Surgery, chemotherapy, and radiation have long been dubbed the three pillars of effective cancer therapy. However, gastrointestinal stromal tumors (GIST) pose a unique challenge compared to other cancers of the gastrointestinal tract – they are resistant to traditional chemotherapy, and radiation treatment does not improve the risk of relapse after surgical resection. Instead, GIST is one of the best examples of a cancer that benefits from the fourth pillar of cancer therapy – using targeted small molecules that block specific signaling pathways in cancer cells.
The hallmark of GIST is that interstitial cells of Cajal acquire pathologic activating mutations in the c-KIT receptor tyrosine kinase, leading to constitutive activation of the pathway. Accordingly, blockade of KIT signaling with tyrosine kinase inhibitors such as imatinib leads to clinical responses in nearly 80% of patients with GIST. However, 50% of patients with advanced GIST acquire resistance to imatinib within two years. Although other tyrosine kinase inhibitors like sunitinib, regorafenib, and others have demonstrated benefit in the imatinib-resistant setting, most patients eventually become resistant to tyrosine kinase inhibitors and are in desperate need of new therapies.
The reason for failure of most tyrosine kinase inhibitors is that GIST cells acquire secondary mutations that allow them to circumvent the blockade in KIT signaling, often utilizing other cellular pathways. The rapid development of resistance in cancer cells to either chemo-therapy or targeted therapy is one of the main reasons that immune-directed therapies are so appealing. At least in theory, the immune system has the adaptability to recognize and attack numerous different antigens, maintaining memory and constant surveillance. However, cancer cells have evolved various mechanisms to evade the immune system, avoiding detection and attack.
The focus of modern immunotherapy is to counteract these evasion mechanisms, and allow the immune system to recognize and attack cancer cells.The development of modern immunotherapy strategies, including checkpoint inhibitors and adoptive T cell therapies has led to remarkable responses in dozens of refractory solid and hematologic malignancies. With reports of long-term remissions in previously incurable diseases like metastatic melanoma, non-small cell lung cancer, and acute lymphoblastic leukemia, it is no surprise that immunotherapy is now being heralded as the fifth pillar of cancer care.
In this review, we will summarize the available laboratory data on the role of the immune system in GIST and highlight ongoing clinical trials focused on immune-directed treatments for GIST patients.
Overview of the Immune System and Potential
Applications of Immunotherap
The immune system is constantly autoregulating between pro-inflammatory and anti-inflammatory states, both globally and focally within tumor deposits. As summarized in the Figure, an effective anti-tumor response requires the following: 1. recognition of tumor-specific proteins called neo-antigens, 2. induction of the body’s innate and adaptive immune machinery, 3. effective migration of the key immune cells into the tumor deposits, and 4. maintenance of the attack at the tumor-immune cell interface.
At every phase of immune activity, there are a variety of natural mechanisms to regulate the inflammatory response. This occurs through changes in pro- and anti-inflammatory cytokines, expression of suppressive checkpoint proteins on antigen-presenting cells and effector T cells, and emergence of suppressive phenotypes including exhausted or anergic T cells, T regulatory cells (Tregs), M2 macrophages, and myeloid-derived suppressor cells. This highly complex interplay has been reviewed in depth1,2 and is summarized in the Table.
Most cancer cells are initially recognized by the immune system, specifically cytotoxic CD8+ T cells, as foreign based on abnormal protein production from mutations in DNA, that lead to neoantigens. Because of this, most cancerous cells are initially immunogenic, meaning that they are recognized and destroyed by the immune system before forming clinically detectable tumors. However, some tumor cells may inherently be poorly recognized by the immune system, or they develop ways to avoid the immune system attack, known as immune evasion. Over time, the immunogenicity of the cancer cells changes, a phenomenon known as immunoediting.3 Ultimately, the less-immunogenic cancer cells will overcome the immune system leading to clinically detectable and immunoresistant tumors.
Cancer cells can evade immune system recognition and attack by three main mechanisms. First, immunoedited cancer cells tend to lose expression of key immunogenic neoantigens. They may also stop expressing the Class I MHC complex, which is required for recognition of neoantigens by cytotoxic CD8+ T cells. Secondly, cancer cells can produce the same immunosuppressive cytokines and express checkpoint proteins that normally function to blunt the immune response. For example, many tumors including GIST secrete vascular endothelial growth factor (VEGF) that not only facilitates angiogenesis, but also promotes an immunosuppressive tumor microenvironment that limits immune cell recruitment, infiltration and activation.
Finally, even if immune cells successfully infiltrate the tumors, cytotoxic T cells and macrophages may express check-point proteins that shut down further activation, evolve to an anergic state, or become suppressive phenotypes like Tregs. Most tumor cells directly promote an environment that is suppressive rather than inflammatory.
