SCLC accounts for 10–15% of all known cases of lung cancer and belongs to the most malignant tumors (Bray et al., 2018; Walcher et al., 2020). SCLC is characterized by its asymptomatic and rapid progression, as well as by early metastasis. The highest incidence of SCLC is recorded in the 40-60 age group. All types of small cell carcinoma have a poor prognosis even in cases when the diagnosis is made at an early stage (Bray et al., 2018; Walcher et al., 2020). In the vast majority of cases, SCLC develops in smokers, with men having a higher incidence. However, lung cancer is also a serious complication in patients with COPD and accounts for approximately 15% of deaths in patients with COPD (Sekine et al., 2012; Denholm et al., 2014).

Standard treatment for diagnosed SCLC is chemotherapy based on platinum preparations with etoposide and radiation therapy (Wang et al., 2019). Patients whose tumor recurs more than 6 months after fist-line chemotherapy are re-treated using the same regimen. Topotecan, with a response rate of 15–20% and an overall survival rate of 1 year in 30% of cases is a second-line treatment for SCLC. The options for subsequent lines of treatment are limited. CART therapy is a promising approach to treating tumors. CART therapy uses antibodies against inhibitory signaling molecules expressed on tumor cells and immune system cells (Dotti et al., 2014). T-cells modified by chimeric antigen receptors have been shown to be effective in patients with hematological malignant neoplasms (Schubert et al., 2016). In addition, safety and applicability of CART therapy has been confirmed for solid tumors. However, there are no reported clinical trials using CART-cells for the treatment of SCLC (Dotti et al., 2014). This is largely due to the high mutational activity of the tumor. Thus, applicability of known therapeutic approaches to SCLC treatment are rather limited, highlighting the urgent need for new treatment methods.

In this review, we analyzed existing reports on diagnosis and treatment of SCLC and propose a novel potential approach to its treatment.

Patients with diseases of the respiratory system (COPD, chronic bronchitis, pulmonary emphysema) have an increased risk of developing lung cancer. Annually, about 1% of patients with COPD develop lung cancer, and the prevalence of COPD in patients with lung cancer is 8–50% (Sekine et al., 2012). Thus, COPD is an independent risk factor for lung cancer (Denholm et al., 2014). Most studies so far investigated the link between COPD and NSCLC. However, there is little data on the association of COPD with the development of SCLC. This is mainly due to the lower prevalence of SCLC compared to NSCLC (Ju et al., 2018).

COPD and lung cancer are different diseases, but they share similar pathogenetic mechanisms, including epithelial-mesenchymal transition, inflammation, oxidative stress, and DNA damage. In COPD, inflammation is found in the airways and airspace and is inherently destructive (Houghton, 2013). In patients with COPD, chronic inflammation is a key feature that positively correlates with disease severity (Hogg et al., 2004) and may be a potential factor in the development of lung cancer in proximal and distant tissues (Parris et al., 2019). Cells such as neutrophils, macrophages, CD4⁺ and CD8⁺ lymphocytes are involved in the pathogenesis of COPD and lung cancer (Barnes et al., 2003; Paone et al., 2011; Gomes et al., 2014).

Recent reports point to different specific characteristics of immune cells in lung cancer and COPD. In patients with COPD, alveolar macrophages with the M1 phenotype have been found to polarize to the Th1 phenotype via interferon-γ. This cytotoxic Th1 phenotype is favorable for the tumor microenvironment, but does not occur in solid tumors, where immune cells are predominantly represented by the Th2 phenotype, activated by M2 macrophages (Shaykhiev et al., 2009). Changes in the composition of immune cells may be a useful biomarker in COPD patients with a precancerous condition. It has been shown that an increase in Th1-polarized CD4⁺ T cells in the lung tissue is associated with the severity of COPD and may be useful in assessing disease progression (Barnes et al., 2003; Paone et al., 2011; Gomes et al., 2014). On the other hand, these inflammatory cells with specific characteristics act as targets and suggest the possibility of targeted therapy for COPD and, separately, precancerous condition and lung cancer.

