Nucleophilic radiosynthesis of 2-[18F]fluoro-2-deoxy-D-galactose from Talose triflate and biodistribution in a porcine model

Experimental non-alcoholic fatty liver disease causes regional liver functional deficits as measured by the capacity for galactose metabolism while whole liver function is preserved

BACKGROUND

Increasing incidence of non-alcoholic fatty liver disease (NAFLD) calls for improved understanding of how the disease affects metabolic liver function.

AIMS

To investigate in vivo effects of different NAFLD stages on metabolic liver function, quantified as regional and total capacity for galactose metabolism in a NAFLD model.

METHODS

Male Sprague Dawley rats were fed a high-fat, high-cholesterol diet for 1 or 12 weeks, modelling early or late NAFLD, respectively. Each NAFLD group (n = 8 each) had a control group on standard chow (n = 8 each). Metabolic liver function was assessed by 2-[¹⁸F]fluoro-2-deoxy-D-galactose positron emission tomography; regional galactose metabolism was assessed as standardised uptake value (SUV). Liver tissue was harvested for histology and fat quantification.

RESULTS

Early NAFLD had median 18% fat by liver volume. Late NAFLD had median 32% fat and varying features of non-alcoholic steatohepatitis (NASH). Median SUV reflecting regional galactose metabolism was reduced in early NAFLD (9.8) and more so in late NAFLD (7.4; p = 0.02), both significantly lower than in controls (12.5). In early NAFLD, lower SUV was quantitatively explained by fat infiltration. In late NAFLD, the SUV decrease was beyond that attributable to fat; probably related to structural NASH features. Total capacity for galactose elimination was intact in both groups, which in late NAFLD was attained by increased fat-free liver mass to 21 g, versus 15 g in early NAFLD and controls (both p ≤ 0.002).

CONCLUSION

Regional metabolic liver function was compromised in NAFLD by fat infiltration and structural changes. Still, whole liver metabolic function was preserved in late NAFLD by a marked increase in the fat-free liver mass.

Application of Metabolic Reprogramming to Cancer Imaging and Diagnosis

Cellular metabolism governs the signaling that supports physiological mechanisms and homeostasis in an individual, including neuronal transmission, wound healing, and circadian clock manipulation. Various factors have been linked to abnormal metabolic reprogramming, including gene mutations, epigenetic modifications, altered protein epitopes, and their involvement in the development of disease, including cancer. The presence of multiple distinct hallmarks and the resulting cellular reprogramming process have gradually revealed that these metabolism-related molecules may be able to be used to track or prevent the progression of cancer. Consequently, translational medicines have been developed using metabolic substrates, precursors, and other products depending on their biochemical mechanism of action. It is important to note that these metabolic analogs can also be used for imaging and therapeutic purposes in addition to competing for metabolic functions. In particular, due to their isotopic labeling, these compounds may also be used to localize and visualize tumor cells after uptake. In this review, the current development status, applicability, and limitations of compounds targeting metabolic reprogramming are described, as well as the imaging platforms that are most suitable for each compound and the types of cancer to which they are most appropriate.

N-(4-[18F]fluorobenzyl)cholylglycine, a potential tracer for positron emission tomography of enterohepatic circulation and drug-induced inhibition of ileal bile acid transport. A proof-of-concept PET/CT study in pigs

INTRODUCTION

Enterohepatic circulation (EHC) of conjugated bile acids is an important physiological process crucial for bile acids to function as detergents and signal carriers. Perturbation of the EHC by disease or drugs may lead to serious and life-threatening liver and gastrointestinal disorders. In this proof-of-concept study in pigs, we investigate the potential of N-(4-[¹⁸F]fluorobenzyl)cholylglycine ([¹⁸F]FBCGly) as tracer for quantitative positron emission tomography (PET) of the EHC of conjugated bile acids.

