Ibrutinib’s off-target mechanism: cause for dose optimization

ABSTRACT

Ibrutinib (Imbruvica®, 2013) is a Bruton’s tyrosine kinase (BTK) inhibitor approved for multiple B-cell malignancies and cGVHD. Its treatment is associated with increased risk of cardiac adverse events. Atrial fibrillation is a common cause of therapy discontinuation and interruptions, which have been correlated with shorter progression-free survival in chronic lymphocyte leukemia (CLL) patients. Recently, Xiao et al. identified that ibrutinib-mediated atrial fibrillation is likely due to off-target CSK inhibition. Given promising in vitro and in vivo evidence of maintained biological activity in CLL at lower-than-labeled ibrutinib doses, this elucidated mechanism substantiates the case to investigate alternative dosing schedules. The potential to minimize ibrutinib’s off-target effects while conserving response warrants further discussion and investigation of optimal ibrutinib dosing.

Introduction

Ibrutinib is an FDA-approved (Imbruvica®, 2013) Bruton’s tyrosine kinase (BTK) inhibitor indicated for treatment of chronic lymphocytic leukemia (CLL)/small lymphocytic lymphoma (SLL), Waldenström’s macroglobulinemia (WM), previously treated mantle cell lymphoma (MCL), previously treated marginal zone lymphoma (MZL), and previously treated chronic graft-versus-host disease (cGVHD).^1–5 The approved ibrutinib dose is 420 mg daily (QD) in CLL/SLL, WM, and cGVHD and 560 mg QD in MCL and MZL.⁶ The FDA deemed safety profiles from early registration trials of MCL⁴ and CLL⁷ at these doses acceptable.⁸ However, approval was granted with the acknowledgment of a limited safety database and a recommendation to investigate lower effective doses in future clinical development.⁸ FDA approval for cGVHD, MZL, and WM followed, given acceptable safety data in early trials.^5,9,10

Although ibrutinib has demonstrated impressive efficacy across these indications^2–5 and initially no major safety concerns, retrospective toxicity studies indicate that its current treatment schedule is associated with an increased risk of cardiac adverse events (AEs).^11–13 As of 2019, 303 sudden death cases due to cardiac events^11,12 had occurred in ibrutinib-treated patients, as well as high incidence of supraventricular arrhythmias, central nervous system hemorrhagic events, heart failure, and conduction disorders.¹³ Of these AEs, a primary concern is atrial fibrillation^13–16 with a pooled incidence rate of 3.3 per 100 person-years compared to 0.84 per 100 person-years among non-ibrutinib treated patients.¹³

Ibrutinib-associated atrial fibrillation has been reported as a common cause of therapy discontinuation and interruption,¹⁴ and dose interruption has been associated with worse outcomes in CLL patients.¹⁷ Barr et al. retrospectively analyzed the clinical impact of dose adherence in the phase 3 RESONATE trial of CLL/SLL patients who received 420 mg QD.¹⁷ Higher dose intensity, defined as patients closely adhering to the treatment regimen, correlated with longer progression-free survival (PFS)¹⁷ compared to patients who required a prolonged discontinuation or maintained lower dose intensity. Despite substantial retrospective association between ibrutinib treatment and atrial fibrillation risk, previously no known mechanism linked the two.

Recently, Xiao et al. identified a mechanism by which ibrutinib mediates atrial fibrillation: C-terminal Src kinase (CSK) inhibition.¹⁸ This paper will summarize and contextualize findings of Xiao et al. with respect to the concerning rates of cardiotoxicity in patients receiving ibrutinib and the need to assess clinical efficacy with alternative dosing regimens. The opportunity to improve ibrutinib’s therapeutic range has previously been asserted.¹⁹ The authors call for FDA post-marketing regulation through a randomized dose-ranging study of ibrutinib,¹⁹ and findings of Xiao et al. outlined herein support this proposition.

Ibrutinib-mediated atrial fibrillation via CSK inhibition

Xiao et al. first established a murine model of ibrutinib-mediated atrial fibrillation with daily injections of 25 mg/kg for 4 weeks. Atrial fibrillation was more easily induced in mice treated with ibrutinib compared to the vehicle. Histological imaging and increased expression of fibrotic and inflammatory markers in ibrutinib-treated mice further indicated atrial damage.

Ibrutinib potently inhibits multiple kinases,²⁰ therefore Xiao et al. investigated whether ibrutinib mediated its effects via BTK or an off-target kinase. An ibrutinib-treated mouse model lacking BTK function demonstrated increased atrial fibrillation inducibility, suggesting atrial fibrillation was induced by an off-target kinase. Chemoproteomic profiling in cardiac tissue, via isolation of biotinylated kinases in the presence and absence of ibrutinib, identified five kinase targets of ibrutinib. Additionally, mice were treated with a second-generation BTK inhibitor, acalabrutinib, which has no recorded association with atrial fibrillation. Identification of two of these five cardiac kinases as specific targets of acalabrutinib narrowed atrial fibrillation-related candidates to three: FYN, MEK5, and CSK. Mice treated independently with FYN and MEK5 inhibitors for 4 weeks showed no increase in AF inducibility, isolating CSK as the most likely relevant target.

