Prostate cancer (PCa) is the most common cancer and the second most common cause of cancer-related death in American men [1▪]. Although screening for PCa with prostate-specific antigen (PSA) testing reduces death from prostate cancer, it has limited specificity for detecting clinically significant disease (CSC) [2▪▪], leading to an overwhelming number of men being subjected to unnecessary invasive biopsies, and the subsequent over diagnosis and overtreatment of indolent disease . It has become clear that we need better tools to help us more reliably distinguish between men who need a biopsy because they are at risk of having a cancer that may threaten their quantity or quality of life versus those who can safely avoid a biopsy as they are at low risk of having such a tumor.
Furthermore, although more men are appropriately choosing active surveillance as an initial management strategy for prostate cancer , issues with tumor heterogeneity and multifocality create uncertainty regarding the risk of missing a significant tumor that would not be safe to follow.
Given the clear need, there are several diagnostic and prognostic tests that have emerged to aid in assessing the risk of harboring a significant tumor. The aim of this article is to introduce and review the current evidence supporting serum and urine biomarkers for PCa detection and surveillance, with an emphasis on markers that identify CSC.
The 4Kscore is a noninvasive blood test that evaluates the measurement of four kallikrien proteins (total PSA, free PSA, intact PSA, and human kallikrein 2), combined with clinical parameters (age, digital rectal examination (DRE) findings, and history of prior biopsy) to provide a personalized probability of having grade group 2 or higher prostate cancer . The 4Kscore has been validated in two independent multiinstitutional prospective studies. The first was conducted in 26 academic and community-based urology practices across the United States, whereas the second was conducted in 8 Veteran's Affairs Medical Centers, where 56% of the cohort was African American. Despite the differences between these study populations, the 4Kscore showed good calibration and discrimination [area under curve (AUC) > 0.80] in both trials, while reducing a significant number of biopsies. Although the 4Kscore was meant to be used as a continuous risk prediction score, there is evidence to support a 7.5% cut-off for defining a population that is unlikely to harbor an adverse prostate cancer. When combining data from both trials, using a 4Kscore of 7.5% or more to decide on biopsy resulted in a one-third reduction in the number of men who underwent a biopsy, while missing absolutely no grade group 4 or higher cancers, and only 8.6% of grade group 2 or 3 tumors [6▪].
Although there are no prospective studies evaluating the 4Kscore for long-term endpoints, retrospective studies using prospective cohorts with banked blood and long-term follow-up have shown that men with a 4Kscore of 7.5% or less had a very low risk of prostate cancer metastasis or mortality 15–20 years later [7,8▪].
The 4Kscore has been tested retrospectively in 718 men undergoing active surveillance in the Canary Prostate Active Surveillance Study. The study found that the 4Kscore helped in selecting men who are likely to reclassify to grade group 2 or higher cancer on the first confirmatory biopsy, but not on subsequent biopsies .
Prostate health index
The Beckman Coulter Prostate Health Index (PHI) is a mathematical formula based on concentrations of three PSA isoforms: total PSA (tPSA), free PSA (fPSA) and [-2]proPSA (p2PSA). The Food and Drug Administration (FDA) approved PHI in 2012 for the early detection of PCa as result of a large multicenter prospective validation study involving 892 men with a PSA between 2 and 10 ng/ml . The study found that higher PHI scores were linked to a higher risk of prostate cancer with an AUC of 0.703 for the detection of any cancer, and 0.724 for the detection of grade group 3 or higher prostate cancer.
Although most studies have compared PHI with PSA, Loeb et al evaluated 728 men who underwent biopsy in a multiinstitutional, prospective and found that adding PHI to the prostate cancer risk calculator and European randomized study of screening for prostate cancer risk calculator improved the AUC from 0.577 to 0.697 (P < 0.001) and 0.650 to 0.711 (P = 0.014), respectively for clinically significant prostate cancer detection. The 2019 National Comprehensive Cancer Network guidelines on early prostate cancer detection suggests that a PHI greater than 35 may be helpful in deciding whether to proceed with a biopsy or not in men prior to an initial or repeat biopsy of the prostate [12▪].
We were unable to find any prospective studies of PHI in men undergoing active surveillance. However, several studies looking at PHI in men who were eligible for active surveillance but underwent radical prostatectomy have shown that higher PHI scores were linked to more aggressive histology at surgery, suggesting that PHI may be helpful in detecting who may be at high risk for reclassification. However, its utility in monitoring tumor growth beyond this is unknown .
