By Richard A. Stein, MD, PhD
Chimeric antigen receptors (CARs), engineered to provide T cells with the ability to target novel antigens, have emerged in recent years as a novel and promising therapeutic strategy. Six CAR T-cell therapies have been approved by the U.S. Food and Drug Administration (FDA) to date for patients with advanced, relapsed, or refractory hematologic malignancies. But realizing the benefits of CAR T-cell therapies in treating solid tumors has been more challenging, for reasons that include the immunosuppressive tumor microenvironment and the heterogeneity of tumor antigens. Increasing efforts are focusing on generating CAR T-cell therapies that benefit patients with solid malignancies, and on expanding the technology to other medical conditions, such as autoimmune and infectious diseases.
Allogeneic cells from the placenta
Historically, the placenta has been viewed simply as a vascular interface between the mother and the developing conceptus. The placenta’s ability to create an immune-privileged location and its wealth of stem cells have not been sufficiently appreciated, yet the placenta could be key to developing therapies for diseases that are challenging to manage.
“The placenta is nature’s professional universal donor tissue, and the biology of its cells would make it the perfect starting material for broadly deployable cell therapies,” says Robert J. Hariri, MD, PhD, the chairperson, founder, and CEO of Celularity. “We know that the placenta from a healthy newborn has gone through nature’s quality control process. So, when the CAR T-cell therapies started to emerge, it was very clear to me that the placental immune cells could be a great platform to engineer constructs.
“Both in the laboratory and in the clinic, we have found that placental cells … do not stimulate an alloreactive immune response. In our experience, patients have not developed alloantibodies and do not become sensitized or unresponsive to subsequent dosing. These benefits fit the CAR T-cell world perfectly, and this is where I think the field is going to move. Cell therapy may require repeat dosing, as opposed to expecting one dose to have its full biological activity.”
Celularity has several biomaterial products on the market under FDA designation, and a range of cell therapy products as candidates in clinical trials. The lead Celularity therapeutic program, CYNK-001, is based on a placental-derived unedited natural killer cell that was developed as a cryogenic, off-the-shelf allogeneic product for several solid cancers, multiple myeloma, and COVID-19.
Celularity scientists found that placental natural killer cells specifically target stress antigen–expressing cells, which include not only cancer cells but also virally infected and senescent cells, and this opened a broad opportunity to use them for additional medical conditions. Several CYNK-CAR candidates that rely on genetically engineered natural killer cells derived from human placental hematopoietic stem cells are currently in preclinical development at Celularity for hematological and solid cancers.
Additional advantages of using the placenta for cancer immunotherapy include its availability, scalability, and engineerability. “And the beauty of getting a cell from the placenta,” Hariri observes, “is that the DNA does not have a lot of epigenetic baggage, which would otherwise create variability among donors.”
Harnessing EBV-specific T cells
At least 95% of adults 40 years and older have been infected with the Epstein-Barr virus (EBV). In most people infected with EBV, a large repertoire of memory T cells is generated that controls the virus. However, individuals with weakened immune systems may develop EBV-driven malignancies (such as Burkitt lymphoma or nasopharyngeal cancer) or autoimmune diseases (such as multiple sclerosis).
EBV-specific T cells are central to the cell therapies that are being developed by Atara Biotherapeutics. The company collects immune cells from healthy donors, then develops preparations that are enriched with EBV-specific T cells. These preparations may serve as treatments directly, targeting EBV-infected cells and thereby fighting the root cause of EBV-associated diseases. Alternatively, the EBV-specific T cells in these preparations may be modified with CARs to create CAR T-cell therapies that can target many non-EBV-associated diseases.
“An important nuance about our platform is the versatility of EBV-specific T cells,” says Cokey Nguyen, PhD, CSO at Atara Biotherapeutics. “We can take advantage of EBV-specific T cells with or without next-generation CAR design.”
ATA3219, Atara’s lead allogeneic CAR T-cell product, is currently in preclinical development. It targets CD19-positive B-cell malignancies, and it has several attributes designed to improve efficacy, safety, and persistence over current autologous and allogeneic CAR T-cell approaches.
To produce ATA3219, Atara obtains B cells from healthy donors and transforms these cells with EBV to create an EBV-positive lymphoblastoid cell line, which is then used to generate preparations that are enriched with EBV-specific T cells. A retroviral vector is then used to insert a CD19-targeting optimized CAR into the cells.
The manufacturing process enriches for a memory, or less differentiated, T-cell phenotype that has been clinically correlated with improved and durable clinical responses. ATA3219 has a next-generation design that uses the 1XX costimulatory domain, which refers to a CAR with only the ITAM1 region, to enhance the expansion of the CAR T cells and their functional persistence.
Also in Atara’s pipeline is ATA3271, an allogeneic CAR T-cell immunotherapy that targets solid tumors expressing mesothelin, an antigen present in mesothelioma, triple-negative breast cancer, and other solid tumors. ATA3271 is currently in preclinical development.
A key decision in designing CAR products for solid tumors involves armoring, which refers to further engineering the cells to express surface ligands or secrete cytokines that target the tumor microenvironment. Many studies have confirmed the safety of the PD-1 blockade as armoring to overcome checkpoint inhibition. “That is why we use a PD-1-dominant negative receptor, which is part of our first-generation armoring,” Nguyen notes.
Additional differentiating factors of the Atara platform include the lack of T-cell receptor or HLA gene editing, the ability to employ advanced processing technology, the possibility of adapting the technology to various therapeutic targets, and an off-the-shelf approach. With the aim of minimizing graft-versus-host reactions, the Atara team uses partial HLA matching between donors and recipients.