CSK is a nonreceptor protein kinase that inactivates Src family kinases through phosphorylation of lysine residues on the C-terminus. Xiao et al. established a cardiac-specific CSK knockout model that developed increased atrial fibrillation inducibility. Histological studies showing cardiac fibrosis, monocyte infiltration into the atria, and increased concentration of circulating inflammatory biomarkers further indicate that inactivation of CSK mediates atrial fibrillation. These data suggest that CSK is the kinase target for ibrutinib-mediated atrial fibrillation.

To reinforce the likelihood that CSK inhibition is linked to atrial fibrillation events in ibrutinib-treated patients, Xiao et al. performed a disproportionality analysis including five FDA-approved anticancer CSK inhibitors and 11 weak/noninhibiting agents. Atrial fibrillation reporting rates of each drug were compared to all other drugs in VigiBase, a pharmacovigilance database. C_max/IC₅₀ values were significantly associated with atrial fibrillation (p = .003). Furthermore, comparison of the reporting rate of atrial fibrillation among CSK inhibitors (C_max>IC₅₀) versus non-inhibitors (C_max<IC₅₀) resulted in a significant reporting odds ratio (p < .0001). This analysis remained significant (p < .0001) even after removal of ibrutinib from the CSK inhibitor group.

Findings of Xiao et al. indicate that ibrutinib-mediated atrial fibrillation is very likely due to CSK inhibition. Murine knockout models and chemoproteic profiling experiments identify CSK as the likely target and disproportionality analyses corroborate these findings by demonstrating the significance between CSK inhibition and reported atrial fibrillation in patient cases. Determination of this direct mechanistic link reinforces the case to reassess ibrutinib’s labeled dose.

Ibrutinib dose optimization

Concerning rates of cardiotoxicities coupled with an established mechanistic association between AF and ibrutinib treatment warrant a discussion as to whether the current dosing schedule is optimal. Early in vitro and clinical data in CLL patients suggest lower doses of ibrutinib could likely maintain efficacy. A phase 1b/2 trial of ibrutinib’s efficacy in CLL demonstrated that BTK receptors reached full occupancy by ≥ 2.5 mg/kg.²¹ The proposed dose of 420 mg QD equates to a mean dose of 7.22 mg/kg per day,²¹ 2.4-fold higher. CLL patients achieved maximal overall response rate (ORR) at doses ≥ 2.5 mg/kg and an increasing steady trough concentration did not affect the proportion of ORR.²¹ Furthermore, flat exposure (AUC) and dose-response relationships with efficacy across a range of doses (140–560 mg QD) were observed.²¹ Notably, based on these findings, the FDA recommended evaluation of lower doses in future clinical development with their initial approval.⁸

Additionally, total BTK expression has shown a significant time-dependent decline at 2, 4, and 12 weeks of ibrutinib treatment in CLL patients compared to baseline.²² Because ibrutinib’s mechanism of action is irreversible covalent inhibition of BTK, the stoichiometry of ibrutinib to BTK receptors may inform optimal dosing. A decrease in receptor concentration following a few weeks of therapy suggests a lower dose of ibrutinib may sufficiently saturate receptors.

Given comparable in vitro and clinical efficacy data at lower doses in CLL patients,²⁰ as well as a time-dependent decrease of BTK expression,²² Chen et al. conducted a pilot study to identify the optimal ibrutinib dose using intra-patient stepwise dose reductions in CLL patients following the first cycle at full dose (420 mg QD).²³ Ibrutinib biological activity was maintained with dose reductions to 280 mg QD and 140 mg QD in subsequent cycles.²³ BTK occupancy remained 99% at each dose level, and no difference was seen in BTK signaling markers. More comprehensive studies are needed to assess clinical endpoints; however, these findings suggest the potential for dose reduction in CLL patients, which may be applicable in other indications.

Currently, a clinical trial (NCT04771507) is recruiting CLL patients to assess the safety and efficacy of an “ON-OFF” ibrutinib dosing strategy.²⁴ Investigators cite economic burden of long-term ibrutinib treatment, evidence of stable partial remission after discontinuation of ibrutinib, and emerging data on BTK mutations leading to loss of ibrutinib efficacy as justification for this study.²⁴ Findings from this trial have the potential to lower drug costs and risk of ibrutinib-mediated side effects by reducing cumulative dose.

Conclusion

Retrospective toxicity studies have identified concerning rates of cardiotoxicities in ibrutinib-treated patients. Atrial fibrillation has been reported as the most common cause of therapy discontinuation, and discontinuation has been associated with shorter PFS in CLL patients. Efficacy, as assessed by receptor occupancy, signaling markers, and ORR, has been achieved at lower-than-approved doses in CLL, and a similar outcome may occur in other indications. BTK receptor expression decreases in a time-dependent manner in CLL patients receiving 420 mg QD and intra-patient dose reductions following a full-dose cycle maintained biological activity.

The recent identification by Xiao et al. of the ibrutinib-mediated AF mechanism involving CSK inhibition should re-ignite this dose optimization debate. With potential to minimize off-target toxicities while conserving response, further dose reduction studies to assess clinical end points are warranted. While ibrutinib has demonstrated excellent efficacy, a clear mechanistic link to atrial fibrillation, a side effect that has led to severe and sometimes fatal cardiac events, necessitates a broader discussion of optimal ibrutinib dosing in oncology clinics.

Funding Statement

This study was paid for by grant [ZIC SC 006537] through the National Institutes of Health.

Disclosure of potential conflicts of interest

No potential conflict of interest was reported by the author(s).