METHODS

The biodistribution of [¹⁸F]FBCGly was investigated by PET/CT in domestic pigs following intravenous and intraileal administration of the tracer. Hepatic kinetics were estimated from PET and blood data using a 2-tissue compartmental model and dual-input of [¹⁸F]FBCGly. The ileal uptake of [¹⁸F]FBCGly was investigated with co-injection of nifedipine and endogenous cholyltaurine. Dosimetry was estimated from the PET data using the Olinda 2.0 software. Blood, bile and urine samples were analyzed for possible fluorine-18 labelled metabolites of [¹⁸F]FBCGly.

RESULTS

[¹⁸F]FBCGly was rapidly taken up by the liver and excreted into bile, and underwent EHC without being metabolized. Both nifedipine and endogenous cholyltaurine inhibited the ileal uptake of [¹⁸F]FBCGly. The flow-dependent hepatic uptake clearance was estimated to median 1.2 mL blood/min/mL liver tissue. The mean residence time of [¹⁸F]FBCGly in hepatocytes was 4.0 ± 1.1 min. Critical organs for [¹⁸F]FBCGly were the gallbladder wall (0.94 mGy/MBq) and the small intestine (0.50 mGy/MBq). The effective dose for [¹⁸F]FBCGly was 36 μSv/MBq.

CONCLUSION

We have shown that [¹⁸F]FBCGly undergoes EHC in pigs without being metabolized and that its ileal uptake is inhibited by nifedipine and endogenous bile acids. Combined with our previous findings in rats, we believe that [¹⁸F]FBCGly has potential as PET tracer for assessment of EHC of conjugated bile acids under physiological conditions as well as conditions with perturbed hepatic and ileal bile acid transport.

Effect of stereotactic body radiotherapy on regional metabolic liver function investigated in patients by dynamic [18F]FDGal PET/CT

Purpose

Stereotactic body radiotherapy (SBRT) is increasingly used for treatment of liver tumors but the effect on metabolic liver function in surrounding tissue is largely unknown. Using 2-deoxy-2-[¹⁸F]fluoro-d-galactose ([¹⁸F]FDGal) positron emission tomography (PET)/computed tomography (CT), we aimed to determine a dose–response relationship between radiation dose and metabolic liver function as well as recovery.

Procedures.

One male subject with intrahepatic cholangiocarcinoma and five subjects (1 female, 4 male) with liver metastases from colorectal cancer (mCRC) underwent [¹⁸F]FDGal PET/CT before SBRT and after 1 and 3 months. The dose response was calculated using the data after 1 month and the relative recovery was evaluated after 3 months. All patients had normal liver function at time of inclusion.

Results

A linear dose–response relationship for the individual liver voxel dose was seen until approximately 30 Gy. By fitting a polynomial curve to data, a mean TD₅₀ of 18 Gy was determined with a 95% CI from 12 to 26 Gy. After 3 months, a substantial recovery was observed except in tissue receiving more than 25 Gy.

Conclusions

[¹⁸F]FDGal PET/CT makes it possible to determine a dose–response relationship between radiation dose and metabolic liver function, here with a TD₅₀ of 18 Gy (95% CI 12–26 Gy). Moreover, the method makes it possible to estimate metabolic recovery in liver tissue.

Hepatic regeneration following radiation-induced liver injury is associated with increased hepatobiliary secretion measured by PET in Göttingen minipigs

Normal liver tissue is highly vulnerable towards irradiation, which remains a challenge in radiotherapy of hepatic tumours. Here, we examined the effects of radiation-induced liver injury on two specific liver functions and hepatocellular regeneration in a minipig model. Five Göttingen minipigs were exposed to whole-liver stereotactic body radiation therapy (SBRT) in one fraction (14 Gy) and examined 4–5 weeks after; five pigs were used as controls. All pigs underwent in vivo positron emission tomography (PET) studies of the liver using the conjugated bile acid tracer [N-methyl-¹¹C]cholylsarcosine ([¹¹C]CSar) and the galactose-analogue tracer [¹⁸F]fluoro-2-deoxy-d-galactose ([¹⁸F]FDGal). Liver tissue samples were evaluated histopathologically and by immunohistochemical assessment of hepatocellular mitosis, proliferation and apoptosis. Compared with controls, both the rate constant for secretion of [¹¹C]CSar from hepatocytes into intrahepatic bile ducts as well as back into blood were doubled in irradiated pigs, which resulted in reduced residence time of [¹¹C]CSar inside the hepatocytes. Also, the hepatic systemic clearance of [¹⁸F]FDGal in irradiated pigs was slightly increased, and hepatocellular regeneration was increased by a threefold. In conclusion, parenchymal injury and increased regeneration after whole-liver irradiation was associated with enhanced hepatobiliary secretion of bile acids. Whole-liver SBRT in minipigs ultimately represents a potential large animal model of radiation-induced liver injury and for testing of normal tissue protection methods.

