PEG chains were covalently linked at the glutamine (Gln) protein residue’s carbox­amide group using the transglutaminase (TGase) enzyme (Mero etal. 2009). TGases are an enzyme type that can catalyze an acyl transfer between two proteins using the glutamyl group of glutamine as the acyl donor and a primary amine as the acyl acceptor, typically the ε-amino of lysine. It is important to emphasize that glutamine cannot be altered chemically without tagging other residues or impairing the struc­ture of the protein. Because TGase has stringent constraints for the amino acid sequence and substrate exibility, this enzymatic conjugation is selective (Fontana etal. 2008). Thus, just one or two of the many glutamines in a protein typically meet the requirements for TGase catalysis.

PEGylated proteins must be puried to extract the end product from complex mix­tures. It is necessary to separate the specic PEG–protein conjugate from unreacted proteins, proteins that have been over-PEGylated, unreacted PEG reagent, and addi­tional reagents that will eventually be incorporated into the PEGylation mixture. Typically, one seeks to take advantage of variances in physicochemical features while constructing a bioseparation method. Variability in charge, hydrodynamic radii, hydrophobicity, and, in some circumstances, afnity is used for the separation of the target PEG-protein. The complexity of the PEGylation mixture often deter­mines how effectively the purication procedure produces the necessary homogene­ity of the product (Veronese etal. 1997).

2012). Unfortunately, the separation speed is slowed down by the diffusion limita-

tion of column-based IEC based on porous particle chromatography. PEGylated proteins typically have a binding capability on IEC media that is weaker than native proteins. Both charge shielding and an increase in hydrodynamic radius, which reduces diffusivity, have been used to explain this phenomenon. While this makes it simple to separate native proteins and PEGylated proteins practically, it is challeng­ing to fractionate multiple PEGylated forms.

The two most crucial elements that affect the concentration of PEG-modied thera­peutics in blood circulation are molecular weight and injection site. The elimination rates from the injection point are intraperitoneal (IP)>subcutaneously (SC)>intra­muscularly (IM) [109]. The drug delivered as IM injection forms a reservoir at the injection site and progressively diffuses into the bloodstream; to a smaller degree, the same is true for the drug administered SC.As PEG’s molecular mass increases, the elimination rate for SC and IM reduces. The pharmacokinetic characteristic of the PEG adducts is signicantly inuenced by the mode of delivery (Veronese and Mero 2008).

Analysis techniques for PEG protein adducts and PEG reagents are needed at differ­ent phases of the development process to create a safe and effective PEGylated ther­apy. To successfully develop PEGylated therapies, properly characterizing PEG reagents, which serve as a critical raw material, is imperative. PEG reagent quality can vary signicantly depending on molecular weights, polydispersity, impurity presence, and activation degree. NMR is often employed to determine the functional groups both qualitatively and quantitatively, and it is the release method chosen by all manu­facturers to determine the terminal activity of activated PEGs. A derivatization pro­cess is required to achieve UV absorbance because most PEG reagents are UV transparent and non-uorescent. Reversed-phase (RP)-HPLC, for instance, can be used to test methoxy-PEG aldehyde after it has been derivatized with 4-aminobenzoic acid (Zhang et al. 2014b). The variation in molecular mass of PEG chains can be determined using reverse phase chromatography and Size Exclusion Chromatography in conjunction with corona detection mode, enabling impurity detection in the nal PEG reagent. The molecular weight of PEG, which affects the protein’s nal half-life and directly impacts the bioavailability of PEGylated medicines, is another aspect of PEG that needs to be carefully managed (Wang etal. 2020b). The same technique is employed to identify polydispersity and the major peak fraction in PEG.