Liquid gold: blood-based biopsies make headway

Applying liquid biopsies now and in the future

There are a plethora of potential applications for liquid biopsies^3,7 (Figure 1), and probably the most exciting among them is the potential for screening for and early detection of cancer. The fact that ctDNA and CTCs have both been shown to be present from the earliest stages of disease has sparked interest in the possibility of developing simple blood tests to identify tumors before they become detectable by other methods and at a point at which they may be curable.

Given that both are present at such low levels within the circulation and are particularly sparse at earlier stages of disease, current technologies may lack the specificity and sensitivity for this application at present. However, numerous clinical trials are ongoing.

For CTCs, simple enumeration has been the most extensively investigated application to date. Numerous studies have shown that the number of CTCs in the bloodstream has prognostic significance in various different tumor types. Three such studies led to the first regulatory approval for a CTC detection system (Table 1 and Table 2).^8-10

One area in which liquid biopsies could really come into their own is in providing more real-time analysis of tumors. This is something that has proven particularly challenging with tissue biopsies because repeating these invasive procedures is problematic. But the ease of repeat blood draws means that serial liquid biopsies could be performed and might offer the possibility of monitoring disease progression and evolution over the course of disease and particularly in response to treatment.

Indeed, studies have shown that in addition to baseline CTC counts, changes in CTC number during treatment are also prognostic. There was improved survival among patients whose CTC counts decreased below a threshold value during treatment and vice versa. This is also an approved use for CellSearch though at present it is not widely clinically implemented.¹²

Clinical utility remains elusive

The ultimate goal would be for liquid biopsies to have an impact on treatment decisions, allowing oncologists to change management strategy based on predicted sensitivity or resistance to therapy, so-called clinical utility. Thus far, clinical utility has proved elusive, though liquid biopsies using ctDNA to evaluate tumor genotype have come closest.

The Cobas EGFR Mutation Test v2 recently became the first ctDNA-based liquid biopsy to receive regulatory approval. It was approved as a companion diagnostic to identify patients with advanced non–small-cell lung cancer (NSCLC) who have specific mutations in the epidermal growth factor receptor (EGFR) gene and are therefore eligible for treatment with the EGFR inhibitor erlotinib.¹³

Approval was based on comparison of EGFR mutation identification rates using plasma ctDNA samples and tumor tissue samples from patients enrolled in the phase 3 ENSURE trial, which compared the efficacy of erlotinib with chemotherapy as first-line therapy in patients with advanced NSCLC. Of the 217 patients enrolled in the trial, 98.6% of patients had both tumor biopsy and plasma ctDNA samples available for testing. Concordance between the two types of biopsy in identifying patients with EGFR mutations was high and patients with EGFR positivity according to liquid biopsy results demonstrated improved progression-free survival when treated with erlotinib.¹⁴

The results of a large-scale genomic analysis of various different types of tumors using ctDNA were also recently presented at the 2016 American Society of Clinical Oncology meeting. Blood samples from more than 15,000 patients with 50 different tumor types, including advanced lung cancer (37%), breast cancer (14%), and CRC (10%), were collected and compared with either available tumor biopsy samples from the same cases (n = 398) or, in the majority of cases, with The Cancer Genome Atlas database, which uses tumor biopsies to perform genome-wide sequencing studies. Both types of biopsy revealed very similar mutation patterns when the Guardant360 next-generation sequencing test, which targets 70 genes, was applied. In particular, when EGFR, BRAF, KRAS, ALK, RET, and ROS1 mutations were identified by tumor tissue biopsy, the same mutations were reported in 94%-100% of plasma samples.¹⁵

Studies of the clinical utility of ctDNA and CTCs are among ongoing clinical trials of liquid biopsies (Tables 2 and 3). The potential for using CTCs to guide treatment decisions has become particularly relevant in breast cancer in light of results showing that patients with primary tumors that are negative for human epidermal growth factor receptor 2 (HER2) amplification, an important biomarker in breast cancer, may have CTCs that are HER2-positive, in up to 30% of cases. These patients may therefore still benefit from HER2-targeted therapy.¹⁶

The DETECT studies are the first phase 3 trials in which treatment decisions are being based on the phenotypic characteristics of CTCs. DETECT III (NCT01619111) is comparing lapatinib in combination with standard therapy with standard therapy alone in patients with HER2-negative metastatic breast cancer who have HER2-positive CTCs, whereas DETECT IV (NCT02035813) is enrolling patients with HER2-negative, hormone receptor-positive metastatic breast cancer and persistent HER2-negative CTCs to receive standard endocrine therapy and the mammalian target of rapamycin inhibitor everolimus.