Immunotherapy for non-small-cell lung cancer

Abstract

Activation of the host immune system represents an attractive treatment approach for cancers. In non-small-cell lung cancer (NSCLC), a variety of immunotherapies, including nonspecific immune stimulants, vaccines and checkpoint inhibitors, have been evaluated in clinical trials. Several randomized Phase III trials have failed to demonstrate clinical benefit from nonspecific immune stimulants and vaccines in the overall trial populations. Activity of vaccines in subsets of patients in these trials needs further evaluation. Unlike vaccines aimed at stimulating a cellular immune response to antigens differentially expressed in cancers, checkpoint inhibitors aim at overcoming immune inhibitory signals in the tumor microenvironment via pharmacological inhibition of immune checkpoints – a crucial tumoral immune escape mechanism. Early clinical trials of checkpoint inhibitors showed promising results with some durable responses. Better understanding of the mechanisms of immunosuppression specific to NSCLC will be crucial for successful patient selection for immunotherapy.

Keywords::

• immune checkpoint inhibitors
• immunotherapy
• non-small-cell lung cancer
• vaccines

1. Introduction

Conceptual progress over the last decade has led to the recognition of immunoevasion as an emerging hallmark of cancer. Cancer cells are thought to escape immune destruction by disabling components of the immune system that have been dispatched to eliminate them [1]. Some of the better characterized tumor immune escape mechanisms include the loss of major histocompatibility complex antigen (HLA) expression, activation of regulatory T cells, upregulation of immune checkpoints and immunosuppressive cytokines [2]. Therefore, activation of the immune system against cancer represents an attractive treatment approach. However, until recently, antitumor effects of immunotherapy among solid tumors were limited to melanoma, renal and prostate cancers.

Although lung cancer was historically thought to be a nonimmunogenic tumor, there is evidence indicating the presence of both cellular and humoral antitumor immune responses in non-small-cell lung cancer (NSCLC). Experimental models of lung cancer demonstrate the presence of immune dysregulation at multiple levels [3].

The goal of immunotherapy is the induction of a humoral or cellular immune response against cancer. The cellular immune response consists of generation of CD4⁺ and CD8⁺ cytotoxic T lymphocytes (CTL) that can selectively destroy cancer cells by targeting tumor-associated antigens. Clinical development of immunotherapy for NSCLC has involved three broad classes of agents: nonspecific immune stimulants, vaccines and immune checkpoint inhibitors [4].

2. Nonspecific immune stimulants

Nonspecific immune stimulants that have been evaluated in Phase III trials in NSCLC include agonists of Toll-like receptors (TLRs), which mediate the induction of adaptive immunity against invading pathogens, and talactoferrin alfa, a recombinant human lactoferrin that is known to have several immunomodulatory functions. In a Phase III, randomized, double-blind, placebo-controlled trial (n = 742), talactoferrin failed to improve the overall survival (OS) of previously treated stage IIIB or IV NSCLC patients [5]. Two randomized, double-blind, placebo-controlled Phase III trials that combined a TLR agonist PF-3512676 with chemotherapy in chemotherapy-naive patients failed to meet the primary objective of improving the OS and were associated with more toxicities in the treatment arms [6,7].

3. Vaccines

Cancer vaccines aim at stimulating a cellular immune response to antigens differentially expressed in cancers. Antigen-specific vaccines and tumor cell vaccines have undergone Phase III testing in NSCLC. The liposomal BLP25 (L-BLP25, tecemotide) is a peptide-based vaccine composed of a synthetic mucin 1 (MUC-1) lipopeptide and monophosphoryl lipid A, an immunoadjuvant in a liposomal delivery system. L-BLP25 targets core peptide of MUC-1, which is overexpressed in NSCLC. MUC-1 has been associated with cell transformation, migration, immunosuppression, resistance to oxidative stress-induced apoptosis and chemotherapy resistance. A randomized Phase III trial compared L-BLP25 with placebo in patients with unresectable stage III NSCLC with nonprogressive disease after sequential or concurrent chemoradiotherapy (n = 1513) [8]. The study did not show OS benefit with L-BLP25 over placebo (median 25.6 months for L-BLP25 vs. 22.3 months; hazard ratio [HR] = 0.88; 95% confidence interval [CI]: 0.75 – 1.03; p = 0.123). Among patients who received concurrent chemoradiotherapy (n = 806), those who received L-BLP25 had longer OS compared with those who received placebo (median 30.8 months for L-BLP25 vs. 20.6 months; HR = 0.78; 95% CI: 0.64 – 0.95; p = 0.016).

