Alliance of the Titans: An Effective Combination of a TKI with CAR T Cells

Main Text

Hepatocellular carcinoma (HCC) is the most common malignancy of the liver, with approximately 841,000 new cases diagnosed and 782,000 deaths in the world in 2018.¹ Complete surgical removal (tumor resection or liver transplantation) can be curative, but systemic treatments for patients with advanced disease are less effective.² Thus, there is a clear unmet need for new effective therapies for patients with HCC. Sorafenib is a multi-tyrosine kinase inhibitor (TKI), used as first line therapy for advanced stage HCC, providing 2.8 months improvement in event-free survival (EFS) compared to placebo.³ Other TKIs, including lenvatinib or regorafenib, provide comparable benefit for upfront or salvage therapy of patients with HCC, respectively.4, 5 The question remains how to improve the efficacy of these treatments with the goal of not just extending survival, but also curing patients. Recently, immunotherapy with T cells expressing chimeric antigen receptors (CARs) have shown remarkable efficacy in patients with hematologic malignancies6, 7 and the approach shows promise for patients with solid tumors. In this issue of Molecular Therapy, Wu et al.⁸ examined the effect of combining sorafenib with glypican-3 (GPC3)-CAR T cells in preclinical models of HCC. The authors showed that, compared to either treatment alone, the combination of sorafenib and human CAR T cells have superior antitumor activity against HCC.

GPC3 is an attractive immunotherapeutic target of HCC because it is expressed on the majority of HCC but not normal mature tissues.⁹ Targeting GPC3 with monoclonal antibodies,¹⁰ vaccines,¹¹ or GPC3-CAR T cells¹² has been safe, but the antitumor responses have been modest. In this new study, the team first focused on examining the effect of combining these two therapies using murine CAR (muCAR) T cells. They found that sorafenib inhibits the cytotoxicity, cytokine production, and proliferation of muCAR T cells in a dose-dependent manner. They next tested the combination in an established murine model of HCC in which smaller tumors can be effectively controlled by muCAR T cells but larger tumors can overcome these therapeutic cells. At subtherapeutic doses of sorafenib, the combination with CAR T cells resulted in enhanced T cell infiltration into tumor tissues and superior antitumor activity compared to either treatment alone. The effect was, at least in part, mediated through interleukin-12 (IL-12) produced by tumor-associated macrophages, leading to increased cancer cell apoptosis. Higher doses of sorafenib—resembling therapeutic doses in humans—inhibited the antitumor function of murine GPC3-CAR T cells, and this latter finding using muCAR T cells could have been a major barrier for further clinical development. To explore whether human T cells exhibit a similar sensitivity to sorafenib, Wu et al.⁸ first compared peripheral blood mononuclear cell and mouse spleen-derived CAR T cells. Interestingly, they demonstrated that the cytolytic activity, cytokine production, and proliferation of human GPC3-CAR (huCAR) T cells were not inhibited at therapeutic concentrations of sorafenib and that their antitumor function was significantly improved when dosed in combination with sorafenib in xenograft models of HCC in immunocompromised murine models. The results of Wu et al.⁸ are highly relevant for patients with HCC and for the immunotherapy field at large.

As newer immunotherapeutic strategies show promise in patients with relapsed or refractory neoplasms, moving these strategies to the upfront setting will be important to benefit a larger number of patients, and when possible, to simplify or shorten upfront therapy (i.e., children with newly diagnosed acute lymphoblastic leukemia [ALL] may be treated for over 3 years with chemotherapeutic agents). For many cancer types, including ALL, upfront regimens are effective against the cancer cells but can directly affect T cell survival and function, complicating the swift transition of CAR T cells into the upfront setting. In contrast, upfront systemic therapy of HCC is based on TKIs, including sorafenib. As shown by Wu et al.,⁸ sorafenib does not inhibit human CAR T cell function and the combination can enhance the antitumor activity of either treatment alone; thus, clinical translation may be successful without compromising either therapy. Wu et al.⁸ used a single dose of GPC3-huCAR T cells and a 2-week course of sorafenib beginning on day 11 post-tumor inoculation in immunodeficient mice. In patients, sorafenib is given continuously until side effects or progression of the tumor prevents further administration, and this continuous dosing schedule can result in improved EFS.³ The question remains whether the continuous administration of sorafenib in combination with GPC3-CAR T cells would be even more effective. The next logical step will be to evaluate the combination of GPC3-CAR T cells and sorafenib in patients with HCC. Based on the results of Wu et al.,⁸ combining GPC3-CAR T cells with 2-week sorafenib dosing may be used, or perhaps using continuous sorafenib administration may be also considered if additional preclinical studies provide further justification.

The difference between the sensitivity of murine and human CAR T cells to sorafenib is a striking finding of Wu et al.⁸ The study of murine (and other animal) models has provided tremendous insight into the intricate processes of the immune system over the past several decades. It is also clear that significant differences exist between the human and murine immune systems.¹³ How should the immunotherapy field balance the use of murine models to make conclusions about human cells? Genetic engineering strategies will continue to evolve and enhance the antitumor properties of human effector cells. It will be important that new approaches that rely on the manipulation of the biology and function of such effector cells carefully examine human cells as well. Wu et al.⁸ found that sorafenib in combination with muCAR T cells induced IL-12 production in tumor-associated macrophages, which contributed, at least in part, to the observed antitumor effect in the immunocompetent murine model. The human tumor microenvironment is particularly challenging to study, and understanding immune escape mechanisms already present or evolving in response to immune-mediated attack will be critical, especially for the effective immunotherapy of solid tumors. Confirming the presence of the described IL-12-dependent process in humans and defining other mechanisms of human tumor immune microenvironment interference and specific immune escape mechanisms should enable the design of effective countermeasures. Post-CAR T cell infusion biopsies may help decipher these mechanisms and, with recent development in single cell analytical tools, the immunotherapy field certainly has an exciting time ahead.14, 15

Conflicts of Interest

A.H. has pending patent applications covering GPC3-CARs.