Ibrutinib (Imbruvica™) is an irreversible, potent inhibitor of Bruton's tyrosine kinase (BTK). Over the last few years, ibrutinib has developed from a promising drug candidate to being approved by FDA for the treatment of three B cell malignancies, a truly remarkable feat. Few, if any medicines are monospecific and ibrutinib is no exception; already during ibrutinib's initial characterization, it was found that it could bind also to other kinases. In this review, we discuss the implications of such interactions, which go beyond the selective effect on BTK in B cell malignancies. In certain cases, the outcome of ibrutinib treatment likely results from the combined inhibition of BTK and other kinases, causing additive or synergistic, effects. Conversely, there are also examples when the clinical outcome seems unrelated to inhibition of BTK. Thus, more specifically, adverse effects such as enhanced bleeding or arrhythmias could potentially be explained by different interactions. We also predict that during long‐term treatment bone homoeostasis might be affected due to the inhibition of osteoclasts. Moreover, the binding of ibrutinib to molecular targets other than BTK or effects on cells other than B cell‐derived malignancies could be beneficial and result in new indications for clinical applications.

BTK and ITK are cytoplasmic tyrosine kinases of crucial importance for B and T cell development, with loss-of-function mutations causing X-linked agammaglobulinemia and susceptibility to severe, frequently lethal, Epstein-Barr virus infection, respectively. Over the last few years, considerable efforts have been made in order to develop small-molecule inhibitors for these kinases to treat lymphocyte malignancies, autoimmunity or allergy/hypersensitivity. The rationale is that even if complete lack of BTK or ITK during development causes severe immunodeficiency, inactivation after birth may result in a less severe phenotype. Moreover, therapy can be transient or only partially block the activity of BTK or ITK. Furthermore, a drug-induced B cell deficiency is treatable by gamma globulin substitution therapy. The newly developed BTK inhibitor PCI-32765, recently renamed Ibrutinib, has already entered several clinical trials for various forms of non-Hodgkin lymphoma as well as for multiple myeloma. Experimental animal studies have demonstrated highly promising treatment effects also in autoimmunity. ITK inhibitors are still under the early developmental phase, but it can be expected that such drugs will also become very useful. In this study, we present BTK and ITK with their signalling pathways and review the development of the corresponding inhibitors.

Irreversible Bruton tyrosine kinase (BTK) inhibitors, ibrutinib and acalabrutinib have demonstrated remarkable clinical responses in multiple B-cell malignancies. Acquired resistance has been identified in a sub-population of patients in which mutations affecting BTK predominantly substitute cysteine 481 in the kinase domain for catalytically active serine, thereby ablating covalent binding of inhibitors. Activating substitutions in the BTK substrate phospholipase Cγ2 (PLCγ2) instead confers resistance independent of BTK. Herein, we generated all six possible amino acid substitutions due to single nucleotide alterations for the cysteine 481 codon, in addition to threonine, requiring two nucleotide substitutions, and performed functional analysis. Replacement by arginine, phenylalanine, tryptophan or tyrosine completely inactivated the catalytic activity, whereas substitution with glycine caused severe impairment. BTK with threonine replacement was catalytically active, similar to substitution with serine. We identify three potential ibrutinib resistance scenarios for cysteine 481 replacement: (1) Serine, being catalytically active and therefore predominating among patients. (2) Threonine, also being catalytically active, but predicted to be scarce, because two nucleotide changes are needed. (3) As BTK variants replaced with other residues are catalytically inactive, they presumably need compensatory mutations, therefore being very scarce. Glycine and tryptophan variants were not yet reported but likely also provide resistance.