Introduction

Lung cancer has been estimated to be the most common cause of cancer deaths across the world [1]. The healthcare costs and burden attributed to lung cancer were significant as per the global burden of disease study conducted in 2016 [2]. The estimated 5-year survival rate of 17.8% was lower than that of all the other cancers [3]. More than 50% of the lung malignancies are diagnosed at the advanced stage, which causes a high mortality rate with a five-year survival rate of 4% [4]. Unfortunately, early stages of lung cancers can be asymptomatic, which makes early diagnosis misinterpreted and dismissed immediately [5]. Various invasive and non-invasive diagnostic procedures are used for lung cancer. Non- invasive methods such as computer tomography (CT) scans, low-dose CT scans, chest X-rays and positron emission tomography (PET scans) are used to diagnose non-small-cell lung carcinoma (NSCLC) [6]. Constant monitoring of patients exposed to radiation can, however, lead to radiation-induced cancer [7]. The statistics concerning the prevalence and survival rate suggest the novel screening methods to detect lung cancer in earlier stages in the general population and thereby improve lung cancer survival rates.

Although new technologies are being developed, tissue biopsy and characterization of histology have always remained the gold standard for the detection of NSCLC [7]. Currently, the tissue biopsy method is used to obtain molecular information from the tumors of NSCLC patients [8]. However, being invasive and an obstacle to frequent sampling, the efficacy of the tissue biopsy method is limited. In addition, local tissue sampling can be biased due to tumor heterogeneity and difficult to detect distant metastasis. Advances in genomics technologies are slowly shifting the future of diagnosis, prognosis, and selection of drug regimen towards liquid biopsy [9].

Liquid biopsy method is minimally invasive in comparison to tissue biopsy and can deliver real-time dynamics of lung cancer through utilizing biomarkers in the circulation. It includes cell-free DNA (cfDNA), circulating tumor DNA (ctDNA), extracellular vesicles (EVs), mRNA (messenger RNA), miRNA (microRNA), circulating tumor cells (CTCs) and exosomes [9,10,11].

Even though blood-based liquid biopsies have the potential to produce tumor molecular profiles, challenges remain due to the limited amount of plasma ctDNA [12]. Hence, other body fluids that contain ctDNA including malignant pleural effusion (MPE), ascites, and cerebrospinal fluid is being used as potential alternatives for liquid biopsy samples [13]. MPE is rich in tumor cells and its collection is minimally invasive than a tissue biopsy. Therefore, MPE has been explored as a suitable liquid biopsy specimen [14].

The first-line treatment for metastatic NSCLC often involves platinum-based combination therapy such as carboplatin/paclitaxel with or without immunotherapy. However, in many cases, it is observed that patients with advanced NSCLC developed mutations in the tyrosine kinase domain of the epidermal growth factor receptor (EGFR) which could confer sensitivity to the available targeted therapy for EGFR tyrosine kinase inhibitors (TKI) such as gefitinib and erlotinib [15]. It has also been reported that the EGFR T790M mutation can be detected in ctDNA before clinical progression, thus indicating an early intervention [16]. Hence, testing for mutations in EGFR proves to be an essential step in the treatment decision pathway [15]. The first published case report in 2005 used MPE for targeting EGFR mutation as a liquid biopsy specimen [17, 18]. Cobas® EGFR Mutation Test v2 (Roche Molecular Systems, Pleasanton, CA, USA) was the first liquid biopsy test approved by the U. S. Food and Drug Administration in 2016 [19].

Recent innovations such as next-generation sequencing (NGS), droplet digital polymerase chain reaction (ddPCR), and microfluidic chip-based technologies have developed promising clinical test results, which recommend that liquid biopsy-based technologies will be a mainstay for precision medicine in the future.

This review focuses on the effectiveness of using MPE as a specimen for liquid biopsy for treatment monitoring of lung cancer. The study was framed after an exhaustive literature search from 84 articles collected through PubMed and Google search using the keywords ‘liquid biopsy’, ‘pleural effusion’, ‘malignant pleural effusion’, ‘lung cancer’, ‘cfDNA’, ‘ctDNA’, ‘extracellular vesicles’ ‘miRNA’ in combination with the Boolean operators “AND/OR.”

