In vitro metabolism of the epoxide substructure of cryptophycins by cytosolic glutathione S-transferase: species differences and stereoselectivity

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-(18)F-(fluoromethyl)-thiazol-4-yl)ethynyl)benzonitrile ((18)F-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 (18)F-SP203 in humans and to determine from the distribution of radioactivity in bone structures with various proportions of bone and red marrow whether (18)F-SP203 undergoes defluorination.

METHODS

Whole-body images were acquired for 5 h after injecting (18)F-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 (18)F-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 microSv/MBq) was unusually high, the effective dose (17.8 microSv/MBq) of (18)F-SP203 was typical of that of other (18)F radiotracers.

CONCLUSION

(18)F-SP203 causes an effective dose in humans typical of several other (18)F radioligands and undergoes little defluorination.

Radiodefluorination of 3-fluoro-5-(2-(2-[18F](fluoromethyl)-thiazol-4-yl)ethynyl)benzonitrile ([18F]SP203), a radioligand for imaging brain metabotropic glutamate subtype-5 receptors with positron emission tomography, occurs by glutathionylation in rat brain

Metabotropic glutamate subtype-5 receptors (mGluR5) are implicated in several neuropsychiatric disorders. Positron emission tomography (PET) with a suitable radioligand may enable monitoring of regional brain mGluR5 density before and during treatments. We have developed a new radioligand, 3-fluoro-5-(2-(2-[(18)F](fluoromethyl)thiazol-4-yl)ethynyl)benzonitrile ([(18)F]SP203), for imaging brain mGluR5 in monkey and human. In monkey, radioactivity was observed in bone, showing release of [(18)F]-fluoride ion from [(18)F]SP203. This defluorination was not inhibited by disulfiram, a potent inhibitor of CYP2E1. PET confirmed bone uptake of radioactivity and therefore defluorination of [(18)F]SP203 in rats. To understand the biochemical basis for defluorination, we administered [(18)F]SP203 plus SP203 in rats for ex vivo analysis of metabolites. Radio-high-performance liquid chromatography detected [(18)F]fluoride ion as a major radiometabolite in both brain extract and urine. Incubation of [(18)F]SP203 with brain homogenate also generated this radiometabolite, whereas no metabolism was detected in whole blood in vitro. Liquid chromatography-mass spectrometry analysis of the brain extract detected m/z 548 and 404 ions, assignable to the [M + H](+) of S-glutathione (SP203Glu) and N-acetyl-S-l-cysteine (SP203Nac) conjugates of SP203, respectively. In urine, only the [M + H](+) of SP203Nac was detected. Mass spectrometry/mass spectrometry and multi-stage mass spectrometry analyses of each metabolite yielded product ions consistent with its proposed structure, including the former fluoromethyl group as the site of conjugation. Metabolite structures were confirmed by similar analyses of SP203Glu and SP203Nac, prepared by glutathione S-transferase reaction and chemical synthesis, respectively. Thus, glutathionylation at the 2-fluoromethyl group is responsible for the radiodefluorination of [(18)F]SP203 in rat. This study provides the first demonstration of glutathione-promoted radiodefluorination of a PET radioligand.

Marine pharmacology in 2005-2006: antitumour and cytotoxic compounds

During 2005 and 2006, marine pharmacology research directed towards the discovery and development of novel antitumour agents was reported in 171 peer-reviewed articles. The purpose of this article is to present a structured review of the antitumour and cytotoxic properties of 136 marine natural products, many of which are novel compounds that belong to diverse structural classes, including polyketides, terpenes, steroids and peptides. The organisms yielding these bioactive marine compounds included invertebrate animals, algae, fungi and bacteria. Antitumour pharmacological studies were conducted with 42 structurally defined marine natural products in a number of experimental and clinical models which further defined their mechanisms of action. Particularly potent in vitro cytotoxicity data generated with murine and human tumour cell lines were reported for 94 novel marine chemicals with as yet undetermined mechanisms of action. Noteworthy is the fact that marine anticancer research was sustained by a global collaborative effort, involving researchers from Australia, Belgium, Benin, Brazil, Canada, China, Egypt, France, Germany, India, Indonesia, Italy, Japan, Mexico, the Netherlands, New Zealand, Panama, the Philippines, Slovenia, South Korea, Spain, Sweden, Taiwan, Thailand, United Kingdom (UK) and the United States of America (USA). Finally, this 2005-2006 overview of the marine pharmacology literature highlights the fact that the discovery of novel marine antitumour agents continued at the same active pace as during 1998-2004.