Variability of Proliferation and Diffusion in Different Lung Cancer Models as Measured by 3′-Deoxy-3′-¹⁸F-Fluorothymidine PET and Diffusion-Weighted MR Imaging

Abstract

Molecular imaging allows the noninvasive assessment of cancer progression and response to therapy. The aim of this study was to investigate molecular and cellular determinants of 3′-deoxy-3′-18 Ffluorothymidine ( 18 F-FLT) PET and diffusion-weighted (DW) MR imaging in lung carcinoma xenografts. Methods: Four lung cancer cell lines (A549, HTB56, EBC1, and H1975) were subcutaneously implanted in nude mice, and growth was followed by caliper measurements. Glucose uptake and tumor proliferation were determined by 18 F-FDG and 18 F-FLT PET, respectively. T2-weighted MR imaging was performed, and the apparent diffusion coefficient (ADC) was determined by DW MR imaging as an indicator of cell death. Imaging findings were correlated to histology with markers for tumor proliferation (Ki67, 5-bromo-2′-deoxyuridine [BrdU]) and cell death (caspase-3, terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling). The expression of human equilibrative nucleoside transporter 1 (hENT1), thymidine kinase 1 (TK1), thymidylate synthase, and thymidine phosphorylase (TP) were analyzed by Western blot and immunohistochemistry. Thymidine levels were determined by liquid chromatography-mass spectrometry. Results: Xenografts varied with respect to in vivo growth rates. MR imaging and PET revealed intratumoral heterogeneities, which were confirmed by histology. 18 F-FLT uptake differed significantly between tumor lines, with A549 and H1975 demonstrating the highest radiotracer accumulation (A549, 8.5 ± 3.2; HTB56, 4.4 ± 0.7; EBC1, 4.4 ± 1.2; and H1975, 12.1 ± 3.5 maximal percentage injected dose per milliliter). In contrast, differences in 18 F-FDG uptake were only marginal. No clear relationship between 18 F-FLT accumulation and immunohistochemical markers for tumor proliferation (Ki67, BrdU) as well as hENT1, TK1, or TS expression was detected. However, TP was highly expressed in A549 and H1975 xenografts, which was accompanied by low tumor thymidine concentrations, suggesting that tumor thymidine levels influence 18 F-FLT uptake in the tumor models investigated. MR imaging revealed higher ADC values within proliferative regions of H1975 and A549 tumors than in HTB56 and EBC1. These ADC values were negatively correlated with cell density but not directly related to cell death. Conclusion: A direct relationship of 18 F-FLT with proliferation or ADC with cell death might be complicated by the interplay of multiple processes at the cellular and physiologic levels in untreated tumors. This issue must be considered when using these imaging modalities in preclinical or clinical settings.

Response Monitoring with [18F]FLT PET and Diffusion-Weighted MRI After Cytotoxic 5-FU Treatment in an Experimental Rat Model for Colorectal Liver Metastases

“…The uptake of [ 18 F]FLT has been linked to cellular proliferation; however, [ 18 F]FLT is not incorporated into the DNA [10]. Its accumulation depends on activity of the ENT1 transporter, the rate of phosphorylation by TK1, and the level of endogenous thymidine [15, 22, 23]. Therefore, [ 18 F]FLT uptake does not necessarily reflect tumor cell proliferation.…”

“…Plasma, liver, and tumor samples of two rats from each group were collected to determine the in vivo thymidine levels. For this, a liquid-chromatography—mass spectrometry (LC-MS/MS) method for the quantitative analysis of thymidine in tumor tissue homogenate was developed by the PK/Bioanalytics Core Facility at the CRUK Cambridge Institute [15, 16]. The biodistribution of In-111-labeled MG1 was determined after dissection as described previously [13].…”

Comparison of Positron Emission Tomography Using 2-[18F]-fluoro-2-deoxy-D-glucose and 3-deoxy-3-[18F]-fluorothymidine in Lung Cancer Imaging

“…Cytological examination indicated that lung cancer cells incorporated much more 18 F-FLT than 11 C-DG during the S-phase of the cell division cycle. [10]…”

Thymidine Metabolism as a Confounding Factor for 3′-Deoxy-3′-18F-Fluorothymidine Uptake After Therapy in a Colorectal Cancer Model

“…Thymidine phosphorylase (TP) affects [ 18 F]FLT uptake by degrading thymidine, whereas [ 18 F]FLT is resistant to the activity of this enzyme (10). It is well recognized that thymidine competes with [ 18 F]FLT for cellular retention (11) and that this competition can be modified by TP (12,13). Moreover, TP activity plays a role in tumor angiogenesis, induces metastasis, and protects cancer cells against apoptosis (14).…”