M Frantzi and A Latosinska contributed equally
Introduction
Prostate Cancer (PCa) is the most commonly diagnosed malignancy of genitourinary system in men, with the number of newly diagnosed cases yearly exceeding one million worldwide and 400,000 patients in Europe alone [1]. At the same time, more than 300,000 patients succumb to the disease yearly, placing this malignancy as fifth main cause of cancer mortality in men worldwide [2]. The diagnosis of PCa is currently based on serum prostate specific antigen (PSA) testing, digital rectal examination (DRE), and nowadays often multiparametric magnetic resonance imaging (mpMRI), followed by multi-core prostatic biopsy in case of suspicious findings of DRE and PSA levels [3]. However, multiple factors not related to prostate malignancy affect the level of blood PSA (eg inflammation, infection, benign prostate hyperplasia). PSA lacks specificity, especially in the intermediate range (4.0–10.0 ng/mL), the so called "gray zone", and only around 25% of these patients are confirmed with PCa after biopsy.
Although introduction of PSA testing resulted in increased and earlier detection of PCa, this was not accompanied by a concordant improvement in the mortality rates. Further, PSA testing increased the detection rate of low malignant PCa unlikely to progress in the absence of treatment. The inability to accurately discriminate between patients with indolent and aggressive PCa (that requires treatment), has resulted in overtreatment of PCa patients. This infers a serious socioeconomic burden, as a result of side effects and high associated treatment costs. Active surveillance (AS), which implies intervention only in patients with tumor progression, is considered a valid alternative to immediate therapy and overtreatment of patients with low risk disease. Around 45% of patients initially diagnosed with PCa experience slow-growing forms (Gleason score<7, PSA≤10 ng/mL), thus are eligible for AS [4,5]. Selection of patients for active surveillance currently relies on repeated invasive biopsies, a series of PSA measurements, and DRE. Although various criteria are used for patient selection and follow-up during AS, incorrect patient exclusion or misclassification becomes a concern.
As such, non-invasive biomarkers (indicators of presence/ severity of disease) have the potential to improve PCa management through the following ways: a) by guiding biopsy and improving on the false positive rate of PSA, b) by guiding intervention for patients on AS (Figure 1), and c) by predicting response to treatment and progression to metastatic castration-resistant PCa. A biomarker approach would have a positive contribution in reducing unnecessary biopsies and overtreatment and decrease associated costs. Urine as a biofluid offers the advantage of noninvasive collection and increased stability in comparison to blood/serum.
Therefore, not surprisingly, more than 800 studies have been published in the last 5 years on urinary biomarkers in the context of PCa. However, only a few biomarkers have been implemented in PCa management, in part due to inappropriate study designs, lack of appropriately powered validation studies, and absence of a demonstrated benefit.
Figure 1. Overview of the proposed clinical context of use for urinary biomarkers in prostate cancer management. Implementation of biomarkers will allow: 1) guidance of initial biopsy, 2) discrimination between patients with insignificant and significant prostate cancer, as well as 3) guidance on the decision-making for patients on active surveillance. The most promising urinary biomarkers for the specific context of use are presented in Table 1.
PCA3
Among urinary biomarkers, prostate cancer antigen 3 (PCA3), also know as DD3, is highly prominent. PCA3 is a noncoding mRNA that is exclusively expressed in human prostate tissue with levels increased in malignancy. PCA3 has been extensively studied as a urinary RNA marker for guiding the first biopsy, as well as repeated biopsies, to improve on the accuracy of PCa detection. PCA3 has been mentioned in the latest European Association of Urology (EAU) guidelines for its main indication to determine whether a repeated biopsy is needed after an initially negative biopsy. Nevertheless, the value of PCA3 in monitoring AS is still under investigation [3]. In several studies, PCA3 has demonstrated superior performance compared to serum PSA, and therefore it has been proposed as a test to be offered to asymptomatic patients with PSA levels between 2.5-10ng/ml to avoid unnecessary biopsies [6].
TMPRSS2-ERG
TMPRSS2-ERG is a gene fusion of the transmembrane protease serine 2 and the ERG proto-oncogene, which are located very close on the same chromosome. The fusion is predominant in prostate cancer. When applying a cut-off of 10 copies, the TMPRSS2-ERG urinary test presented an AUC of 0.84 and outperformed the ERSPC risk calculator (AUC=0.80) in a multicentric study including more than 400 PCa patients [7]. TMPRSS2-ERG performed similarly to PCA3 (AUC=0.83) and improved sensitivity from 68 to 76% when PCA3 and TMPRSS2-ERG were combined [7].
microRNAs
microRNAs (miRNAs) are small non-coding RNAs of generally up to 22 nucleotides that are, among other things, involved in the regulation of translation by mediating RNA silencing and/or post-transcriptional regulation. Several miRNAs have been investigated in the context of PCa. As an example, miR-148a and miR-375 showed AUC values of 0.78 and 0.68, respectively, for discriminating benign prostatic hyperplasia from PCa in urine samples from 166 patients [8]. Increased performance was achieved when the miRNAs were combined in multimarker models. Further, when combined with PSA, the overall AUC was improved to 0.85 [8]. However, currently, multiple different miRNAs have been reported, with very little, if any, consistency between the reports, and a lack of follow-up studies. While the initial data appear promising, verification in large, prospective studies is still pending.
Peptide biomarker panels
Following this principle to apply multi-marker panels, a mass spectrometry (CE–MS/ proteomics)- based peptide panel was initially introduced for patients with suspicious PSA and/or DRE results prior to intended biopsy. The test simultaneously determines 12 peptide biomarkers indicative for PCa. The performance of the approach was investigated in two blinded prospective studies on over 400 patients [9,10]. In combination with age and percent-free PSA in a diagnostic nomogram, the 12-peptide biomarker panel resulted in 91% sensitivity at 69% specificity [9]. Direct comparison with PSA revealed improved diagnostic accuracy of the 12-peptide biomarker panel (AUC 0.72) in comparison to total PSA (AUC of 0.60), as well as the percent-free PSA (AUC 0.69) [10]. As a follow-up to these studies, a CE–MS-based urinary peptide marker panel was established, which in combination with the current clinical prognosticators, improves detection clinically significant PCa. In a cohort of 823 PCa patients with low levels of PSA (<15ng/ml), this urinary biomarker panel was able to discriminate between clinically significant and insignificant cancer with an accuracy of 75% (AUC=0.75). Combination with age and the ERSPC risk calculator resulted in an AUC value of 0.82.
Conclusion
In conclusion, several urinary biomarkers of different origins show potential for implementation in PCa management. The most commonly addressed clinical context is to guide the first or repeated biopsies, aiming to reduce unnecessary biopsies in PCa diagnosis or AS monitoring. Only a few studies are addressing the need for prediction of treatment response and the progression to mCRPC. The most promising biomarkers and candidates are listed in Table 1. Of the multiple potential biomarkers that were published within the last 5 years, only few were followed up in appropriate, well-powered validation studies. A prospective, well-powered, multicenter study testing the most promising biomarker candidates in parallel is urgently required to assess their true value, and guide implementation of those biomarkers that hold true value in patient management.