Evaluation of a novel radiotracer for positron emission tomography imaging of reactive oxygen species in the central nervous system.

Introduction: Few, if any, radiotracers are available for the in vivo imaging of reactive oxygen species (ROS) in the central nervous system. ROS play a critical role in normal cell processes such as signaling and homeostasis but overproduction of ROS is implicated in several disorders. We describe here the radiosynthesis and initial ex vivo and in vivo evaluation of [C]hydromethidine ([C]HM) as a radiotracer to image ROS using positron emission tomography (PET).

Methods: [C]HM and its deuterated isotopologue [C](4) were produced using [C]methyl triflate in a one-pot, two-step reaction and purified by high performance liquid chromatography. Ex vivo biodistribution studies were performed after tail vein injections of both radiotracers. To demonstrate sensitivity of uptake to ROS, [C]HM was administered to rats treated systemically with lipopolysaccharide (LPS). In addition, ex vivo autoradiography and in vivo PET imaging were performed using [C]HM on rats which had been microinjected with sodium nitroprusside (SNP) to induce ROS.

Results: [C]HM and [C](4) radiosyntheses were reliable and produced the radiotracers at high specific activities and radiochemical purities. Both radiotracers demonstrated good brain uptake and fast washout of radioactivity, but [C](4) washout was faster. Pretreatment with LPS resulted in a significant increase in brain retention of radioactivity. Ex vivo autoradiography and PET imaging of rats unilaterally treated with microinjections of SNP demonstrated increased retention of radioactivity in the treated side of the brain.

Conclusions: [C]HM has the attributes of a radiotracer for PET imaging of ROS in the brain including good brain penetration and increased retention of radioactivity in animal models of oxidative stress.