Biological effects and receptor binding affinities of new pseudononapeptide bombesin/GRP receptor antagonists with N‐terminal d‐Trp or d‐Tpi

In an attempt to produce more powerful (effective) bombesin/GRP receptor antagonists, the D forms of Trp or Trp analog (Tpi) were introduced at position 6 in two pseudononapeptides, Leu¹³ψ(CH₂NH)Leu¹⁴-bombesin(6-14) and Leu¹³ψ(CH₂NH)Phe¹⁴-bombesin (6-14). These antagonists were tested for their ability to inhibit basal and gastrin releasing peptide (GRP) (14-27)-induced amylase release from rat pancreatic acini in a superfusion assay. They were also assessed for the inhibition of ¹²⁵I-Tyr⁴-bombesin binding to Swiss 3T3 and small cell lung carcinoma cell line H-345 and the mitogenic response of Swiss 3T3 cells induced by GRP(14-27). The peptides, when given alone, did not stimulate amylase secretion, but were able to inhibit gastrin releasing peptide (14-27)-induced amylase release. All of the antagonists showed strong binding affinities for Swiss 3T3 and H-345 cells and suppressed the GRP(14-27)-induced increase of [³H]thymidine incorporation into DNA of Swiss 3T3 cells at nanomolar concentrations. Antagonist D-Tpi⁶,Leu¹³ψ(CH₂NH)Leu¹⁴-bombesin (6-14)(RC-3095) was slightly more potent in these assays than D-Trp⁶,Leu¹³ψ(CH₂NH)Leu¹⁴-bombesin (6-14)(RC-3125). Nevertheless, D-Trp⁶,Leu¹³ψ(CH₂NH)Phe¹⁴-bombesin (6-14) showed the highest binding affinity for Swiss 3T3 and H345 cells and it was the most potent inhibitor of GRP(14-27)-induced amylase secretion. This antagonist RC-3420 was particularly effective in inhibiting the growth of Swiss 3T3 cells, exhibiting an IC₅₀ value less than 1 nM. Our work indicated that the substitution of D-Trp and D-Tpi at position 6 of the pseudononapeptide bombesin analogs (ψ^13-14), in which the Met¹⁴ residue is replaced by Leu or Phe, results in potent bombesin/GRP antagonists with improved in vivo activity.