Leu-enkephalin

Leu-enkephalin is an endogenous opioid peptide neurotransmitter with the amino acid sequence Tyr-Gly-Gly-Phe-Leu that is found naturally in the brains of many animals, including humans. It is one of the two forms of enkephalin; the other is met-enkephalin. The tyrosine residue at position 1 is thought to be analogous to the 3-hydroxyl group on morphine. Leu-enkephalin has agonistic actions at both the μ- and δ-opioid receptors, with significantly greater preference for the latter. It has little to no effect on the κ-opioid receptor.
A nasal spray formulation of leu-enkephalin is under development by Virpax Pharmaceuticals for the treatment of pain and post-traumatic stress disorder. As of November 2023, it is up to the preclinical stage of development for these indications.