Malignant pleural effusion

Pleural effusion or “water on the lungs” is the build-up of extra fluid in the space between the lungs and chest wall, which is referred to as pleural space. Pleural effusion is a convenient clinical sample with high clinical diagnostic significance [18]. MPE is a common in metastatic lung cancer and is defined as the presence of malignant cells in the pleural space. The incidence of MPE in the USA is estimated as more than 150,000 cases annually [20]. The common causes of MPE are lung cancer (37.5%), breast cancer (16.8%), and lymphoma (11.5%) [21]. It is assessed that 8 to 15% of lung cancer patients have MPE, in which lung adenocarcinoma is more associated because it grows in the lung periphery and easily invades the pleural cavity which shows the advanced disease stage or progression [22]. In contrast to other invasive techniques like tissue biopsy, MPE is very easy to collect. Moreover, the mutation rate in patients with MPE related to lung adenocarcinoma were much higher compared to mutation rates in surgically resected specimen [21]. It may be an alternative source supplying useful information about the mutation status of the EGFR gene. If EGFR gene mutation determination can be achieved with more attainable pleural effusion samples, then targeted drug therapy will be possible for advanced NSCLC patients, which will contribute to vital clinical and practical value [23].

Diagnosis and collection of malignant pleural effusion

Malignancies in the lung may be asymptomatic or patients may experience cough, dyspnoea and decreased exercise tolerance during early stages. In these cases, MPE could be detected on imaging studies. Standard chest radiographs can detect as little as 50 mL of pleural fluid (PF) on a lateral view, which is used for the diagnosis of MPE [24]. Pleural effusions are evaluated using chest ultrasonography because it detects small volumes (5 mL) of pleural fluid which can identify MPE following thoracentesis and chest-catheter insertion [25]. Patients with suspected MPE can be detected using CT scans and better imaging of soft tissues can be detected with magnetic resonance imaging (MRI) [26]. MRI with triple-echo pulse sequences shows highly sensitive for small effusions thus can differentiate from exudative and transudative effusions [27]. Thoracentesis is a routinely performed minimally invasive method used in patients with PE to relieve their clinical symptoms. It also used for diagnosis and possible further ancillary tests, like molecular studies [28]. Tissue biopsy, which was considered to be the gold standard for the molecular detection and decision making regarding the treatment of NSCLC patients before 2016 might fail sometimes because of multiple reasons like unavailability of the tumor tissue due to its invasiveness or failure in getting enough of tumor tissues for further gene detection [29]. For the patients resistant to targeted therapy, re-biopsy is very difficult because of the suboptimal clinical conditions. On the other hand, liquid biopsy has proven to be comparatively non-invasive, easily accessible and repeatable for conducting molecular profile testing and monitoring drug resistance. The combination of imaging techniques like CT or MRI along with the molecular techniques involving the liquid biopsy can increase the sensitivity, cost-effectiveness and early detection of lung cancers [30].

Studies conducted by using malignant cells (cell blocks or cell pellets) from MPE using real-time PCR concluded with high concordance in comparison with tumor samples in terms of sensitivity and specificity [31]. In contrast, only a few studies have investigated genetic mutations in MPE samples using amplification refractory mutation system (ARMS) and/or PCR [32,33,34] or by next-generation sequencing (NGS) method [35,36,37,38,39].

Circulating cancer biomarkers in malignant pleural effusion

Cell-free DNA

Generally, normal and cancer cells release small fragmented DNA into the circulation, during the course of events such as necrosis and apoptosis [12, 40,41,42]. These fragments of DNA are known as cfDNA irrespective of the cell origin. cfDNA released by cancerous cells is referred to as ctDNA. ctDNA is a good source to investigate tumor mutations, changes in methylation, CNVs, or viral sequences associated with the tumor [43,44,45,46,47]. ctDNA can be isolated in serum and plasma. However, they are reported to be circulating in other body fluids like breast milk, urine, stool, sputum, cerebrospinal fluid, peritoneal fluid, lymphatic fluid, bone marrow, ascites and even in gastric and biliary juice [10].

A study by Kimura et al. indicated that cfDNA in pleural effusion fluid can act as a predictor of EGFR mutations in patients and correlate to the EGFR tyrosine kinase inhibitor responsiveness in patients [39]. Kawahara et al. reported that the cytology cfDNA approach has high sensitivity and specificity (88% and 100%, respectively) when compared to the analysis DNA isolated from tumor tissue, which suggests that this can act as an accurate method for detecting the activating EGFR mutations [48]. Numerous studies have shown the efficacy of using cfDNA from PE and MPE as a source of cfDNA for investigating tumor mutations in lung cancer and advanced lung cancer patients respectively. We have summarized these studies in Table 1.

