Courtesy Pictures of Elements, via Wikipedia
Antimony is a metallic chemical element (atomic number 51). Although it's poisonous and causes symptoms similar to arsenic poisoning, it's far less toxic than most heavy metals. As a result, it's an ingredient in some pharmaceuticals, particularly those that are used against parasites, or in emetics.
In Simple Explanation, antimony was used in an attempt to treat leishmaniasis.
In Runaways, the clinic patients were suffering from antimony poisoning.