Douglas Hanahan and Robert Weinberg acknowledged the importance of the immune system in cancer development in 2011, when they added immune evasion to their list of "hallmark" abilities that are essential for the transformation of normal cells into cancerous ones.
CTLA-4 indicates cytotoxic T lymphocyte-associated antigen 4; DC, dendritic cell; IDO, indoleamineâ€‘2, 3â€‘dioxygenase; IL, interleukin; NK, natural killer.
In addition to being expressed on tumor cells, IDO is also expressed on DCs in the tumor-draining lymph nodes. IDO-activated DCs set off a cascade of events that can impair immune responses.
Douglas Hanahan and Robert Weinberg acknowledged the importance of the immune system in cancer development in 2011, when they added immune evasion to their list of “hallmark” abilities that are essential for the transformation of normal cells into cancerous ones. Researchers have begun to uncover some of the complex mechanisms employed by cancer cells that enable them to fly under the radar of the host’ s immune system, and a new class of immunotherapeutic anticancer agents has emerged as a result.
Among them are agents that target immune checkpoints—inhibitory signaling pathways that switch off the T cells of the immune system. Indoleamine (2,3)-dioxygenase (IDO) was recently identified as a checkpoint protein involved in generating the immunosuppressive tumor microenvironment that supports tumor growth. IDO inhibitors have been developed that have shown promising antitumor activity in early clinical trials, and appear to be synergistic in combination with chemotherapy and other forms of immunotherapy.
What Is IDO?
IDO is an enzyme with two isoforms (IDO1 and IDO2) that acts at the first step in the metabolic pathway that breaks down the essential amino acid tryptophan. In the late 1990s, it was discovered that IDO played a role in modulation of the immune response when researchers demonstrated that IDO expression prevented T cell-driven immune rejection of a genetically distinct fetus in pregnant female mice.
IDO exerts its immunomodulatory effects by shutting down the effector T cells of the immune system. There are several, not necessarily mutually exclusive theories as to how exactly it does this. The tryptophan starvation theory posits that the local depletion of tryptophan by IDO leads to cell cycle arrest and death of T cells (which are sensitive to decreases in tryptophan levels), while the tryptophan metabolite theory suggests that certain downstream metabolites of tryptophan are toxic to T cells. IDO expression also directly activates the regulatory T cells (a subset of T cells whose major function is to shut down T cell-mediated immunity at the end of an immune reaction).
Since the overall outcome of IDO function is an inhibition of the immune response, it joins the proteins cytotoxic T lymphocyte antigen-4 (CTLA-4) and programmed death 1 (PD-1) in the group known as immune checkpoint proteins. These proteins help to keep the immune system in check and bring an immune reaction to an end at the appropriate time.
How IDO Contributes to Immune Evasion
There has been substantial interest in the checkpoint proteins in recent years (Table). The ability to evade the immune system has been added to the list of hallmark capabilities acquired by normal cells that drives their transformation into a malignant state. One of the ways in which cancer cells are able to evade the immune system is by hijacking the checkpoint proteins; overexpression of these proteins on tumor cells enables a tumor to dampen down the immune response against it.
Stage of Development
Cytotoxic T-lymphocyte antigen 4 (CTLA-4)
The first immune-checkpoint receptor to be clinically targeted, CTLA-4 is a receptor expressed exclusively on T cells, where it regulates the early stages of T-cell activation. It acts as an “off switch” counteracting the activity of the T cell co-stimulatory receptor CD28 and dampening T- cell activation.
Ipilimumab (Yervoy) Bristol-Myers Squibb
FDA-approved for melanoma; phase I, II, and III trials ongoing for a wide variety of different cancers, including prostate cancer, pancreatic cancer, and B-cell lymphoma (NCT01057810, NCT01729806, NCT01473940, NCT01896869)
Tremelimumab (formerly ticilimumab) MedImmune LLC
Phase III trials in advanced melanoma discontinued; ongoing phase I and II trials in hepatocellular carcinoma, melanoma, and mesothelioma (NCT01853618, NCT01103635, NCT01843374, NCT01655888)
Programmed death-1 (PD-1)
PD-1 is expressed on the surface of activated T cells. As with CTLA-4, it acts as an inhibitory receptor. Binding to its ligands (PD-L1 and PD-L2), which are expressed on the surface of antigen-presenting cells, leads to dampening of T-cell activation.
