Liquid biopsy on the horizon in immunotherapy of non-small cell lung cancer: current status, challenges, and perspectives

Abstract

Non-small cell lung cancer (NSCLC) is one of the most threatening malignancies to human health and life. In most cases, patients with NSCLC are already at an advanced stage when they are diagnosed. In recent years, lung cancer has made great progress in precision therapy, but the efficacy of immunotherapy is unstable, and its response rate varies from patient to patient. Several biomarkers have been proposed to predict the outcomes of immunotherapy, such as programmed cell death-ligand 1 (PD-L1) expression and tumor mutational burden (TMB). Nevertheless, the detection assays are invasive and demanding on tumor tissue. To effectively predict the outcomes of immunotherapy, novel biomarkers are needed to improve the performance of conventional biomarkers. Liquid biopsy is to capture and detect circulating tumor cells (CTCs), circulating tumor DNA (ctDNA) and exosomes in body fluids, such as blood, saliva, urine, pleural fluid and cerebrospinal fluid as samples from patients, so as to make analysis and diagnosis of cancer and other diseases. The application of liquid biopsy provides a new possible solution, as it has several advantages such as non-invasive, real-time dynamic monitoring, and overcoming tumor heterogeneity. Liquid biopsy has shown predictive value in immunotherapy, significantly improving the precision treatment of lung cancer patients. Herein, we review the application of liquid biopsy in predicting the outcomes of immunotherapy in NSCLC patients, and discuss the challenges and future directions in this field.

Fig. 1. Different immunotherapy approaches.

They mainly include immune checkpoint inhibitors, adoptive immune cell therapy, tumor vaccine and cytokine-based therapy. Among the immune checkpoint inhibitors, PD-1/PD-L1 inhibitors block the interaction between PD-1 and PD-L1, preventing the inhibition of anti-tumor activity of cytotoxic T cells and thus eliminating tumor cells. Similarly, CTLA-4 inhibitors enable T cells to proliferate massively and attack tumor cells by binding CTLA-4 molecules. As shown in the figure, the currently approved drugs for NSCLC are nivolumab, pembrolizumab, cemiplimab targeting PD-1; atezolizumab, durvalumab targeting PD-L1 and ipilimumab, tremelimumab targeting CTLA-4. Adoptive immune cell therapy is exemplified by chimeric antigen receptor (CAR) T-cell therapy. Technicians isolate and purify T cells from the blood of a tumor patient. A viral vector containing a CAR that recognizes tumor cells and activates T cells is genetically engineered into the T cells, transforming T cells into CAR-T cells. The CAR-T cells are cultured in vitro to expand in large numbers, and then reinfused into the patient’s body, where they can specifically recognize tumor cells and efficiently kill them through immune effect. As for cancer vaccines, one of them involves removing cancer cells from a patient’s tumor, isolating cancer antigens and mixing these antigens with adjuvants that enhance the immune response, and injecting them back into the patient. The immune system can then recognize and attack cancer cells. Another type of cancer vaccine uses dendritic cells. Dendritic cells are removed from the blood and loaded with cancer antigens outside the body. The dendritic cells then take up the cancer antigens and post them on the cell surface. When injected back into the body, these antigen-rich dendritic cells trigger an immune response to the cancer. Finally, cytokine-based therapy suppresses tumors by directly administering exogenous cytokines that effectively activate immune cells in the body or counteract immune suppression.

Fig. 2. A timeline illustrates the immune checkpoint inhibitors (ICIs) approved by the Food and Drug Administration for NSCLC to date.

mNSCLC metastatic NSCLC, TPS tumor cell proportion score, TC tumor cell, IC immune cell.

Fig. 3. Specimen types and promising biomarkers for liquid biopsy.

A variety of body fluids such as cerebrospinal fluid, saliva, pleural fluid, peripheral blood, ascites, and urine are all sources of specimens for liquid biopsy. Blood-based liquid biopsy is currently the most important research direction. The main detection biomarkers are ctDNA, CTCs, exosomes, miRNAs and TCR repertoire, etc. In addition, feces-based biomarkers such as gut microbiota are also promising. ctDNA circulating tumor DNA, CTCs circulating tumor cells, miRNAs microRNAs, TCR T-cell receptor.