Altanserin, a fluorobenzoyl derivative related to ketanserin, was reported to be a potent antagonist of 5-HT2A receptors with >100-fold selectivity over D2/3 receptors, 5-HT1A, 5-HT6, and 5-HT7 (9, 10). This led to the development of 3-{2-[4-(4-[(18)F]fluorobenzoyl)-1-piperidyl]ethyl}-2-sulfanyl-3H-quinazolin-4-one ([(18)F]altanserin) as a useful tool for 5-HT2A receptor PET imaging in vivo (11). 5-HT2A antagonists bind to the total pool of receptors, whereas 5-HT2A agonists bind only to the high-affinity functional state of the receptor but may be more important in disease states because the high affinity sites are the ones that transmit the intracellular signals.