Novel fenoterol derivatives as suppressors of ERK1/2 phosphorylation in melanoma

Melanoma is a highly aggressive cancer with limited therapeutic options. In melanoma cell lines, β 2 AR activation leads to the suppression of pro-proliferatory MAPK/ERK signaling. This suggests that fenoterol, a potent β 2 AR agonist, can be useful in the treatment of melanoma. Here, we investigated the structure–activity relationship (SAR) of fenoterol derivatives for β 2 AR-depedent ERK suppression. We used the UACC-647 human-derived melanoma cell line to screen a panel of chiral compounds based on the fenoterol scaffold. The levels of phosphoactive ERK1/2 were quantified by immunoblotting. Anti-tumorigenic activity of the drugs was assessed using MTS assay and zebrafish xenograft model. ( R , R ′) stereoisomers of fenoterol derivatives exhibited the highest suppression of ERK phosphorylation, followed by ( R , S ′) and ( S , R ′) isomers, with ( S , S ′) isomers being the least active, mirroring trends observed in β 2 AR binding and cAMP accumulation. SAR analysis revealed that modifications to the α′ alkyl chain and the 4′-hydroxy moieties significantly affected ERK-inhibitory activity of fenoterol derivatives. ( R , R ′)-4′-hydroxy-1-naphthylfenoterol [( R , R ′)-HNF] was the most active inhibitor of ERK activation with an IC 50 of 0.03 nM. However, it showed minimal anti-proliferative activity in the zebrafish xenograft model. In contrast, ( R , R ′)-4′-methoxy-1-naphthylfenoterol [( R , R ′)-MNF], a weaker ERK inhibitor (IC 50 = 0.17 nM), significantly reduced cell viability in vitro (IC 50 = 14.07 µM) and tumor growth in vivo. SAR for fenoterol-mediated ERK inhibition in melanoma was defined, highlighting the importance of the ( R , R ′) stereochemistry. Crucially, potent ERK inhibition does not directly predict anti-tumorigenic activity, as shown by ( R , R ′)-HNF. The pronounced anti-proliferatory effect of ( R , R ′)-MNF, despite weaker ERK inhibition, implies additional mechanisms beyond ERK modulation.