[18 F]-Fluoro-2-deoxy-D-galactose positron emission tomography/computed tomography as complementary imaging tool in patients with hepatocellular carcinoma

BACKGROUND & AIMS

Positron emission tomography (PET) with the liver-specific tracer [¹⁸ F]-fluoro-2-deoxy-D-galactose (¹⁸ F-FDGal) can be used for imaging of hepatocellular carcinoma (HCC). Curative intended and locoregional treatments of HCC require absence of extrahepatic disease. The aim of this prospective study was to determine whether adding ¹⁸ F-FDGal PET/CT to standard work-up changes the planned treatment in patients with HCC deemed suitable for curative or locoregional treatment.

METHODS

Fifty patients with HCC were included at our tertiary liver centre. The primary study outcome was a change in treatment strategy. A subgroup of 29 patients was also examined with [¹⁸ F]-fluoro-2-deoxy-D-glucose (¹⁸ F-FDG) PET/CT for comparison.

RESULTS

¹⁸ F-FDGal PET/CT detected eight extrahepatic HCC metastases in six patients (12%), which were primarily not detected by ceCT or MRI. These findings led to a change in treatment in five patients (10%). One of the eight extrahepatic HCC foci was also detected by ¹⁸ F-FDG PET/CT. A total of 85 malignant intrahepatic foci were examined, 12 of these were new findings by ¹⁸ F-FDGal PET/CT which had a sensitivity of 71%, highest for large foci. None of the additional intrahepatic foci found by ¹⁸ F-FDGal PET changed the planned treatment.

CONCLUSIONS

For the detection of extrahepatic HCC metastases, ¹⁸ F-FDGal PET/CT was superior both to standard clinical work-up with contrast-enhanced CT, and/or MRI, and to ¹⁸ F-FDG PET/CT in patients deemed suitable for locoregional treatment. ¹⁸ F-FDGal PET/CT led to a change in the planned treatment in 10% of the patients whereas ¹⁸ F-FDG PET/CT did not change the planned treatment in any patient.

Non-alcoholic steatohepatitis, but not simple steatosis, disturbs the functional homogeneity of the liver - a human galactose positron emission tomography study

BACKGROUND

The disease severity of non-alcoholic fatty liver disease (NAFLD) and the distinction between simple steatosis and non-alcoholic steatohepatitis (NASH) are based on the pathohistological presence of steatosis, inflammation, ballooning and fibrosis. However, little is known about the relation between such structural changes and the function of the afflicted liver.

AIMS

To investigate in vivo effects of hepatic fat fraction, ballooning and fibrosis on regional and whole liver metabolic function assessed by galactose elimination in NASH and simple steatosis.

METHODS

Twenty-five biopsy-proven, nondiabetic patients with NAFLD (13 NASH with low-grade fibrosis, 12 simple steatosis with no fibrosis) underwent 2-[¹⁸ F]fluoro-2-deoxy-d-galactose positron emission tomography and magnetic resonance imaging-derived proton density fat fraction of the liver. Nine healthy persons were included as controls.

RESULTS

In the NASH patients, the standardised hepatic uptake of 2-[¹⁸ F]fluoro-2-deoxy-d-galactose was reduced to 13.5 (95% confidence interval, 12.1-14.9) as compared with both simple steatosis and controls (16.4 (15.6-17.1), P < 0.001). Thus, the NASH patients had reduced regional metabolic liver function. The liver fat fraction diluted the standardised uptake equally in NASH and simple steatosis but the fibrosis and ballooning of NASH were associated with a further decrease. Moreover, the NASH livers exhibited increased variation in their standardised uptake values (coefficient of variation 13.8% vs 11.6% in simple steatosis and 10.2% in controls, P = 0.02), reflecting an increased functional heterogeneity.

