Site-specific chemical modification with polyethylene glycol of recombinant immunotoxin anti-Tac(Fv)-PE38 (LMB-2) improves antitumor activity and reduces animal toxicity and immunogenicity

Improved partitioning in aqueous two-phase system of tyrosine-tagged recombinant lactate dehydrogenase

The partitioning of Bacillus stearothermophilus lactate dehydrogenase (LDH) in an aqueous two-phase system was studied. Particularly, the influence of tyrosine tags on the partitioning was evaluated. The hydrophobic effect, caused by the addition of tyrosine residues, was determined in a system based on dextran and the thermoseparating ethylene oxide-propylene oxide random copolymer (EO30PO70). Five different LDH variants were constructed with N-terminal tags containing tyrosines (Y3 and Y6), tyrosines and prolines (Y3P2 and Y6P2), and only prolines (P2). LDH fused with tags containing tyrosines increased the partitioning coefficient, and the more tyrosines added to the protein, the larger improvement in partitioning. When prolines were added between the tyrosine-rich tag and the protein, a further increased partitioning was obtained. The enhanced partitioning was attributed to the rigid structure of the proline, which in turn led to an increase in the exposure of the tag to the surroundings. The best tyrosine tag, Y6P2, increased the partition coefficient four times and additionally, a higher thermostability was observed.

Mono-N-terminal poly(ethylene glycol)-protein conjugates

A site-directed method of joining proteins to poly(ethylene glycol) is presented which allows for the preparation of essentially homogeneous PEG-protein derivatives with a single PEG chain conjugated to the amine terminus of the protein. This selectivity is achieved by conducting the reductive alkylation of proteins with PEG-aldehydes at lower pH. Working examples demonstrating the application of this method to improve the delivery characteristics and therapeutic value of several proteins are provided.

Enzymatic procedure for site-specific pegylation of proteins

We have developed a novel methodology for site-specific pegylation of proteins by use of transglutaminase (TGase). In this methodology, alkylamine derivatives of poly(ethyleneglycol) (PEG) could be site-specifically incorporated into intact or chimeric proteins without decreasing the bioactivities. The incorporation site of the TGase-catalyzed modification is limited to the substrate Gln residues for TGases. The high homogeneity of the constructed conjugates and the ability to design conjugates with suitable incorporation sites will improve the applicability of PEG-protein conjugates for clinical use.

Bioactivities of ricin retained and its immunoreactivity to anti-ricin polyclonal antibodies alleviated through pegylation

Ricin has long been employed to construct immunotoxins, whose efficacy was often undermined by immunogenicity. Pegylation (modification of proteins with polyethylene glycol, PEG) was one of those recently developed approaches to circumvent immunogenicity of legions of drugs. Herein, pegylation of ricin was found to have barely changed the RNA N-glycosidase activity and protein synthesis inhibiting activity of ricin, but remarkably altered the cytotoxicity of ricin on hepatoma cell line (BEL7404) or the immunoreactivity with polyclonal anti-ricin antibodies. This result suggested that the attached PEG or monomethyloxyl polyethylene glycol (mPEG) groups did not hinder ricin from hydrolyzing ribosomal RNA, but indeed covered some areas on the surface of ricin molecule, including those involved in the interaction with cellular receptors and epitopes recognized by polyclonal antibodies. Pegylation, masking certain epitopes of ricin, might contribute to alleviate the immunogenicity of the toxin. Approach in this work, if applied to thereby constructed immunotoxins, would help improve the prospective efficacy of these toxins.

Peptide drug modifications to enhance bioavailability and blood-brain barrier permeability

Peptides have the potential to be potent pharmaceutical agents for the treatment of many central nervous system derived maladies. Unfortunately peptides are generally water-soluble compounds that will not enter the central nervous system, via passive diffusion, due to the existence of the blood-brain barrier. Peptides can also undergo metabolic deactivation by peptidases, thus further reducing their therapeutic benefits. In targeting peptides to the central nervous system consideration must be focused both on increasing bioavailability and enhancing brain uptake. To date multiple strategies have been examined with this focus. However, each strategy comes with its own complications and considerations. In this review we assess the strengths and weaknesses of many of the methods currently being examined to enhance peptide entry into the central nervous system.

Improving the efficacy of antibody-based cancer therapies

A quarter of a century after their advent, monoclonal antibodies have become the most rapidly expanding class of pharmaceuticals for treating a wide variety of human diseases, including cancer. Although antibodies have yet to achieve the ultimate goal of curing cancer, many innovative approaches stand poised to improve the efficacy of antibody-based therapies.

