Advances in new targets for immunotherapy of small cell lung cancer

Abstract

Small cell lung cancer (SCLC) is one of the highly aggressive malignancies characterized by rapid growth and early metastasis, but treatment options are limited. For SCLC, carboplatin or cisplatin in combination with etoposide chemotherapy has been considered the only standard of care, but the standard first‐line treatment only results in 10‐month survival. The majority of patients relapse within a few weeks to months after treatment, despite the relatively sensitive response to chemotherapy. Over the past decade, immunotherapy has made significant progress in the treatment of SCLC patients. However, there have been limited improvements in survival rates for SCLC patients with the current immune checkpoint inhibitors PD‐1/PD‐L1 and CTLA‐4. In the face of high recurrence rates, small beneficiary populations, and low survival benefits, the exploration of new targets for key molecules and signals in SCLC and the development of drugs with novel mechanisms may provide fresh hope for immunotherapy in SCLC. Therefore, the aim of this review was to explore four new targets, DLL3, TIGIT, LAG‐3, and GD2, which may play a role in the immunotherapy of SCLC to find useful clues and strategies to improve the outcome for SCLC patients.

Immune checkpoint inhibitors have altered the treatment paradigm of small cell lung cancer (SCLC). However, the current immunotherapies, such as programmed cell death‐1 (PD‐1)/programmed death ligand 1 (PD‐L1) and cytotoxic T lymphocyte‐associated antigen‐4 (CTLA‐4), brought limited benefits to survival. It is urgent to develop new targets or immunotherapy drugs. This review focuses on multiple new targets of immunotherapy investigated in the field of SCLC, including delta‐like ligand 3 (DLL3), T cell immunoglobulin and immunoreceptor tyrosine‐based inhibitory motif structural domains (TIGIT), lymphocyte activation gene‐3 (LAG‐3), and disialoganglioside (GD2), and others.

INTRODUCTION

As one of the leading causes of cancer death, ¹ small cell lung cancer (SCLC) accounts for ~15%–17% of all lung cancers. ² It is one of the most aggressive malignant tumors, characterized by rapid tumor growth and early distant metastases. ³ , ⁴ Clinically, SCLC is categorized as either limited stage SCLC (LS‐SCLC) or extensive stage SCLC (ES‐SCLC), with 5‐year survival rates of 10%–13% and 1%–2%, respectively. ⁵ ES‐SCLC accounts for approximately two‐thirds of all SCLCs and most diagnoses are accompanied by distant metastases. In most cases, chemoradiotherapy remains the mainstay of treatment, and a high initial response to this treatment is often observed. ⁶ , ⁷ Unfortunately, most patients experience an early recurrence and have a poor prognosis. ⁸ For many years, cisplatin or carboplatin in combination with etoposide chemotherapy has been considered the only standard of care, with a median survival of just about 10 months. ⁹ The outcome of SCLC patients has also not improved significantly over the past three decades, making SCLC an intractable disease.

In the past decade, immunotherapy has brought new light to first‐line treatment for SCLC, increasing survival rates and improving patient outcomes. ³ Multiple studies have demonstrated potential benefits of immunotherapy for SCLC. Immune checkpoint inhibitors bind to cell surface proteins to block immunosuppressive signaling, such as programmed death receptors (PD‐1), programmed death ligand 1 (PD‐L1), or cytotoxic T lymphocyte‐associated antigen 4 (CTLA‐4). However, the improvement in survival of SCLC patients with PD‐1/PD‐L1 inhibitors remains limited and has not been groundbreaking. The IMpower133 study demonstrated that the addition of the anti‐PD‐L1 monoclonal antibody (mAb) atezolizumab to carboplatin and etoposide improved progression‐free survival (PFS) and overall survival (OS) in patients with ES‐SCLC compared to chemotherapy alone (mPFS: 5.2 vs. 4.3 months, mOS: 12.3 vs. 10.3 months), but merely resulted in a two‐month survival benefit. ¹⁰ The KEYNOTE‐604 study demonstrated that in the first‐line treatment of patients with ES‐SCLC, pembrolizumab plus etoposide and platinum (EP) improved PFS in patients compared to placebo plus EP, but overall survival only showed a trend toward benefit and did not reach the threshold of significance (mPFS: 4.5 vs. 4.3 months, mOS: 10.8 vs. 9.7 months). ¹¹

It is believed that SCLCs are homogeneous and are characterized almost by functional inactivation of the tumor suppressors RB1 and TP53. ¹² , ¹³ However, four transcriptional subtypes of SCLC have been proposed based on the elevated expression of transcription factors ASCL1 (SCLC‐A), NEUROD1 (SCLC‐N), and POU2F3 (SCLC‐P) and inflammatory characteristics (SCLC‐I). ¹⁴ , ¹⁵ SCLC‐A and SCLC‐N are neuroendocrine tumors characterized by low numbers of infiltrating immune cells, while SCLC‐P and SCLC‐I are non‐neuroendocrine tumors with increased immunogenicity. ¹⁶ Different molecular subtypes are associated with different therapeutic sensitivity. It has been found that SCLC‐I tumors benefit more from immune checkpoint blockade targeting the PD‐1/PD‐L1 axis and that SCLC‐N tumors are often associated with MYC amplification. ¹⁷ , ¹⁸ Further, Gay et al. demonstrated that transcription shifts from SCLC‐A to SCLC‐I during platinum treatment. ¹⁸ Consequently, chemotherapy resistance caused by intratumor heterogeneity may change how we approach treatment.

