Later-Generation Cellular Therapies Look Promising in Hematologic Malignancies

Mehdi Hamadani, MD

As more chimeric antigen receptors (CAR) undergo clinical development for the treatment of hematologic malignancies, their approval for use in a wider array of malignancies is inevitable. Currently, CAR T-cell therapy has its limits, with benefits mostly seen in hematologic tumors, leading investigators to look towards later-generation CAR T cells and other adoptive cellular therapies. CAR T-cell development involves transducing T cells from the patient or a healthy donor with a synthetic T-cell receptor (TCR) engineered to bind to a specific antigen on the surface of tumor cells. CARs also include costimulatory intracellular signaling domains necessary for expansion and persistence of the T cells.^1-3

Currently approved CAR T-cell products for hematologic malignancies, all of which are indicated for aggressive, advanced, or relapsed/refractory (R/R) disease, include axicabtagene ciloleucel (axicel; Yescarta) for diffuse large B-cell lymphoma (DLBCL), other B-cell lymphomas, and follicular lymphoma (FL); tisagenlecleucel (Kymriah) for adults with DLBCL and young adults with B-cell precursor acute lymphoblastic leukemia (ALL); lisocabtagene maraleucel (liso-cel; Breyanzi) for DLBCL, high-grade B-cell lymphomas, and FL; brexucabtagene autoleucel (Tecartus) for mantle cell lymphoma; and idecabtagene vicleucel (Abecma) for multiple myeloma (MM).^4-8

Mehdi Hamadani, MD, a professor of internal medicine at the Medical College of Wisconsin and director of the Adult Blood and Marrow Transplant Program at Clinical Cancer Center at Froedtert Hospital in Milwaukee, calls CAR T cells a very relevant therapeutic tool for lymphoma treatment.

“There are obvious advantages of an autologous product. Your immune system is not going to reject an autologous blood product. It’s going to be curative for some patients, and the toxicity profile is generally manageable. Most centers have enough experience to work with these toxicity signals,” Hamadani said.

Toxicities include potentially life-threatening cytokine release syndrome (CRS) and neurologic toxicities^4-8 and hematologic toxicities, including hemophagocytic lymphohistiocytosis/macrophage activation syndrome and cytopenia.⁸ Other shortfalls include limited antitumor activity and antigen escape (loss or downregulation of target antigen expression).¹

Axi-cel, tisagenlecleucel, liso-cel, and brexucabtagene autoleucel are all anti-CD19 CAR T-cell therapies.^4-7 Antigen escape causes CD19-negative relapse in approximately 30% of patients with lymphoma treated with these therapies, Hamadani said, rendering ineffective other CD19-directed therapies recently approved for aggressive lymphomas that might have been used for subsequent treatment (FIGURE1).

There are also logistical challenges. It takes time to collect patient cells, ship them to the manufacturing facility, generate the CAR T cells, and ship them back.

“The turnaround between sending the product and getting the product back would be 3 to 5 weeks, depending on the commercial product. Some patients with lymphoma can wait 3 to 5 weeks. Some can’t. Some patients can have disease progression or even death while waiting for the product to come back,” Hamadani said, adding that complete manufacturing failures occur up to 10% of the time.

Sometimes the product does not meet prespecified FDA quality-control measures. Those CAR T cells are called “out-of-spec” products.

Jason Westin, MD, MS

Other cellular therapies in development for hematologic malignancies may help address limitations of the current CAR T-cell therapies, said Jason Westin, MD, MS, director of Lymphoma Clinical Research, Department of Lymphoma/Myeloma, at The University of Texas MD Anderson Cancer Center in Houston.

“There are many different types of next- generation cell therapies, but they share certain properties,” Westin said. “They generally have a faster production time, result in increased cell fitness, and have lower risk for toxicities, especially longer term, in comparison with the existing generation of therapies. Some of the most innovative approaches may result in autologous cell therapies that are delivered in near ‘off-the-shelf’ timelines, which may [avoid] the risk from an allogeneic product and have greater persistence.”

Next-Generation CAR T Cells

Idecabtagene, which targets B-cell maturation antigen (BCMA), is the only approved CAR T-cell product not targeting CD19.⁸ Hematologic malignancies that do not express CD19 and CD19-negative relapse in patients treated with CD19 CAR T cells could be addressed by targeting different antigens.

A CAR T-cell therapy directed at an alternative B-cell antigen, CD22, is in a phase 1 trial for B-cell ALL treatment (NCT02315612). Recently reported results showed that, of 208 patients, 62% had received a prior CD19 CAR T-cell treatment and 88% had received any prior CD19-targeted therapy; more than half had CD19-negative disease. CD4/ CD8 T-cell selection was used to enhance CAR T-cell manufacturing feasibility and reduce product variability. However, this resulted in increased inflammatory toxicities, requiring dose de- escalation. Nevertheless, the complete remission rate was 70%, supporting further development.⁹

Another CAR T-cell product in development targets CD33 and is being tested in a phase 1/2 trial (NCT03971799) recruiting children and young adults with acute myeloid leukemia.

