3.1. Proteomic biomarkers
Novel protein biomarkers have been reported to be able to detect early-stage pancreatic cancer. Leucine-rich alpha-2 glycoprotein 1 (LRG1) is a glycoprotein that is part of the leucine-rich repeat (LRR) family of proteins. It is primarily involved in protein interactions, signal transduction, cell adhesion and development, and the promotion of new blood vessels. The overexpression of LRG1 has been correlated with poor survival and late tumor stage; furthermore, LRG1 promotes the viability, proliferation and invasion of pancreatic tumor cells [
34,
35,
36,
37,
38]. Another biomarker tissue inhibitor is metalloproteinase 1 (TIMP-1), which is normally expressed to regulate of cell proliferation and apoptosis, with a sensitivity of 47.1%, a specificity of 69.2%, and AUC of 0.64 reported in diagnosing pancreatic cancer [
39,
40]. Transthyretin (TTR) a thyroid hormone carrier (thyroxin and tri-iodothyronine), has been reported to be increased by more than 1.5-fold in the serum of pancreatic ductal adenocarcinoma (PDAC) patients compared to normal controls, with a sensitivity of 90.5, a specificity of 47.6, and AUC of 0.75 [
41]. ICAM-1 is a glycoprotein involved in cell adhesion and a macrophage chemo attractant; several studies have evaluated the use of ICAM-1 as an early diagnostic tool for pancreatic cancer. By using a cut-off value of 878.5 u/mL, ICAM-1 exhibited a sensitivity, specificity, and AUC of 82%, 82.26%, and 0.851 respectively [
42]. Another biomarker reported to have potential in the early detection of pancreatic cancer is osteoprotegerin (OPG), which has a role in bone homeostasis. Shi, et al. reported that OPG is upregulated in cancerous pancreatic tissue and the expression of which is even higher in patients with new-onset diabetes [
43,
44,
45].
3.2. Combination of biomarkers
The use of multiple biomarkers or biomarker panels for early diagnosis has been proposed in several studies (
Table 1). The use of a single tumor marker is reported to have a high probability of false positives and false negatives [
46,
47]. Park, et al. were able to report a sensitivity of 82.5%, a specificity of 92.1%, and an AUC of 0.93 (P < 0.01) when using a proteomic multi-marker panel including LRG1, TTR, and CA19-9, which was 10% higher compared to CA19-9 alone [
48]. In another study by using a panel of three biomarkers, CA 19-9, Intercellular Adhesion Molecule 1 (ICAM-1), and osteoprotegerin (OPG) researchers were able to discriminate healthy patients from patients with PDAC with a sensitivity of 88%, a specificity of 90%, and a AUC of 0.93 [
49]. In a Korean study, Kim et al. were able to create a new biomarker combination consists of ApoA1, CA125, CA19-9, CEA, ApoA2, and TTR, with a sensitivity, specificity, and area under curve of 93%, 96%, and 0.993, respectively [
50]. Interestingly, all six biomarkers that used are part of a pan-diagnostic kit that is commercially available in Korea to diagnose seven cancers, hepatocellular carcinoma, breast cancer, lung cancer, gastric cancer, colon cancer, prostate cancer, and ovarian cancer. In a case-control study, Mellby et al. were able to achieve a sensitivity and specificity of 94% and 95% in differentiating stages I and II from normal controls with an AUC of 0.96, by using biomarker signatures that consists of 29 biomarkers [
51].
3.3. miRNA
Micro-RNA (miRNA) is single-stranded RNA that was discovered in 1993, which consists of 19-25 nucleotides. These nucleotides are then transcribed into miRNA with 21-2 nucleotides [
52,
53]. miRNAs belong to non-coding RNA, and although miRNAs are not translated into proteins, they still play a crucial role in the development and function of the normal human body, including cell division, differentiation, apoptosis, and angiogenesis. miRNAs can be identified according to their location cytoplasmic or nuclear and length, i.e., small (<200 base-pairs) or long (>200 base-pairs) [
23,
54]. miRNA has been correlated with tumorigenesis and progression through apoptosis escape, epithelial mesenchymal transition (EMT), invasion, and poor clinical outcomes. The EMT is a process by which epithelial cells lose cell to cell adhesion and gain invasive properties similar to mesenchymal cells, which is important in the metastasis of pancreatic cancer [
55,
56].
