for ITK could be obtained (30c) albeit at the expense of potency
(IC 50
ITK
¼ 60 nM vs. IC 50
BTK
¼ 1,050 nM).
Another series of covalent ITK inhibitors exemplified by compound 31 (Fig. 13b)
was reported by researchers at GSK [85]. This key compound possessed high
potency (IC 50 ¼ 5 nM at 1 mM ATP), a more pronounced selectivity against BTK
and a higher k inact /K I ratio (5.2 Â 10
5 M
À1 s
À1 ) compared to the aforementioned
inhibitors. Covalent binding was verified by jump-dilution, cellular washout, and
X-ray crystallography using a close analog (PDB: 4KIO). Compound 31 demonstrated low reactivity toward GSH, favorable PK properties for inhaled dosing, and
prevented anti-CD3-induced T-cell activation in rat lung tissue (p.i.). In human
PBMCs, the compound further suppressed the production of T H 1, T H 2, and T H 17
cytokines.
Other kinases with a cysteine at the F2 position that have been addressed by
rationally designed covalent inhibitors include JAK3, BMX, and MKK7. Efforts
toward highly isoform-selective JAK3 inhibitors culminated in the development of
the clinical candidate PF-06651600 (32, Fig. 14a) and have been summarized in a
separate chapter of this book and a recent review [86]. Notably, efforts published
very recently from researchers at Pfizer showed that JAK3 Cys909 is also amenable
to covalent-reversible targeting with cyanamides (e.g., compound 33a) [17]. The
latter compound features excellent isoform selectivity (>245-fold vs. other JAKs at
1 mM ATP), high potency (IC 50 ¼ 11 nM at 1 mM ATP), and efficient inactivation
kinetics (k inact /K I ¼ 1.9 Â 10
5 M
À1 s
À1 ) while being reasonably stable against GSH.
X-ray crystallography (e.g., with the analog 33b) unambiguously confirmed
isothiourea formation (Fig. 15a).
A distinct series of α-cyanoacrylamide-based covalent-reversible JAK3 inhibitors
(exemplified by 34a and b) with excellent isoform and kinome selectivity has been
developed by Forster et al. [87, 88]. Remarkably, both the covalent and the
non-covalent complexes coexist in the X-ray crystal structure of 34b bound to
JAK3 (Fig. 15b). The nitrile substituent of these compounds opens up a rare
induced-fit pocket formed by Arg911, Arg953, and Asp912 which constitutes an
additional selectivity filter contributing to the excellent selectivity of this compound
class in the kinome.
Covalent inhibitors for other kinases with an F2 cysteine have also been reported.
Inhibitors that target the kinase BMX include, for example, the dual BMX/BTK
inhibitor BMX-IN-1 (35, Fig. 14b) [89], or the type II inhibitor CHMFL-BMX-078
(36) [90] featuring increased selectivity against BTK. As mentioned before, the
MAP kinase kinase MKK7, one of the two activators of the c-Jun N-terminal kinases
(JNKs) features several cysteines in the active site: one at the F2 position (Cys218),
one at the D1 position (Cys276), one at the P3 position (Cys147), and one at the O3
subsite (Cys296). The F2 cysteine, which is also addressed by LL-Z1640-2 (9, see
Figs. 6 and 7), has recently been targeted by indazole-derived inhibitors exemplified
by 37 (MKK7-COV-2, Fig. 14c) discovered in a virtual screening campaign using
DOCKovalent [91, 92]. Even more recently, ibrutinib-derived 1,2,3-triazoles (e.g.,
38) addressing MKK7-Cys218 have been reported by the Rauh group [93].
Covalent Kinase Inhibitors: An Overview
63
(IC 50
ITK
¼ 60 nM vs. IC 50
BTK
¼ 1,050 nM).
Another series of covalent ITK inhibitors exemplified by compound 31 (Fig. 13b)
was reported by researchers at GSK [85]. This key compound possessed high
potency (IC 50 ¼ 5 nM at 1 mM ATP), a more pronounced selectivity against BTK
and a higher k inact /K I ratio (5.2 Â 10
5 M
À1 s
À1 ) compared to the aforementioned
inhibitors. Covalent binding was verified by jump-dilution, cellular washout, and
X-ray crystallography using a close analog (PDB: 4KIO). Compound 31 demonstrated low reactivity toward GSH, favorable PK properties for inhaled dosing, and
prevented anti-CD3-induced T-cell activation in rat lung tissue (p.i.). In human
PBMCs, the compound further suppressed the production of T H 1, T H 2, and T H 17
cytokines.
Other kinases with a cysteine at the F2 position that have been addressed by
rationally designed covalent inhibitors include JAK3, BMX, and MKK7. Efforts
toward highly isoform-selective JAK3 inhibitors culminated in the development of
the clinical candidate PF-06651600 (32, Fig. 14a) and have been summarized in a
separate chapter of this book and a recent review [86]. Notably, efforts published
very recently from researchers at Pfizer showed that JAK3 Cys909 is also amenable
to covalent-reversible targeting with cyanamides (e.g., compound 33a) [17]. The
latter compound features excellent isoform selectivity (>245-fold vs. other JAKs at
1 mM ATP), high potency (IC 50 ¼ 11 nM at 1 mM ATP), and efficient inactivation
kinetics (k inact /K I ¼ 1.9 Â 10
5 M
À1 s
À1 ) while being reasonably stable against GSH.
X-ray crystallography (e.g., with the analog 33b) unambiguously confirmed
isothiourea formation (Fig. 15a).
A distinct series of α-cyanoacrylamide-based covalent-reversible JAK3 inhibitors
(exemplified by 34a and b) with excellent isoform and kinome selectivity has been
developed by Forster et al. [87, 88]. Remarkably, both the covalent and the
non-covalent complexes coexist in the X-ray crystal structure of 34b bound to
JAK3 (Fig. 15b). The nitrile substituent of these compounds opens up a rare
induced-fit pocket formed by Arg911, Arg953, and Asp912 which constitutes an
additional selectivity filter contributing to the excellent selectivity of this compound
class in the kinome.
Covalent inhibitors for other kinases with an F2 cysteine have also been reported.
Inhibitors that target the kinase BMX include, for example, the dual BMX/BTK
inhibitor BMX-IN-1 (35, Fig. 14b) [89], or the type II inhibitor CHMFL-BMX-078
(36) [90] featuring increased selectivity against BTK. As mentioned before, the
MAP kinase kinase MKK7, one of the two activators of the c-Jun N-terminal kinases
(JNKs) features several cysteines in the active site: one at the F2 position (Cys218),
one at the D1 position (Cys276), one at the P3 position (Cys147), and one at the O3
subsite (Cys296). The F2 cysteine, which is also addressed by LL-Z1640-2 (9, see
Figs. 6 and 7), has recently been targeted by indazole-derived inhibitors exemplified
by 37 (MKK7-COV-2, Fig. 14c) discovered in a virtual screening campaign using
DOCKovalent [91, 92]. Even more recently, ibrutinib-derived 1,2,3-triazoles (e.g.,
38) addressing MKK7-Cys218 have been reported by the Rauh group [93].
Covalent Kinase Inhibitors: An Overview
63