Objectives: Serum tPSA lacks specificity. The DD3(PCA3) gene is highly specific for prostate cancer and is detectable in prostate cancer cells shed into urine after rectal palpation. A newly developed nucleic acid sequence based amplification assay (uPM3) for detecting DD3(PCA3) RNA in urine samples was evaluated prospectively in patients referred for prostate cancer detection.
Methods: The uPM3 assay simultaneously detects the relative expression of DD3(PCA3) RNA and PSAmRNA as a marker for prostate cells in urine. Urine samples were collected after attentive digital rectal palpation prior to transrectal guided prostate biopsy. Samples were provided as a single void specimen (20-30 ml), stabilized in phosphate buffer and centrifuged. Lysis was performed on cell pellets DD3(PCA3) RNA and PSAmRNA were extracted and amplification was performed using isothermic nucleic acid based amplification (NASBA). The two targets were detected in real-time using specific beacons as probes in a thermostated spectrofluorimeter. Parameters of the amplification curve were defined after a logistic curve fitting routine and a classification tree model was constructed to predict the outcome of patients (i.e. cancer and non-cancer).
Results: 201 patients were included in this prospective study. 158/201 analyzed urine samples contained enough prostate cells sufficient for DD3(PCA3) analysis (79% adequacy rate). Prostate cancer was found in 62 (39%) of the evaluable patients. Overall sensitivity, specificity, positive predictive value and negative predictive value for the uPM3 assay at a cut-off 0.5 probability were 82%, 76%, 67% and 87% respectively as compared to 98%, 5%, 40% and 83% respectively for tPSA (at a cutoff of 2.5 ng/ml). In the tPSA categories <4, 4-10 and >10 ng/ml sensitivity was 73%, 84% and 84% and specificity was 61%, 80% and 70%, respectively. The AUC (area under the curve) was 0.87 (CI 0.81-0.92).
Conclusion: The uPM3 assay showed excellent clinical performances and a specificity far superior to tPSA.