The conjugation of proteins with polyethylene glycol and other polymers

The emerging role of ricin A-chain immunotoxins in leukemia and lymphoma

Since MRD is the major cause for relapses of malignant diseases, strategies utilizing ITs to target tumor cells surviving conventional treatment have attracted scientific and clinical interest. Many different ITs against various blood-borne as well as solid malignancies have demonstrated specific potent anti-tumor effects in vitro and in animal models. Some of these have already undergone clinical phase I/II-trials. The dose-limiting toxicities of RTA ITs include manifestation of VLS presenting as decreased urinary sodium excretion, hypoalbuminemia, fatigue, hypotonia, myalgia, pulmonary edema, or rhabdomyolysis. Problems encountered clinically include the development of HAMA, HARA, and HACA and the selection of antigen-deficient malignant clones. Most clinical trials performed with ITs so far were conducted in heavily pretreated patients presenting with high tumor burdens. Thus, the responses observed with ITs in these trials are very encouraging and warrant further exploration.

Properties of solid supports

Many supports including composite materials and functionalized surfaces are available for solid-phase synthesis. In the process of selecting the proper support it is important to consider the optimal performance during solid-phase synthesis. For most purposes the mechanically stable beaded gel resins are preferred. These resins are homogeneous, and the loading and physical and chemical properties can easily be varied. Optimal properties have been obtained by radical polymerization of end group acryloylated long-chain polyethylene glycols. However, polystyrene resins or amide bond free PEG-based resins may be more suited for general organic synthesis where reactivity of radicals, carbenes, carbanions, carbenium ions, or strong Lewis acids have to be considered. Loading of the resins can have a dramatic effect on the outcome of a synthesis and has to be considered separately for each synthesis. Synthesis of long peptides with 50-100 amino acids imposes completely different requirements on the performance, swelling, and loading than a large-scale synthesis of, for example, the pentapeptide enkephalin. Automated multiple synthesizers constructed for columns of beaded gel or composite supports are available from many suppliers. It is therefore expected that the optimization of support properties will continue in order to meet new synthetic challenges. In the synthesis for solid-phase screening of binding of biomolecules to ligands directly on the resin beads, it is an advantage if the resin is not permeable to the biomolecule so unbound molecules can easily be removed by washing. This is the case with polystyrene-based resins, but they do, however, often show nonspecific adhesion of proteins owing to the hydrophobic character of the polystyrene. Modification of the functional groups of polystyrene with polyethylene glycol as spacers for synthesis of the binding ligands can increase the available ligand concentration on the bead surface and eliminate most of the nonspecific adhesion. In contrast to binding studies, solid-phase assays of enzymes require beads that are permeable to the enzyme, as the progress of reaction can be followed and the product of reaction analyzed. The available amount on the surface of the polystyrene-based beads (approximately 0.3%) is not enough for product analysis. Therefore, in the case of enzyme assays, highly swelling permeable PEG-based gel resins or functionalized surfaces of a polar and porous matrix are preferred.

Photodynamic effect of polyethylene glycol-modified fullerene on tumor

Fullerence (C60) efficiently generates singlet oxygen when irradiated with light, and thus should have a photodynamic effect on tumors, if it is accumulated in the tumor tissue. To explore tumor targeting of C60, we chemically modified the water-insoluble C60 with polyethylene glycol (PEG), not only to make it soluble in water, but also to enlarge its molecular size. When injected intravenously into mice carrying a tumor mass in the back subcutis, the C60-PEG conjugate exhibited higher accumulation and more prolonged retention in the tumor tissue than in normal tissue. The conjugate was excreted without being accumulated in any specific organ. Following intravenous injection of C60-PEG conjugate or Photofrin to tumor-bearing mice, coupled with exposure of the tumor site to visible light, the volume increase of the tumor mass was suppressed and the C60 conjugate exhibited a stronger suppressive effect than Photofrin. Histological examination revealed that conjugate injection plus light irradiation strongly induced tumor necrosis without any damage to the overlying normal skin. The antitumor effect of the conjugate increased with increasing irradiation power and C60 dose, and cures were achieved by treatment with a dose of 424 micrograms/kg at an irradiation power of 107 J/cm2. These findings indicate that PEG-modified C60 is a candidate agent for photodynamic tumor therapy.

