Biodistribution and radiation dosimetry of a positron emission tomographic ligand, ¹⁸F-SP203, to image metabotropic glutamate subtype 5 receptors in humans

Purpose A new PET ligand, 3-fluoro-5-(2-(2- ¹⁸F-(fluoromethyl)- thiazol-4-yl)ethynyl)benzonitrile ( ¹⁸F-SP203), is a positron emission tomographic radioligand selective for metabotropic glutamate subtype 5 receptors. The purposes of this study were to estimate the radiation-absorbed doses of ¹⁸F-SP203 in humans and to determine from the distribution of radioactivity in bone structures with various proportions of bone and red marrow whether ¹⁸F-SP203 undergoes defluorination. Methods Whole-body images were acquired for 5 h after injecting ¹⁸F-SP203 in seven healthy humans. Urine was collected at various time points. Radiation-absorbed doses were estimated by the Medical Internal Radiation Dose scheme. Results After injecting ¹⁸F-SP203, the two organs with highest radiation exposure were urinary bladder wall and gallbladder wall, consistent with both urinary and fecal excretion. In the skeleton, most of the radioactivity was in bone structures that contain red marrow and not in those without red marrow. Although the dose to red marrow (30.9 μSv/MBq) was unusually high, the effective dose (17.8 μSv/MBq) of ¹⁸F-SP203 was typical of that of other ¹⁸F radiotracers. Conclusion ¹⁸F-SP203 causes an effective dose in humans typical of several other ¹⁸F radioligands and undergoes little defluorination.