Atara has evaluated findings for over 500 patients who received EBV-specific T-cell immunotherapies. According to Nguyen, the findings suggest that these immunotherapies can be safe. He says, “We have not detected any instances of the host rejecting the T cells.” In addition, there are indications of efficacy. However, with CAR T-cell therapies, defining the efficacious dose of cells remains difficult. Also, it is still a challenge to generate cells in sufficient numbers. “This is something that we do not talk about often enough as a field,” Nguyen says.
Innovation through lateral CAR constructs
The lead asset at Leucid Bio, LEU011, is a CAR T cell directed against NKG2D ligands, a family of eight targets that are upregulated by various forms of cellular stress. “The majority of malignant tumors express NKG2D ligands,” notes John Maher, MD, PhD, the company’s founder and CSO.
LEU011 is currently in preclinical development. Leucid anticipates starting a basket Phase I trial on LEU011 later this summer that will include patients who have any type of solid tumor that expresses NKG2D ligands, and who have exhausted conventional medical approaches.
A differentiating feature of LEU011 is Leucid’s proprietary lateral CAR structure. The company says that this structure delivers better durability and functional persistence, and that it outperforms previous CAR designs in clinical trials.
“We found that in order to design CARs that deliver an optimum blend of signaling, we need to build them out, rather than up,” Maher says. “This positions the signaling modules to their natural location next to the plasma membrane.”
To overcome key challenges in CAR T-cell treatment of solid and blood tumors, Leucid takes an integrated approach. Specifically, Leucid brings together a lateral CAR T-cell platform and a CAR T-cell armor approach. LEU011 incorporates a lateral CAR and a homing receptor. With the homing receptor, which is designed to recognize small molecules released from the tumor microenvironment, LEU011 is better equipped to move from the bloodstream into a targeted tumor.
“The cells will home in on the tumor more efficiently and reside in vital organs for a shorter period of time,” Maher says. “We are hoping that this will improve efficacy and make the product safer.”
The very broad design of LEU011 should be beneficial for many solid tumors in which NKG2D ligands are detected from archival material. “It is difficult to know in advance which tumor will be the most suitable one to treat,” Maher admits. “This is the reason for the very broad design, but we hope to learn quickly about which patient groups will benefit most.”
Another asset in the Leucid pipeline, LEU001, is a pan-ErbB CAR T-cell product that is currently in a Phase I trial for recurrent head and neck squamous cell carcinoma. “We chose head and neck cancer because the main clinical problem in this patient group is locally advanced or locally recurrent tumor formation,” Maher explains. Terminal-stage head and neck cancers are particularly challenging due to their extremely rapid growth, with tumors doubling in size in some patients over six weeks. However, these are accessible tumors that the therapeutic can be injected into, minimizing systemic toxicity.
“Having now treated 19 patients with terminal disease, we have not detected any dose-limiting toxicities, and 10 participants achieved stable disease,” Maher reports. “One of the patients who achieved stable disease subsequently enrolled in another clinical trial that administered an oncolytic virus with pembrolizumab and achieved complete remission that persisted for 3.5 years.”
CAR T-cell therapies are currently administered in tertiary referral centers, which restricts access for many patients. “Even once we have effective CAR T-cell solutions for solid tumors, delivering them to patients will be a major challenge,” Maher says.
Relying on innate immunity
Takeda Pharmaceuticals’ immunotherapy programs are built on two major themes. “As part of our programs targeting solid tumors, we focus on approaches that enhance innate immunity through type I interferon biology, while our redirected immunity strategies focus on cells and cell engagers,” says Christopher Arendt, PhD, head of Takeda’s Oncology Cell Therapy and Therapeutic Area Unit. The heterogeneous expression of antigens on solid tumors, even in the same patient, and the presence of the same antigens in healthy tissue continue to represent major obstacles in the field. “As we reflect on solid tumors,” Arendt relates, “we increasingly recognize their complexity on multiple levels.”
In late 2021, Takeda acquired GammaDelta Therapeutics, a developer of immunotherapies based on gd T cells. Within this group of cells, a major focus at Takeda is on Vd1 gd T cells, which are the predominant type of gd T cells in adult peripheral tissues. Vd1 gd T cells show potent cytotoxicity, can recognize both malignant and virally infected cells, and are associated with antitumor responses in clinical studies.
In early 2022, Takeda acquired Adaptate Biotherapeutics, a developer of antibody-based therapeutics to modulate Vd1 gd T cells. Through this acquisition, Takeda obtained Adaptate’s platform for developing gd T cell engagers that can generate immune responses at cancer sites without affecting healthy cells.
A unique asset in Takeda’s redirected immunity platform is TAK-102, which was licensed from Noile-Immune Biotech and is now being tested in Japan, in an open-label, dose-escalation Phase I trial that is recruiting adult patients with previously treated solid tumors that express GPC3. The strategy behind TAK-102 relies on the ability of the cells to migrate to the tumor that has GPC3 and deliver two payloads to stimulate both innate and adaptive immunity in the tumor microenvironment. “We will see more innate cell–tailored engineering strategies for solid tumors,” Arendt predicts.
Also in 2021, Takeda acquired Maverick Therapeutics and its unique, conditionally activated T-cell engager platform, which is designed to target a broad range of solid tumors with highly specific and potent activity while limiting toxicities in normal tissues. TAK-186 is in a Phase I/II study for the treatment of EGFR-expressing solid tumors, and TAK-280 is in a Phase I/II study for the treatment of patients with B7-H3-expressing solid tumors.