Researchers Identify Better Blood Test for Prostate Cancer
(special interest story)
New studies of a blood protein identified recently by investigators at Johns Hopkins University suggest that there soon may be changes in the way men are screened for prostate cancer, a disease that kills tens of thousands of men every year. Testing for this protein, which is called early prostate cancer antigen-2 (EPCA-2) could curtail unnecessary biopsies and can differentiate disease that has spread outside the prostate from cancer within the prostate, according to the investigators who conducted this study.
Current standards of screening and testing for prostate cancer focus on the blood protein prostate-specific antigen (PSA) along with a digital rectal examination. Men who have more than 2.5 nanograms per milliliter of PSA are considered at risk for prostate cancer. However, PSA testing often erroneously highlights noncancerous conditions (false positives) and can miss some cases of cancer (false negatives), according to Robert Getzenberg, Ph.D., who led the study assessing the predictive value of measuring EPCA-2.
Due to elevated PSA levels, approximately 1.6 million men undergo prostatic biopsies in the United States annually, and roughly 80 percent of these men have negative results, says Getzenberg. He adds that, of the entire population of men in the United States who have been tested for PSA, an estimated 25 million have elevated PSA levels and a biopsy of the prostate that did not reveal any prostate cancer. Conversely, roughly 15 percent of men with prostate cancer go undetected because their PSA levels are below the cutoff level, according to Getzenberg.
However, in a study published in the journal Urology, Getzenberg and his colleagues introduce evidence in support of EPCA-2 testing as a more accurate way to identify cancer in the prostate. "A blood test based on EPCA-2 may greatly improve our ability to accurately detect prostate cancer early and minimize the number of false positives, therefore lowering the number of unnecessary biopsies," says Getzenberg. "In addition, this is the first time we have a test that effectively distinguishes between men with cancer confined to the prostate and those whose disease has spread outside of the gland."
Getzenberg and his team measured EPCA-2 levels in the blood of 330 patients separated into several groups: men with normal PSA levels and no evidence of disease, men with elevated PSA levels but who had negative biopsies, men with a common noncancerous prostate condition known as benign prostatic hypertrophy (BPH) who did not receive biopsies for prostate cancer, men with prostate cancer but with normal PSA levels, men with prostate cancer confined to the prostate, men with prostate cancer that had invaded outside of the gland at the time of surgery, and a diverse group of patients with benign conditions of other organs as well as individuals with other cancer types.
Patients with an EPCA-2 cutoff level of 30 nanograms per milliliter or higher were considered to be at risk for prostate cancer. This cutoff was based on a pilot study of 30 blood samples, which was then applied throughout the larger study.
Results showed that the EPCA-2 test was negative in 97 percent of the patients who did not have prostate cancer. Men with no evidence of disease (regardless of their PSA levels), as well as the control group of patients with other cancer types and benign conditions, all had EPCA-2 levels below the cutoff. In contrast, in a multiinstitutional study published in 2003 in the Journal of Urology, PSA levels between 4 and 10 nanograms per milliliter were shown to be accurate in identifying patients without prostate cancer only 19 percent of the time.
In addition, 77 percent of the BPH patients had a level of EPCA-2 lower than the cutoff point. Getzenberg says this is well within the likely percentage range of BPH patients who are prostate-cancer free. He says this result was encouraging since BPH is often associated with elevated PSA levels, leading to misdiagnosis and unnecessary biopsies.
When it came to correctly identifying patients with prostate cancer, EPCA-2 levels at or above the cutoff were detected in 90 percent of the men with organ-confined prostate cancer and in 98 percent of the men with disease outside the prostate. Overall, in this study, the EPCA-2 test detected 94 percent of the men with prostate cancer.
Results of the study also revealed that EPCA-2 levels were significantly higher in patients whose cancers had spread outside the prostate compared with those with disease confided to the gland. EPCA-2 was dramatically better at separating these groups than were PSA levels, according to Getzenberg. "This is important, since cancer that has spread outside the prostate is more deadly, which makes it even more crucial to have a tool that detects it early," says Getzenberg.
An optimized version of the assay, evaluated in a separate set of 55 patients, supported the earlier findings. Finally, the EPCA-2 test identified 78 percent of the men with prostate cancer in the group with PSA levels below the accepted cutoff level of 2.5 nanograms per milliliter. According to their PSA levels, these were all "healthy men," but EPCA-2 was able to show that they had prostate cancer.
EPCA-2 is the second prostate cancer marker identified by Getzenberg and his team that has outperformed PSA. Last year, they discovered an unrelated, tissue-based test, EPCA-1, that also proved effective at identifying prostate cancer. The only commonality between these markers is that they were discovered using the same approach. Getzenberg says the efficacy of EPCA-1 as a test of biopsy samples is currently being evaluated. He adds that larger clinical trials for EPCA-2 are planned that could make this test available to the public in approximately 18 months.
Prostate cancer is the most common type of cancer found in U.S. men. The American Cancer Society estimates that there will be approximately 218,890 new cases of prostate cancer in the United States in 2007, and 27,050 men will die of this disease.
This work, which was supported by the National Cancer Institute, is detailed in the paper "EPCA-2: A highly specific serum marker for prostate cancer." An abstract of this paper is available through PubMed.