Solid phase synthesis and evaluation of Tyr-Tic-Phe-Phe(p-NHCOCH₂Br) ([Phe(p-bromoacetamide)⁴]TIPP), a potent affinity label for δ opioid receptors

Derivatives of the δ opioid receptor selective peptide Tyr-Tic-Phe-Phe-OH (TIPP) containing a p-bromoacetamide moiety on the phenyl ring of Phe³ or Phe⁴ were prepared by solid phase synthesis. [Phe(p-NHCOCH₂Br)⁴]TIPP exhibited high affinity for cloned δ receptors (IC₅₀ = 5.4 nM), and incubation with only 2.5 nM resulted in 85% wash resistant inhibition of radioligand binding to δ receptors. Therefore, this peptide is a potent affinity label for further study of δ opioid receptors.