The prognosis for patients with SCLC remains a major clinical dilemma. Immunotherapy for SCLC also still faces problems such as limited benefits, few beneficiaries, and lack of typical predictive markers. Therefore, the development of new immunotherapeutic targets or drugs is urgently required. In response to this, four new targets are currently being investigated in SCLC, including delta‐like ligand 3 (DLL3), T cell immunoglobulin and ITIM structural domains (TIGIT), lymphocyte activation gene‐3 (LAG‐3), and disialoganglioside (GD2). This review focuses on new therapies, preclinical studies, and clinical data related to these four new immunotherapy targets with the intention of contributing fresh ideas to the treatment of SCLC in the future.

DELTA‐LIKE LIGAND 3

DLL3 and the Notch signaling pathway in SCLC

Delta‐like ligand 3 (DLL3) is a single transmembrane protein that attaches to the cell surface and is a member of the Notch family of ligands. There are four Notch receptors (Notch 1, 2, 3, and 4) and five ligands (Jagged 1 and 2, and Delta‐like 1, 3, and 4) in the Notch system. ¹⁹ , ²⁰ DLL3 binds to the Notch receptor to exert its biological function. Unlike other Notch ligands, DLL3 does not activate the Notch receptor when it binds to it but instead inhibits signaling through the Notch pathway. ²¹ The DLL3 protein binds to a variety of Notch receptors in different types of tumors and therefore promotes or inhibits tumorigenesis and development during cell proliferation, differentiation, and apoptosis. ²² , ²³ , ²⁴ , ²⁵ As a result of DLL3 binding to the Notch1 receptor, Notch signaling activation will be inhibited, causing HES1/HEY1 transcription to be upregulated, and ASCL1 inhibited. ²⁶ ASCL1 is a transcription factor required for normal lung neuroendocrine cell growth and development as well as an oncogenic factor in SCLC. ²⁷ , ²⁸ Among SCLC expressing ASCL1, DLL3 is significantly upregulated and abnormally transported to the cell surface, ²⁹ and the widespread and specific expression of DLL3 on SCLC tumor cells makes it an ideal therapeutic target. Studies have confirmed that DLL3 is positively correlated with the expression of ASCL1, and Zhu et al. found the expression of DLL3 in all clinicopathological cases of SCLC‐A. ³⁰ , ³¹ Currently, it has been found that the target DLL3 has advantages over other therapies affecting Notch signaling. ²⁶

There is an increasing use of T cell‐based immunotherapies in cancer patients, and immune checkpoint inhibitors are being developed that target SCLC. In addition, T cell retargeting therapies that use the intrinsic power of the immune system to target DLL3 have provided new ideas for the treatment of SCLC. Bispecific T cell engagers (BiTEs) based on antibodies and chimeric antigen receptor (CAR) cell therapies are examples of such therapies. The fact that DLL3 is highly expressed on the cell surface of high‐grade lung neuroendocrine tumors, including SCLC, but rarely in normal tissues, makes it a potential target for T cell redirection. ²⁹ , ³² A study of 63 SCLC patients revealed that 83% of tumor samples were positive for DLL3 expression by immunohistochemistry (IHC), with 32% exhibiting high levels of expression. ³³

DLL3‐targeted BiTE molecules and clinical experience

BiTE is a class of bispecific antibodies that have significant antitumor effects by targeting and activating T cells. In BiTE, two single‐chain variable fragments (ScFvs) are connected by a flexible fusion junction, which recognizes T cell surface proteins CD3ε and specific tumor cell surface antigens, respectively. ³⁴ It is through this structure that BiTE can physically connect T cells and tumor cells, resulting in the formation of immune synapses and T cell activation, which is characterized by CD3 aggregation, the proliferation of T cells, and the release of pore‐forming granzymes and perforins. The sequence of events ultimately leads to apoptosis of the tumor cells and an amplification of the T cell response. ³⁵ , ³⁶ , ³⁷ In addition to retargeting cytotoxic T cells, BiTE drugs also prevent further cytotoxicity of normal cells. ³⁸ Furthermore, BiTE molecular activity does not require specific T cell receptors or peptide–MHC complexes and may be able to overcome the immunosuppressive environment of tumors. ³⁸ In preclinical models, BiTE showed strong antitumor activity and was superior to conventional mAb and other forms of bispecific antibodies. ³⁹ In addition, it has been demonstrated that bispecific antibodies rapidly induce specific cytotoxic activity in unstimulated T cells even at very low potent antibody concentrations (10–100 pg/mL). ⁴⁰

Tarlatamab (AMG757)

Tarlatamab is a bispecific T cell adherent (BiTE) antibody that targets delta‐like ligand 3 (DLL3) on SCLC and the CD3 complex on T cells, fusing to the Fc structural domain to extend its pharmacokinetic half‐life. Tarlatamab showed significant efficacy in SCLC cell lines and mouse models. In patient‐derived tumor xenograft (PDX) studies, tarlatamab induced significant tumor regression (83%–98%) and a significant reduction in tumor volume. ⁴¹ Tarlatamab significantly inhibits tumor growth even at low doses in a disseminated in situ model of SCLC. ⁴¹ The use of SCLC cell lines in an in vitro T cell‐dependent cytotoxicity assay revealed that low molar concentrations of tarlatamab were able to direct T cells to kill DLL3‐positive cancer cells in vitro. ⁴² In addition, tarlatamab was well tolerated in GLP toxicology studies, with no adverse reactions reported at doses up to 4.5 mg/kg QW. ³²

