Feasibility of experimental BT4C glioma models for somatostatin receptor 2-targeted therapies

High folate receptor expression in gliomas can be detected in vivo using folate-based positron emission tomography with high tumor-to-brain uptake ratio divulging potential future targeting possibilities

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 ([¹⁸F]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 [¹⁸F]FOL or 2-deoxy-2-18F-fluoro-D-glucose ([¹⁸F]FDG) on 19 and 32-days post glioma grafting. Two subjects were also PET/CT imaged with [¹⁸F]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. [¹⁸F]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 [¹⁸F]FOL while the [18F]FDG values were significantly lower giving 2.9 ± 0.6 (P<0.0001). Immunostaining demonstrated an increased presence of FR-α in the BT4C gliomas versus the contralateral brain tissue, while FR-β was present only on glioma periphery. Human sections assayed showed similar FRs expression characteristics.

Conclusion

This study shows upregulation of FR-α inside glioma regions in both human and animal tissue, providing a biochemical basis for the observed increased [¹⁸F]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.

Contemporary Mouse Models in Glioma Research

Despite advances in understanding of the molecular pathogenesis of glioma, outcomes remain dismal. Developing successful treatments for glioma requires faithful in vivo disease modeling and rigorous preclinical testing. Murine models, including xenograft, syngeneic, and genetically engineered models, are used to study glioma-genesis, identify methods of tumor progression, and test novel treatment strategies. Since the discovery of highly recurrent isocitrate dehydrogenase (IDH) mutations in lower-grade gliomas, there is increasing emphasis on effective modeling of IDH mutant brain tumors. Improvements in preclinical models that capture the phenotypic and molecular heterogeneity of gliomas are critical for the development of effective new therapies. Herein, we explore the current status, advancements, and challenges with contemporary murine glioma models.

Efficient cartridge purification for producing high molar activity [18F]fluoro-glycoconjugates via oxime formation

INTRODUCTION

¹⁸F-fluoroglycosylation via oxime formation is a chemoselective and mild radiolabeling method for sensitive molecules. Glycosylation can also improve the bioavailability, in vivo kinetics, and stability of the compound in blood, as well as accelerate clearance of biomolecules. A typical synthesis procedure for ¹⁸F-fluoroglycosylation with [¹⁸F]FDG (2-deoxy-2-[¹⁸F]fluoro-d-glucose) and [¹⁸F]FDR (5-deoxy-5-[¹⁸F]fluoro-d-ribose) involves two HPLC (high performance liquid chromatography) purifications: one after ¹⁸F-fluorination of the carbohydrate to remove its labeling precursor, and a second one after the oxime formation step to remove the aminooxy precursor. The two HPLC purifications can be time consuming and complicate the adaptation of the synthetic strategy in nuclear medicine applications and automated synthesis. We have developed a procedure in which SPE (solid phase extraction) and resin purification methods replace both of the needed HPLC purification steps.

METHODS

We used [¹⁸F]FDR and [¹⁸F]FDG as prosthetic groups to radiolabel two aminooxy-modified model molecules, a tetrazine and a PSMA (prostate specific membrane antigen) inhibitor. After fluorination, the excess carbohydrate precursor was removed by derivatizing it with 4,4'-dimethoxytrityl chloride (DMT-Cl). The DMT moiety increases the hydrophobicity of the unreacted precursor making the separation from the fluorinated precursor possible with simple C18 Sep-Pak cartridge. For removal of the aminooxy precursor, we used a commercially available aldehyde resin (AminoLink, Thermo Fisher Scientific). C18 Sep-Pak SPE cartridge was used to separate [¹⁸F]FDR and [¹⁸F]FDG from the ¹⁸F-fluoroglycoconjugate end product.

RESULTS

[¹⁸F]FDR and [¹⁸F]FDG were efficiently purified from their precursors, free fluorine-18, and other impurities. The aldehyde resin quantitatively removed the unreacted aminooxy precursors after the oxime formation. The fluorine-18 labeled oxime end products were obtained with high radiochemical purity (>99%) and molar activity (>600 GBq μmol^-1).

CONCLUSIONS

We have developed an efficient cartridge purification method for producing high molar activity ¹⁸F-glycoconjugates synthesized via oxime formation.

Comparison of Somatostatin Receptor 2-Targeting PET Tracers in the Detection of Mouse Atherosclerotic Plaques

PURPOSE

Rupture-prone atherosclerotic plaques are characterized by accumulation of macrophages, which have shown to express somatostatin type 2 receptors. We aimed to investigate whether somatostatin receptor-targeting positron emission tomography (PET) tracers, [(68)Ga]DOTANOC, [(18)F]FDR-NOC, and [(68)Ga]DOTATATE, can detect inflamed atherosclerotic plaques.

PROCEDURES

Atherosclerotic IGF-II/LDLR(-/-)ApoB(100/100) mice were studied in vivo and ex vivo for tracer uptake into atherosclerotic plaques. Furthermore, [(68)Ga]DOTANOC and [(68)Ga]DOTATATE were compared in a head-to-head setting for in vivo PET/X-ray computed tomography (CT) imaging characteristics.

RESULTS

Ex vivo uptake of [(68)Ga]DOTANOC and [(68)Ga]DOTATATE in the aorta was higher in atherosclerotic mice compared to control C57Bl/6N mice, while the aortic uptake of [(18)F]FDR-NOC showed no genotype difference. Unlike [(18)F]FDR-NOC, [(68)Ga]DOTANOC and [(68)Ga]DOTATATE showed preferential binding to atherosclerotic plaques with plaque-to-wall ratio of 1.7 ± 0.3 and 2.1 ± 0.5, respectively. However, the aortic uptake and aorta-to-blood ratio of [(68)Ga]DOTANOC were higher compared to [(68)Ga]DOTATATE in in vivo PET/CT imaging.

CONCLUSION

Our results demonstrate superior applicability for [(68)Ga]DOTANOC and [(68)Ga]DOTATATE in the detection of atherosclerotic plaques compared to [(18)F]FDR-NOC.