Chapter 48: Immuno-Oncology

The ability of the immune system to recognize and eradicate cancer was first postulated in the 19th century; however, proof of principle remained elusive until the 20th century. The effect of infection on tumor regression was observed as early as 1884 by Anton Chekov (¹); following this, William Coley developed a mixture of killed bacteria that were used to treat various types of cancer between the 1890s and 1960s with mixed clinical benefit. In addition, the concept of using bacterial elements was validated with Food and Drug Administration (FDA) approval of intravesical administration of BCG, which is an FDA-approved therapy that leads to nonspecific inflammatory immune responses and clinical benefit for patients with superficial bladder cancer (²). The discovery of the major histocompatibility complex (MHC) and T-cell receptor (TCR) in the 1980s provided insight into T-cell function that led to a number of clinical trials (^3,4). Unfortunately, many of the early clinical trials failed due to incomplete understanding of T-cell function. Further research led to understanding of costimulatory and coinhibitory molecules, which led to improved strategies in the field of cancer immunotherapy, including chimeric antigen receptor (CAR) T cells and immune checkpoint therapies, resulting in clinical success that turned the tide in favor of immunotherapy.

The basic principles that guide cancer immunology are (a) immune surveillance, (b) immune editing, and (c) immune tolerance (⁵). Immune surveillance involves scanning and eliminating the transformed nascent cells by the immune system. This theory was bolstered by the finding of tumor-specific antigen that could be identified by cytotoxic T cells (⁶). Immune editing is a process that involves elimination in which the immune system acts as an extrinsic tumor suppressor (similar to the original concept of immunosurveillance); second, equilibrium occurs, in which tumor cells survive but are held in check by the immune system; and third, in escape, tumor cell variants with either reduced immunogenicity or the capacity to attenuate or subvert immune responses grow into clinically apparent cancers. Escape theory also gave credence to the concept of immune tolerance by which cancer cells exploit the body’s immune system and use it for their continuous immune evasion and subsequent growth and proliferation.

The immune compartments that aid in tumor elimination and promote tumor evasion are listed in Table 48-1. The guiding theory for development of immunotherapy is to promote antitumor immune factors and attenuate protumor factors.

Immune cells, such as cytotoxic T cells, are one of the main effector mechanisms of tumor killing and elimination. Cell-based therapy involves increasing the number and efficiency of immune cells that are capable of recognizing and killing the tumor cells

Adoptive T-Cell Therapy

Adoptive T-cell therapy (ACT) involves adoptive transfer of autologous tumor-infiltrating lymphocytes (TILs) to the tumor-bearing host (^7,8). It involves isolation and ex vivo rapid expansion of TILs from tumor, followed by infusion of large numbers of expanded autologous TILs as depicted in Fig. 48-1. Therapy with TILs has been especially promising for patients with stage III and IV unresectable melanoma, and its efficacy has been further increased with combined interleukin (IL) 2 therapy and preconditioning of the host with chemotherapy agents such as Cytoxan and fludaribine. Objective response has been reported in more than 50% of patients (^8,9). The transition of ACT into the clinical setting, however, has not been without difficulties. These can be considered in two groups: factors relating to difficulties in generating appropriate products for adoptive transfer and factors relating to host or tumor resistance to transferred populations. Recent advances in development of artificial antigen-presenting cells (aAPCs) have helped to overcome many technical issues with rapid TIL expansion (¹⁰). Further, TIL therapy has been shown to be particularly beneficial in patients pretreated with other immunotherapy agents such as IL-2 or checkpoint blockade like anti-CTLA4 (anti–cytotoxic T lymphocyte-associated antigen 4) (⁸).

Chimeric Antigen Receptor T Cell

In an effort to increase the affinity of T cells to tumor antigen, CAR T cells are engineered that contain a synthetic antigen receptor with specificity to a tumor cell marker coupled with T-cell signaling domain, resulting in high-affinity recognition of the tumor antigen (¹¹). The CAR T cells are particularly effective against cancer with known tumor antigen. The CAR T cells with specificity for CD19 have shown activity in chronic lymphocytic lymphoma (¹²). Recently, a phase III study demonstrated the CD19-coupled CAR T cells were particularly effective against relapsed refractory acute lymphoblastic leukemia (ALL) (^13,14). Autologous T cells transduced with lentiviral vector expressing a chimeric antigen receptor with specificity for the B-cell antigen CD19, coupled with CD137/4-1 BB (a costimulatory receptor in T cells) and CD3-zeta (a signal transduction component of the T-cell antigen receptor) signaling domains were infused in patients with relapsed or refractory ALL and resulted in complete remission in 90% of cases. A potential drawback of CAR T-cell therapy is exaggerated immune response against normal tissue expressing the antigen; therefore, careful dose escalation studies are necessary to predict the therapeutic window. Further, in many cancers the tumor antigen is not known, making CAR T-cell therapy ineffective against undefined tumor antigens.