The expression of miRNAs is affected by the deletion, amplification, translocation and integration of DNA during carcinogenesis. As a consequence miRNAs may be detected or overexpressed in certain cancers and used as biomarkers [
23]. miRNAs can be detected in blood serum, plasma, cells, and tissues using reverse transcription-quantitative PCR (RT-qPCR), in situ hybridization, next-generation sequencing, and miRNA micro arrays [
21,
56,
57]. In a four-stage study conducted by Zhou, et al. using qRT-PCR assays, they were able to identify six-miRNA signatures, including miR-122-5p, miR-125b-5p, miR-192-5p, miR-193b-3p, miR-221-3p, and miR-27b-3p, that could discriminate pancreatic cancer patients from normal controls with an AUC of 0.977 (95% CI: 0.894–0.979; sensitivity = 88.7%; and specificity = 89.1%) [
58]. They also reported that miR-125b-5p could be used as an independent biomarker in predicting the survival rates of pancreatic cancer patients.
Serum miR-25 has been reported to be overexpressed in patients with PDAC. High levels of miR-25, and miR 25-3p suppresses PH domain leucine-rich repeat protein phosphatase 2 (PHLPP2), which results in the malignant phenotype of pancreatic cells via the activation of oncogenic AKT-p70S6K signaling. The overexpression of miR-25-3p has been correlated with a worse prognosis in pancreatic cancer patients [
59]. The overexpression of miR-25 has also been reported in gastric cancer, lung cancer, and cholangiocarcinoma; other studies have suggested that miR-25 serves as a tumor suppressor in thyroid cancer and colon cancer [
60,
61,
62,
63,
64]. When miR-25 was combined with CA19-9 to differentiate pancreatic cancer patients from normal controls, an AUC-ROC of 0.985, a sensitivity of 97.5%, and a specificity of 90.11% were achieved; for the detection of stages I and II tumors, miR-25 and CA19-9 were able to detect 40 out of 42 patients (95.24%). These results suggest that miR-25 may be used as a new biomarker for the early detection of pancreatic cancer [
65].
Schultz. et al, were able to identify two panels of miRNAs that are dysregulated in pancreatic cancer [
66]. Panel 1 consisted of miR-145, miR-150, miR-223, and miR-636, and panel 2 consisted of miR-26b, miR-34a, miR-122, miR-126 miR-145, miR-150, miR-223, miR-505, miR-636, and miR-885.5p. These miRNA panels were able to distinguish pancreatic cancer patients from healthy subjects. Using panel 1, an AUC of 0.86 (95% CI: 0.82-0.90), a sensitivity of 0.85 (95% CI: 0.79-0.90), and a specificity of 0.64 (95% CI: 0.57-0.71) were achieved. Using panel 2 an AUC of 0.93 (95% CI: 0.90-0.96), a sensitivity of 0.85 (95% CI; 0.79-0.90), and a specificity of 0.85 (95% CI: 0.80-0.85) were achieved. Interestingly, when combined with CA19-9, both panels were able to detect stages IA-IIB pancreatic cancer with the following performance; panel 1 an AUC of 0.83 (95% CI: 0.76-0.90); panel 2 an AUC of 0.91 (95% CI: 0.86-0.95). In a similar study conducted by Johansen, et al. they used 4-panels, panel I (7 miRNAs), panel II (9 miRNAs), panel III (5 miRNAs), and panel IV (12 miRNAs). Patients with pancreatic cancer in panels I and II were compared to patients with chronic pancreatitis and healthy people combined; in contrast, patients with pancreatic cancer in panels III and IV were compared to healthy participants (
Table 2). Panels I and III were designed to be robust to technical variation, and panels II and IV included all significant miRNAs from a multivariate model, thus it representing the upper limit in terms of training [
67]. The best panel to discriminate stages I and II pancreatic cancer from healthy subjects was panel II combined with serum CA19-9, exhibiting a sensitivity of 0.77 (0.69-0.84), a specificity of 0.94 (0.90-0.96), and an AUC of 0.93 (0.90-0.96). It is noteworthy that the aforementioned studies did not share any miRNA in their panels, except for miR-25.
