18F-Labeled ApoE mimetic peptide for ABCA1 imaging

1033 Objectives: Small Apolipoprotein E (ApoE) mimetic peptides have been shown to increase ATP-binding cassette transporter A1 (ABCA1)-mediated cholesterol efflux. CS-6253, a 26-residue peptide modeled after the C-terminus of ApoE, is a proposed ABCA1 agonist that reversed hypolipidation of ApoE in an Alzheimer’s disease model, and reversed brain pathology and cognitive deficits; ABCA1 is a viable therapeutic target in Alzheimer’s disease for treatment with peptides such as CS-6253. A novel peptide, ACMT, is a proposed ABCA-1 agonist; radiolabeling this peptide will allow for monitoring of ABCA1 activity in the brain using PET imaging. Methods: 19F-ACMT and 18F-ACMT were synthesized via a CuAAC reaction strategy, using ACMT modified at the side chain of a lysine residue as the azide and a fluorine-containing alkyne. ABCA1 mediated cholesterol efflux was measured in BHK cell lines transfected with a mifepristone switch to express ABCA1. Cells were labeled with 3H-labeled cholesterol (1 μCi/mL) and ABCA1 was induced with mifepristone, then cells were treated with the relevant peptides. The efflux of cholesterol was assessed by the ratio of cholesterol in the medium to total cholesterol (medium and cell lysate), measured by scintillation counter. PET-CT images were taken of normal BALB/c mice (n = 3) at 30 and 60 minutes after injection of 18F-ACMT. Mice were euthanized after the 60-minute scan and biodistribution of 18F-ACMT was measured using gamma counter. Results: 19F-ACMT was synthesized in high purity (≥ 99%). 18F-ACMT was radiosynthesized in excellent radiochemical purity (≥ 99%). Circular dichroism shows both ACMT and 19F-ACMT have alpha-helical structures, suggesting that the secondary structure of ACMT is not changed due to labeling. Unlabeled ACMT increased cholesterol efflux in BHK cells by 51% compared to mifepristone alone, while 19F-ACMT treatment led to an increase of 44%. These values are comparable to the increase of efflux due to CS6253 treatment (49%), and recombinant APOE-ɛ3 (64%). PET-CT imaging shows low brain uptake in normal BALB/c mice at 30 minutes after injection, with only slightly higher uptake at 1 hour. In the biodistribution studies, brain uptake was relatively low (%ID/g: 0.9%), while tracer levels in blood and lung were higher (%ID/g: 11.9% and 12.7%, respectively). Conclusions: We have successfully synthesized 18F-labeled peptide (18F-ACMT) in excellent radiochemical purity. ACMT appears to be as effective as CS-6253 in inducing ABCA1-mediated cholesterol efflux, and labeling the peptide with a fluorine-containing triazole does not have a significant effect on the peptide’s secondary structure and activity. In vivo studies showed relatively low brain uptake in normal BALB/c mice. Test of 18F-ACMT in animal disease models is underway to demonstrate the potential of 18F-ACMT as a promising imaging agent. Acknowledgement: This work was supported by the L.K. Whittier Foundation.