In recent years, macromolecular conjugation of PEG has emerged as an effective strategy to alter the pharmacokinetics (PK) of various drugs, thereby enhancing their therapeutic potential. However, PEGylation results in loss of binding affinity due to steric hindrance interfering with drug-target binding interactions. The pharmacodynamic (PD) properties of drugs are measured at the molecular level by parameters such as receptor binding affinity or enzyme activity. This loss of pharmacodynamics is offset by the longer circulating half-life of the drug, so the effect of PEG is to alter the balance between pharmacodynamic and pharmacokinetic properties, compensating for the decreased binding affinity by increased systemic exposure. And the resulting changes in PK-PD have in some cases enabled the development of otherwise untapped drugs, and in others the improvement of existing drugs.
Key characteristics of PEG polymers include PEG molecular weight (MW), branching, and end group chemistry. Factors such as the modification site of PEG, the size of PEG, and the linkage between peptide and PEG can all affect the biological activity. The size and hydrophobicity of the end groups are the keys to determining the size of the binding affinity. The kinetics of proteins attached to polymers are substantially affected by the kinetics of the polymer itself. Therefore, before evaluating a specific PEG-protein conjugate, it is necessary to analyze the plasma kinetics and tissue distribution of PEG and PEG-protein conjugates separately.
1. Absorb
PEG is an amphiphilic substance with a long chain that can bind multiple water molecules. It is bulky and may not easily enter the blood circulation from the gastrointestinal tract environment, so it is less absorbed. In the topical administration test, the transdermal absorption rate of PEG also depends on their molecular weight. Low molecular weight PEG can enter the body through intact skin to a low degree, while PEG with molecular weight higher than 4000 Da can only be absorbed by the body when the protective barrier of the skin is damaged. PEG with a molecular weight of 2000 is considered to be the cut-off value that can be taken up by epithelial cell membranes via paracellular transport or endocytosis.
At the injection site, PEG-50 retained longer than PEG-6 at the injection site after intramuscular and subcutaneous injections, indicating that the absorption of PEG from intramuscular and subcutaneous injection sites is molecular weight dependent. However, for intraperitoneal injection, the in vivo uptake of PEG of different molecular weights is very similar. The difference in the curves of different molecular weight PEG drugs is mainly related to the pore size of the renal vascular bed.
2. Distribution
PEGylation may alter the tissue distribution of the drug due to the physicochemical changes it induces in modifying the parent drug. The relative preferential distribution of PEGylated drugs in certain tissue sites can be regarded as the basis of drug targeting, and the molecular weight of PEG is an important factor in determining the targeting properties. Macromolecules with prolonged circulation accumulate in tumor tissue in large numbers. For PEG molecules of 10 KDa or larger, the relative uptake in tumor tissues was higher than in normal tissues. The drug-PEG-liposome combination can significantly alter the biodistribution of the parent drug and specifically bind to targeted tumor cells in vivo. Studies have shown that PEG-coated doxorubicin-loaded liposomes significantly increase the distribution of tumor sites. In addition, compared with the non-PEGylated control group, the distribution of PEGylated nanospheres used for regional lymph node imaging diagnosis was significantly changed, and the localization of the target site was enhanced.
3. Metabolism
PEG is generally considered to be a non-biodegradable polymer, but reports clearly show that PEG can be oxidatively degraded by various enzymes, such as alcohol dehydrogenase, aldehyde dehydrogenase, and cytochrome p450-dependent oxidase. Phase I metabolism of PEG is mainly mediated by alcohol dehydrogenase and aldehyde dehydrogenase. Molecular weight has an important effect on the phase I metabolism of PEG. About 25% of the dose of PEG 400 was metabolized in vivo, but metabolism decreased with increasing MW. PEG with molecular weight less than 400 can be converted into toxic metabolites in vivo by alcohol dehydrogenase, while PEG used for drug or preparation modification has a larger molecular weight and is rarely degraded by enzymes.
PEGylation affects the metabolism of the attached drug by two mechanisms, shielding plasmin through steric hindrance and reducing RES phagocytosis. Therefore, after the drug is modified by PEG, the PEG polymer with larger particle size can also be metabolized by the enzyme, but the speed of biotransformation is significantly slower than that of system elimination.
In addition, the linkages between PEG and the drug also play a role in the metabolism of the parent drug because they determine the release rate of the parent drug.
4. Excretion
PEGylation may lead to physicochemical changes of the parent drug compound, which may lead to a reduction in the efficiency of the drug clearance process. In general, therapeutic proteins have a very limited lifespan in circulation due to efficient elimination mechanisms in vivo, such as proteolysis, specific cell-mediated protein degradation pathways, and capture by RES.
The circulating half-life (t1/2) of PEG increases with increasing molecular weight. For example, when the molecular weight was increased from 6 kDa to 50 kDa, the t1/2 increased from 18 minutes to 16.5 hours. PEG with a molecular weight of 40 to 50 kDa can delay the glomerular filtration of small molecules. For example, the systemic clearance rate reached 6.6–29.2 lit/hr after intravenous injection of IFN-α, and when conjugated to 5 kDa linear PEG, the systemic clearance rate was significantly reduced to 2.5–5 lit/hr.
Protein clearance depends on the net ionic charge of the protein at physiological pH, its molecular weight, and the presence of protein-specific receptors in the cell responsible for protein uptake. PEG molecules and PEG-protein conjugates with molecular weights less than 20 kDa are cleared in urine, while larger molecular weight PEG-protein conjugates are cleared slowly in urine and feces. With the increase of the molecular weight of PEG adhering to the drug, the main elimination route shifted from the renal route to the hepatic route.
Due to the ease of elimination of PEG, PEG was not significantly toxic over a wide dose range. PEG itself is only toxic at high doses parenterally. When the PEG dose exceeds the renal maximum load, the kidney is the only target organ attacked by PEG. Acute tubular necrosis has been reported in patients receiving intravenous lorazepam (Ativan) after a cumulative dose of 240 g of PEG 400.
