Analysis of the regional uptake of radiolabeled deoxyglucose analogs in human tumor xenografts.

UNLABELLED: It has been shown in vitro that the cell uptake of (18)F-FDG, a tracer of glucose metabolism, increases under hypoxia. This is consistent with increased glycolytic metabolism. We have previously shown that in ischemic heart ex vivo the rates of uptake of (18)F-FDG and 2-(14)C-deoxy-D-glucose ((14)C-2DG) are both reduced. In this study, we investigated this effect in tumors by comparing the microdistribution of (18)F-FDG and (14)C-2DG in hypoxic and normoxic regions. METHODS: Mice (MF1) bearing LS174T human tumor xenografts were injected with premixed (18)F-FDG (100 MBq), (14)C-2DG (0.37 MBq), and pimonidazole hydrochloride (60 mg/kg). After 30, 60, and 120 min, tissues (n = 4) were taken and counted for whole-body biodistribution. Tumors were frozen, sectioned, and exposed to phosphor image plates to obtain a quantitative digital image of radionuclide distribution. Sections were then stained to reveal tumor pathophysiology: Hematoxylin and eosin staining demonstrated viable and necrotic regions, and immunohistochemical staining detected pimonidazole metabolism in hypoxic cells. The images of radionuclide microdistribution and histology were then coregistered and analyzed to assess radionuclide trapping throughout the tumor on a pixel-by-pixel basis. The Pearson correlation coefficients between the 2 radionuclides were calculated. The relative amounts of nuclide were then analyzed in viable and necrotic regions and in normoxic and hypoxic regions. RESULTS: Whole-body biodistributions for the 2 radiotracers were similar. A high Pearson correlation coefficient was obtained for the 2 radionuclides throughout the tumors (r = 0.85 +/- 0.10, P < 0.0001), indicating a highly similar microdistribution. When the tumors were divided into viable and necrotic regions, the ratio of mean counts per pixel was 1.96 (P < 0.0001), whereas for hypoxic versus normoxic regions it was 1.26 (P < 0.0001). There was no significant difference in selectivity for hypoxia between the 2 radiotracers (P = 0.86). CONCLUSION: The tumor microdistribution of deoxyglucose in viable, hypoxic, and necrotic regions show that there was little change in the microdistribution of deoxyglucose throughout this time course. This study extends previous in vitro work and confirms the selectivity of deoxyglucose for viable cells over necrotic regions and for hypoxic cells over normoxic regions in vivo.

Rapid solid phase synthesis and biodistribution of 18F-labelled linear peptides.

A rapid method for radiolabelling short peptides with 18F ( t(1/2)=109.7 min) for use in positron emission tomography (PET) was developed. Linear peptides (13mers) were synthesised using solid phase peptide synthesis and 9-fluorenylmethoxycarbonyl (Fmoc) chemistry. The peptides were assembled on a solid-phase polyethylene glycol-polystyrene support using the "hyper acid labile" linker xanthen-2-oxyvaleric acid and were labelled in situ with 4-[19F]- or 4-[18F]fluorobenzoic acid. Optimum coupling of 4-[19F]fluorobenzoic acid to the peptidyl resin was achieved within 2 min using N-[(dimethylamino)-1 H-1,2,3-triazolo[4,5-b]pyridin-1-ylmethylene]- Nmethylmethanaminium hexafluorophosphate N-oxide (HATU/DIPEA), and optimum cleavage was achieved within 7 min using trifluoroacetic acid/phenol/water/Triisopropylsilane at 37 degrees C. The linear peptides were rapidly labelled with 4-[18F]fluorobenzoic acid with an overall radiochemical yield of 80%-90% (decay corrected), a radiochemical purity of >95% without HPLC purification and an overall synthesis time of 20 min. This novel method was used to label peptides containing the arginine-glycine-aspartic acid (RGD) motif, the binding site of many integrins. In vitro studies showed that the fluorobenzoyl prosthetic group had no deleterious effect on the ability of these peptides to inhibit the binding of human cells via integrins. Biodistribution studies in tumour-bearing mice showed that although the linear peptides were rapidly removed from the circulation by the liver and kidneys, there was a transient and non-RGD-dependent accumulation in the tumour of both the test and the control peptides. The use of more selective peptides with a longer half-life in the circulation combined with this rapid labelling technique will significantly enhance the application of peptides in PET.