eUpdate 62-2: Blinatumomab: A CD19/CD3-Bispecific T-Cell Engaging (BiTE) Antibody for B-Cell Leukemia Immunotherapy—A First-in-Class Agent that Directs T Cells to Tumor Cell Targets

Blinatumomab (blincyto) is a first-in-class bispecific antibody with high affinity toward both CD19 (expressed primarily on surface B cells) and CD3 (found on T lymphocytes) receptors. Blinatumomab received accelerated approval for treatment of Philadelphia chromosome-negative (Ph–) B-cell acute lymphoblastic leukemia (B-ALL) in December 2014.¹ It was the second cancer immunotherapeutic to gain breakthrough therapy designation and accelerated approval in 2014 (see Update: Programmed Cell Death-1 Pathway Checkpoint Inhibition: Immuno-oncology for Advanced Melanoma).

Rational Development and Key Features of BiTE Antibodies

Directing native T cells to engage and clear specific tumor cells in vivo has long been a goal of antitumor immunotherapy. BiTE antibodies represent the first immunotherapy with the ability to bind to and sustain T-cell engagement in vivo using an anti-CD3 T-cell single-chain antibody linked to a second tumor-specific antibody.² The first bispecific single-chain antibody was described in 1995 and was developed with the goal of directing CD3 T cells to tumor cells expressing the epithelial cell adhesion/activating molecule (EpCAM) cell surface antigen in colorectal cancer.^{3, 4} A promising cancer-specific target, EpCAM expression is prevalent in cancers of epithelial origin, and a BiTE therapy (MT110, eFigure 62-2.1B) is currently in phase I clinical evaluation for lung, gastric, colorectal, breast, prostate, and ovarian cancers.^{5, 6} A related bispecific trifunctional monoclonal antibody, catumaxomab, is approved in Europe and currently in trials in the U.S.⁷

Blinatumomab features a CD3 T-cell engaging single-chain antibody linked to a CD19 single-chain antibody (eFigure 62–2.1A). In normal hematopoiesis, the CD19 cluster differentiation epitope is expressed on B cells beginning at the pro-B cell stage in development and persists until differentiation into terminal plasma cells.⁸ B-cell lineage malignancies include leukemias and lymphomas, and, importantly, many express the CD19 epitope. Moreover, the majority of patients with these malignancies are not well managed on conventional chemotherapies, with a significant percentage succumbing to disease relapse.⁹ A different B cell–specific therapeutic, rituximab [an anti-CD20 monoclonal antibody (mAb)], has been added to standard chemotherapy for non-Hodgkin lymphoma and diffuse large B-cell lymphoma (DLBCL); however, it is not curative as a single agent.^{10, 11} Blinatumomab differs from the classic mAb approach by directing a cytotoxic T lymphocyte to a predefined B-cell target.

Mechanism of Action. Blinatumomab is a CD19-to-CD3-linked bispecific single-chain antibody that can facilitate interactions of cytotoxic T cells and malignant or normal CD19 B cells for tumor cell killing (eFigure 62–2.1A).^{12, 13} Preclinical evaluation of blinatumomab demonstrated clustering of malignant chronic lymphocytic leukemia cells in close proximity to T lymphocytes in vitro in the presence of blinatumomab (eFigure 62–2.1B), whereas no clustering was seen with a single anti-CD3 antibody or an anti-CD3/CD28 bead.¹⁴ Additional studies showed serial engagement of CD3⁺ T cells, inducing lysis of multiple CD19⁺ target cells over time at picomolar concentrations in vitro.^{15, 16} T cell–directed lysis is achieved by the release of granzyme into the cytolytic synapse and perforin-mediated entry into the target cell, activating target T-cell caspases and initiating a rapid apoptotic response.⁹

Clinical Efficacy. Prior to approval by the FDA, the safety and efficacy of blinatumomab in two ongoing phase II clinical trials were published.^{17, 18} Thirty-six patients with adult relapsed or refractory B-ALL were treated in the initial trial (NCT01209286), which began in 2010 and used a 4-week continuous intravenous (IV) infusion, followed by 2-week treatment-free interval. Patients were divided into three cohorts for safety evaluation in the first stage for dose-finding purposes, followed by an extension stage of one cohort (eTable 62–2.1). This single-arm study demonstrated complete remission (CR) or CR with partial hematologic recovery (CRh) in 25 patients (69%).¹⁸ An additional 189 patients were treated in a second trial (NCT01466179), registered in 2011. Drug administration in this single-arm study was 9 μg/day by continuous IV infusion for the first 7 days, followed by 28 μg/day by continuous IV infusion for 4 weeks, followed by a 2-week holiday interval; treatment was continued for up to 5 total cycles. Analysis after 2 cycles of treatment revealed 81 patients (43%) achieved CR or CRh.¹⁷ Inclusion criteria for the second, larger study differed from the first, with a higher leukemic burden (≥5% versus ≥10% blasts in marrow) required. Finally, response to blinatumomab was studied in nine pediatric relapsed or refractory B-ALL patients on a compassionate-use basis. Patients with no further therapeutic options were administered either 5 or 15 μg/m²/day on a 4-week continuous infusion schedule. CR was seen in four patients after the first cycle, and an additional two patients responded following a second cycle. The remaining three patients did not respond.¹⁹ In the three studies described above, patients who achieved remission, as measured by CR or CRh, went on to receive a hematopoietic stem cell transplant (between 33% and 56% of total individuals on study).

