APRIL CAR T-cells for dual targeting of BCMA and TACI

B-cell maturation antigen (BCMA) is a promising target in Multiple Myeloma (MM) and is the current chosen target for numerous MM-directed CAR T-cells, as well as therapeutic monoclonal antibodies – see MM Hub article. However, expression can vary between patients and several studies suggest it has a low target density which further decreases at relapse, thereby potentially escaping therapies and minimizing clinical efficacy.

To address this issue, Lydia Lee and colleagues from the Department of Haematology, UCL Cancer Institute, London, UK, in collaboration with Autolus Ltd, have developed a third generation CAR T-cell construct using the truncated form of a proliferation-inducing ligand (APRIL), the natural ligand for both BCMA and TACI (transmembrane activator and CAML interactor). The resultant construct has been termed ACAR and was tested both in-vitro and in-vivo (with a mouse model). The study was published in Blood in December 2017.

 Key Findings:

• Primary CD138+ bone marrow (BM)-derived MM cells from 50 patients (pts) were found to express BCMA: Median = 1060 (range 105–8323 ABC)
• Co-expression of TACI was found on 39/50 pts: Median = 333 (range 0-21301)
• Co-expression of BCMA and TACI was found in 78% of pts
• The expression pattern of TACI was similar to that of BCMA and limited to mostly lymphoid compartment, therefore not expected to pose a problem with normal tissue
• Three APRIL-based chimeric antigen receptors (ACAR) were designed, comprising a truncated APRIL fused to a spacer domain, a CD28 transmembrane and tripartite 29 endodomain (CD28-OX40-CD3ζ)
• Three ACARs were generated with different spacers: the hinge of human IgG1 (ACAR-H), the stalk of human CD8α (ACAR-CD8) or CH2 and CH3 domains of human IgG1 (ACAR-Fc)
• In in-vitro assays using SUPT1 cells modified to express high levels of BCMA (SUPT1^BCMA) and high levels of TACI (SUPT1^TACI), ACAR-CD8 and ACAR-H transduced T-cells demonstrated superior activity (higher target cytolysis, cytokine release and effector proliferation) compared to ACAR-Fc transduced T-cells
• Flow cytometry confirmed that ACAR-H and ACAR-CD8 constructs could drive target cytolysis at low antigen densities and at low effector to target (E: T) ratios, in the presence of soluble APRIL, BCMA, and TACI
• ACAR-H vs ACAR-CD8 constructs resulted in a similar percentage of primary MM cell death: 72.9±12.2% vs7±5.4% (mean± SEM cytolysis relative to control, N = 5)
• On the basis of similar efficacy, ACAR-H was selected for further in vivo studies due to being smaller and simpler than ACAR-CD8
• Using an intramedullary myeloma mouse model, significant tumor clearance was observed in ACAR-H treated mice observed at day 12 post CAR T-cell injection, and by immunohistochemistry (IHC) compared to two control cohorts (EGFRvIII CAR P < 0.05 and untreated P < 0.001)
• Using NSG mice engrafted with a mix of SUPT1^BCMA and SUPT1^TACI cell, model for ACAR-mediated tumor control despite BCMA down-regulation was modeled
• The most effective tumor clearance was observed in ACAR treated mice compared to those receiving BCMA-CAR or NT T-cells

In summary, this study provides a strong rationale for the further clinical investigation of a dual targeting CAR. ACAR displayed target cytolysis at low E: T ratios in vitro and rapid tumor regression in an intramedullary murine myeloma model (48 hours post-ACAR T-cell injection). Additionally, no normal tissue toxicity was observed in vivo and no obvious off-target effects. A-CAR could prove to be especially beneficial for patients with low levels of tumor BCMA, as well as heavily pre-treated patients that may experience antigen down-regulation.