What is "fluorophen"

Fluorophen, or fluorofen, is a fluorinated analogue of phenazocine, an opioid drug of the benzomorphan group, which was developed as a radioligand for the purpose of labeling opioid receptors during PET scans (with F). Unlike most other benzomorphan derivatives, fluorophen acts as a full agonist of the opioid receptors with preferential affinity for the μ-opioid receptor (approximately 6x that of morphine), similar but slightly lower affinity for the δ-opioid receptor (equipotent to [[enkephalin|[D-Ala, D-Leu]enkephalin]]), and very low affinity for the κ-opioid receptor.