Dosage and Administration. Blinatumomab is approved for use in patients who weigh at least 45 kg. A single cycle of treatment is 4 weeks of infusions, followed by a 2-week treatment-free interval. Patients should be pretreated with 20 mg dexamethasone 1 hour prior to the start of each blinatumomab cycle or prior to a dose increase or interruption. A 9 μg/m²/day 24-hour infusion is given over the first week of treatment, followed by 28 μg/m²/day for the remaining weeks of cycle 1. Future cycles are restarted at 28 μg/m²/day.²⁰ Induction for up to 2 cycles is followed by an additional consolidation treatment of up to 3 additional cycles, with minimum residual disease level determining response and treatment continuation.²¹

Pharmacokinetics. In a study of 212 patients with relapsed or refractory ALL, who received at least 1 dose of ≤28 μg/day, steady-state serum concentration (C_SS) was achieved within 1 day following continuous IV infusion and remained stable over time.²⁰ C_SS was proportional to a dose of 5-90 μg/m²/day. The mean C_SS for 9 μg/m²/day was 211 pg/mL, whereas the 28 μg/m²/day mean C_SS was 621 pg/mL. Estimated volume of distribution at terminal phase was 4.52±2.89 L following continuous infusion. The mean clearance was 2.92±2.83 L/hour, and the mean half-life (t_½) was 2.11±1.42 hours.²⁰

Toxicity. Blinatumomab includes a black box warning for cytokine release syndrome (CRS) and neurological toxicities.²⁰ Overall, serious adverse reactions were present in 65% of patients. Serious CRS-related toxicities included pyrexia (62%), nausea (25%), and headache (36%), among others. CRS may be due in part to hemophagocytic lymphohistiocytosis (also known as macrophage activation syndrome) and may partially be mitigated by the administration of tocilizumab (an anti-IL6 receptor antibody approved for use in rheumatoid arthritis).²² Neurological toxicities were reported in 50% of patients within 7 days of starting treatment. Fifteen percent of neurological toxicities were severe, manifesting as encephalopathy, convulsions, speech disorders, disturbances in consciousness, confusion, disorientation, and coordination and balance disorders. Treatment interruption or discontinuation is recommended to resolve these clinically reversible events. Out of six patients who discontinued treatment due to epilepsy or convulsions [and saw amelioration of these adverse events (AEs) within 72 hours of treatment discontinuation], three were able to resume blinatumomab therapy with anti-seizure prophylaxis.¹⁸ Preparation and administration errors were also observed, with one patient receiving 133-fold the recommended therapeutic dose.²⁰ A blincyto Risk Evaluation and Mitigation Strategy (REMS) website was established to aid healthcare providers in the management and reporting of AEs associated with this drug (http://www.blincytorems.com/).

Outlook of BiTE Antibodies and CD19 Lymphocyte-directed Therapies

BiTE antibodies are produced as a fully functional recombinant protein and are purified from large cultures of eukaryotic cell lines, such as Chinese hamster ovary (CHO) cells.^{4, 23} The smaller size (50 kDa versus 150 kDa) and lower per-patient dose of BiTE antibodies provide a significant manufacturing cost reduction compared with traditional full-sized mAbs (e.g., rituximab administration of 375 mg/m²).²⁴ Additional BiTE antibodies are in clinical trials for a variety of tumor epitopes, including the previously mentioned EpCAM in solid tumors (MT110), carcinoembryonic antigen (CEA) in gastrointestinal (GI) adenocarcinoma (MT111), and prostate-specific membrane antigen (PSMA) (AMG112) in prostate cancer, while even more are in preclinical development [e.g., CD33 for acute myeloid leukemia (AML) and melanoma-associated chondroitin sulfate proteoglycan (MCSP) for melanoma].²⁵ Evidence also suggests that bispecific antibodies may facilitate transport of peripheral lymphocytes across the blood-brain barrier in the central nervous system (CNS), as seen in epidermal growth factor receptor variant III (EGFRvIII)-positive brain tumors.²⁶ The intracerebral accumulation of BiTE antibodies or activated peripheral T cells (mediating CNS inflammation) may offer a potential explanation for the CNS toxicities observed with blinatumomab.^{26, 27} While bispecific antibodies represent a great step forward in targeted immunotherapy, caution should be exercised in their use, considering the significant occurrence of severe AEs. Interestingly, CRS is not limited only to blinatumomab patients—it is also seen with great frequency in the treatment of leukemia and lymphoma with another CD19-targeted therapy: chimeric antigen-receptor T-cell (CAR-T) therapy.²⁸ CAR-Ts are produced by transduction of the patient’s own T cells with a recombinant target receptor for therapy. Several CD19-directed CAR-Ts are in trials for leukemia and lymphoma with promising results thus far and will be direct competitors of blinatumomab.^{29, 30} In fact, several patients reported to be refractory to blinatumomab have responded to CAR-T (CTL019) therapy.²⁹ Overall, it is clear that this next generation of immuno-oncology therapeutics provides a significant therapeutic advantage, but adverse effects must be carefully monitored.