A novel noninvasive method for assessing glutathione-conjugate efflux systems in the brain

Brain efflux systems export such conjugated metabolites as glutathione (GSH) and glucuronate conjugates, generated by the detoxification process, from the brain and serve to protect the brain from harmful metabolites. The intracerebral injection of a radiolabeled conjugate is a useful technique to assess brain efflux systems; however, this technique is not applicable to humans. Hence, we devised a novel noninvasive approach for assessing GSH-conjugate efflux systems using positron emission tomography. Here, we investigated whether or not a designed proprobe can deliver its GSH conjugate into the brain. Radiolabeled 6-chloro-7-methylpurine (7m6CP) was designed as the proprobe, and (14)C]7m6CP was prepared by the reaction of 6-chloropurine with (14)C]CH(3)I as a model of (11)C]CH(3)I. The radiochemical yield and purity of (14)C]7m6CP were 10-20% and greater than 99%, respectively. High brain uptake (0.8% ID/g) at 1 min was observed, followed by gradual radioactivity clearance from the brain for 5-60 min after the injection of (14)C]7m6CP into rats. Analysis of metabolites confirmed that the presence of (14)C]7m6CP was hardly observed, and 80% of the radioactivity was identical to its GSH conjugate for 15-60 min. The brain radioactivity was single-exponentially decreased during the period of 15-60 min post-injection of (14)C]7m6CP, and the first-order efflux rate constant of the conjugate, estimated from the slope, was 0.0253 min(-1). These results showed that (1) (14)C]7m6CP readily entered the brain, (2) it efficiently and specifically transformed to the GSH conjugate within the brain, and (3) after (14)C]7m6CP disappearance, the clearance of radioactivity represented the only efflux of GSH conjugate. We conclude that 7m6CP can deliver the GSH conjugate into the brain and would be useful for assessing GSH-conjugate efflux systems noninvasively.