Esterification of 4-(4,5-dihydro-1H-imidazol-2-ylamino)benzoic acid methanesulfonate (I) with 6-amidino-2-naphthol methanesulfonate (II) by means of dimethylaminopyridine (DMAP) and dicyclohexylcarbodiimide (DCC) in dry pyridine.