The Efficacy and Safety of Programmed Death-1 and Programmed Death Ligand 1 Inhibitors for the Treatment of Hepatocellular Carcinoma: A Systematic Review and Meta-Analysis

Background Hepatocellular carcinoma (HCC) is often diagnosed at an advanced stage where only systemic treatment can be offered. The emergence of immune checkpoint inhibitors (ICIs) provides hope for the treatment of HCC. In this study, we performed a meta-analysis to provide evidence for the efficacy and safety of ICIs in the treatment of HCC. Methods The following databases and websites were searched: Embase, PubMed, Cochrane Library and ClinicalTrials.gov. The primary endpoints were response rate (RR), disease control rate (DCR), progression-free survival (PFS) and overall survival (OS). Results Finally, twelve studies were included in this meta-analysis. When the corresponding outcome indicators and their 95% confidence intervals (CIs) were pooled directly, the overall RR, DCR, PFS and OS were 0.17 (0.15-0.19, I2 = 56.2%, P=0.009), 0.58 (0.55-0.61, I2 = 75.9%, P<0.001), 3.27 months (2.99-3.55, I2 = 73.0%, P=0.001), 11.73 months (10.79-12.67, I2 = 90.3%, P<0.001). Compared to the control group, treatment with ICIs significantly improved RR, PFS and OS, the OR and HRs were 3.11 (2.17-4.44, P<0.001), 0.852 (0.745-0.974, P=0.019) and 0.790 (0.685-0.911, P=0.001), respectively. However, no significant improvement in DCR was found in ICIs treatment in this meta-analysis. Conclusion HCC patients would benefit from ICIs treatment, however, more studies are needed in the future to provide more useful evidence for the treatment of HCC by programmed death-1 (PD-1) or programmed death ligand 1 (PD-L1) inhibitors.


INTRODUCTION
Primary liver cancer is the sixth most common tumor in the world and the fourth leading cause of cancer-related death, of which 75% to 85% are hepatocellular carcinoma (HCC) (1). Chronic infection with hepatitis C virus (HCV) or hepatitis B virus (HBV) is the leading cause of hepatocellular carcinoma (2). Additionally, HCC is often diagnosed at an advanced stage where only systemic treatment can be offered (3). Although many measures have been taken, the incidence of HCC has increased during the last decade globally and increases progressively with advancing age in all populations (4).
For a long time, there has been a lack of effective systemic therapy for advanced HCC. In the past decade, sorafenib was the only approved first-line agent for patients with unresectable or metastatic hepatocellular carcinoma (3,5,6). However, the benefits of sorafenib as the first-line standard treatment were limited. In the global and Asian phase III studies, compared with the placebo group, the median overall survival (OS) of patients in the sorafenib group was only extended by about 2 months, and the objective response rate (ORR) was only 2%-3.3%, and it often causes adverse events (7,8). Targeted agents currently used in patients with HCC, such as sorafenib, regorafenib, and lenvatinib, are multikinase inhibitors, which have lower response rates and higher therapeutic resistance than targeted therapy agents in other cancers (9).
The emergence of immune checkpoint inhibitors (ICIs) provides hope for the treatment of hepatocellular carcinoma. ICIs are designed to block immunosuppressive receptors expressed on the surface of T lymphocytes such as cytotoxic Tlymphocyte-associated antigen 4, programmed death receptor-1 (PD-1), and the programmed death-ligand 1 (PD-L1) expressed   on tumor cells and tumor-infiltrating immune cells (10). At present, immunotherapy, together with surgery, radiotherapy, chemotherapy and targeted therapy, has become the mainstay of the treatment of malignant tumors. Therapeutic monoclonal antibodies targeting PD-1 or PD-L1 have demonstrated notable clinical efficacy in the treatment of various advanced cancers, including non-small-cell lung cancer (NSCLC), melanoma, hepatocellular carcinoma et al. (11). In this study, the existing literature on the treatment of HCC with PD-1 or PD-L1 inhibitors was retrieved, and a metaanalysis was conducted to provide evidence for the efficacy and safety of PD-1/PD-L1 inhibitors in the treatment of HCC.

MATERIALS AND METHODS
We conducted this meta-analysis according to the Preferred Reporting Items for Systematic Review and Meta-Analysis guidelines (PRISMA) (12).

Data Sources and Searches
The following databases and websites were searched: Embase, PubMed, Cochrane Library and ClinicalTrials.gov. Key words used were: hepatocellular carcinoma; PD-1/PD-L1 inhibitors, nivolumab, pembrolizumab, camrelizumab, tislelizumab, atezolizumab. The time limit was from the establishing of the databases to October 2020. References in the eligible articles would also be searched when necessary.

