2-hydroxyl-8-methoxy-5-methylphenanthridinium chloride (11k) showed good inhibitory activity with an IC50 value of 0.12 mM. Preliminary structure–activity relationship indicated that DDC inhibitory potential of 5-methyl phenanthridium derivatives correlated with the π-electro densities on CN double bond of iminium cation. The hydroxyl group on compound 11k possibly contributed to the formation of hydrogen bond between