Hearing the words “it might be cancer” paired with your doctor’s concerned look is life-altering. Getting to the heart of a diagnosis usually requires a surgical biopsy—removal and examination of the suspicious tissue for signs of cancer.

Less invasive diagnostics, called liquid biopsies, bring more choices to doctors and patients. Liquid biopsies are today’s reality thanks to our ability to isolate molecules from body fluids. These diagnostic innovations pair technology with the latest in biomarkers knowledge. They are rapidly gaining acceptance as a reliable way to screen for cancer and to monitor disease progression and response to treatment.

A liquid biopsy is a test that is able to detect the presence of cancer using blood, urine, saliva, or other bodily fluid as the sample rather than tissue from a specific organ. The technique is possible because cancerous tissues shed cells, DNA, and tiny lipid-encased compartments called exosomes. Liquid biopsies detect the presence of these cancer-associated biomarkers.

Now that you know a liquid biopsy can be used as a diagnostic, let’s explore three types of liquid biopsies in various phases of development that may detect three different cancer biomarkers.

The first type of liquid biopsy is one that detects the cell-free DNA biomarker. When cells in the body die, including tumor cells, cell-free DNA (cfDNA) is released. A cell-free DNA liquid biopsy seeks to identify the DNA found in cancer cells. This cell-free DNA diagnostic identifies cancer-specific mutations using PCR or next-generation sequencing analysis. Listed below are the companies that are working in the cell-free DNA liquid biopsy space:

Cardiff Oncology, formerly Trovagene (San Diego, CA), analyzes cfDNA found in urine samples, which patients collect at home. Currently, Trovagene has tests detecting mutations associated with melanoma, colon cancer, and non-small cell lung cancer, as well as the presence of viral DNA for the diagnosis of the human papillomavirus. Exact Sciences Laboratories (Madison, WI) uses at-home collection in their colon cancer test, which analyzes cfDNA in stool samples for cancer-associated DNA.

Genomic Health (Redwood City, CA) currently markets tissue-based genomic tests for detecting and classifying breast and prostate cancer. It is developing cfDNA-based tests for cancers of the breast (blood sample) and bladder (urine sample).

Qiagen (Hilden, Germany) is developing cfDNA liquid biopsy diagnostics in partnership with pharmaceutical companies such as AstraZeneca (London, U.K.), Tokai Pharmaceuticals (Boston, MA), Novartis (Basel, Switzerland), and Eli Lilly (Indianapolis, IN).

The second type of liquid biopsy detects exosomes. Exosomes are lipid-encased vesicles that contain cellular protein, DNA, and RNA and typically have surface proteins specific to their native cell. These attributes, combined with the fact that they are found in many different body fluids, make exosomes a very attractive possibility for liquid biopsy. The idea is to capture exosomes based on tumor-specific surface markers or to collect exosomes and identify them as cancer-associated by examining the enclosed DNA or RNA.

Aptly named Exosome Diagnostics (Cambridge, MA) has exosome-based urine and blood tests for prostate cancer on the market and another in development for lung cancer. Qiagen has a partnership with Exosome Diagnostics to help develop additional exosome-based cancer diagnostics to complement their work with cfDNA.

The final category of liquid biopsy is perhaps the most obvious—circulating tumor cells (CTCs), or cells splintered from a tumor and circulating in the bloodstream. CTCs are identified by the tumor-specific proteins on their surfaces. The challenge lies in detecting CTCs: some estimates classify them as rare as one circulating tumor cell per billion normal cells!

Janssen Diagnostics (Raritan, NJ) currently markets CellSearch, the single FDA-approved test that allows physicians to identify early CTCs from blood samples. Monoclonal antibodies (mAbs) capable of recognizing proteins on the surface of migrating tumor cells are chemically linked to magnetic nanoparticles and then added to a patient’s blood sample. These tumor-specific mAbs grab hold of the CTCs and a strong magnetic field is then applied to the sample, isolating the captured cells for identification and analysis. CellSearch (Huntington Valley, PA) is currently used to monitor the efficacy of breast, prostate, and colorectal cancer treatments. A higher number of CTCs detected may indicate a higher incidence of metastasis or a less-than-effective treatment route if used to quantify cancer therapy success.

Another way to identify CTCs may be cell size—CTCs tend to be significantly larger than other cells in the blood. This size differential may be exploited in a microfluidics-based approach to cell separation.

Researchers at National University in Singapore (Singapore) and MIT (Cambridge, MA) have developed a microfluidics chip that routes cells from a blood sample into different channels based on cell size. Although still in the clinical research phase, this approach shows promise for capturing a wide range of CTCs.

Epic Sciences (San Diego, CA) adopts a “no cell left behind” game plan thanks to technology developed by the Scripps Research Institute (La Jolla, CA). Automated fluorescence microscopy identifies the CTCs in blood samples placed on microscope slides. A detailed analysis of three million cells per slide is performed, each blood sample yielding approximately twelve slides. This technology may potentially hone in on the presence of a single CTC. Genomic Health uses Epic Sciences’ CTC-detecting platform in their Oncotype-Dx AR-V7 Nucleus Detect test, which determines whether or not prostate cancer patients’ tumors have developed mutations that make them resistant to common types of treatment.

Ultimately, the best liquid biopsies may contain a combination of all the above approaches. Today, liquid biopsies are mainly used for monitoring the progress of or response to the treatment of already diagnosed cancers rather than as initial diagnostic tests. A major goal in the field is to develop tests that can be used routinely to detect cancer in seemingly healthy people, which should translate to better treatment outcomes.

Biocept (San Diego) is leading the way by developing liquid biopsies that analyze both cfDNA and CTCs. Biocept currently markets liquid biopsy tests for the detection of lung cancer, breast, colorectal, gastric, prostate, and melanoma. Biocept also has its eye on combination liquid biopsies for both colon cancer and melanoma.

Research published by a group at Johns Hopkins University (Baltimore, MD) suggests that a liquid biopsy test that detects both cancer-associated cfDNA and proteins known to be characteristic of certain types of cancer may be better at detecting cancer early on than those that look at just one biomarker. Dubbed Cancer-SEEK, the tool can detect many different cancers, including ovary and liver, and may soon begin testing as a screening tool.

As the technologies to detect cfDNA, cancer-specific exosomes, and CTCs progress, we can expect to see an increasing number of liquid biopsies available, making detecting and treating a range of cancers less invasive and more manageable.