PEGylated Proteins: How Much Does Molecular Weight Matter?
Diego Fornasari
Abstract
Polyethylene glycols (PEGs) are inert polymers of repeating ethylene oxide subunits. Attaching PEGs to therapeutic proteins may reduce the protein’s immunogenicity and antigenicity, improve solubility and stability, slow protein degradation, and increase the half-life (t½). This usually results in less frequent administration, improved quality of life and convenience, and potentially better adherence and lower costs. The advantages and disadvantages of PEGylated proteins differ according to the structure of the PEG moiety, particularly its molecular weight. The larger the PEG molecular weight, the longer the t½ and time to steady state. PEGs have low toxicity and undergo minimal metabolism. The PEG moiety usually undergoes renal elimination and is excreted in urine, but with greater molecular weights, renal elimination declines and biliary excretion increases. Because PEG molecules are not broken down, there is potential for PEGs to accumulate in the cytoplasm, forming vacuoles, mostly in macrophages, although this does not affect their function. The risk of vacuolation increases with molecular weights > 30 kDa. However, even high molecular weight PEGs are used at doses markedly lower than the European Medicines Agency safety threshold for paediatric use. People can develop antibodies to PEGs, and this may increase the overall clearance of the PEGylated protein if antibody levels are sufficiently high (> 500 ng/mL according to one modelling study). In conclusion, it is important for physicians to understand how PEG molecular weight and architecture can impact stability, immunogenicity, glomerular filtration and cellular uptake, to better understand the overall safety, efficacy and pharmacological profile of PEGylated proteins. The effectiveness and safety of proteins used to treat medical conditions are affected by the immune response they may generate. Attaching a polymer to the protein can protect it from the immune system and slow down its removal from the body (elimination). A common polymer used for this purpose is polyethylene glycol (PEG). ‘PEGylation’ changes the physicochemical properties of the protein but does not affect the protein structure or biological activity. Different lengths and configurations of PEG polymer chains are used to PEGylate proteins. This article describes how the PEG size (molecular weight) affects the properties of the therapeutic protein. The higher the molecular weight, the slower the process of elimination, so products containing large PEGs are retained in the body for longer and require less frequent dosing. This is a major advantage for treatments taken over long periods (e.g. clotting factors for haemophilia), significantly improving the convenience of treatment and patient quality of life. Small PEGs are eliminated by the kidneys in urine, but larger PEGs are eliminated through bile and eventually faeces. There is a risk that larger PEGs may accumulate in cells (a process called vacuolation), although this does not generally affect the normal functioning of the cell. Animal studies suggested that problems could arise if large PEGs formed vacuoles in the choroid plexi in the brain. However, the European Medicines Agency has set safety standards to limit this possibility, and even the largest PEGs in current use fall well within these safety standards.