Complex sentence: HDMP-28 or methylnaphthidate is a piperidine based stimulant drug, closely related to methylphenidate, but with the phenyl ring replaced by naphthalene.
Split into simpler sentences: HDMP-28 or methylnaphthidate is a piperidine based stimulant drug. It is closely related to methylphenidate, but with the phenyl ring replaced by naphthalene.