In this chapter, the history of covalent JAK3 inhibitors will be reviewed followed
by the detailed discussion of case studies on how covalent targeting of Cys909
enabled isoform- and kinome-wide selectivity for this promising therapeutic target.
Keywords Chemical probes, Covalent inhibitors, Covalent-reversible inhibitors,
Cysteine targeting, Inflammation, Isoform selectivity, Janus kinase 3
1 JAK3 and the Janus Kinase Family
Although covalent kinase inhibitors have mostly been used in oncology so far,
the covalent inhibition of kinases regulating immune response is currently under
investigation for the treatment of inflammatory and autoimmune disorders [1].
In this context, substantial efforts have been made to selectively address Janus
kinase (JAK) 3, a member of the JAK family of non-receptor tyrosine kinases [2].
JAKs are composed of seven domains termed Janus homology domains (JH1–7, see
Fig. 1a). The name Janus kinase arose from the JAKs’ domain structure featuring
a pseudokinase domain (JH2) adjacent to the catalytic domain (JH1), representing
the two faces of the Roman god Janus [3, 4]. All four members of this kinase family,
JAK1–3 and tyrosine kinase (TYK) 2, are involved in immune signaling via the
JAK–STAT (signal transducers and activators of transcription) pathway. JAKs
are the intracellular effectors of type I and II cytokine receptors, which are
devoid of intrinsic kinase activity [5]. They associate with the intracellular receptor
domains where they signal as homo- and heterodimers or heterotrimers. Upon
receptor activation, a conformational change positions the JAKs near to each
other promoting cross-phosphorylation and thereby activation (Fig. 1b). The fully
active JAKs phosphorylate the receptor to enable the recruitment of STAT
proteins via the receptor’s SH2 domains. The STATs are subsequently phosphorylated by the JAKs. This phosphorylation event promotes STAT dimerization and
translocation to the nucleus where the STAT proteins function as transcription
factors [4]. JAK3 only associates with cytokine receptors featuring the
interleukin-2 receptor subunit gamma, also termed common gamma chain (γ c ), i.e.,
interleukin (IL)-2, IL-4, IL-7, IL-9, IL-15, and IL-21 receptors [6, 7]. At
these receptors, JAK3 functions exclusively as a heterodimeric pair with JAK1. In
contrast, all heterodimeric combinations of JAK1, JAK2, and TYK2 are possible,
and even heterotrimeric combinations of the latter JAKs as well as homodimeric
JAK2 pairs occur at certain receptor types [4]. Moreover, JAK3 expression is mainly
limited to hematopoietic and epithelial cells highlighting the restricted and specific
role of JAK3 compared to other Janus kinases [8, 9].
Inactivation of JAK3 by loss-of-function mutations leads to a pathology called
JAK3-SCID (severe combined immunodeficiency), which is characterized by a
lack of T cells and NK cells and the presence of nonfunctional B cells [10].
226
M. Gehringer and M. Forster
by the detailed discussion of case studies on how covalent targeting of Cys909
enabled isoform- and kinome-wide selectivity for this promising therapeutic target.
Keywords Chemical probes, Covalent inhibitors, Covalent-reversible inhibitors,
Cysteine targeting, Inflammation, Isoform selectivity, Janus kinase 3
1 JAK3 and the Janus Kinase Family
Although covalent kinase inhibitors have mostly been used in oncology so far,
the covalent inhibition of kinases regulating immune response is currently under
investigation for the treatment of inflammatory and autoimmune disorders [1].
In this context, substantial efforts have been made to selectively address Janus
kinase (JAK) 3, a member of the JAK family of non-receptor tyrosine kinases [2].
JAKs are composed of seven domains termed Janus homology domains (JH1–7, see
Fig. 1a). The name Janus kinase arose from the JAKs’ domain structure featuring
a pseudokinase domain (JH2) adjacent to the catalytic domain (JH1), representing
the two faces of the Roman god Janus [3, 4]. All four members of this kinase family,
JAK1–3 and tyrosine kinase (TYK) 2, are involved in immune signaling via the
JAK–STAT (signal transducers and activators of transcription) pathway. JAKs
are the intracellular effectors of type I and II cytokine receptors, which are
devoid of intrinsic kinase activity [5]. They associate with the intracellular receptor
domains where they signal as homo- and heterodimers or heterotrimers. Upon
receptor activation, a conformational change positions the JAKs near to each
other promoting cross-phosphorylation and thereby activation (Fig. 1b). The fully
active JAKs phosphorylate the receptor to enable the recruitment of STAT
proteins via the receptor’s SH2 domains. The STATs are subsequently phosphorylated by the JAKs. This phosphorylation event promotes STAT dimerization and
translocation to the nucleus where the STAT proteins function as transcription
factors [4]. JAK3 only associates with cytokine receptors featuring the
interleukin-2 receptor subunit gamma, also termed common gamma chain (γ c ), i.e.,
interleukin (IL)-2, IL-4, IL-7, IL-9, IL-15, and IL-21 receptors [6, 7]. At
these receptors, JAK3 functions exclusively as a heterodimeric pair with JAK1. In
contrast, all heterodimeric combinations of JAK1, JAK2, and TYK2 are possible,
and even heterotrimeric combinations of the latter JAKs as well as homodimeric
JAK2 pairs occur at certain receptor types [4]. Moreover, JAK3 expression is mainly
limited to hematopoietic and epithelial cells highlighting the restricted and specific
role of JAK3 compared to other Janus kinases [8, 9].
Inactivation of JAK3 by loss-of-function mutations leads to a pathology called
JAK3-SCID (severe combined immunodeficiency), which is characterized by a
lack of T cells and NK cells and the presence of nonfunctional B cells [10].
226
M. Gehringer and M. Forster