Radiosynthesis and preclinical evaluations of [18F]AlF-RESCA-5F7 as a novel molecular probe for HER2 tumor imaging.

HER2 overexpression is associated with various tumor types and prompted the development of targeted therapies. Previously, iso-[²¹¹At]SGMAB-5F7 was developed as a HER2-targeted alpha therapy agent, demonstrating promising therapeutic efficacy in the preclinical stage. Aiming for an ¹⁸F-labeled tracer for companion diagnostics in clinical translation, we employed the Al¹⁸F-RESCA strategy in our current work and investigated whether [¹⁸F]AlF-RESCA-5F7 could visualize HER2 expression in vivo. [¹⁸F]AlF-RESCA-5F7 was attained with high radiochemical purity (> 99%) and molar activity in the range of 16.5 ± 8.8 GBq/μmol (n = 8). Compared to previously reported radiotracers that contained 5F7 as the HER2-targeting carrier and fluorine-18 as the positron-emitting isotope, the radiosynthesis was simplified to one single step within 30 min. The dissociation constant of [¹⁸F]AlF-RESCA-5F7 was determined as 3.3 nM via saturation binding assay using SKOV3 ovarian carcinoma cells. Tumor uptake of the novel tracer in Balb/c nude mice bearing SKOV3 xenografts was 4.69 ± 1.51, 3.34 ± 0.82 and 3.77 ± 0.99 %ID/g at 1, 2, and 4 h post-injection. Even though high retention of radioactivity was seen in the kidneys, micro-PET/CT imaging of [¹⁸F]AlF-RESCA-5F7 delineated the tumor up to 4 h post-injection with minimal activity in the gallbladder, intestines, and bone. This study suggests that [¹⁸F]AlF-RESCA-5F7 is a promising HER2 PET radiotracer with an eased radiolabeling method. Whether [¹⁸F]AlF-RESCA-5F7 could work as a companion diagnostic agent to assist in patient stratification and treatment monitoring of iso-[²¹¹At]SGMAB-5F7 warrants further investigation.