ImmunoPET imaging of amyloid-beta in a rat model of Alzheimer’s disease with a bispecific, brain-penetrating fusion protein

Abstract

Background Hijacking the transferrin receptor (TfR) is an effective strategy to transport amyloid-beta (Aβ) immuno-positron emission tomography (immunoPET) ligands across the blood–brain barrier (BBB). Such ligands are more sensitive and specific than small-molecule ligands at detecting Aβ pathology in mouse models of Alzheimer’s disease (AD). This study aimed to determine if this strategy would be as sensitive in rats and to assess how TfR affinity affects BBB transport of bispecific immunoPET radioligands. Methods Two affinity variants of the rat TfR antibody, OX26, were chemically conjugated to a F(ab′)2 fragment of the anti-Aβ antibody, bapineuzumab (Bapi), to generate two bispecific fusion proteins: OX265-F(ab′)2-Bapi and OX2676-F(ab′)2-Bapi. Pharmacokinetic analyses were performed 4 h and 70 h post-injection of radioiodinated fusion proteins in wild-type (WT) rats. [124I]I-OX265-F(ab′)2-Bapi was administered to TgF344-AD and WT rats for in vivo PET imaging. Ex vivo distribution of injected [124I]I-OX265-F(ab′)2-Bapi and Aβ pathology were assessed. Results More [125I]I-OX265-F(ab′)2-Bapi was taken up into the brain 4 h post-administration than [124I]I-OX2676-F(ab′)2-Bapi. [124I]I-OX265-F(ab′)2-Bapi PET visualized Aβ pathology with significantly higher signals in the TgF344-AD rats than in the WT littermates without Aβ pathology. The PET signals significantly correlated with Aβ levels in AD animals. Conclusion Affinity to TfR affects how efficiently a TfR-targeting bispecific fusion protein will cross the BBB, such that the higher-affinity bispecific fusion protein crossed the BBB more efficiently. Furthermore, bispecific immunoPET imaging of brain Aβ pathology using TfR-mediated transport provides good imaging contrast between TgF344-AD and WT rats, suggesting that this immunoPET strategy has the potential to be translated to higher species.

Receptor-mediated drug delivery of bispecific therapeutic antibodies through the blood-brain barrier

“…Affinity dematuration produced a variant, designated OX26–76, with a low affinity for the rat TfR and a KD of 76 nM ( Thom et al, 2018 ). The brain uptake of the OX26–76 was compared to the brain uptake of the wild type OX26 antibody, designated OX26–5, which had high affinity for the TfRMAb and a KD of 5 nM ( Bonvicini et al, 2022 ). The brain uptake in the rat of the OX26–5 was 10-fold higher than the brain uptake of the OX26–76, which was at the background level ( Bonvicini et al, 2022 ).…”

“…The brain uptake of the OX26–76 was compared to the brain uptake of the wild type OX26 antibody, designated OX26–5, which had high affinity for the TfRMAb and a KD of 5 nM ( Bonvicini et al, 2022 ). The brain uptake in the rat of the OX26–5 was 10-fold higher than the brain uptake of the OX26–76, which was at the background level ( Bonvicini et al, 2022 ). The low affinity variants of the OX26 antibody were produced by mutations of amino acids within the CDRs of the variable regions of the OX26 antibody ( Thom et al, 2018 ).…”

Imaging of Tauopathies with PET Ligands: State of the Art and Future Outlook

“…The pioneering method of Wang et al, consists of using transferrin receptor (TfR)-mediated transcytosis [76][77][78]. OX265 and OX2676, two variants of mouse anti-rat Tfr (rTfR), were synthesized and linked to bapineuzumab (Bapi), an anti-Aβ antibody F(ab')2 fragment, generating OX265-F(ab')2-Bapi and OX2676-F(ab')2-Bapi [79]. The two compounds were radiolabeled with 124 I and 125 I.…”