Novel PET Radiotracers for Potential Use in Management of Lymphoma

Overview of PET Tracers for Brain Tumor Imaging

This article provides an overview of the key considerations for the development and application of molecular imaging agents for brain tumors and the major classes of PET tracers that have been used for imaging brain tumors in humans. The mechanisms of uptake, biological implications, primary applications, and limitations of PET tracers in neuro-oncology are reviewed. The available data indicate that several of these classes of tracers, including radiolabeled amino acids, have imaging properties superior to those of (18)F-fluorodeoxyglucose, and can complement contrast-enhanced magnetic resonance imaging in the evaluation of brain tumors.

Comparative Analysis between [(18)F]Fludarabine-PET and [(18)F]FDG-PET in a Murine Model of Inflammation

Lymphoma research has advanced thanks to introduction of [(18)F]fludarabine, a positron-emitting tool. This novel radiotracer has been shown to display a great specificity for lymphoid tissues. However, in a benign process such as inflammation, the uptake of this tracer has not been questioned. Indeed, in inflammatory zones, elevated glucose metabolism rate may result in false-positives with [(18)F]FDG-PET Imaging. In the present investigation, it has been argued that cells, involved in inflammation, might be less avid of [(18)F]fludarabine. To generate inflammation, Swiss mice were intramuscularly injected with 0.1 mL of turpentine oil into the right front paw. Imaging sessions with (18)F-labeled tracers named above were conducted on days 5 and 25 after inoculation. For each animal, volumes of interest (VOI), delineating the muscle of the inflamed (IP) and normal paws (NP), were determined on PET scans. For characterization of inflammation, muscle samples from IP and NP were stained with hematoxylin and eosin (H&E). In early (day 5) inflammation, [(18)F]FDG accumulation was 4.00 ± 1.65 times greater in the IP than in the contralateral NP; for [(18)F]fludarabine, this IP/NP ratio was 1.31 ± 0.28, resulting in a significant difference between radiotracer groups (p < 0.01). In late (day 25) inflammation, the IP/NP ratios were 2.07 ± 0.49 and 1.03 ± 0.07, for [(18)F]FDG and [(18)F]fludarabine, respectively (p < 0.001). [(18)F]Fludarabine showed significantly weaker uptake in inflammation when compared with [(18)F]FDG. This encouraging finding suggests that [(18)F]fludarabine-PET might well be a robust approach for distinguishing tumor from inflammatory tissue, avoiding false-positive PET results and thus enabling an accurate imaging of lymphoma.

Evaluation of the specificity of [(18)F]fludarabine PET/CT in a xenograft model of follicular lymphoma: comparison with [(18)F]FDG and impact of rituximab therapy

Background

[¹⁸F]Fludarabine is a novel positron emission tomography (PET) radiotracer for imaging lymphoma. The purpose of this preclinical study was to evaluate the robustness of [¹⁸F]fludarabine during rituximab therapy. In addition, a comparison was made between [¹⁸F]fludarabine and [¹⁸F]fluorodeoxyglucose ([¹⁸F]FDG) with regard to their concordance with histologically derived data.

Methods

CB17-SCID mice bearing human follicular DOHH-2 lymphoma were treated once weekly with rituximab (10 mg/kg) or physiological saline over 3 weeks. To obtain the tracer uptake in the metabolically active volume of the tumour (MAVT), a background-level threshold was applied to the volume of interest (VOI) defined on computed tomography (CT) image. The tumour uptake analysis was performed with MAVT-based segmentation for data analysis of sequential [¹⁸F]fludarabine PET/CT studies and with total tumour-based segmentation for comparison with histologically derived data.

Results

The correlation between the MAVT and [¹⁸F]fludarabine accumulation (%ID) in those viable tissues was equally significant for both vehicle- or rituximab-treated mice; for these latter, the presence of lymphoid tissues at the end of imaging sessions was confirmed histologically. A stronger correlation was demonstrated between quantitative values extracted from [¹⁸F]fludarabine-PET and histology (r² = 0.91, p < 0.001) when compared to [¹⁸F]FDG-PET (r² = 0.55, p = 0.03).

Conclusions

[¹⁸F]Fludarabine uptake in the follicular lymphoma model compared favourably with [¹⁸F]FDG in terms of specificity for PET imaging and also remained robust for persistent viable tissues following rituximab therapy. [¹⁸F]Fludarabine PET/CT may be a promising approach to evaluate lymphoma, including their surveillance during therapy.

Electronic supplementary material

The online version of this article (doi:10.1186/s13550-015-0101-7) contains supplementary material, which is available to authorized users.