Initial pre-clinical research has shown that the PCA3 gene is over-expressed only in malignant prostate tissue. Gen-Probe's prototype APTIMA PCA3 test may, therefore, prove superior to prostate specific antigen (PSA) testing in certain clinical situations. Because PSA is produced by both malignant and non-malignant tissue, PSA tests tend to produce many "false positive" results that cannot be confirmed by biopsy.
"The promising results of these initial studies are consistent with our performance targets for the APTIMA PCA3 assay and give us confidence in our ongoing and future research," said Harry Rittenhouse, Ph.D., Gen-Probe's senior director of cancer research.
The first study, presented at the Oak Ridge meeting last week, assessed the ability of physicians to collect "informative" urine samples that could be tested for PCA3 messenger RNA (mRNA) expression levels. In the study, physicians collected urine samples from men before and after an attentive digital rectal exam (DRE). An attentive DRE involves three sweeps on each side of the prostate. The men were divided into three groups: those less than 50 years old with no known prostate disease (N=24); those undergoing prostate biopsy (N=45); and those who had undergone radical prostatectomy (prostate removal) more than 1 year before the study (N=15). The third group of men served as controls.
The researchers defined informative samples as those having sufficient prostate cells to yield a valid clinical result, as measured by PSA mRNA expression. After the attentive DRE, PSA gene expression could be quantified from all the samples collected. Samples that were collected before the DRE had a lower level of PSA mRNA expression, yet still demonstrated an informative test rate approaching 90%.
"One challenge for molecular urine tests is obtaining enough prostate cells to ensure a valid result," said lead author Leonard Marks, M.D., clinical associate professor of urology at UCLA and medical director, Urological Sciences Research Foundation. "In this study, combining an attentive DRE with Gen-Probe's powerful molecular detection technologies yielded an informative rate of 100%. This high informative rate provides an advantage over other methods of molecular detection in urine and reduces the number of repeat specimen collections required. Significantly, results from patients in the post-prostatectomy control group showed that PSA and PCA3 mRNA were nearly undetectable, demonstrating the excellent analytical specificity of the assay."
The second study, also presented at the Oak Ridge meeting, evaluated the clinical performance of the prototype APTIMA PCA3 assay. In the study, physicians collected urine samples from men with elevated PSA scores who were scheduled for biopsy (N=93) or radical prostatectomy (N=26). After biopsy, 53 of the 119 men were diagnosed with prostate cancer. A third group of men who had undergone radical prostatectomy more than 1 year before the study (N=31) served as controls. The urine samples were collected before and after an attentive DRE, with centrifuged (pelleted) and uncentrifuged (whole) urine stabilized in a lysis buffer for analysis.
For the whole urine samples collected after a DRE, sensitivity and specificity of the prototype APTIMA PCA3 assay were 66% and 75%, respectively, when the ratio of PCA3 to PSA mRNA was used to determine the presence of cancer. Similar results were obtained using pelleted samples.
"We believe that the prototype APTIMA PCA3 assay demonstrated promising sensitivity and specificity in this study, which also showed that similar results can be obtained with or without centrifuging urine samples collected after a DRE," Dr. Rittenhouse said.
The third study, which was presented at the AACR annual meeting yesterday, was conducted using the same 150 patient samples described in the previous poster. In the study, researchers evaluated whether biopsy results correlated more closely with the absolute level of PCA3 mRNA, or with the ratio of PCA3 mRNA to PSA mRNA. As described above, sensitivity and specificity of the prototype APTIMA PCA3 assay were 66% and 75%, respectively, when the ratio of PCA3 to PSA mRNA was used to determine the presence of cancer. Sensitivity and specificity results were lower using the absolute level of PCA3 mRNA, confirming the value of calculating a ratio of PCA3 to PSA mRNA. In addition, expression levels of PCA3 and PSA mRNA were near negative control levels for men who had undergone radical prostatectomy, again demonstrating the analytical specificity of the assay.
"This study validates our research strategy to maximize the accuracy of the APTIMA PCA3 assay by using it to calculate the ratio of PCA3 to PSA mRNA expression," Dr. Rittenhouse said.
Gen-Probe acquired exclusive worldwide diagnostic rights to the PCA3 gene from DiagnoCure in November of 2003. The companies maintain a license and collaboration agreement under which they are developing, and Gen-Probe will market, the APTIMA PCA3 assay.
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