TY - JOUR T1 - Radiosynthesis and in vitro evaluation of [<sup>11</sup>C]Shield-1, a PET probe for imaging CAR T cells that express the iCasp9 suicide gene <strong/> JF - Journal of Nuclear Medicine JO - J Nucl Med SP - 61 LP - 61 VL - 59 IS - supplement 1 AU - Katheryn Lohith AU - Soojin Kwon AU - Prashanth Padakanti AU - Sandeep Thanna AU - Catherine Hou AU - Chi-Chang Weng AU - Mark Sellmyer AU - Robert Mach AU - Michael Farwell Y1 - 2018/05/01 UR - http://jnm.snmjournals.org/content/59/supplement_1/61.abstract N2 - 61Objectives: Recently, T cells engineered with chimeric antigen receptors (CARs) that target tumor-associated antigens have shown striking activity in a variety of malignancies. However, assessment of treatment efficacy in solid tumors has proved to be challenging, since the fate and localization of the administered cells cannot be assessed directly. We developed [11C]Shield-1, a novel PET probe that binds to F36V-FKBP (a mutant human FK506 binding protein) with high affinity and specificity, to noninvasively track CAR T cells that have been transduced with the iCasp9 suicide gene, in which the recruitment domain of caspase 9 has been replaced by F36V-FKBP. Since iCasp9 is only activated by dimeric ligands such as rimiducid, monomeric ligands such as Shield-1 do not activate the suicide gene. We plan to utilize the iCasp9 system, which is currently in clinical use, as both a safety switch and a PET imaging reporter gene. Methods The radiosynthesis of [11C]Shield-1 was carried out by methylating the synthesized phenol precursor with [11C]methyl iodide in the presence of base (12N NaOH) at 80 °C for 5 min followed by HPLC purification. In vitro cell-uptake studies with [11C]Shield-1 were performed using two cell lines (HEK293 and HCT116) transduced with F36V-FKBP, with untransduced cells as a control. Blocking studies were performed using 10 μM FK506 (Tacrolimus), an immunosuppressive drug that binds to both FKBP and F36V-FKBP. To assess clearance from blood and major organs, biodistribution studies were carried out at 2 min, 25 min, and 45 min following intravenous administration of [11C]Shield-1 in BALB/c mice. Results [11C]Shield-1 was produced with high radiochemical purity (&gt;99%) and specific activity (&gt;1,800 mCi/μmol). [11C]Shield-1 uptake in F36V-FKBP transduced cells was 3 - 4 fold higher at 40 min compared to untransduced cells and cells blocked with FK506. Biodistribution studies revealed rapid clearance of [11C]Shield-1 from the blood, with low uptake in muscle, spleen, and bone, and increasing uptake in the small bowel over time due to hepatic clearance. Conclusion [11C]Shield-1 was successfully synthesized and its in vitro evaluation showed high and specific uptake in cells expressing the F36V-FKBP reporter gene. Future studies will explore the ability of iCasp9 to serve as a dual function suicide-reporter gene for CAR T cells in a mouse tumor model. We expect that clinical translation of [11C]Shield-1 will be relatively straightforward given that iCasp9 is already in clinical use as a suicide gene in CAR T cells. Research Support: Stand Up To Cancer Innovative Research Grant (SU2C-AACR-IRG-15-17) and RSNA Research Scholar Grant (RSCH1510). ER -