Cardio-Oncology: A Win-Win Situation: How Solving the Mystery of an Ibrutinib Off-Target Effect Reveals New Insights Into Atrial Fibrillation Mechanisms

Farrukh T. Awan, MD; Dan Tong, MD, PhD; Vlad G. Zaha, MD, PhD


Circulation. 2020;142(25):2456-2458. 

The development of targeted molecular cancer therapies, such as kinase inhibitors, has revolutionized cancer treatment and improved the prognosis of many malignancies. Whereas these are better tolerated than less selective chemotherapy, the overall outcomes of some molecular therapies are impacted by cardiovascular events ranging from 6% to 20% in clinical trials.[1–4] A cardio-oncological clinical approach to the recognition of specific cardiovascular phenotypes, coupled with mechanistic studies, may not only lead to the development of next-generation molecules with improved onco-specificity but also unveil fundamental cardiac molecular mechanisms.[5]

Ibrutinib, the first irreversible BTK (Bruton tyrosine kinase) inhibitor, has been a tremendous advance in the treatment of various lymphoid malignancies including mantle cell lymphoma and chronic lymphocytic leukemia, improving patient outcomes with a fairly well-tolerated therapeutic option that can be continued indefinitely until disease progression or intolerable toxicity.[6] Similar oncological outcomes have been then reported with newer, more specific, and arguably better tolerated BTK inhibitors, such as acalabrutinib[2,7] and zanubrutinib,[8] which can be used in patients who discontinue ibrutinib because of intolerable adverse events.[9] In addition, second-generation compounds, such as MK-1026 (ARQ531) and Loxo-305, have been developed to overcome resistance to first-generation BTK inhibitors and are currently being evaluated in clinical trials.[6,8]

Relevant cardiovascular risks of ibrutinib therapy emerged in the analysis of large ibrutinib clinical trials that indicated an increase in the incidence of atrial fibrillation (AF), with a particularly increased risk in patients having previous history of AF.[1,3,10] Moreover, ibrutinib has been shown to significantly increase the risk of ventricular arrhythmias, as well as incident or worsening hypertension.[11] Ibrutinib poses a unique challenge in the management of patients with AF because it also results in increased risk of clinically relevant bleeding events. This represents a major management conundrum, because applying the current AF management guidelines for protecting against embolic stroke in these patients may lead to an increased risk of a catastrophic bleed, necessitating development of specific management guidelines.

Multiple studies evaluating various kinases have been performed in an effort to elucidate the mechanism of ibrutinib-induced AF, but they were unable to definitely validate a target. In this issue of Circulation, Xiao et al[12] elegantly chronicle their discovery process of unintended targets of ibrutinib, showing that ibrutinib increases AF risk via an off-target inhibitory effect on CSK (C-terminal Src kinase). Their study established and characterized a murine model of ibrutinib-induced AF and reported increased atrial refractory periods, atrial fibrosis, and dilation in an inflammatory milieu, with a preserved left ventricular ejection fraction. In an effort to identify ibrutinib-specific targets in cardiac myocytes, they evaluated the impact of ibrutinib in the induction of atrial fibrillation in BTKxid mice that harbor a mutant and nonfunctional BTK and demonstrated persistence of the AF phenotype consistent with a lack of involvement of BTK in its pathophysiology.

They further used chemoproteomic analysis to evaluate specific kinases inhibited by ibrutinib and, by using serial in vivo inhibitor studies, established CSK as the leading ibrutinib-induced arrhythmogenic target. Conditional, cardiomyocyte-specific CSK knockout mice recapitulated a similar inflammatory infiltrate in the cardiac myocytes and left atrial enlargement, with preserved ventricular function. They also demonstrated a cytokine profile similar to that seen in ibrutinib-treated mice and an overlapping gene expression pattern suggesting mechanistic similarities between CSK loss and ibrutinib use. Importantly, the AF outcome was not observed with acalabrutinib in these murine models. Furthermore, a large international, pharmacovigilance database was queried to evaluate and confirm the increased incidence of AF in patients treated with kinase inhibitors with nonspecific CSK targeting.