Co-expression of PD-1, T-cell immunoglobulin, and molecule-3, containing the mucin domain (TIM-3) on CD8⁺ T-cells, increases with the severity of COPD (Biton et al., 2018). PD-1 blockade is considered as one of the treatment approaches to both reduce the severity of COPD and reduce the likelihood of developing lung cancer. Recent studies have demonstrated an increase in progression-free survival among patients with advanced NSCLC with concomitant COPD receiving anti-PD-1 therapy (Biton et al., 2018; Chalela et al., 2018). The effectiveness of anti-PD-1 therapy among COPD patients is explained by immunological dysregulation leading to increased expression of immune checkpoints among T-cells (Chalela et al., 2018; Mark et al., 2018).

Various approaches to the treatment of lung cancer, including SCLC, have been presented above. The overwhelming majority of treatment options focus on patients with a localized tumor process (limited stage) and a widespread tumor process (advanced stage). This is largely due to the ability to diagnose lung cancer during this period of the development of the disease, including in patients with COPD. Meanwhile, precancerous conditions in patients without and with COPD are practically not covered. This is due to a very fine line of the transition from COPD to SCLC, and due to the lack of approaches to diagnose this process. During the searching for potential biomarkers of the precancerous condition, we turned our attention to such an important pathological process in COPD as systemic endothelial dysfunction (Green and Turner, 2017) with increasing apoptosis of lung endothelial cells (Peinado et al., 2006). An expected result of this is a decrease in alveolar vascularization (Wedzicha et al., 2016). Pulmonary microvascular blood flow and vascular density are already reduced in mild COPD, including in areas of the lungs without overt emphysema (Hueper et al., 2015). In patients with severe COPD, destruction of the microvascular bed is increasing (Hueper et al., 2015). It is known that in patients with pulmonary emphysema, a decrease in the number of endothelial progenitor cells in the blood was noted, and this indicator was the lowest in pan lobular emphysema (Doyle et al., 2017). The results of our own research confirm these data. We propose that damage to the lung endothelium is a factor in the progression of COPD. We can already point out several parameters within the endothelium that can be used as prognostic biomarkers of complications. These include changes of the numbers of endothelial cells, endothelial progenitor cells, pericytes, and smooth muscle cells (Pakhomova et al., 2020). In this regard, angiogenesis can be beneficial for the pulmonary vasculature and alveolar epithelium in pulmonary emphysema and COPD, and thus, presumably, it is possible to prevent the negative development of events: the transition of the disease to a precancerous condition, induction of lung cancer.

One of the promising methods for eliminating tumor cells may be the use of T-cells with a chimeric antigen receptor (CAR). CARs are modular synthetic receptors with a fragment of a monoclonal antibody designed to selectively bind to specific antigens on the surface of the plasma membrane of tumor cells. The result of the interaction of CAR with a tumor antigen is the activation of T-cell antitumor response (Stern et al., 2020). First generation CARs contained only the CD3ζ, or Fc gamma signaling domains (Figure 1). Over time, technology for generating CART cells has improved significantly, which made it possible to add co-stimulating domains such as CD28, 4-1BB, or OX4. Second-generation CARs have been supplemented with one domain while more than two domains can be added to modern CARs (Srivastava and Riddell, 2018).

As noted above, CART cells are designed to recognize specific tumor antigens and eradicate tumor cells. A CD19-specific CART cell therapy has been shown to be effective in B-cell malignancies, including acute lymphoblastic leukemia (ALL), chronic lymphocytic leukemia (CLL), and non-Hodgkin’s lymphoma (NHL) (Schubert et al., 2016). However, no definite positive results have been achieved in the treatment of solid tumors with CART cells, in particular, due to the difficulties in the selection of T-cells for the formation of CAR, the selection of a molecular and cellular target, and the recruitment of CART cells. These difficulties and limitations have yet to be overcome.

