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:

more »

... ne patients with liver cirrhosis were studied in connection with a previous study with determination of hepatic intrinsic clearance of 18 F-FDGal (V Ã max =K Ã 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 18 F-FDGal was calculated as (V Ã max =K Ã m )/(V max /K m ). On a second day, a dynamic 18 F-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 18 F-FDGal (K Ã 1gal ) from linear analysis of data (Gjedde-Patlak method). The maximum hepatic removal rate of galactose was estimated from 18 F-FDGal PET data (V PET max ) 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 from healthy subjects, K m did not differ (P 5 0.77), whereas both V max and V max /K m were significantly lower in patients (both P , 0.01). Mean LC for 18 F-FDGal was 0.24, which was significantly higher than the mean LC of 0.13 in healthy subjects (P , 0.0001). Mean K Ã 1gal 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 5 0.85). Mean hepatic V PET max 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.