In Despicable Me 2, the serum that turns Minions purple is called PX-41.
PX-41 is an experimental mutagen developed by the villainous El Macho. When Minions are exposed to it, they become aggressive, purple-haired creatures with enlarged appetites and altered behavior. The transformation creates the film’s central threat because the Minions’ loyalty and comic helpfulness are replaced by destructive instincts.
Gru and his allies work to stop the spread of the serum and restore the affected Minions. The antidote is an important part of the resolution, allowing the transformed characters to return to their normal yellow appearance. The code is sometimes misremembered as another letter-number combination, but the film identifies the compound as PX-41.
Despicable Me 2 was released in 2013. It expands the world introduced by the first film by pairing Gru with the Anti-Villain League and developing his relationship with Lucy Wilde.