Brief Article pubs.acs.org/molecularpharmaceutics
Comparative Analysis between [18F]Fludarabine-PET and [18F]FDGPET in a Murine Model of Inflammation Narinée Hovhannisyan,*,†,‡,§ Martine Dhilly,†,‡,§ Stéphane Guillouet,†,‡,§ Michel Leporrier,†,‡,§ and Louisa Barré†,‡,§ †
CEA, DSV/I2BM, LDM-TEP group, GIP Cyceron, Boulevard Henri Becquerel, BP 5229, 14074 Caen Cedex, France UNICAEN, UMR6301-ISTCT, F-14032 Caen, France § UMR ISTCT 6301, LDM-TEP group, GIP Cyceron, Boulevard Henri Becquerel, BP 5229, 14074 Caen Cedex, France ‡
ABSTRACT: Lymphoma research has advanced thanks to introduction of [18F]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 [18F]FDGPET Imaging. In the present investigation, it has been argued that cells, involved in inflammation, might be less avid of [18F]fludarabine. To generate inflammation, Swiss mice were intramuscularly injected with 0.1 mL of turpentine oil into the right front paw. Imaging sessions with 18F-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, [18F]FDG accumulation was 4.00 ± 1.65 times greater in the IP than in the contralateral NP; for [18F]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 [18F]FDG and [18F]fludarabine, respectively (p < 0.001). [18F]Fludarabine showed significantly weaker uptake in inflammation when compared with [18F]FDG. This encouraging finding suggests that [18F]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. KEYWORDS: [18F]fludarabine, [18F]FDG, PET, inflammation
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Although the clinical management with [18F]FDG-PET is expanding in oncology, [18F]FDG uptake is not sufficiently tumor specific. The specificity of [18F]FDG uptake has been criticized because it measures glucose metabolism which increases in states such as inflammation or infection.4−6 Moreover, [18F]FDG-PET has even been proposed for detection of inflammatory lesions.7,8 The goal of this present work was to investigate the uptake of the adenine nucleoside analogue [18F]fludarabine in turpentineinduced inflammatory tissue in comparison to the glucose analogue [18F]FDG.
INTRODUCTION
Recently, [18F]fludarabine has been introduced as a PET probe for lymphoma imaging; the rationale for the development of the drug, fludarabine, as a radiotracer was based on the remarkable property of this purine analogue which displays a high degree uptake by lymphoid cells, as revealed by in vitro and in vivo studies.1,2 The usefulness and robustness of this radiotracer for the preclinical evaluation of lymphoma as well as for the surveillance during therapy have been studied previously.2,3 [18F]Fludarabine has shown selective accumulation in lymphoid tissues even in tumors with necrotic and fibrotic components that follow rituximab treatment. Furthermore, with comparative analyses [18F]fludarabine has been shown to have a greater specificity for lymphoid tissues when compared to [18F]FDG, which, despite its failings, is as of now the most commonly used radiotracer for PET imaging in lymphoma. [18F]Fludarabine has also demonstrated a greater washout from normal tissues which results in an enhanced contrast between cancerous and healthy tissues. © XXXX American Chemical Society
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EXPERIMENTAL SECTION Radiochemical Synthesis. [18F]Fludarabine was synthesized in house, and its radiosynthesis has been previously described;1 [18F]FDG was obtained from Cyclopharma S.A. Received: January 19, 2016 Revised: April 8, 2016 Accepted: April 14, 2016
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DOI: 10.1021/acs.molpharmaceut.6b00050 Mol. Pharmaceutics XXXX, XXX, XXX−XXX
Brief Article
Molecular Pharmaceutics
Figure 1. Illustrations (3D) of representative PET/CT fused scans. The same mouse (on day 5 after inoculation) imaged with (a) [18F]FDG and (b) [18F]fludarabine. The mouse is seen from below and from the right side. The inflamed area is shown with dotted lines.
Figure 2. Quantitative PET data analysis. Uptake (defined as IP/NP ratio) in inflammation for [18F]FDG and [18F]fludarabine on (a) day 5 and (b) day 25 (the paired samples are connected with gray lines) and (c) combined results from two examinations (the horizontal line near the middle of the rectangle indicates the median). *** p < 0.001.
Inflammatory Model. All procedures are in accordance with national legislation under the European directive (86/609/ EU); details are given in a previous publication.2 Permission was accorded for the experimental procedures by the competent regional committee on animal ethics (approval CENOMEXA 1112-26). Swiss mice were housed under constant environmental conditions with 12/12 h light−dark cycles. Food and water were provided ad libitum. To induce inflammation, mice (n = 10) were inoculated intramuscularly with 0.1 mL of turpentine oil (Sigma-Aldrich, Saint Quentin Fallavier, France) into the right front paw and monitored for the duration of the study. At the end of the investigation, the mice were euthanized under deep anesthesia. PET/CT Imaging and Data Analysis. The anesthesia and PET scanning parameters applied during this investigation can be found in our earlier report.3 The first imaging session was conducted on day 5 after inoculation. Mice were injected with [18F]FDG (11.55 ± 1.22 MBq, n = 9) via the caudal vein and imaged with small-animal PET/CT system (Inveon, Siemens, Knoxville, TN, USA) 70−90 min post radiotracer administration. The next day, [18F]fludarabine was injected with a similar activity (12.00 ± 0.79 MBq, n = 6, of which five were paired with [18F]FDG) and the PET scan was performed 40− 60 min post radiotracer administration. The scan periods were chosen to replicate the conditions needed for imaging of tumor.2,3 The same protocol was followed for the second imaging session, conducted on day 25 post inoculation. A low dose CT scan was used for attenuation correction and for the adjustment of the VOIs defined on PET image. For each front paw, a VOI was delineated manually on the coronal plane of the PET image (PMOD 3.6, PMOD
Technologies, Zurich, Switzerland). The quantification was based on mean activity over the region delineated with an isocontour at 50% of the maximum pixel value of the initial VOI. Differences between the radiotracer groups were tested for statistical significance using Student’s t test for independent samples and the paired t test for pairwise comparisons (Prism 4.03, GraphPad Software, USA). P values
