Prostate cancer is the most common malignancy and second most common cause of cancer death in men in the U.S., yet there are no available biomarkers to guide treatment decisions. Amir Goldkorn, MD, a medical oncologist at Keck Medicine of USC and assistant professor of medicine at the Keck School of Medicine of USC, addressed this challenge in a phase 3 multi-center trial conducted through the NCI Southwest Oncology Group.

In the study, 1,200 men with metastatic prostate cancer were randomized to receive one of two hormonal therapy regimens. At the start of treatment, blood samples were sent to Dr. Goldkorn’s team at the USC Norris Comprehensive Cancer Center Liquid Biopsy Core, who measured the number of CTCs in the blood.

In an oral presentation at the American Society of Clinical Oncology’s Annual Meeting in May, Dr. Goldkorn presented the results: The number of baseline CTCs prior to treatment strongly predicted who will respond to hormonal therapy and for how long. For example, a man with 0 CTCs was 6-fold more likely to have a complete response than a man with 5 or more CTCs, whereas a man with 5 or more CTCs was nearly 4-fold more likely to experience disease progression than a man with 0 CTCs. Based on these results, CTC counts may become a valuable noninvasive biomarker in men with metastatic prostate cancer who are starting treatment. Men with few or no detectable CTCs would be likely to have a robust, long-lasting response to hormonal therapy, whereas those with higher CTC numbers would be less likely to experience a favorable response and may benefit from earlier, more aggressive therapies or enrollment in clinical trials.