Reproducibility of 3'-deoxy-3'-(18)F-fluorothymidine microPET studies in tumor xenografts in mice

Abstract

3'-Deoxy-3'-(18)F-fluorothymidine ((18)F-FLT) has been used to image tumor proliferation in preclinical and clinical studies. Serial microPET studies may be useful for monitoring therapy response or for drug screening; however, the reproducibility of serial scans has not been determined. The purpose of this study was to determine the reproducibility of (18)F-FLT microPET studies.

Methods: C6 rat glioma xenografts were implanted into nude mice (n = 9) and grown to mean diameters of 5-17 mm for approximately 2 wk. A 10-min acquisition was performed on a microPET scanner approximately 1 h after (18)F-FLT (1.9-7.4 MBq [50-200 muCi]) was injected via the tail vein. A second microPET scan was performed approximately 6 h later on the same day after reinjection of (18)F-FLT to assess for reproducibility. Most of the mice were studied twice within the same week (for a total of 17 studies). Images were analyzed by drawing an ellipsoidal region of interest (ROI) around the tumor xenograft activity. Percentage injected dose per gram (%ID/g) values were calculated from the mean activity in the ROIs. Coefficients of variation and differences in %ID/g values between studies from the same day were calculated to determine the reproducibility after subtraction of the estimated residual tumor activity from the first (18)F-FLT injection.

Results: The coefficient of variation (mean +/- SD) for %ID/g values between (18)F-FLT microPET scans performed 6 h apart on the same day was 14% +/- 10%. The difference in %ID/g values between scans was -0.06% +/- 1.3%. Serum thymidine levels were mildly correlated with %ID/g values (R(2) = 0.40). Tumor size, mouse body weight, injected dose, and fasting state did not contribute to the variability of the scans; however, consistent scanning parameters were necessary to ensure accurate studies, in particular, controlling body temperature, the time of imaging after injection, and the ROI size.

Conclusion: (18)F-FLT microPET mouse tumor xenograft studies are reproducible with moderately low variability. Serial studies may be performed to assess for significant changes in therapy response or for preclinical drug development.

FIGURE 1

Competition between various concentrations of thymidine and ³H-FLT in C6 rat glioma cells. Accumulation values represent tracer activity relative to activity in medium, normalized to total protein. Thymidine concentrations are plotted on log scale. Error bars represent 1 SD for triplicate samples.

FIGURE 2

Correlation between mouse serum thymidine levels determined by LC-MS/MS and ¹⁸F-FLT accumulation in 6 nude mice with C6 rat glioma xenografts. Accumulation values represent %ID/g values from ROI analysis of tumor xenografts from microPET images.

FIGURE 3

3D volume renderings of nude mouse C6 tumor xenograft scans showing reproducibility of ¹⁸F-FLT microPET. (Left) First scan. (Right) Second scan obtained 6 h later, after second injection of ¹⁸F-FLT. Mean %ID/g values are shown adjacent to tumor xenografts.

FIGURE 4

3D volume renderings of nude mouse C6 tumor xenograft scans obtained 1 h (left) and 4 h (right) after single ¹⁸F-FLT injection by microPET and showing changes in tumor activity and tracer distribution over time. Mean %ID/g values are shown adjacent to tumor xenografts.

FIGURE 5

3D volume renderings of nude mouse C6 tumor xenograft scans displaying temperature dependence of ¹⁸F-FLT microPET at 2 different temperatures, 24°C (left) and 35°C (right). Mean %ID/g values are shown adjacent to tumor xeno-grafts.