The clinical trial results for KYMRIAH and YESCARTA had already been published in the New England Journal of Medicine
By Anamika Ghosh, Ph.D., and Dana Gheorghe, Ph.D., DRG Oncology
A novel and exciting approach to cancer treatment, CAR T cell therapies bring forth a new paradigm in cancer immunotherapy, wherein a patient’s own T cells are bioengineered to express chimeric antigen receptors (CARs) that identify, attach to, and subsequently kill tumor cells.
Novartis’ Kymriah, the first ever such therapy to receive regulatory approval for the treatment of B-cell acute lymphoblastic leukemia (ALL), a hematological malignancy, entered the U.S. market in August 2017 and was followed in October 2017 by Gilead/Kite Pharma’s Yescarta — also a CAR T cell therapy — targeting diffuse large B-cell lymphoma (DLBCL) and primary mediastinal large B-cell lymphoma (PMBCL), subtypes of non-Hodgkin’s lymphoma (NHL). Kymriah was subsequently granted an FDA label expansion to include its use in patients with DLBCL in May 2018. Geographic expansion soon followed, with Kymriah receiving marketing authorization from the EU in August 2018 and from Japan’s MHLW in March 2019 for treatment of B-cell ALL and DLBCL and Yescarta receiving EU approval in August 2019 for treatment of DLBCL and PMBCL.
The landmark approvals and clinical success of Kymriah and Yescarta opened new and encouraging avenues for developers of cellular immunotherapies. Research in the field of CAR T cells has progressed rapidly, and novel technologies to address areas left unaddressed by Kymriah and Yescarta have started streaming into the research arena.
This article aims to focus on the barriers to widespread commercial adoption of the currently available CAR T cell therapies and how these weaknesses are presenting opportunities for developers of the next generation of CAR T cells.
Limitations Directly Affecting Patients
Life-Threatening Adverse Events
Close patient monitoring is a crucial part of the treatment protocol for both Kymriah and Yescarta, as the therapies are associated with high-risk side effects such as cytokine release syndrome (CRS) and CAR T-related encephalopathy syndrome (CRES). CRS, a type of systemic inflammatory response, is typically characterized by high fever, lower-than-normal blood pressure, and difficulty breathing. CRES, a toxic encephalopathic state, often manifests with symptoms of confusion and delirium, seizures, and cerebral edema. Administration of CAR T cells must be followed by strict adherence to patient safety protocols to ensure that proper measures are taken to immediately manage these high-risk side effects.
Wait During Vein-To-Vein Time
The manufacturing process of autologous CAR T cells requires leukapheresis, followed by extraction of patients’ T cells, transportation to the manufacturing facility, genetic engineering to incorporate CARs, and transportation of the finished product back to the treatment center. The highly personalized therapy is then administered to the patient. The period in between, referred to as vein-to-vein time, ranges between three and four weeks for both Yescarta and Kymriah. This period can be daunting for the patients awaiting treatment and renders these CAR T cells unsuitable for patients with rapidly progressing disease.
Treatment Is Restricted To Heavily Pretreated Patients
Patients must have progressed on at least two lines of systemic therapies to be eligible for Kymriah or Yescarta treatment. Heavily pretreated patients can be weakened by progressing disease and prior therapies and thus be unable to withstand the severe side effects of CAR T cells. Thus, the eligible patient pool to qualify for these therapies gets further limited to heavily pretreated patients with good performance status.
Limitations Directly Affecting Healthcare Practitioners
Complex Patient Referral Pathway
Because of the complex nature of the therapy and its associated high-risk side effects, access to CAR T cells is highly regulated, being available only at certified centers. Primary care oncologists must refer eligible patients to CAR T cell therapy specialists, a process that hinders the widespread adoption of CAR T cell therapy. To offset this complexity, Gilead is now training its oncology representatives to inform physicians about CAR T cells, encourage identification of Yescarta-eligible patients, and help them with patient referrals.
Accreditation Of CAR T-Cells Specialty Centers And Training Of Hospital Staff
The FDA mandates CAR T cells be available only through a restricted and regulated program, in certified centers and administered by trained healthcare providers (HCPs) who adhere to risk evaluation and mitigation strategies (REMS) guidelines. Training of HCPs is a mandatory step toward getting a center certified as a CAR T cell specialist center. The long training process and the increasing demand for CAR T cells, however, are increasing patient waiting lists as new centers await certification.
Lack Of Clarity In Placement Of CAR T Cells In Treatment Practice
Novel drug classes with limited clinical data, such as CAR T cells, require research to ascertain some practical aspects of patient treatment in the commercial setting. Some physicians are skeptical about prescribing CAR T cells, as they are unsure about this therapy’s place in the treatment algorithm and its impact on further lines of therapy.
Limitations Associated With Complicated Manufacturing Process
Failure In Production
Being a highly personalized therapy, the complex, multistep process of generating autologous CAR T cells increases the risk of production failure, an event that delays and, in some instances, even denies access to the therapy.