Data Extraction and Quality Assessment
Two independent investigators screened the articles and extracted the data. If there was any disagreement, it would be resolved through discussion between the two investigators or by a third investigator. The data extracted were: publication year, countries, trial names, study registration no., inhibitors used, number of patients and their median ages (years), RR, DCR, PFS and OS.

Statistical Analysis
The data was analyzed by Stata 14.0 (StataCorp), Excel (Microsoft office 2016) and SPSS 21.0 (IBM SPSS Statistics). I 2 statistic was used to evaluate the heterogeneity among studies. If I 2 <50% or P>0.10, then the heterogeneity was considered to be low and fixed-effects model was applied. Otherwise, the randomeffects model was applied. For the single-arm study, outcomes were pooled to get overall RR, DCR, PFS and OS. Hazard ratios (HRs) were used to analyze the PFS and OS and odd ratios (ORs) were used to analyze the RR and DCR. P<0.05 indicated that the results were statistically significant.

Search Results and Study Quality Assessment
This meta-analysis searched a total of 317 studies, and finally 12 studies were included, among which 8 were single-arm studies, 2 were RCTs and 2 were retrospective cohort studies. Figure 1 displayed the flow chart of study selection. The PD-1 or PD-L1 inhibitors involved in these studies were nivolumab (7 studies), pembrolizumab (3 studies), camrelizumab (1 study), cemiplimab (1 study) and tislelizumab (1 study). The characteristics of included studies were shown in Table 1. Table 2 displayed the characteristics of the patients in the studies included in this meta-analysis. This meta-analysis focused on 4 outcomes: response rates (RR), disease control rates (DCR), progression-free survival (PFS) and overall survival (OS). For single-arm studies, the corresponding outcome indicators and their 95% confidence intervals (CIs) were pooled directly due to the lack of control group data. Data from RCTs or cohort studies would also be pooled with single-arm studies if they reported the same outcome indicators. There were 11,11,7,8 Table 3.

DISCUSSION
In recent years, the inhibition of PD-1 and PD-L1 pathway has emerged as one of the most potential therapeutic strategies in a variety of cancers, such as melanoma, lung cancer, renal cell carcinoma, head and neck squamous cell carcinoma, etc. (20).  However, no significant improvement in DCR was found in ICIs treatment in this meta-analysis, which may be due to the small number of RCTs or cohort studies included in this study.
Although immunotherapy has achieved certain results, the efficacy of treating some patients with ICI single drug is not ideal. Therefore, similarly to the treatment strategies that were commonly used against other malignant tumors, researchers       (21). However, immunotherapy, as a drug class, boosts the body's natural defense against cancer. These drugs have adverse effects, collectively known as immune-related adverse events, that represent immune effects on normal tissue that can result from misdirected stimulation of the immune system (24). There were also a lot of adverse events reported in the included studies, such as infection, rash, pruritus, reactive cutaneous capillary endothelial proliferation (RCCEP), increased aspartate aminotransferase (2,6,16).
There were also many deficiencies in this meta-analysis. Firstly, although 12 studies were finally included in this study, 8 of them were single-arm studies and only 2 were RCTs, and the lack of comparison data made it difficult to provide solid evidence of the efficacy and safety in the treatment of HCC with ICIs. Most of the studies included in this meta-analysis were single-arm studies, which only provided information on patients treated with ICIs, but did not have a control group for comparison. The direct merging of the single-arm studies data with the comparative studies data in the article might make the results of this metaanalysis unstable. At the same time, it was very difficult for us to perform the bias analysis due to the lack of studies with comparison data in this meta-analysis. Secondly, all the ICIs used in the included studies were PD-1 inhibitors (nivolumab, pembrolizumab, camrelizumab, cemiplimab and tislelizumab), so that the efficacy and safety of PD-L1 inhibitors in the treatment of HCC could not be analyzed, such as atezolizumab, durvalumab et al. Besides, the classification of HCC, the dosage and method of administration of ICIs were not identical, which could affect the reliability of the meta-analysis results. In general, there were still few studies on the treatment of HCC with PD-1 or PD-L1 inhibitors, especially the high-quality RCT studies that would reveal the efficacy and safety of PD-1 or PD-L1 inhibitors in the treatment of HCC. The good news is that there are a lot of studies going on right now, such as KEYNOTE-394 (25) (pembrolizumab plus best supportive care vs. placebo plus best supportive care), RATIONALE-301 (26) (tislelizumab vs sorafenib), etc. It is hoped that in the future, more objective and rich data can be obtained from these studies, so as to provide more useful evidence for the treatment of HCC by PD-1 or PD-L1 inhibitors.

DATA AVAILABILITY STATEMENT
The original contributions presented in the study are included in the article/supplementary material. Further inquiries can be directed to the corresponding author.