Pidilizumab (CT-011) CureTech Ltd
Phase II trials in a variety of hematologic malignancies and solid tumors, including pancreatic cancer, prostate cancer, metastatic melanoma, and follicular lymphoma (NCT01435369, NCT00904722, NCT01313416, NCT01420965)
Nivolumab (BMS-936558) Bristol-Myers Squibb)
Phase III trials in advanced melanoma, metastatic renal cell carcinoma, and metastatic NSCLC; phase I and II trials in a variety of cancers (NCT01844505, NCT01668784, NCT01673867)
Lambrolizumab (MK-3475) Merck
Phase II/III trial in NSCLC; phase I trial in progressive, locally advanced, or metastatic melanoma or NSCLC (NCT01905657, NCT01295827)
One of the 2 ligands that activates the PD-1 pathway.
BMS-936559 Bristol-Myers Squibb
Phase I trial in multiple cancers (NCT00729664)
Phase I and II trials in metastatic melanoma, NSCLC, and locally advanced or metastatic hematologic malignancies and solid tumors (NCT01656642, NCT01846416, NCT01375842)
Same as above.
Phase I trials in advanced cancer (NCT01352884)
Lymphocyte activation gene 3 (LAG3)
Receptor that inhibits T-cell activation.
IMP321 (ImmuFact) Immutep
Ongoing phase I/II in melanoma; completed phase I trials in breast cancer and renal cell carcinoma (NCT01308294, NCT00349934, NCT00351949)
B7 homolog 3 (B7-H3)
Another member of the B7 superfamily that includes CTLA-4 and PD-1. B7-H3 is a ligand that inhibits T-cell activation.
Phase I trial in multiple refractory cancers (NCT01391143)
Indoleamine (2,3)-dioxygenase (IDO) (PD-1)
A metabolic enzyme expressed by both tumor and tumor-infiltrating cells. IDO modulates T-cell behavior by depleting amino acids that are essential for their function.
Indoximod NewLink Genetics Corporation
Phase II trials of metastatic breast cancer and refractory metastatic prostate cancer in combination with sipuleucel-T (NCT01792050, NCT01560923)
INCB024360 Incyte Corporation
Phase II trial of biochemical-recurrent ovarian cancer and a phase I/II trial in unresectable or metastatic melanoma (NCT01685255, NCT01604889)
NSCLC indicates non-small cell lung cancer. For more information on clinical trials, visit www.ClinicalTrials.gov.
IDO overexpression has been observed in a wealth of tumor types, including prostate, colorectal, pancreatic, and cervical tumors, and is correlated with poorer survival in many cases. Increased IDO protein levels drive growth arrest and apoptosis of the effector T cells, a group of immune cells that includes cytotoxic T cells,helper T cells, and natural killer (NK) cells that mediate the immune system’s ability to destroy pathogens. By reducing the number of effector T cells, IDO overexpression prevents the immune system from effectively destroying cancer cells.
Not only is IDO expressed on tumor cells, but it also is present on cells in the surrounding microenvironment, such as dendritic cells (DCs) in the tumor-draining lymph nodes (Figure). DCs are responsible for activating the T cells by presenting them with foreign antigens. However, DCs that overexpress IDO actually inhibit the activation of effector T cells and drive their conversion into regulatory T cells, thereby further suppressing the immune response. By driving IDO overexpression, tumors are able to create an immunosuppressive microenvironment in which to reside.
