How to use liquid biopsies to treat patients with cancer

Abstract

Precision medicine is now pivotal to design patients' specific treatment strategies with the aim of prolonging progression and overall survival. In this regard, invasive tumor tissue testing has so far been the golden standard for making cancer diagnosis, but has limitations. Cell-free tumor DNA (ctDNA), a form of liquid biopsy, is a noninvasive biomarker that can be isolated from patients' blood and other biofluids. An increasing body of evidence has demonstrated clinical utility of plasma ctDNA profiling to select patients for genomic-driven therapies. Analyses of mutations in plasma ctDNA have shown high accuracy and more rapid identification of mutations, allowing matching patients for specific therapies with equivalent clinical efficacy to that of the tissue profiling. In the clinical setting, ctDNA has been recently implemented to select patients with specific genomic alterations to targeted treatments, and a few molecular tests have been approved for use in non-small-cell lung, prostate, ovarian, and breast cancers. However, standardization of ctDNA collection, storage, and analysis methods would be critical to facilitate the wide adoption of ctDNA technology in routine clinical practice. This review summarizes how we can exploit ctDNA analysis to treat cancer patients, and explains how the results should be interpreted. In addition, we focus on how ctDNA could be used in the future as a marker of minimal residual disease to guide adjuvant therapy, as an immuno-oncology biomarker in patients treated with immune checkpoint blockade drugs, and as an early cancer detection marker to screen the asymptomatic population.

Keywords: cancer treatment; circulating tumor DNA; early detection; genomics; liquid biopsy; next generation sequencing; noninvasive.

Figure 1

Plasma cell-free tumor DNA (ctDNA) clinical utility as a liquid biopsy. (A) ctDNA can be shed by various tumor types, including breast, lung, prostate, and ovarian cancers. Each lesion can harbor different alterations (trunk mutation, blue, shared by all the clones as it occurs early in the tumorigenesis; driver mutation, green, and resistance mutation, red, which can emerge after treatment). Once in the blood, ctDNA can be collected by venipuncture, and the sample is then processed to obtain plasma. After isolation, ctDNA can be assessed to investigate for molecular alterations by two approaches: the single-gene analysis (PCR-based methods such as droplet digital PCR, upper panel), or the multigene panel analysis (next-generation sequencing, lower panel). Four tests have by now been approved by the US Food and Drug Administration (FDA) for clinical use: the therascreen PIK3CA RGQ PCR Kit and the cobas EGFR Mutation Test v2, and FoundationOne Liquid CDx and Guardant360 CDx, for the single-gene and multigene analysis, respectively. (B) ctDNA as a biomarker with different dynamics, where the percentage of mutational allelic frequency is quantified throughout all the collected ctDNA timepoints. Upper panel: ctDNA is assessed for three biomarkers: (i) for patients' monitoring during treatment, with a trunk mutation, as a marker of tumor burden, blue line; (ii) for therapy selection, with a driver mutation, as a marker of tumor response to therapy, green line; and (iii) with a resistance mutation, as a marker of tumor progression and resistance to therapy, red line. Lower panel: ctDNA used as a marker of early tumor detection and diagnosis in patients with early stage cancer or in asymptomatic population (purple line). In addition, ctDNA can be used to detect minimal residual disease (MRD, blue line) in cancer patients undergoing surgery with curative intent, with or without prior neoadjuvant treatment. If ctDNA after surgery is still detected, then the patient could be guided to receive adjuvant therapy and monitored for further disease recurrence; if ctDNA after surgery is not detected, then the patient could be considered disease free.