In October 2017, Amgen initiated an open‐label phase I study of tarlatamab (DeLLphi300, NCT03319940) aimed at evaluating the safety, tolerability, and pharmacokinetics of tarlatamab in patients with progressive SCLC or relapse after platinum‐based chemotherapy. ⁴³ On July 19, 2022, 107 patients had been treated, of whom 97 (90.7%) experienced treatment‐related adverse events (TRAE). The most common treatment‐related adverse event was cytokine release syndrome, which occurred in 52% of patients. The objective remission rate (ORR) was 23.4% (95% CI: 15.7–32.5), including two complete and 23 partial remissions. The median duration of response (DOR) was 12.3 months (95% CI: 6.6–14.9). The disease control rate was 51.4% (95% CI: 41.5–61.2). The median progression‐free survival (mPFS) and median overall survival (mOS) were 3.7 months (95% CI: 2.1–5.4) and 13.2 months (95% CI: 10.5 to not reached), respectively. ⁴⁴ Despite the low remission rate of tarlatamab, the median duration of remission and median survival exceeded 1 year, which is encouraging.

Furthermore, four studies have been conducted to assess the efficacy of tarlatamab in SCLC (NCT05361395, NCT04885998, NCT05740566, and NCT05060016).

BI 764532

There is another BiTE construct, BI764532, which displays potent antitumor activity in DLL3‐positive tumor cells in preclinical studies. In a human T cell transplantation mouse model, BI 764532 induced potent, DLL3‐dependent tumor cell lysis and T cell infiltration of tumor tissues, leading to complete tumor regression. ⁴⁵ In addition, upregulation of PD‐1, PD‐L1, and LAG‐3 was observed in this preclinical study, suggesting potential synergistic effects with ICIs. ⁴⁵

The NCT04429087 study is the first human trial for BI 764532, which aims to assess the safety, efficacy, and highest tolerated dose of the drug. In this study, BI 764532 is being investigated as monotherapy against SCLC and other neuroendocrine carcinomas that are positive for DLL3. ⁴⁶ The data were presented at the ASCO Congress in 2023. As of December 28, 2022, 90 patients were treated with ≥1 dose of BI 764532, 52% of whom had SCLC, with a median duration of 43 days (1–443 days), and 25 were still on treatment. Safety data showed an 86% incidence of TRAE, most of which were grade 1–2 and manageable with symptomatic management. Maximum tolerated dose (MTD) was not achieved. For efficacy data, the objective remission rate (ORR) for all regimens in patients with SCLC (n = 24) or NEC (n = 23) receiving ≥target dose of BI 764532 was 33% and 22%, respectively. One patient with LCNEC was available for efficacy assessment and achieved partial remission (PR). Moreover, tumor shrinkage was observed in all patients receiving ≥90 μg/kg of the drug. BI 764532 demonstrated clinically manageable tolerability and the study is ongoing. ⁴⁷

There are also two ongoing clinical studies, NCT05879978 and NCT05882058.

QLS31904

QLS31904 is also a drug that targets DLL3/CD3, with target indications for a variety of advanced solid tumors including SCLC. NCT05461287 is a phase I study evaluating the safety and tolerability of QLS31904.

Other T cell engagers (TCE)

HPN328 is a tri‐specific T cell activating construct (TriTAC), which has a mechanism of action similar to BiTE drugs but contains an additional intermediate single domain antibody (sdAb) to prolong the half‐life of the drug. The preclinical studies have demonstrated that HPN328 inhibits the growth of subcutaneous NCI‐H82 SCLC xenograft tumors in mice. In a single‐dose pilot toxicity study in cynomolgus monkeys, HPN328 was shown to be well tolerated at both 1 and 10 mg/kg doses. ⁴⁸ NCT04471727 is an ongoing phase I/II a study being evaluated in patients with metastatic SCLC and other neuroendocrine cancers associated with DLL3 expression. Interim data demonstrated that HPN328 was well tolerated and clinically active with a manageable rate of adverse events and no grade ≥3 cytokine release syndrome (CRS). Tumor shrinkage was observed, including one confirmed PR. ⁴⁹

SCLC cells also express CD47, which inhibits macrophage‐mediated killing of tumor cells. PT217, a DLL3/CD47 bispecific antibody, specifically targets SCLC cells in two ways. First, PT217 blocks CD47‐SIRPα interaction and stimulates macrophages to phagocytose tumor cells. Second, the functional Fc of PT217 was found to be highly effective in NK‐cell‐mediated cytotoxicity as well as macrophage‐mediated phagocytosis. ⁵⁰ PT217 has shown potent tumor inhibitory activity in preclinical mouse models and a favorable safety profile in nonhuman primate (NHP) model studies, and the FDA has granted PT217 Orphan Drug status as a potential therapeutic option for patients with SCLC.