Most CAR T-cell therapies in clinical trials contain only 1 costimulatory domain. MB-106, a fully human, third-generation, CD20- directed CAR T-cell product with 2 costimulatory domains is in an ongoing trial (NCT03277729) for B-cell non-Hodgkin lymphoma (NHL) and chronic lymphocytic leukemia (CLL). Initial lack of response has been ameliorated by manufacturing changes that included combined culture of CD4- and CD8-positive cells. Investigators have reported a 93% overall response rate (ORR) and a 67% complete response (CR) rate. This has been observed at particularly higher dose levels, which are also associated with faster CAR T-cell expansion; no relapses have been observed in patients with CRs. Use in combination or in sequence with a CD19 CAR T therapy has been proposed.^10,11

Another strategy to counter antigen escape is to target 2 or more antigens.1 Multitargeting has been proposed to address CD19- negative and CD22-dim (diminished expression) relapses in ALL.² One such bispecific CD19/CD22 CAR T-cell product is in a phase 1 trial in relapsed/refractory (R/R) B-cell ALL (NCT03185494).¹² All 6 patients treated in this small trial experienced CRs, although 3 experienced relapse within 3 to 10 months. Only 1 relapse was due to antigen escape (CD19-negative/CD22-dim); the other 2 relapses were associated with decreased CAR T-cell persistence. Three of the 6 patients have an ongoing response, and toxicity has been manageable.¹²

A novel CAR molecule targeting both CD19 and CD22 is in ongoing phase 1 studies in R/R B-cell malignancies: 1 for adults (NCT03233854) and 1 for children and young adults (NCT03241940). Autolus Therapeutics has multiple CAR T products in development, including a CD19/CD22 product (AUTO3) investigated in AMELIA (NCT03289455), a completed phase 1/2 trial in B-cell ALL.13 In the ongoing ALEXANDER study (NCT03287817) of this bicistronic CAR T product in R/R DLBCL, the anti–PD-1 immunotherapy pembrolizumab (Keytruda) is being added to AUTO3 treatment to reduce the likelihood of relapse.¹⁴

A bispecific anti-CD20, anti-CD19 CAR T product (LV20.19 CAR T cells) is in a phase 1 trial in patients with R/R B-cell malignancies (NHL and CLL; NCT03019055). This product is manufactured on site in a local stem cell processing laboratory, which allows prompt infusion of fresh, noncryopreserved CAR T cells. In the trial, the 28-day overall response rate was 82% (18 of 22 patients), with 64% CRs. The median overall survival was 20.3 months. There was no association between clinical responses and baseline CD19 or CD20 expression in tumors. CD19 antigen escape did not appear to underlie lack of response; loss of CD19 expression was not observed in patients who experienced relapse or treatment failure. Treatment of patients who previously received unsuccessful CD19 CAR T-cell therapy remains a challenge; of 5 such patients in this trial, only 1 achieved adequate CAR T-cell expansion and a CR. The LV20.19 CAR T cells contain the same murine anti-CD19 domain as the commercial CD19 CAR T-cell products those 5 patients previously had received, suggesting that prior exposure led to immune-mediated rejection of the murine portion, representing an obstacle to retreatment.¹⁵ Long-term follow-up (NCT03375619) continues.

TCR-Engineered T-Cell Therapy

Another type of adoptive T-cell therapy, TCR-engineered T-cell therapy, uses genetically modified T cells with a TCR that specifically binds to a tumor-associated antigen. Unlike CAR T cells, TCR T cells interact with a peptide antigen presented by major histocompatibility complex (MHC), also called human leukocyte antigen (HLA), making them more sensitive than CAR T cells and more effective in some contexts, particularly for treating solid tumors.^16,17

A major advantage of TCR T cells over CAR T cells is that target antigens are not limited to cell surface proteins; TCR T cells can also bind intracellular proteins digested into peptides and presented by MHC. Additionally, the risk of CRS is reduced. Advantages of CAR T cells include their MHC/HLA independence. One barrier to TCR T-cell therapy is identifying targets that are sufficiently cancer specific to avoid off-target toxicities. TCR T-cell products in development that target a single antigen are subject to antigen escape in much the same way that single-target CAR T cells are. However, targeting antigens essential to maintaining the malignant phenotype, such as leukemia- initiating fusions, should overcome this barrier, as would the infusion of more than 1 TCR T-cell product or a single product that expresses multiple TCRs.¹⁷ Ongoing trials of TCR T-cell therapies in hematologic malignancies are summarized in the TABLE.

