Since the U.S. Food and Drug Administration's (FDA's) initial approval of the anti-CD20 antibody rituximab in 1997 to treat relapsed or refractory low-grade B cell lymphomas, monoclonal antibodies have been a mainstay of therapy for many hematologic malignancies. On its own, rituximab facilitates apoptosis of the CD20-expressing cells; it also mediates antibody-dependent cellular cytotoxicity by engaging effector immune cells, including natural killer cells, to lyse the targeted tumor cells.
In the 22 years since rituximab's approval, many monoclonal antibodies to treat both hematologic malignancies and solid tumor cancers have entered the market, including gemtuzumab ozogamicin for CD33-positive acute myeloid leukemia, ofatumumab to target CD20 in patients with chronic lymphocytic leukemia (CLL), daratumumab to target CD38 in patients with multiple myeloma, and many others.
Using protein engineering tricks that became feasible only during the 1990s, researchers can now create multifunctional antibodies that bring together antibody fragments in novel ways to create so-called bispecific antibodies that have two targets instead of one. Furthermore, these bispecific antibodies can be made to scale for clinical production.1
"These agents target both malignant cells, based on the expression of a specific marker, and the patient's own T cells, based on a T cell–specific marker," explained Farhad Ravandi, MD, from the University of Texas MD Anderson Cancer Center, who has taken part in several clinical trials of bispecific antibodies. "The agent acts as a bridge bringing these T cells into proximity of malignant cells and activating the T cells to kill malignant cells. This is a very elegant form of immunotherapy."
ASH Clinical News spoke with Dr. Ravandi and other researchers and clinicians about the biology of bispecific antibodies, their advantages and disadvantages, and how these new drug entities are faring in the clinic.
Bispecific Antibodies 101
Monoclonal antibodies have two arms that each recognize the same target antigen. In contrast, bispecific antibodies are engineered hybrid molecules with two unique binding domains, each of which recognizes a unique target. The first type of bispecific antibody to reach the clinic is a bispecific T cell engager. Each arm of these antibodies consists of a single-chain variable fragment – an engineered fusion protein and not an actual fragment of an antibody. One fragment recognizes and binds to a tumor antigen while the other recognizes and binds to an antigen on a T cell or other effector immune cells that can kill tumor cells. The two binding domains are joined by a short, flexible linker peptide. Bispecific antibodies also can be designed to bind to other non–T cell immune cells.
Because antibodies are biologics, requiring that living cells assemble parts of the antibody correctly; an initial challenge in making bispecific antibodies was being able to incorporate antibody subunits from two different antibodies into a single, functional unit with a stable configuration. Cells can generate these dual molecules in any one of 10 configurations, of which only one is correct and fully functional.
To get around this issue, researchers at California-based pharmaceutical manufacturer Amgen, which has several bispecific antibodies under investigation, have engineered antibodies with positive and negative electric charges at certain parts of the structure to promote correct assembly. Subunits fit together only in certain configurations to create a functional molecule.2
"These drugs are exciting because they are the result of innovative protein and antibody engineering," commented Charles Sentman, PhD, from the Geisel School of Medicine at Dartmouth University, where his lab develops bispecific antibodies and other forms of immunotherapy.
As of November 2019, the FDA has approved one bispecific antibody for the treatment of cancer: blinatumomab. The drug binds to CD3 on T cells and the CD19 antigen on tumor cells.
The drug entered phase I trials in 2001 for the treatment of patients with acute lymphocytic leukemia (ALL). Thirteen years later, the FDA granted marketing approval for the treatment of adults and children with relapsed or refractory B cell precursor ALL. That indication was expanded in 2018, through the accelerated approval pathway, to include patients with B cell precursor ALL whose disease is in remission but is positive for measurable residual disease (MRD).
T cells need both the antigen-specific signal for activation and a secondary signal, known as co-stimulation, that immune cells use to mount a fully effective immune response. A unique feature of bispecific T cell engagers, in contrast to the endogenous immune response or to chimeric antigen receptor (CAR) T cells, is that these molecules don't appear to require a co-stimulation signal to activate T cells against tumors.
One theory of why bispecific antibodies may not require the co-stimulation signal for full T cell activation is that these molecules predominantly activate memory T cells, which require less stimulation to mount a cytotoxic antitumor response. The T cells are then able to cluster together more effectively, triggering the necessary signal.3
With bispecific antibodies, the addition of a costimulatory signal has the potential to boost antitumor activity, according to Paulina Velasquez, MD, from St. Jude Children's Research Hospital in Memphis, Tennessee. "The field is exploring strategies that incorporate costimulatory molecules, including expression of one or more of the costimulatory molecules as part of the bispecific antibody," Dr. Velasquez told ASH Clinical News. "I think that these additions could potentially make the T cell responses more robust and longer-lasting."
