kinome but a relatively low degree of conservation in the so-called cysteinome (i.e.,
the entire set of cysteines present in the kinome) [7]. In addition, irreversible
covalent binding eliminates the competition with natural ligands, substrates or
co-factors (i.e., ATP in the case of kinases) thereby increasing overall efficacy
[7]. Given the low millimolar ATP concentrations in cells, the lack of competitivity
after covalent bond formation is a key advantage. Due to their time-dependent
binding behavior, efficient irreversible inhibitors can achieve full target occupancy
even at very low concentrations, provided that the exposure time is long enough.
Thus, lower doses may be sufficient to achieve equivalent therapeutic efficacy
compared to reversibly binding drugs. At the same time, the target protein remains
inhibited until its function is restored by de novo synthesis. If the re-synthesis rate is
not too high, persistent target engagement leads to a decoupling of pharmacokinetics
(PK) from pharmacodynamics (PD) which can translate into prolonged dosage
intervals even for high-clearance compounds and a lower side effect burden due to
a decreased overall exposure [2]. Since covalent bond formation can be used as a
powerful promoter of potency and selectivity, it can also enable the reduction of
molecule size and lipophilicity. Thereby, molecular obesity [10] may be prevented,
and physicochemical properties be improved.
Covalent-reversible targeting strategies can be a viable alternative when off-target
labeling, GSH-mediated clearance, or haptenization is an issue, when sustained
target engagement causes mechanism-based side effects, but also when high turnover targets need to be addressed. Ideally, the unmodified inhibitor dissociates after
protein degradation to be “recycled” by engaging with a newly translated target
protein. Moreover, this approach offers the potential to benefit from advantages of
irreversible covalent inhibitors (e.g., prolonged target occupancy, increased potency,
and selectivity) at a decreased risk of drug safety issues [11]. Remarkably, the target
residence times of covalent-reversible inhibitors can be fine-tuned by both warhead
chemistry and stabilization of the complex via non-covalent interactions, thus
providing tailor-made solutions for the desired application. However, although
these features hold the promise of enabling the design of better and safer
kinase-targeted drugs, no covalent-reversible kinase inhibitors have been approved
so far and critical evaluation of the benefit–risk balance in comparison to traditional
non-reactive ligands will still be necessary.
As mentioned above, irreversible covalent inhibition is a non-equilibrium process. Due to its time-dependent nature, it can be accurately described neither by the
equilibrium dissociation constant K i (¼ k off /k on ) nor by IC 50 values [1]. TCI binding
usually involves two steps (Fig. 1). In the first step, the inhibitor reversibly binds the
target while covalent bond formation takes place in the second step. Importantly,
only the first of these two steps is ATP-competitive. For irreversible covalent
binders, the (reversible) binding affinity is described by the constant K I being the
inhibitor concentration required to achieve the half-maximal rate of covalent inactivation (¼ k inact /2). It should be noted that K I does not equal K i although these
values converge for k inact << k off . The first-order rate constant k inact defines the
maximal potential rate of covalent inactivation, i.e., the rate of covalent bond
formation at full occupancy with the reversibly bound ligand. Accordingly, k inact
represents a measure for the efficiency of the covalent inactivation step. k inact
Covalent Kinase Inhibitors: An Overview
45
the entire set of cysteines present in the kinome) [7]. In addition, irreversible
covalent binding eliminates the competition with natural ligands, substrates or
co-factors (i.e., ATP in the case of kinases) thereby increasing overall efficacy
[7]. Given the low millimolar ATP concentrations in cells, the lack of competitivity
after covalent bond formation is a key advantage. Due to their time-dependent
binding behavior, efficient irreversible inhibitors can achieve full target occupancy
even at very low concentrations, provided that the exposure time is long enough.
Thus, lower doses may be sufficient to achieve equivalent therapeutic efficacy
compared to reversibly binding drugs. At the same time, the target protein remains
inhibited until its function is restored by de novo synthesis. If the re-synthesis rate is
not too high, persistent target engagement leads to a decoupling of pharmacokinetics
(PK) from pharmacodynamics (PD) which can translate into prolonged dosage
intervals even for high-clearance compounds and a lower side effect burden due to
a decreased overall exposure [2]. Since covalent bond formation can be used as a
powerful promoter of potency and selectivity, it can also enable the reduction of
molecule size and lipophilicity. Thereby, molecular obesity [10] may be prevented,
and physicochemical properties be improved.
Covalent-reversible targeting strategies can be a viable alternative when off-target
labeling, GSH-mediated clearance, or haptenization is an issue, when sustained
target engagement causes mechanism-based side effects, but also when high turnover targets need to be addressed. Ideally, the unmodified inhibitor dissociates after
protein degradation to be “recycled” by engaging with a newly translated target
protein. Moreover, this approach offers the potential to benefit from advantages of
irreversible covalent inhibitors (e.g., prolonged target occupancy, increased potency,
and selectivity) at a decreased risk of drug safety issues [11]. Remarkably, the target
residence times of covalent-reversible inhibitors can be fine-tuned by both warhead
chemistry and stabilization of the complex via non-covalent interactions, thus
providing tailor-made solutions for the desired application. However, although
these features hold the promise of enabling the design of better and safer
kinase-targeted drugs, no covalent-reversible kinase inhibitors have been approved
so far and critical evaluation of the benefit–risk balance in comparison to traditional
non-reactive ligands will still be necessary.
As mentioned above, irreversible covalent inhibition is a non-equilibrium process. Due to its time-dependent nature, it can be accurately described neither by the
equilibrium dissociation constant K i (¼ k off /k on ) nor by IC 50 values [1]. TCI binding
usually involves two steps (Fig. 1). In the first step, the inhibitor reversibly binds the
target while covalent bond formation takes place in the second step. Importantly,
only the first of these two steps is ATP-competitive. For irreversible covalent
binders, the (reversible) binding affinity is described by the constant K I being the
inhibitor concentration required to achieve the half-maximal rate of covalent inactivation (¼ k inact /2). It should be noted that K I does not equal K i although these
values converge for k inact << k off . The first-order rate constant k inact defines the
maximal potential rate of covalent inactivation, i.e., the rate of covalent bond
formation at full occupancy with the reversibly bound ligand. Accordingly, k inact
represents a measure for the efficiency of the covalent inactivation step. k inact
Covalent Kinase Inhibitors: An Overview
45