Toxicities of Immunotherapy for Small Cell Lung Cancer

Small cell lung cancer (SCLC), composing 15–20% of lung cancer, is a fatal disease with extremely poor prognosis. In the past two decades, etoposide platinum doublet chemotherapy remained the only choice of therapy, with disappointing overall survival ≤1 year for the metastatic disease. Novel treatments including immunotherapy are urgently needed and extensively explored. Recently, in two phase III trials, atezolizumab and durvalumab were shown to bring survival benefit to patients. While immunotherapy brings better outcome, it is accompanied by adverse events different from traditional treatments. Although these immune-related adverse events (irAEs) are generally mild and can be managed, some irAEs (myocarditis, pneumonitis) may be severe and even life-threatening. Accompanying with the increasing application of immunotherapy in clinical practice, the irAEs should not be overlooked. In this review, the irAEs profile in clinical trials of immunotherapy for SCLC will be summarized, also its unique features compared with irAEs in other malignancies will be explored. This review may be helpful for the appropriate clinical use of immunotherapy for SCLC.

Immune checkpoint helps to maintain the immune stability, while during carcinogenesis it is hijacked by tumors to evade immune surveillance. Immune checkpoint inhibitors (ICIs) including antibodies against cytotoxic T-lymphocyte antigen 4 (CTLA-4) or programmed cell death 1 (PD-1)/ programmed cell death ligand 1 (PD-L1) act to reverse the immunosuppression imparted by tumor cells, either by blocking CTLA-4 pathway or interrupting the interaction between PD-1 and PD-L1 (8,9). ICI has been widely used in a variety of malignancies, including non-small cell lung cancer, melanoma, triple-negative breast cancer, and non-Hodgkin lymphoma etc. (8,(10)(11)(12). Especially in SCLC which has a notorious reputation of poor prognosis, PD-L1 inhibitors including atezolizumab and durvalumab show promising efficacy (9,13).
While immunotherapy brings better outcome, it is accompanied by adverse events different from traditional treatments. Mounted immune response is directed to not only tumor, but also normal tissues and causes immune-related adverse events (irAEs) (14). Although these irAEs are generally mild and can be managed, some irAEs (myocarditis, pneumonitis) may be severe and even life-threatening (15,16).
Recently, in two phase III trials (IMPower 133 and CASPIAN), atezolizumab and durvalumab were shown to bring survival benefit to patients (9,13). Accompanying with the increasing application of immunotherapy in clinical practice, the irAEs should not be overlooked. In SCLC, due to the poor life expectance, also the high incidence of neurological complications, it is intriguing to ask whether the irAEs would be different from other tumors. This review provided a brief summary of irAEs from published clinical trials in the field of SCLC treatment.

OVERVIEW OF SCLC
SCLC is a distinct form of lung cancer, with dominant component of neuroendocrine tumor cells, and early and frequent distant metastases (17). Mutations in p53 gene (TP53) and retinoblastoma1 gene (RB1) are universal genetic events in SCLC (18). Studies also showed although SCLC harbors a high tumor mutational burden, tumor infiltrating lymphocytes are scarce in the microenvironment (19). Neither SCLC tends to express PD-L1, as it is found ≤20% tumor cells express PD-L1 (>1%) (20,21).
Etoposide plus platinum combination chemotherapy is recommended for metastatic SCLC patients (extensive stage, ES). For those in the limited stage (LS, non-metastatic), chest radiation at a dose of 45 Gy administered in 1.5 Gy fractions twice-daily for 30 days with chemotherapy, followed by prophylactic cranial irradiation is recommended (22). For relapsed or platinum-refractory SCLC, topotecan was the only approved drug by FDA in second-line treatment (23). Meanwhile, clinical trials on inhibitors of PARP, EZH2, WEE1, DLL3, and Aurora kinase etc. are all actively ongoing at this time, which is beyond the scope of this review (24). Here, we restrict our focus on the clinical data of the immunotherapy for SCLC.

