Liquid biopsy in pancreatic cancer: the beginning of a new era

Abstract

With dismal survival rate pancreatic cancer remains one of the most aggressive and devastating malignancy. Predominantly, due to the absence of a dependable methodology for early identification and limited therapeutic options for advanced disease. However, it takes over 17 years to develop pancreatic cancer from initiation of mutation to metastatic cancer; therefore, if diagnosed early; it may increase overall survival dramatically, thus, providing a window of opportunity for early detection. Recently, genomic expression analysis defined 4 subtypes of pancreatic cancer based on mutated genes. Hence, we need simple and standard, minimally invasive test that can monitor those altered genes or their associated pathways in time for the success of precision medicine, and liquid biopsy seems to be one answer to all these questions. Again, liquid biopsy has an ability to pair with genomic tests. Additionally, liquid biopsy based development of circulating tumor cells derived xenografts, 3D organoids system, real-time monitoring of genetic mutations by circulating tumor DNA and exosome as the targeted drug delivery vehicle holds lots of potential for the treatment and cure of pancreatic cancer. At present, diagnosis of pancreatic cancer is frantically done on the premise of CA19-9 and radiological features only, which doesn't give a picture of genetic mutations and epigenetic alteration involved. In this manner, the current diagnostic paradigm for pancreatic cancer diagnosis experiences low diagnostic accuracy. This review article discusses the current state of liquid biopsy in pancreatic cancer as diagnostic and therapeutic tools and future perspectives of research in the light of circulating tumor cells, circulating tumor DNA and exosomes.

Keywords: circulating tumor cells; circulating tumor nucleic acids; exosomes; liquid biopsy; pancreatic cancer.

Figure 1. Application of circulating biomarkers

(A) Application of blood-based liquid biopsy analysis over the span of pancreatic cancer management, peripheral venous blood is collected from the patients for isolation of circulating tumor cells (CTCs), circulating tumor DNA (ctDNA) and exosomes. These circulating biomarkers may be applied to guide initial diagnosis, treatment monitoring or planning, prognosis prediction and developing a new targeted therapy for patients with pancreatic cancer. (B) Functional studies with CTCs and development of CTC— derived xeno-grafts (CDXs), patient-derived tumor xeno-graft (PDTX) and 3D organoids model from CTCs for dynamic monitoring of PC and development of new targeted drugs after its molecular characterization and genomic analysis. (C) Clinical application of ctDNA as a tool for therapy monitoring. ctDNA can be obtained from plasma for genomic analysis, drug testing and use in personalized medicine according to the genomic and epigenomic alteration. (D) Clinical use of exosomes for drug development after genomic and immunological testing. Moreover, use of exosome as a drug delivery vehicle where it can be loaded with drugs, siRNAs, gene etc.

Figure 2. Tumor heterogeneity and clonal evolution during treatment

(A) Diagram showing the evolutional clonal architecture in pancreatic cancer (PC) at diagnosis and relapse. Of note, at diagnosis, the clonal and subclonal diversity evolved from a common ancestral tumor stem cell. The clonal evolution may follow linear or branched evolution, however, branched evolution is probably more likely to contribute to tumor heterogeneity. Additionally, drug treatment instigates a bottleneck effect, where resistant subclones will survive and proliferate to form a heterogeneous tumor. (B) During systemic successive targeted therapy assessed by longitudinal liquid biopsies may identify an actionable genetic alteration, therapy response or progression. In the event that progression is identified, the clinician may be able to switch treatment to target arising clones that carry additional mutations that were identified by the ctDNA analysis. At the start of targeted therapy, all cells in the patient's with PC have actionable genetic mutations (clone 1). The administration of treatment 1 targets the clone 1. longitudinal liquid biopsy analysis demonstrates an initial decrease in the clone 1 during treatment 1, yet uncovers the evolution of new clone (clone 2 and clone 3) causing resistance to treatment 1. The clone 2 and clone 3 can be targeted with treatment 2, where longitudinal liquid biopsy analysis uncovers a decrease in the frequency of resistance clone 2 and clone 3, during this time, however, other genetic alterations clone 4 and clone 5 increases in frequency. These clones 4 and 5 are resistant to treatment 2, yet is sensitive to treatment 3. During treatment 3, the frequency of the clone 4 and clone 5 decreases, while residual earlier resistant clones may persist to give rise to therapeutic resistance.