Biological properties of a new fluorescent biphalin fragment analogue

Previous studies of structure-activity of biphalin defined fragments which expressed the full biological potency of the parent compound. The most simple fragment was Tyr-D-Ala-Gly-Phe-NH-NH←X, where X = Phe, but it also could be other hydrophobic amino acids. This paper presents data that replacement of the phenylalanine with a dansyl (X = DNS) groups gives an analogue (AA2016) that fully preserves the high affinity of the initial analogue for both μ and δ opioid receptors. In the tail flick test in rats, intrathecal injection of the compound produces strong antinociception, comparable to the parent biphalin. Because AA2016 contains a strong fluorescent group, it can be a very useful tool for prospective studies in vivo, including biological barrier permeability, tissue distribution, metabolism and receptor-ligand complex formation.