The therapeutic efficacy of CART anti-cancer therapy largely depends on the choice of T-cell subpopulations and the nature of their modification. In particular, CART cells as the final cell product must have the ability to proliferate in order to obtain a cell mass sufficient for anti-cancer therapy (Wang et al., 2018). In CART therapy, attempt shave been made to use various CD4⁺ and CD8⁺ T cells (Golubovskaya and Wu, 2016). It is known that CD4⁺ T-cells mediate systemic immunity, which is important for long-term tumor elimination (Wang et al., 2018). CD8⁺ T-cells have the potential not only to recognize, but also to destroy tumor cells (Gascón et al., 2020). Although following CAR modification, cytotoxic CD8⁺ T-cells mediate the direct eradication of tumor cells, also CD4⁺ T helper cells (Th cells) have been identified as highly efficient and clinically important T-cells (Stock et al., 2019). Recent studies indicate that CD4⁺ CART cells have a cytotoxic ability comparable to cytotoxic CD8⁺ CART cells (Stock et al., 2019), and can increase the proliferation of CD8⁺ T cells (Yang Y. et al., 2017). Based on data inferred from T cell biology, trials have been designed and conducted using defined mixtures of CD8 and CD4 T cells to generate CAR products that have demonstrated success in CD19⁺ leukemias and lymphomas (Gascón et al., 2020).

Cancer immunotherapy can include the use of different subpopulations of memory T-cells and effector T-cells in CART therapy. Subpopulations of CD4⁺ memory cells (Th1, Th2, Th9, Th17, Th22, Treg, and Tfh) and effector CD8⁺ cells differ in many parameters including expression of extracellular (CD25, CD45RO, CD45RA, CCR-7, L-selectin or CD62L, etc.) and intracellular markers (FOXP3), epigenetic and genetic factors, and lastly metabolic pathways (catabolism and anabolism). By influencing these parameters, CART therapy can be improved (Golubovskaya and Wu, 2016). It is believed that during immunotherapy, it is important to maintain and/or create a certain ratio between CD4⁺ Th-cells and cytotoxic CD8⁺ T-cells. The ratio of CD4⁺ T-cells/CD8⁺ T-cells in the cell product can affect the effectiveness of antitumor CART therapy. To resolve this issue, the desired cell subpopulations must be isolated, and CARs need to be separately modified. As a result, it is possible to adjust the ratio of CD4⁺ T cells/CD8⁺ T cells in a patient specific manner (Stock et al., 2019). However, this approach complicates the production process.

The differentiation status of T-cells proposed for modification plays a certain role in achieving a positive effect during CART therapy. There is evidence that transfusion of a large number of undifferentiated CART cells has a beneficial effect on treatment (Stock et al., 2019). Undifferentiated CART cells are characterized by high proliferative activity, high viability in vivo, and long-term antitumor activity. The therapeutic characteristics of the cell product are influenced not only by the T-cell construct included in the CAR, but also by the culture conditions during the ex vivo expansion of the T cells as well. When obtaining the required cell mass of T-cells, cytokines, and pharmacological inhibitors of specific differentiation pathways are used. Currently a standardized and optimal production process for obtaining CART cells has not yet been established. Thus, defined and effective criteria for predicting the response to CART therapy are absent.

Since blood samples are easier to obtain than a lung biopsy, the study of the antigenic profile (immunophenotype) of immune cells circulating in the blood in order to predict the immune response has recently become an increasingly frequent method. Patients with a good response to checkpoint inhibitors usually have fewer T-cells in the peripheral blood compared to patients who do not respond to therapy (Gascón et al., 2020). The initial ratio of T-cell subpopulations, including the content of poorly differentiated cells, in the blood of patients is reflected in the result of CART therapy. In particular, patients with large numbers of circulating tumor cells (CTCs) may have a small number of poorly differentiated T-cells in their blood. As a result, CTCs can be the preferred target for exogenous cytokines compared to CART cells. In such a situation, the cytotoxic activity of T cells against the tumor will be significantly reduced (Gattinoni et al., 2005). Therefore, to provide additional cytokine therapy during CART therapy, it is desirable to select T-cells for CAR modification in patients with a small number of CTCs in the blood.

