Synthesis and binding characteristics of tritiated tipp[Φ], a highly specific and stable delta opioid antagonist

The pseudopeptide H-Tyr-TicΦ[CH 2-NH]Phe-Phe-OH (TIPP[Φ]) is a δ opioid antagonist with high δ receptor affinity and unprecedented δ selectivity. TIPP[Φ] was radiolabelled by catalytic tritiation of its precursor [Tyr(3′,5′-I 2) 1]TIPP[Φ]. The resulting radioligand, [ 3H]TIPP[Φ], had a specific activity of 1.77 TBq/mmol (47.9 Ci/mmol) and showed high stability against enzymatic degradation. [ 3H]TIPP[Φ] binding to rat brain membranes was saturable and Scatchard analysis indicated a single binding site with a K d of 0.98 nM and a B max of 105.4 fmol/mg. A study of [ 3H]TIPP[Φ] binding displacement by various receptor-selective opioids showed the expected rank order of potency ( δ ⪢ μ > κ). [ 3H]TIPP[Φ] represents an excellent new radioligand for δ receptor labelling studies in vitro and in vivo.