PET quantification of specific binding of carbon-11-nicotine in human brain.

Abstract

Previous work on the PET measured uptake of (S)-[11C]nicotine presents conflicting findings as to whether it reflects specific binding. We studied the uptake of (R)-[11C]nicotine and (S)-[11C]nicotine in normal volunteers at baseline conditions and after a challenge with unlabeled (S)-nicotine to decrease the concentration of free binding sites or with CO2 to increase perfusion. We analyzed the data using two- and three-compartment models. We found tissue pharmacokinetics of (R)- and (S)-[11C]nicotine are adequately described by the two-compartment model. (S)-nicotine challenge induced small but statistically significant reductions in distribution volume (DV) of both (R)- and (S)-[11C]nicotine. The changes in DV could not be attributed to perfusion changes because DV was not affected by CO2 challenge. Although the reduction in DV indicates sensitivity of [11C]nicotine to status of nicotinic binding sites, the small magnitude of the reduction suggests that most nicotine uptake is nonspecific. Although differences in DV attributable to specific binding were detected, (R)- and (S)-[11C]nicotine are relatively poor tracers for studying nicotinic binding sites using PET.