Role of N- and C-terminal substituents on the CCK-B agonist-antagonist pharmacological profile of Boc-Trp-Phg-Asp-Nal-NH2 derivatives Original Research Article

Among the CCK derivatives, the tetrapeptide Boc-Trp-Phg-Asp-Nal-NH2 (1) behaves as a short potent CCK-B agonist which led to the development of an efficient peptidase-resistant CCK-B antagonist by bismethylation of its terminal CONH2 group. Further modifications of the N- and C-terminal moieties of 1 have been performed and are described in this paper, together with the pharmacological profile of the novel synthesized compounds. Introduction of more bulky substituents than NalNH2 on the C-terminal part decreased the CCK-B receptor binding affinity. In the series of N-protected tetrapeptides X30-Phg31-Asp32-Nal33-N(CH3)2, the Boc-substituent was shown to be optimal among the N-protecting groups Boc, 2Adoc, propionyl or acetyl when X = Trp. On the other hand, when X = αMeTrp, its optimal N-protecting group was 2Adoc and its configuration was preferentially d. In the newly synthesized compounds, 13: 2Adoc-d-αMeTrp-Phg-Asp-NalN(CH3)2 and 16: 2Adoc-d-αMeTrp-Phg-Asp-NalNH2 had the best CCK-B receptor affinities (K1 = 3.5 and 3.4 nM, respectively) and were selected for further biological evaluation. Interestingly, when tested for their capacity to influence inositol phosphate formation, induced by CCK8 in CHO cells transfected with the rat CCK-B receptor, compound 13 behaved as a full CCK-B antagonist with an IC50 value of 18 ± 1 nM, being as potent as the antagonists L-365,260 and PD-134,308 (IC50 values respectively, 39 ± 17 and 30 ± 2 nM), whereas compound 16 was found to behave as a partial CCK-B agonist. Indeed 16 behaved as an antagonist on the firing rate of rat CA1 hippocampal neurons and acted as an agonist in the pentagastrin stimulated gastric acid secretion (EC50 = 12 nmol/kg) in anesthetized rats. Compound 13 in contrast, was found to inhibit the pentagastrin action at a dose (ID50 = 0.56 μmol/kg) similar to the potent antagonist PD-134,308 (ID50 = 0.4 μmol/kg). The antagonist/agonist properties of compounds 13 and 16 show that both N- and C-terminal substituents modulate the pharmacological properties in the Boc-CCK4 derivatives presented here.