Circulating Tumor Cell Blood Test Shows Promise in Prostate Cancer
A preliminary report of the IMMC38 prostate cancer trial at Sloan Kettering Hospital in New York City confirms that the Cellsearch ™ Circulating Tumor Cell Test (CTC) count can predict whether patients are responding favorably to chemotherapy. Using the CTC count test results patients can be divided into two groups, those who show a favorable response to therapy and those who are responding poorly. In a report published in the February 11 2009 issue The Lancet, Sloan Kettering's lead investigator, Dr. Howard Scher, reports that CTC blood test results, used in conjunction with several other parameters, can be used to predict mortality risk in patients with inoperable metastatic prostate cancer.
The Cellsearch ™ test measures tumor cells in comparison to most blood tests for cancer, which measure proteins known as tumor markers that are produced by tumor cells. Tumor marker tests, such as the prostate specific antigen (PSA) test have offered little value in evaluating a therapeutic response and have been subject to numerous interpretation errors and problems with cross-reactivity from other proteins.
The Cellsearch ™ test first received FDA approval in January 2004 as a diagnostic laboratory test for identifying and counting CTCs in blood samples of patients with metastatic breast cancer. Results of early trials using this test showed good predictive value and have been published in The New England Journal of Medicine in 2004 and in Clinical Cancer Research in 2006. The studies concluded that the presence of CTCs in patients with metastatic breast cancer in the patient's first follow-up visit predicts ineffective therapy. In November 2007 the test was approved by the FDA for use in patients with metastatic colorectal cancer.
In February 2008, the FDA approved the Cellsearch ™ test for monitoring tumor cells in patients with metastatic prostate cancer. Prior to FDA approval, the test was evaluated in patients with metastatic prostate cancer at the Nevada Cancer Institute. Results of the Nevada study were presented at the American Association of Cancer Research (AACR) annual meeting in San Diego California in April 2008.
In the Sloan Kettering IMMC38 trial, Dr. Scher used blood tests to measure the enzyme lactic dehydrogenase (LDH) and also PSA along with CTC count test results to assess the response of patients with metastatic prostate cancer to chemotherapy. In this trial a circulating tumor cell count of 4 or less per 7.5 ml of blood indicated a favorable prognosis whereas a CTC count of 5 or more cells identified a poor treatment response. High LDH and PSA values and a high CTC count, as well as low albumin and hemoglobin values, before beginning treatment were associated with higher mortality. Changes in PSA results alone were only weakly associated with risk and were not significant until 12 weeks into treatment.
Dr. Scher emphasized that further clinical trials are needed to determine if the presence of circulating tumor cells definitively offers prognostic value. He reported that continuing trials of CTC counts in metastatic breast cancer have shown mixed results.