NK Cell Therapies

Natural killer (NK) cells are also being exploited in cellular therapies for hematologic malignancies. NK cells are lymphoid cells that play a role in the innate immune response to infectious agents and malignant cells. Unlike T cells, they do not need prior antigen exposure or MHC expression to kill transformed cells. Normal cells are spared because NK cells recognize critical self- antigens expressed on them.³

Sources of NK cells can be autologous or allogeneic, such as donor peripheral blood, umbilical cord blood, and stem cells, including induced pluripotent stem cells. Though considered less prone than other allogeneic cells to both graft-vs-host disease (GVHD) and host-vs-graft rejection, NK cells may be susceptible to immunosuppression by the tumor microenvironment. Because they occur in small numbers, NK cells need cytokine support to survive more than 1 to 2 weeks, and they often require ex vivo expansion and activation. Although repeated infusions of expanded donor NK cells may address the short life span, patients with R/R acute leukemias may not be able to wait for expansion.^3,18

NK cells derived from induced pluripotent stem cells can be readily expanded, but as immature cells they may lack some killing function. Cell line–derived NK cells have malignant potential and require irradiation. The creation of universal-donor, allogeneic NK cell banks may overcome some limitations with the option to provide educatable CAR NK cells for off-the-shelf therapy.^3,18

CAR NK cells in trials for B-cell leukemias and lymphomas target CD19; BCMA is a target of CAR NK cells designed to treat MM. In a phase 1/2 study of R/R B-cell hematologic malignancies (NCT03056339), CD19-directed CAR NK cells derived from cord blood and engineered with a suicide gene safety switch produced responses in 8 of 11 patients. The NK cells were expanded ex vivo and were HLA mismatched with the recipient. Neither GVHD nor CRS occurred in any patient.^3,19 Although these CAR NK cells were transduced using a retroviral vector,¹⁹ nonviral strategies to genetically modify NK cells using electroporation or transposons and CRISPR/Cas9 have led to increased efficiency and stable genomic insertion. Other CAR NK cell therapies in development for hematologic malignancies have CD33, CD20, CD38, CD123, CLL1, and FLT3 as single targets; dual-targeted products are directed to CD19/CD22.¹⁸

Other Novel Products

Other allogeneic products in development can address the limitations of turnaround time and manufacturing failures associated with current autologous CAR T-cell therapies. “The allogeneic platform gives you the ability, in theory, [to] readily get the product, but for an allogeneic CAR you still have to check donor-specific antibodies in the patient, because if the patient has premade antibodies against the CAR, they will reject it. In addition, they may cause GVHD, as in an allogeneic transplant,” Hamadani said.

ALLO-501A, an allogeneic anti-CD19 CAR T-cell therapy, is being tested in patients with R/R large B-cell lymphoma in the ongoing ALPHA-2 trial (NCT04416984). Proprietary gene editing technology was used to disrupt TRAC, a gene that encodes part of the TCR; without functioning TCRs, the infused CAR T cells cannot recognize and attack allogeneic antigens, which should eliminate GVHD. Similarly, the CD52 gene was knocked out via editing to allow the addition of ALLO647, a humanized anti-CD52 monoclonal antibody, to the lymphodepletion regimen to selectively deplete CD52-positive host T cells while preventing host rejection of the CD52-negative CAR T cells, enabling their expansion and persistence.²⁰ Although the ALPHA-2 results were from a small cohort, they showed that ALLO-501A’s efficacy mirrored that of ALLO-501, a previous iteration containing rituximab recognition domains, in the ALPHA trial of patients with R/R NHL (NCT03939026). Additionally, ALLO-501A therapy with ALLO-647 lymphodepletion was well tolerated, with no GVHD, and redosing at disease progression was safe and may provide clinical benefit.²⁰

PBCAR0191, another off-the-shelf, allogeneic CD19-targeting CAR T-cell therapy, is in an ongoing early-phase trial in patients with R/R NHL and B-cell ALL (NCT03666000). The trial is comparing the standard lymphodepletion regimen to an enhanced one with extended fludarabine and higher-dose cyclophosphamide. The results in the R/R NHL cohort favored enhanced lymphodepletion, with 50-fold greater peak CAR T-cell expansion and deeper, more durable clinical responses, including a higher CR rate.²¹

Looking Ahead

More data are needed to demonstrate the utility of the newer cellular therapy platforms, whether they be dual- targeting, third-generation, or off-the-shelf allogeneic products.

Cost is an issue that often goes unmentioned. “Cost will become an increasingly important barrier for cellular therapy to be deployed at scale and potentially could be a reason to choose one approach over another if efficacy is similar,” Westin said.

Hamadani agreed that cost is a major concern. “People keep saying, ‘Our CAR is going to be cheaper,’ and so far every single CAR that gets approved is more expensive than the last one, and when they see that the next is more expensive, they raise their price,” he said. “That is what happened with liso-cel. When lisocel [was] approved, it was more expensive than axi-cel, and axi-cel recently increased their price to match liso-cel. Allogene promises that its AlloCAR T therapies are going to be cheaper.”

Other unmet needs include more effective cell therapies that provide more durable remissions for more patients.

Hamadani said he sees durable remission in approximately 40% of patients receiving CAR T-cell therapy, meaning that 60% will experience a relapse. “More effective cellular therapy platforms are needed, safer platforms are needed, cheaper platforms are needed,” Hamadani said.

“The cell therapy field for patients with lymphoma is moving very fast, potentially with CAR T cells becoming a new standard of care for patients with first relapse of aggressive lymphomas,” Westin said. “Cell therapy may not be right for every patient with lymphoma, but it is a powerful new weapon that sets the example for the potential for cellular therapy against all cancers as we break down more barriers.”

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