title: Quantitative PET Reporter Gene Imaging with [11C]Trimethoprim.
abstract: There is a need for improved methods to image genetically engineered cells, including immune cells used for cell-based therapy. Given the genetic manipulation inherent to gene therapy, the use of a reporter protein is a logical solution and positron emission tomography (PET) can provide the desired sensitivity and spatial localization. We developed a broadly applicable PET imaging strategy based on the small bacterial protein E. coli dihydrofolate reductase (Ec dhfr) and its highly specific small molecule inhibitor, trimethoprim (TMP). The difference in TMP affinity for bacterial compared to mammalian DHFR suggests that a TMP radioligand would have a low background in unmodified mammalian tissues and high retention in Ec dhfr engineered cells, providing high contrast imaging. Here, we describe the in vitro properties of [11C]TMP and show over 10-fold increased signal in transgenic Ec dhfr cells compared to control. In a mouse xenograft model, [11C]TMP rapidly accumulated in Ec dhfr carrying cells within minutes of intravenous administration. Moreover, [11C]TMP can identify less than a million xenografted cells in a small volume in tissues other than the abdominal compartment. This limit of detection is a clinically relevant number and bodes well for clinical translation especially given that [11C]TMP is an isotopologue of clinically approved antibiotic.
authors.affiliation: Department of Radiology, University of Pennsylvania, Philadelphia, PA 19104, USA. Electronic address: mark.sellmyer@uphs.upenn.edu.
authors.forename: Mark A
authors.initials: MA
authors.lastname: Sellmyer
authors.affiliation: Department of Radiology, University of Pennsylvania, Philadelphia, PA 19104, USA.
authors.forename: Iljung
authors.initials: I
authors.lastname: Lee
authors.affiliation: Department of Radiology, University of Pennsylvania, Philadelphia, PA 19104, USA.
authors.forename: Catherine
authors.initials: C
authors.lastname: Hou
authors.affiliation: Department of Radiology, University of Pennsylvania, Philadelphia, PA 19104, USA.
authors.forename: Brian P
authors.initials: BP
authors.lastname: Lieberman
authors.affiliation: Department of Radiology, University of Pennsylvania, Philadelphia, PA 19104, USA.
authors.forename: Chenbo
authors.initials: C
authors.lastname: Zeng
authors.affiliation: Department of Radiology, University of Pennsylvania, Philadelphia, PA 19104, USA.
authors.forename: David A
authors.initials: DA
authors.lastname: Mankoff
authors.affiliation: Department of Radiology, University of Pennsylvania, Philadelphia, PA 19104, USA. Electronic address: rmach@mail.med.upenn.edu.
authors.forename: Robert H
authors.initials: RH
authors.lastname: Mach
mesh_terms: D000818:Animals; D002250:Carbon Radioisotopes; D002460:Cell Line; D017930:Genes, Reporter; D051379:Mice; D057054:Molecular Imaging; D049268:Positron-Emission Tomography / Q000379:methods*; D012680:Sensitivity and Specificity; D014295:Trimethoprim; D055114:X-Ray Microtomography
chemical_list: D002250:Carbon Radioisotopes; D014295:Trimethoprim
keywords: PET; cell therapy; chemical biology; imaging; molecular imaging; reporter gene