The focus of modern immunotherapy aims to counteract many of these evasion mechanisms, such as checkpoint inhibitors which block the immunosuppression resulting from the PD-1/PD-L1 interaction. However, checkpoint inhibitors are not effective for everyone, and counterintuitively, activity is not dependent on expression of PD-1/PD-L1 by the tumor cells. A better understanding of the mechanisms used to avoid immune recognition by a particular patient’s GIST may help us to customize immunotherapy approaches, similarly to how we use mutation testing to customize targeted therapy.
The Role of the Immune System in GIST –
Immune cells have the ability to infiltrate GIST tumor deposits, but lead to a generally suppressive environment.
One of the features that has been suggested to predict better prognosis, as well as a better response to immune-directed therapies in other types of cancer is the presence of infiltrating immune cells in metastatic tumor deposits.5 This information has led to the classification of tumors into “hot” and “cold” tumors, which likely require different types of immune-based therapies to induce immune responses.6 For example, a tumor without immune cell infiltration might be postulated to benefit from anti-VEGF therapies to try to optimize immune cell infiltration prior to induction of the immune activity with checkpoint inhibitors.
Our group has recently reviewed the reported literature evaluating immune cell infiltration in GIST.7,8 The most abundant infiltrating immune cells in GIST are tumor-associated macrophages (TAM), including alternatively-activated type 2 TAM and an immature macrophage phenotype expressing Ki-M1P. CD3+ T cells and suppressive Tregs are also found in GIST, with a higher CD3/CD8 to Treg ratio correlating with improved outcomes.9 Interestingly, while the presence of immune cells tends to be associated with metastatic sites, unfavorable locations, and more aggressive proliferative in- dices, these tumors also tend to have superior progression-free survival. While overall the immune milieu seems to consist of largely suppressive phenotypes, it suggests that these tumors were at one point impacted by the immune system, and one could argue that immune therapies to reverse immunosuppression and restore the balance to a pro-inflammatory state may be more likely to impact the out- comes of these patients.
Imatinib Directly Impacts Immune Cells
in GIST Through Effects on IDO
The work from Dr Ronald DeMatteo and colleagues was recently presented at the American Society of Clinical Oncology (ASCO) 2016 Annual Meeting. Using a mouse model of GIST, the animals were treated with imatinib and a variety of changes in the immune cells within the tumors were noted.9 They found that in tumors treated with imatinib, a much higher ratio of anti-tumor CD8+ cytotoxic T cells to suppressive Tregs was seen. After looking at changes in mRNA in these treated tumors, a marked reduction in transcription of the enzyme indoleamine 2,3-dioxygenase (IDO), which catalyzes the conversion of tryptophan into metabolites that stabilize Tregs and suppress cytotoxic effector T cell activity was observed. Expression of IDO1 and a down-stream protein, KYN, were reported to be associated with improved responses to chemotherapy in soft tissue sarcomas at ASCO 2016 by Dr Toulmonde and colleagues (Abstract 11008).
Dr DeMatteo’s team went on to treat mice with MT-1, a more specific inhibitor of IDO. While IDO inhibitors alone produced only modest slowing of tumor growth, these effects were dramatically increased when combined with checkpoint inhibitors. Based on this observation, mice were treated with combination therapy with imatinib plus a checkpoint inhibitor of CTLA-4. Mice demonstrated less tumor growth when they received the combination of imatinib plus CTLA-4 inhibitor compared to either drug alone. This concept was recently explored in a clinical trial of dasatinib plus ipilimumab for GIST patients.
The results of this phase I study were recently reported, and showed 7/13 patients having partial responses by Choi criteria and an additional 3/13 patients with stable disease.10 Interestingly, one patient with PDGFR D842V mutation has achieved stable disease and remains on study for greater than 13.9 months. Another patient with SDH-deficient GIST achieved Choi partial response and remained on study for 47 weeks. However, most patients had previously received multiple TKIs and were resistant to KIT inhibitors which likely lessened the potential effects of the combination therapy. Further investigations of alternative KIT inhibitors, or studies with anti-PD-1 checkpoint inhibitors are ongoing or in planning stages.
Expression of Checkpoint Proteins in GIST Tumors
The expression of checkpoint proteins by human GIST cells including PD-L1 and TIM-3 has now been reported in several articles and reviewed in detail elsewhere.7,8,11 Dr. DeMatteo and colleagues also evaluated the presence of check- point proteins on tumor-infiltrating immune cells in human GIST samples collected at surgical excision.12 A higher proportion of tumor-infiltrating lymphocytes expressed the checkpoints PD-1, LAG-3, and TIM-3 relative to circulating immune cells. Higher expression of checkpoint proteins was seen in tumors that were resistant to imatinib.