AF continues to be the most common sustained clinical arrhythmia, with a complex and incompletely understood molecular pathophysiology. Focal ectopic activity and electric reentry are commonly considered as the 2 major determinants of AF initiation and maintenance. Abnormalities in calcium handling, including leaky sarcoplasmic reticulum and increased NCX (Na+/Ca2+ transmembrane exchanger) currents, lead to delayed after-depolarizations and trigger ectopic activities. Electric and structural atrial remodeling provide a vulnerable substrate for reentry activities, and the remodeling process has been shown to be influenced at the molecular level by stress signaling kinase pathways[13] and CaMKII (calmodulin-dependent protein kinase II).[14]

As with many great studies, the study by Xiao et al also opens further questions. How does CSK suppression lead to AF? As pointed out by the authors, in tumor cells, CSK suppresses the activities of SFKs (Src family tyrosine kinases) by phosphorylating residues on the C-terminal end. SFKs play multiple critical roles in a wide variety of cells and tissues. However, our understanding of the role CSK/SFKs play in cardiomyocytes is quite limited. Does CSK suppression lead to AF because of loss of inhibition on SFKs? Does the CSK/SFK signaling pathway play a general role in AF? Based on the atrial phenotype and RNAseq data obtained from ibrutinib-treated as well as CSK knockout mice, increased inflammatory signaling may be the culprit. Previous studies have shown that enhanced activity of the NLRP3 inflammasome plays a critical role in AF pathogenesis.[15] Whether CSK/SFK signaling triggers inflammasome activation in the atria awaits further investigation.

Excessive CaMKII activation is associated with changes in atrial membrane excitability, calcium mishandling, atrial remodeling, and inflammation. It is interesting that increased CaMKII phosphorylation was also observed in the atria of ibrutinib-treated mice. Is there an interaction between the CaMKII pathway and CSK/SFK signaling? Furthermore, although no significant electric remodeling was observed in ibrutinib-treated mice, previous studies have reported association between CSK/SFK signaling and potassium channel activity, as well as gap junction protein expression and function, both of which play critical roles in AF pathogenesis. Therefore, further characterization of the CSK/SFKs cascade in atrial electric remodeling is warranted (Figure).


Molecular signaling pathways engaged by ibrutinib in lymphoid tumor cells vs cardiomyocytes and putative connections of CSK (C-terminal Src kinase) with molecular substrates involved in initiation and maintenance atrial fibrillation.
CaMKII indicates calcium/calmodulin-dependent protein kinase II; ICa, calcium channels; INa, sodium channels; NCX, sodium-calcium exchanger; RYR, ryanodine receptor; and SR, sarcoplasmic reticulum.

This report serves as a major advance in the understanding of the pathophysiology of AF related to ibrutinib. It not only details the cardiac structural derangements observed with the use of ibrutinib but also the inflammatory cytokine alterations and real-world pharmacovigilance data to establish CSK targeting as a valid mechanism of ibrutinib-induced AF. Interestingly, ibrutinib treatment was also shown to increase systolic and diastolic blood pressures in mice treated with ibrutinib, hence suggesting a possible mechanism of non–AF-related cardiac adverse events. Whether a similar outcome was observed in CSK knockout mice is uncertain from the report and requires further investigation. Moreover, this report was not designed to provide mechanistic explanations about the higher incidence of AF in patients with a previous history of AF. It is unclear whether an underlying cardiac injury or predisposition somehow primes the kinome profile to promote these outcomes. Importantly, the lack of observed ventricular dysfunction and ventricular tachyarrhythmias also raises important questions about potential alternate pathways being active in ventricular tachycardia, fibrillation, and sudden cardiac death events, all of which are shown to occur at increased frequency, especially in older patients treated with ibrutinib. Additional studies are therefore essential in detailing the metabolic, electric, and inflammatory pathway cross-talk that will explain the entire scope of cardiovascular toxicity observed with ibrutinib use. An improved understanding will allow rational discovery of agents that are more selective toward the malignant target without the off-target toxicities observed in first-generation compounds. This study is another great example of conceptual bridges in cardio-oncology, of how investigation at cellular and molecular levels in targeted cancer therapy can reveal fundamental cardiac biological paradigms.