CONCLUSIONS

In NASH, the regional metabolic liver function was lower and more heterogeneous than in both simple steatosis and healthy controls. Thus, NASH disturbs the normal homogeneous metabolic function of the liver.

Quantitative PET of liver functions

Improved understanding of liver physiology and pathophysiology is urgently needed to assist the choice of new and upcoming therapeutic modalities for patients with liver diseases. In this review, we focus on functional PET of the liver: 1) Dynamic PET with 2-deoxy-2-[18F]fluoro-D-galactose (18F-FDGal) provides quantitative images of the hepatic metabolic clearance Kmet (mL blood/min/mL liver tissue) of regional and whole-liver hepatic metabolic function. Standard-uptake-value (SUV) from a static liver 18F-FDGal PET/CT scan can replace Kmet and is currently used clinically. 2) Dynamic liver PET/CT in humans with 11C-palmitate and with the conjugated bile acid tracer [N-methyl-11C]cholylsarcosine (11C-CSar) can distinguish between individual intrahepatic transport steps in hepatic lipid metabolism and in hepatic transport of bile acid from blood to bile, respectively, showing diagnostic potential for individual patients. 3) Standard compartment analysis of dynamic PET data can lead to physiological inconsistencies, such as a unidirectional hepatic clearance of tracer from blood (K1; mL blood/min/mL liver tissue) greater than the hepatic blood perfusion. We developed a new microvascular compartment model with more physiology, by including tracer uptake into the hepatocytes from the blood flowing through the sinusoids, backflux from hepatocytes into the sinusoidal blood, and re-uptake along the sinusoidal path. Dynamic PET data include information on liver physiology which cannot be extracted using a standard compartment model. In conclusion, SUV of non-invasive static PET with 18F-FDGal provides a clinically useful measurement of regional and whole-liver hepatic metabolic function. Secondly, assessment of individual intrahepatic transport steps is a notable feature of dynamic liver PET.

Metabolic liver function in humans measured by 2-18F-fluoro-2-deoxy-D-galactose PET/CT-reproducibility and clinical potential

Background

PET/CT with the radioactively labelled galactose analogue 2-¹⁸F-fluoro-2-deoxy-D-galactose (¹⁸F-FDGal) can be used to quantify the hepatic metabolic function and visualise regional metabolic heterogeneity. We determined the day-to-day variation in humans with and without liver disease. Furthermore, we examined whether the standardised uptake value (SUV) of ¹⁸F-FDGal from static scans can substitute the hepatic systemic clearance of ¹⁸F-FDGal (K_met, mL blood/min/mL liver tissue/) quantified from dynamic scans as measure of metabolic function.

Four patients with cirrhosis and six healthy subjects underwent two ¹⁸F-FDGal PET/CT scans within a median interval of 15 days for determination of day-to-day variation. The correlation between K_met and SUV was examined using scan data and measured arterial blood concentrations of ¹⁸F-FDGal (blood samples) from 14 subjects from previous studies. Regional and whole-liver values of K_met and SUV along with total metabolic liver volume and total metabolic liver function (total SUV, average SUV multiplied by total metabolic liver volume) were calculated.

Results

No significant day-to-day differences were found for K_met or SUV. SUV had higher intraclass correlation coefficients than K_met (0.92–0.97 vs. 0.49–0.78). The relationship between K_met and SUV was linear. Total metabolic liver volume had non-significant day-to-day variation (median difference 50 mL liver tissue; P = 0.6). Mean total SUV in healthy subjects was 23,840 (95% CI, 21,609; 26,070), significantly higher than in the patients (P < 0.001).

Conclusions

The reproducibility of ¹⁸F-FDGal PET/CT was good and SUV can substitute K_met for quantification of hepatic metabolic function. Total SUV of ¹⁸F-FDGal is a promising tool for quantification of metabolic liver function in pre-treatment evaluation of individual patients.

