Introduction
Non-invasive imaging techniques such as positron emission tomography (PET) are extremely important for cancer detection and characterization especially for difficult to biopsy or extremely delicate organs such as the brain. The folate analogue 1,4,7-triazacylononane-1,4,7-triacetic acid-conjugated folate radiolabeled with aluminum fluoride-18 ([F-18]FOL) has been previously shown to accumulate preferentially in tumor cells with an overexpression of folate receptors (FRs) and here was investigated for its ability to detect orthotopic gliomas in a rat model. In addition, we studied the expression of FRs in human glioblastoma samples to investigate if an analogous relationship may exist.

Methods
Nine BDIX rats were injected with BT4C rat glioma cells into the right hemisphere of the brain. Animals were imaged with gadolinium-enhanced magnetic resonance imaging at on days prior to PET/computed tomography (CT) imaging. Animals were divided into two groups, and were PET/CT imaged with either [F-18]FOL or 2-deoxy-2-18F-fluoro-D-glucose ([F-18]FDG) on 19 and 32-days post glioma grafting. Two subjects were also PET/CT imaged with [F-18]FOL on day 16. Biodistribution was studied and brains were cryosectioned for autoradiography, immunofluorescence, and histological studies. Patient-derived paraffin-embedded glioblastomas were sectioned and stained with similar methods.

Results
PET imaging showed an increase of [18F]FOL tumor-to-brain uptake ratio (TBR) over the study duration from day 16/19 (3.3 +/- 0.9) increasing to 5.7 +/- 1.0 by day 32. [F-18]FDG PET-imaged rats had a consistent TBR of 1.6 +/- 0.1 throughout the study. Ex vivo autoradiography results revealed an exceptionally high TBR of 116.1 +/- 26.9 for [F-18]FOL while the [18F]FDG values were significantly lower giving 2.9 +/- 0.6 (P<0.0001). Immunostaining demonstrated an increased presence of FR-alpha in the BT4C gliomas versus the contralateral brain tissue, while FR-beta was present only on glioma periphery. Human sections assayed showed similar FRs expression characteristics.

Conclusion
This study shows upregulation of FR-alpha inside glioma regions in both human and animal tissue, providing a biochemical basis for the observed increased [F-18]FOL uptake in animal PET images. These results suggest that FRs targeting imaging and therapeutic compounds may possess clinically relevant translational abilities for the detection and treatment of gliomas.