Other than serum and pancreatic tissue samples, miRNAs can also be found in feces, urine, and saliva. miR-143, miR-223 and miR-30 can be found in urine and detected in stage I cancer. The combination of miR-143 and miR-30 showed a sensitivity and specificity of 83.3% and 96.2%, along with an AUC of 0.92 [
68,
69]. The measurement of miR-1246 and miR-4644 in saliva has been studied to distinguish pancreatic cancer patients from healthy controls, with AUC values for the ROC curves of 0.814 (P = 0.008) and 0.763 (P = 0.026), respectively; when miR-1246 and miR-4644 were combined, the AUC increased to 0.833 (P = 0.005) [
70]. Salivary miRNAs were reported to be stable due to the protection of exosomes. In the stool samples of pancreatic cancer patients, miR-21 and miR-155 were found to be overexpressed (P = 0.0049 and P = 0.0112, respectively), with a lower expression of miR-216 levels (P = 0.0002). The combination of miR-21, miR-155, and miR-216 for pancreatic cancer screening exhibits a sensitivity of 83.3%, a specificity of 83.3%, and an AUC of 0.866 (95% CI: 0.7722-0.9612) [
71].
3.4. Circulating DNA
Circulating-free tumor DNA (ctDNA) was first described in 1948, and it has been postulated that DNA release via the necrosis, apoptosis, and lysis of circulating tumor cells (CTCs) and micro-metastasis contributes to the presence of ctDNA [
72,
73,
74]. ctDNA consists of 170-181 base pairs and is found in body fluids at a very low concentrations ranging from 1 to 100 ng/mL, depending on the type and tumor burden [
47,
75]. Due to its low concentration in body fluids, it requires high analytical sensitivity and specificity for detection. The methods used to detect ctDNA include real-time PCR, automatic sequencing, mass spectrometry genotyping, next-generation sequencing (NGS), and digital PCR platforms (such as digital droplet PCR, (ddPCR)) with the sensitivity of these methods greatly varies, ranging between 0.01% and 15% [
76,
77,
78,
79].
ctDNA has been reported to be higher in patients with pancreatic cancer. In particular, Shapiro, et al. detected ctDNA levels as low as 25 ng/mL and DNA levels above 100 ng/mL as the upper normal limit using radioimmunoassay DNA quantification [
80]. The KRAS gene has received significant attention in terms of ctDNA mutations because it is highly mutated in pancreatic cancer [
77]. Samples from 26 pancreatic cancer patients were assessed for 54 of their genes, and it was reported that KRAS, TP53, APC, FBXW7, and SMAD4 may be able to detect PDAC [
81]. ctDNA KRAS mutation for the diagnosis of PDAC was reported to have a sensitivity of 47% and a specificity of 87%, and when combined with CA19-9, it had a sensitivity of 98% and specificity of 77% [
82]. On the contrary, Cohen et al. reported that CA19-9 was superior to ctDNA for the detection of stages I and II PDAC [
83]. Studies of ctDNA have reported mixed results. In a study of 26 cancer patients, it was found that KRAS, TP53, APC, FBXW7, and SMAD4 mutations were found in 90% of matched tumor biopsies utilizing NGS technology. The diagnostic accuracy was reported to be 97.7%, with an average sensitivity of 92.3%, and a specificity of 100% across all five investigated genes [
84]. On the contrary, Pishavian et al. reported that overall concordance between blood and tissue samples was only 25%using NGs assays, and mutations in KRAS were only detected in 29% of blood samples in comparison to 87% in tumor tissue biopsies [
85]. Similarly, in another study researchers evaluated the correspondence of KRAS mutations in pancreatic cancer tissue as well as ctDNA and KRAS mutations in serological markers, reporting that KRAS mutations were detected in 70% of neoplastic tissue samples, though none were found in ctDNA samples [
86].