DATA ACQUISITION
All relevant articles are identified by using the keywords "small cell lung cancer," "SCLC," "immunotherapy," "CTLA-4," "PD-1," "PD-L1," "clinical trial" on Pubmed, clinicaltrials.gov, Embase and Web of science. Abstracts and presentation were also reviewed from major conference including ASCO (https://www.asco.org/) and ELCC (https://www.esmo.org/) from 2015 to 2020.The literature or abstract was viewed, and those with only protocol design or lack of AEs results were excluded. Finally, fifteen studies involving ICIs for SCLC therapy with full description of the AEs were selected.

First Line
The first one being tested was ipilimumab, a fully human monoclonal antibody for CTLA-4. Following a successful phase II study (NCT00527735), a phase III trial (CA184-156) investigated the efficacy and safety of ipilimumab combined with chemotherapy (25,26). However, the addition of ipilimumab failed to demonstrate any improvement in neither OS, ORR, nor duration of response. IMPower133 was a phase III trial to investigate the efficiency of atezolizumab (a humanized monoclonal PD-L1 antibody) combined with chemotherapy. The combination regimen showed benefit in both PFS (5.2 m vs 4.7 m) and OS (12.3 m vs 10.3 m) (9). A similar good outcome was also achieved by durvalumab, another high-affinity human IgG1 monoclonal antibody for PD-L1. In the phase III CASPIAN study, the combination of durvalumab and chemotherapy achieved an OS of 13.3 m (13). The results of PD-1 antibodies seemed less favorably. KEYNOTE-604 was a phase III trial to investigate the efficacy of pembrolizumab (a humanized monoclonal IgG4 antibody) in ES-SCLC patients (27). The results showed that pembrolizumab significantly improves PFS, while OS narrowly had significant difference. Nivolumab is another monoclonal antibody for PD-1. A phase II randomized study (EA5161) evaluated the combination of nivolumab with EP for the ES-SCLC patients. Preliminary results were reported in ASCO 2020, nivolumab significantly improved PFS (5.5 and 4.7 m, p = 0.047) in treated population while OS was no statistical difference (11.3 and 8.5 m, p = 0.14) (28).

Maintenance
A phase III study (CHECKMATE-451) tested either nivolumab monotherapy, or nivolumab plus ipilimumab, or placebo as maintenance therapy after platinum-based first-line chemotherapy. However, nivolumab has a shorter OS compared with placebo (29). Another phase II, single-arm trial (NCT02359019) studied pembrolizumab as maintenance therapy. The 1-year PFS and OS rates were only 13 and 37% respectively (30).

Second Line
Salvage therapy for the relapsed SCLC is more difficult. At least two randomized controlled trial tested the efficacy of immunotherapy. In IFCT-1603 study, atezolizumab monotherapy was compared with topotecan or re-induction of initial chemotherapy (31). A phase III trial CHECKMATE-331 investigated the efficacy and safety of nivolumab monotherapy in the second line of therapy (32). Both trials demonstrated no superiority of immunotherapy over traditional chemotherapy.
Monotherapy seems inappropriate, and following studies tested combination therapy. In a multi-center, single arm, phase II study (NCT02551432), pembrolizumab was combined with chemotherapy drug paclitaxel (33). In another phase II study (NCT02484404), Durvalumab was tested in combination with olaparib (PARP inhibitors) (34). The preliminary reports of these small sample sized showed promising results.

Third Line or Later
Some early, small-scale studies were performed in these very latestaged patients. Nivolumab was the first ICIs approved by FDA for third-line therapy of SCLC, based on the results of C H E C K M A T E -0 3 2 i n 2 0 1 6 ( 3 5 ) . K E Y N O T E -0 2 8 (NCT02054806) and KEYNOTE-158 studies both tested pembrolizumab in the third line therapy. Based on the results, pembrolizumab monotherapy was approved to SCLC in thirdline or later (36,37).

