1) Opioid peptide 
Frakefamide (INN) is a synthetic, fluorinated opioid tetrapeptide with the amino acid sequence Tyr-D-Ala-(p-F)Phe-Phe-NH2 which acts as a peripherally-specific, selective μ-opioid receptor agonist. Despite its inability to penetrate the blood-brain-barrier and enter the central nervous system, frakefamide has potent analgesic effects and, unlike ...