Belagenpumatucel-L (Lucanix™) is an allogeneic tumor cell vaccine, which consists of four irradiated NSCLC cell lines that have been modified with TGF-β2 antisense gene plasmid. TGF-β helps tumors escape host immunosurveillance by a variety of mechanisms. Belagenpumatucel-L was evaluated as maintenance therapy after first-line chemotherapy and/or chemoradiotherapy in patients with stage III or IV NSCLC (n = 532) in a randomized Phase III trial [9]. The study did not show OS benefit with belagenpumatucel-L over placebo (median 20.3 months for belagenpumatucel-L vs 17.8 months; HR = 0.94; p = 0.59). The OS improved by 7.3 months with belagenpumatucel-L in patients who were randomized within 12 weeks of completing chemotherapy (n = 305) (median 20.7 months with belagenpumatucel-L vs. 13.4 months; HR = 0.75; p = 0.083).

Melanoma-associated antigen 3 (MAGE-A3), a peptide on the HLA-A1 molecule, is recognized by CTL and is expressed almost exclusively on tumor cells. MAGE-A3 is a peptide-based vaccine consisting of the recombinant antigen ProtD-MAGE-A3/His (a fusion protein containing protein D, a lipoprotein present on the surface of Haemophilus influenzae B, MAGE-A3 protein and a polyhistidine tail) and a proprietary immunological adjuvant. A Phase III, randomized, double-blind, placebo-controlled trial (n = 2312) evaluated MAGE-A3 vaccine in completely resected stage IB, II or IIIA MAGE-A3-expressing NSCLC [10]. The trial did not meet its first and second co-primary end points, which was an improvement in the disease-free survival (DFS) in the overall MAGE-A3 positive population and in patients who did not receive adjuvant chemotherapy. Assessment of a third co-primary end point, DFS in a gene-signature-positive population, was not feasible due to insufficient treatment effect.

4. Immune checkpoint inhibitors

Immune checkpoints are molecules expressed on the surface of immune cells including T lymphocytes that modulate the immune response to antigens via inhibitory or stimulatory signaling to T cells. Two most extensively studied immune checkpoints in lung cancer are cytotoxic T-lymphocyte antigen-4 (CTLA-4) and programmed cell death-1 (PD-1). Activation of both receptors causes downregulation and inhibition of immune responses.

Ipilimumab, an anti-CTLA-4 antibody, was evaluated in a randomized Phase II study in combination with chemotherapy [11]. Patients with chemotherapy-naive NSCLC (n = 204) were randomized to receive paclitaxel and carboplatin with either placebo or ipilimumab in one of the following two regimens: concurrent ipilimumab (four doses of ipilimumab plus paclitaxel and carboplatin followed by two doses of placebo plus paclitaxel and carboplatin) or phased ipilimumab (two doses of placebo plus paclitaxel and carboplatin followed by four doses of ipilimumab plus paclitaxel and carboplatin). The study met its primary objective of improving the immune-related progression-free survival (ir-PFS) for phased ipilimumab compared with control (HR = 0.72; p = 0.05) but not for concurrent ipilimumab (HR = 0.81; p = 0.13). Phased ipilimumab, concurrent ipilimumab and control were associated with a median ir-PFS of 5.7, 5.5 and 4.6 months, respectively. There was no significant difference in the OS between the three arms. In the phased ipilimumab arm, improvements in ir-PFS appeared to be greater in patients with squamous histology (HR = 0.55; 95% CI: 0.27 – 1.12) than in patients with nonsquamous histology (HR = 0.82; 95% CI: 0.52 – 1.28).

