Thiol-Reactive PODS-Bearing Bifunctional Chelators for the Development of EGFR-Targeting [(18)F]AlF-Affibody Conjugates

Site-selective bioconjugation of cysteine-containing peptides and proteins is currently achieved via a maleimide–thiol reaction (Michael addition). When maleimide-functionalized chelators are used and the resulting bioconjugates are subsequently radiolabeled, instability has been observed both during radiosynthesis and post-injection in vivo, reducing radiochemical yield and negatively impacting performance. Recently, a phenyloxadiazolyl methylsulfone derivative (PODS) was proposed as an alternative to maleimide for the site-selective conjugation and radiolabeling of proteins, demonstrating improved in vitro stability and in vivo performance. Therefore, we have synthesized two novel PODS-bearing bifunctional chelators (NOTA-PODS and NODAGA-PODS) and attached them to the EGFR-targeting affibody molecule Z(EGFR:03115). After radiolabeling with the aluminum fluoride complex ([(18)F]AlF), both conjugates showed good stability in murine serum. When injected in high EGFR-expressing tumor-bearing mice, [(18)F]AlF-NOTA-PODS-Z(EGFR:03115) and [(18)F]AlF-NODAGA-PODS-Z(EGFR:03115) showed similar pharmacokinetics and a specific tumor uptake of 14.1 ± 5.3% and 16.7 ± 4.5% ID/g at 1 h post-injection, respectively. The current results are encouraging for using PODS as an alternative to maleimide-based thiol-selective bioconjugation reactions.