Circadian rhythm impacts preclinical FDG-PET quantification in the brain, but not in xenograft tumors

The inner clock of biological organisms plays a pivotal role and has strong effects on metabolic processes such as glucose consumption. Since the commonly used positron emission tomography (PET) tracer (18)F-flourodeoxygucose (FDG) is a glucose analogue, it is not surprising that the FDG distribution in mice and humans has been shown to succumb to daily rhythms. In preclinical studies, the circadian rhythm of animals is often not considered, and studies are performed at different times of day. Only a few studies have analyzed the effect of the circadian rhythm on FDG uptake in mice, and none of these studies included human tumor xenografts. Therefore, it is not known how strongly a preclinical tumor study is influenced by the time of day. In this work, the effect of the circadian rhythm on FDG uptake in human tumor xenografts and other organs was analyzed. CD1 nu/nu mice were kept for three weeks under a 12 h light/12 h dark rhythm and then injected s.c. with PC3 or A431 tumor cells. When the tumors had reached an appropriate volume, FDG-PET scans were performed on different animal groups (n = 4–5) every 4 h over a time period from 8 A.M. to 8 P.M. Tracer uptake in the tumors and in other organs was determined based on the PET scans and biodistribution studies. The standardized uptake value and %injected dose/cc of the tumors remained constant over the whole observed time period, and no statistically significant differences were determined according to the PET analysis. In the brain, we found a small but statistically significant increase from noon to 4 P.M., which led to a decrease in the tumor-to-brain ratio. No evidence for an effect of the circadian rhythm on FDG uptake could be found in subcutaneous tumors, however, in brain studies the circadian rhythm needs to be considered.