In Despicable Me 2, the Minion transformation serum is called PX-41.
The Anti-Villain League investigates a purple, aggressive Minion after a series of unusual events at a shopping mall. Gru and Lucy Wilde eventually discover that El Macho stole PX-41 and used it to transform captured Minions into destructive purple creatures.
The serum creates a dramatic visual contrast between the familiar yellow Minions and their purple forms. Gru and his allies must develop a counteragent before the transformed Minions can be used to attack the world.
PX-41 is a fictional chemical compound in the film, not a real-world product. The sequence is also one of the movie’s most recognizable examples of the franchise’s cartoon science and slapstick action.