Fluorinated mu-opioid receptor agonists

A fluorinated compound of Formula (I):

• a deuterated or tritiated variant thereof, or a pharmaceutically acceptable salt thereof,
• wherein X is optionally present and, when present, is —O—, —S—, or —NR4—,
• wherein each of R1, R2, R3, and R4 is independently selected from hydrogen, a substituted or unsubstituted C1-6 alkyl group, a substituted or unsubstituted C2-6 alkenyl group, a substituted or unsubstituted C2-6 alkynyl group, a substituted or unsubstituted C1-6 heteroalkyl group, a substituted or unsubstituted C3-6 cycloalkyl group, a substituted or unsubstituted C3-6 heterocycloalkyl group, a substituted or unsubstituted aryl group, a substituted or unsubstituted heteroaryl group, a substituted or unsubstituted alkaryl group, a substituted or unsubstituted arylalkyl group, and a combination thereof, and
• wherein at least one C—H bond on the compound of Formula (I) is replaced with a C—F bond or a C—18F bond.