18F-FDG and 18F-FLT-PET imaging for monitoring everolimus effect on tumor-growth in neuroendocrine tumors: studies in human tumor xenografts in mice

Abstract

Introduction: The mTOR inhibitor everolimus has shown promising results in some but not all neuroendocrine tumors. Therefore, early assessment of treatment response would be beneficial. In this study, we investigated the in vivo and in vitro treatment effect of everolimus in neuroendocrine tumors and evaluated the performance of 18F-FDG and the proliferation tracer 18F-FLT for treatment response assessment by PET imaging.

Methods: The effect of everolimus on the human carcinoid cell line H727 was examined in vitro with the MTT assay and in vivo on H727 xenograft tumors. The mice were scanned at baseline with 18F-FDG or 18F-FLT and then treated with either placebo or everolimus (5 mg/kg daily) for 10 days. PET/CT scans were repeated at day 1,3 and 10.

Results: Everolimus showed significant inhibition of H727 cell proliferation in vitro at concentrations above 1 nM. In vivo tumor volumes measured relative to baseline were significantly lower in the everolimus group compared to the control group at day 3 (126±6% vs. 152±6%; p = 0.016), day 7 (164±7% vs. 226±13%; p<0.001) and at day 10 (194±10% vs. 281±18%; p<0.001). Uptake of 18F-FDG and 18F-FLT showed little differences between control and treatment groups, but individual mean uptake of 18F-FDG at day 3 correlated with tumor growth day 10 (r2 = 0.45; P = 0.034), 18F-FLT mean uptake at day 1 correlated with tumor growth day 7 (r2 = 0.63; P = 0.019) and at day 3 18F-FLT correlated with tumor growth day 7 (r2 = 0.87; P<0.001) and day 10 (r2 = 0.58; P = 0.027).

Conclusion: Everolimus was effective in vitro and in vivo in human xenografts lung carcinoid NETs and especially early 18F-FLT uptake predicted subsequent tumor growth. We suggest that 18F-FLT PET can be used for tailoring therapy for neuroendocrine tumor patients through early identification of responders and non-responders.

Figure 1. Study design.

Figure 2. MTT Assay.

In vitro inhibition of neuroendocrine tumor cell proliferation by everolimus using MTT assay.

Figure 3. Tumor growth of everolimus and vehicle treated human H727 xenografts in nude mice.

A significant effect of everolimus treatment was found on day 3, 7 and 10 compared with the control group. Tumor volume was found by manual drawing on the tumor boundary on CT images. N = 18 tumors/group. *) P<0.05, **) P<0.01, ***) P<0.001, treatment versus control group at same day.

Figure 4. PET uptake in H727 xenografts.

Uptake of FDG (upper panel) and FLT (lower panel) after treatment of H727 xenografts with everolimus or vehicle. N = 8–10 tumors/group. *) P<0.05 vs. control group on same day, #) P<0.05 and ##) P<0.01 vs. baseline of same group. P-values are Bonferroni corrected.

Figure 5. PET uptake and Tumorvolume.

Comparison of early FLT PET on day 1 with late CT volume on day 10 in same animal. Correlations for the whole group between early PET and late CT volumes are shown in Figure 6.

Figure 6. Early PET correlated to later tumorgrowth.

Correlations of individual tumor uptake (SUVmean) day 1 and day 3 and subsequent tumor growth until day 7 and 10. Left panel: mean uptake of FDG at day 3 correlated significantly with tumor growth until day 10, (n = 10). Right panel: mean uptake of FLT at day 1 correlated significantly with tumor growth until day 7 and FLT uptake at day 3 correlated with tumor growth until both day 7 and 10 (n = 8 tumors). Tumor growth is expressed as volume relative to baseline, 95% confidence intervals are shown. Correlations with SUVmax show same trend (not shown).