Structure–activity relationships indicate that the presence of a lipophilic amino acid side chain is fundamental to achieve good potencies. The l-Trp derivative (20, PCM126) was the most potent antagonist of the series disrupting EphA2–ephrinA1 interaction and blocking EphA2 phosphorylation in prostate cancer cells at low μM concentrations, thus being significantly more potent than LCA. Compound 20 is among