How to Assess CAR-T Cell Therapies Preclinically
Chimeric antigen receptor (CAR)-T cell therapies are a new and novel immuno-oncology treatment modality, only recently approved for patient use. As with any new treatment approach, efficacy needs to be effectively monitored, as well as safety with potentially life-threatening toxicities linked to CAR-T cell therapy. This post looks at the preclinical models and tools available for ensuring efficient assessment and translation of new CAR-T cell therapies to the clinic.
Two CAR-T Cell Therapies Currently Approved, More Indications on the Way
CAR-T cell are autologous or allogeneic T cells specifically targeting antigens or markers expressed by tumor cells. In August last year, the US Food and Drug Administration (FDA) issued a historic approval for the first cancer cell therapy using CAR-T cells, with the treatment targeting the CD19 antigen. Kymriah™ (tisagenlecleucel) was approved for the treatment of patients up to 25 years of age with relapsed and/or refractory B-cell precursor acute lymphoblastic leukemia (ALL) based on a remarkable overall remission rate of 82.5%.
These agents have also shown considerable promise in patients with B-cell lymphoma, and the first approval in this indication followed 2 months after Kymriah. The second anti-CD19 CAR-T cell therapy, Yescarta™ (axicabtagene ciloleucel), was approved for the treatment of adult patients with relapsed and/or refractory large B-cell lymphoma, based on an objective response rate of 72%.
More recently, the FDA granted a priority review to a supplemental biologics license application (sBLA) for Kymriah to be used in treating adult patients with relapsed or refractory diffuse large B-cell lymphoma (DLBCL) who either relapse or are not eligible for an autologous stem cell transplant (ASCT). The sBLA is based on the phase 2 JULIET study, in which the overall response rate (ORR) to Kymriah was 53.1% in adult patients.
Toxicity of CAR-T Therapies Needs to Be Carefully Monitored
Despite these breakthrough and encouraging results, potentially life-threatening toxicities are intimately associated with CAR-T cell therapy. A particularly severe complication is cytokine release syndrome (CRS), but encephalopathy and hemophagocytic lymphohistiocytosis also occur. However, in comparison with adverse events seen with immune checkpoint inhibitor use - mostly delayed-onset autoimmune adverse events -the toxicities of CAR-T cells tend to be acute, less diverse, and therefore more predictable.
Preclinical Safety, as well as Efficacy, Assessments are Essential
In vivo assessment is of pivotal importance for investigating the factors that may affect the efficacy of CAR-T cells. CAR gene delivery approaches into T cells, in vitro culture conditions, constituents of the CAR construct, and the types of host T cells all have a great impact on CAR-T efficacy and safety.
Therefore, the preclinical in vivo assessment of the efficacy and safety of CAR-T cells are essential for dosage determination and risk management for this promising immunotherapy type.
Antitumor Specificity is Usually Confirmed In Vitro
Most preclinical studies investigating CAR-T cell function focus on the specificity and potency of antitumor activity. Specificity is generally confirmed via in vitro assays, demonstrating that the CAR-T cells exhibit a different functional response between the target antigen and nonspecific antigens.
The advantage of the CAR is its ability to redirect T cell effector function in the absence of HLA-restriction. In consequence, the effector function of allogeneic T cells can be easily examined without the need for matched targets.
As a result of allo-recognition, background activity is usually equivalent between cells transduced with irrelevant CARs and non-transduced T cells, determined in vitro as fold increase above this background.
Such assays demonstrate CAR function, and support translation to an animal model system. It should be highlighted, however, that background allo-recognition has not been related to antitumor activity in vivo in a number of xenograft tumor studies.
Clinical Translation Requires In Vivo Assessment, Often in Immunocompromised Mice despite Limited Human T Cell Engraftment
For clinical translation, regulatory authorities generally require information relating to the specific agent to be used in the clinical trial, meaning for CAR-T that testing should be assessed in immunocompromised mice. But human T cell engraftment in the majority of these mice is limited, due to the disability of the residual elements in the mouse immune system.
Nonetheless, a certain number of studies have shown the efficacy of CAR-T cell function using Nude, NOD/SCID or SCID/Beige animals(1–4). More recently, highly immunocompromised NOG®/NSG™ mice (NOD/SCID IL-2Rγ-/-) have allowed efficient human T cell engraftment.
However, CAR-T cells can drive xeno-graft versus host disease (xGVHD) in 50 days, with the occurrence associated with the persistence of the transduced cells conditioned by the cytokines used to culture the cells prior to injection into mice. This limits the ability to investigate the long-term effects of CAR T cells in this model(5–7).
In addition, the majority of these studies target human antigens with restricted expression to human tumor cells used to challenge mice in the preclinical model. Therefore, off target or targeting of the antigen which may be expressed on normal healthy tissue cannot be investigated.
Immunocompetent or Transgenic Mouse Models can Overcome some Immunocompromised Model Issues, but use Mouse Homologues
To get around this issue, immunocompetent mouse models have been used with the target antigen of the mouse homologue, although the interactions of ‘mouse CAR-mouse homologue’ may differ from the equivalent human configuration, as well as disparity between expression profiles of mouse and human protein homologues.
The utilization of transgenic animals is another relevant approach, where the expression of the human antigen is controlled by a physiologically relevant promoter where the human CAR can be evaluated.
NHPs can be Used for Safety Testing when Mouse Homologues are Absent
When mouse homologues are absent, safety testing in non-human primates (NHP) may also be useful to identify potential on-target/off-tumor toxicities and prioritize molecules that can be targeted safely with CAR-T cells(8-9).
Combined Preclinical Approaches for Effective Efficacy and Safety Testing
These combined approaches have been used to successfully demonstrate CAR T-cell function and safety against a wide diversity of target antigens. With the growing interest in this exciting field, new and effective treatments will continue to emerge and require asseesment in the upcoming years.
References and Further Reading:
- Cheadle et al. The combination of cyclophosphamide and human T cells genetically engineered to target CD19 can eradicate established B-cell lymphoma. Br J Haematol 2008;142(1):65-8.
- Hillerdal et al. Systemic treatment with CAR-engineered T cells against PSCA delays subcutaneous tumor growth and prolongs survival of mice. BMC Cancer 2014;14:30.
- Parente-Pereira et al. Trafficking of CAR-engineered human T cells following regional or systemic adoptive transfer in SCID beige mice. J Clin Immunol 2011;31(4):710-8.
- Zhao et al. Multiple injections of electroporated autologous T cells expressing a chimeric antigen receptor mediate regression of human disseminated tumor. Cancer Res 2010;70(22):9053-61.
- Alcantar-Orozco et al. Potential limitations of the NSG humanized mouse as a model system to optimize engineered human T cell therapy for cancer. Hum Gene Ther Methods 2013;24(5):310-20.
- Hannon et al. Infusion of clinical-grade enriched regulatory T cells delays experimental xenogeneic graft-versus-host disease. Transfusion 2014;54(2):353-63.
- Zhou et al. Humanized NOD-SCID IL2rg–/– mice as a preclinical model for cancer research and its potential use for individualized cancer therapies. Cancer Lett 2014;344(1):13-19.
- Berger et al. Safety of targeting ROR1 in primates with chimeric antigen receptor-modified T cells. Cancer Immunol Res 2015;3(2):206-16.
- Künkele et al. Preclinical Assessment of CD171-Directed CAR T-cell Adoptive Therapy for Childhood Neuroblastoma: CE7 Epitope Target Safety and Product Manufacturing Feasibility. Clin Cancer Res 2017;23(2):466-477.