PET Imaging Agents for Protoporphyrin IX

Project Summary Primary brain tumor is a major central nervous system malignancy with 23,800 estimated total new cases in the US in 2017 resulting in 16,700 deaths. High grade glioblastomas (HGG; WHO grades III and IV) in particular carry a dismal prognosis.

Glioblastomas are known for their molecular and cellular heterogeneity which contributes to the highly aggressive malignant behaviors of tumor cells, including high proliferation, infiltration and development of resistance to treatment.

Therefore, non-invasive imaging approaches that can provide molecular and metabolic information of the glioblastomas are important to characterize a specific tumor for subtype specific and individualized treatments.

As a non-invasive clinical imaging modality, positron emission tomography (PET) is capable of molecular and metabolic imaging applications, however, its application in brain tumor is limited.

The clinical standard of fluorodeoxyglucose (FDG) PET has low specificity and is not appropriate for brain tumor imaging given the high FDG uptake in normal brain tissue.

Currently, there is a significant unmet need in molecular imaging probes for imaging highly heterogeneous and infiltrating gliomas.

The overall objective of this exploratory project is to develop a fluorine-18 labeled analog of 5- aminolevulinic acid (5-ALA), which is an FDA approved imaging tracer for intraoperative imaging of gliomas, to provide molecular and metabolic information of the glioma.

This innovative new PET imaging capability can be used to characterize a tumor for subtype specific diagnosis and clinical decisions on individualized treatments as well as monitoring the treatment responses.

The central hypothesis of this proposal is that a fluorine-18 labeled 5-ALA will enable PET imaging for glioma metabolism and activity on tumorigenicity and cellular proliferation based upon tumor cell metabolizing 5-ALA to produce protoporphyrin IX in highly heterogeneous and infiltrating gliomas.

This hypothesis and scientific premise of the project are supported by our preliminary findings that: 1) in vivo pre-clinical PET evaluation of 13N-5-ALA with rats bearing intracranial 9L gliomas demonstrated highly selective uptake of 13N-5-ALA within the tumor, and tumor specific contrast without background; 2) racemic 3-fluoro-5-ALA shows greater substrate reactivity for protoporphyrin IX synthesis in glioma cells than 5-ALA.

Thus, our exploratory development of this novel PET tracer for brain tumor imaging will focus on the following specific aims: 1. to synthesize and in vitro characterize R- and S-3-fluoro-5-ALA; 2. to develop radiolabeling methods for the candidate R- and S-3-[18F] fluoro-5-ALA ligands; and 3. to determine whether developed R- and S-3-[18F] fluoro-5-ALA ligand candidates can be selectively taken up in the intracranial tumors in rodent orthotopic models of glioblastomas.