[18F]FLT tumor PET imaging

HCT-116 (5 × 10⁶) cells were injected on the back of female nu/nu-BALB/c athymic nude mice. When xenografts reached ~100 mm³, HCT-116 tumor-bearing mice were treated with C1A or BEZ235 alone or in combination. At 48 h post treatment, the animals were scanned on a dedicated small animal PET scanner (Siemens Inveon PET module; Siemens, Erlangen, Germany) following a bolus intravenous injection of 3.7 MBq of [¹⁸F]fluorothymidine ([¹⁸F]FLT) and tumor uptake analyzed as previously described.³² Dynamic emission scans were acquired in list-mode format over 60 min. The acquired data were then sorted into 0.5 mm sinogram bins and 19 time frames for image reconstruction, which was done by filtered back projection. Cumulative images of the data (30–60 min) were used for visualization of radiotracer uptake and to define the regions of interest (ROIs) across several slices with the Siemens Inveon Research Workplace software (3D ROIs were defined for each tumor). The count densities were averaged for all ROIs at each of the 19 time points to obtain a time versus radioactivity curve. The radiotracer uptake in tumor regions was normalized to that of the heart of the same animal. The normalized uptake value at 60 min post injection (NUV60) was used for comparisons. The area under the NUV curve was calculated as the integral of NUV from 0 to 60 min.