Tumors express a vast array of antigens. Cancer vaccines typically consist of a known tumor antigen with an immunostimulatory formulation that could activate the tumor antigen-specific lymphocyte population, leading to eradication of cells expressing this antigen.

Although therapeutic cancer vaccines have impressive antitumor activity in various animal models, their clinical benefit in cancer patients has to date been minimal (¹⁵). However, optimization of a cancer vaccine with a combinatorial strategy resulted in significant success of the cancer vaccine, most notably two prostate cancer vaccines: Provenge and Prostvac (^16,17). Provenge (sipuleucel-T) is a cancer vaccine based on dendritic cells (DCs) that was approved in 2010. Provenge is prepared from patient’s peripheral blood mononuclear cells (PBMCs) pulsed with a fusion protein of granulocyte-macrophage colony-stimulating factor (GM-CSF) and the prostate cancer antigen prostatic acid phosphatase (PAP), with the rationale that on injection GM-CSF–activated DCs will present the antigen (PAP) to T cells in a more efficient manner. Provenge increased the median survival by 4.1 months compared to placebo in metastatic prostate cancer. Prostvac is another prostate cancer vaccine that is composed of vaccinia and fowlpox vector that contain the transgenes for prostate-specific antigen (PSA) and multiple T-cell costimulatory molecules (TRICOM) (¹⁸). The TRICOM consists of costimulatory molecules B7.1, leukocyte function-associated antigen 3 (LFA-3), and intercellular adhesion molecule 1 (ICAM-1). The PSA-TRICOM vaccines infect antigen-presenting cells (APCs) and generate proteins that are expressed on the surface of APCs that present antigen to T cells, resulting in antigen-specific tumor killing. E75 (nelipepimut-S) is a human leukocyte antigen (HLA) A2/A3–restricted immunogenic peptide derived from the HER2 protein, which was used in a phase I/II trial involving patients with node-positive or high-risk node-negative breast cancer expressing any level of Her-2 to prevent recurrence (¹⁹).

Although there remains significant room for improvement in the design and delivery of cancer vaccines, these clinical successes have led to the proof of principle that cancer vaccines are effective. Few of the cancer vaccines that are approved or are currently in clinical trials are listed in Table 48-2.

Immunostimulatory Agents

Dendritic cells are professional APCs that are critical for effective T-cell stimulation. Both DCs and T cells possess an array of stimulatory molecules that are necessary for sustained and durable immune response, making them potential therapeutic targets.

Dendritic Cell–Based Immunostimulatory Agents

Toll-like receptors (TLRs) are a group of 13 receptors present in DCs that have distinct ligands, and the receptor-ligand interaction leads to the activation of DCs, inducing the expression of type I interferons, cytokines (eg, IL-12), and costimulatory molecules (eg, CD80, CD86, and CD40) that are critical for T-cell activation (^20,21). Clinical trials with TLR agonists have shown promise, particularly in combination with other therapeutic modalities. The imidazoquinolone Imiquimod ligates TLR7; in several clinical trials, topical application resulted in an 80% to 90% clearance rate for superficial basal cell carcinoma (^22,23). Systemic administration of TLR7 and TLR9 agonist as monotherapy in melanoma and renal cell carcinoma had a strong immune response but failed to demonstrate objective clinical response (^24,25). Although they failed as monotherapy, their capacity to boost the immune system makes them a suitable adjuvant therapy. Both CpG and imiquimod induced increased levels of tumor antigen-specific T cells in patients with melanoma and prostate cancer vaccinated with recombinant protein tumor antigen NY-ESO1 showing promise in combination therapy (^26,27).