Commercial Scalability Challenges
With each product representing a fresh manufacturing batch, the production of autologous CAR T cells that meet commercial demand and anticipated label and geographical expansions, while maintaining product quality and clinical equivalence, remains a challenge.
Limitations Due To Exceptionally High Therapy Cost And Complicated Payer Policies
In the United States, CMS recently raised reimbursement of the total cost of CAR T cell therapies from 50 percent to 65 percent, effective from 2020. Treating physicians, however, maintain that given the extremely high cost of therapy (ranging between $373,000 and $475,000 per infusion) and patient management (which can go as high as, and sometimes also over, $0.5 million), the reimbursement gap remains unsustainable and is a huge impediment to patient access. Novartis offers outcomes-based pricing for Kymriah (only for the treatment of B-cell ALL) — an agreement that ties the therapy’s clinical success to its payment. However, this arrangement does not include the hospital expenses associated with the therapy. While the access and reimbursement policies are being ironed out, the queue of patients waiting for insurance clearance is continuing to grow.
Opportunities & Developments
Despite the challenges listed above, the overall attitude about CAR T cells is decidedly positive. Investors are convinced that CAR T cells are a revolutionary cancer treatment. While physicians indicate that the safety issues that are synonymous with CAR T cell therapy are a huge concern and call for an urgent solution, research is already underway to devise solutions that can address the pain points of the currently available CAR T cells. Some noteworthy concepts and developments are discussed below.
Improving Safety
Advanced Safety Mechanisms
Being “live” drugs, many of the safety issues of CAR T cells are attributed to the difficulty in controlling the cells’ proliferation and activation, which can lead to symptoms of an immune system in overdrive. Various companies are employing novel techniques to address this problem. Researchers are working on tunable CAR T cells whose proliferation, concentration, activation, and elimination can be regulated with an inducer agent. For example, Juno Therapeutics’ lisocabtagene maraleucel contains a truncated form of epidermal growth factor (EGFR), EGFRt, that enables rapid elimination of these CAR T cells using cetuximab, an EGFR inhibitor. Bellicum Pharma’s CAR T cell candidate, BPX-601, employs an inducible MyD88/CD40 activation switch, and the therapeutic effect and level of activation of BPX-601 can be modulated by regulating the concentration of a small-molecule inducer, rimiducid. Similarly, Autolus’ AUTO-2 and AUTO-4 can be turned off by administering monoclonal antibody rituximab. Autolus is also developing next-generation CAR T cells for solid tumors that incorporate a suicide cassette called rapaCasp9 that is controlled by rapamycin, a compound with a better tissue penetration and faster effect than rituximab.
Improving On-Target/Off-Tumor Targeting And Overcoming Risk Of Resistance Due To Antigen Loss
Tumor plasticity leading to loss or modulation of antigen is one of the primary tumor escape mechanisms that results in development of resistance to antineoplastic therapies. To overcome this risk, researchers are developing bi-specific [e.g., Autolus’ AUTO-2 (TACI/BCMA-specific), AUTO-3 (CD19/CD22-specific)] and multi-targeted [e.g., Celyad’s CYAD-01 (NKG2D receptor-specific)] CAR T cells. It is expected that such multi-targeted CAR T cells will have better on-target/off-tumor specificity and will thus have lesser side effects than single-targeted CAR T cells.
Expanding The Scope Of Treatment
Going Beyond CD19-Targeting
Both Yescarta and Kymriah are CD19-targeting CAR T cells, and many emerging CAR T-cell therapy developers are continuing to focus on this antigen. CD19, a target expressed mostly on B-cells, has served as an excellent target for the first generation of successful CAR T cells; however, researchers are gradually beginning to shift their focus to other tumor antigens with the aim of expanding the scope of cancer treatment beyond B-cell hematological malignancies. Some of the most advanced and noteworthy of this new wave of CAR T cells are bluebird bio’s bb2121 (BCMA-specific for multiple myeloma), Mustang Bio’s MB-102 (CD123-targeting for AML), and Juno Therapeutics’ JCAR018 (CD22-targeting for follicular lymphoma and B-cell ALL).
Treating Solid Tumors
Solid tumors are undeniably a much larger market (and hence, attractive to investors) than hematological malignancies, and being able to launch a successful CAR T-cell therapy in a solid tumor indication represents a holy Grail. Achieving success in solid tumors, however, is an enormous challenge because of target antigen heterogeneity, a general lack in specific cell surface antigens, physical barriers (like dense stroma or obscure tumor location), and immunosuppressive microenvironment. One of the approaches being adopted to overcome some of these challenges is intratumoral delivery of CAR T cells [e.g., Mustang Bio’s MB-103 for glioma, Leucid Bio’s 4ab T1E28z+ T-cells for squamous cell carcinoma of the head and neck (SCCHN)]. Other researchers are focusing their efforts on well-established solid tumor antigens (such as CEA-targeting CAR T cells by Sorrento Therapeutics for metastatic liver tumors and Novartis’ mesothelin-targeting NIU-440 for various mesothelin-positive cancers). To improve tumor targeting and potency, development is also focused on multi-targeted CAR T cells, such as Aurora BioPharma’s AU-105 (HER2/CMV antigen targeting) or multifunctional CAR T cells [e.g., Celyad’s CYAD-01 (NKG2D receptor-specific) and Baylor College of Medicine’s GD2-targeted Epstein-Barr virus-specific cytotoxic T lymphocytes (CTLs)].
