Bispecific antibody, anti-CD3/anti-CLL-1 for AML treatment

Bispecific antibodies are antibodies that combine the specificities of two antibodies that simultaneously target different antigens¹.  The use of antibodies as an immunotherapeutic approach for cancer treatment has been considered a viable strategy, since bispecific antibodies have demonstrated clinical activity. The anti-cancer effects of bispecific antibodies have been achieved by their binding to CD3 thus causing polyclonal CD3⁺ T-cells to get closer to a tumor target².

The C-type Lectin-like Molecule-1 (CLL-1) is normally present on the surface of myeloid cells and has been shown to be overly expressed in Acute Myeloid Leukemia (AML) hence a therapeutic target for AML. Furthermore the use of a bispecific antibody that targets CLL-1 could be effective in redirecting the human immune system to treat AML. In order for bispecific antibodies to exert an optimal therapeutic effect, the pharmacokinetic (PK) properties of these agents have to be considered. At present the main pharmacokinetic concerns relating to bispecific molecules stem from their lack of Fc domain function which is necessary for extended circulation.³ A bispecific antibody with an improved pharmacokinetic profile might help to overcome some of the current shortfalls.

Steven R. Leong and colleagues based in San Francisco, USA, published results from their study on bispecific antibodies in February 2017 in Blood.

The authors describe a new humanized IgG1 CD3 T-cell- Dependent Bispecific (TDB) antibody that targets CLL-1. They investigated both the PK properties and safety profile of this novel formulation in both human AML cell lines and animal models.

The key results of the study are:

• Three TDBs were developed comprising of anti-CLL-1 paired with different affinities of anti-CD3; CLL1/CD3Low (CLL1/CD3L), CLL1/CD3High (CLL1/CD3H) and CLL1/CD3VeryHigh (CLL1/CD3VH)
• CLL1/CD3H and CLL/CD3L TDB decreased the percentage survival of CLL-1+ AML cell lines in a dose-dependent manner
• CLL/CD3L TDB had a higher EC50 by four-to 100 fold compared to the CLL/CD3H
• CLL/CD3H was more effective in lysing AML CD11b+/CD33+ and CD11b-/CD33+ target cells compared to control Non-Targeting (NT) TDBs with an EC₅₀ ranging from 0.45-3 ng/ml ; n = 2
• Rapid depletion of human CLL-1+ cells was observed in the blood, bone marrow and spleen in 2xBAC-Tg GEMM mice (a transgenic mice generated to co-express human CD3 and CLL-1) administered with all affinities of CLL1/CD3 TDBs
• There was an increase in CD4+ and CD8+ T- cell activation in 2xBAC-Tg GEMM mice treated with all affinities of CLL1/CD3 TDBs compared to NT-TDBs control
• CLL1/CD3H and CLL1/CD3VH showed two to four times faster clearance and decrease in exposure compared to control in 2xBAC-Tg GEMM mice.
• Cynomologous monkeys (expresses CD3 and CLL-1) administered CLL1/CD3L developed vascular shock, fever and elevated levels of pro-inflammatory mediators compared to monkeys administered CLL1/CD3L

In summary, anti-CD3/anti-CLL-1 bispecific antibody can deplete CLL-1+ target cells in animal models. 

The authors concluded their study by highlighting that the humanized IgG anti-CD3 targeting human CLL-1 had a low liability of off target destruction of HSCs and a superior PK properties. They further suggested that targeted therapy with anti-CD3/anti-CLL-1 can potentially be effective in AML patients.

Abstract

Acute myeloid leukemia (AML) is a major unmet medical need. Most patients have poor long-term survival, and treatment has not significantly changed in 40 years. Recently, bispecific antibodies that redirect the cytotoxic activity of effector T cells by binding to CD3, the signaling component of the T-cell receptor, and a tumor target have shown clinical activity. Notably, blinatumomab is approved to treat relapsed/refractory acute lymphoid leukemia. Here we describe the design, discovery, pharmacologic activity, pharmacokinetics, and safety of a CD3 T cell-dependent bispecific (TDB) full-length human IgG1 therapeutic antibody targeting CLL-1 that could potentially be used in humans to treat AML. CLL-1 is prevalent in AML and, unlike other targets such as CD33 and CD123, is not expressed on hematopoietic stem cells providing potential hematopoietic recovery. We selected a high-affinity monkey cross-reactive anti-CLL-1 arm and tested several anti-CD3 arms that varied in affinity, and determined that the high-affinity CD3 arms were up to 100-fold more potent in vitro. However, in mouse models, the efficacy differences were less pronounced, probably because of prolonged exposure to TDB found with lower-affinity CD3 TDBs. In monkeys, assessment of safety and target cell depletion by the high- and low-affinity TDBs revealed that only the low-affinity CD3/CLL1 TDB was well tolerated and able to deplete target cells. Our data suggest that an appropriately engineered CLL-1 TDB could be effective in the treatment of AML..