Abstract 2048: Characterization of tumor hypoxia by photoacoustic imaging and limitations of bioluminescence imaging in a Mia PaCa2-luc orthotopic model of pancreatic carcinoma
Authors: Raes Florian , Barre Thomas , Jose Jithin , Trochet Philippe , Lerondel Stéphanie , Pape Alain L. 
Published: May 2015

Abstract
Background
Photoacoustic (PA) imaging is a recent promising modality combining advantages of ultrasounds (resolution) and photon imaging (sensitivity), thus providing anatomical and functional data. PA can be implemented by using the natural endogenous contrast agent hemoglobin, providing informations such as the oxygen saturation of hemoglobin (SO2). In the field of oncology, when tumors are growing, the central area becomes hypoxic. This phenomenon does not interfere with tumor growth because cancer cells can switch from aerobic metabolism (using O2) to anaerobic metabolism. Hypoxia is a crucial parameter that needs to be investigated and quantified as it is responsible for radio and chemoresistances. Among tumor cell lines, the MIA PaCa2 cell line (pancreatic adenocarcinoma) appeared quite relevant for a preclinical approach.
The aim of this work was to assess the hypoxic status of tumors in a MIA PaCa-2 Luc+ orthotopic cancer model, and to compare bioluminescent signal and tumor volumes in a longitudinal study.
Methods
The tumor proliferation was monitored by bioluminescence imaging (BLI) and tumor volume with Ultrasound (US). BLI was performed using the IVIS-Lumina II imaging system (PerkinElmer) whereas US and PA with the VEVO LAZR system (VisualSonics). For the experiments, human ductal adenocarcinoma MIA PaCa-2-luc model was orthotopically implanted in nude mice. For the experiment, 7 days after engraftments, tumors were explored both by BLI and US 3 times a week during 12 weeks. Moreover, tumors were investigated by PA imaging so that average values of SO2 were calculated.
Results and Discussion
BLI was selected as it is one of the reference techniques to assess tumor growth in vivo. Overall, our data demonstrated that in our model there was a correlation between BLI and US measurements during the first four weeks of the study. But we observed a significant decrease in BLI signals whereas tumor volumes were continuously increasing. Bioluminescence reaction needs ATP and O2 to occur. In hypoxic situations, light emission becomes not proportional to tumor volume, due to the dependence of luciferin-luciferase reaction towards the metabolism. It appeared that BLI was no longer relevant in the study of advanced stages of MiaPaCa-2 tumors. PA imaging confirmed that there was hypoxia into tumors, approximately since the 5th week. So we were able, thanks to this approach, to accurately map the areas of hypoxia within the tumor.
These results confirm that BLI should be used with caution, in particular during an evaluation of a therapeutic drug. The combination of these three methods (BLI, US, PA) should therefore allow to precisely characterize orthotopic tumor models including the control of the appearance or location of a hypoxic state within tumors, while specifying the limits of bioluminescence.
Citation Format: Florian Raes, Thomas Barre, Jithin Jose, Philippe Trochet, Stéphanie Lerondel, Alain Le Pape. Characterization of tumor hypoxia by photoacoustic imaging and limitations of bioluminescence imaging in a Mia PaCa2-luc orthotopic model of pancreatic carcinoma. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 2048. doi:10.1158/1538-7445.AM2014-2048