Positive effects of polyethylene glycol conjugation to generation-4 polyamidoamine dendrimers as macromolecular MR contrast agents

Macromolecules conjugated with polyethylene glycol (PEG) acquire more hydrophilicity, resulting in a longer half-life in circulation and lower immunogenicity. Two novel conjugates for MRI contrast agents were synthesized from a generation-4 polyamidoamine dendrimer (G4D), 2-(p-isothiocyanatobenzyl)-6-methyl-diethylenetriaminepentaacetic acid (1B4M), and one or two PEG molecules with a molecular weight of 20000 Da (PEG(2)-G4D-(1B4M-Gd)(62) (MW: 96 kD), PEG(1)-G4D-(1B4M-Gd)(63) (MW: 77 kD)). Their pharmacokinetics, excretion, and properties as vascular MRI contrast agents were evaluated and compared with those of G4D-(1B4M-Gd)(64) (MW: 57 kD). PEG(2)-G4D-(1B4M-Gd)(62) remained in the blood significantly longer and accumulated significantly less in the liver and kidney than the other two preparations (P < 0.01). Although the blood clearance was slower, PEG(2)-G4D-(1B4M-Gd)(62) was excreted more readily without renal retention than the other two preparations. In conclusion, the positive effects of PEG conjugation on a macromolecular MRI contrast agent were found to be prolonged retention in the circulation, increased excretion, and decreased accumulation in the organs.

Phase 1 study of polyethylene glycol formulation of interferon alpha-2B (Schering 54031) in Philadelphia chromosome-positive chronic myelogenous leukemia

Interferon alpha (IFN-alpha) therapy improves prognosis in Philadelphia chromosome (Ph)-positive chronic myelogenous leukemia (CML). Polyethylene glycol (PEG) attached to IFN-alpha prolongs its half-life and may offer better therapy. The aims of this phase 1 study were to define the maximal tolerated dose (MTD), dose-limiting toxicities (DLTs), and response with PEG IFN-alpha-2b. Twenty-seven adults with Ph(+) CML in chronic or accelerated phases, in whom IFN-alpha treatment had failed, were studied. Patients had hematologic (9 patients) or cytogenetic resistance (12 patients) or intolerance to IFN-alpha (6 patients). PEG IFN-alpha-2b was given as a weekly subcutaneous injection starting at 0.75 microg/kg weekly and escalating to 1.5, 3, 4.5, 6, 7.5, and 9.0 microg/kg. The MTD was defined at 7.5 to 9 microg/kg; DLT included severe fatigue, neurotoxicity, liver function abnormalities, and myelosuppression. Longer administration of PEG IFN-alpha-2b resulted in chronic side effects not observed earlier, which defined the MTD and DLT. The proposed phase 2 dose of PEG IFN-alpha-2b was 6 microg/kg weekly. Among 19 patients with active disease, 7 (37%) achieved complete hematologic response (CHR); 2 (11%) had a cytogenetic response (complete). Among 8 patients treated in CHR, 7 (87%) improved cytogenetic response to complete (4 patients) or partial (3 patients). All 6 patients intolerant to IFN-alpha tolerated PEG IFN-alpha-2b; 4 improved their cytogenetic response. The results show that PEG IFN-alpha-2b is easier to deliver (once weekly), better tolerated, and perhaps more effective than IFN-alpha.

Recombinant immunotoxins for cancer therapy

Recombinant immunotoxins consist of Fv regions of tumour-selective antibodies fused to toxins found in bacteria, plants or fungi. These toxins must be modified to remove normal-tissue binding sites but to retain all other functions of cytotoxicity. The recombinant antibody fragments target the modified toxin to cancer cells which are killed, either by direct inhibition of protein synthesis, or by concomitant induction of apoptosis. Cells that are not recognised by the antibody fragment because they do not carry the tumour antigen, are spared. Many factors influence the in vivo antitumour activity of recombinant immunotoxins. Among them are considerations of which types of cancer may be the best targets for immunotoxin therapy as well as tumour specificity of the antigen that is targeted by the recombinant antibody. Other relevant issues are the affinity of immunotoxins and their ability to enter and penetrate into tissues and tumours, which in turn is dependent on the size of the protein. A great deal of protein-engineering is required to stabilise the recombinant antibody moiety of immunotoxins, since stability of the molecules is crucial for good clinical efficacy. Excellent activity and specificity can be observed for many recombinant immunotoxins in in vitro assays using cultured cancer cells as well as in animal tumour models. Ongoing clinical trials provide examples where the promising preclinical data correlate with successful results in experimental cancer therapy.