What if you could count tiny bits of tumor in the bloodstream to predict whether a drug is working against cancer, rather than measuring the tumor itself? That's what researchers are doing in a formal collaboration with the Food and Drug Administration as part of an effort to make clinical trials faster, more efficient and less expensive.The research team, led by Howard Scher, MD, of Memorial Sloan-Kettering Cancer Center in New York, investigated whether tests that trap circulating tumor cells, or CTCs, could determine whether patients with advanced prostate cancer responded to Zytiga (abiraterone), a drug that targets a protein in the production of testosterone, which stimulates cancer cell growth. CTCs represent about one cell in a billion in the blood stream, Scher says.In the study of 1,195 patients, the median overall survival rate improved by 4.6 months. The researchers tested the blood of 972 patients at the beginning of the trial, and 723 patients after three months, finding that Zytiga reduced the number of CTCs and extended patients' lives.Prostate cancer is common in older men, and the standard screening test for prostate-specific antigen, or PSA, is unreliable because it sometimes rises even when a patient is benefiting from treatment, Scher says. But measuring CTCs predict a better prognosis and overall survival as early as four weeks after treatment.The researchers are continuing their work to define biological signals known as biomarkers that can predict which patients are the best match for drugs or give an early indication that treatments are working.