Developments of polymer conjugation for protein delivery

G. Pasut
University of Padova, IT

Keywords: protein delivery

Summary:

The current proteomic era is generating great expectations for the consequences connected to discoveries on the role and activity of new proteins, which can represent new targets for highly specific therapies, or might even, themselves, become new drugs. Proteins as therapeutic agents are already holding a large share of newly approved drugs and are used in almost every field of medicine. This growing success, however, is not devoid of pitfalls, and often the therapeutic application of a new protein is hampered by unexpected shortcomings and/or poor therapeutic outcomes. Common issues are, for example low stability in vivo, short half-life and immunogenicity. Advanced approaches of protein delivery allowed the half-life prolongation of these fragile products. Different approaches have been investigated in the last few years to improve the pharmacokinetic and pharmacodynamic profile of biotech drugs. Among these, polymer conjugation can be considered one of the leading delivery approach for protein and it has already produced several conjugates presently in clinical practice. The rationale for polymer conjugation is the possibility to prolong the plasma half-life of therapeutically active agents by increasing their hydrodynamic volume and hence reducing the kidney excretion rate. Furthermore, polymer chains can prevent the approach of antibodies, proteolytic enzymes or cells on the surface of conjugated proteins, an effect obtained by the steric hindrance of polymer chains. Site selective conjugation is mandatory for a successful project of polymer conjugation. In this direction many chemical approaches have been settled with PEG, such as N-terminal-, thiol- and disulfide bridge-PEGylation. Recently, an active area of research is the development of enzymatic methods of polymer conjugation. Microbial transglutaminase (TGase) is an enzyme of particular interest because can catalyzes an acyl transfer between the glutamyl group of a glutamine (acyl donor) and a primary amine (acyl acceptor), usually the ε-amino group of a lysine but also the amino group of mPEG-NH2. mTGase-mediated PEGylation presents two important advantages: i) the modification of glutamine, a residue that otherwise cannot be modified with chemical methods and ii) the obtainment of highly homogenous conjugate isomer mixtures because no more than one or two glutamines, of all those present in a protein, can be a substrate for the enzyme. In the field of PEGylation many constrains are coming from the increasing number of patents. To overcome this aspect and to generate a second generation of polymer–protein conjugates we are looking to develop a new concept of polymer conjugation. In fact, so far most of the advantage of PEG have been reached thanks to both its high hydrodynamic volume and the steric hindrance of its polymer chains. The preparation of new polymers able to exploit further properties is in our opinion a promising way to reach the second generation of bioconjugates.