Human radiation dosimetry of 6-[18F]FDG predicted from preclinical studies.

PURPOSE: The authors are developing 6-[(18)F]fluoro-6-deoxy-D-glucose (6-[(18)F]FDG) as an in vivo tracer of glucose transport. While 6-[(18)F]FDG has the same radionuclide half-life as 2-[(18)F]fluoro-2-deoxy-D-glucose (2-[(18)F]FDG) which is ubiquitously used for PET imaging, 6-[(18)F]FDG has special biologic properties and different biodistributions that make it preferable to 2-[(18)F]FDG for assessing glucose transport. In preparation for 6-[(18)F]FDG use in human PET scanning, the authors would like to determine the amount of 6-[(18)F]FDG to inject while maintaining radiation doses in a safe range. METHODS: Rats were injected with 6-[(18)F]FDG, euthanized at specified times, and tissues were collected and assayed for activity content. For each tissue sample, the percent of injected dose per gram was calculated and extrapolated to that for humans in order to construct predicted time-courses. Residence times were calculated as areas under the curves and were used as inputs to OLINDA/EXM in order to calculate the radiation doses. RESULTS: Unlike with 2-[(18)F]FDG for which the urinary bladder wall receives the highest absorbed dose due to urinary excretion, with 6-[(18)F]FDG there is little urinary excretion and osteogenic cells and the liver are predicted to receive the highest absorbed doses: 0.027 mGy/MBq (0.100 rad/mCi) and 0.018 mGy/MBq (0.066 rad/mCi), respectively. Also, the effective dose from 6-[(18)F]FDG, i.e., 0.013 mSv/MBq (0.046 rem/mCi), is predicted to be approximately 30% lower than that from 2-[(18)F]FDG. CONCLUSIONS: 6-[(18)F]FDG will be safe for use in the PET scanning of humans.

A colorimetric assay method to measure acetyl-CoA synthetase activity: application to woodchuck model of hepatitis virus-induced hepatocellular carcinoma.

A new spectrophotometric method for quantitation of acetyl-CoA synthetase (ACAS) activity is developed. It has been applied for ACAS assay in the liver tissues of a woodchuck model of hepatitis virus-induced hepatocellular carcinoma (HCC). The assay is based on the established pyrophosphate (PPi) detection system. ACAS activity is indexed by the amount of PPi, the product of ACAS reaction system of activated form of acetate (acetyl-CoA) with ACAS catalysis. PPi is determined quantitatively as the amount of chromophore formed with molybdate reagent, 1-amino-2-naphthol-4-sulfonic acid in bisulfite and 2-mercaptoethanol. PPi reacts with molybdate reagent to produce phosphomolybdate and PPi-molybdate complexes. 2-mercaptoethanol is responsible for color formation which has the peak absorbance at 580 nm. This method was sensitive from 1 to 20 nmol of PPi in a 380-mul sample (1-cm cuvette). A ten-fold excess of Pi did not interfere with the determination of PPi. To study the major metabolic pathways of imaging tracer [1-(11)C]-acetate in tumors for detection of HCC by Positron Emission Tomography (PET), the activity of one of the key enzymes involved in acetate or [1-(11)C]-acetate metabolism, ACAS was assayed by this newly developed assay in the tissue samples of woodchuck HCCs. A significant increase of ACAS activity was observed in the liver tissues of woodchuck HCCs as compared with neighboring regions surrounding the tumors (P<0.05). The respective ACAS activities in the subcellular locations were also significantly higher in HCCs than in the surrounding tissues (P<0.05) (total soluble fraction: 876.61+/-34.64 vs. 361.62+/-49.97 mU/g tissue; cytoplasmic fraction: 1122.02+/-112.39 vs. 732.32+/-84.44 mU/g tissue; organelle content: 815.79+/-100.77 vs. 547.91+/-97.05 mU/ g tissue; sedimentable fragment: 251.92+/-51.56 vs. 90.94+/-18.98 mU/ g tissue). The finding suggests an increase in ACAS activity in the liver cancer of woodchuck models of HCC as compared to that in the normal woodchuck liver. The developed assay is rapid, simple and accurate and is suitable for the investigation of ACAS activity under physiologic and pathophysiologic conditions.

Hexokinase and glucose-6-phosphatase activity in woodchuck model of hepatitis virus-induced hepatocellular carcinoma.

2-Deoxy-2-[(18)F]fluoro-D-glucose ([(18)F] FDG) is used for PET imaging of woodchuck (Marmota monax) model of hepatocellular carcinoma (HCC). The usefulness of FDG on this animal model needs to be validated according to the hypothesized mechanisms. In this study, two key enzymes involved in glucose or [(18)F] FDG metabolism, hexokinase (HK) and glucose-6-phophatase (G6Pase), were examined for their enzymatic activities in the woodchuck models of HCC, which has not been studied before. After dynamic PET scans, woodchuck liver tissue samples were harvested and the homogenate was centrifuged. The supernatant was used for HK activity assay and the microsomal pellet was used for G6Pase assay. HK and G6Pase activities were measured by means of colorimetric reactions via kinetic and end-point assays, respectively. Total protein content was measured by the Bradford method and used to normalize all enzyme activities. HK and G6Pase activities in woodchuck HCC will be used to correlate with in vivo PET imaging data. The woodchuck model of HCC had significantly increased levels of HK in the livers compared to the age-matching healthy woodchuck (7.96 +/- 1.27 vs. 2.74 +/- 0.66 mU/mg protein, P < 0.01) and significantly decreased levels of G6Pase compared to healthy woodchuck (40.35 +/- 19.28 vs. 237.01 +/- 17.32 mU/mg protein, P < 0.01), reflecting an increase in glycolysis. In addition, significant differences were found in HK and G6Pase activities between HCC liver region (HK: 7.96 +/- 1.27 mU/mg protein; G6Pase: 40.35 +/- 19.28 mU/mg protein) and surrounding normal liver region (HK: 2.98 +/- 0.92 mU/mg protein; G6Pase: 140.87 +/- 30.62 mU/mg protein) in the same woodchuck model of HCC (P < 0.01). Our study demonstrated an increased HK activity and a decreased G6Pase activity in liver of the woodchuck models of HCC as compared to normal woodchuck liver.

Measuring gluconeogenesis using a low dose of 2H2O: advantage of isotope fractionation during gas chromatography.

The contribution of gluconeogenesis to glucose production can be measured by enriching body water with (2)H(2)O to approximately 0.5% (2)H and determining the ratio of (2)H that is bound to carbon-5 vs. carbon-2 of blood glucose. This labeling ratio can be measured using gas chromatography-mass spectrometry after the corresponding glucose carbons are converted to formaldehyde and then to hexamethylenetetramine (HMT). We present a technique for integrating ion chromatograms that allows one to use only 0.05% (2)H in body water (i.e., 10 times less than the current dose). This technique takes advantage of the difference in gas chromatographic retention times of naturally labeled HMT and [(2)H]HMT. We discuss the advantage(s) of using a low dose of (2)H(2)O to quantify the contribution of gluconeogenesis.