Affinity-Tuned Albumin Hitchhiking Extends the Bioorthogonal Capture Window in Pretargeting Radiotheranostics

Pretargeted radiotheranostics decouple tumor recognition from radionuclide delivery and utilize in vivo click chemistry to achieve targeted delivery of radioactive payloads. A major barrier to efficient radionuclide delivery via the pretargeting approach is the suboptimal pharmacokinetics of the small-molecule radioligand, which rapid clearance restricts the in vivo reaction window for bioorthogonal capture, thereby also limiting clinical translation. Herein, we investigated an in situ albumin engagement strategy to modulate the pharmacokinetics of a clickable radioligand in a PSMA-targeted, polymer-assisted bispecific antibody pretargeting system. By incorporating an albumin binder into the small-molecule radioligand, systemic circulation was prolonged to expand the bioorthogonal capture window, without perturbing target affinity or receptor biology. This design may offer a modular advantage, as the clickable radioligand can be optimized independently while different pretargeting agents may, in principle, be adapted for alternative receptor systems. Within the PSMA-targeting BsAb/HBP system, the lead candidate (DOTA-mTz-sALB), compared to the non-albumin binding construct, increased tumor-associated uptake from ∼1.3 to ∼4.7%ID g^-1 (∼3.5-fold increase) and improved tumor-to-liver ratios from ∼0.1 to ∼0.6 (∼6-fold increase), while systemic background largely resolves within 24-48 h. Together, these results support albumin engagement as a modular pharmacokinetic-tuning element to improve pretargeted radiotheranostic delivery.