DLL3‐targeted CAR therapies

CAR‐T therapy

Chimeric antigen receptor (CAR) T cell therapy is another strategy for utilizing a patient's T cells to treat cancer. These are patient‐derived T cells that have been genetically modified to recognize tumor cells by expressing receptors for tumor antigens, resulting in T cells activating and expanding to kill tumor cells. ⁵¹ CAR‐T cells have proven clinically effective in treating patients with hematological malignancies, laying the foundation for them to become an important component of cancer treatment. ⁵²

AMG 119 is a permissive cell therapy drug in which the patient's autologous T cells are genetically modified in vitro to express a transmembrane CAR that specifically targets DLL3 and redirects cytotoxic T cells to DLL3‐positive cells. In a SCLC xenograft model, AMG 119 CAR‐T cells were found to be effective at killing DLL3‐expressing SCLC cells in vitro and inhibiting tumor growth in vivo. ⁵³ Based on preclinical studies, AMG 119 demonstrated significant antitumor activity in an in vivo mouse model. ³² In NCT03392064, an open‐label phase I study, the primary objective is to evaluate the safety and tolerability of AMG 119 in patients with relapsed SCLC whose disease has progressed following platinum‐based chemotherapy. In the study, five participants experienced TRAEs of different grades: one participant suffered a grade 1 TRAE, two participants suffered grade 2 TRAEs, and one participant suffered a grade 3 TRAE. Among the evaluable participants (n = 4), one experienced a confirmed partial remission (PR) 1.1 months after the first dose. An additional subject with stable disease had a 16% reduction in target lesions from baseline. Furthermore, the participant who achieved PR demonstrated a rapid decrease in total circulating tumor cell levels within 7 days of treatment initiation. The median progression‐free survival was 3.7 months (range, 1.1–6.7) and the median overall survival was 7.4 months (range, 4.6–18.9). In summary, AMG 119 is the first CAR‐T cell therapy for SCLC with a manageable safety profile and promising antitumor activity. Unfortunately, registration is currently suspended due to low patient numbers but may resume in the future. ⁵⁴

LB2102 is also a DLL3‐targeted CAR‐T cell treatment for SCLC. A First‐in‐Human Dose Escalation and Cohort Expansion Study is ongoing (NCT05680922).

CAR‐NK therapy

A CAR‐NK‐92 cell has also been developed to target DLL3 on SCLC cells in addition to T cells. In a recent study, DLL3‐specific NK‐92 cells were investigated for their therapeutic potential in SCLC. Coculturing DLL3+ SCLC cells with DLL3CAR‐NK‐92 cells resulted in significant in vitro cytotoxicity and cytokine production. An H446‐derived lung metastasis model treated with DLL3 CAR‐NK‐92 cells demonstrated antitumor activity at a good safety threshold. Additionally, DLL3+ SCLC xenografts showed a significant tumor invasion of DLL3 CAR‐NK‐92 cells. ⁵⁵ In light of these findings, DLL3 CAR‐NK‐92 cells may be a potential treatment option for SCLC. Table 1 summarizes ongoing clinical trials of DLL3‐targeting therapies including patients with SCLC.

TABLE 1.

Ongoing clinical trials of DLL3‐targeting therapies for SCLC.

Drug	Status	Tumor type	Phase	ClinicalTrials. gov identifier	Treatment cohorts	Enrollment
DLL3/CD3 TCE
Tarlatamab (AMG 757)	Recruiting	SCLC	I	NCT03319940	Tarlatamab +/− pembrolizumab/CRS mitigation strategies	392
	Active, not recruiting	Relapsed/refractory SCLC	II	NCT05060016	Tarlatamab	222
	Active, not recruiting	SCLC	I	NCT04885998	Tarlatamab + AMG 404	23
	Recruiting	SCLC	III	NCT05740566	Tarlatamab	700
	Recruiting	ES‐SCLC	I	NCT05361395	1. Tarlatamab + atezolizumab + carboplatin + etoposide 2. Tarlatamab + atezolizumab 3. Tarlatamab + durvalumab	340
BI 764532	Recruiting	SCLC and other neuroendocrine neoplasms	I	NCT04429087	BI 764532	193
	Recruiting	SCLC and other neuroendocrine neoplasms expressing DLL3	I/II	NCT05879978	BI 764532 + ezabenlimab	30
	Not yet recruiting	SCLC and other neuroendocrine neoplasms expressing DLL3	II	NCT05882058	BI 764532	120
QLS31904	Recruiting	Advanced solid tumors, including SCLC	I	NCT05461287	QLS31904	290
TriTAC
HPN328	Recruiting	SCLC	I/II	NCT04471727	HPN328 +/− atezolizumab	162
DLL3/CD47 TCE
PT217	Recruiting	Advanced or metastatic relapsed/refractory SCLC, large cell NEC, neuroendocrine prostate cancer, and gastroenteropancreatic neuroendocrine tumors	I	NCT05652686	PT217	58
CAR‐NK therapy
DLL3‐CAR‐NK cells	Recruiting	ES‐SCLC	I	NCT05507593	DLL3‐CAR‐NK cells	18
CAR‐T therapy
AMG 119	Suspended	SCLC	I	NCT03392064	AMG 119	6
LB2102	Not yet recruiting	ES‐SCLC and large cell NEC of the lung	I	NCT05680922	LB2102	41

Abbreviations: CAR, chimeric antigen receptor; DLL3, delta‐like canonical Notch ligand 3; ES‐SCLC, extensive‐stage SCLC; NEC, neuroendocrine carcinoma; SCLC, small cell lung cancer; TCE, T cell engager; TriTAC, tri‐specific T cell‐activating construct.

