Tritium labelling of two highly selective agonists for CCK-B receptors : [3H]propionyl-Tyr(SO3Na)-gNle-mGly-Trp-(N-Me)Nle-Asp-Phe-NHsub (2) ([3H]pBC 264) [3H]propionyl-γD.Glu-Tyr(SO3H)-Nle-D.Lys-Trp-Nle-Asp-Phe-NH2 ([3H]pBC 254)

Among the CCK-B receptor agonists reported to date, the two modified peptides BC 264 and BC 254 display a high affinity and selectivity for this binding site and are highly protected from enzymatic degradation. Recently, we reported the biological properties of a tritiated analog of this agonist, [3H]pBC 264, which fullfils all the criteria required for in vitro as well as in vivo studies of the CCK-B receptor. On the other hand, BC 254 displays a high affinity for the CCK-B binding sites in the guinea-pig (Ki = 0.56 nM) while its affinity in the rat is more than 60-fold lower, a difference which could be due to the occurrence of CCK-B receptor subtypes. In the present paper, we report the synthesis of [3H]pBC 264 and of the new tritiated ligand [3H]pBC 254 using [3H] NPS (N-succinimidyl[2,3-3H]propionate) as labelling agent. These two probes have high specific activity (70-100 Ci/mmol) and will enable extensive studies of the CCK-B receptors to be carried out. (author)