Background
The CD19/CD3 bispecific T-cell engager (BiTE
®) antibody construct blinatumomab is an adaptor protein that allows T cells to recognize specifically CD19-expressing B cells [
1], thereby directing the cytotoxic potential of the T cell towards the targeted B cell. Numerous preclinical studies have demonstrated this mode of action, showing complete target cell lysis at very low blinatumomab concentrations and effector-to-target cell ratios along with tumor eradication in xenograft models [
2]. Clinical response to blinatumomab treatment has been evaluated in relapsed/refractory non-Hodgkin’s lymphoma (NHL), relapsed/refractory acute lymphoblastic leukemia (ALL), and minimal residual disease (MRD)-positive ALL [
3‐
8]. In an exploratory phase II dose-finding study in relapsed/refractory ALL, 69% of patients achieved complete remission (CR) or CR with partial hematologic recovery (CRh) within two treatment cycles. The study identified the blinatumomab target dose as 15 µg/m
2/day using a 1-week run-in phase at 5 µg/m
2/day for mitigation of first-dose effects [
6]. In long-term follow-up analysis, T-cell expansion was associated with long-term survival [
9]. In the subsequent large confirmatory phase II study, 43% of patients with relapsed/refractory ALL achieved CR/CRh within the first two cycles of blinatumomab treatment [
7].
The first comprehensive pharmacokinetic (PK) and pharmacodynamic (PD) analysis in response to blinatumomab treatment was conducted in patients who were in complete hematologic remission after receiving treatment for ALL but maintained MRD-positive disease, an indicator of chemotherapy resistance [
10]. T-cell and B-cell distribution kinetics and markers of blinatumomab mode of action in patients with relapsed/refractory ALL, an aggressive disease with a very poor prognosis [
11,
12], have not yet been studied. However, evaluating blinatumomab-induced PD effects in this setting is an important first step in elucidating potential biomarkers for clinical outcomes. Furthermore, PD analyses may contribute to the management of adverse events associated with the blinatumomab mode of action. For example, treatment-induced cytokine release may cause rare events of cytokine release syndrome (CRS), and blinatumomab treatment has been associated with changes in liver enzyme parameters [
6,
7]. Medications or factors related to hepatic injury/dysfunction and cholestasis or biliary obstruction may cause liver enzyme elevations above normal levels even in otherwise healthy individuals [
13‐
16]. Thus, detailed serum chemistry, including liver enzymes such as alkaline phosphatase (AP), alanine aminotransferase (ALT), aspartate aminotransferase (AST), total bilirubin, and gamma-glutamyl transferase (GGT), provides important information on a patient’s liver function in response to drug treatment and may reveal drug-induced hepatocellular, cholestatic or mixed liver injury [
17‐
19].
In the present study, we analyzed for the first time changes in liver enzymes and markers of inflammation and coagulation in response to blinatumomab treatment administered to patients with relapsed/refractory ALL in the exploratory phase II dose-finding study. Furthermore, we performed a detailed assessment of the behavior of peripheral T and B cells, neutrophils, and thrombocytes and characterized the release of cytokines and the T-cell effector molecule granzyme B.
Methods
Patients
Detailed inclusion/exclusion criteria are published elsewhere [
6]. Briefly, adult patients with relapsed/refractory ALL were eligible if they expressed the B-precursor phenotype and had >5% leukemic blasts in the bone marrow. Relapse was defined as reappearance of disease after CR of 28-day duration; refractory disease was defined as not having achieved CR after induction and/or consolidation treatment. A total of 36 patients were enrolled and treated with blinatumomab. ClinicalTrials.gov identifier: NCT01209286.
Study design
Study design and dose cohorts are described in detail elsewhere [
6]. Briefly, this was an open-label, multicenter, phase II study with Simon 2-stage design investigating the efficacy, adverse events, PK, and PD of blinatumomab in patients with relapsed/refractory ALL. Patients received blinatumomab continuous IV infusion at a flat dose of 15 µg/m
2/day (
n = 7; cohort 1), or a stepwise dose of 5‒15 µg/m
2/day (5 µg/m
2/day for the first 7 days and 15 µg/m
2/day thereafter;
n = 5 in cohort 2a;
n = 18 in extension cohort 3) or 5‒15‒30 µg/m
2/day (as in cohort 2a with an additional dose step to 30 µg/m
2/day in week 3;
n = 6; cohort 2b) over 4 weeks followed by a 2-week treatment-free period (one cycle). Patients who achieved CR or CRh within the first two cycles could receive up to three additional treatment cycles (induction and consolidation). The core study period included screening plus the treatment period (up to five cycles).
