after tissue and intravenous injection of naked plasmid DNA (pDNA).

METHODS

pDNA encoding firefly luciferase was injected directly into the liver, kidney, spleen, skin and muscle, or into the tail vein of mice, and electric pulses were then applied to one of these organs. The distribution of transgene expressing cells was evaluated using pDNA encoding beta-galactosidase.

RESULTS

Tissue injection of pDNA produced a significant degree of transgene expression in any tissue with the greatest amount in the liver, followed by kidney and spleen. The expression in these organs decreased quickly with time, and muscle showed the greatest expression at 7 days. Electroporation significantly increased the expression, and the expression level was comparable among the organs. Intravenous injection of pDNA followed by electroporation resulted in a significant expression in the liver, spleen, and kidney but not in the skin or muscle.

CONCLUSIONS

Electric gene transfer to the liver, kidney, and spleen can be an effective approach to obtain significant amounts of transgene expression by either tissue or intravenous injection of pDNA, whereas it is only effective after tissue injection as far as skin- or muscle-targeted gene transfer is concerned.

The use of chitosan as a condensing agent to enhance emulsion-mediated gene transfer

Previously we have formulated a new cationic emulsion, composed of 3beta [N-(N',N'-dimethylaminoethane) carbamoyl] cholesterol and dioleoylphosphatidyl ethanolamine, castor oil and Tween 80, and it efficiently delivered plasmid DNA into various cancer cells with low toxicity. Chitosan is a natural cationic polysaccharide and is able to form polyelectrolyte complexes with DNA, in which the DNA is condensed and protected against nuclease degradation. Based on these facts, chitosan was used as a condensing agent to enhance the transfection efficiency of cationic emulsion-mediated gene delivery vehicle. The particle size, zeta potential and transmission electron micrographs of DNA/emulsion complexes were observed before and after condensation by chitosan. In vitro transfection efficiency of naked or precondensed DNA/emulsion (pcDNA/E) complexes was investigated in human hepatoma cells (HepG2) using flow cytometer, confocal microscope and western blot. In addition, in vivo gene transfer was also evaluated as GFP mRNA expression by reverse transcriptase-polymerase chain reaction. The size of transfection complexes was reduced after the condensation of DNA by chitosan. Moreover, when the pcDNA/E complexes were administered into the mice, the GFP mRNA expression was prolonged in liver and lung until day 6. These results suggest that the use of chitosan enhance the in vitro transfection efficiency and extend in vivo gene transfer.

Encapsulation of porphyrins and chlorins in biodegradable nanoparticles: the effect of dye lipophilicity on the extravasation and the photothrombic activity. A comparative study

In the present work, we performed a preclinical inter-comparison study using several photosensitizers with the goal of optimizing photodynamic therapy (PDT) for the treatment of choroidal neovascularization (CNV) associated with age-related macular degeneration. The tested molecules were the porphyrins meso-tetraphenylporphyrin (TPP) and meso-tetra-(4-carboxyphenyl)-porphyrin (TCPP), and the chlorins pheophorbide-a (Pheo-a) and chlorin e(6) (Ce(6)). Each of these molecules was entrapped in biodegradable nanoparticles (NP) based on poly(d,l-lactic acid). The influence of the degree of lipophilicity on the incorporation efficiency of the drug in the NPs, and on the dye leakage from blood vessels as well as on the photothrombic efficiency was investigated using the chick chorioallantoic membrane (CAM) as in vivo model. NP characterization showed that the dye was more effectively entrapped in the polymeric matrix when its degree of lipophilicity increased. While less lipophilic compounds (TCPP, Ce(6)) extravasate rather easily, the more lipophilic dyes (TPP, Pheo-a) tend to remain inside the blood vessels. After injection of a drug dose of 1 mg/kg body weight and a drug-light application interval of 1 min, irradiation with light doses ranging from 5 to 20 J/cm(2) led to the highest photothrombic efficiency when using the NPs loaded with the most lipophilic molecule (TPP). The latter induced vascular damage, which was significantly higher than that observed with the other molecules tested. Thus, in addition to minimal leakage from blood vessels, the TPP in NP formulation exhibited photothrombic efficiency similar to Visudyne which was also tested in the CAM model.

Cytotoxicity of cis-Platinum(II) Conjugate Models. The Effect of Chelating Arms and Leaving Groups on Cytotoxicity: A Quantitative Structure−Activity Relationship Approach

Thirteen newly synthesized or resynthesized diamine-platinum(II) complexes were characterized, and their cytotoxic activities (IC50) were tested on parental and resistant ovarian cancer cell lines. They represent models of conjugates between biologically active vectors and cytotoxic Pt(II) moieties within the "drug targeting and delivery strategy". Three drugs, routinely employed in the clinical treatment of cancer, namely, cisplatin, carboplatin, and oxaliplatin, were also included in the study as controls. The quantitative structure-activity relationship approach provides simple regression models able to predict log(1/IC50) of diamine-platinum(II) complexes on both parental and resistant ovarian cancer cell lines. The 16 complexes were characterized using 197 molecular descriptors, after which the best regression models relating a subset of these descriptors to the log(1/IC50) in the two cancer cell lines were calculated. Models with four variables proved to be endowed with very good predictive ability Q2(LMO-50%) > or = 85.6%, making it possible to discard 50% of the molecules from the test set following for cross-validation procedure. A four-variable regression model also proved effective in predicting the resistance index RI, Q2(LMO-50%) = 84.4%.