Biophysical methods for the determination of antibody-antigen affinities

Binding specificities against virtually any antigen can be isolated from antibody libraries displayed on filamentous phage. The determination of antibody-antigen affinity constants and binding kinetics is an important part of antibody characterization, and may be predictive of antibody performance in biotechnological applications. This article, intended as a guideline for the scientist who isolates a novel antibody and wishes to characterize its binding properties, presents the authors' view on widely used methodologies for the quantitative determination of antibody-antigen interactions.

PEG thiazolidine-2-thione, a novel reagent for facile protein modification: conjugation of bovine hemoglobin

A novel PEG linker that employs a thiazolidine-2-thione group has been synthesized. Kinetic studies done on this compound demonstrate a relatively long half-life compared to those of traditional succinimidyl linkers. This new PEG derivative reacts with proteins under mild conditions and was utilized to conjugate bovine hemoglobin (bHb) to provide a PEG amide-linked protein. The physical characteristics of this conjugate were compared with those of the known PEG carbamate-linked bHb.

Macromolecular versus small-molecule therapeutics: drug discovery, development and clinical considerations

Recent advances in biomedical science in general, and molecular biology in particular, have provided a greater understanding of pathogenesis at the molecular and (sub)cellular level. In turn, this has stimulated the development of macromolecular, mechanism-based therapeutic agents, ranging from recombinant proteins, to oligonucleotides, to genes/gene fragments. The factors essential for the successful development of this new class of therapeutic agents are not necessarily the same as those for the development of conventional small organic molecules. This review mentions several issues relating to the development of macromolecular drugs, and emphasizes the key issue of drug transport and delivery.

Generation of soluble and active subtilisin and alpha-chymotrypsin in organic solvents via hydrophobic ion pairing

With very low concentrations of anionic detergents, such as sodium dodecyl sulfate (SDS) and Aerosol OT (AOT), it is possible to solubilize proteases in organic solvents, while retaining enzymatic activity. For example, the SDS-subtilisin BPN' complex catalyzes transesterification of Ac-Phe-OMe in ethanol with a kcat/Km of 36 M-1 s-1 for mutant M1 and 39 M-1 s-1 for the wild type. By comparison, M1 suspended in ethanol is approximately 1000-fold less active, with a kcat/Km of 0.03 M-1 s-1. Similarly, AOT complexes of alpha-chymotrypsin were found to be approximately 1000 times more active (kcat/Km = 100-350 M-1 s-1) than the suspended enzyme.

Poly(ethylene glycol)-serum albumin hydrogel as matrix for enzyme immobilization: biomedical applications

Poly(ethylene glycol)-albumin hydrogels were implanted in mice in subcutaneous position to study their biocompatibility. After one month of implantation, the fibrous capsule formed around the implant was thin and the inflammatory tissue was limited. Acid phosphatase (AP) was selected to evaluate the hydrogel as matrix for enzyme immobilization. AP-hydrogels were prepared using activated PEG (PEGa) of different molecular weights (M.W. 4,600 to 20,000) to evaluate the effect of the matrix composition on the activity of AP. The apparent Km of the immobilized AP was 16 to 20 times higher than the Km of the soluble enzyme. The apparent Km value decreases with the increase of the chain length of the PEGa used. This can be correlated to an increase in the hydrogel porosity. The operational stability of the AP was markedly improved after immobilization by 110 to 160 times according to the PEGa molecular weight involved. Also, asparaginase (ASNase) was immobilized in PEGa (M.W. 10,000)-albumin-hydrogel as a model for in vivo bioreactor. ASNase hydrogels were implanted in the peritoneal cavity of rats; 7 days later, 75% of the initial enzyme activity were retrieved.

Recent developments in immunotoxin therapy

Immunotoxin (IT) research has been ongoing for 15 years. During the past 2 years, work has focused on several areas: on improvements and developments in first- and second-generation ITs; the preparation of new immunotoxin constructs with anti-tumor activity; novel animal models for preclinical evaluation of immunotoxins; and clinical trials, which are now entering Phase II or III in humans.

Pertechnetate release from a water/oil microemulsion and an aqueous solution after subcutaneous injection in rabbits

A water-oil microemulsion and an aqueous solution, both carrying pertechnetate, were injected subcutaneously in rabbits; release was observed by imaging the administration sites with a gamma-camera. Disappearance from the injection site of pertechnetate in aqueous solution was about ten times faster than that of pertechnetate in a microemulsion.