T CELL IMMUNORECEPTOR WITH IG AND ITIM DOMAIN (TIGIT)

Structure, expression, and function of TIGIT

TIGIT, a novel immune checkpoint receptor with inhibitory function, is a member of the Ig superfamily, along with WUCAM, Vstm3, and VSIG9. It consists of an extracellular immunoglobulin variable structural domain, a type I transmembrane structural domain, and an intracellular short structural domain containing an immunoreceptor tyrosine‐based inhibitory motif (ITIM) and an immunoglobulin tyrosine tail (ITT)‐like motif. ⁵⁶ , ⁵⁷ , ⁵⁸ In response to chronic antigenic stimulation, T cell dysfunction or exhaustion is accompanied by the upregulation of many IRs, including programmed cell death receptor 1 (PD‐1) and T cell immune receptor with immunoglobulin and ITIM structural domains (TIGIT). ⁵⁹ As with the PD‐1 receptor, TIGIT can limit antitumor immune responses in cancer. ⁶⁰ According to a study using anti‐TIGIT monoclonal antibodies, TIGIT was expressed on NK, NKT, CD8+, Treg, and memory CD4+ T cells. ⁶¹ One high‐affinity ligand CD155 (poliovirus receptor, PVR), and two weaker‐affinity ligands, CD112 (poliovirus receptor‐related 2, PVRL2) and CD113 (PVRL3), have been identified to date. ⁶²

There are several ways in which TIGIT may exert immunosuppressive effects. First, as a ligand for CD155, TIGIT can downregulate T cell responses extracellularly. Yu et al. demonstrated that the interaction of TIGIT with CD155 promotes the downregulation of T cell responses in tolerogenic dendritic cells and that CD155 signaling in human monocyte‐derived dendritic cells leads to increased secretion of IL‐10 and decreased secretion of the proinflammatory cytokine IL‐12. ⁵⁷ Second, it may interfere with costimulation on a cell‐intrinsic level. DNAM‐1 is a costimulatory molecule that promotes the function of cytotoxic lymphocytes. ⁶³ TIGIT binds CD155 with higher affinity than DNAM‐1 and therefore may perform better than DNAM‐1 in its interaction with CD155. ⁶⁴ In addition, TIGIT molecules may also interfere with CD226 costimulatory signaling. Similar to the CD28‐CTLA‐4 axis, CD226‐TIGIT inhibits CD226‐mediated costimulatory signaling and can compete with CD155 in T cells. ⁵⁸ , ⁶⁵ , ⁶⁶ Third, TIGIT can also transmit inhibitory signals directly to effector cells. It has been found that TIGIT is expressed in all human NK cells and directly inhibits NK cytotoxicity through its ITIM. ⁶¹ , ⁶⁷ Treg cells that express Foxp3+ are inhibitory components of the adaptive immune response. There is evidence that TIGIT+ Tregs suppress T cells more effectively than TIGIT‐ Tregs. ⁶⁸ , ⁶⁹

Preclinical experience of TIGIT

In preclinical studies, it has been demonstrated that blocking TIGIT inhibits tumor growth, as evidenced by a reduction in tumor volume and an increase in overall survival in different mouse tumor models. ⁷⁰ In this regard, anti‐TIGIT treatment alone only resulted in a small sustained tumor improvement and modest survival benefit, but anti‐TIGIT treatment synergized with PD‐1 blockade to increase efficacy. In the MC38 mouse model, PD‐1 inhibitors alone had limited efficacy, but all mice showed complete tumor regression after anti‐TIGIT treatment. ⁷¹ The combination of anti‐PD‐1 and anti‐TIGIT significantly improved the survival rate of mice in the established GL261 glioblastoma model, with 17% of mice showing long‐term survival. ⁷¹ Additionally, monotherapy with anti‐TIGIT or anti‐PD‐L1 was not sufficient to reduce tumor load and only prolonged median survival by ~3 days in the CT26 mice. However, the coblockade of TIGIT and PD‐L1 significantly reduced tumor growth, with a 75% reduction in mean tumor volume. ⁷² According to these findings, TIGIT blockade may be an effective treatment option for SCLC in combination with PD‐1 blockade.

Targeting TIGIT in SCLC

Tiragolumab

Tiragolumab, a novel cancer immunotherapy targeting TIGIT, is the first anti‐TIGIT molecule to be granted breakthrough therapy designation (BTD) by the US FDA. It has been achieved in phase I and phase II trials in a variety of solid malignancies, particularly NSCLC. ⁷³ However, it suffered a setback in SCLC, with tiragolumab failing to meet the trial's primary endpoint in a phase III study (NCT04256421). SKYSCRAPER‐02 was a double‐blind, randomized study to assess whether the antitumor effect and survival benefit of the atezolizumab + CE (carboplatin/etoposide) combination therapy could be augmented by the addition of tiragolumab in 490 patients with ES‐SCLC. ⁷⁴ The coprimary endpoints were investigator‐assessed overall survival (OS) and progression‐free survival (PFS) in the primary analysis set, which excluded patients with existing brain metastases or a history of brain metastases. In the primary analysis set consisting of 397 patients, median PFS was 5.4 months (95% CI: 4.7–5.5) in the tiragolumab arm and 5.6 months (95% CI: 5.4–5.9) in the placebo arm; while median OS was 13.6 months (95% CI: 10.8–14.9) and 13.6 months (95% CI: 12.3–15.2), with no separation of Kaplan–Meier curves (HR = 1.04; 95% CI: 0.79–1.36; p = 0.7963). There were no statistical differences in PFS or OS in either the primary or complete analysis set. In the complete analysis set, the ORR was 70.8% (95% CI: 64.6%–76.3%) and 65.6% (95% CI: 59.3%–71.4%) for the tiragolumab and placebo groups, respectively. The median duration of response (DOR) was 4.2 months (range 4.1–4.4) and 5.1 months (4.4–5.8), respectively. The results showed that although the remission rate was higher in the tiragolumab group, it did not meet the expected response rate for first‐line treatment.

