Dual-Tracer Autoradiography and Positron Emission Tomography (PET) Scans Using In-Yolk-Sac Tracer Delivery in the Chicken Chorioallantoic Membrane (CAM) Tumor Model.

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.