Advantages of a Bispecific Molecule
There are more than a dozen bispecific antibodies for cancer treatment in clinical trials. One of the draws of these immune system–targeting molecules is that, unlike the customized CAR T cell therapies that take weeks to prepare for an individual patient, bispecific antibodies are off-the-shelf products.
"Ultimately, more patients will likely benefit from bispecific antibodies than CAR T cell therapies because they're just so much easier to use," said Max Topp, MD, from the University Hospital of Würzburg in Germany, who has taken part in clinical trials of blinatumomab and other bispecific antibodies. Still, Dr. Topp said, bispecific antibodies have some disadvantages related to administration. For example, blinatumomab has a short half-life and requires that the drug is given to patients by a continuous intravenous infusion using a pump, sometimes for longer than seven days at a time. "This can be quite cumbersome for patients," he noted.
Another advantage of a bispecific antibody such as blinatumomab, according to Dr. Sentman, is its multifaceted ability to stimulate the adaptive arm of the immune response. The antibody engages and activates T cells, which then further triggers the release of cytokines that attract additional immune cells, including macrophages and natural killer cells, to target the tumor.
"What we're learning is that the cytokines are key to activating a full immune response and bringing in other immune cells, besides T cells, to the tumor," he said. "Also, so far, it has been easier to treat hematologic malignancies than solid tumors. Part of that is related to the issue of access to solid tumors."
"These drugs are exciting because they are the result of innovative protein and antibody engineering."
—Charles Sentman, PhD
The downside of bispecific antibodies' immune stimulation is the risk of cytokine release syndrome (CRS), which occurred in about 15% of patients with relapsed/refractory ALL enrolled in clinical trials of blinatumomab. However, the antibody is still given as an outpatient therapy. "The immune system is powerful and, if you overactivate it, it can be difficult to control," said Dr. Sentman. The good news is that blinatumomab-related CRS can be prevented by using a stepwise dosing schedule, monitoring patients for increased cytokines (including IL-10 and IL-6), and administering a corticosteroid prior to the start of therapy.
Most of the available safety information on bispecific antibodies comes from the clinical trial data from blinatumomab studies and, thus far, its toxicity has been shown to be reversible.4 Like bispecific antibodies, CAR T cell therapies also galvanize T cells against tumors and can result in CRS and neurotoxicity. But, so far, CRS and other toxicities seen with blinatumomab are both lower in frequency and severity than those seen with CD19-targeting CAR T cell therapies.5-8
Once CAR T cells are administered into the body, they cannot be taken back because they are live cells. But this is not true for blinatumomab, which has a short half-life and requires a prolonged IV infusion that can be halted immediately if adverse effects become severe, limiting the potential toxicity. As new, longer-acting bispecific antibodies are being developed, patients may require prophylactic treatment with tocilizumab, an anti-IL6 antibody that is used to manage CRS, Dr. Topp acknowledged.
A Magic Bullet?
There is another important consideration to bringing bispecific antibodies into routine clinical use: Any cancer therapy that relies on activating a patient's own T cells to kill tumor cells requires that the T cells can do their job.
"Bispecific antibodies depend on a patient having T cells with good antitumor activity," explained Dr. Velasquez. "Cancer patients with few comorbidities and who have received no, or few, cancer therapies, have T cells that likely are in better shape to mount an antitumor response. However, in those who have heavily pretreated disease, it is hard to predict whether their T cells will be efficacious."
As with any cancer therapy, potential resistance also is likely to be an issue. "We can infer resistance mechanisms with bispecific antibodies by looking at those seen with blinatumomab and CAR T cell therapies," she added. With immunotherapies that target a specific antigen such as CD19, for example, the tumor could express a mutated CD19 gene that results in a loss of CD19 at the surface. This has occurred in approximately 20% of patients treated with the two FDA-approved CD19-targeting agents.9 "We know that immune escape will likely be a challenge in some patients," Dr. Velasquez said. "One potential solution is to target multiple antigens at the same time, or perhaps, sequentially, to attempt to bypass this form of resistance."