Improved photodynamic activity of porphyrin loaded into nanoparticles: an in vivo evaluation using chick embryos

Hydrophobic porphyrins are potentially interesting molecules for the photodynamic therapy (PDT) of solid cancers or ocular vascularization diseases. Their pharmaceutical development is, however, hampered by their lipophilicity, which renders formulation difficult especially when intravenous administration is needed. Encapsulation of a lipophilic derivative of porphyrin, the meso-tetra(p-hydroxyphenyl)porphyrin (p-THPP), into polymeric biodegradable poly(D,L-lactide-co-glycolide) (PLGA) nanoparticles proved to enhance its photodynamic activity against mammary tumour cells when compared to free drug. In order to further investigate these carriers, the efficacy of the encapsulated drug was assessed on the chick embryo chorioallantoic membrane (CAM) model. First, we identified a suitable solvent for the drug in terms of p-THPP solubility and tolerability by chick embryos. This solution was used as a reference. Then, the fluorescence pharmacokinetics and the photodynamic effects of the porphyrin on CAM vessels were evaluated after intravenous administration of either a p-THPP solution (free drug) or the drug loaded into nanoparticles. The results showed that: (i) the drug remained longer in the vascular compartment when incorporated into nanoparticles and (ii) vascular effects of p-THPP after light irradiation were enhanced with nanoparticle carriers. These results are discussed taking into account the extravasation of intravascular circulating photosensitizers and its influence on PDT performance.

Polymeric nanogels produced via inverse microemulsion polymerization as potential gene and antisense delivery agents

Polymeric nanogel vectors were developed for cellular gene and antisense delivery. Inverse microemulsion polymerization was utilized to synthesize biocompatible nanogels with controlled size, morphology, and composition. The chemical composition, size, polydispersity, stability, and swelling behavior of the nanogels were investigated by NMR, light scattering, transmission electron microscopy, and atomic force microscopy. The cell viability, uptake, and physical stability of nanogel-DNA complexes were evaluated under physiological conditions. Monodisperse nonionic and cationic nanogels were produced with controllable sizes ranging from 40 to 200 nm in diameter. The nanogels demonstrated extended stability in aqueous media and exhibited low toxicity in cell culture. Cationic nanogels formed monodisperse complexes with oligonucleotides and showed enhanced oligonucleotide uptake in cell culture. The nanogels synthesized in this study demonstrate potential utility as carriers of oligonucleotides and DNA for antisense and gene delivery.

Macrophage-mediated activation of camptothecin analogue T-2513-carboxymethyl dextran conjugate (T-0128): possible cellular mechanism for antitumor activity

Camptothecin (CPT) analogue T-2513-carboxymethyl (CM) dextran conjugate (T-0128) suppressed human tumor xenografts that were refractory to CPTs. This improvement was explained by its altered pharmacokinetics, but the cellular mechanism of action is still not clear. For this reason, in the present study we examined the determinants of T-0128 action at the cellular level. In vitro tests showed that T-0128 was inactive, indicating that the requirement for its activity lies in the release of linked T-2513, accompanied by the cellular uptake of the conjugate. The accumulation varied between cell lines: tumor cells, including Walker-256 carcinoma and B16 melanoma, showed only a marginal uptake and an undetectable drug release in the medium. In contrast, macrophage-like cells, such as J774.1, internalized T-0128 very efficiently, and liberated T-2513. With regard to the mode of accumulation, fluid-phase pinocytosis seems to be a key factor based on the followings: a similar cell-specificity existed in the uptake of FITC dextran, a marker of fluid-phase pinocytosis. Also, the macrophage uptake of T-0128 increased almost linearly with its medium concentration and was insensitive to dextran sulfate, a ligand for macrophage scavenger receptor. Comparative efficacy studies of T-0128 in the presence and absence of macrophages demonstrated that macrophages increased the efficacy of T-0128. The enhancement could be explained in terms of increases in the amount of released T-2513. Overall, these results lead us to the conclusion that T-0128 acts like a Trojan horse with the help of macrophages: T-0128 is taken up by macrophages in tumor tissues, and the liberated T-2513 kills tumor cells.

Time- and dose-limiting erysipeloid rash confined to areas of lymphedema following treatment with gemcitabine--a report of three cases

Gemcitabine is a deoxycytidine analog with broad antitumor activity. Its main toxicities include myelosuppression, flu-like symptoms, bronchospasms and mild skin rash. We report three cases, in which the patients developed time- and dose-limiting erysipeloid skin reactions confined to areas of impaired lymphatic drainage after application of gemcitabine. Three patients with metastatic tumors (breast cancer, endometrial cancer and non-small cell lung cancer) received weekly infusions of gemcitabine (1000 mg/m2). All patients suffered from lymphedema of different origin and developed an erysipeloid erythema 40-48 h after chemotherapy within their preexisting lymphedema. Genuine erysipela was ruled out by laboratory tests and clinical observation. The skin reaction was repeatedly observed and faded after 14 days without specific treatment. Although the pathogenesis of the observed reaction is unclear, it is suspected that the skin symptoms were caused by gemcitabine or its metabolites. Gemcitabine is usually metabolized fast and excreted renally. In areas with impaired lymphatic drainage pharmakocinetics might be altered: inactivation happens slower and the drug might accumulate in the s.c. and cutaneous tissue, thus increasing local toxicity. Clinical judgement and biochemical parameters can help to tell apart genuine erysipela and the erysipeloid reaction.

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.