Potential for Synergy
Given its role in sheltering tumors from the immune system, IDO makes an ideal target for anticancer immunotherapy. Cancer immunotherapy typically involves the stimulation of a patient’s immune system so that it recognizes tumor cells as foreign and attacks them—usually through immunization with a vaccine or administration of a therapeutic antibody.
administration of a therapeutic antibody. However, researchers are beginning to appreciate that not only do we need to present the immune system with antigens to attack, but we also need to overcome tumor-induced tolerance of these antigens that antagonizes the function of immunotherapy and limits its efficacy. This is where IDO-targeted agents could potentially truly come into their own. Because IDO expression helps to create this state of tolerance, IDO inhibition could enhance the efficacy of other immunotherapies as well as chemotherapy and radiation therapy.
Development of IDO Inhibitors
There are a number of chemicals and other small molecules that have been found to act as IDO inhibitors. These include those based on natural products, such as the cabbage extract brassinin, the marine hydroid extract annulin B, and the marine sponge extract exiguamine A. Synthetic derivatives for each of these substances are being prepared and evaluated in preclinical testing.
IDO can also be inhibited by molecular analogs of its substrate, tryptophan, and this is where the most significant clinical development has occurred. 1-methyl tryptophan (1-MT) is a tryptophan mimetic found as two stereoisomers: dextrorotary (D) and levorotary (L). The L isomer significantly inhibits IDO1, while the D isomer more significantly inhibits IDO2. The D isomer was selected for clinical translation by NewLink Genetics Corporation and the National Cancer Institute as indoximod.
Indoximod was well tolerated (the most frequent adverse events were fatigue and anemia) and showed promising signs of anticancer activity. Four of 22 patients experienced partial responses and 9 of 22 experienced stable disease in a phase I dose-escalation study in patients with advanced solid tumors (NCT01191216). To evaluate the potential of indoximod to enhance the efficacy of other therapies, it was also evaluated in a phase I trial in combination with a DC vaccine directed against p53. Results presented at the 2013 American Society of Clinical Oncology Annual Meeting in Chicago suggested that the immunotherapy sensitized patients to subsequent therapies. Seven of 19 patients treated with subsequent chemotherapy had an objective response, including 6 of 11 patients who received gemcitabine-based therapy (NCT01042535). Indoximod is currently being evaluated in a phase II, double-blind, randomized, placebo-controlled trial in patients with breast cancer (NCT01792050). NewLink Genetics also has an active program directed at synthesizing other IDO pathway inhibitors.
A second IDO inhibitor is in clinical trials. INCB024360 is an orally available hydroxyamidine small-molecule inhibitor. Unlike 1-MT-based inhibitors, hydroxyamidine inhibitors also inhibit tryptophan (2,3)-dioxygenase (TDO), an enzyme with identical activity to IDO. A recent study implicated TDO in the development of cancer in a similar manner to IDO, and many in the field believe it will be important to inhibit both IDO and TDO to overcome any compensatory mechanism. INCB024360 is in phase II trials in patients with unresectable or metastatic melanoma (NCT01604889) and ovarian cancer (NCT01685255).
Several other companies also are directing preclinical research programs at IDO inhibitor development. iTeos Therapeutics, a spinoff of the Ludwig Institute for Cancer Research and the de Duve Institute at the Université catholique de Louvain in Belgium, was launched two years ago specifically to develop novel immunotherapies for cancer treatment. It has both IDO and TDO inhibitors in preclinical development. ToleroTech Inc and Bio-Matrix Scientific Group Inc are developing small interfering (si)RNA-based cancer vaccines designed to treat cancer by silencing immunosuppressive molecules such as IDO. While Tolerotech is evaluating the value of these agents in melanoma, Bio-Matrix is focusing on breast cancer.
Because it contains heme iron in its active site that is easily inactivated by oxidation, IDO is only active in a low-oxygen (hypoxic) environment. A number of hybrid hypoxia-targeting IDO inhibitors (eg, TX-2274) have been designed to take advantage of this fact and are in preclinical development. Meanwhile, the search for other, more effective IDO-targeting agents continues, aided by the recent elucidation of the crystal structure of the IDO protein. The future looks bright for the inhibition of checkpoint proteins as a therapeutic strategy in oncology.
Jane de Lartigue, PhD, is a freelance medical writer and editor based in Davis, California.