Frakefamide
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 centrally-acting opioids like morphine, does not produce respiratory depression, indicating that its antinociceptive effects are mediated by peripheral μ-opioid receptors. It was under development for the treatment of pain by AstraZeneca and Shire but was shelved after phase II clinical trials.