2-[(18)F]fluoro-2-deoxy-D-galactose PET/CT of hepatocellular carcinoma is not improved by co-administration of galactose

INTRODUCTION

PET with [(18)F]fluoro-2-deoxy-D-galactose ((18)F-FDGal) is a promising imaging modality for detection of hepatocellular carcinoma (HCC). However, it can be difficult to distinguish small intrahepatic HCC lesions from surrounding liver tissue. Ut the competitive inhibition that galactose shows towards hepatic (18)F-FDGal metabolism, we tested the hypothesis that co-administration of galactose, at near-saturating doses, inhibits (18)F-FDGal metabolism to a greater extent in non-malignant hepatocytes than in HCC cells. This would increase the tumor to background ratio in the (18)F-FDGal PET scans with co-administration of galactose.

METHODS

Three patients known to have HCC underwent two (18)F-FDGal PET/CT scans on consecutive days, one with and one without simultaneous constant intravenous infusion of galactose. On both days, (18)F-FDGal was injected in the beginning of a 45-min dynamic PET scan of the liver followed by a static PET scan from mid-thigh to the top of the skull starting 60-70min after (18)F-FDGal administration. Parametric images of the hepatic metabolic function expressed in terms of hepatic systemic clearance of (18)F-FDGal were generated from the dynamic PET recordings.

RESULTS

Co-administration of galactose did not give significantly better discrimination of the HCC lesions from background. Parametric images of the hepatic metabolic function did not add additional useful information to the detection of HCC lesions compared to the static images of radioactivity concentrations.

CONCLUSION

Co-administration of galactose did not improve the interpretation of the (18)F-FDGal PET/CT images and did not improve the detection of intrahepatic HCC lesions, either using static or parametric images.

Optimal 2-[(18)F]fluoro-2-deoxy-D-galactose PET/CT protocol for detection of hepatocellular carcinoma

BACKGROUND

Positron emission tomography (PET) with the liver-specific galactose tracer 2-[(18)F]fluoro-2-deoxy-D-galactose ((18)F-FDGal) may improve diagnosis of hepatocellular carcinoma (HCC). The aim of this study was to test which of three different (18)F-FDGal PET protocols gives the highest tumour-to-background (T/B) ratio on PET images and thus better detection of HCC tumours.

METHODS

Ten patients with a total of 15 hepatic HCC tumours were enrolled prior to treatment. An experienced radiologist defined volumes of interest (VOIs) encircling HCC tumours on contrast-enhanced CT (ce-CT) images. Three PET/CT protocols were conducted following an intravenous (18)F-FDGal injection: (i) a 20-min dynamic PET/CT of the liver (to generate a 3D metabolic image), (ii) a traditional static whole-body PET/CT after 1 h, and (iii) a late static whole-body PET/CT after 2 or 3 h. PET images from each PET/CT protocol were fused with ce-CT images, and the average standardized uptake values (SUV) in tumour and background liver tissue were used to calculate (T/B) ratios. Furthermore, Tpeak/B ratios were calculated using the five hottest voxels in all hot tumours. The ratios for the three different PET protocols were compared.

RESULTS

For the individual tumours, there was no significant difference in the T/B ratio between the three PET protocols. The metabolic image yielded higher Tpeak/B ratios than the two static images, but it was easier to identify tumours on the static images. One extrahepatic metastasis was detected.

CONCLUSIONS

Neither metabolic images nor static whole-body images acquired 2 or 3 h after (18)F-FDGal injection offered an advantage to traditional whole-body PET/CT images acquired after 1 h for detection of HCC.

Metabolic liver function measured in vivo by dynamic (18)F-FDGal PET/CT without arterial blood sampling

BACKGROUND

Metabolic liver function can be measured by dynamic PET/CT with the radio-labelled galactose-analogue 2-[(18)F]fluoro-2-deoxy-D-galactose ((18)F-FDGal) in terms of hepatic systemic clearance of (18)F-FDGal (K, ml blood/ml liver tissue/min). The method requires arterial blood sampling from a radial artery (arterial input function), and the aim of this study was to develop a method for extracting an image-derived, non-invasive input function from a volume of interest (VOI).