A striking feature was that patients were highly variable in the degree of expression; some patients had very high levels of PD-1 whereas others had very low expression. As seen in other cancers, this suggests that there may be particular patients with very high expression of checkpoint proteins along with immune cell infiltration, and it is tempting to speculate that these patients would be most critical to include in clinical trials exploring checkpoint inhibitors and other types of immunotherapy.
It is still unclear whether checkpoint expression on tumor cells as compared to expression on infiltrating immune cells is more important for effect from checkpoint inhibitors. In many cancers, expression of PD-L1 or PD-1 on immune effector cells predicts response to PD-1 antibodies, including bladder cancer and non-small cell lung cancer, however in melanoma, patients lacking expression can still respond to these drugs. We are awaiting the follow-up correlative data from the recently reported Phase II clinical trial, SARC028, studying pembrolizumab in patients with bone and soft tissue sarcomas. These results will determine whether responders in this trial exhibited tumor or TIL PD-1/PD-L1 expression. GIST patients were not included in this study, thus biomarkers of GIST patients who may respond to immunotherapy are still unknown.
VEGF May Impact the Immune System in GIST
Vascular endothelial growth factor (VEGF) and related cofactors are known to drive tumor angiogenesis, and the resulting aberrant blood vessel networks are critical for tumor growth and metastasis. GIST also depends on VEGF signaling. High expression of VEGF correlates with inferior PFS after treatment with imatinib,13 and the most effective therapies in imatinib-resistant GIST are TKIs that also inhibit VEGFR, like sunitinib and regorafenib. Critically, Dmitry Gabrilovich and others have shown that VEGF and related proteins like HIF-1αcan directly suppress immune responses within the tumor microenvironment by inhibiting dendritic cell maturation and antigen presentation, blocking migration of lymphocytes across endothelium into tumor deposits, and promoting accumulation of suppressive myeloid-derived suppressor cells (MDSC), tumor-associated macro-phages (TAM) and Tregs.14-18
As we have discussed previously, GIST that is resistant to imatinib shows upregulation of checkpoint proteins compared to untreated tumors. Thus, one may speculate that the worse survival of GIST patients with high VEGF may be related to suppression of the immune system. By inhibiting VEGF, we might improve immune cell infiltration into the tumors which has been shown to be a positive factor in GIST outcomes. Combinations of VEGF blockade plus immune checkpoint inhibitors has improved anti-tumor response in other cancers like melanoma and renal cell carcinoma.19,20 This is the rationale for our ongoing Phase II clinical trial of the VEGFR inhibitor axitinib plus pembrolizumab, which is currently open to accrual and includes GIST patients (NCT02636725).
Adoptive T cell Therapy for GIST
Another strategy for immunotherapy is the concept of adoptive T cell therapy. Rather than relying on the native immune system’s ability to recognize antigens of interest and to stimulate an effective adaptive response, adoptive T cell therapy bypasses these steps by engineering a patient’s own T cells to express a T cell receptor specific for the target of interest. The expanded T cell population is then returned to the patient. This strategy has been used effectively in acute lymphoblastic leukemia and lymphomas and has been shown to produce patient responses in NY-ESO-1 positive synovial sarcomas.21
Dr Steven Katz and colleagues have recently reported the development of a KIT-specific chimeric antigen receptor (CAR) for transduction into human T cells.22 These engineered T-cells were effective at suppressing GIST growth in mice models. While these advances are promising, the reported toxicities with adoptive T cell therapy from cross-reaction against normal human tissues are not trivial, and further testing and development will be required before these strategies are ready for clinical testing.
Clinical Opportunities for the Exploration
of Immunotherapy in GISTs
With the excitement surrounding immunotherapy in medical conferences as well as the media, a key question is when GIST patients and their providers might want to consider participating in a clinical trial. This decision is particularly difficult given that many trials require patients to stop using tyrosine kinase inhibitors which we generally continue without interruption even in the setting of resistant disease. GIST patients in general do best with uninterrupted suppression of KIT, even when the disease has developed resistance pathways. We have learned that response to checkpoint inhibitors can take months to manifest, and patients with GIST who abruptly discontinue TKIs can often experience a flare and rapid growth of their disease. Even if patients ultimately respond to immunotherapy, the short-term interval growth can be problematic and lead to medical complications like gastrointestinal obstruction. While laboratory evidence is promising for potential benefit from immunotherapy for GIST, there is still minimal clinical data to guide patients and physicians in whether the science will translate to patient responses in the clinic.
