Synthesis and preliminary evaluation of an ¹8F-labeled oleic acid analog for PET imaging of fatty acid uptake and metabolism.

INTRODUCTION: Imaging fatty acid uptake and utilization has broad impact in investigating myocardial diseases, hepatic functions, tumor progression, and the metabolic state of adipose tissue. The SPECT tracer (123)I-15-(p-iodophenyl)-3-(R,S)-methylpentadecanoic acid (BMIPP) is a clinically used nuclear medicine tracer to image myocardial uptake of fatty acid. Although ((18)F-5) has been in clinical use for PET imaging of adipose tissue as well as the myocardium, here we developed a click oleate analog to compare to FTO, with the goal of improved stability to defluorination and suitability for imaging myocardial uptake and oxidation of fatty acids. METHODS: A rapid and convenient synthetic approach for a precursor to a (18)F-labeled oleate analog using click chemistry was developed and evaluated for PET imaging in fasted mice. RESULTS: The overall yield for the preparation of the labeling precursor of the clicked oleate analog was 12%. This precursor was efficiently radiolabeled with F-18 in 17% non-decay-corrected radiochemical yield. PET/CT imaging and biodistribution results show that this fatty acid analog had reasonable heart uptake (0.94±0.28 %ID/g at 0.5 h p.i.) and heart-to-muscle ratio (2.05±0.39 at 0.5h p.i.) and is a potential lead for developing new PET tracers to image fatty acid uptake and utilization using click chemistry methodologies. The synthetic route to FTO was optimized to three steps from known starting materials. CONCLUSION: While the uptake of the clicked oleic acid analog was sufficient for visualizing the myocardium in mice, the preliminary metabolism data suggest that only a fraction of the uptake was due to fatty acid beta-oxidation. Studies are under way to explore the uptake/oxidation mechanism and kinetics.

Evaluation of a triple-helical peptide with quenched FluorSophores for optical imaging of MMP-2 and MMP-9 proteolytic activity.

Matrix metalloproteinases (MMP) 2 and 9, the gelatinases, have consistently been associated with tumor progression. The development of gelatinase-specific probes will be critical for identifying in vivo gelatinoic activity to understand the molecular role of the gelatinases in tumor development. Recently, a self-assembling homotrimeric triple-helical peptide (THP), incorporating a sequence from type V collagen, with high substrate specificity to the gelatinases has been developed. To determine whether this THP would be suitable for imaging protease activity, 5-carboxyfluorescein (5FAM) was conjugated, resulting in 5FAM3-THP and 5FAM6-THP, which were quenched up to 50%. 5FAM6-THP hydrolysis by MMP-2 and MMP-9 displayed kcat/KM values of 1.5 × 104 and 5.4 × 103 M-1 s-1, respectively. Additionally 5FAM6-THP visualized gelatinase activity in gelatinase positive HT-1080 cells, but not in gelatinase negative MCF-7 cells. Furthermore, the fluorescence in the HT-1080 cells was greatly attenuated by the addition of a MMP-2 and MMP-9 inhibitor, SB-3CT, indicating that the observed fluorescence release was mediated by gelatinase proteolysis and not non-specific proteolysis of the THPs. These results demonstrate that THPs fully substituted with fluorophores maintain their substrate specificity to the gelatinases in human cancer cells and may be useful in in vivo molecular imaging of gelatinase activity.