inhibits the SR-protein kinase SRPK1 (IC 50 ¼ 11 nM), an m-phenylsulfonyl fluoride
moiety was introduced to address Tyr227, a unique tyrosine adjacent to the solventexposed front region of the SRPK1 active site. The obtained inhibitor SRPKIN-1
(88b) showed similar SRPK1 activity (IC 50 ¼ 36 nM) while retaining only moderate
potency on ALK (IC 50 ¼ 195 nM). No kinetic analysis was provided to deduce the
contribution of covalent bond formation to the observed activity. However, washout
experiments supported a covalent binding mode and MS experiments confirmed the
predicted labeling of Tyr227. Cellular profiling revealed good selectivity for SRPK1
and SRPK2 and the inhibitor suppressed neovascularization in a choroidal
neovascularization (CNV) model.
3 Summary and Outlook
The field of covalent protein kinase inhibitor research has matured over the last
years. Covalent targeting strategies have enabled the design of chemical probes with
excellent selectivity in the kinome and the benefits of TCIs have successfully been
implemented in drug discovery. Covalent approaches have been used, for example,
to boost potency and break resistance (e.g., with covalent EGFR inhibitors), to
achieve durable target occupancy (e.g., with covalent BTK inhibitors), or to promote
excellent selectivity over closely related enzymes (e.g., with covalent JAK3 or
FGFR4 inhibitors). The recent FDA approval of six irreversible kinase inhibitors
impressively highlights the utility of covalent design strategies for the discovery of
new medicines. Although covalent kinase inhibitors have almost exclusively been
approved for cancer treatment so far, substantial efforts are currently being made to
develop such drugs for non-oncology indications, especially for the treatment of
inflammatory and autoimmune disorders.
In the current practice, covalent kinase targeting largely relies on limited warhead
chemistry. Generally, attenuated Michael acceptors such as α,β-unsaturated amides
are used to address non-catalytic cysteines in a (quasi)-irreversible fashion. However, more diverse warhead types are increasingly receiving attention [9]. Most
notably, covalent-reversible targeting approaches relying, for example, on dually
activated Michael acceptors, but also on other electrophiles such as cyanamides or
aldehydes, are gaining importance as highlighted by the inhibitors PRN1008 and
FGF401 (see Figs. 12b and 24b) which are currently under clinical investigation.
Furthermore, attempts to address other nucleophilic amino acids inside or proximal
to the ATP binding pocket (e.g., Lys and Tyr) or cysteine moieties at allosteric sites
have recently been successful and such approaches are expected to gain traction in
the near future. Although the structure-guided modification of known reversible
inhibitors, which is facilitated by the steeply increasing amount of publicly available
X-ray crystal structures, is still by far the most common strategy for the discovery of
kinase TCIs, alternative concepts such as the screening of reactive fragments or
DNA-encoded libraries show great promise. Methods for estimating the extent of
specific and non-specific labeling by irreversible covalent inhibitors in living
82
M. Gehringer
moiety was introduced to address Tyr227, a unique tyrosine adjacent to the solventexposed front region of the SRPK1 active site. The obtained inhibitor SRPKIN-1
(88b) showed similar SRPK1 activity (IC 50 ¼ 36 nM) while retaining only moderate
potency on ALK (IC 50 ¼ 195 nM). No kinetic analysis was provided to deduce the
contribution of covalent bond formation to the observed activity. However, washout
experiments supported a covalent binding mode and MS experiments confirmed the
predicted labeling of Tyr227. Cellular profiling revealed good selectivity for SRPK1
and SRPK2 and the inhibitor suppressed neovascularization in a choroidal
neovascularization (CNV) model.
3 Summary and Outlook
The field of covalent protein kinase inhibitor research has matured over the last
years. Covalent targeting strategies have enabled the design of chemical probes with
excellent selectivity in the kinome and the benefits of TCIs have successfully been
implemented in drug discovery. Covalent approaches have been used, for example,
to boost potency and break resistance (e.g., with covalent EGFR inhibitors), to
achieve durable target occupancy (e.g., with covalent BTK inhibitors), or to promote
excellent selectivity over closely related enzymes (e.g., with covalent JAK3 or
FGFR4 inhibitors). The recent FDA approval of six irreversible kinase inhibitors
impressively highlights the utility of covalent design strategies for the discovery of
new medicines. Although covalent kinase inhibitors have almost exclusively been
approved for cancer treatment so far, substantial efforts are currently being made to
develop such drugs for non-oncology indications, especially for the treatment of
inflammatory and autoimmune disorders.
In the current practice, covalent kinase targeting largely relies on limited warhead
chemistry. Generally, attenuated Michael acceptors such as α,β-unsaturated amides
are used to address non-catalytic cysteines in a (quasi)-irreversible fashion. However, more diverse warhead types are increasingly receiving attention [9]. Most
notably, covalent-reversible targeting approaches relying, for example, on dually
activated Michael acceptors, but also on other electrophiles such as cyanamides or
aldehydes, are gaining importance as highlighted by the inhibitors PRN1008 and
FGF401 (see Figs. 12b and 24b) which are currently under clinical investigation.
Furthermore, attempts to address other nucleophilic amino acids inside or proximal
to the ATP binding pocket (e.g., Lys and Tyr) or cysteine moieties at allosteric sites
have recently been successful and such approaches are expected to gain traction in
the near future. Although the structure-guided modification of known reversible
inhibitors, which is facilitated by the steeply increasing amount of publicly available
X-ray crystal structures, is still by far the most common strategy for the discovery of
kinase TCIs, alternative concepts such as the screening of reactive fragments or
DNA-encoded libraries show great promise. Methods for estimating the extent of
specific and non-specific labeling by irreversible covalent inhibitors in living
82
M. Gehringer