Neuroleptic binding sites: specific labeling in mice with [18F]haloperidol, a potential tracer for positron emission tomography.

Haloperidol labeled with fluorine-18 (T 1/2 = 110 min, positron emission 97%), prepared yielding .04 Ci/millimole by the Balz-Schiemann reaction, was evaluated in a murine model as a potential radiotracer for noninvasive determination, by positron-emission tomography, of regional concentrations of brain dopamine receptors in patients. As the haloperidol dose in mice was increased from 0.01 to 1000 micrograms/kg, the relative concentration of [18F]haloperidol (microCi per g specimen/microCi per g of body mass), at one hour after injection decreased from 30 to 1.0 in the striatum and from 8.0 to 1.0 in the cerebellum. The striatal radioactivity, plotted as relative concentration against log of dose, decreased sigmoidally, presumably reflecting competition between labeled and unlabeled haloperidol for a single class of accessible binding sites. Because the cerebellum is relatively deficient in dopamine receptors, the observed decrease in cerebellar radioactivity may reflect a saturable component of haloperidol transport into brain. The high brain concentrations and the unexpectedly high striatum-to-cerebellum concentration ratios (greater than 4 at haloperidol doses less than or equal to 1 microgram/kg) suggest that [18F]haloperidol warrants further investigation as a potential radiotracer for dopamine receptors.