METHODS

Dynamic (18)F-FDGal PET/CT data from 16 subjects without liver disease (healthy subjects) and 16 patients with liver cirrhosis were included in the study. Five different input VOIs were tested: four in the abdominal aorta and one in the left ventricle of the heart. Arterial input function from manual blood sampling was available for all subjects. K*-values were calculated using time-activity curves (TACs) from each VOI as input and compared to the K-value calculated using arterial blood samples as input. Each input VOI was tested on PET data reconstructed with and without resolution modelling.

RESULTS

All five image-derived input VOIs yielded K*-values that correlated significantly with K calculated using arterial blood samples. Furthermore, TACs from two different VOIs yielded K*-values that did not statistically deviate from K calculated using arterial blood samples. A semicircle drawn in the posterior part of the abdominal aorta was the only VOI that was successful for both healthy subjects and patients as well as for PET data reconstructed with and without resolution modelling.

CONCLUSIONS

Metabolic liver function using (18)F-FDGal PET/CT can be measured without arterial blood samples by using input data from a semicircle VOI drawn in the posterior part of the abdominal aorta.

18F-glutathione conjugate as a PET tracer for imaging tumors that overexpress L-PGDS enzyme

Lipocalin-type prostaglandin D synthase (L-PGDS) has been correlated with the progression of neurological disorders. The present study aimed at evaluating the imaging potency of a glutathione conjugate of fluorine-18-labeled fluorobutyl ethacrynic amide ([¹⁸F]FBuEA-GS) for brain tumors. Preparation of [¹⁸F]FBuEA-GS has been modified from the -4-tosylate derivative via radiofluorination in 5% radiochemical yield. The mixture of nonradioactive FBuEA-GS derived from a parallel preparation has be resolved to two isomers in a ratio of 9∶1 using analytic chiral reversed phase high performance liquid chromatography (RP-HPLC). The two fluorine-18-labeled isomers purified through nonchiral semipreparative RP-HPLC as a mixture were studied by assessing the binding affinity toward L-PGDS through a gel filtration HPLC, by analyzing radiotracer accumulation in C6 glioma cells, and by evaluating the imaging of radiotracer in a C6 glioma rat with positron emission tomography. The inhibition percentage of the production of PGD₂ from PGH₂ at the presence of 200 µM of FBuEA-GS and 4-Dibenzo[a,d]cyclohepten-5-ylidene-1-[4-(2H-tetrazol-5-yl)butyl]piperidine (AT-56) were 74.1±4.8% and 97.6±16.0%, respectively. [¹⁸F]FBuEA-GS bound L-PGDS (16.3–21.7%) but not the isoform, microsomal prostaglandin E synthase 1. No binding to GST-alpha and GST-pi was observed. The binding strength between [¹⁸F]FBuEA-GS and L-PGDS has been evaluated using analytic gel filtration HPLC at the presence of various concentrations of the cold competitor FBuEA-GS. The contrasted images indicated that the radiotracer accumulation in tumor lesions is probably related to the overexpression of L-PGDS.

The lumped constant for the galactose analog 2-18F-fluoro-2-deoxy-D-galactose is increased in patients with parenchymal liver disease

The galactose analog 2-(18)F-fluoro-2-deoxy-d-galactose ((18)F-FDGal) is a suitable PET tracer for measuring hepatic galactokinase capacity in vivo, which provides estimates of hepatic metabolic function. As a result of a higher affinity of galactokinase toward galactose, the lumped constant (LC) for (18)F-FDGal was 0.13 in healthy subjects. The aim of the present study was to test the hypothesis of a significantly different LC for (18)F-FDGal in patients with parenchymal liver disease.

METHODS

Nine patients with liver cirrhosis were studied in connection with a previous study with determination of hepatic intrinsic clearance of ¹⁸F-FDGal (V*(max/K*(m)). The present study determined the hepatic removal kinetics of galactose, including hepatic intrinsic clearance of galactose (V(max)/K(m)) from measurements of hepatic blood flow and arterial and liver vein blood galactose concentrations at increasing galactose infusions. LC for ¹⁸F-FDGal was calculated as (V*(max)/K*(m))/(V(max)/K(m)). On a second day, a dynamic ¹⁸-FDGal PET study with simultaneous infusion of galactose (mean arterial galactose concentration, 6.1 mmol/L of blood) and blood samples from a radial artery was performed, with determination of hepatic systemic clearance of ¹⁸F-FDGal (K*(+gal) from linear analysis of data (Gjedde-Patlak method). The maximum hepatic removal rate of galactose was estimated from ¹⁸F-FDGal PET data (V(max)(PET)) using the estimated LC.

