Tritium labelling of highly selective probes for δ-opioid receptors: [3H]Tyr-D-Ser(O-t-Bu)-Gly-Phe-Leu-Thr(DSTBULET) and [3H]Tyr-D-Ser(O-t-Bu)-Gly-Phe-Leu-Thr(O-t-Bu)(BUBU)

The introduction of bulky residue(s) in linear enkephalin-related hexapeptides represents a new approach in the design of selective probes for δ-opioid receptors, displaying the appropriate criteria to investigate biological and pharmacological properties of the assumed binding site (δ) of endogenous enkephalins. The selectivities and high affinities of Tyr-D-Ser(O-t-Bu)-Gly-Phe-Leu-Thr(DSTBULET) and especially Tyr-D-Ser(O-t-Bu)Gly-Phe-Leu-Thr(O-t-Bu) (BUBU) associated with a satisfactory resistance to peptidases, make them the most suitable δ-probes reported to date. In the present paper, we report the synthesis of DSTBULET and BUBU under tritiated forms with high specific radioactivities. These radio-labelled probes will enable extensive in vitro and in vivo investigations of δ-opioid receptors properties to be carried out. (author)