This invention relates to novel compounds that are pyrazinoisoquinoline derivatives, and pharmaceutically acceptable salts thereof. More specifically, this invention relates to novel pyrazinoisoquinoline derivatives that are derivatives of praziquantel, such as compounds of formula (I): or pharmaceutically acceptable salts thereof. This invention also provides compositions comprising one or more compounds of this invention and a carrier and the use of the disclosed compounds and compositions in methods of treating diseases and conditions that are beneficially treated by administering an antihelminthic agent, such as praziquantel.