(68^Ga)-GaPSMA-11 PET/MR-imaging of Malignant Intra-axial Brain Tumors

Overview

PSMA is a transmembrane protein specifically expressed in the vascular endothelium of malignant brain tumors, most notably glioblastoma and not in healthy brain parenchyma. It has been shown to be involved in (neo)angiogenesis and endothelial cell invasion. By means of 68Ga-labeled PSMA ligands, investigators are able to non-invasively visualize/quantify PSMA expression in glioblastoma (neo)vasculature in vivo by means of PET. The primary aim of this study is to confirm PSMA as suitable diagnostic and potential theranostic target in patients with intra-axial brain tumors by means of \[68Ga\]Ga-PSMA-HBEC-CC (\[68Ga\]Ga-PSMA-11) PET. The secondary aim is to assess whether uptake is increased with intra-arterial injection in those tumors that show uptake after intravenous injection of \[68Ga\]Ga-PSMA-11.

Brain tumors have a major impact due to both high morbidity and mortality. Primary brain tumors (glioma) have a low incidence (5-7:100,000), but the highest loss of life years compared to other cancer types. Glioblastoma without mutations in the Isocitrate Dehydrogenase gene (IDHwt), are the most frequent and the most aggressive of all primary malignant brain tumors. They account for 48% of all the primary malignant brain tumors and 15% of all primary brain tumors. Treatment consists of resection, radiotherapy and systemic chemotherapy. Nevertheless the current prognosis is still poor with a 1-, 2- and 5-year survival of respectively 40%, 17% and 6%. Secondary brain tumors (brain metastases) are frequent, occurring in 15% of cancer patients (incidence 50-70:100,000). The prevalence of brain metastases is rising due to better extracranial local cancer control with systemic treatment resulting in more surviving patients with good local tumor control, but with brain metastases. A recently discovered potential target for (localized) therapy of glioblastoma is PSMA. PSMA was originally found to be specifically expressed in normal prostate, and overexpressed in almost all stages of prostate cancer. In recent years, PSMA was also found to be expressed in tumor-associated (neo)vasculature in many other solid tumors, including glioblastoma. PSMA is distinct from other vascular targets, such as vascular endothelial growth factor, endoglin, or the integrins involved in the general process of angiogenesis, as these latter are not specific to tumor vasculature. PSMA, in contrast, is not expressed in normal vasculature and is only minimally expressed in a subset of normal glial cells. PSMA therewith represents the most specific (neo)vascular target for glioblastoma currently known. This specificity makes PSMA an ideal target for delivery of a cytotoxic agent or a radionuclide designed to tumor vasculature destruction. Particularly in intra-axial brain tumors such as glioblastoma, therapeutic targeting of (neo)vasculature is attractive as it may be exposed to a therapeutic agent much more readily than the tumor parenchyma itself, which is protected by the blood-brain barrier. The primary aim of this first PSMA-PET study performed in the Netherlands is to confirm PSMA as suitable diagnostic and theranostic target in patients with intra-axial brain tumors by means of \[68Ga\]Ga-PSMA-HBEC-CC (\[68Ga\]Ga-PSMA-11) PET. The secondary aim is to assess whether uptake is increased with intra-arterial injection in those tumors that show uptake after intravenous injection of \[68Ga\]Ga-PSMA-11. Eligible for this non-treatment study are patient, aged \>18 years old, with enhancing glioma or brain metastases scheduled for (re-)resection. Upon informed consent, patients will undergo intravenous injection of \[68Ga\]Ga-PSMA-11, followed by synchronously combined PET-MR scans at 45-60 min post-injection to determine \[68Ga\]Ga-PSMA tumor uptake and dosimetry over time. Based on the known expression profile of PSMA in glioblastoma, it is expected that significant tumor-uptake will be seen in 50% of included patients. In case the tumor shows significant uptake, patients will subsequently undergo local intra-arterial injection of \[ 68Ga\]GaPSMA-11, with an interval of two weeks, performed by a trained neuro-interventional radiologist, immediately followed by a repeat PET/MRI scan. Tumor uptake values of each of the scans will be compared to determine the optimal route of administration. \[68Ga\]Ga-PSMA-11 uptake values of the tumor will be compared to the degree of PSMA expression determined by IHC. Based on the few studies published thus far, we expect significant expression of PSMA to be present in progressive/recurrent enhancing glioma and brain metastases. We expect approximate 50% positive scans and in these, increased uptake upon intra-arterial injection compared to intravenous injection. The results of this pilot study will aid in the development of future PSMA-targeting chemo- /radionuclide therapies for malignant brain tumors. Therapeutic PSMA PET imaging studies have not yet been performed in patients with primary brain tumors, but studies in patients with (cerebral) metastases from prostate cancer show promising clinical results and acceptable toxicity.