This compound, now being named as at least one of the toxic agents in the Menu Foods recall, is compound derived from pteridine, a compound first identified in butterflies' wings1. With that "innocent" start, we are learning that this is a very toxic compound; to both animals and humans.
This compound is classed as a "folic acid antagonist", which is used to interfere (antagonize) the action of folic acid, a form of vitamin B. This compound has been used as an anticancer agent, but has largely been replaced by another drug, methotrexate, due to its "unpredictable toxicity"2 This compound was also used as an human abortifacient in the 1950's (US).
Clinical Signs of Exposure to Aminopterin
From Wikipedia article on Aminopterin:
"Signs and symptoms of acute exposure to aminopterin may include nausea, vomiting, anorexia, stomatitis (inflammation of the oral mucosa), pharyngitis (inflammation of the pharynx), erythematous rashes (red rashes on the skin), hyperpigmentation (abnormally increased pigmentation), chills, fever, gastrointestinal hemorrhage, and renal failure (especially in high doses). May induce abortion in pregnant women." (This is a discussion of human symptoms.)
This compound is not registered as a rodenticide in the United States and is considered "highly toxic by ingestion". (EPA, 1998)3