The addition of a methoxy methyl on the 4-piperidine and thesubstitution of the phenethyl ring for an ethyl-substitutedtetrazole-one yielded a compound with about one fourth toone third the potency of fentanyl. Although lesspotent, it has a quicker onset of action, a shorter duration ofaction, and thus a better, safety profile for use as an anestheticadjunct. The piperidine amine has a pKa of 6.5 compared withfentanyl’s pKa of 8.4. This results in a higher proportion ofunionized drug for alfentanil leading to quicker penetrationthrough the blood-brain barrier and thus onset of action.Alfentanil (Alfenta) is available as an IV injection for use asan analgesic adjunct for induction of general anesthesia and tomaintain analgesia during general surgical procedures.