Off-The-Shelf CAR T Cells To Address Logistic Challenges And Waiting Periods
Most of the logistic challenges associated with the complex manufacturing process of the current generation of autologous CAR T cells will likely get addressed with allogeneic, off-the-shelf CAR T cells. Allogeneic CAR T cells are generated from healthy donor cells that are better in both quality and quantity than cells derived from patients. These CAR T cells will be readily available for patients, thus reducing the gap between prescribing and administering the therapy. This would be especially beneficial for patients with rapidly progressive disease. Additionally, as each batch of allogeneic CAR T cells could be used to treat multiple patients, the overall therapy costs would diminish, and the scalability challenges would be overcome. However, anticipated safety challenges, like graft-versus-host disease (GvHD) and immune rejection, cannot be disregarded. Developers of allogeneic CAR T cells are testing various gene editing techniques to generate universal CAR T cells. For example, CRISPR Therapeutics’ CTX-110 employs clustered regularly interspaced short palindromic repeat (CRISPR)/Cas9 multiplexed gene editing technique to eliminate T cell receptor (TCR) and major histocompatibility complex class I (MHC-I) expression, thereby minimizing the risk of GvHD and recognition and rejection by a patient’s own T cells. In Servier/Allogene Therapeutics’ UCART19, TRAC and CD52 genes are disrupted, thereby allowing administration in non-HLA (human leukocyte antigen)-matched patients.
Increasing Potency
Increasing CAR T Cells’ Persistence With Defined Cell Composition
Biological characteristics of different subsets of T cells can be exploited to attain distinct characteristics in CAR T cells. For example, Poseida Therapeutics’ P-BCMA-101 is enriched in T-stem cell memory (Tscm) cells. Tscm cells are long-lived, are multipotent, and gradually produce T-effector cells; these properties are anticipated to render CAR T cells with more durable therapeutic response than the current CAR T cells, which are composed largely of the short-lived T-effector cells. Baylor College of Medicine is employing NK-T cells (GD2-targeting, IL15-expressing CAR NK-T cells) that are known to co-localize with tumor-associated macrophages (TAMs) and can effectively permeate into solid tumor tissues. City of Hope and NCI are collaboratively developing CAR T cells based on T-central memory (Tcm)-enriched CD8+ T cells that are known to have better persistence and migration potential to secondary lymphoid tissues than standard T cells.
Overcoming Immunosuppressive Tumor Microenvironment
Tumors with immunosuppressive environs, referred to as immunotherapy-cold tumors, present a particularly difficult challenge for immunotherapies. To address this challenge, CAR T cell developers are coming up with novel mechanisms to combine CAR T cells with pro-inflammatory cytokines. One of the techniques being employed to offset the side effects of systemic administration of cytokines is the incorporation of the cytokine gene within the CAR T-cell construct. An example of such an approach is Juno Therapeutics’ MUC16-targeting, IL12-secreting “armored” CAR T cells – JCAR-020, currently in an early-phase trial in solid tumors. Another interesting concept being tested by Baylor College of Medicine is the TGFβ-resistant (TGFβ being an immunosuppressing cytokine) HER2-targeting, Epstein-Barr virus (EBV)-specific cytotoxic T lymphocytes (EBV-specific CTLs).
Combination With Immune Checkpoint Inhibition To Overcome T-Cell Exhaustion
Immune checkpoints can attenuate the activity of CAR T cells and quicken T cell exhaustion. CAR T cell developers are addressing this challenge by testing the combination of CAR T cells with immune checkpoint inhibitors (e.g., Autolus’ AUTO-3 in combination with Merck & Co.’s Keytruda), by incorporating an immune checkpoint inhibitor-secretory gene within the CAR construct (e.g., Marino Biotechnology’s PD1 shRNA-expressing iPD1-CD19-eCAR T cells), or by creating immune checkpoint-resistant CAR T cells (e.g., Innovative Cellular Therapeutics’ dominant negative PD1 CAR T cells, ICTCAR-014).
Conclusion
CAR T cells show immense potential, but they also face substantial challenges to more widespread adoption. Since their launch, sales of Yescarta and Kymriah have been increasing at a relatively slow pace, with barriers such as reimbursement, patient selection and access, and manufacturing issues hindering their commercial success. These hurdles will need to be overcome in order to fully capitalize on the potential of these therapies. Nevertheless, encouraged by the clinical activity demonstrated by Kymriah and Yescarta, researchers have turned their focus to immune cells other than T cells, such as macrophages and NK cells. While researchers are fine-tuning cellular immunotherapies with novel concepts or technologies, the medical community is eagerly waiting for the therapy that can address all the limitations of the currently approved CAR T cells
