Assessing hypoxia in animal tumor models based on pharmocokinetic analysis of dynamic FAZA PET

Abstract

Positron emission tomography (PET) allows non-invasive detection and mapping of tumor hypoxia. However, slow tracer kinetics and low resolution, results in limited tumor-to-normal tissue contrast and the risk of missing areas where hypoxic cells are intermixed with necrosis. The shape of tumor time activity curves (TACs), as deduced from dynamic scans, may allow further separation of tumors/tumor sub-volumes that are inseparable based on static scans. This study was designed to define the added value of dynamic scans. Material and methods. Three squamous cell carcinoma tumor models were grown in mice. Mice were injected with the ¹⁸F-labeled PET hypoxia-tracer fluoroazomycin arabinoside (FAZA) and the immunologically-detectable hypoxia-marker pimonidazole, and PET scanned dynamically for three to six hours. Subsequently, microregional tracer retention (autoradiography) and the distribution of pimonidazole-retaining cells (immunohistology) and necrosis were analyzed in tumor tissue sections. Dynamic PET data were analysed based on a two-compartment model with irreversible tracer binding generating estimates of the putative hypoxia surrogate markers k₃ (tracer trapping rate constant) and K_i (influx rate constant from plasma into irreversible bound tracer). Results/Discussion. High tumor-to-reference tissue ratios and a strong linear correlation (R∼0.7 to 0.95) between density of hypoxic cells and FAZA concentration was observed three hours after tracer administration, suggesting that late time PET images provides an accurate measure of hypoxia against which kinetic model estimates can be validated. Tumor TACs varied widely (ranging from distinctly wash-out to accumulative type) among tumor types although pimonidazole-stainings revealed extensive hypoxia in all models. Kinetic analysis of tumor sub-volumes showed that k₃ correlated poorly with late time FAZA retention regionally in two of the three tumor models. The influx rate constant K_i displayed far less variability and correlated strongly with late time FAZA retention (hypoxia) in two of three tumor models, whereas a non-consistent relationship was observed in the last tumor model. Our study demonstrates the potential usefulness of dynamic PET, but also that a simple two-compartment model may be inappropriate in some tumor models.

Acknowledgements

We thank Ms. M. Simonsen from the PET-centre (Aarhus University Hospital, Aarhus, Denmark), Mr. T. Nielsen, Ms. I. M. Horsman, Mr. M. Johannsen, Ms. D. Grand, Ms. P Schjerrbeck, Ms. M.V. Bjerre and Ms. M. Kristiansen from the Department of Experimental and Clinical Oncology (Aarhus University Hospital, Aarhus, Denmark) and Lene H. Skjærris from the Institute of Pathology (Aarhus University Hospital, Aarhus, Denmark) for excellent technical and practical assistance. This study was supported by the EC FP6 project BIOCARE (LSHC-CT-2204-505785) and EC FP7 project METOXIA (Project no. 222741), and CIRRO - The Lundbeck Foundation Center for Interventional Research in Radiation Oncology, and The Danish Council for Strategic Research.

Declaration of interest: The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.