Development of brain-penetrable antibody radioligands for in vivo PET imaging of amyloid-β and tau

Abstract

IntroductionAlzheimer's disease (AD) is characterized by the misfolding and aggregation of two major proteins: amyloid-beta (Aβ) and tau. Antibody-based PET radioligands are desirable due to their high specificity and affinity; however, antibody uptake in the brain is limited by the blood–brain barrier (BBB). Previously, we demonstrated that antibody transport across the BBB can be facilitated through interaction with the transferrin receptor (TfR), and the bispecific antibody-based PET ligands were capable of detecting Aβ aggregates via ex vivo imaging. Since tau accumulation in the brain is more closely correlated with neuronal death and cognition, we report here our strategies to prepare four F-18-labeled specifically engineered bispecific antibody probes for the selective detection of tau and Aβ aggregates to evaluate their feasibility and specificity, particularly for in vivo PET imaging.MethodsWe first created and evaluated (via both in vitro and ex vivo studies) four specifically engineered bispecific antibodies, by fusion of single-chain variable fragments (scFv) of a TfR antibody with either a full-size IgG antibody of Aβ or tau or with their respective scFv. Using [18F]SFB as the prosthetic group, all four 18F-labeled bispecific antibody probes were then prepared by conjugation of antibody and [18F]SFB in acetonitrile/0.1 M borate buffer solution (final pH ∼ 8.5) with an incubation of 20 min at room temperature, followed by purification on a PD MiniTrap G-25 size exclusion gravity column.ResultsBased on both in vitro and ex vivo evaluation, the bispecific antibodies displayed much higher brain concentrations than the unmodified antibody, supporting our subsequent F18-radiolabeling. [18F]SFB was produced in high yields in 60 min (decay-corrected radiochemical yield (RCY) 46.7 ± 5.4) with radiochemical purities of >95%, confirmed by analytical high performance liquid chromatography (HPLC) and radio-TLC. Conjugation of [18F]SFB and bispecific antibodies showed a 65%–83% conversion efficiency with radiochemical purities of 95%–99% by radio-TLC.ConclusionsWe successfully labeled four novel and specifically engineered bispecific antibodies with [18F]SFB under mild conditions with a high RCY and purities. This study provides strategies to create brain-penetrable F-18 radiolabeled antibody probes for the selective detection of tau and Aβ aggregates in the brain of transgenic AD mice via in vivo PET imaging.

[125I]IPC-Lecanemab: Synthesis and Evaluation of Aβ-Plaque-Binding Antibody and Comparison with Small-Molecule [18F]Flotaza and [125I]IBETA in Postmortem Human Alzheimer’s Disease

“…Despite the large size of antibodies leading to low uptake, their high affinity for Aβ plaques with therapeutic implications makes imaging highly significant [32][33][34]. Several efforts have been made to increase their brain uptake [35][36][37][38]. These efforts alter the antibody targeting Aβ, such as conjugating to a transferrin receptor, to enable receptor-mediated transcytosis across the BBB for imaging mice models of AD [37,38].…”

“…Several efforts have been made to increase their brain uptake [35][36][37][38]. These efforts alter the antibody targeting Aβ, such as conjugating to a transferrin receptor, to enable receptor-mediated transcytosis across the BBB for imaging mice models of AD [37,38]. Although the brain uptake of the antibodies is very low, preliminary efforts have been made to target a functionalized antibody that was preinjected in AD mice using the PET radioisotope copper-64 [39].…”