The search for inhibitors of the Janus kinase family (JAK1, JAK2, JAK3 and TYK2) has
been ongoing for several decades and has resulted in a number of JAK inhibitors being approved for
use in patients, such as tofacitinib for the treatment of autoimmune diseases such as Rheumatoid Arthritis
(RA). Although initially thought to be a JAK3 selective inhibitor, tofacitinib was subsequently
found to possess significant activity to inhibit JAK1 and JAK2 which has contributed to some adverse
side effects. A selective JAK3 inhibitor should only have an effect within the immune system since
JAK3 is solely expressed in lymphoid tissue; this makes JAK3 a target of interest in the search for
treatments of autoimmune diseases. A method to obtain selectivity for JAK3 over the other JAK family
members, which has attracted more scientific attention recently, is the targeting of the active site cysteine
residue, unique in JAK3 within the JAK family, with compounds containing electrophilic warheads
which can form a covalent bond with the nucleophilic thiol of the cysteine residue. This review encompasses
the historical search for a covalent JAK3 inhibitor and the most recently published research
which hasn’t been reviewed to date. The most important compounds from the publications reviewed
the activity and selectivity of these compounds together with some of the more important biological
results are condensed in to an easily digested form that should prove useful for those interested in the
field.