The World Health Organization criteria or Response Evaluation Criteria in Solid Tumors, which is conventionally used for response assessment of cytotoxic agents, may not fully assess the clinical activity of immunotherapies. The immune-related response criteria (ir-RC) adapt the standard RC to include the potential for delayed clinical responses and apparent increases of tumor burden, which may precede responses in patients receiving immunotherapy. ir-PFS, the primary objective of the above-discussed trial [11], is assessed from the time of randomization to immune-related progressive disease (ir-PD) or death. ir-PD is defined as an increase in tumor burden ≥ 25% relative to the minimum recorded tumor burden that is confirmed on repeat assessment no less than 4 weeks from the date first documented. The core difference of ir-RC from conventional RC is that it allows patients with new lesions but a decrease in baseline lesions to continue receiving therapy [12].

PD-1, another key immune-checkpoint receptor, is expressed by activated T and B cells and is structurally similar to CTLA-4 but has distinct biological functions and ligand specificity. PD-1 is engaged by its ligands PD-L1 and PD-L2 within the tumor microenvironment. When activated T cells expressing PD-1 encounter PD-L1 or PD-L2, the T-cell effector functions are diminished. Nivolumab, an anti-PD-1 antibody, was evaluated in a Phase I trial where it was administered intravenously once every 2 weeks in 8-week cycles to patients with selected advanced cancers [13]. In the NSCLC expansion cohorts treated at 1, 3 and 10 mg/kg, response rates were 18% (14 of 76 patients). Responses were observed across squamous (objective response rates [ORR] = 33%; 6 of 18 patients) and nonsquamous (ORR = 12%; 7 of 56 patients) histologies.

BMS-936559, an anti-PD-L1 antibody, was evaluated in a Phase I trial where it was administered intravenously once every 2 weeks in 6-week cycles in patients with selected advanced cancers [14]. In the NSCLC expansion cohorts treated at 3 and 10 mg/kg, response rates were 10% (5 of 49). Responses in the squamous and nonsquamous histologies were 8% (1 of 13) and 11% (4 of 36), respectively. A number of other anti-PD-1 (e.g., pembrolizumab) and anti-PD-L1 (e.g., MPDL-3280A) antibodies have demonstrated preliminary antitumor activity in NSCLC. In a Phase I study of 38 previously treated NSCLC patients, pembrolizumab 10 mg/kg administered intravenously once every 3 weeks resulted in 24% immune-related response [15]. Preliminary evidence suggests an association between tumor cell PD-L1 expression, the presence of intratumoral immune cell infiltrates and PD-1 receptor expression by tumor-infiltrating lymphocytes [16].

5. Expert opinion

At this time, immunotherapies remain investigational in NSCLC. Failure of several large randomized trials of antigen-specific and allogeneic tumor cell vaccines to meet their primary end points are illustrative of the challenges to clinical evaluation of immunotherapy in NSCLC. The inability to provide clinical benefit despite induction of cellular immune responses to antigens differentially expressed on cancers is suggestive of humoral and cellular immune dysregulation in NSCLC. That subsets of patients derived benefit with some vaccines [7,8] indicates the importance of patient selection. A better understanding of the mechanisms of immunosuppression in lung cancer will be crucial for successful patient selection for immunotherapy.

Pharmacological inhibition of immune checkpoints, which form a crucial immune escape mechanism for the tumor, have yielded promising results in early-phase clinical trials. Preliminary studies have also identified potential biomarkers which may identify patients who may respond to this strategy. For example, tumor cell PD-L1 expression may be selected for patients with an improved response to PD-1 axis inhibitors [13]. However responses have also been seen in patients whose tumors have minimal PD-L1 expression [15] raising concerns that excluding the ‘marker-negative’ patient population from treatment might exclude potential responders. Moreover, reports of tumor cell PD-L1 expression vary widely in the methodologies used including anti-PD-L1 antibodies, staining techniques and thresholds for positivity. Further studies are needed to validate tumor PD-L1 expression as a biomarker for response.

Combinatorial strategies involving vaccines or immune-checkpoint inhibitors with other immune-based therapies, chemotherapy or radiation are under active investigation and may successfully overcome the immunosuppressive tumor milieu. For example, concurrent radiotherapy and CTLA-4 blockade was recently reported to induce tumor regression via immune-mediated abscopal effects [17]. Further studies are needed to understand immune escape mechanisms specific for NSCLC as mechanisms observed in other tumors may not be generalizable to NSCLC due to the proinflammatory and immunosuppressive effects of tobacco smoke, the major risk factor for lung cancer.

Declaration of interest

This work was supported by the Intramural Research Program of the NIH, National Cancer Institute. The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.