Response measurement
The primary endpoint was achievement of CR or CRh within the first two treatment cycles. CR was defined as bone marrow blasts ≤5%, no evidence of disease, and full recovery of peripheral blood counts (platelets >100,000/µL, hemoglobin [Hb] ≥11 g/dL, absolute neutrophil count [ANC] >1500/µL); CRh was defined as bone marrow blasts ≤5%, no evidence of disease, and partial recovery of peripheral blood counts (platelets >50,000/µL, Hb ≥7 g/dL, ANC >500/µL). Bone marrow blast count was quantified by a central laboratory at screening and after each treatment cycle.
Anti-blinatumomab antibodies
Serum samples for detection of anti-blinatumomab antibodies were collected at baseline (predose), at the end of infusion of each treatment cycle, and at the end-of-core-study visit. Anti-blinatumomab antibodies were measured with a validated electrochemiluminescence immunoassay (Meso Scale Discovery, Rockville, MD, USA). Briefly, serum samples (undiluted and prediluted 1:100 in the respective predose serum in order to avoid potential hook effects) were diluted 1:10 in phosphate-buffered saline and then incubated with 0.5 µg/mL each of biotin- and ruthenium-conjugated blinatumomab (prepared using MSD SULFO-TAG™ [Meso Scale Discovery] following the manufacturer’s instructions) for at least 1 h at room temperature. Samples were then added to a streptavidin-coated 96-well microtiter plate (Meso Scale Discovery) blocked with 5% bovine serum albumin in phosphate-buffered saline at room temperature and incubated for 0.5–2 h to allow formation of antibody complexes. Anti-blinatumomab antibodies in patient serum bound to biotin-conjugated/streptavidin-captured blinatumomab were recognized by ruthenium-conjugated blinatumomab. After washing with phosphate-buffered saline plus 0.05% Tween 20 and application of Reading Buffer (Meso Scale Discovery), signals were measured using a Sector Imager 2400 analyzer (Meso Scale Discovery) and normalized against a predose serum sample tested in parallel. Polyclonal goat anti-blinatumomab antibodies (Biogenes, Berlin, Germany) were included as positive control. Positive serum samples were retested in a competitive inhibition assay determining percent change in assay signal with and without blinatumomab preincubation.
Pharmacokinetics
Blood samples were collected before, during, and after infusion: baseline (day 1), days 3, 8, 15, 22, and 29 in all cohorts; and at additional time points (day 8 + 2 h, day 8 + 6 h; days 9, 10, and 17) in some cohorts. Biologically active concentrations of blinatumomab in serum were analyzed as described previously [
20], using an assay based on CD69 upregulation on the surface of newly activated T cells after dual binding of blinatumomab to HPB-ALL T cells and Raji B-lymphoma cells. The dose-dependent increase of CD69 expression was measured using a fluorescence-activated cell sorter (FACS) instrument (FACSCalibur or FACSCanto II; BD Biosciences, Heidelberg, Germany). Data were analyzed using GraphPad Prism 6 software (GraphPad Software, La Jolla, CA, USA) or SoftMax Pro software (MDS Analytical Technologies, Sunnyvale, CA, USA). The assay was internally validated; the lower limit of quantification (LLOQ) was 50 pg/mL. The mean steady state concentration (C
ss) of blinatumomab in serum from individual patients was calculated from available data points at exposure plateau in each 4-week treatment period. For each dose cohort, the data from individual cycles were included as independent data points.
Serum chemistry
Blood samples for evaluation of clinical laboratory parameters were collected during the screening period (day −21 to day 0), before treatment start (baseline [day 1]), and during treatment (days 2, 3, 8, 15, 22, 29) for up to 5 cycles. AST, ALT, GGT, lactate dehydrogenase (LDH), total bilirubin, and C-reactive protein (CRP) were analyzed using samples from all time points. Patient inclusion criteria were <5× upper limit of normal (ULN) for AST, ALT or AP and <1.5 × ULN for total bilirubin [
7]. Numbers of white blood cells and thrombocytes were determined from differential blood count. D-dimer concentrations were measured only in samples collected during the first two treatment cycles. No threshold values for treatment discontinuation were defined for the laboratory parameters.
