This article mentioned circulating cancer cells may be a biomarker for prognosis.
Cells shed by a metastatic castration-resistant prostate cancer into the blood may be a robust measure of how well treatment is working, a researcher said here.
Working on a clinical trial of a new drug, the researchers found that the number of such cells in the blood predicted overall survival, according to Howard Scher, MD, of Memorial Sloan-Kettering Cancer Center in New York City.
If the finding is validated in future trials, the method could speed the development of new drugs, as well as aiding prognosis, Scher said at the annual meeting of the American Society of Clinical Oncology.
Scher said the analysis was planned as part of a phase III trial of abiraterone acetate (Zytiga), which was approved in April to treat men with metastatic castration-resistant cancer who have progressed on treatment with docetaxel (Taxotere).
Abiraterone inhibits the so-called CYP17 enzyme and thereby prevents androgen synthesis by the tumor itself, one of the ways by which prostate cancer escapes from drug therapy, Scher said.
Essentially, he told reporters, "the cancers themselves learn how to produce their own androgens," making hormone-blocking drugs less effective.
Analysis showed a significant improvement in overall survival associated with the drug -- a median of 15.8 months, compared with 11.2 months, Scher said.
During the study, the researchers measured circulating tumor cells, using the Veridex Cell Search system, both at baseline and during therapy. The cells were part of a biomarker panel that included lactate dehydrogenase, prostate specific antigen, and alkaline phosphatase.
The cell count was unfavorable if the number yielded by the cell search system was five or greater and favorable if it was below five, he said.
In some patients, treatment with abiraterone "converted" unfavorable counts to favorable ones as early as four weeks after starting therapy. The treatment effect was seen in up to 40% of patients, Scher said.
In multivariate analysis, baseline cell counts, "conversion" from unfavorable to favorable during treatment, and lactate dehydrogenase predicted overall survival.
An analysis adjusting for change in cell count almost completely eliminated any treatment effect, changing the hazard ratio from a significant 0.74 to a non-significant 0.97, Scher reported.
"We're all trying to hone in on a surrogate of survival," she said, because determining actual survival in clinical trials "can take so long to assess."
"To have a biomarker that is very individualized within a patient and... to show that in a statistically significant way that this correlates with survival is very promising," she said.
The study was supported by Cougar Biotechnology. Scher reported financial links with Cougar Biotechnology and Veridex.
Smith reported financial links with Biogen Idec, Celgene, Cephalon, Genentech, GlaxoSmithKline, and Spectrum Pharmaceuticals.
Primary source: Journal of Clinical Oncology