PEG linkers (polyethylene glycol linkers) play an indispensable role as "invisible foundational building blocks" in radionuclide drug conjugates (RDCs). Although seemingly just auxiliary components, they directly impact the stability, targeting efficiency, and metabolic fate of RDC drugs in the body. Within the complex structure of an RDC, the PEG linker serves as a critical bridge connecting the targeting ligand (e.g., antibodies, peptides) to the chelator that complexes the radionuclide. Its characteristics are crucial for constructing a successful RDC molecule.
Firstly, PEG linkers can significantly improve the physicochemical properties and pharmacokinetic profiles of RDC drugs. PEG chains are highly hydrophilic; incorporating them into RDC molecules can effectively enhance the overall water solubility of the conjugate. This is particularly important for hydrophobic peptides or small molecule ligands, helping to prevent drug molecule aggregation and improve biodistribution. Furthermore, by precisely controlling the PEG chain length (e.g., PEG4, PEG8, PEG12), researchers can fine-tune the linker's flexibility and spatial structure. This helps reduce the steric hindrance caused by the chelator and radionuclide on the targeting ligand's activity, ensuring the ligand can efficiently and accurately recognize and bind to targets on tumor cells.
Secondly, PEG linkers play a decisive role in optimizing the in vivo biodistribution of RDC drugs and improving their therapeutic index. Studies have shown that PEGylation can accelerate the clearance of RDC drugs from the blood while enhancing their uptake and retention in tumor sites. This achieves a higher tumor-to-normal tissue (e.g., kidney, liver, bone marrow) radioactive absorbed dose ratio, effectively widening the therapeutic window. For instance, by introducing a short PEG4 linker between an antibody and a chelator, scientists successfully developed bivalent DOTA-radiohaptens (like "Gemini"). These constructs demonstrated excellent tumor uptake and retention in pretargeted radioimmunotherapy while maintaining rapid renal clearance, enabling curative tumor treatment with lower radioactivity. Similarly, in targeted alpha therapy research for prostate cancer, ⁴Ce/²²⁵Ac-Macropa-PEG4-YS5 conjugates containing a PEG4 linker showed the highest tumor uptake and best anti-tumor efficacy compared to analogs without PEG or with overly long linkers.
Finally, the application of PEG linkers also facilitates the exploration of novel RDC designs and multifunctional platforms. For example, PEG linkers can be used to construct peptide-based multimerization platforms (such as PEGibodies). By forming nanoparticles, these platforms increase the binding affinity of the ligand to its receptor, significantly prolonging drug retention time in tumors without necessarily extending the circulation half-life in the blood. In targeted alpha therapy research for EpCAM-positive tumors, site-specific conjugation of the antibody HEA125 was achieved using a maleimide-PEG4 linker. The resulting [²²⁵Ac]Ac-Macropa-PEG4-HEA125 conjugate exhibited high immunoreactivity, excellent in vitro stability, and potent in vivo anti-tumor activity with minimal off-target accumulation. These examples fully demonstrate the critical role PEG linkers play in advancing RDC drugs towards greater precision, safety, and efficacy.
