ML-18 is a non-peptide bombesin receptor subtype-3 antagonist which inhibits lung cancer growth

Bombesin receptor subtype (BRS)-3 is a G protein coupled receptor (GPCR) for the bombesin (BB)-family of peptides. BRS-3 is an orphan GPCR and little is known of its physiological role due to the lack of specific agonists and antagonists. PD168368 is a nonpeptide antagonist for the neuromedin B (NMB) receptor (R) whereas PD176252 is a nonpeptide antagonist for the gastrin releasing peptide (GRP) R and NMBR but not BRS-3. Here nonpeptide analogs of PD176252 e.g. the S-enantiomer ML-18, and the R-enantiomer, EMY-98, were investigated as BRS-3 antagonists using lung cancer cells. ML-18 and EMY-98 inhibited specific 125I-BA1 (DTyr-Gln-Trp-Ala-Val-βAla-His-Phe-Nle-NH2)BB6-14 binding to NCI-H1299 lung cancer cells stably transfected with BRS-3 with IC50 values of 4.8 and >100 μM, respectively. In contrast, ML-18 bound with lower affinity to the GRPR and NMBR with IC50 values of 16 and >100 μM, respectively. ML-18 (16 μM), but not its enantiomer EMY-98, inhibited the ability of 10 nM BA1 to elevate cytosolic Ca2+ in a reversible manner using lung cancer cells loaded with FURA2-AM. ML-18 (16 μM), but not EMY-98, inhibited the ability of 100 nM BA1 to cause tyrosine phosphorylation of the EGFR and ERK in lung cancer cells. ML-18 but not EMY-98 inhibited the proliferation of lung cancer cells. The results indicate that ML-18 is a nonpeptide BRS-3 antagonist that should serve as a template to improve potency and selectivity.