[18F]FDG and [18F]FLT uptake in human breast cancer cells in relation to the effects of chemotherapy: an in vitro study

Abstract

Increased 2'-deoxy-2'-[18F]fluoro-D-glucose (FDG) uptake is the most commonly used marker for positron emission tomography in oncology. However, a proliferation tracer such as 3'-deoxy-3'-[18F]fluorothymidine (FLT) might be more specific for cancer. 3'-deoxy-3'-[18F]fluorothymidine uptake is dependent on thymidine kinase 1 (TK) activity, but the effects of chemotherapeutic agents are unknown. The aim of this study was to characterise FDG and FLT uptake mechanisms in vitro before and after exposure to chemotherapeutic agents. The effects of 5-fluorouracil (5-FU), doxorubicin and paclitaxel on FDG and FLT uptake were measured in MDA MB231 human breast cancer cells in relation to cell cycle distribution, expression and enzyme activity of TK-1. At IC50 concentrations, 5-FU resulted in accumulation in the G1 phase, but doxorubicin and paclitaxel induced a G2/M accumulation. Compared with untreated cells, 5-FU and doxorubicin increased TK-1 levels by >300. At 72 h, 5-FU decreased FDG uptake by 50% and FLT uptake by 54%, whereas doxorubicin increased FDG and FLT uptake by 71 and 173%, respectively. Paclitaxel increased FDG uptake with >100% after 48 h, whereas FLT uptake hardly changed. In conclusion, various chemotherapeutic agents, commonly used in the treatment of breast cancer, have different effects on the time course of uptake of both FDG and FLT in vitro. This might have implications for interpretation of clinical findings.

Figure 1

Distribution of MDA MB231 cells after exposure to IC₅₀ concentrations of 5-fluorouracil (), doxorubicin (), paclitaxel (▪) and untreated cells (□), in (A) sub-G1, (B) G1, (C) S and (D) G2/M phases.

Figure 2

Thymidine kinase (TK) activities in MDA MB231 breast cancer cells (A) without, and with addition of the TK-2 inhibitors (B) dCTP and (C) KIN52. Cells were incubated with IC₅₀ concentrations of 5-fluorouracil (), doxorubicin () and paclitaxel (▪), and compared with untreated cells (□).

Figure 3

Relative thymidine kinase (TK)-1 activity in MDA MB231 breast cancer cells after addition of (A) dCTP and (B) KIN52. Cells were exposed to IC₅₀ concentrations of 5-fluorouracil (), doxorubicin () and paclitaxel (▪), and compared with untreated cells (□). % cells are normalised to TK activity before addition of dCTP or KIN52 (set to 100%).

Figure 4

Lineweaver–Burk plots showing the effects of 3′-deoxy-3′-[¹⁸F]fluorothymidine (FLT) on thymidine (dThd) phosphorylation, catalysed by (A) thymidine kinase (TK)-1 at (▵) 25 μM and (◊) 10 μM, and (B) TK-2 at (▵) 2.5 μM, (°) 5 μM and (*) 10 μM of FLT, compared with thymidine alone (•). dThd, thymidine; V, reaction rate.

Figure 5

Typical Western blot for thymidine kinase (TK)-1 levels in CEM and MDA MB231 cells after treatment with cytotoxic agents compared with untreated cells, β-actin was used as loading control.