Marked differences are seen between Ro 18-5364 (sulfoxide) and Ro18-5362 (sulfide) with regard to inhibitory activity[1]. The sulfoxide Ro 18-5364 is a potential metabolite of the IND Ro18-5362,and is a powerful inhibitor of gastric mucosal (H++K+)-ATPase, decreasing enzymatic activity with an apparent Ki of 0.1 μM[2].