Dual-Tracer Autoradiography and Positron Emission Tomography (PET) Scans Using In-Yolk-Sac Tracer Delivery in the
Emil L Villumsen1,2, Signe Bauenmand3, Marie B Thuesen1
1Comparative Medicine Lab, Department of Clinical Medicine, Aarhus University, 8200 Aarhus, Denmark.
Abstract:
Background: Routine use of the chorioallantoic membrane (CAM) tumor model in nuclear imaging studies is hampered by small tumors, embryonic movements and laborious volume-restricted intravenous tracer/drug administration. We sought a workaround by using fast-growing tumors, high-resolution autoradiography and non-intravenous tracer administration. Methods: Dekalb White chicken eggs were grafted with C3H mammary carcinoma fragments or MOC2 oral squamous cell carcinoma fragments from donor mice. The tumor uptake of 18F-fluorodeoxyglucose (FDG) following in-yolk-sac injection, dripping after CAM scoring or allantoic cavity injection was evaluated using positron emission tomography (PET) and autoradiography. Using in-yolk-sac injection, eggs were administered different tracer mixtures, namely (1) pimonidazole (hypoxia-marker), FDG and 14C-2-deoxyglucose (14C-2DG), (2) pimonidazole, FDG and 14C-acetate or (3) pimonidazole, the hypoxia-selective tracer 18F-fluoroazomycin-arabinoside (FAZA) and 14C-2DG. For comparison, tumor-bearing mice were administered FDG/14C-acetate/pimonidazole. Gross tumor uptake was evaluated using PET. Tumor cryosections were analyzed using dual-tracer autoradiography. Complementary autoradiograms were co-registered, covered by a square grid (0.5 × 0.5 mm). Pearson correlation coefficients (PCC) were calculated from scatterplots. Results: C3H tumors reached a mean weight (with 95% confidence interval) of 0.32 g (0.28-0.37 g), while for MOC2, it was 0.19 g (0.09-0.29 g). In-yolk-sac tracer injection was simple and effective, producing high tracer uptake and contrast 3 h post-administration. Spatial tracer overlap (PCC) was: FDG vs. 14C-2DG, 0.95-0.97; FAZA vs. 14C-2DG, 0.71-0.79 and FDG vs. 14C-acetate, 0.26-0.84 (0.15-0.76 in mice). Pimonidazole revealed tumor hypoxia. Conclusions: Direct-grafting from donor mice generated larger tumors than previously reported. In-yolk-sac tracer administration was practical and allowed larger injected volumes. Autoradiography revealed that: (1) FDG and 14C-2DG can be used interchangeably, (2) 14C-2DG was elevated in FAZA-positive areas, suggesting that in some tumors FDG-PET may provide information on the intratumoral distribution of hypoxic areas, and (3) FDG and 14C-acetate showed variable overlap. We conclude that in-yolk-sac tracer injection and autoradiography simplify and optimize CAM-based nuclear imaging research.