Prostate cancer antigen (PCA3)
PCA3 is a large-chain RNA molecule, mapped to chromosome 9q21–22 within intron 6 of the PRUNE2 gene, found to be overexpressed in cancerous prostate tissue as compared with benign tissue. PCA3 mRNA level is measured in urine sediment after DRE and the ratio of PCA3 mRNA to PSA mRNA multiplied by 1000 generates the PCA3 score .
The initial validation study for PCA3 was conducted in 233 men with a prior negative biopsy and PCA3 had an AUC of 0.68 versus 0.52 for PSA for detecting cancer . Using a cut-off of 35, PCA3 had a sensitivity of 58% and a specificity of 72%; however, scores between men with indolent versus aggressive prostate cancer were not statistically different.
Although PCA3 is a reliable tool for detection of PCa, studies have noted that there is no correlation with aggressiveness of the cancer or clinical tumor stage . In a multicenter prospective study of 443 men, PCA3 incrementally improved the AUC for prostate cancer detection from 0.79 to 0.83 when added to the European Randomized Study of Screening for Prostate Cancer risk calculator. However, it did not appear to improve the detection of clinically significant prostate cancer. PCA3 is currently FDA-approved in patients with a prior negative biopsy.
PCA3 was assessed in 260 men undergoing active surveillance, where it was found to significantly add to the prediction of grade group reclassification in a multivariable model. Although the authors found that initial and subsequent PCA3 was associated with reclassification, the rate of change in PCA3 score was not, suggesting the PCA3 may be helpful in selecting men who are likely to reclassify, but its use in monitoring tumor progression may be limited .
ExoDx Prostate (IntelliScore) is a liquid biopsy that examines exosomal RNA in non-DRE, first catch urine to detect the presence of grade group 2 or higher prostate cancer. Exosomes are small membrane vesicles that are secreted by many cell types, including tumor cells. The test uses a proprietary algorithm that combines the relative weighted expression of a three gene-signature (Exosomal RNA or PCA3, TMPRSS2: ERG, SPDEF) and gives individual risk scores from 0 to 100, in which a score greater than 15.6 is associated with increased likelihood of grade group 2 or higher prostate cancer. The key validation study involved a 255 patient development cohort in which the gene expression signature was associated with improved discrimination of grade group 2 cancer. The signature was tested on an independent validation dataset consisting of 519 men above the age of 50 years with a PSA of 2–20 who were referred for an initial or repeat biopsy of the prostate. The study showed the IntelliScore improved a clinical model consisting of PSA, age, and family history from an AUC of 0.63--0.73 for detecting grade group 2 or higher prostate cancer. The study reported that a cut-off greater than 15.6, had a negative predictive value of 91% and a sensitivity of 92%, allowing 27% of biopsies to be avoided, with only 8% of high-grade PCa being missed . This cut point was validated further in a independent cohort of 503 men with a PSA between 2 and 10 ng/ml, where it avoided 26% of biopsies, missing only 7% of grade group 2 or higher cancer, giving it a negative predictive value of 89%. The cut-point is undergoing prospective validation for biopsy decision-making in the second phase of this clinical trial [19▪]. We are unaware of any studies of ExoDx Prostate test in an active surveillance population.
SelectMDx is a test that involves measuring DLX1 and HOX6 mRNA levels in post-DRE urine. It was initially described by Leyten et al., and validated by van Neste et al. who collected urine samples from 905 patients in two prospective trials. It has been shown that when combined to clinic factors (PSA, PSA density, family history, and history of prostate biopsy) it has an AUC of 0.90 (95% confidence interval (CI) 0.85 – 0.95) for identifying high-grade PCa (Gleason 7 or above) . It also decreased the rate of unnecessary biopsies by 53% with a negative predictive value of 98% for Gleason greater than 7 . Furthermore, recent literature revealed that it was cost-effective as a stratification tool for initial prostate biopsy in Netherlands [23▪]. We are not aware of any studies evaluating the use of the SelectMDx test in an active surveillance population.
The Mi-prostate score (MiPS) score is a urine-based test from the University of Michigan that combines the measurements of TMPRSS2:ERG and PCA3 in post-DRE urine with clinical information from the PCPT risk calculator. The test reports the quantitative risk of prostate cancer and aggressive prostate cancer. In the primary validation study, the model was trained in 733 men undergoing biopsy at 3 US academic medical centers, and validated in an independent cohort of 1244 men presenting for biopsy at 7 community-based clinics within the United States. The study found the addition of the urinary markers to PSA or the PCPT risk calculator improved the AUC for the detection of overall and aggressive prostate cancer . We are not aware of any studies evaluating the MiPS score in active surveillance.
