Biodistribution of [11C]methylaminoisobutyric acid, a tracer for PET studies on system A amino acid transport in vivo

[N-methyl-¹¹C]!-Methylaminoisobutyric acid (¹¹C-MeAIB) is a potentially useful tracer for positron emission tomography (PET) studies on hormonally regulated system A amino acid transport. ¹¹C-MeAIB is a metabolically stable amino acid analogue specific for system A amino acid transport. We evaluated the biodistribution of ¹¹C-MeAIB in rats and humans to estimate the usefulness of the tracer for in vivo human PET studies, for example, on regulation of system A amino acid transport and on tumour imaging. Healthy Sprague-Dawley rats (n=14) were killed 5, 20, 40 or 60 min after the injection of ¹¹C-MeAIB, and the tissue samples were weighed and counted for ¹¹C radioactivity. Ten lymphoma patients with relatively limited tumour burden underwent whole-body (WB) PET imaging with ¹¹C-MeAIB. In addition, three other patients had dynamic PET scanning of the head and neck area, and the tracer uptake was quantitated by calculating the kinetic influx constants (K_i values) for the tracer. In animal studies, the highest activity was detected in the kidney, pancreas, adrenal gland and intestines. In humans, the highest activity was found in the salivary glands, and after that in the kidney and pancreas, similar to the results in animal studies. Rapid uptake was also detected in the skeletal muscle. In the graphical analysis, linear plots were obtained, and the mean fractional tracer uptake values (K_i) of the parotid glands (n=3) and cervical muscles (n=3) were 0.039ǂ.008 min^-1 and 0.013ǂ.006 min^-1, respectively. The K_i value of the tumour (n=1) was 0.064 min^-1. Higher uptake of ¹¹C-MeAIB into the tumour tissue was encountered. These results encourage further ¹¹C-MeAIB PET studies in humans on the physiology and pathology of system A amino acid transport and on tumour detection.