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

Derivatives of the delta -opioid receptor-selective peptide antagonist H-Ty r-Tic-Phe-Phe-OH (TIPP) containing an isothiocyanate moiety at the para pos ition of either Phe(3) or Phe(4) were prepared as potential affinity labels for delta -opioid receptors. The;synthesis was accomplished using a genera l solution-phase synthetic procedure which allows for introduction of affin ity labeling groups late in the synthesis of a variety of small peptide sub strates. The target peptides and their corresponding amines were then evalu ated in radioligand binding experiments using Chinese hamster ovary (CHO) c ells expressing delta- and mu -opioid receptors. The peptides [Phe-(P-NCS)( 3)]TIPP (2) and [Phe(p-NCS)(4)]TIPP (4) showed affinity for delta -receptor s 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 d-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)]-TIP P (4) represent two affinity labels that may prove useful in the study of d elta -opioid receptors.