Temporary cessation of ibrutinib results in reduced grade 3‐4 infections and durable remissions—Interim analysis of an on‐off‐repeat Phase 1b/2 study in patients with chronic lymphocytic leukemia

Abstract Ibrutinib is used continuously in CLL. This phase 1b/2 study interim analysis explored on‐off‐repeat dosing to reduce toxicity. After 12 months, 16/22 patients (73%) remained in first off‐phase irrespective if initial CR/PR or TP53 aberration. Grade 3‐4 infections were reduced from 55% to 5% during a similarly long off‐phase (P < .01). Treg and exhausted T‐cells increased (P = .01). Six patients restarted ibrutinib at early progression and remain drug‐sensitive. Our interim analysis shows a durable off‐phase in most patients, with reduced infections and cost‐saving potential. If toxicity‐driven permanent cessation of ibrutinib will be affected will be explored in the extended study.

ibrutinib tolerability may be crucial to delay the need for alternative therapy. Ibrutinib may not be cost-effective with the current standard dosing [7] and repetitive on-off dosing may also have significant impact on total drug costs.

Study design
We report here early interim results from a Phase 1b/2 multicenter, prospective, single-arm, investigator-initiated study of CLL patients who were in remission following continuous ibrutinib therapy, designed to investigate the safety and efficacy of repeated responseguided treatment interruptions. The primary study endpoints were safety, that is, adverse events (AEs) and relationship of AEs to inter-  thereafter. Blood counts with differential are performed every second week during the first two months of every off-phase and monthly thereafter, and monthly during every on-phase. In case of early signs of PD, a more comprehensive evaluation, which included a CT and bone marrow biopsy is performed before ibrutinib is restarted. With every subsequent on-off cycle the same comprehensive evaluation is performed.
Immunomonitoring is performed during the first off-phase (baseline, 2 and 4 weeks, 3, 6, and 12 months). Materials and methods are included as supplement.

RESULTS AND DISCUSSION
We report here the early preplanned interim analysis of the Phase 1b/2 study. A total of 22 patients were enrolled during 20 months at three sites. Four patients were included in an early run-in safety period and had to complete 6 months of the first off-phase before additional patients could be included in the study. Patients characteristics are shown in Table 1. Median age was 73.5 years (range 52-86), nine patients (41%) had TP53-mutation and/or 17p deletion, and eight patients (36%) received ibrutinib as first-line treatment.
Of the 22 patients, 11 (50%) entered the study with a PR and 11 with CR, which was minimal residual disease (MRD)-positive in all cases ( Figure 1A). Rapid progressive disease (rebound) or other safety signals were not observed. Sixteen patients (73%) remained in their first offphase without early signs of PD at the 12 months follow-up ( Figure 1A).
Median time to progression has not been reached. There was tendency toward a longer off-phase among patients with clinical CR, even though many patients with PR at inclusion also remained off drug at the 12month follow-up ( Figure 1A). Cytogenetic status was not related to length of first off-phase, but longer follow-up and extended number of patients will be required. Six patients (27%) showed early signs of progression (for criteria see "Study design" above) and restarted ibrutinib after a median of 7 months (range 4-29); all responded with shrinking lymph nodes and/or improving blood counts. In one patient, the second response was deeper (MRD-positive CR) than achieved prior to study inclusion (PR). However, this patient experienced increasing lymphocytosis after 7 months in the second off-phase and restarted ibrutinib and has achieved a new PR.
We thereafter compared side effects observed during 12 months before study inclusion and during the first off-phase, that is, a similarly long time period off-drug ( We assume that the lower incidence of infections was ascribable to the patients being off the drug. Several studies have shown improved humoral immunity (especially IgA) during ibrutinib treatment. [11] Whether humoral immunity may improve following cessation of ibrutinib will be object of future investigations including vaccine trials.
We previously showed that most peripheral blood T-cell subsets decline during ibrutinib treatment, which correlates with a decreasing disease burden. [12] No significant changes in the numbers of CD4 + and CD8 + T-cell subsets were observed following ibrutinib cessation ( Figure 1B and C). NK cells (CD3 − CD56 + ) decreased during the first 2 weeks (P = .04) but then increased (P = .008 at 12 months) (Fig-

Patient characteristics at study inclusion (n = 22)
During the first off-phase, n (%) 1 (5) ure 1D). Exhausted T cells (CD3 + PD-1 + TIM3 + ) (P = .01 after 2 weeks) and regulatory T cells (P = .01 after 3 months) gradually increased during the first off-phase ( Figure 1E, F). Such immune defects are likely to reflect the tumor burden. [13][14][15] There are several limitations in our study. First, the data presented here is an early interim analysis, that is, on limited number of patients and need to be confirmed in more patients (ongoing). Since cessation of a BTK inhibitor in responding patients is outside the current treatment recommendation, we decided to perform an early interim analysis to check safety signals and trends for efficacy to decide whether continuation of the study is justified. Our early data suggest that severe infections may be reduced and that the first drug-free period exceeded one year in most patients, which is considered as clinically meaningful. Furthermore, all re-treated patients remain drug-sensitive. Second, the study design may not be optimal; an alternative design would be to use a predefined length of ibrutinib therapy or a predefined depth of remission before drug cessation. However, our early data suggest that neither length of therapy nor degree of remission may be critical for the length of first off-phase. Third, even though the costsaving potential is obvious in a short term perspective, longer follow-up (including more patients and repeated on-off cycles) will be required to determine all aspects of our dosing strategy-only a randomized comparison can provide the final answer on whether ibrutinib and other BTKi should be continued until PD or if they can safely be halted.
Since half of patients treated with ibrutinib continuously in a real-world F I G U R E 1 Response-guided on-off dosing in ibrutinib-treated CLL patients and immunomonitoring during the first year of the initial off-phase. (A) Duration of ibrutinib treatment prior to study enrollment (Months = 0) and duration of the respective response-guided interruptions (off; in orange) and resumptions (on; in blue). Quality of response (CR or PR) at the time of ibrutinib cessation is indicated at the end of each on-phase. Absolute numbers of (B) CD3 + CD4 + , (C) CD3 + CD8 + , (D) CD3 − CD56 + , (E) CD3 + PD-1 + TIM3 + , and (F) CD4 + CCR4 + CD25 + CD127 low lymphocytes, respectively, in the peripheral blood during the first year of the initial off-phase as measured by flow cytometry. Panels B to F show data from 14 patients at ibrutinib cessation (study baseline) and the indicated period thereafter. Stars indicate a statistically significant difference between the respective timepoints and baseline as established by Wilcoxon signed-rank test. *P ≤ .05; **P ≤ .01; ***P ≤ .001; CR: complete remission; PR: partial remission; MRD; minimal residual disease; PD-1: programmed cell death protein 1; TIM3: T cell immunoglobulin and mucin domain-containing protein 3; Treg: regulatory T cell; ♦: patients with 17 p deletion and/or TP53 mutation setting had stopped ibrutinib permanently already at 18 months [4] we suggest that an alternative dosing strategy shall be developed to minimize permanent withdrawals, thereby possibly delay the time to need of alternative therapy.

AUTHOR CONTRIBUTIONS
JL and AÖ designed the study, treated the patients, analyzed the data, and wrote the manuscript. TAM designed and performed laboratory experiments, analyzed the data, performed statistical analysis, and wrote the manuscript. MK, LH, and AU treated the patients and approved the manuscript. HM analyzed the immunological data and approved the manuscript. TW analyzed the clinical data and approved the manuscript. KH designed and performed laboratory experiments, analyzed the data, and approved the manuscript. MP designed laboratory experiments and wrote the manuscript.