gabapentin impurity b ; 1-cyanocyclohexaneacetic acid ; gabapentin related compound b (30 mg) ((1-cyanocyclohexyl)acetic acid) ; gabapentin related material b ; 2-(1-cyanocyclohexyl)acetic acid ; (1-cyanocyclohexyl)acetic acid ; gabapentin usp rc b ; gabapentin ep impurity b