Partial Reconstitution of Humoral and Cellular Immunity in Patients with Chronic Lymphocytic Leukemia Treated with Acalabrutinib
Authors: Pleyer Christopher , Sun Clare C. , Niermann Pia , Tian Xin , Ahn Inhye E. , Valdez Janet , Lotter Jennifer , Izumi Raquel , Hamdy Ahmed , Wiestner Adrian 
Published: October 2019

Abstract
Introduction
Immune dysregulation in chronic lymphocytic leukemia (CLL) contributes to a high rate of infections and morbidity. We previously reported that treatment of CLL with ibrutinib, a Bruton tyrosine kinase (BTK) inhibitor, leads to partial reconstitution of humoral immunity and fewer infections, especially in patients who achieved a ≥50% increase in serum IgA levels. Acalabrutinib is also an irreversible BTK inhibitor that is more selective than ibrutinib and has demonstrated safety and efficacy in the treatment of relapsed or refractory CLL. It is currently unclear how the increased specificity of acalabrutinib affects immune reconstitution and infection rates. We assessed the immunological impact of acalabrutinib in patients with CLL treated with single-agent acalabrutinib.
Methods
Samples originated from a phase 2, single-center trial studying acalabrutinib 100 mg twice daily (BID) or acalabrutinib 200 mg once daily (QD) in patients with relapsed/refractory (RR) CLL or high-risk, treatment naïve (TN) CLL (chromosome 17p deletion or mutation in TP53 or NOTCH1) (NCT02337829). Patients who received at least 6 months of acalabrutinib and had paired longitudinal data available were included in the analyses. Patients receiving IV immunoglobulin replacement were excluded from analysis of IgG levels. Additionally, patients with detectable monoclonal IgG, IgA and/or IgM proteins on serum immunofixation were excluded from analysis of the corresponding immunoglobulin isotype. The analysis of free light chains was stratified based on k or λ restriction of CLL cells determined by flow cytometry. Immunohistochemical staining of bone marrow biopsies was performed: T cell numbers were estimated by CD3 staining and the degree of CD68-positive macrophage extensions in contact with CLL cells were semi-quantitatively assessed on a scale from 0 (no extensions) to 4 (maximum number of extensions). The Wilcoxon signed-rank test and the Mann-Whitney U test were used to compare paired and unpaired data, respectively. Differences in the rate of infection between groups were examined using the Cox regression model for recurrent events.
Results
Serum IgA levels increased as early as 3 months after the initiation of acalabrutinib (median increase 35.7%, P = .0001), with levels sustained up to 24 months (Figure 1), whereas serum IgG and IgM levels were not affected by acalabrutinib. There was no difference (P > .05) in IgA, IgG, IgM trend between TN or RR CLL. Furthermore, there was no difference (P > .05) in IgA, IgG and IgM trends between patients treated with QD compared to BID dosing of acalabrutinib. Among 20 k-restricted and 18 λ-restricted CLL cases, the involved (tumor-derived) free light chain was elevated at baseline and trended toward the normal range after 3 months of acalabrutinib therapy consistent with an anti-tumor effect (k: median decrease 55.4%; P < .0001 and λ: median decrease 49.1%; P = .0003). The uninvolved free light chain did not change (P > .05). Peripheral blood CD3+, CD4+ and CD8+ T cell counts were elevated above the laboratory reference range at baseline and normalized after 6 months (CD4+ median decrease 49.2%; P = .0074 and CD8+ median decrease 54.8%; P = .003). T cell numbers in the bone marrow did not appreciably change. However, treatment-induced changes of the immune microenvironment were apparent in tumor-macrophage interactions. At baseline, macrophages tightly interacted with CLL cells, often with multiple podocytes making contact with CLL cells. On acalabrutinib, we observed a decrease in these macrophage podocyte interactions (P = .0007). At a median follow-up of 20 months, 31 (68.9%) patients developed a total of 68 infections, including 7 (10.3%) grade 3 and 1 (1.5%) grade 4 infections. Patients with superior immune reconstitution, as defined by an increase in serum IgA of ≥ a median of 36% from baseline to 3 months, had a significantly lower rate of infections (risk ratio = 0.52, P = 0.029).
Conclusions
These data indicate that acalabrutinib allows for partial reconstitution of humoral and cell-mediated immunity and disrupts macrophage-CLL cell interaction in the bone marrow microenvironment in patients with CLL. Furthermore, acalabrutinib did not interfere with uninvolved free light chains, suggesting that acalabrutinib selectively inhibits CLL B-cells and does not impair normal B-cell function.

Disclosures
Izumi: Acerta Pharma: Employment, Equity Ownership, Patents & Royalties: Acerta Pharma, various patents for ACP-196. Hamdy:Acerta Pharma: Employment, Equity Ownership, Membership on an entity's Board of Directors or advisory committees, Patents & Royalties: Acerta Pharma, various patents for ACP-196. Wiestner:Pharmacyclics LLC, an AbbVie Company: Research Funding.