Generation of CAR in T-cells begins with the collection of unstimulated leukocytes from the blood using leukapheresis. Isolation of the required cell population from the total fraction of lymphocytes is carried out taking into account their functional and structural features. This is based mainly on the assessment of surface markers CD4, CD8, CD25, or CD62L (Levine, 2015). Isolation of the patient’s autologous antigen-presenting cells (APCs) for subsequent activation requires additional multi-stage manipulations, which make it difficult to obtain active CART cells in the required period of time. In this regard, alternative approaches have been developed to increase the efficiency. These include the use of beads coated with MABs specific to the CD3 and CD28 antigens (Suhoski et al., 2007). During the activation process, cells are incubated with a viral vector, which is used to deliver RNA encoding a chimeric antigen receptor into T-cells, followed by reverse transcription into DNA and insertion into the patient’s T-cell genome. Thereafter, CAR begins to be expressed on the surface of the patient’s T-cells (Montini et al., 2009). The final stage of the protocol for the modification of T-cells in vitro is obtaining the required amount for administration to the patient (Gattinoni et al., 2005). Part of the cell material is cryopreserved and, if necessary, thawed to confirm its characteristics and to conduct repeated cell therapy (Levine, 2007).

The resistance of tumors to a number of therapeutic agents is well-known. Resistance is based on the functional and structural heterogeneity of the cellular composition of the tumor. It should be noted that it is possible for a tumor to acquire resistance to subsequent therapy with CART cells and to relapse. After a T-cell therapy with CD19-CAR, 70–90% of patients with B-cell malignant neoplasms developed a stable clinical remission. Subsequent clinical trials showed a loss and/or suppression of CD19 antigen expression in 70% of treated patients, which was considered causative for the occurrence of relapses after treatment (Levine, 2007; Maude et al., 2015).

An ideal situation for CART cell therapy is when the target antigen is overexpressed on all or the vast majority of tumor cells and is not expressed on the surface of healthy cells. However, the majority of tumor-associated antigens (TAA) are expressed on the cell membrane of patients with various types of cancer, including NSCLC and SCLC. Many of them are expressed at high levels and by healthy cells. Thus, CART cells might attack healthy tissue, which can lead to serious side effects (Zhong et al., 2020). Importantly, even a slight expression of tumor associated antigens on the surface of healthy cell membranes leads to damage and dysfunction of the lungs, blood system, and gastrointestinal tract and can be life-threatening (Bonifant et al., 2016). Therefore, to reduce the risk of side effects, CART cells should be as selective as possible. One of the options for increasing selectivity is an appropriate choice of the isoform of the target antigen. Expression of one antigen isoform can be observed on tumor cells, and another isoform on healthy cells. Examples for this strategy are CART therapies targeting EGFRvIII and CD44v8-10 (Morgan et al., 2012; Kim and Ryu, 2017). Improving the recognition of the target antigen by CART cells also allows reducing cellular toxicity to healthy tissues, as well as the incidence of relapses and tumor resistance to CART therapy. These methods are based on the creation of CART cells capable of recognizing several antigens, which significantly increases the specificity and efficiency (Walcher et al., 2020).

Various independent research groups have designed CARs containing two independent domains (Grada et al., 2013; Zah et al., 2016). These CAR T cells work according to the “OR-gate” and can be activated by two antigens and the binding of either antigen induces T-cell activation. One OR-gate strategy utilizes the pooled mixture of two populations of CART cells (CAR pool), each expressing a monospecific CAR. A variation on this theme is to sequentially administer two different CART cell products. Another strategy is the co-expression of two separate CARs in each T cell (dual CAR). Yet another approach uses tandem bispecific CARs (TanCAR) that comprise two scFv domains separated by a linker on one receptor chain, and this strategy was shown to be functionally superior to both the CAR pool and dual-CAR approaches (Hegde et al., 2016; Zah et al., 2020). Under conditions of the developing resistance risk due to a decrease or termination of the expression of the target antigen of the cancer cell, this type of CAR T cell construction can be effective. In particular, CARs targeting two B-cell specific antigens: CD19 and CD22, or CD19 and CD20. These CARs are less sensitive to tumor cell resistance in absence of CD19 (Shah et al., 2019). The disadvantage of this approach is the increased likelihood of a toxic effect in relation to healthy cells and tissues, since with an increase in the number of CAR application points, the number of healthy target cells naturally increases.

