The Evolution of Therapies Targeting Bruton Tyrosine Kinase for the Treatment of Chronic Lymphocytic Leukaemia: Future Perspectives

SIMPLE SUMMARY: The development of drugs that successfully target Bruton tyrosine kinase (BTK) represents a major scientific and clinical advance for the treatment of chronic lymphocytic leukaemia (CLL). This article will begin by reviewing the scientific observations that underpinned the targeting of this kinase in CLL. It will then discuss the evolution of BTK inhibitors, from the initial studies with ibrutinib, to the development of more specific and noncovalently binding BTK inhibitors, focusing on how different agents can be sequenced in patients who are resistant or intolerant to one of these drugs. Finally, this article will also review the concept of BTK degraders and offer insights into the future direction of the field. ABSTRACT: The development of inhibitors of Bruton tyrosine kinase (BTK) and B-cell lymphoma 2 (BCL2) has resulted in a paradigm shift in the treatment of chronic lymphocytic leukaemia (CLL) over the last decade. Observations regarding the importance of B-cell receptor signalling for the survival and proliferation of CLL cells led to the development of the first-in-class BTK inhibitor (BTKi), ibrutinib, for the treatment of CLL. Despite being better tolerated than chemoimmunotherapy, ibrutinib does have side effects, some of which are due to the off-target inhibition of kinases other than BTK. As a result, more specific inhibitors of BTK were developed, such as acalabrutinib and zanubrutinib, which have demonstrated equivalent/enhanced efficacy and improved tolerability in large randomized clinical trials. Despite the increased specificity for BTK, side effects and treatment resistance remain therapeutic challenges. As these drugs all bind covalently to BTK, an alternative approach was to develop noncovalent inhibitors of BTK, including pirtobrutinib and nemtabrutinib. The alternative mechanisms of BTK-binding of these agents has the potential to overcome resistance mutations, something that has been borne out in early clinical trial data. A further step in the clinical development of BTK inhibition has been the introduction of BTK degraders, which remove BTK by ubiquitination and proteasomal degradation, in marked contrast to BTK inhibition. This article will review the evolution of BTK inhibition for CLL and offer future perspectives on the sequencing of an increasing number of different agents, and how this may be impacted on by mutations in BTK itself and other kinases.