TIGIT is a potentially powerful immune checkpoint in tumor immunotherapy. SKYSCRAPER‐02 study did not meet its primary endpoint, PFS, but it cannot be dismissed based solely on the results of one study. The screening of the study population, drug dosage, and dosing regimen all may have influenced the results of the study. At the same time, the data on PFS and OS reported in the study provide further support for the use of atezolizumab plus chemotherapy as a first‐line treatment for ES‐SCLC.

Tiragolumab is currently being studied in two other clinical trials (NCT04665856 and NCT04308785).

Other TIGIT antibodies

Three other TIGIT antibodies, IBI939, ociperlimab (BGB‐A1217), and vibostolimab (MK‐7684A), are currently undergoing studies for the treatment of SCLC. All related clinical trials are summarized in Table 2.

TABLE 2.

Clinical trials of TIGIT‐targeting therapies for SCLC.

Drug	Status	Tumor type	Phase	Clinical trials. gov identifier	Treatment cohorts	Target	Enrollment
IBI939	Completed	NSCLC and SCLC	I	NCT04672356	IBI939 + sintilimab	TIGIT PD‐1	19
Ociperlimab (BGB‐A1217)	Active, not recruiting	LS‐SCLC	II	NCT04952597	1. Ociperlimab + tislelizumab + chemoradiotherapy 2. Tislelizumab + chemoradiotherapy 3. Concurrent chemoradiotherapy	TIGIT PD‐1	126
Tiragolumab	Active, not recruiting	SCLC	III	NCT04665856	1. Tiragolumab + atezolizumab + chemotherapy 2. Placebo + atezolizumab + chemotherapy	TIGIT PD‐1	123
	Active, not recruiting	SCLC	III	NCT04256421	1. Tiragolumab + atezolizumab + chemotherapy 2. Placebo + atezolizumab + chemotherapy	TIGIT PD‐1	490
	Active, not recruiting	SCLC	II	NCT04308785	1. Atezolizumab + tiragolumab 2. Atezolizumab + placebo	TIGIT PD‐1	24
Vibostolimab (MK‐7684A)	Active, not recruiting	SCLC	III	Keyvibe‐008 NCT05224141	1. Pembrolizumab + vibostolimab + chemotherapy 2. Atezolizumab + chemotherapy	TIGIT PD‐1	450

Abbreviations: LS‐SCLC, limited‐stage SCLC; NSCLC, non‐small cell lung cancer; PD‐1, programmed cell death‐1; SCLC, small cell lung cancer; TIGIT, T cell immunoreceptor with Ig and ITIM domains.

LYMPHOCYTE ACTIVATION GENE‐3

Structure and mechanism of action of LAG‐3

Lymphocyte activation gene‐3 (LAG‐3, CD233) is on chromosome 12 and encodes a type I transmembrane protein. It is composed of three regions: the extracellular region, the transmembrane region, and the intracellular region. ⁷⁵ The extracellular region of LAG‐3 consists of four immunoglobulin superfamily (IgSF) structural domains, D1, D2, D3, and D4. And there is a long amino acid sequence, called the “linker peptide”, between D4 and the transmembrane region in LAG‐3. The intracellular region of LAG‐3 consists of ~60 amino acid residues, including FSAL in the juxtamembrane region, KIEELE in the central region, and several other amino acid sequences. ⁷⁶ The intracellular structural domain of LAG‐3 has been associated with T cell proliferation and cytolytic functions. ⁷⁷ Workman et al. reported that lysine residues of the KIEELE sequence inhibited LAG‐3‐mediated activation of T cell antigens in 3A9 hybridomas. ⁷⁸

Similar to PD‐1 and CTLA‐4, LAG‐3 is not expressed on initial T cells. However, its expression can be induced on CD4+ and CD8+ T cells after antigenic stimulation. ⁷⁹ T cell expansion is negatively regulated by LAG‐3. ⁸⁰ In tumors, T cells are continuously stimulated, leading to T cell depletion and the expression of LAG‐3. As a result, T cells become less sensitive, thereby losing their ability to kill tumor cells. Moreover, T cells expressing both LAG‐3 and PD‐1 are depleted to a more significant extent than T cells expressing PD‐1 alone. ⁸¹ In Foxp3+ Treg cells, LAG‐3 is constitutively expressed. ⁸² It has been shown, both in vitro and in vivo, that the LAG‐3 antibody inhibits Treg suppression. ⁸² The inhibitory effect of LAG‐3 was found to be strongly and positively correlated with the amount of LAG‐3 on the surface of the cell (R ² = 0.9287). ⁸³ It has been reported that inhibiting LAG‐3 can enhance T cell antitumor activity. ⁸⁴

LAG‐3 is an inhibitory coreceptor structurally similar to CD4. The major histocompatibility complex class II (MHC‐II) molecules are ligands for LAG‐3, and LAG‐3 binds MHC‐II with ~100‐fold higher binding affinity than CD4 (Kd: 60 nM). ⁸⁵ , ⁸⁶ It has been suggested that LAG‐3 inhibits T cell activation by interfering with CD4 binding to MHCII. ⁸⁵ , ⁸⁷ Conversely, there are also data suggesting that the negative regulatory function of LAG‐3 is mediated through its cytoplasmic structural domains, rather than competing with CD4 for MHC‐II binding. ⁷⁸ , ⁸⁸