Another approach to reduce the toxic effect on healthy cells is to design CAR T cells that work according to the “NOT gate.” Here, a T-cell is engineered to have two specific CARs, one of which is activating and the other is inhibitory (iCAR). The principle of this method is when a T-cell expressing both CAR and iCAR meets a cell expressing only the target CAR antigen, it kills the target, but when both CAR and iCAR antigens are encountered on the same cell, negative iCAR regulatory signaling will predominate or attenuate CAR signaling and reduce the activation of T-cells. This approach is possible due to the fact that iCARs have an extracellular domain of antigen recognition, but their intracellular domains, instead of containing activating domains, carry signaling domains from immune inhibitory receptors such as (programmed cell death 1) PD-1 and cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) (Fedorov et al., 2013). The disadvantage of this approach is that it is impossible to separate healthy cells from tumor cells using a single surface marker.

Another method that improves the accuracy of targeting CART cells to tumor cells is an approach to designing CART cells that work according to the “AND” gate, which requires the simultaneous activation of two receptors for the manifestation of an immune response. In this approach, activation of one receptor by a tumor antigen induces the expression of a second CAR, which triggers the destruction of tumor cells. A similar approach is implemented using the synthetic receptor Notch (synNotch) (Roybal et al., 2016b). SynNotch is a synthetic transmembrane receptor, with a cytoplasmic domain that equipped with a transcriptional regulator that is released from the membrane when the ligand interacts with the receptor. Subsequently, it enters the cell nucleus, thereby activating the transcription of target genes (Morsut et al., 2016). In this scenario, synNotch controls the expression of CAR. This CAR triggers T-cell cytotoxicity and shows significant advantages over CAR T cells that have specificity for only one antigen. In preclinical studies, a higher accuracy of the destruction of tumor cells has been shown. Importantly, healthy tissues of animals expressing only one antigen remained unaffected (Roybal et al., 2016a).

Table 1 shows clinical trials with lung cancer as indication (https://clinicaltrials.gov/) registered as of September 2021 (ClinicalTrials, 2021).

In the overwhelming majority of cases, these studies are focused on therapy of NSCLC. Notably, no clinical trials have been conducted involving CART therapy for the treatment of SCLC. Moreover, there are no clinical trials focusing on adoptive T-cell therapy at the 4 phase. We found only an alone clinical trial using CART-cells for the treatment of SCLC (Phase 1, NCT03392064). Chen et al. demonstrated that Delta-like 3-targeted (DLL3) bispecific antibody plus PD-1 inhibition was effective in controlling SCLC growth (Chen et al., 2020). However, investigators have recognized four major distinct subcategories of SCLC using unbiased RNA-sequencing: SCLC-A (it is characterized by transcription factors expressions such as high ASCL1, high DLL3, and protein Schlafen 11 expression), SCLC-P (it is characterized by transcription factors expressions such as high POU2F3 expression), SCLC-N (it is characterized by transcription factors expressions such as high expression of NEUROD1, high expression of somatostatin receptor 2), and SCLC-Y (it is characterized by YAP1 expression) (Sayed and Blais, 2021). Recent studies have also pointed to the presence of an immune-dependent subtype, referred to as SCLC-I with increased immune infiltration (higher antigen presentation + immune cell infiltration) (Rudin et al., 2019; Gay et al., 2021). These subcategories were divided into those with (SCLC-A, SCLC-N) or without (SCLC-P, SCLC-I) neuroendocrine differentiation. Distinct features and therapeutic vulnerabilities were noted in each sub-category. Therefore CART cell therapy targeting only DLL3 presumably may be effective in an only subcategory of SCLC-A.

