Review Question - QID 213883

A medicinal chemist performs research on the molecular structure of various opiate compounds in order to discover novel clinically useful members of this drug class. He creates a compound that appears to decrease the plasma conductance of calcium in cells. When administering this drug to rodents, he finds that the drug does not appear to affect the pain tolerance or level of sedation of these animals. He then combines this compound with morphine and administers the mixture to animals and finds that the animals are equally sedated with or without this new compound. Finally, he administers a radiolabeled compound and finds that the compound appears to be concentrated selectively in the medulla of the brainstem. Which of the following medication is most analogous to the activity of this new compound?