Radiosynthesis and preliminary evaluation of an 18F-labeled ApoE mimetic peptide CS-6253 for PET imaging of ABCA1.

Objectives: ATP-binding cassette transporter A1 (ABCA1) is a viable therapeutic target in Alzheimer’s disease for treatment with peptides such as CS-6253. CS-6253 is a 26-residue peptide derived from the C-terminus of Apolipoprotein E (ApoE) that has been shown to increase ABCA1-mediated cholesterol efflux. Radiolabeling CS-6253 will allow for monitoring of ABCA1 activity in the brain using PET imaging.

Methods: CS-6253 contains a lysine residue (Lys-25) at the C-terminal side that can be modified while retaining the bioactivity of the peptide, which allows for radiolabeling. An azide-modified amino acid was incorporated into CS-6253 by solid-phase peptide synthesis to form the radiolabeling precursor (Azido-CS-6253). ¹⁸F-CS-6253 was radiosynthesized via a CuAAC reaction strategy, using Azido-CS-6253 and an ¹⁸F-containing alkyne. The stability of ¹⁸F-CS-6253 in mouse serum was determined at 1 and 2 hours after incubation. PET-CT images were performed in normal nude mice (n = 3) at 1 and 2 hours after tail vein injection of ¹⁸F-CS-6253. Biodistribution of ¹⁸F-CS-6253 in major organs was measured using PET.

Results: ¹⁸F-CS-6253 was radiosynthesized with a radiochemical purity of ≥ 99%. The stability results show that ¹⁸F-CS-6253 is stable in mouse serum after 2 h incubation. PET imaging shows that brain uptake of ¹⁸F-CS-6253 at 1 h post-injection (pi) was 2.10 ± 0.08 %ID/g and 1.93 ± 0.05 %ID/g after 2 h. Uptake of ¹⁸F-CS-6253 in the liver was 12.93 ± 1.60 %ID/g at 1 h pi and 12.40 ± 1.35 %ID/g at 2 h pi. Kidney uptake was 11.43 ± 0.68 and 10.10 ± 0.81 %ID/g at 1 h and 2 h, respectively, and muscle uptake was 1.43 ± 0.12 %ID/g at 1 h and 1.67 ± 0.09 %ID/g at 2 h. The brain-to-muscle uptake ratio of ¹⁸F-CS-6253 was measured to be 1.47 ± 0.14 at 1 h and 1.16 ± 0.04 at 2 h, respectively.

Conclusions: ¹⁸F-CS-6253 has been successfully synthesized with excellent radiochemical purity. Brain uptake of ¹⁸F-CS-6253 was observed by PET in normal mice. Tests of ¹⁸F-CS-6253 in animal disease models are underway to demonstrate the potential of ¹⁸F-CS-6253 as an ABCA1 imaging agent in various phenotypes. Research Support: This work was supported by R01AG067063 and R21AG056518 from the National Institute of Aging.