One of these hydrogen bonds is formed between a heteroaryl nitrogen atom and
Leu905, the other between the primary amide and the backbone carbonyl oxygen of
Glu903. The benzyl linker adopts a similar conformation as in a previously mentioned compound from Gray et al. (48, see Fig. 15), which is further stabilized by an
intramolecular hydrogen bond between the amide carbonyl oxygen and the NH
of the secondary amino group.
Besides the classical acrylamide headgroup, a variety of other warheads were
attached to this scaffold, including less common ones like the vinylsulfonamide (68)
or a pentafluorophenoxy acetamide (69). However, these electrophiles, except the
vinylsulfonamide in 68, impaired JAK3 inhibitory activity to different extents, and
all of them failed to achieve potent inhibition of IL-2-stimulated T cell proliferation
(IC 50 values of 5–25 μM).
Additional characterization of such pyrrolo[1,2-b]pyridazine-derived compounds
in comprehensive kinome panels or in vivo has not been provided yet, but follow-up
reports including such data were announced by the authors.
3 Summary
Substantial efforts have been made to develop covalent kinase inhibitors for the
treatment of inflammatory and autoimmune disorders. Here, JAK3 was among
the most prominent targets, but specific and drug-like inhibitors for this enzyme
with a covalent mode of action have only appeared in the literature during the last
5 years. However, a tremendous amount of work has been published in this field
since 2014 providing a considerable set of covalent-reversible and irreversible JAK3
inhibitors with excellent selectivity within the JAK family and the kinome. Previous
non-covalent inhibitors, which were supposed to be JAK3-selective typically suffered from limited cellular selectivity or lack of potency in cells due to the higher
ATP affinity of JAK3 when compared to other JAKs. The latter is not taken into
account by the typical IC 50 determinations employing ATP concentrations at (or
close to) the enzymes’ K m values. Experiments with such compounds have previously raised the question, whether selective JAK3 inhibition is enough to block
STAT phosphorylation, sparking a discussion whether JAK1 had a dominant role
over JAK3 in the signaling of γ c cytokine receptors. The newly developed covalent
JAK3 inhibitors, however, have resolved this issue and can now be used for
the detailed investigation of JAK3-dependent JAK-STAT signaling. Based on
the reports discussed in this chapter, we conclude that selective JAK3 inhibition
is sufficient to suppress downstream signaling. If major issues such as limited in vivo
stability, e.g., due to GSH/GST-mediated extrahepatic clearance, can be overcome,
and long-term safety is proven, there is a considerable chance for specific covalent
JAK3 inhibitors to become effective anti-inflammatory and immunosuppressive
drugs with limited side effects outside the immune system. In this light, the results
of further clinical studies with the currently most advanced compound,
PF-06651600, and potential follow-up candidates are eagerly awaited.
Covalent Janus Kinase 3 Inhibitors
251
Leu905, the other between the primary amide and the backbone carbonyl oxygen of
Glu903. The benzyl linker adopts a similar conformation as in a previously mentioned compound from Gray et al. (48, see Fig. 15), which is further stabilized by an
intramolecular hydrogen bond between the amide carbonyl oxygen and the NH
of the secondary amino group.
Besides the classical acrylamide headgroup, a variety of other warheads were
attached to this scaffold, including less common ones like the vinylsulfonamide (68)
or a pentafluorophenoxy acetamide (69). However, these electrophiles, except the
vinylsulfonamide in 68, impaired JAK3 inhibitory activity to different extents, and
all of them failed to achieve potent inhibition of IL-2-stimulated T cell proliferation
(IC 50 values of 5–25 μM).
Additional characterization of such pyrrolo[1,2-b]pyridazine-derived compounds
in comprehensive kinome panels or in vivo has not been provided yet, but follow-up
reports including such data were announced by the authors.
3 Summary
Substantial efforts have been made to develop covalent kinase inhibitors for the
treatment of inflammatory and autoimmune disorders. Here, JAK3 was among
the most prominent targets, but specific and drug-like inhibitors for this enzyme
with a covalent mode of action have only appeared in the literature during the last
5 years. However, a tremendous amount of work has been published in this field
since 2014 providing a considerable set of covalent-reversible and irreversible JAK3
inhibitors with excellent selectivity within the JAK family and the kinome. Previous
non-covalent inhibitors, which were supposed to be JAK3-selective typically suffered from limited cellular selectivity or lack of potency in cells due to the higher
ATP affinity of JAK3 when compared to other JAKs. The latter is not taken into
account by the typical IC 50 determinations employing ATP concentrations at (or
close to) the enzymes’ K m values. Experiments with such compounds have previously raised the question, whether selective JAK3 inhibition is enough to block
STAT phosphorylation, sparking a discussion whether JAK1 had a dominant role
over JAK3 in the signaling of γ c cytokine receptors. The newly developed covalent
JAK3 inhibitors, however, have resolved this issue and can now be used for
the detailed investigation of JAK3-dependent JAK-STAT signaling. Based on
the reports discussed in this chapter, we conclude that selective JAK3 inhibition
is sufficient to suppress downstream signaling. If major issues such as limited in vivo
stability, e.g., due to GSH/GST-mediated extrahepatic clearance, can be overcome,
and long-term safety is proven, there is a considerable chance for specific covalent
JAK3 inhibitors to become effective anti-inflammatory and immunosuppressive
drugs with limited side effects outside the immune system. In this light, the results
of further clinical studies with the currently most advanced compound,
PF-06651600, and potential follow-up candidates are eagerly awaited.
Covalent Janus Kinase 3 Inhibitors
251