PET (18F-FDG) and SPECT (99mTc-1) agents in preclinical models of melanoma metastasis in mice

Objectives The imaging of melanoma with [¹⁸F]-FDG and [^99mTcOAADT]-(CH₂)₂-N(Et)₂ (^99mTc-1) was studied using both a subcutaneous tumor model and a B16/F10 metastasis model where small melanoma lesions occur in mouse lungs.

Results [¹⁸F]-FDG displayed subcutaneous tumor uptake of 4.92±0.24, 6.28±0.96 and 3.98±0.73 %ID/g at 1, 3, and 6 h, respectively, while ^99mTc-1 displayed a significant and comparable tumor uptake of 7.6±1.3, 6.6±2.16 and 4.72±1.9 %ID/g.In the metastasis model, [¹⁸F]-FDG displayed tumor-lung uptake of 2.03±0.58, 3.67±0.69, 2.09±0.2, and 1.78±0.5 %ID/g at 0.5, 1, 2, and 3 h respectively, compared to normal lung uptake of 1.67±0.1, 2.56±0.4, 2.12±0.3 and 2.1±0.26 %ID/g. ^99mTc-1 displayed a significant tumor uptake in the melanotic lesions with the tumor uptake of 10.35±1.6 and 3.82±0.9 %ID/g at 2 and 3 h p.i, respectively, compared to 2.13±0.20 and 0.85±0.26 %ID/g observed in the normal lung. Exvivo radiophosphor imaging of the excised tumor-lungs also confirmed the uptake differences between [¹⁸F]-FDG and the ^99mTc-1 complex. Distinct visualization of the small tumor lesions within the mouse lungs was also achievable with ^99mTc-1 by µSPECT imaging.

Conclusions In both murine melanoma models [¹⁸F]-FDG displayed a significant washout from the melanoma tumors over time, presumably due to the phosphatase activity in these tumors. The lack of [¹⁸F]-FDG accumulation observed in the small metastatic lesions in the lungs may also be due to a higher blood-flow-dependent washout. ^99mTc-1, however, displayed high and specific uptake in melanotic melanomas in both tumor models.