Dynamic PET evaluation of elevated FLT level after sorafenib treatment in mice bearing human renal cell carcinoma xenograft

Abstract

Background: Sorafenib, an oral multikinase inhibitor, has anti-proliferative and anti-angiogenic activities and is therapeutically effective against renal cell carcinoma (RCC). Recently, we have evaluated the tumor responses to sorafenib treatment in a RCC xenograft using [Methyl-³H(N)]-3'-fluoro-3'-deoxythythymidine ([³H]FLT). Contrary to our expectation, the FLT level in the tumor significantly increased after the treatment. In this study, to clarify the reason for the elevated FLT level, dynamic 3'-[¹⁸F]fluoro-3'-deoxythymidine ([¹⁸F]FLT) positron emission tomography (PET) and kinetic studies were performed in mice bearing a RCC xenograft (A498). The A498 xenograft was established in nude mice, and the mice were assigned to the control (n = 5) and treatment (n = 5) groups. The mice in the treatment group were orally given sorafenib (20 mg/kg/day p.o.) once daily for 3 days. Twenty-four hours after the treatment, dynamic [¹⁸F]FLT PET was performed by small-animal PET. Three-dimensional regions of interest (ROIs) were manually defined for the tumors. A three-compartment model fitting was carried out to estimate four rate constants using the time activity curve (TAC) in the tumor and the blood clearance rate of [¹⁸F]FLT.

Results: The dynamic pattern of [¹⁸F]FLT levels in the tumor significantly changed after the treatment. The rate constant of [¹⁸F]FLT phosphorylation (k₃) was significantly higher in the treatment group (0.111 ± 0.027 [1/min]) than in the control group (0.082 ± 0.009 [1/min]). No significant changes were observed in the distribution volume, the ratio of [¹⁸F]FLT forward transport (K₁) to reverse transport (k₂), between the two groups (0.556 ± 0.073 and 0.641 ± 0.052 [mL/g] in the control group).

Conclusions: Our dynamic PET studies indicated that the increase in FLT level may be caused by the phosphorylation of FLT in the tumor after the sorafenib treatment in the mice bearing a RCC xenograft. Dynamic PET studies with kinetic modeling could provide improved understanding of the biochemical processes involved in tumor responses to therapy.

Keywords: 3′-[18F]fluoro-3′-deoxythymidine ([18F]FLT); Dynamic PET; Renal cell carcinoma xenograft; Sorafenib; Tumor proliferation.

Fig. 1

Blood time activity curves (input function) for [¹⁸F]FLT in the left ventricle. a Control group. b Sorafenib-treated group. c Average of each group (SUV ± SD). d Transaxial image of [¹⁸F]FLT PET across the heart of the mouse at first flame (0–15 s). A cuboid ROI (1.5 × 1.5 × 2.0 mm³) was drawn on the left ventricle (LV) region on a CT image and projected to every PET image to obtain the LV TAC (filled blue region)

Fig. 2

Compartment model of [¹⁸F]FLT in the tumor tissue. K₁, k₂, k₃, and k₄ are the kinetic rate constants between the compartments. Cp blood concentration of [¹⁸F]FLT, Ce exchangeable [¹⁸F]FLT concentration in the tissue, Cm phosphorylated [¹⁸F]FLT metabolites in the tissue, Cmet concentration of [¹⁸F]FLT metabolites in the arterial plasma. FLTMP FLT-monophosphate, TLTDP FLT-diphosphate, FLTTP FLT-triphosphate, FLT-gluc FLT-glucuronide

Fig. 3

Time activity curves in the tumor following [¹⁸F]FLT injection. a Control group. b Sorafenib-treated group. c Average of each group (SUV ± SD)

Fig. 4

PET images of [¹⁸F]FLT (horizontal sections) at 110–120 min postinjection in the mice bearing the tumor. a Control group. b Sorafenib-treated group. Filled arrows show the tumor region and blank arrows show the bladder. Artifacts due to extremely high radioactivity accumulation were observed in the bladder area. c SUVs of [¹⁸F]FLT in the tumor at 110–120 min postinjection