PX-41 is a potent mutagen created by the supervillain El Macho. It turns anyone injected with it into an giant white-haired purple rage monster, whether they’re an articially created life form or a dangerous Mexican supervillain.
Both PX-41 and its antidote were created by Dr. Nefario, an elderly British mad scientist in the employ of El Macho (although he betrayed him to create the antidote). All known samples are nor presumably in the custody of the Anti-Villain League, although Dr. Nefario could no doubt whip up more easily enough.