6533b857fe1ef96bd12b3a97

RESEARCH PRODUCT

Reinvestigation of the synthesis and evaluation of [N-methyl-11C]vorozole, a radiotracer targeting cytochrome P450 aromatase

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Abstract Introduction We reinvestigated the synthesis of [ N -methyl- 11 C]vorozole, a radiotracer for aromatase, and discovered the presence of an N -methyl isomer which was not removed in the original purification method. Herein we report the preparation and positron emission tomography (PET) studies of pure [ N -methyl- 11 C]vorozole. Methods Norvorozole was alkylated with [ 11 C]methyl iodide as previously described and also with unlabeled methyl iodide. A high-performance liquid chromatography (HPLC) method was developed to separate the regioisomers. Nuclear magnetic resonance (NMR) spectroscopy ( 13 C and 2D-nuclear Overhauser effect spectroscopy NMR) was used to identify and assign structures to the N -methylated products. Pure [ N -methyl- 11 C]vorozole and the contaminating isomer were compared by PET imaging in the baboon. Results Methylation of norvorozole resulted in a mixture of isomers (1:1:1 ratio) based on new HPLC analysis using a pentafluorophenylpropyl bonded silica column, in which vorozole coeluted one of its isomers under the original HPLC conditions. Baseline separation of the three labeled isomers was achieved. The N-3 isomer was the contaminant of vorozole, thus correcting the original assignment of isomers. PET studies of pure [ N -methyl- 11 C]vorozole with and without the contaminating N-3 isomer revealed that only [ N -methyl- 11 C]vorozole binds to aromatase. [ N -methyl- 11 C]Vorozole accumulated in all brain regions with highest accumulation in the aromatase-rich amygdala and preoptic area. Accumulation was blocked with vorozole and letrozole consistent with reports of some level of aromatase in many brain regions. Conclusions The discovery of a contaminating labeled isomer and the development of a method for isolating pure [ N -methyl- 11 C]vorozole combine to provide a new scientific tool for PET studies of the biology of aromatase and for drug research and development.