Another attractive target for enhancing DC function is CD40, which is present on APCs, including DCs, and ligation of CD40 with its ligand CD40L present on T cells is critical for T-cell priming (²⁸). CD40 targeting agents as monotherapy showed modest clinical benefit in non-Hodgkins lymphoma and melanoma (²⁹). To increase its efficacy, a novel approach utilized electroporation to introduce messenger RNA encoding CD40 ligand, constitutively active TLR4, CD70, and multiple melanoma tumor antigens into autologous DCs (TriMix-DC). Tumor regressions were observed in 6 of 17 patients who had received interferon-α-2b in combination with TriMix-DC (³⁰). Further, combination of anti-CD40 antibody and the chemotherapy agent gemcitabine showed tumor regression in both humans and mice (³¹).

T-Cell Based Immunostimulatory Agents

Engagement of the TCR on T cells with MHC serves as a first signal for T-cell activation; the second signal is mediated by the binding of costimulatory molecules on the T-cell surface to B7 proteins (such as CD80 or CD86) on APCs. Both signals are critical for effective T-cell activation, proliferation, and migration, making costimulatory molecules an interesting therapeutic target for sustained immune response (^32,33).

Greater attention has been focused on 41BB (CD137) and OX40 (CD134). Preclinical studies with antimouse anti-CD137 have shown promising activity in combination with antitumor antibody. Most recently, it was shown that targeting anti-CD137 increases the efficacy of cetuximab (anti–epidermal growth factor receptor monoclonal antibody [mAb]) in murine xenograft models, making it another suitable target for combination therapy (³⁴). Clinical studies with PF-05082566 (Pfizer, anti-CD-137) in combination with rituximab in B-cell Lymphoma and MK-3475 (anti–programmed cell death 1 [PD-1]) in solid tumor are ongoing. Further, a phase I clinical trial using a mouse mAb that agonizes human OX40 signaling in patients with advanced cancer showed that patients treated with one course of the anti-OX40 mAb (9B12) had an acceptable toxicity profile and regression of at least one metastatic lesion in 12 of 30 patients (³⁵).

Immune Checkpoint Blockade

T-cell activation is tightly controlled by immune-suppressive cells and cytokines as well as by the coinhibitory molecules present in T cells, such as CTLA-4 or PD-1 (see Fig. 48-2) (³⁶). The CTLA-4 is expressed by activated CD4 and CD8 T cells, and it competes with costimulator CD28 for binding to its ligands (B7 proteins). Binding of CTLA-4 to B7 proteins interrupts CD28 costimulatory signals and serves as a negative regulator of T-cell responses.

For many years, it was widely accepted that T-cell responses can be turned “on” via T-cell receptor and CD28 costimulation, but the concept of turning “off” T-cell responses did not exist until the discovery of CTLA-4. It was a paradigm shift in cancer immunotherapy to move away from vaccine strategies aimed at turning on T-cell responses to immunotherapy strategies aimed at turning off T-cell inhibitory pathways (³⁷). Preclinical studies with anti-CTLA-4 antibodies demonstrated rejection of syngeneic transplanted tumors in mice.

These preclinical studies led to the eventual development of an antibody to block human CTLA-4 (ipilimumab), which was subsequently shown in a phase III randomized, controlled trial in patients with metastatic melanoma to improve the median overall survival (³⁸). Importantly, additional studies have shown that a subset of patients treated with anti-CTLA-4 had durable clinical responses lasting 10 or more years (³⁹). This trial led to the approval of ipilimumab by the FDA in March 2011 for the treatment of patients with metastatic melanoma. A number of factors have been proposed to potentially serve as biomarkers for response to ipilimumab therapy. Recent studies suggest that an increase in TILs correlates with clinical response of anti-CTLA-4 therapy (⁴⁰). In addition, sustained Inducible T-cell COStimulator (ICOS) expression on CD4 T cells has also been observed to correlate with survival of patients with melanoma treated with anti-CTLA-4 therapy (⁴¹).

Consistent with this observation, increased frequency of ICOS⁺ CD4 T cells can also serve as a pharmacodynamic biomarker for anti-CTLA-4 therapy (⁴²). ICOS is one of the costimulatory receptors of T cells. A preclincial study showed that engagement of the ICOS pathway markedly enhanced efficacy of CTLA-4 blockade in cancer immunotherapy (⁴³). This finding provides a potential mechanism for future clinical studies with agonistic signaling through ICOS in combination with blockade of CTLA-4.

