Molecular Tracer and Imaging Core Facility

thumbnail showing two bright blobs that represent radioactive tracers on a PET scan

The PET Radiochemistry and Imaging Core Facility develops novel methods for incorporating radionuclides and fluorophores into molecules for the study of biologically important processes. All of our projects are in support of principal investigator-initiated research. We seek to collaborate with clinical and biological investigators who share our goal of providing new imaging tools for clinical studies and the study of biological processes. Our research efforts are driven by a desire to improve understanding of human biology and disease and to generate tools that have clinical application. We have produced probes with applications to many important biological processes, including inflammation, metabolism, proliferation, angiogenesis, metastasis, lymphogenesis, and apoptosis.

Research Topics
Lab Resources
Staff
Publications

Small Molecule Probes
There are numerous targets for drug-like molecules that are of interest due to their role in human disease. Imaging studies can be utilized as a research tool to help determine optimum drug dosing and to elucidate biochemical function. In addition, imaging can be used for diagnosis and staging of disease and for monitoring treatment response. We have developed methods for the incorporation of fluorine-18 and bromine-76 into small molecule tracers with applications for studies of brain function and disease and for diagnosis, staging, and treatment management in oncology. Adenosine A2a and A3 receptors, corticotrophin releasing hormone (CRH) ligands, muscarinic, and serotonin ligands, CXCR4 are a few examples.

PET images in rat brain following administration of [18F]-MRS5425 (8.6±2.2 MBq). Top shows the chemical structure of [18F]-MRS5425; this molecule is an adenosine 2A receptor ligand. Lower panel shows (left) normal rat and (right) 6-OHDA-lesioned rat. Arrow points to increase uptake on the side of the 6-OHDA lesion.
Targeting Employing Proteins and Peptides
Antibodies are specific molecules that bind to a particular antigen. Imaging of radiolabeled antibodies has been investigated for many years. The challenge with antibody imaging is the long half-life in the blood. One approach is to radiolabel with longer lived radionuclides. The non-traditional PET radioisotopes such as Br-76, Y-86, Cu-64, Zr-89, and I-124 and SPECT radionuclides such as I-123 and In-111 offer longer half-lives that may be more useful for this application.

An alternative approach is to develop smaller proteins or peptides that display high binding specificity but have more rapid pharmacokinetics. Peptides and proteins can be functionalized first in order to allow site-specific labeling with radiohalides or radiometals as well as fluorophores. We have conducted conjugation reaction of peptides and proteins with various macrocyclic chelators to allow radiolabeling with radiometals such as Cu-64, Ga-68, and Zr-89. We have developed or adapted from the literature small organic prosthetic groups labeled with fluorine-18 for conjugation with proteins and peptides.

Insulinoma INS-1 mouse tumor xenograft model imaged with [18F]-exendin-4. Nose is up in all images. Images represent one slice through region of tumor xenograft. Arrow shows location of tumor in the images following blocking.
Analytical Techniques
We have expertise in the use of highly sensitive, high resolution mass spectrometry coupled with HPLC (LC-MS) to confirm the identity of all of our tracers, from small molecules up to small proteins and oligonucleotides. In addition, we have experience using LC-MS to assist in the identification of metabolites. The results allow a qualitative assessment of the propensity of radiolabeled metabolites to interfere with interpretation of images, an assessment of species differences anticipated as the compounds move toward eventual clinical use, and can assist in the preparation of improved ligands by modification of chemical structure to alter the metabolism. The mass spectrometry results have been utilized to assist in the development and validation of thin-layer chromatography (TLC) and/or extraction methods to provide parent corrected plasma input functions for PET modeling. The high quantitative sensitivity of the mass spectrometer for some chemical structures can allow determination of biodistribution without the need to synthesize the radiolabeled compound. The sensitivity is sufficient to quantify the membrane binding and cellular internalization of ligand molecules. We have extended this technology to study metabolism pathways of radiolabeled and fluorescently labeled peptides and proteins.
Liquid chromatography-mass spectrometry for NOTA-MAL-Cys40-exendin-4.

The Molecular Tracer and Imaging Core Facility is physically located in building 10; radiochemical laboratories are located within the NIH cyclotron facility. The radiochemistry laboratory has four hot cells for conducting high level radiochemical syntheses. High level automated radiosyntheses can be conducted with fluorine-18 utilizing either a GE single reactor synthesis system or an Eckert & Ziegler two reactor synthesis system. We have the ability to conduct low-level manual synthesis including thermal and microwave assisted synthesis. The staff has significant experience in conducting multistep chemical synthesis of small drug-like molecules and developing strategies for incorporation of F-18 (including aluminum fluoride complexes), Cu-64, Ga-68, Y-86, Zr-89 or Br-76 radionuclides. The staff also possesses experience in organic synthesis and peptide/protein conjugation with chelators and fluorophores. The laboratory is well equipped for conducting organic synthetic reactions and subsequent purification and analysis procedures. We have the following chemical analysis capabilities: NMR, HPLC, GC-MS and LC-MS. We obtain radionuclides from the Cyclotron Facility of the PET Department, Warren Grant Magnuson Clinical Center and from commercial vendors. Gallium-68 is obtained from a commercial Ge-68/Ga-68 generator.

Photograph of a hot cell for high level radiochemical syntheses

A Capintec, Inc. hot cell for high level radiochemical syntheses.

Photo of lab equipment for radiochemical synthesis

General Electric Nucleophilic Fluorination Module for automated radiochemical synthesis.

The Core Facility also has laboratory space on the B3 level of the Magnuson Center appropriate for live animal experiments, tissue preparation, radioactivity counting, and live animal imaging. The Facility maintains two instruments for PET, a Siemens Inveon PET/CT system and a Sofie G4 PET/Xray device. The Siemens Inveon can be operated as separate PET and CT scanners or in docked configuration for optimal co-registration of PET and CT. In addition, the Facility maintains two systems for optical imaging; a CRI Maestro II and an IVIS Lumina II.”