RESULTS

The mean hepatic V(max) of galactose was 1.18 mmol/min, the mean K(m) was 0.91 mmol/L of blood and the mean V(max)/K(m) was 1.18 L of blood/min. When compared with values of healthy subjects, K(m) did not differ (P = 0.77), whereas both V(max) and V(max)/K(m) were significantly lower in patients (both P < 0.01). Mean LC for ¹⁸LF-FDGal was 0.24, which was significantly higher than the mean LC of 0.13 in healthy subjects (P < 0.0001). Mean K*(+gal) determined from the PET study was 0.019 L of blood/min/L of liver tissue, which was not significantly different from that in healthy subjects (P = 0.85). Mean hepatic V(max)(PET) was 0.57 mmol/min/L of liver tissue, which was significantly lower than the value in healthy subjects (1.41 mmol/min/L of liver tissue (P < 0.0001).

CONCLUSION

Disease may change the LC for a pet tracer, and this study demonstrated the importance of using the correct LC.

Regional metabolic liver function measured in patients with cirrhosis by 2-[¹⁸F]fluoro-2-deoxy-D-galactose PET/CT

BACKGROUND & AIMS

There is a clinical need for methods that can quantify regional hepatic function non-invasively in patients with cirrhosis. Here we validate the use of 2-[(18)F]fluoro-2-deoxy-d-galactose (FDGal) PET/CT for measuring regional metabolic function to this purpose, and apply the method to test the hypothesis of increased intrahepatic metabolic heterogeneity in cirrhosis.

METHODS

Nine cirrhotic patients underwent dynamic liver FDGal PET/CT with blood samples from a radial artery and a liver vein. Hepatic blood flow was measured by indocyanine green infusion/Fick's principle. From blood measurements, hepatic systemic clearance (Ksyst, Lblood/min) and hepatic intrinsic clearance (Vmax/Km, Lblood/min) of FDGal were calculated. From PET data, hepatic systemic clearance of FDGal in liver parenchyma (Kmet, mL blood/mL liver tissue/min) was calculated. Intrahepatic metabolic heterogeneity was evaluated in terms of coefficient-of-variation (CoV, %) using parametric images of Kmet.

RESULTS

Mean approximation of Ksyst to Vmax/Km was 86% which validates the use of FDGal as PET tracer of hepatic metabolic function. Mean Kmet was 0.157 mL blood/mL liver tissue/min, which was lower than 0.274 mL blood/mL liver tissue/min, previously found in healthy subjects (p<0.001), in accordance with decreased metabolic function in cirrhotic livers. Mean CoV for Kmet in liver tissue was 24.4% in patients and 14.4% in healthy subjects (p<0.0001). The degree of intrahepatic metabolic heterogeneity correlated positively with HVPG (p<0.05).

CONCLUSIONS

A 20-min dynamic FDGal PET/CT with arterial sampling provides an accurate measure of regional hepatic metabolic function in patients with cirrhosis. This is likely to have clinical implications for the assessment of patients with liver disease as well as treatment planning and monitoring.

Bringing physiology into PET of the liver

Several physiologic features make interpretation of PET studies of liver physiology an exciting challenge. As with other organs, hepatic tracer kinetics using PET is quantified by dynamic recording of the liver after the administration of a radioactive tracer, with measurements of time-activity curves in the blood supply. However, the liver receives blood from both the portal vein and the hepatic artery, with the peak of the portal vein time-activity curve being delayed and dispersed compared with that of the hepatic artery. The use of a flow-weighted dual-input time-activity curve is of importance for the estimation of hepatic blood perfusion through initial dynamic PET recording. The portal vein is inaccessible in humans, and methods of estimating the dual-input time-activity curve without portal vein measurements are being developed. Such methods are used to estimate regional hepatic blood perfusion, for example, by means of the initial part of a dynamic (18)F-FDG PET/CT recording. Later, steady-state hepatic metabolism can be assessed using only the arterial input, provided that neither the tracer nor its metabolites are irreversibly trapped in the prehepatic splanchnic area within the acquisition period. This is used in studies of regulation of hepatic metabolism of, for example, (18)F-FDG and (11)C-palmitate.

