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)

Abstract
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.

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