Disulfide bridge based PEGylation of proteins

In situ maleimide bridging of disulfides and a new approach to protein PEGylation

The introduction of non-natural entities into proteins by chemical modification has numerous applications in fundamental biological science and for the development and manipulation of peptide and protein therapeutics. The reduction of native disulfide bonds provides a convenient method to access two nucleophilic cysteine residues that can serve as ideal attachment points for such chemical modification. The optimum bioconjugation strategy utilizing these cysteine residues should include the reconstruction of a bridge to mimic the role of the disulfide bond, maintaining structure and stability of the protein. Furthermore, the bridging chemical modification should be as rapid as possible to prevent problems associated with protein unfolding, aggregation, or disulfide scrambling. This study reports on an in situ disulfide reduction-bridging strategy that ensures rapid sequestration of the free cysteine residues in a bridge, using dithiomaleimides. This approach is then used to PEGylate the peptide hormone somatostatin and retention of biological activity is demonstrated.

Coated cross-species antibodies by mannosamine-biotin adduct confer protection against snake venom without eliciting humoral immune response

Passive immunization with cross-species antibodies triggers the patient's immune response, thereby preventing repeated treatment. Mannosamine-biotin adduct (MBA) has been described as a masking agent for immunogenic reduction and here, the immunogenicity and biological activity of MBA-coated horse anti-viper venom (hsIgG) were compared to those of uncoated or PEGylated hsIgG. In in vitro tests, hsIgG binding was not affected by MBA conjugation. The immune response to hsIgG-MBA was about 8-fold and 32-fold lower than to PEG-coated and uncoated hsIgG, respectively. In vivo, hsIgG-MBA showed efficient venom-neutralization activity. We thus demonstrate the feasibility of using MBA as a masking agent for passive immunization with cross-species antibodies.

Protein modification, bioconjugation, and disulfide bridging using bromomaleimides

The maleimide motif is widely used for the selective chemical modification of cysteine residues in proteins. Despite widespread utilization, there are some potential limitations, including the irreversible nature of the reaction and, hence, the modification and the number of attachment positions. We conceived of a new class of maleimide which would address some of these limitations and provide new opportunities for protein modification. We report herein the use of mono- and dibromomaleimides for reversible cysteine modification and illustrate this on the SH2 domain of the Grb2 adaptor protein (L111C). After initial modification of a protein with a bromo- or dibromomaleimide, it is possible to add an equivalent of a second thiol to give further bioconjugation, demonstrating that bromomaleimides offer opportunities for up to three points of attachment. The resultant protein-maleimide products can be cleaved to regenerate the unmodified protein by addition of a phosphine or a large excess of a thiol. Furthermore, dibromomaleimide can insert into a disulfide bond, forming a maleimide bridge, and this is illustrated on the peptide hormone somatostatin. Fluorescein-labeled dibromomaleimide is synthesized and inserted into the disulfide to construct a fluorescent somatostatin analogue. These results highlight the significant potential for this new class of reagents in protein modification.

Phosphine-mediated one-pot thiol-ene "click" approach to polymer-protein conjugates

We employ water-soluble organic phosphines as key reagents in a one-pot synthetic protocol where a (poly)peptide disulfide bridge is first reduced followed by subsequent reaction of the two thiols in situ with poly(monomethoxy ethylene glycol)(meth)acrylates (p(mPEG)(M)A); we use salmon calcitonin (sCT) as a disulfide bridge-containing peptide, which contains a disulfide bridge-Cys1-Cys7-that can be reduced to give two sulfhydryl units available for thiol functionalisation; bioactivity is retained.

Synthesis of linear polyether polyol derivatives as new materials for bioconjugation

Linear polyether polyol (PEP) consisting of glycidol as repeating units is a flexible hydrophilic aliphatic polymer. The polyether main chain is similar to the widely used, biocompatible polymer poly(ethylene glycol) (PEG). While linear PEG has one or two terminal hydroxyl group(s), linear PEP distinguishes itself by the large number of pendant hydroxyl groups along the polyether main chain. We propose that this property of PEP represents a major advantage over PEG, namely, by providing multiple anchorage points and increasing the possibility for introducing different functional groups. As a first step to establishing PEP as a bioconjugation material, we modified the pendant hydroxyl groups on PEP and prepared a series of mono- and heterobifunctional derivatives with the potential to join various drug entities and biomolecules. The synthesis methods and the results of characterization are reported here.

High-purity discrete PEG-oligomer crystals allow structural insight

To great (monodisperse) lengths: An improved synthesis of purer ethylene glycol (EG) oligomers allows access to 16- and 32-mers pure enough for multiple incorporation, and also to the longest (48-mer) discrete EG oligomer yet reported. The high purity enables the first crystallizations and hence the first glimpses of secondary 3(10)-helical PEG structures.

Photosensitiser delivery for photodynamic therapy. Part 2: systemic carrier platforms

BACKGROUND

The treatment of solid tumours and angiogenic ocular diseases by photodynamic therapy (PDT) requires the injection of a photosensitiser (PS) to destroy target cells through a combination of visible light irradiation and molecular oxygen. There is currently great interest in the development of efficient and specific carrier delivery platforms for systemic PDT.

OBJECTIVE

This article aims to review recent developments in systemic carrier delivery platforms for PDT, with an emphasis on target specificity.

METHODS

Recent publications, spanning the last five years, concerning delivery carrier platforms for systemic PDT were reviewed, including PS conjugates, dendrimers, micelles, liposomes and nanoparticles.

RESULTS/CONCLUSION

PS conjugates and supramolecular delivery platforms can improve PDT selectivity by exploiting cellular and physiological specificities of the targeted tissue. Overexpression of receptors in cancer and angiogenic endothelial cells allows their targeting by affinity-based moieties for the selective uptake of PS conjugates and encapsulating delivery carriers, while the abnormal tumour neovascularisation induces a specific accumulation of heavy weighted PS carriers by enhanced permeability and retention (EPR) effect. In addition, polymeric prodrug delivery platforms triggered by the acidic nature of the tumour environment or the expression of proteases can be designed. Promising results obtained with recent systemic carrier platforms will, in due course, be translated into the clinic for highly efficient and selective PDT protocols.