Preclinical experience of LAG‐3

In an in vivo study in mice with head and neck squamous cell carcinoma (HNSCC), LAG‐3‐specific antibodies were found to enhance the antitumor response of CD8+ T cells and reduce the number of immunosuppressive cells, retarding tumor growth in a manner associated with enhanced systemic antitumor responses. ⁸⁹ According to another study, tumor‐infiltrating lymphocytes expressed LAG‐3 in mice, and blocking LAG‐3 inhibited tumor growth. In human soft tissue sarcoma (STS) tissue samples, immunohistochemical staining also demonstrated a correlation between LAG‐3 expression and high pathological grade, advanced tumor stage, and poor prognosis. ⁹⁰

Dual blockade of LAG‐3 and PD‐1 enhances neuroimmune responses. ⁹¹ A mouse model of ovarian cancer was used to examine the effect of combining LAG‐3 blockade with PD‐1 blockade on tumor growth. Data suggest that PD‐1 and LAG‐3 are associated with rapid transport to the immune synapse, which results in a synergistic inhibitory effect on T cell signaling. Through the dual blockade of LAG3 and PD1, antitumor immunity is enhanced in the ovarian tumor microenvironment by increasing T cell infiltration, improving T effector function, and decreasing immunosuppressive Treg infiltration. ⁹²

Clinical application of LAG‐3 in SCLC

There are several studies of immunotherapeutic agents targeting LAG‐3 as monotherapy or in combination with anti‐PD‐1 agents.

Monoclonal antibody

INCAGN02385 and LAG525 are all monoclonal antibodies developed by Incyte that block the binding of LAG‐3 to MHCII. INCAGN02385 showed acceptable tolerability and pharmacokinetic profile in studies with cynomolgus monkeys. ⁹³

NCT03365791 was a phase II study evaluating the safety and efficacy of LAG525 in combination with PDR001 for the treatment of advanced solid and hematological tumors. As of January 7, 2019, 76 patients were treated, including 16 with SCLC. During the trial, LAG525 demonstrated effective antitumor activity, with 24‐week clinical benefit rates of 0.27 in SCLC, meeting the primary endpoint. ⁹⁴

Bispecific antibody

XmAb®22 841 is an anti‐CTLA4‐LAG‐3 bispecific antibody developed by Xencor that enhances T cell response and proliferation. NCT03849469 is a phase I, ascending dose and extension study designed to determine the maximum tolerated and recommended dose of XmAb®22 841 monotherapy or in combination with pembrolizumab. It is intended for patients with advanced solid tumors, including SCLC.

See Table 3 for clinical trials of LAG‐3‐targeting therapies for SCLC patients.

TABLE 3.

Clinical trials of LAG‐3‐targeting therapies for SCLC.

Drug	Status	Tumor type	Phase	ClinicalTrials. gov identifier	Intervention	Target	Enrollment
XmAb®22 841 (Bavunalimab)	Completed	Advanced solid tumors	I	NCT03849469	XmAb22841 +/− pembrolizumab	Anti‐(CTLA‐4 × LAG‐3) + Anti‐(PD‐1 × ICOS)	78
INCAGN02385	Completed	Advanced solid tumors and lymphomas	I	NCT03538028	INCAGN02385	Anti‐LAG‐3	22
LAG525	Completed	Advanced solid tumors and hematological malignancies	II	NCT03365791	PDR001 + LAG525	Anti‐PD‐1 + Anti‐LAG‐3	76

Abbreviations: CTLA‐4, cytotoxic T lymphocyte‐associated antigen‐4; ICOS, inducible costimulator; LAG‐3, lymphocyte‐activation gene 3; PD‐1, programmed cell death‐1; SCLC, small cell lung cancer.

DISIALOGANGLIOSIDE

Structure and expression of GD2

As a glycolipid, GD2 acts as a cell–cell receptor by interacting with glycan‐binding proteins on neighboring cells. ⁹⁵ In contrast to normal tissues, GD2 is overexpressed in neuroblastoma, sarcoma, and SCLC. ⁹⁶ Neuroblastoma patients treated with anti‐GD2 antibodies experience a significantly improved prognosis and longer survival. ⁹⁷ , ⁹⁸ Yoshida et al. found that the expression of GD2 varied greatly in lung cancer, with characteristic expression in SCLC and low or no expression in NSCLC. ⁹⁹ Therefore, GD2 is an attractive target for the immunotherapy of SCLC. GD2 promotes cancer cell proliferation, adhesion, migration, and invasion, as well as conferring antiapoptotic properties. ¹⁰⁰ , ¹⁰¹ When GD2 is overexpressed in the tumor microenvironment, immune escape is promoted and lymphocyte immunity is suppressed. ¹⁰² , ¹⁰³

Preclinical studies of GD2

In preclinical studies, monoclonal antibodies, bispecific antibodies, and CAR‐T targeting GD2 demonstrated promising antitumor activity. Both the value‐adding ability and invasive activity of GD2‐transfected cells were significantly enhanced, and cell growth was significantly inhibited by the addition of monoclonal antibody. ⁹⁹ Mujoo et al. and Mueller et al. demonstrated that mouse anti‐GD2 monoclonal antibodies 14.G2a and ch14.18 have a high affinity for GD2‐positive neuroblastoma cells and can effectively direct the antibody‐dependent cell‐mediated cytotoxicity. ¹⁰⁴ , ¹⁰⁵ The data suggest that the monoclonal antibody 14.18 binds strongly to SCLC cell lines and mediates tumor burn killing both in vivo and in vitro. ¹⁰⁶ The development of BsAbs that target GD2 is also expected to have a promising future in anticancer therapy. T cell armed with hu3F8‐BsAb was shown to kill a human tumor cell line in vitro and to inhibit the growth of NK cells and patient‐derived xenografts in vivo in mice. ¹⁰⁷ , ¹⁰⁸ CAR‐T cell therapy targeting GD2 is also a promising option for lung cancer patients. Results indicate that optimized GD2‐CAR T cells exhibit effective antitumor activity in vivo and in vitro against lung cancer models. ¹⁰⁹ This makes GD2 a promising candidate for immunotherapy as a treatment for SCLC.

