CAR-T Cell Therapy: Refining the Approach in Solid Tumors

By Jacqueline Carrico, BS, MD Candidate

Chimeric antigen receptor T-cells, better known as CAR-T cells, are being used as a novel anticancer therapy. CAR-T cells are engineered T-cells which express a modified antigen-receptor. Each chimeric antigen receptor contains 3 domains: an extracellular binding domain, a transmembrane hinge domain, and an intracellular activation or costimulatory domain. The extracellular portion is the single chain variable fragment (scFv), made up of an antibody-derived heavy chain and light chain, which ultimately recognize specific tumor antigens. The intracellular activation domain allows T-cell activation upon binding of an antigen to the scFv.

CAR-T therapy has shown promising results in hematologic malignancies, in particular with acute lymphoblastic leukemia (B-ALL) with up to a 90% complete remission rate. However, there has been limited efficacy of CAR-T therapy in solid tumor models. Therefore, recent efforts have been made to identify the unique barriers to success in solid tumors. The leading hypothesis is that the tumor microenvironment (TME) is immunosuppressive and there is limited CAR-T trafficking to the tumor site. Targeting metabolic pathways in the TME, chemotaxis and adhesion, stromal cell signaling and cytokines, have shown promising results in improving the success of CAR-T therapy in solid tumor models.

Targeting Nutrient Deprivation in the TME

A major hallmark of the TME is nutrient deprivation. One potential target for improving the nutrient status in the TME is Indoleamine 2,3-dioxygenase (IDO). IDO catalyzes degradation of tryptophan to kynurenine. Amino acid starvation prevents T-cell proliferation and survival, meanwhile kynurenine buildup impairs CAR-T efficacy. Ninomiya et. al. showed that Fludarabine and Cyclophosphamide treatment improved the efficacy of CAR-T therapy by decreasing IDO expression.¹ This is just one example of altering metabolic pathways in the TME to confer a survival advantage to CAR-T cells.

Attracting CAR-T Cells to the Tumor Site

The efficacy of CAR-T cells is known to be directly proportional to the density and persistence of CAR-T expression. Protein Kinase A (PKA) is a downstream effector of PGE₂ and Adenosine, and inhibition of PKA has been shown to improve CAR-T chemotaxis, infiltration, adhesion, and antitumor activity.² Newick et. al. inhibited PKA by expressing the RIAD peptide, which displaces PKA from its anchoring protein Ezrin, disrupting the interaction with adenylyl cyclase. However, there is some concern about the possible immunogenicity of the RIAD peptide in vivo.

Immunocytochemistry/Immunofluorescence: Ezrin Antibody [NBP1-86863] - Immunofluorescent staining of human cell line A-431 shows localization to the plasma membrane.

Another promising target for improving chemotaxis is Heparanase, an enzyme that breaks down heparan sulfate proteoglycans, necessary for T-cell chemotaxis and accumulation. CAR-T cells lose heparanase during their engineering process, and induction of heparanase has been shown to improve tumor infiltration and CAR-T cell survival.³

Targeting Stromal Cells and Cytokines

Cancer associated stromal cells are non-malignant support cells which secrete various cytokines and chemokines to promote tumor growth, metastasis, and angiogenesis. Fibroblast activating protein (FAP) is expressed largely by stromal cells, and its inhibition was shown to enhance the endogenous CD4+ response and improve the antitumor activity of CAR-T cells in vivo. However, there have been mixed results with inhibition of FAP, with one trial ending due to the presence of severe cachexia and lethal bone toxicities.⁴ Another study using a different scFv was also efficacious and did not result in these toxicities. Further studies will be necessary to understand the safety of inhibiting FAP.⁵

There are well defined interactions between the various T-cell subtypes and unique cytokines.
IL-12, for example, induces the Th1 CD4+ response, supports CD8+ clonal expansion, activates tumor infiltrating lymphocytes, recruits NK cells, and inhibits regulatory T-cells. Addition of IL-12 may help support CAR-T proliferation and antitumor activity, as well as enhance the endogenous immune response.

Importance

Given the remarkable results of CAR-T therapy in hematologic malignancies, it may also be a promising therapy for solid tumors. The tumor microenvironment poses unique challenges to the success of CAR-T therapy. Improving chemotaxis and adhesion, as well as proliferation and survival in the TME, will be critical to improving the success of CAR-T therapy in solid tumors.

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Jacqueline Carrico, MD Candidate
University of Colorado, School of Medicine
Jackie is completing her medical training in Anesthesiology and has a diverse background in basic science, translational, and clinical research.

References

1. Ninomiya S, Narala N, Huye L, Yagyu S, Savoldo B, Dotti G, Heslop HE, Brenner MK, Rooney CM, Ramos CA. Tumor indoleamine 2,3-dioxygenase (IDO) inhibits CD19-CAR T cells and is downregulated by lymphodepleting drugs. Blood. 2015;125:3905–16.
2. Newick K, O’Brien S, Sun J, Kapoor V, Maceyko S, Lo A, Pure E, Moon E, Albelda SM. Augmentation of CAR T-cell trafficking and antitumor efficacy by blocking protein kinase a localization. Cancer Immunol Res. 2016;4:541–51.
3. Caruana I, Savoldo B, Hoyos V, Weber G, Liu H, Kim ES, Ittmann MM, Marchetti D, Dotti G. Heparanase promotes tumor infiltration and antitumor activity of CAR-redirected T lymphocytes. Nat Med. 2015;21:524–529.
4. Tran E, Chinnasamy D, Yu Z, Morgan RA, Lee CC, Restifo NP, Rosenberg SA. Immune targeting of fibroblast activation protein triggers recognition of multipotent bone marrow stromal cells and cachexia. J Exp Med. 2013;210:1125–35.
5. Kakarla S, Chow KK, Mata M, Shaffer DR, Song XT, Wu MF, Liu H, Wang LL, Rowley DR, Pfizenmaier K, Gottschalk S. Antitumor effects of chimeric receptor engineered human T cells directed to tumor stroma. Mol Ther. 2013;21:1611–20.
6. Scarfò I, Maus MV. Current approaches to increase CAR T cell potency in solid tumors: targeting the tumor microenvironment. J Immunother Cancer. 2017;5:28.