DISCUSSION
This review summarized 15 trials in SCLC immunotherapy, including phase III (n = 5) and phase I/II trials (n = 10, Figure 1). Among them, IMpower133, CASPIAN, CA184-156, KEYNOTE-604 and EA5161 evaluated the efficacy of atezolizumab, durvalumab, ipilimumab, pembrolizumab, or nivolumab, when combined with chemotherapy. CheckMate-331 and IFCT-1603 tested the efficacy of nivolumab and atezolizumab monotherapy in 2nd-line. Six trials investigated efficacy and safety of ICIs in later-line or maintenance treatment. Most trials were performed in ICIs combined with chemotherapy. More studies are ongoing ( Table 1).
In CheckMate-451 trial, the frequency of irAEs of the nivolumab plus ipilimumab group was higher than that of nivolumab group. Not only occurrence, but the severity (frequency of ≥G3 irAEs) was also worse in the combo therapy. Similarly, immunotherapy plus chemotherapy showed better efficacy in IMpower133 and CASPIAN study, but at the price of more irAE events. Furthermore, adding ipilimumab to this combination brought no additional benefit, but significantly higher toxicities.
The exact pathophysiology of irAEs is unclear, but the toxicity between CTLA-4 and PD-L1/PD-1 inhibitors is quite different. Pituitary cells translocate to express CTLA-4. The CTLA-4 antibody binds to the pituitary and induces lymphocyte infiltration, and tissue destruction is triggered (38,39). PD-L1 was highly expressed on the surface of myocardial cells in two patients with immune myocarditis, leading to the recognition of myocardial and tumor surface antigens by the same T cell clone, which ultimately cause destruction of organ (16). In Keynote001 trial, 10 patients were newly diagnosed with hypothyroidism after receiving pembrolizumab, and eight of them were diagnosed with anti-thyroid antibody (40). It was suggested that irAE may be associated with autoantibodies. CTLA-4 Inhibitors reduce the number and activity of Treg cells, resulting in increased activity of TH17 cells and increased IL-17 release, contributing to the onset of immune-related colitis (41)(42)(43).
Because SCLC is a kind of neuroendocrine tumor, also autoimmune encephalitis was frequently reported for this disease, we proposed there might be an increased occurrence of neuromuscular toxicity during the immunotherapy. To test this hypothesis, we performed a pooled analysis of the reported neuromuscular toxicity from the above trials. We found less occurrence in the control group, compared with that in immunotherapy group ( Figure 3A). To confirm this observation, we performed a similar analysis in NSCLC trials. Conversely, immunotherapy and control groups had comparable toxicity ( Figure 3B). This further supported the notion the neuromuscular toxicity of immunotherapy was specifically restricted in SCLC.
We paid special attention to the fatal toxicities. Immunotherapy and chemotherapy had a similar incidence of treatment-related death for SCLC patients. Totally 36 and 27 death events occurred from seven head-to-head trials respectively (Figure 4). From all the trials, the most common reason of reported death were sepsis (n = 7) and pneumonitis (n = 7), followed by multiorgan failure (n = 3), hematologic      disease (n = 2), cardiotoxicity (n = 3), hepatitis (n = 3), and other unspecified cause (n = 2).

CONCLUSION
This paper reviewed the current status of immunotherapy in SCLC. Immunotherapy brings new hope to this formidable disease, and also unprecedented toxicity profile. Immunotherapy combined with either chemotherapy or other immunotherapies, led to higher occurrence of AE than immunotherapy alone. The toxicity of immunotherapy in SCLC seemed to be different with those in NSCLC, esp. for neuromuscular toxicity. This review may be helpful for the appropriate clinical use of immunotherapy for SCLC.

AUTHOR CONTRIBUTIONS
Z-YD and YZ contributed conception. YF drafted the manuscript, Z-YD reviewed the manuscript, and P-PW edited the manuscript. All authors contributed to the article and approved the submitted version.