A Plethora of Trial Options
Most of the bispecific antibodies in development for hematologic malignancies are now in early-stage clinical trials. At the 2019 European Hematology Association Congress, Dr. Topp presented results from the first-in-human trial of escalating doses of AMG 420, a bispecific T cell engager that binds B cell maturation antigen on multiple myeloma cells and CD3 on T cells. Among 42 patients with relapsed/refractory multiple myeloma, thirteen (31%) responded to treatment, including six patients who had an MRD-negative complete response.10
In another phase I trial of patients with relapsed/refractory B cell NHL, treatment with REGN1979, an anti-CD20, anti-CD3 bispecific antibody, induced responses across disease subtypes, including a 100% response rate among the 10 enrolled patients with follicular lymphoma and a 42% response rate in the 24 patients with diffuse large B cell lymphoma.11 Based on these results, investigators are planning a phase II clinical trial in these and other B cell NHL subtypes.
Another molecule in development is XmAb14045, an anti-CD123, anti-CD3 bispecific antibody that is being evaluated for relapsed or refractory acute myeloid leukemia and other CD123-expressing hematologic malignancies. The FDA put the antibody's phase I trial on clinical hold in February 2019 due to two patient deaths, one from CRS and one from pulmonary edema that was considered possibly related to XmAb14045 treatment. The trial resumed in April with the addition of a protocol to monitor and treat for CRS. The bispecific antibodies AMG 339, CD20-TCB, and mosunetuzumab also are under investigation.
Just as razor companies have added more and more blades to their products, antibody manufacturers have begun to develop more complex bispecific antibodies. One such compound is AMV564, a tetravalent CD33/CD3 antibody that has four different binding domains and is in clinical trials for leukemia.
The investigators who spoke with ASH Clinical News agreed that, despite the progress and the many clinical trial options available, there is still much to be learned about the biology and clinical activity of bispecific antibodies. "This field has evolved quite rapidly, but we still have relatively little knowledge of the mechanism of how these bispecific molecules engage with the immune system to kill tumors," said Dr. Sentman. "We need to understand the biology better." —By Anna Azvolinsky
References
- Husain B, Ellerman D. Expanding the boundaries of biotherapeutics with bispecific antibodies. BioDrugs. 2018;32:441-64.
- Amgen Science. Bispecific Antibody. Accessed October 29, 2019, from www.amgenscience.com/items/bispecific-antibodies.
- Huehls AM, Coupet TA, Sentman CL. Bispecific T-cell engagers for cancer immunotherapy. Immunol Cell Biol. 2015;93:290-6.
- Topp MS, Kufer P, Gökbuget N, et al. Targeted therapy with the T-cell-engaging antibody blinatumomab of chemotherapy-refractory minimal residual disease in B-lineage acute lymphoblastic leukemia patients results in high response rate and prolonged leukemia-free survival. J Clin Oncol. 2011;29:2493-8.
- Teachey DT, Lacey SF, Shaw PA, et al. Identification of predictive biomarkers for cytokine release syndrome after chimeric antigen receptor T-cell therapy for acute lymphoblastic leukemia. Cancer Discov. 2016;6:664-79.
- Martinelli G, Boissel N, Chevallier P, et al. Complete hematologic and molecular response in adult patients with relapsed/refractory Philadelphia chromosome-positive B-precursor acute lymphoblastic leukemia following treatment with blinatumomab: results from a phase II, single-arm, multicenter study. J Clin Oncol. 2017;35:1795-802.
- Goebeler ME, Knop S, Viardot A, et al. Bispecific T-cell engager (BiTE) antibody construct blinatumomab for the treatment of patients with relapsed/refractory non-Hodgkin lymphoma: final results from a phase I study. J Clin Oncol. 2016;34:1104-11.
- Slaney CY, Wang P, Darcy PK, Kershaw MH. CARs versus BiTEs: a comparison between T cell-redirection strategies for cancer treatment. Cancer Discov. 2018;8:924-34.
- Shah NN, Fry TJ. Mechanisms of resistance to CAR T cell therapy. Nat Rev Clin Oncol. 2019;16:372-85.
- Topp M, Duell J, Zugmaier G, et al. Evaluation of AMG 420, an anti-BCMA bispecific T-cell engager (BiTE) immunotherapy, in R/R multiple myeloma (MM) patients: updated results of a first-in-human (FIH) phase 1 dose escalation study. Abstract #S825. Presented at the 24th European Hematology Association Annual Congress, June 15, 2019; Amsterdam, The Netherlands.
- Topp MS, Arnason J, Advani R, et al. Clinical activity of REGN1979, an anti-CD20 X anti-CD3 bispecific antibody (AB) in patients (PTS) with (W/) relapsed/refractory (R/R) B-cell non-Hodgkin lymphoma (B-NHL). Hematol Oncol. 2019;37(suppl 2):90-92.
