The Radiochemistry Group has research interests in the development of novel methods for incorporation of radionuclides into tracers for the study of biologically important processes. Our research efforts are driven by the desire to translate our work into clinically relevant human diseases. Thus, we seek to collaborate with clinicians that share our goal of providing new imaging tools with utility in patients. We contributed to the preparation of INDs of compounds now used at the NIH Clinical Center (FC-WAY and FPTZTP) and an RGD based peptide agent at Stanford University.

 

There are numerous targets for drug-like molecules that are of interest for their role in human disease. Imaging studies can be utilized as a research tool to help determine optimum drug dosing and elucidation of biochemical function. In addition imaging studies can be clinically relevant for diagnosis of disease, staging of disease, and treatment of 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. Recent studies have focused on ligands for Adenosine A2a receptor that is believed to play a key role in Parkinson’s Disease and adenosine A3 receptor, which is a marker of inflammation. We also have a long standing interest in the study of the stress response; recent efforts have focused on corticotrophin releasing hormone (CRH) ligands radiolabeled with either Br-76 or F-18.

Proteins and Peptides

Antibodies are the most specific molecules for binding to their 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-tradition PET radioisotopes such as Br-76, Cu-64, Zr-89, and I-124 offer longer half-lives that may be more useful for this application. The second approach is to develop small proteins or peptides that display high binding specificity but have more rapid pharmacokinetics.

Peptide Labeling

We have developed small organic prosthetic groups labeled with fluorine-18 for conjugation with proteins and peptides. These small molecules contain either a maleimide that can be specifically attached via a free cysteine sulfhydryl residue or an N-hydroxysuccinimidyl ester that can be attached to a reactive amine (i.e. lysine). The fluorine-18 labeled maleimide has been applied to the preparation of an Affibody molecule for the study of human epidermal growth factor receptor type 2 (HER2) for the imaging of this receptor in expressing tumors and to study the receptor change as a function of treatment. A second approach for labeling peptides is to functionalize the peptide first. Examples include adding an azide function to allow radiolabeling via a [18F]fluoroalkyne using “click” chemistry, incorporating a hydroxylamine functionality to allow labeling using [18F]fluorobenzaldehyde with formation of stable oximes, or incorporating a chelating moiety to allow radiolabeling using positron emitting radiometals, such as Ga-68, Cu-64, Zr-89, or Y-86. Metal chelation of Tc-99m and In-111 would allow imaging using SPECT.

Analytical Techniques

Our laboratory has been one of the leaders in the use of highly sensitive high resolution mass spectrometry coupled with HPLC to identify radiometabolites from novel imaging agents. In the development of novel small molecule imaging agents, we typically use hepatocytes from rat, monkey, and human to generate metabolites. The metabolites are analyzed by HPLC-MS and structures are proposed based on acquired data and literature precedence. 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 TLC and/or extraction methods to provide parent corrected plasma input functions for PET modeling. The high sensitive of the mass spectrometer for some chemical structures can allow determination of biodistribution without the need to synthesize the radiolabeled compound. We intend to pursue this technology to metabolite analysis of radiolabeled peptides and proteins. 

Staff