Table 1 Liquid biopsy studies in pleural effusion

Extracellular vesicles

EVs are migratory packages of the cells carrying nucleic acids, lipids, and metabolites and helps in the exchange of these materials among the cells. Because of this activity, EVs are considered responsible for cancer development as they have the ability for altering the tumor microenvironment [49]. EVs have a dual lipid membranous coat with double-stranded DNA in tumor exomes which act as a biomarker for the detection of cancer and they are released in several biological fluids, such as plasma, saliva, urine, PE, and bronchoalveolar lavage fluid [50]. A number of studies have reported the presence of the double-stranded DNA in the tumor-derived extracellular vesicles. In a study, Jong et al. demonstrated that DNA derived from EV is considerably very efficient for EGFR genotyping compared to cfDNA in lung malignancy. This suggests that the exosomal or extracellular DNA can act as an alternative biomarker for the detection and diagnosis of cancers [50]. However, only a few studies have demonstrated the translational value of exosomal DNA for the potential benefit as a biomarker in cancer [51,52,53]. The fragments carrying mutations of kirsten rat sarcoma (KRAS) and p53 have been detected in the exosomes from pancreatic cancers [53]. Moreover, the exosomal DNAs also have been reported to have mutations in EGFR and BRAF (V600E) [51]. Microvesicles (MVs) also known as exosomes are released from tumor cells in the circulation and pleural effusion. Park et al. reported that MVs derived from pleural effusion of NSCLC patients can support the development of new liquid biopsy tools [54].

microRNA

EVs are rich in diverse contents such as DNA, mRNA, miRNA, lipids and proteins [55]. miRNAs are a part of non-coding RNAs that are never expressed as proteins but are crucial for regulating the transcription, translation processes and gene coding for the proteins [56]. miRNAs are able to function as an oncogene or as a suppressor [57]. The oncogenic effect of miRNA is due to deletion or silencing of mRNA expression of oncogene inhibitors, silencing of suppressor gene amplification or up-regulation of expression in malignant cells. The absence or lack of miRNA can lead to tumorigenesis in lungs due to severe cellular abnormalities [58]. 52.5% miRNA genes are located in cancer-associated genomic regions or in fragile sites. Mutations occurring in these fragile sites are associated with oncogenesis. Example, miRNA-15a and miRNA-16/1, which are coded by genes located in a fragile site on chromosome 13 (area 14.2) [59, 60]. Reduced or lack of expression of these miRNAs were detected in patients with mantle cell lymphoma (MCL), multiple myeloma (MM), B-cell lymphoma, chronic lymphocytic leukemia (CLL), and prostate cancer [60]. This shows a significant association between abnormal miRNA expression and cancer prognosis [59].

miRNAs are observed to be exclusively expressed in most of the cancers including lung cancer. These have also been reported to be circulating in the blood and serum of patients with lung cancers [61,62,63]. Studies have reported the expression of miRNA from pleural effusion of NSCLC patients [55, 64]. NSCLC patients were found to have a decline in levels of let-7 miRNA. This down-regulation of let-7 expression was reported to be suggestively associated with venous invasion, cancer metastasis, advanced TNM stages, worse prognosis and lower post-operative survival in NSCLC patients [65]. miRNA-520b also plays a role as tumor suppressor gene in the progress of NSCLC. Its expression is downregulated in lung cancer tissue and was negatively associated with tumor development, TNM staging and lymph node metastasis [66]. miRNAs can be used in the differential diagnosis of MPEs and non-PEs. A recent study has shown that miRNAs were found to be altered in MPE and significantly associated with overall survival. In a study done by Wang et al. the low expression of miRNA-93, miRNA-134, miRNA-151, miRNA-345 and high expression of miRNA-100 were associated with poor survival [67]. Other studies have reported that significantly lower expression of miRNA-22, miRNA-134, and miRNA-185, were present in patients with MPE as compared to those with non-MPE [57]. Another study has shown reduced expression of miRNA-198 in MPE-associated lung adenocarcinoma compare to non-malignant effusions [68].

It is found that, 12 miRNAs were overexpressed (miRNA-17, miRNA-21, miR-106, miR-146, miR-155, miR-191, miR-192, miRNA-203, miRNA-205, miRNA-210, miRNA-212, and miRNA-214) between the tumor and normal specimens [69]. A study by Rabinowits et al. concluded that there is potential for diagnosis of lung cancer based on the differences in the sequences of miRNA expression. This study also suggested that circulating exosomal miRNA could be useful as a screening test for lung adenocarcinoma [70]. The upregulation of multiple miRNAs (including miRNA-484, miRNA-320, let-7a, and miRNA-125a-5p) was found in malignant pleural mesothelioma compared to benign asbestos-related pleural effusion, providing a potential role of miRNA in diagnosis and management [71]. Furthermore, the divergent expression levels of miRNA-182 and miRNA-210 are seen in MPEs which is associated with lung adenocarcinoma compared to benign pleural effusion [72].

