Mapping cocaine binding sites in human and baboon brain in vivo

The first direct measurements of cocaine binding in the brain of normal human volunteers and baboons have been made by using positron emission tomography (PET) and tracer doses of [N‐¹¹C‐methyl]‐(–)‐cocaine ([¹¹C]cocaine). Cocaine's binding and release from brain are rapid with the highest regional uptake of carbon‐11 occurring in the corpus striatum at 4–10 minutes after intravenous injection of labeled cocaine. This was followed by a clearance to half the peak value at about 25 minutes with the overall time course paralleling the previously documented time course of the euphoria experienced after intravenous cocaine administration. Blockade of the dopamine reuptake sites with nomifensine reduced the striatal but not the cerebellar uptake of [¹¹C]cocaine in baboons indicating that cocaine binding is associated with the dopamine reuptake site in the corpus striatum. A comparison of labeled metabolites of cocaine in human and baboon plasma showed that while cocaine is rapidly metabolized in both species, the profile of labeled metabolites is different, with baboon plasma containing significant amounts of labeled carbon dioxide, and human plasma containing no significant labeled carbon dioxide. These studies demonstrate the feasibility of using [¹¹C]cocaine and PET to map binding sites for cocaine in human brain, to monitor its kinetics, and to characterize its binding mechanism by using appropriate pharmacological challenges.