A liquid biopsy for metastatic prostate cancer

Metastatic prostate cancer can progress in different ways. In some men the disease advances rapidly, while other men have slower-growing cancer and a better prognosis. Researchers are developing various tools for predicting how fast prostate cancer might progress. Among the most promising are assays that count circulating tumor cells (CTCs) in blood samples.

Prostate cancer spreads by shedding CTCs into the bloodstream, so higher counts in blood generally reflect worse disease. Sometimes referred to as a liquid biopsy, the CTC assay can help doctors decide if patients should get standard or more aggressive treatment. Just one CTC assay is currently on the market for prostate cancer. Called CellSearch, its use is so far limited to men with late-stage metastatic cancer for whom hormonal therapies are no longer effective.

Using CTC data

Hormonal therapies block testosterone, a hormone that drives prostate tumors to grow. Research shows that high CTC counts predict poorer survival and faster disease progression among patients with metastatic prostate cancer who become resistant to this form of treatment. But new research shows CTC counts are also predictive for early-stage metastatic prostate cancer that still responds to hormonal therapy.

Why is that important? Because the earlier doctors can predict a cancer's trajectory, the better their ability to select patients who could benefit from more powerful (and potentially more aggressive) drug combinations or a clinical trial. Conversely, men who are older or frail might be treated less aggressively if doctors had better insights into their prognosis.

How the study was done

The investigators collected blood samples from 503 newly-diagnosed patients with hormonally-sensitive metastatic prostate cancer who had enrolled in a clinical trial with experimental hormonal therapies. The team collected baseline samples at trial registration, and then another set of samples after the treatments were no longer working. CTC counts were divided in three categories:

more than 5 CTCs per 7.5 milliliters (mLs) of blood between 1 and 4 CTCs per 7.5 mLs of blood zero CTCs per 7.5 mLs of blood.

What the research showed

Results showed that men with higher baseline CTC counts fared worse regardless of which cancer drugs they were taking. Median survival for men with 5 or more CTCs per sample was 27.9 months compared to 56.2 months in men with 1 to 4 CTCs. There weren't enough patient deaths among those with 0 CTCs to calculate a survival rate.

Observations and comments

"Still to be determined is how this type of testing compares with more traditional evaluations of disease advancement, such as x-rays, bone scans, and other types of imaging. Ready access to cancer cells in blood that, in turn, eliminate the need for more invasive biopsy procedures of metastatic deposits will be a welcome addition - especially if future studies show that CTCs inform more precise treatment choices."

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