Liquid biopsy using plasma vs pleural effusion fluid in lung cancer

Many studies have demonstrated that the yield of cfDNA in the supernatant of pleural effusion fluid is comparatively higher than pleural effusion sediment and plasma samples. A study conducted by Lin Tong et al. showed that concentrations of cfDNA in PE supernatants were higher than plasma, which is median: 278.1 ng/ml PE and median: 20.4 ng/ml plasma respectively. PE supernatants (98.4%) are rich in tumor DNA than plasma (87%) and PE sediments (90.5%) [73]. About 93% of the tissue determined driver mutations were observed in PE-cfDNA while only 62% were observed in plasma cfDNA. PE-cfDNA also reported the highest detection of EGFR mutations (71%) whereas it was comparatively as low as 59% in plasma cfDNA [73]. On the other hand, another study reported the level of plasma cfDNA was particularly higher in non-small cell lung carcinoma when associated to benign lung tumors, which acted as the basis to discriminate between NSCLC and benign lung tumors showing 91% sensitivity and 68% specificity [74]. It has also been demonstrated the sensitivity of detection of ctDNA during the early stages of lung cancer was about 50% [74]. Studies have shown that PE-cfDNA can robustly detect EGFR, KRAS and EGFR resistance mutations [75, 76]. A study conducted by Song Z et al. support the potential utility of PE-exoDNA, which can be used as an alternative source for genetic testing [77].

The plasma protein has also been reported for early diagnosis of lung malignancy. On the contrary, the pleural effusion is observed in advance stages of cancer, which makes it unavailable for the detection of lung cancer in an early stage [73]. A combination of 16 driver gene mutations in the ctDNA and 8 circulation plasma proteins called CancerSEEK was evaluated in a study for the early diagnosis of various cancers like lung cancer [30]. This combination included cancer antigen 125 (CA-125), myeloperoxidase (MPO), carcinoembryonic antigen (CEA), cancer antigen 19-9 (CA19-9), hepatocyte growth factor (HGF), pro-lactin (PRL), tissue inhibitor of metalloproteinases 1 (TIMP-1) protein levels, and osteopontin (OPN). The study reported that the sensitivity of CancerSEEK in detecting the presence of early cancers was really high with 70% for median and 60% for lung cancer. This method also distinguished the original organ of cancer effectively [30, 78].

Discussion

Pleural effusion is an extra fluid buildup in the space between the lungs and the chest wall. It is observed in cases of lymphoma and cancers of breast, lung, and ovary [79]. However pleural effusion is also seen in cases other than cancer, such as in tuberculosis. As both the type of pleural effusion fluids (malignant and tuberculous) have similar biochemical profiles, differentiating them is quite difficult. A study reported that adenosine deaminase (ADA) of pleural fluid, which is an enzyme of macrophages and activated T lymphocytes, is sensitive biomarker in discriminating between MPE and tuberculous pleural effusion (TPE). Also, the percentage of polynuclear leukocyte in MPE was reported to be higher than TPE [80]. Out of 5888 MPE samples, the most common cause of MPE was reported to be in lung cancer (35.6%) [81].

Identifying biomarkers in pleural effusion can be a non-invasive approach to screening lung cancers. The identification of makers like EGFR, Kirsten rat sarcoma (KRAS), v- Raf murine sarcoma viral oncogene homolog B (BRAF) and translocation in gene rearrangement rat osteosarcoma (ROS1), anaplastic lymphoma kinase (ALK), can predict the nature of malignancy and assess the suitability of patient for gene-targeted therapy [79]. The importance of EGFR mutations has been known for the indicators for better clinical outcomes in NSCLC patients receiving EGFR TKIs. Interestingly, it is reported that 30–50% of patients developed T790M mutation (threonine 790 with methionine) which is a resistant mutation to EGFR TKIs [48].

Currently, ctDNA is the target of liquid biopsy methods. As noted earlier ctDNA are highly fragmented pieces of DNA from the tumors and their concentration makes about less than 0.1% to over 10% of the total cfDNA [9]. The detection of such a low concentration of DNA is quite challenging, particularly at an early stage. To solve this problem, many recent strategies have been emerged [16]. Several PCR based technologies such as ddPCR and BEAMing (beads, emulsion, amplification, and magnetic) have been developed which show high sensitivity ranging between 1% and 0.001% [16, 82, 83]. However digital PCR techniques are not suitable for larger studies because it may miss substantial information [16]. NGS based platforms such as CAPP-Seq (cancer personalized profiling by deep sequencing, TAm-Seq (tagged amplicon deep sequencing), AmpliSeq, and Safe-Seq (safe sequencing system) could help resolve this issue and allow wide ranging application of liquid biopsy [13, 16, 83, 84]. As discussed in this review, MPE supernatants could be used as an excellent source for biomarkers in lung cancer patients. The investigation of these circulating biomarkers may help in detect early disease progression which could be a vital step towards decreased mortality due to lung cancer.

Conclusion

cfDNA, miRNAs and exosomes offers a better alternative for genomic profiling in malignant pleural effusion when tumor tissues are not available for assessing tumor genomics in advanced lung cancer patients. Moreover, PE-cfDNA could possibly be used for EGFR mutation detection and treatment response as an alternative to plasma-cfDNA in advanced malignancy stage, which suggests that malignant pleural effusion could be used as a potential alternative for liquid biopsy.