Additionally, a common misconception is that since immunotherapy is designed to boost the patient’s own natural immune system to fight cancer, the side effects of these treatments will be less toxic than chemotherapy or targeted therapies. While most patients on checkpoint inhibitors tolerate these drugs well, there can be significant auto-immune toxicities that can lead to life-threatening inflammation or even death, particularly with checkpoint inhibitor combinations. Adoptive T cell therapy can be even more toxic, and the ongoing trials of CAR T cells and other engineered T cell therapies have led to treatment-related deaths in some patients from overwhelming inflammation. It is critical that patients and providers are educated on the potential adverse reactions prior to embarking on clinical trials studying these agents.
Regardless, for GIST patients without other treatment options, there are now several clinical trials that utilize immunotherapy, especially novel combinations involving tyrosine kinase inhibitors, that will help to answer whether immunotherapy will have a role in management of advanced GIST. The list below shows clinical trials for which GIST patients are eligible. Additional information can be found at http://clinicaltrials.gov.
- Phase I, imatinib plus ipilimumab (NCT01738139). Offered at MD Anderson Cancer Center. Metastatic or unresectable GIST, patients with mutations in exon 13 V654X, 14 T6701, 17 D816X and all exon 18 muta-tions will not be eligible for enrollment.
- Phase II (randomized), nivolumab (anti-PD-1) with or without ipilimumab (anti-CTLA4) for patients with metastatic GIST, (NCT02880020). Offered at UCLA. GIST that is unresectable and has progressed on imatinib are eligible.
- Phase II, metronomic cyclophosphamide and pembrolizumab (PEMBROSARC), (NCT02406781). Offered in various locations in France. GIST-specific arm, progression after imatinib and sunitinib.
- Phase II, durvalumab (anti-PD-L1) and tremeli-mumab (anti-CTLA4), (NCT02815995). Offered at MD Anderson Cancer Center. GIST eligible for “other sarcoma” arm.
- Phase I/IIa, pexidartinib (PLX3397, inhibits KIT among other targets) plus pembrolizumab (NCT024 52424). Offered in Arizona, California, Michigan, Texas. GIST eligible for Phase II portion only.
- Phase II, axitinib (anti-VEGFR) plus pem-brolizumab (NCT02636725). Offered at Sylvester Comprehensive Cancer Center, Miami. Metastatic/unresectable GIST refractory to at least one TKI are eligible.
- Phase I, Intuvax (intratumor injections) plus suni-tinib, NCT02686944. Offered in Sweden. Metastatic/unresectable GIST refractory to imatinib and sunitinib.
- Phase I, enoblituzumab (anti-B7-H3) and pembro-lizumab, (NCT02475213). All solid tumors eligible but must be B7-H3 positive (requires additional testing.) Offered in Texas, Maryland, New York, Pennsylvania, and Florida.
- Phase I, TLR4 Agonist GLA-SE and Radiation Therapy (NCT02180698). Offered at Fred Hutchinson Cancer Center (Seattle). Open for various types of soft tissue sarcoma.
- Phase I, GDC0919 (IDO inhibitor), (NCT02048709). Offered in Georgia. All solid tumors eligible.
- Phase I/II, In Situ vaccination with tremelimumab (anti-CTLA4) and IV durvalumab (anti-PD-L1) plus PolyICLC, (NCT02643303). Offered at Ludwig Institute, New York. All solid tumors.
Summary and Conclusions
In summary, modern immunotherapy has generated incredible excitement and hope for patients with cancer that has not responded to traditional chemotherapy or targeted treatments. However, there is still much that is unknown about which patients are most likely to benefit. Gastrointestinal stromal tumors are among the best-studied sarcomas in the laboratory with significant evidence that the immune system is a critical component of the body’s ability to repress and attack GIST.
Thus, the investigation of immunotherapy as a potential treatment strategy to fight GIST is critical. The decision to participate in a clinical trial is not one that is to be taken lightly, particularly given the relative newness of modern immunotherapy drugs and the significant toxicities that can occur. However, with the increasing number of scientifically-driven immunotherapy clinical trials, GIST patients now have the opportunity to take part in one of the newest frontiers in cancer research.
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Key words: immunotherapy, GIST, aKIT signaling, checkpoint inhibitors, adoptive T cell therapies, PD-1/PD-L1, pembrolizumab, imatinib.
Address for correspondence: Breelyn A. Wilky, MD, Assistant Professor – Sarcoma Program, Sylvester Comprehensive Cancer Center, University of Miami Miller School of Medicine,1120 NW 14th Street, Suite 610E, Miami, FL 33136 E-mail: email@example.com