2-Naphthalenesulphonyl L-aspartyl-(2-phenethyl)amide (2-NAP)--a selective cholecystokinin CCKA-receptor antagonist

1. The in vitro pharmacological characterization of the sodium salt of 2-naphthalenesulphonyl 1-aspartyl-(2-phenethyl)amide [2-NAP], a hydrophilic compound derived from the C-terminal aspartate-phenylalanine dipeptide of cholecystokinin (CCK), is described. 2. 2-NAP behaved as a competitive antagonist of sulphated cholecystokinin octapeptide (CCK-8) at CCKA-receptors in both intact tissue bioassays (guinea-pig gall bladder, pancreas and ileum, human and rabbit gall bladder) and a radioligand displacement assay (guinea-pig pancreatic cells). The mean pKB, over assays, was 6.5. 3. Compared to the other assays, the rabbit gall bladder assay gave a significantly higher pKB estimate [7.0] for 2-NAP and a significantly lower estimate [8.9] for devazepide (formerly L-364,718 and MK-329), a well-characterized CCKA-receptor antagonist; these anomalous results suggest that a different class of CCKA-receptors may be involved. 4. 2-NAP, was found to be highly selective, having at least 300 fold greater affinity for CCKA-receptors than for 50 other pharmacological loci, including gastrin/CCKB, as estimated by bioassay or radioligand displacement.