Lymphocyte subpopulations
Lymphocyte subpopulations were measured either in density gradient-separated peripheral blood mononuclear cells, prepared as described previously [
10], or in whole peripheral blood at screening, before treatment start (baseline), and at various time points during the infusion periods as well as at the end of the core study and at follow-up visits. Briefly, peripheral blood was collected more frequently in cohorts 1 and 2a: within 1 h before and at 2, 6, 24, and 48 h after treatment start and dose step, if any, and again once weekly until end of infusion. In cohorts 2b and 3, peripheral blood was only collected within 1 h before and 48 h after treatment start and dose step(s), and again once weekly until end of infusion. Lymphocyte subpopulations were analyzed by flow cytometric determination of different cell surface markers, obtaining measures of the relative cellular composition of the blood sample using an eight-color FACSCanto II instrument (BD Biosciences), a five-color FC500 instrument (Beckman Coulter, Brea, CA, USA), or a ten-color FACS NAVIOS instrument (Beckman Coulter). Fluorescent dye-labelled monoclonal antibodies were used to detect the following cell surface markers: CD45 (lymphocytes); CD19 (B cells); CD3, CD4, CD8 (T cells); CD69 (T-cell activation); CD45RA, CD28, CCR7 (memory T-cell subsets). T-cell adhesiveness was assessed by binding of soluble ICAM-1-Fc fusion proteins (R&D Systems, Abingdon, UK) to (activated) LFA-1 on T cells, with subsequent detection by goat anti-human IgG, Fc-FITC (Dianova, Hamburg, Germany). By combining percentage values of certain lymphocyte subpopulations with the absolute lymphocyte number determined by differential blood count, the absolute numbers of the respective subpopulations were calculated.
Cytokines and granzyme B
Peripheral blood cytokine levels of interleukin (IL)-2, IL-4, IL-6, IL-10, TNF-α, and IFN-γ were monitored by measuring the respective markers in serum using the FACS-based BD Cytometric Bead Array Human Th1/Th2 Kit II (BD Biosciences) following the manufacturer’s instructions. Signals were measured using a FACSCanto II instrument and the FACS Diva evaluation software (BD Biosciences). Cytokine concentrations were calculated with the FCAP array software (Soft Flow Inc., St Louis Park, USA). The assay was internally validated; the LLOQ was 125 pg/mL and the limit of detection (LOD) was 20 pg/mL. Serum samples were collected approximately 1 h before infusion start (baseline [day 1]); at 2, 6, 24, 48, and 168 h after treatment start; and at these same time points after each dose step (if applicable) in each treatment cycle for up to five cycles. For calculations of mean cytokine peak concentrations (Cmax) across all patients who received a starting dose of 5 or 15 µg/m2/day during treatment week 1, or 15 µg/m2/day as second dose during treatment week 2, and for calculations of mean and standard deviation of cytokine levels per cycle, values below LLOQ were included as such; values below LOD were set to ½ LOD to allow logarithmic plotting. Restarted cycles following treatment interruption were considered as new cycles.
Granzyme B was monitored in parallel to cytokines in treatment cycle 1. Serum samples were collected at the time points described above. Granzyme B was measured in triplicate using the Human Granzyme B Platinum ELISA kit (eBioscience, San Diego, USA). The assay was internally validated with an LLOQ of 100 pg/mL and an LOD of 30 pg/mL. Calculations of mean granzyme B peak concentrations and mean granzyme B levels per cycle were performed as described above.
Body temperature
Peak body temperature was measured before treatment start (baseline); every 4 h within the first 12 h of infusion start; in the morning and evening on treatment day 2 and 3; and once on day 8, 15, 22 and 29. Peak body temperature was analyzed for the first treatment week in patients who received a starting dose of 5 or 15 µg/m2/day in cycle 1 and for the second treatment week in patients who received 15 µg/m2/day as the second dose in cycle 1.
Statistical analysis
All data were summarized using descriptive statistics. Data are presented as mean ± standard deviation (SD) or as median with 25th and 75th percentile.
