Synthesis and evaluation of isothiocyanate-containing derivatives of the δ-opioid receptor antagonist Tyr-Tic-Phe-Phe (TIPP) as potential affinity labels for δ-opioid receptors

Derivatives of the δ-opioid receptor-selective peptide antagonist H-Tyr-Tic-Phe-Phe-OH (TIPP) containing an isothiocyanate moiety at the para position of either Phe³ or Phe⁴ were prepared as potential affinity labels for δ-opioid receptors. The synthesis was accomplished using a general solution-phase synthetic procedure which allows for introduction of affinity labeling groups late in the synthesis of a variety of small peptide substrates. The target peptides and their corresponding amines were then evaluated in radioligand binding experiments using Chinese hamster ovary (CHO)cells expressing δ- and μ-opioid receptors. The peptides [Phe(p-NCS)³]TIPP (2) and [Phe(p-NCS)⁴]TIPP (4) showed affinity for δ-receptors comparable to the parent compound TIPP (IC₅₀ = 12 and 5 nM, respectively, vs 6 nM for TIPP). Both peptides 2 and 4 were able to inhibit radioligand binding to δ-receptors in a wash-resistant manner at a concentration of 10 nM. Therefore, the peptides [Phe(p-NCS)³]TIPP (2) and [Phe(p-NCS)⁴]TIPP (4) represent two affinity labels that may prove useful in the study of δ-opioid receptors.