Currently, the use of ctDNA as a diagnostic tool is limited due to the low amount of detectable ctDNA in the early stage of disease [
87]. However, ctDNA has been reported to be correlated with tumor burden and could be used as a tool to predict treatment response and surveillance in advanced cases [
88]. Chen et al. reported that KRAS-mutant ctDNA was correlated with time to progression and overall survival, with detection rates of 93.7% and 86.4% in patients with non-elevated CA19-9. KRAS mutations were also correctly predict 80% of patient response to treatment [
89]. Patients with KRAS-mutant ctDNA were reported to have 6.1 months of disease-free survival in comparison to 16.1 months in patients that had no such mutation, with overall survival times of 13.3 and 27.6 months, respectively (p < 0.001) [
90]. Similarly, a recent study utilizing ddPCR reported that KRAS-mutated ctDNA was correlated with a poorer prognosis, at 170 days vs. 489 days; interestingly, the presence of KRAS mutation in tissue DNA was not associated with survival rates [
91]. A subtype of a specific KRAS mutation, p.G12V, was also associated with shorter survival compared to p.G12D, p.G12R, or wild type variants [
91]. Serial plasma testing of KRAS-mutant ctDNA in advanced PDAC patients receiving chemotherapy seems to allow better monitoring than CA 19.9 [
92]. Longitudinal monitoring of ctDNA has been reported to predict response to therapy and disease progression around 5 months earlier than standard radiological imaging and CA19-9 9 [
93,
94].
The use of ctDNA is still limited because of signature ctDNA concordance between tissue biopsy and liquid biopsy, which greatly varies from 48% to 100% [
90]. Additionally, there is a lack of protocol standardization, the reliability of ctDNA detection methods varies between studies, and there are only a limited number of validation studies available [
77]. Furthermore, since tumor entities other than pancreatic cancer also show mutations low diagnostic sensitivity and specificity have been observed [
47].
3.5. Circulating Tumor Cells (CTC)
Circulating tumor cells, which are intact cells shed by tumors and circulate in the body can be detected in the blood [
95]. After shedding, the circulating tumor cells can disseminate through blood vessels and invade local tissue stoma [
96,
97,
98]. It has been reported that CTCs can be detected before metastasis [
99]. CTCs were reported to be present in whole blood at a ratio of around 1 for every 107 leukocytes per mL, with a half-life estimated at around 1-2.4 hours [
100]. The identification of CTCs includes the process of CD45 depletion to remove leukocytes; then, the enrichment of CTCs is performed via size-based filtration or epithelial cell adhesion molecules (EPCAMs). The actual CTC recognition involves examining cell morphology and measuring the expression of particular gene markers or proteins via the immunofluorescence of molecules specific for CTCs [
47,
100]. Another CTC detection method utilizes genomic, transcriptomic and proteomic approaches; one of the most widely used is FDA-approved Cell Search® [
100]. Methods for detecting CTCs are challenging, as there are only a low number of captured CTCs [
101]. Furthermore, in the case of pancreatic cancer, it has been reported to have a lower detection rate in comparison to other tumors [
77].
Several studies have reported that CTCs may have sufficient sensitivity to detect stage I and II cancer. Kulemann et al. detected CTCs in 80% of early-stage IIA and IIB tumors in 8 out of 10 patients using immunofluorescence for an epithelial-to-mesenchymal transition (EMT) marker and an epithelial antigen cytokeratin (CK); in contrast, CTCs were not found in any of the 10 control patients (p < 0.001) [
102]. Similarly, Xu et al. using negative enrichment (NE), immunofluorescence, and in situ hybridization (FISH) of chromosome 8 (NE-iFISH), were able to detect CTCs in 90% of pancreatic cancer patients; when combined with CA19-9, the diagnostic rate was reported to reach 97.5%, 75% in benign disease, and 73% in early-stage pancreatic cancer [
103]. Furthermore, Rhim et al. reported capturing CTCs in 33% of patients with cystic lesions without a clinical diagnosis of cancer (Sendai criteria), 73% with PDAC, and no detection in patients without cysts or cancer [
104]. Although promising, the use of CTC as an early biomarker is not yet suitable for clinical settings and still requires studies involving higher number of samples.