Interest of Dosing of Ibrutinib in B-Cell Lymphoid Malignancies: Data from a Real-Life, Phase 4 Study
Authors: Calleja Anne , De Barros Sandra , Vinson Camille , Protin Caroline , Oberic Lucie , Gallais Fanny , White-koning Melanie , Chatelut Etienne , Recher Christian , Cadot Sarah , Quillet Mary Anne , Ysebaert Loic , Despas Fabien 
Published: October 2019

Abstract
Introduction: Therapeutic drug monitoring (TDM) entails the measurement of drug concentrations and the individualization of drug dosages or schedules to maximize therapeutic effects and minimize toxicity. Ibrutinib (IBR), the first-in-class inhibitor of BTK (Bruton tyrosine kinase) is approved for the therapy of relapsed/refractory chronic lymphocytic leukemia (R/R CLL), mantle cell lymphoma (MCL) and Waldenström's disease (WM), at the dose of 420-560mg/d.
Drug-drug interactions (DDI), older age, liver diseases have been reported to impact PK parameters of ibrutinib, but dosing is not yet part of clinical practice, despite TDM of imatinib and other kinase inhibitors is routinely used in chronic myeloid leukemia.
In this study, we sought to determine whether TDM of ibrutinib should be proposed for patients, in a preliminary cohort of 73 patients included in the PK-e3i trial (NCT02824159).
Methods: Serial plasma PK samples were collected at steady-state after one month of therapy in 73 patients: before intake (residual concentration), and then at time 0.5-1-2-4-6h. Key PK parameters for ibrutinib and its metabolite DHD-ibrutinib were calculated: Cmax, Cmin, tmax,AUC24h. Analysis of DDI was made by an oncology pharmacist in 49/73 patients. Treatment-related adverse events were monitored by phonecalls given by an oncology nurse (AMA procedure) and during consultations with hematologist (at least twice a month the first 6 months, then monthly until 12 months, then every 3 months), and severity graded according CTCAE version 4 scale. Efficacy of therapy in CLL patients was assessed with an "effect marker" to demonstrate biological efficacy, the redistribution hyperlymphocytosis seen in 70% of patients the first month of therapy.
Results: we reported very similar PK results for ibrutinib as compared to pivotal phase 1 trials in CLL and other B-cell lymphoid malignancies (Advani RH, J Clin Oncol 2013, Byrd JC, New Engl J Med 2013). Mean peak plasma concentrations were observed 1-2h after dosing, Cmax and AUC results showing an important inter-patient heterogeneity. Median Cmax was 150ng/ml (8.2-596ng/ml), and median AUC24h was 412.4 ng h/mL ((32.2-2906 ng h/mL). According to published phase I trials, complete or near complete BTK occupancy was observed in patients with AUCs exceeding 160 ng h/mL, suggesting ibrutinib dose might have been supramaximal in 67 of our patients. Adverse events the first 3 months were seen in 96% (grade 1), 63% (grade 2), 25% (grade 3) and 6.5% (grade 4), respectively. We plotted AUC results for the total cohort of 73 patients (Figure 1), and for 49 patients with adverse events monitoring available at 3 months (9/49 needed drug dose reduction due to toxicity) (Figure 2), both emphasizing the absence of correlations between AUC levels and toxicities. We next splited up toxicities into 10 sub-groups (bleeding, cardiac, liver, muscle, joint, skin, infection, gastro-intestinal, hematologic, neurologic disorders). Again, we could not identify a specific organ toxicity associated with a significant increase of Cmax or AUC24h, nor we could identify a specific DDI signature explaining side effects in our patents (data not shown: 13/49 had CYP3A4 inhibitors, 25/49 had pgp inhibitors).
In 45 CLL patients with PK parameters and lymphocyte counts available after one month of therapy, we made the intriguing observation that lower Cmax correlated with the lack of observable, transient hyperlymphocytosis (a class-effect of ibrutinib, correlating with PFS in the Resonate trial) (Figure 3). Altogether, our data did not find any positive correlation between high ibrutinib exposure and efficacy or safety profile.
Conclusions: our preliminary results suggested that higher Cmax and AUC24h did not correlate neither to efficacy nor to classical toxicities reported with ibrutinib intake. On one hand, we think that dosing intra-cellular concentrations could be more reliable than in plasma. On the other hand, we could consider TDM of ibrutinib in the context of a clinical trial reducing the doses of drug over time, to limit clinical and financial toxicity of this highly efficient drug.

Disclosures
No relevant conflicts of interest to declare.