reactivities compared to analogous anilides [74] which might translate into
decreased promiscuity. Compound 5-rac was effective in an arthritis mouse model
and the more potent R-enantiomer now known as ibrutinib (5, see Fig. 5) was further
developed by Pharmacyclics and Johnson & Johnson. Interestingly, ibrutinib was
later shown to potently inhibit most other kinases with an equivalently positioned
cysteine [45].
Acalabrutinib (6, see Fig. 5), an approved second-generation covalent BTK
inhibitor, features a related imidazo[1,5-a]pyrazine core [45]. In this case, a but-2ynamide warhead linked via the nitrogen atom of an S-configured pyrrolidin-2-yl
substituent was employed. While propiolamides are more reactive than analogous
acrylamides, but-2-ynamides are slightly less reactive [74] making acalabrutinib
more stable toward GSH when compared to ibrutinib (t 1/2 ¼ 5.5 h vs. 1.9 h).
Acalabrutinib (IC 50
BTK
¼ 5.1 nM, k inact /K I ¼ 3.1 Â 10
4 M
À1 s
À1 ) was shown to
be slightly less potent than ibrutinib (IC 50
BTK
¼ 1.5 nM, k inact /K I ¼ 4.8 Â 10
5 M
À1 s
À1 )
but relatively selective against most kinases harboring an F2 cysteine with only
ErbB4 (IC 50 ¼ 16 nM), BMX (IC 50 ¼ 46 nM), and TEC (IC 50 ¼ 126 nM) being
significantly hit (MKK7 was not tested). The compound features a clean kinome
profile, oral availability, and durable target engagement in vivo (99% after 4 and
12 h, 100 mg p.o.) although being rapidly eliminated thus highlighting the disconnection between PD and PK, a key feature of many irreversible inhibitors.
Acalabrutinib showed a tolerable adverse effect profile and gained accelerated
approval by the FDA as second-line therapy for mantle cell lymphoma (MLL)
[75]. Several phase III studies for the treatment of chronic lymphocytic leukemia
(CLL) are ongoing. However, BTK C481S mutation renders both ibrutinib and
acalabrutinib ineffective at the recommended dosage since the decreased potency
in combination with the relatively fast clearance of these compounds precludes
sustained reversible target engagement [76].
Besides the two aforementioned drugs, many other irreversible BTK inhibitors
with distinct chemotypes and good or excellent selectivity profiles (e.g., CHMFLBTK-11 [77], branebrutinib [78], spebrutinib [45], or poseltinib [79], 22–25,
Fig. 12a) have been investigated in preclinical and clinical studies. A covalentreversible approach was chosen by researchers from the Taunton group and Principia
Biopharma [11], who generated α-cyanoacrylamide derivatives of ibrutinib to
engage BTK Cys481. Target residence times could be modulated as a function of
the β-substituent. For example, compound 26a (Fig. 12b) bearing a bulky tert-butyl
group in the β-position retained >50% cellular target occupancy 20 h after washout.
In contrast, occupancy of methyl-capped analog 26b was negligible under the same
conditions. The extended target residence time of compound 26a may be rationalized by additional thermodynamic stabilization of the covalent complex via hydrophobic interactions with the bulky tert-butyl moiety, which simultaneously induces a
conformation with decreased α-CH-acidity while also shielding the Cα-proton from
water thereby impeding its abstraction (see PDB: 4YHF). Further optimization with
special emphasis on improving solubility and PK properties furnished a compound
series exemplified by 27a and 27b. While 27a (IC 50 ¼ 0.7 nM, k inact /
K I ¼ 1.9 Â 10
3 M
À1 s
À1 ) had a residence time of 34 h, key compound 27b
(IC 50 ¼ 1.9 nM, k inact /K I ¼ 4.3 Â 10
2 M
À1 s
À1 ) behaved quasi-irreversible with a
60
M. Gehringer
decreased promiscuity. Compound 5-rac was effective in an arthritis mouse model
and the more potent R-enantiomer now known as ibrutinib (5, see Fig. 5) was further
developed by Pharmacyclics and Johnson & Johnson. Interestingly, ibrutinib was
later shown to potently inhibit most other kinases with an equivalently positioned
cysteine [45].
Acalabrutinib (6, see Fig. 5), an approved second-generation covalent BTK
inhibitor, features a related imidazo[1,5-a]pyrazine core [45]. In this case, a but-2ynamide warhead linked via the nitrogen atom of an S-configured pyrrolidin-2-yl
substituent was employed. While propiolamides are more reactive than analogous
acrylamides, but-2-ynamides are slightly less reactive [74] making acalabrutinib
more stable toward GSH when compared to ibrutinib (t 1/2 ¼ 5.5 h vs. 1.9 h).
Acalabrutinib (IC 50
BTK
¼ 5.1 nM, k inact /K I ¼ 3.1 Â 10
4 M
À1 s
À1 ) was shown to
be slightly less potent than ibrutinib (IC 50
BTK
¼ 1.5 nM, k inact /K I ¼ 4.8 Â 10
5 M
À1 s
À1 )
but relatively selective against most kinases harboring an F2 cysteine with only
ErbB4 (IC 50 ¼ 16 nM), BMX (IC 50 ¼ 46 nM), and TEC (IC 50 ¼ 126 nM) being
significantly hit (MKK7 was not tested). The compound features a clean kinome
profile, oral availability, and durable target engagement in vivo (99% after 4 and
12 h, 100 mg p.o.) although being rapidly eliminated thus highlighting the disconnection between PD and PK, a key feature of many irreversible inhibitors.
Acalabrutinib showed a tolerable adverse effect profile and gained accelerated
approval by the FDA as second-line therapy for mantle cell lymphoma (MLL)
[75]. Several phase III studies for the treatment of chronic lymphocytic leukemia
(CLL) are ongoing. However, BTK C481S mutation renders both ibrutinib and
acalabrutinib ineffective at the recommended dosage since the decreased potency
in combination with the relatively fast clearance of these compounds precludes
sustained reversible target engagement [76].
Besides the two aforementioned drugs, many other irreversible BTK inhibitors
with distinct chemotypes and good or excellent selectivity profiles (e.g., CHMFLBTK-11 [77], branebrutinib [78], spebrutinib [45], or poseltinib [79], 22–25,
Fig. 12a) have been investigated in preclinical and clinical studies. A covalentreversible approach was chosen by researchers from the Taunton group and Principia
Biopharma [11], who generated α-cyanoacrylamide derivatives of ibrutinib to
engage BTK Cys481. Target residence times could be modulated as a function of
the β-substituent. For example, compound 26a (Fig. 12b) bearing a bulky tert-butyl
group in the β-position retained >50% cellular target occupancy 20 h after washout.
In contrast, occupancy of methyl-capped analog 26b was negligible under the same
conditions. The extended target residence time of compound 26a may be rationalized by additional thermodynamic stabilization of the covalent complex via hydrophobic interactions with the bulky tert-butyl moiety, which simultaneously induces a
conformation with decreased α-CH-acidity while also shielding the Cα-proton from
water thereby impeding its abstraction (see PDB: 4YHF). Further optimization with
special emphasis on improving solubility and PK properties furnished a compound
series exemplified by 27a and 27b. While 27a (IC 50 ¼ 0.7 nM, k inact /
K I ¼ 1.9 Â 10
3 M
À1 s
À1 ) had a residence time of 34 h, key compound 27b
(IC 50 ¼ 1.9 nM, k inact /K I ¼ 4.3 Â 10
2 M
À1 s
À1 ) behaved quasi-irreversible with a
60
M. Gehringer