Discussion
This is the first study reporting in detail on changes in laboratory and pharmacodynamic parameters in response to blinatumomab treatment in the setting of relapsed/refractory ALL using data from an exploratory dose-finding phase II study. Changes in most of the evaluated liver and first-dose parameters were characterized by rapid elevations immediately after infusion start and return to baseline at the end of the cycle. Specifically, ALT and AST were moderately elevated after infusion start (<5 times the normal range) but decreased to baseline within the first cycle. Mild to moderately elevated ALT and AST are the most frequently observed clinical transaminase alterations [
19,
21] and are not uncommon in response to medications as described for various other drugs, such as statins and methotrexate [
22]. The observed transiently elevated levels of LDH, which may indicate tissue damage, and D-dimer, a coagulation marker [
23], might be associated with blinatumomab-induced B-cell lysis and adhesion of lymphocytes and platelets to blood vessel endothelium. Similar changes in laboratory parameters, including elevated GGT, have been described for patients with B-cell chronic lymphocytic leukemia receiving the anti-CD20 monoclonal antibody rituximab [
19,
24,
25]. Importantly, the increases in liver and first-dose parameters, including CRP, a marker of infection and inflammatory processes [
26], in response to blinatumomab were transient and reversible, and did not result in treatment interruptions or discontinuations. Furthermore, pronounced elevations in those parameters appear to occur early in the course of treatment when most patients with relapsed/refractory ALL receive blinatumomab in the hospital (week 1 of cycle 1), which would allow for careful monitoring and management, especially with respect to treatment interruptions.
Blinatumomab activates T cells only in the presence of CD19
+ target cells. Incubation of isolated T cells in the absence of target cells even at saturating blinatumomab concentrations has not been shown to cause expression of activation markers CD69, CD25, or release of cytokines [
27‐
29]. Therefore, blinatumomab-induced PD effects in patients with ALL or NHL are most likely caused by activation of T cells after binding of accessible CD19
+ target cells and formation of a cytolytic synapse. Several PD effects have been reported in studies with blinatumomab in patients with MRD-positive ALL [
6,
10] or relapsed/refractory NHL [
20]. In both settings, B-cell depletion, T-cell redistribution, and cytokine elevation were most pronounced during the first days after the start of blinatumomab infusion. The qualitative pattern of PD effects (such as transient T-cell redistribution) that occur shortly after infusion start in each treatment cycle, or after each dose step, was comparable across those studies. In patients with relapsed/refractory ALL, peripheral B cells rapidly declined within the first two treatment days, independent of clinical response to blinatumomab. This pattern of B-cell depletion is similar to that observed in patients with MRD-positive ALL or relapsed/refractory NHL, providing an early marker of blinatumomab activity [
10,
20]. In patients with relapsed/refractory ALL there was an association between T-cell expansion and clinical response to blinatumomab during cycle 1, and T-cell expansion has been associated with long-term survival in this setting [
9]. T-cell counts above baseline may be caused by a delayed return of T cells from target tissue back into peripheral blood, by T-cell proliferation, or both. Increased CD3
+ T-cell counts of responding patients coincided with expanding CD4
+ and CD8
+ T cells and memory T cells, especially during treatment cycle 1. Similarly, T-cell expansion has also been described for MRD-positive ALL and relapsed/refractory NHL. In the MRD-positive ALL study, T-cell expansion above baseline was limited to the first treatment cycle and both CD4
+ and CD8
+ T cells contributed to T-cell expansion, but neither was associated with response [
10,
20]. The present analysis suggests an association between T-cell expansion and clinical response to blinatumomab in relapsed/refractory ALL; however, the results are based on small numbers of patients. Appropriately designed larger studies are required to validate T-cell expansion as a biomarker for clinical response to blinatumomab treatment in this setting.
The redistribution kinetics of peripheral T cells in patients with relapsed/refractory ALL were also comparable to previously published data. As described for MRD-positive ALL and relapsed/refractory NHL [
3,
10,
20], peripheral T cells rapidly disappeared from circulation within the first day of infusion before recovering to baseline after approximately 1 week. This T-cell disappearance is most likely a consequence of increased T-cell adhesion to blood vessel endothelium as evident from the affinity shift in the activated cell adhesion molecule LFA-1 on T cells. Simultaneously, the early activation marker CD69 was upregulated on both CD4
+ and CD8
+ T cells, suggesting that T cells were activated upon encounter with peripheral B cells. Involvement of both CD4
+ and CD8
+ T cells in redistribution and activation has been observed in all clinical studies with blinatumomab conducted to date [
4,
10,
20].