Biological therapy

Two monoclonal antibodies are being evaluated in clinical trials involving patients with SCLC, 124I‐humanized 3F8 and dinutuximab. Naxitamab (humanized‐3F8) is a humanized (IgG1) anti‐GD2 (hu3F8) monoclonal antibody. Hu3F8 showed moderate toxicity, low immunogenicity, and significant antineuroblastoma activity in a phase I clinical study. ¹¹⁰ NCT02307630 is a pilot study to find out how an antibody called 124I‐hu3F8 travels through the body and to tumors. Dinutuximab (ch14.18; Unituxin®) is also a monoclonal antibody that targets GD2 and has been approved by the FDA for the treatment of high‐risk neuroblastomas. Dinutuximab and irinotecan were compared to irinotecan or topotecan alone in patients with refractory or recurrent SCLC in a phase II/ III clinical study. Unfortunately, the results of the study showed that dinutuximab/irinotecan did not improve patient survival or remission rates. ¹¹¹ As a result, further research is needed to determine the optimal dosage and treatment regimen for the use of dinutuximab in SCLC.

Nivatrotamab (Hu3F8‐BsAb) is a humanized anti‐GD2 × CD3 bispecific antibody. In NCT04750239, the drug was evaluated for safety and tolerability in patients with SCLC. The study, however, was terminated after three subjects. Reppel et al. added IL‐15 and inducible caspase 9 (iC9) safety switch gene to promote T cell expression and control unforeseen toxicities respectively. ¹⁰⁹ There is also a clinical study currently in progress, NCT05620342, which is designed to evaluate the safety of iC9‐GD2.CAR.IL‐15T cells in lung cancer patients.

While most of these treatments are in their early stages, they could all provide new strategies for immunotherapy in patients with SCLC. Table 4 summarizes clinical trials of GD2‐targeting therapies for SCLC patients.

TABLE 4.

Clinical trials of GD2‐targeting therapies for SCLC.

Biological therapy	Status	Tumor type	Phase	Clinical trials. gov identifier	Intervention	Target	Enrollment
124I‐Humanized 3F8	Active, not recruiting	Melanoma, neuroblastoma, sarcoma, and solid tumors including SCLC	IIT	NCT02307630	124I‐humanized 3F8	GD2	7
Dinutuximab	Completed	SCLC	II/ III	NCT03098030	Dinutuximab + irinotecan	GD2	483
Nivatrotamab	Terminated	SCLC	I/ II	NCT04750239	Nivatrotamab	GD2 + CD3	3
iC9‐GD2.CAR.IL‐15T‐cells	Recruiting	SCLC and NSCLC	Early I	NCT05620342	iC9‐GD2.CAR.IL‐15T‐cells	GD2 + IL15R	24

Abbreviations: GD2, disialoganglioside GD2; iC9, inducible caspase 9; IIT, investigator‐initiated trials; IL15R, interleukin‐15 receptor; NSCLC, non‐small cell lung cancer; SCLC, small cell lung cancer.

CONCLUSION

SCLC is an aggressive neuroendocrine tumor with poor prognosis. The limitation of current treatment for SCLC pushes the search for novel therapeutic approaches urgently. During the past 30 years, immunotherapy has shown its potential in the treatment of SCLC. Most studies, however, have failed to demonstrate a significant improvement in survival among patients with SCLC when immune checkpoint inhibitors are administered compared to conventional chemotherapy. As a result, it is imperative to continue developing new targets and drugs for immunotherapy in SCLC. DLL3, TIGIT, LAG‐3, and GD2 are promising targets for immunotherapy. In recent years, some new drugs and therapeutic regimens have been tested in clinical studies, and the results have been promising. In SCLC, most of the therapeutic drugs for these four new targets are still in development, and a large amount of data has not yet been published. The safety and effectiveness of these drugs require further research, as well as exploring more effective combinations of therapies and how to prevent relapse. Further studies and better data are expected to bring more survival benefits to patients with SCLC in the future.

AUTHOR CONTRIBUTIONS

All authors had full access to the data in the study and take responsibility for the integrity of the data and the accuracy of the data analysis. Conceptualization: Zitong Zheng and Jiangyong Yu. Investigation: Junling Ma, Runting Kang, Zhen Liu. Writing ‐ Original Draft: Zitong Zheng and Juanjuan Liu. Writing ‐ Review & Editing: all authors.

FUNDING INFORMATION

This work was supported by the National Natural Science Foundation of China (81972199 and 82141107); National High Level Hospital Clinical Research Funding (BJ‐2022‐101 and BJ‐2023‐069).

CONFLICT OF INTEREST STATEMENT

The authors declare that they have no competing interests.

Zheng Z, Liu J, Ma J, Kang R, Liu Z, Yu J. Advances in new targets for immunotherapy of small cell lung cancer. Thorac Cancer. 2024;15(1):3–14. 10.1111/1759-7714.15178