Since its introduction in the late 1990s, PSA has been the ultimate biomarker, changing the face of prostate cancer from a disease we detect so late we are unable to offer curable therapy to a cancer we find so early we are uncertain what to do with it. Although PSA screening has reduced prostate cancer mortality, it has resulted in a significant number of men undergoing unnecessary invasive testing and unwarranted treatments. In an effort to minimize prostate biopsy and reduce overtreatment, the approach of moving from an elevated PSA directly to prostate biopsy has been challenged. Over the past years, there has been a growing body of evidence supporting the incorporation of biomarkers that can be used as a secondary screening tool after an elevated PSA or abnormal DRE to aid in decision-making regarding the need for a biopsy of the prostate. Although various types of markers have contributed to this space, we focus on molecular markers that are commercially available for detecting clinically significant prostate cancer.
The largest limitation when deciding which biomarker to use is the lack of prospective head-to-head trials comparing the various tests. Furthermore, they all have evidence supporting their claim to improve prostate cancer detection, and each touts itself as better than the other, making it very difficult for the urologist to decide which marker would be most appropriate for their practice. Although all of the markers discussed are noninvasive, some require a digital rectal exam, which usually consists of three to four swipes per lobe. Table 1 provides an overview of the most relevant serum and urine biomarkers currently available.
Both the 4Kscore and PHI test have been shown to predict the occurrence of CSC and to reduce the number of unwanted biopsies. Furthermore, both have been extensively validated in multiple cohorts including men undergoing active surveillance for prostate cancer. In a previous retrospective study of 513 men undergoing biopsy, there was no significant difference in the AUC between PHI and 4Kscore (0.71 versus 0.72) for detecting clinically significant prostate cancer . However, this was a retrospective study, and what is really needed is a multicenter prospective study comparing markers using proposed cut-offs to decide on the need for a biopsy. The SelectMDx, Exosome Dx, and MiPS tests all seem very promising and their focus on more aggressive cancer is in line with contemporary practice; however, they are new and require more validation in diverse cohorts. PCA3 has been primarily used in men with a previous negative biopsy; however, recent studies have suggested a limited utility in distinguishing between indolent and aggressive cancers .
Furthermore, several studies have suggested that even within a given test, the cut-off used in one population may not be the most appropriate cut-off for another population [27,28▪]. Therefore, extensive validation in multiple diverse cohorts is critical to us learning which markers, and their respective cut-offs work the best in each population. Finally, as more markers become available in various modalities, there have been several studies that have combined markers and shown superior performance over using either marker alone. For example, we previously validated our experience using mpMRI and the 4Kscore for deciding on the need for a biopsy, and showed that using both tests together allowed better cancer detection and more biopsy reduction compared with using each test separately [29▪▪]. We are also seeing new markers, such as the 4Kscore and MiPS incorporate their test into clinical risk prediction tools as opposed to stand-alone tests like PHI and PCA3.
Although serum PSA remains the mainstay of prostate cancer screening, we have seen the emergence of several biomarkers, which could serve as a secondary test to help decide on the need for a biopsy. All of these tests have evidence to support their claim to reduce unnecessary biopsies and enhance aggressive cancer detection. However, there is a desperate need for head-to-head comparison to gain a better understanding of which markers perform the best in which populations. Although most of these markers may have a utility in predictive active surveillance patients that are likely to reclassify, their benefit for subsequent tumor monitoring is less certain.
Deciding between the multitude of biomarkers can prove challenging for the clinical urologist. There have been promising studies showing the benefits of incorporation of biomarkers prior to biopsy as an adjunct to elevated PSA or abnormal DRE, for diagnosis as well as in active surveillance. Both 4K and PHI have been associated with increased prediction of clinically significant cancer and potentially reducing unwanted biopsies. Newer tests like SelectMDx, Exosome Dx, and MiPS appear to have greater sensitivity for aggressive cancer, which is the primary intention. The urgent need for more comparative studies between these markers will fill lacunae in existing knowledge and make choices easier for the clinician.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
REFERENCES AND RECOMMENDED READING
Papers of particular interest, published within the annual period of review, have been highlighted as:
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* Maria F. Becerra and Abhishek Bhat have contributed equally to this article and will serve as first co-authors.