The effector molecule granzyme B is stored in secretory vesicles of cytotoxic T cells and is released upon formation of a blinatumomab-mediated cytolytic synapse between a T and B cell [
30]. Our data show that in patients with relapsed/refractory ALL, peak levels of granzyme B during the 1st week of blinatumomab infusion were not associated with a clinical response, but its rapid appearance in serum provides additional evidence of the biologic activity of blinatumomab.
This is the first study reporting on the distribution kinetics of neutrophils and thrombocytes in patients with relapsed/refractory ALL in response to blinatumomab. Both responders and nonresponders had low platelet counts before infusion start. Thrombocytopenia is frequently associated with polychemotherapy, a common treatment for relapsed/refractory ALL, and bone marrow infiltration by blast cells, thus reflecting the disease state. After start of blinatumomab treatment, thrombocyte counts decreased further during the first treatment days before recovering and even achieving normal levels in the responder group at the end of cycle 1. The initial platelet redistribution coincided with the described T-cell redistribution and might be explained by an increased adhesiveness of activated blood vessel endothelium, leading to platelet adhesion and disappearance from the circulation. Surprisingly, neutrophils showed the opposite behavior, with mean cell counts initially increasing after infusion start. It can be speculated that this neutrophil spike, which coincides with T-cell redistribution, is caused by other lymphocytes and platelets displacing neutrophils from the blood vessel endothelium, thus releasing them into the circulation.
Blinatumomab treatment frequently causes fever and can cause rare events of CRS or cytokine storm, a consequence of extensive cytokine release following blinatumomab-induced T-cell activation [
6,
7]. Transient elevations of cytokines during the first treatment week have been described for patients with MRD-positive ALL [
10]. Those elevations were limited to cycle 1 and were not associated with clinical response. The overall pattern of released cytokines in the present study was similar; however, the anti-inflammatory cytokine IL-10 was also detectable in cycles 2 and 3, possibly because of the larger tumor load in patients with relapsed/refractory ALL, compared with MRD-positive ALL. The magnitude of IL-6, IL-10, and IFN-γ release was slightly higher in patients who achieved CR/CRh than in nonresponders. It may be speculated that increased IL-10 levels, or even IL-6 or IFN-γ levels, in the responder group were an early sign of clinical response. However, data from a larger number of evaluable patients are required to establish an association between cytokine release and the antileukemic activity of blinatumomab. A possible association may be supported though by the observation that IL-10 does not only possess anti-inflammatory but also immune-stimulatory properties. This has recently been described for pegylated recombinant IL-10, which stimulated the activation, expansion, and cytotoxicity of tumor-infiltrating CD8
+ T cells, while increased levels of immune stimulatory cytokines and antitumor activity were observed in patients with solid tumors who received pegylated recombinant IL-10 [
31].
Blinatumomab first-dose effects, including CRS, in patients with relapsed/refractory ALL have been successfully mitigated using stepwise dosing [
6]. We have shown for the first time that stepwise dosing resulted in a reduction in cytokine release and in lower body temperature, which explains the better tolerability of this dosing regimen compared with flat dosing. Patients with relapsed/refractory ALL who receive stepwise dosing, especially at the beginning of therapy, experienced few cytokine-associated clinical symptoms, such as CRS or cytokine storm [
6,
7]. Our data also support the administration of stepwise dosing when restarting blinatumomab treatment after CRS-associated infusion interruption once the CRS event has resolved.
Authors’ contributions
MK, PK, and GZ contributed to the concept and design of the experiments or of the clinical laboratory data analysis (GZ). The following authors collected data: VN and MK (FACS); AK (peripheral blood cells, granzyme B), YH (PK, cytokines), and AW (anti-blinatumomab antibodies). MST and NG collected the patient data in the clinical study. CH performed the clinical laboratory data and the responder/nonresponder analysis. VN, MK, GZ, MST, and GN interpreted the clinical laboratory data; VN, MK, AK, YH, and AW interpreted all other data; PK and PAB interpreted all data in the broader context of the literature. VN, MK, and AK wrote the manuscript. All authors read and approved the final manuscript.