JAK inhibitor

Janus kinases are enzymes inside cells that pass messages from the immune system's chemical signals into the cell nucleus, where they switch genes on. Those pathways, in the US Food and Drug Administration's words, play a key role in the body's immune and inflammatory responses. A JAK inhibitor is a drug that blocks them, so the signal never arrives and the inflammation it would have caused is reduced.

Because the same pathways carry useful immune signals as well as harmful ones, blocking them has a cost. JAK inhibitors as a class carry a boxed warning, the strongest warning the FDA applies to a medicine, covering serious infections, increased death from any cause, cancers, serious heart and blood vessel problems, and blood clots. Most are taken as a daily tablet, which is why they have opened up oral treatment for diseases that previously needed infusions or injections.