Abstract 4210: Non-invasive molecular imaging of SIRT1-mediated epigenetic regulation in cancer using PET/CT with a novel substrate-type radiotracer 2-[18F]PhAHA
Authors: Bonomi Robin E. , Laws Maxwell , Shavrin Aleksandr , Mangner Thomas , Gelovani Juri G. 
Published: July 2016

SIRT1 is involved in a wide variety of cellular processes and functions and is known to mediate cleavage of the acetyl moiety from the acetylated lysine residues of several proteins, including p53, PPARγ, members of the FOXO family and NF-KB. Therefore, SIRT1 has emerged as an important target for therapy of cancer. However, currently there are no methods for non-invasive monitoring of pharmacodynamics of novel SIRT1 isoform-selective inhibitors in norm and disease. Therefore, there is a need for development of agents for non-invasive imaging of expression and activity of various SIRT1 in vivo.

Previous studies demonstrated that SIRT1 can dephenylacetylate a lysine residue at a rate of ∼56% of its natural deacetylation rate, while other SIRT isoforms cannot cleave an aromatic leaving group. This provided an opportunity to develop a SIRT1-selective imaging agent for PET imaging, which would allow for quantitative measurement of the SIRT1 expression-activity product in vivo. We developed a focused library of compounds to elucidate the structure-activity relationship of SIRT1 and to identify the most selective and efficient imaging substrate for SIRT1. This preliminary screening utilized high-throughput fluorogenic assay with the Carbazol-L-Lysine-Aminomethylcoumarin as a backbone [1]. Subsequently, this backbone was exchanged for 6-aminohexanoicanilide, AHA [2] to develop PET radiotracer. The lead compound, 2-fluorophenylaminohexanoicanilide, was developed in both F-19 (2-FPhAHA) and F-18 (2-[18F]PhAHA) versions. 2-[18F]PhAHA was injected i.v. into Sprague-Dawley (SD) rats followed by dynamic imaging using MicroPET R4 (Siemens, TN), followed by CT imaging on INVEON microCT (Siemens, TN). The kinetics of radiotracer uptake was quantified using Logan graphical analysis [3], which demonstrated differential accumulation of 2-[18F]PhAHA-derived radioactivity in specific areas of the rat brain with high expression of SIRT1 (i.e., the dentate gyrus, CA1, nucleus accumbens, and caudate putamen). Following characterization of 2-[18F]PhAHA-derived radioactivity in the normal rat brain, 2-[18F]PhAHA was administered to SD rats bearing intracerebral 9L gliomas. PET/CT demonstrated significantly increased uptake of 2-[18F]PhAHA-derived radioactivity in the 9L tumors vs. normal brain tissue (P&lt;0.01) and matched with tumor localization as observed with MRI and histological staining.

In summary, SIRT1 activity is upregulated in intracerebral 9L gliomas, as evidenced by increased accumulation of 2-[18F]PhAHA on PET/CT images, which demonstrates the feasibility of this method for visualization and quantification of SIRT1 activity in brain tumors. Also, 2-[18F]PhAHA on PET/CT should allow for future monitoring of pharmacodynamics of SIRT1 inhibitors during anti-cancer therapy.

Citation Format: Robin E. Bonomi, Maxwell Laws, Aleksandr Shavrin, Thomas Mangner, Juri G. Gelovani. Non-invasive molecular imaging of SIRT1-mediated epigenetic regulation in cancer using PET/CT with a novel substrate-type radiotracer 2-[18F]PhAHA. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 4210.