Targeting Bruton tyrosine kinase using non-covalent inhibitors in B cell malignancies

Abstract

B cell receptor (BCR) signaling is involved in the pathogenesis of B cell malignancies. Activation of BCR signaling promotes the survival and proliferation of malignant B cells. Bruton tyrosine kinase (BTK) is a key component of BCR signaling, establishing BTK as an important therapeutic target. Several covalent BTK inhibitors have shown remarkable efficacy in the treatment of B cell malignancies, especially chronic lymphocytic leukemia. However, acquired resistance to covalent BTK inhibitors is not rare in B cell malignancies. A major mechanism for the acquired resistance is the emergence of BTK cysteine 481 (C481) mutations, which disrupt the binding of covalent BTK inhibitors. Additionally, adverse events due to the off-target inhibition of kinases other than BTK by covalent inhibitors are common. Alternative therapeutic options are needed if acquired resistance or intolerable adverse events occur. Non-covalent BTK inhibitors do not bind to C481, therefore providing a potentially effective option to patients with B cell malignancies, including those who have developed resistance to covalent BTK inhibitors. Preliminary clinical studies have suggested that non-covalent BTK inhibitors are effective and well-tolerated. In this review, we discussed the rationale for the use of non-covalent BTK inhibitors and the preclinical and clinical studies of non-covalent BTK inhibitors in B cell malignancies.

Keywords: B cell malignancies; BTK; C481 mutations; Ibrutinib; Non-covalent inhibitors.

Fig. 1

BCR signaling. The binding of antigens to the B cell receptor leads to the phosphorylation of the intracellular immunoreceptor tyrosine-based activation motifs (ITAMs) of CD79A and CD79B. The phosphorylation of CD79A/CD79B initiates SYK activation, which then results in BTK activation and subsequent PLCG2 activation. This signal cascade ultimately leads to the activation of NF-κB and MAPK/ERK pathways, contributing to the survival and proliferation of CLL cells. BTK and PLCG2 mutations are detected in BTK inhibitor-resistant CLL cases

Fig. 2

BCR in B cell development. In the bone marrow, progenitor B (pro-B) cells undergo the rearrangement and development of immunoglobulin heavy-chain variable (V), diversity (D), and joining (J) gene segments to form the pre-BCR. The pre-BCR is an immature form of the BCR providing signals for survival, proliferation, and cellular differentiation. After light-chain gene rearrangement occurs, immature B cells express BCR, leave the bone marrow, and mature in the periphery. Mature B cells undergo somatic hypermutation (SHM) driven by the expression of activation-induced cytidine deaminase (AID) in the germinal center (GC) to complete BCR affinity maturation and antibody diversification. The genotoxic stress induced by SHM may lead to the apoptosis of these B cells. However, the continuous BCR signaling could provide pro-survival signals for these B cells, thereby preventing them from apoptosis. Therefore, B cells deficient in Bruton tyrosine kinase tend to undergo apoptosis during the development. The B cells that have completed affinity maturation and antibody diversification then undergo class-switch recombination (CSR), after which they develop into memory B cells with high-affinity BCRs or plasma cells secreting antibodies. DZ, dark zone; LZ, light zone; FDC, follicular dendritic cell; Tfh cell, T follicular helper cell

Fig. 3

The structural diagram of Bruton tyrosine kinase (BTK). The BTK protein is a 77 kDa protein of 659 amino acids, which contains five different protein interaction domains. There are two critical tyrosine phosphorylation sites, Y223 in the SH3 domain and Y551 in the kinase domain. BTK inhibitors bind to the BTK kinase domain and blocks the catalytic activity of BTK. Currently available covalent BTK inhibitors, including ibrutinib, acalabrutinib, zanubrutinib, and orelabrutinib, selectively bind to C481 residue in the allosteric inhibitory segment of the BTK kinase domain. The non-covalent BTK inhibitors do not bind to C481. For example, ARQ 531 binds to BTK by forming hydrogen bonds with E475 and Y476 residues [56]. Fenebrutinib forms hydrogen bonds with K430, M477, and D539 residues [50]

Fig. 4

Mechanisms for the action of representative BTK inhibitors. a Chemical structure of the covalent BTK inhibitor ibrutinib. b Chemical structure of the non-covalent BTK inhibitor ARQ 531. c Ibrutinib covalently binds to BTK cysteine 481 (C481), competes with ATP in the ATP binding pocket, and inhibits autophosphorylation of BTK. The action of ibrutinib requires its binding to BTK C481, and BTK C481 mutations abrogate the binding of ibrutinib and lead to the resistance to ibrutinib. d ARQ 531 non-covalently interacts with BTK, occupies the ATP binding pocket, and inhibits BTK autophosphorylation. The effect of ARQ 531 does not require its binding to BTK C481. Therefore, ARQ 531 remains active in patients with BTK C481 mutations. c, d were made by using PyMOL 0.99