SCLC accounts for 10–15% of all known cases of lung cancer and is one of the most malignant and incurable tumors. All types of small cell carcinoma have a poor prognosis even when the diagnosis is made at an early stage. Clinical approaches to treating SCLC (surgery, radiation therapy, and chemotherapy) do not lead to patient recovery. High tumor mutational burden (TMB) and high neoantigens production are the main reasons for the progression of treated lung cancer. Until now, CART lymphocytes have not found their application in the treatment of SCLC. This may be due to the prescription of CART lymphocytes when the tumor mass is formed, whereas CART therapy is a more subtle treatment approach than, for example, radiation therapy and chemotherapy, and is designed to target detection and destruction of tumor cells. Thus, there is an urgent need for new approaches to treating SCLC, or revising the tactics of prescribing existing ones.

It is known that COPD increases the risk of NSCLC. There is much less data on the link between COPD and SCLC (Ju et al., 2018). Thus we focused on a group of COPD patients who develop SCLC (Figure 2).

In our opinion, in patients with COPD as a precancerous condition, prevention of SCLC at the stage of transformation of healthy cells into tumor cells seems to be promising. As diagnostic and prognostic markers of lung cancer are associated with COPD, it is proposed to assess the level of neutrophils, cholesterol (Mouronte-Roibás et al., 2019), and alpha1-antitrypsin in peripheral blood (Mouronte-Roibás et al., 2019; Tubío-Pérez et al., 2021). A number of studies indicated the possibility of CSCs using as a diagnostic marker of NSCLC in patients with COPD (Ilie et al., 2014; Marquette et al., 2020). There are few studies examining predictors of SCLC (Huang et al., 2018). In a pilot study, we have demonstrated the need to study different populations of CSCs in breast cancer (Pershina et al., 2021). Similarly, detecting circulating CSCs might by the key to early diagnosis of SCLC.

The first question that arises during the transition to this platform is related to the detection of CTCs (including CSC) in the blood of patients in a precancerous condition. At the stage of precancerous condition, we propose to use already known tumor markers as diagnostic markers of CSC including CD133, CD87, CD44, CD117, ALDH, Sox2, and CRYPTO-1. The need to assess the entire spectrum of antigens is justified by the high heterogeneity of the CSC phenotype in SCLC. Compliance with this requirement puts us on a path towards stratification and personalization from the beginning of the detection of CSCs in blood. Not all tumor antigens are expressed or expressed at the same level in all patients. A number of experts in the field are inclined to believe that when choosing a target marker, attention should be paid to antigens with a significant level of expression and to their ratio.

The appearance of one or a group of tumor markers in the blood of patients with a precancerous condition is a signal for CART therapy. To create CARs, populations of CD3⁺CD4⁺ and CD3⁺CD8⁺ blood T cells are proposed. These cells have proven themselves well as the basis for CARs aimed at other conditions. We further propose to use the expressed tumor markers identified in the patient as targets for modified T cells.

Importantly, potentially negative effects of the therapy need to be considered. To reduce the side effects risk of the CART therapy and to increase the selectivity of CART cells in relation to ROS, specific antigens such as EGFRvIII and CD44v8-10 are proposed in addition to the already identified antigens. Creation of CART cells capable of recognizing multiple antigens can significantly increase the effectiveness of CART therapy. A promising approach to the design of CART cells functioning according to the “AND” gate, which requires the simultaneous activation of two receptors for the manifestation of an immune response. With a positive outcome of CART therapy in patients with a precancerous condition, the need for chemotherapy and radiation therapy may disappear. One of the approaches to increase the effectiveness of SCLC treatment is the modulation of immune response and effect on the tumor microenvironment activity.

We propose a new approach for an increase of CART therapy efficiency in SCLC. However, our review and the proposed strategy have some limitations. To assess their applicability to a clinic, the results discussed here need further validation in pre-clinical studies and multi-center clinical studies focused on cohorts with large numbers of patients.