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Vancouver, British Columbia--PCA3, the gene most over-expressed by prostate cancer, is a promising urine marker for the disease. Moreover, the PCA3 gene test appears to offer much greater cancer specificity than the PSA test, which is presently the best screening test available. Most PSA elevations seen today are actually caused by BPH, not cancer, which is a major limitation with PSA testing for prostate cancer. However, according to researchers at Gen-Probe in San Diego, the PCA3 gene assay, which is currently under development, may provide a test that is cancer-specific, not just "prostate-specific."
Harry Rittenhouse, PhD, senior director of cancer research at Gen-Probe, and his colleagues found that the assay was also sensitive enough to achieve almost a 100% detection rate of prostate cancer cells in urine, thus allowing quantification of PCA3 in all samples collected in urologic clinics in Quebec, Canada, and Los Angeles following attentive digital rectal examination.
"We are looking at the effect of a DRE on the clinical sensitivity and specificity and also on the technical analytical sensitivity," said Dr. Rittenhouse, who presented the study findings at the AUA Western Section's annual meeting in Vancouver.
In the study, 66 men, ranging in age from 27 to 85 years, were assigned to one of three groups: a control group of 22 men <45 years of age, with no prostate disease; 12 men who had previously undergone radical prostatectomy; and 32 men with BPH confirmed by biopsy but no cancer. All men were outpatients at the urologic clinics.
Quantifiable samples for PCA3 were collected from all patients in the study, in contrast to previous studies, where evaluable specimens frequently could not be obtained. A urine specimen is only evaluable for PCA3 if it contains prostate epithelial cells, which is ascertained by the finding of PSA-mRNA in the urine. The results showed a high degree of variability in the amount of cellular material collected, but the attentive DRE increased the evaluable rate to 100% among the control and BPH groups. In the group of men who had undergone prostatectomy, the PCA3 and PSA-mRNA levels were similar to background levels.
Potentially a specific assay
The test's clinical specificity rate is between 75% and 90%, potentially making it better than PSA for excluding prostate cancer in men with only BPH, according to Dr. Rittenhouse.
"PSA really has limited value for that population, which is one of the reasons PSA is becoming a dilemma," he said, adding that the clinical sensitivity of the test needs to be increased. "The challenge with the assay is clinical sensitivity, which is 50% to 60% so far."