Fluorine-18 ImmunoPET Imaging of Antibody Brain Kinetics and Amyloid-Beta Pathology

Bispecific antibodies utilizing the transferrin receptor (TfR) for transport into the brain are being developed for both therapeutic and diagnostic targeting of the amyloid-β (Aβ) protein that deposits in the Alzheimer's disease (AD) brain. In contrast to traditional antibodies, TfR-binding bispecific antibodies display rapid and efficient brain uptake. However, due to differences in pharmacokinetic properties, it has been challenging to directly compare mono- and bispecific antibody brain uptake in vivo. Here, we have studied the Aβ antibody Bapineuzumab (Bapi) and its bispecific variant Bapi-Fab8D3, which contains a fragment of the TfR-binding antibody 8D3, enabling receptor-mediated transcytosis into the brain. Both antibodies were engineered to reduce binding to the neonatal Fc receptor (FcRn), thereby increasing their clearance from the blood. The antibodies were radiolabeled with fluorine-18 (¹⁸F) and administered to wildtype (WT) mice, which were PET scanned in an alternating manner to visualize antibody brain pharmacokinetics over a period of 9 h, followed by ex vivo analyses. Next, the bispecific antibody [¹⁸F]-F-Bapi-Fab8D3^FcRn- was used for PET imaging to quantify Aβ pathology in AD mouse model App ^NL‑G‑F mice at 12 h after antibody administration. [¹⁸F]-F-Bapi ^FcRn- and [¹⁸F]-F-Bapi-Fab8D3^FcRn- had identical blood concentration curves in the WT mice. PET data quantification demonstrated that while the brain concentration of [¹⁸F]-F-Bapi^FcRn- declined, that of [¹⁸F]-F-Bapi-Fab8D3^FcRn- increased throughout the 9 h time period, indicative of its active transport into the brain. PET imaging discriminated App ^NL‑G‑F from WT mice at 12 h after [¹⁸F]-F-Bapi-Fab8D3^FcRn- administration, suggesting that this novel antibody-based tracer may be used for the same-day PET imaging of Aβ.