TRITIUM LABELING OF 2 HIGHLY SELECTIVE AGONISTS FOR CCK-B RECEPTORS - [H-3]PROPIONYL-TYR(SO3NA)-GNLE-MGLY-TRP-(N-ME)NLE-ASP-PHE-NH2 ([H-3]PBC-264) [H-3]PROPIONYL-GAMMA-D.GLU-TYR(SO3H)-NLE-D.LYS-TRP-NLE-ASP-PHE-NH2 ([H-3]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, [H-3]pBC 264, which fulfills 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 (K(i) = 0.56 nM) while its affinity in the rat is more than 60-fold lower, a difference which could be due to thc occurrence of CCK-B receptor subtypes. In the present paper, we report the synthesis of [H-3]pBC 264 and of the new tritiated ligand [H-3]pBC 254 using [H-3] NPS (N-succinimidyl[2,3-H-3]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.