18F-FLT PET as a Surrogate Marker of Drug Efficacy During mTOR Inhibition by Everolimus in a Preclinical Cisplatin-Resistant Ovarian Tumor Model

Targeting the mammalian target of rapamycin (mTOR) pathway is a potential means of overcoming cisplatin resistance in ovarian cancer patients. Because mTOR inhibition affects cell proliferation, we aimed to study whether 3'-deoxy-3'-F-18-fluo-rothymidine (F-18-FLT) PET could be useful for monitoring early response to treatment with mTOR inhibitors in an animal model of cisplatin-resistant ovarian tumor. Methods: BALB/c nude mice bearing subcutaneous human SKOV3 ovarian cancer xeno-grafts were treated with either the mTOR inhibitor everolimus (5 mg/kg) or vehicle, and F-18-FLT PET was performed at baseline, day 2, and day 7 of treatment. F-18-FLT uptake was evaluated by calculation of mean standardized uptake value (SUVmean) corrected for partial-volume effect. Ex vivo immunohistochemistry studies were performed on separate cohorts of mice treated as above and sacrificed at the same time points as for the PET studies. The ex vivo analysis included bromodeoxyuridine incorporation as a marker of cell proliferation, and phosphorylation of ribosomal protein S6 as a downstream marker of mTOR activation. Results: During the treatment period, no significant change in tumor F-18-FLT uptake was observed in the vehicle group, whereas in everolimus-treated mice, F-18-FLT SUVmean decreased by 33% (P = 0.003) at day 2 and 66% (P < 0.001) at day 7, compared with baseline. Notably, the reduction of F-18-FLT uptake observed at day 2 in the everolimus group preceded changes in tumor volume, and a significant difference in F-18-FLT uptake was observed between vehicle and drug-treated tumors at both day 2 (P = 0.0008) and day 7 (P = 0.01). In ex vivo studies, everolimus treatment resulted in a 98% reduction in phosphorylated ribosomal protein S6 immunostaining at day 2 (P = 0.02) and 91% reduction at day 7 (P = 0.003), compared with the vehicle group. Bromodeoxyuridine incorporation was reduced by 65% at day 2 (not significant) and by 41% at day 7 (P = 0.02) in drug versus vehicle groups. Conclusion: Reduction in F-18-FLT uptake correlates well with the level of mTOR inhibition by everolimus in the SKOV3 ovarian tumor model. These data suggest that early treatment monitoring by F-18-FLT PET may be of use in future preclinical or clinical trials evaluating treatment of cisplatin-resistant ovarian tumors by mTOR inhibitors.