Programmed cell death 1 is another negative regulator of T-cell response that is mainly expressed by activated CD4 and CD8 T cells as well as APCs (⁴⁴). In preclinical studies, blocking antibodies against PD-1 resulted in reduction of tumor metastasis and growth in a number of experimental tumor models (^45,46). These preclinical results led to many clinical trials. In a phase I clinical trial, a fully human immunoglobulin (Ig) G4 anti-PD-1 mAb (nivolumab) was evaluated in patients with relapsed or refractory Hodgkin’s lymphoma and demonstrated objective responses in 20 patients (87%), including 17% with a complete response (⁴⁷).

A randomized, controlled, phase III clinical study compared nivolumab versus dacarbazine (chemotherapy) in patients with previously untreated melanoma (without the BRAF mutation), and the overall response rate favored nivolumab (40% vs 14%) (⁴⁸). Another phase III clinical trial compared nivolumab versus chemotherapy (dacarbazine or carboplatin plus paclitaxel) in patients with ipilimumab-refractory advanced melanoma induced an overall response rate of 32% versus 11%, respectively, leading to the accelerated FDA approval of nivolumab for patients with unresectable or metastatic melanoma no longer responding to other drugs (⁴⁹).

Pembrolizumab is another humanized IgG4 mAb targeting PD-1 that demonstrated an overall response rate of 26% in patients with ipilimumab-refractory advanced melanoma in a phase I clinical trial, which prompted its accelerated FDA approval (⁵⁰). Nivolumab also had an overall response rate of 87% in patients with Hodgkin’s lymphoma who had failed brentuximab vedotin (⁴⁷). These studies show that nivolumab and pembrolizumab have significant clinical activity in a variety of heavily pretreated patients with solid tumor malignancies as well as patients with hematological malignancies.

Programmed cell death 1 has two ligands, PD-L1 and PD-L2, with distinct expression profiles. The PD-L1 ligand is expressed not only on APCs but also on T cells, B cells, and nonhematopoietic cells, including tumor cells. Expression of PD-L2 is largely restricted to APCs, including macrophages and myeloid DCs, as well as mast cells. Promising clinical results were observed with drugs targeting PD-L1. A phase I trial with anti-PD-L1 (human IgG4 mAb; BMS-936559) demonstrated an objective response rate of 6% to 17% in patients with advanced non–small cell lung cancer (NSCLC), melanoma, and Renal Cell Carcinoma (RCC) (⁵¹). Another agent targeting PD-L1, MPDL3280A (human IgG1), was engineered with a modification in the Fc domain that eradicates antibody-dependent cellular cytotoxicity. MPDL3280A showed objective response rates of 13% to 26% in multiple solid tumor malignancies, including NSCLC, melanoma, RCC, colorectal cancer, gastric cancer, and head and neck squamous cell carcinoma (⁵²). Remarkably, MPDL3280A also had a 26% objective response rate in bladder cancer (⁵³).

Because anti-CTLA-4 and anti-PD-1 target distinct inhibitory pathways in T cells, preclinical studies have shown that concurrent targeting of CTLA-4 and PD-1 significantly improves therapeutic efficacy when compared to the monotherapies (⁵⁴). A phase I clinical trial evaluated the concurrent treatment of advanced melanoma with ipilimumab plus nivolumab using various doses of both drugs (four cohorts). The objective response rate was 40% when all four cohorts were included and 53% in the cohort representing the maximum dose that was associated with an acceptable level of toxicities. This latter cohort was also associated with unprecedented 1- and 2-year overall survival rates of 94% and 88%, respectively (⁵⁵). These results show that drugs targeting the immune checkpoints, CTLA-4, PD-1, and PD-L1 as monotherapy or in combinations, are likely to become the standard of care in various solid as well as hematologic malignancies.

Recent success with immunotherapy in the management of cancer has given credence to the long-held belief that the immune system can be used to treat cancer. Most immunotherapies currently approved or in clinical trials are being used in the metastatic setting. More clinical trials are warranted to test the efficacy of immunotherapy in the presurgical or neoadjuvant setting. The potential for immunotherapies augmenting the effects of conventional chemotherapy or radiotherapy also requires further exploration. Combinational immunotherapy and the combination of immunotherapy with conventional therapy will be the theme of future cancer treatment. The identification of multiple other immune pathways that can be targeted has led to the development of numerous immunotherapy agents that will require testing as monotherapy and combination therapy in the clinic. Furthermore, this new class of cancer treatment also brings a unique set of side effects, which will require close monitoring of patients and additional work to understand predictive biomarkers of toxicities, as well as studies to guide rational development of combination strategies for optimal patient benefit.