Hepatic galactose metabolism quantified in humans using 2-18F-fluoro-2-deoxy-D-galactose PET/CT

Accurate quantification of regional liver function is needed, and PET of specific hepatic metabolic pathways offers a unique method for this purpose. Here, we quantify hepatic galactose elimination in humans using PET and the galactose analog 2-(18)F-fluoro-2-deoxy-d-galactose ((18)F-FDGal) as the PET tracer.

METHODS

Eight healthy human subjects underwent (18)F-FDGal PET/CT of the liver with and without a simultaneous infusion of galactose. Hepatic systemic clearance of (18)F-FDGal was determined from linear representation of the PET data. Hepatic galactose removal kinetics were determined using measurements of hepatic blood flow and arterial and liver vein galactose concentrations at increasing galactose infusions. The hepatic removal kinetics of (18)F-FDGal and galactose and the lumped constant (LC) were determined.

RESULTS

The mean hepatic systemic clearance of (18)F-FDGal was significantly higher in the absence than in the presence of galactose (0.274 ± 0.001 vs. 0.019 ± 0.001 L blood/min/L liver tissue; P < 0.01), showing competitive substrate inhibition of galactokinase. The LC was 0.13 ± 0.01, and the (18)F-FDGal PET with galactose infusion provided an accurate measure of the local maximum removal rate of galactose (V(max)) in liver tissue compared with the V(max) estimated from arterio-liver venous (A-V) differences (1.41 ± 0.24 vs. 1.76 ± 0.08 mmol/min/L liver tissue; P = 0.60). The first-order hepatic systemic clearance of (18)F-FDGal was enzyme-determined and can thus be used as an indirect estimate of galactokinase capacity without the need for galactose infusion or knowledge of the LC.

CONCLUSION

(18)F-FDGal PET/CT provides an accurate in vivo measurement of human galactose metabolism, which enables the quantification of regional hepatic metabolic function.

The potential use of 2-[¹⁸F]fluoro-2-deoxy-D-galactose as a PET/CT tracer for detection of hepatocellular carcinoma

PURPOSE

The aim of the study was to evaluate the feasibility of using the hepatocyte-specific positron emission tomography (PET) tracer 2-[(18)F]fluoro-2-deoxy-D-galactose (FDGal) as a tracer for hepatocellular carcinoma (HCC).

METHODS

In addition to standard clinical investigations, 39 patients with known HCC or suspected of having HCC underwent a partial-body FDGal PET/CT (from base of skull to mid-thigh). Diagnosis of HCC was based on internationally approved criteria. FDGal PET/CT images were analysed for areas with high (hot spots) or low (cold spots) tracer accumulation when compared to surrounding tissue.

RESULTS

Seven patients did not have HCC and FDGal PET/CT was negative in each of them. Twenty-three patients had HCC and were included before treatment. FDGal PET/CT correctly identified 22 of these patients, which was comparable to contrast-enhanced CT. Interestingly, FDGal PET/CT was conclusive in 12 patients in whom conventional imaging techniques were inconclusive and required additional diagnostic investigations or close follow-up. Nine patients were included after treatment of HCC and in these patients FDGal PET/CT was able to distinguish between viable tumour tissue as hot spots and areas with low metabolic activity as cold spots. FDGal PET/CT detected extrahepatic disease in nine patients which was a novel finding in eight patients.

CONCLUSION

FDGal PET/CT has great clinical potential as a PET tracer for detection of extra- but also intrahepatic HCC. In the present study, the specificity of FDGal PET/CT was 100%, which is very promising but needs to be confirmed in a larger, prospective study.