Background
Screening modalities for prostate cancer include digital rectal exam (DRE) and prostate-specific antigen (PSA) test at a frequency of every twelve months for men ages fifty and over in effort to detect increase risk for adenocarcinoma of prostate.1 PSA is a reliable immunocytochemical marker for adenocarcinoma of the prostate. However, screening and early treatment of prostate cancer have come under scrutiny due to concerns for overdiagnosis of low-risk cancers.2 Guidelines have been updated due to studies to refine the use of PSA, which remains the primary screening test.1-3 Investigations into adjunctive testing may provide opportunities to avoid biopsies and reduce overdiagnosis.2
IsoPSA is a blood-based single parameter, structure-based assay for improved detection of high-grade prostate cancer. The test utilizes partitioning isoforms of prostate-specific antigen with an aqueous two-phase reagent.4 The test aims to improve specificity by testing specific changes in PSA that arise specifically in cancer cells and would not be affected by conditions such as prostate hyperplasia, inflammation, or age that reduce the specificity of the standard PSA assays.4
Evidence Review
Klein et al.4 2017 was an industry-sponsored multi-centered prospective study that included 261 men scheduled for prostate biopsy due to rising PSA level or suspicious digital rectal exam.4 The study was conducted at five different academic and community centers within the United States over 15 months. The primary study endpoint was the presence or absence of cancer and cancer grade as detected by ultrasound or MRI fusion biopsy. Exclusion criteria included serum PSA<2 ng/ml; recent (<72 hours) prostate manipulation, including DRE; recent (<2 weeks) urinary tract infection and/or prostatitis; recent (<30 days) prostate surgery, urinary catheterization, prostate infarction, or endoscopic evaluation; and other urinary tract malignancy. Out of 434 samples collected, 173 were excluded. Reasons for exclusion included prolonged storage greater than ninety days, canceled biopsies, the breach of sample collection protocol, shipping delays, or not meeting study protocol.
The research aimed to assess two clinical performance objectives: the discriminatory power between malignant and non-malignant prostate conditions and between high-grade and low-grade cancers versus benign pathology. This population had an overall prevalence of 53% for prostate cancer, with 33.7% of cancers considered high-grade. They utilized receiver operating characteristic analysis to determine the discriminatory power measurement. In this preliminary analysis, they utilized decision curve analysis to access the clinical utility of the models by comparing the two extremes, no biopsy to biopsy for all (current patient cohort), and comparison against the modified prostate cancer prevention travel risk calculator 2.0. They reported results were: “for cancer versus no cancer endpoint, the area under the curve (AUC) was 0.79 (95% CI 0.73–0.84) for IsoPSA versus 0.61 (95% CI 0.54–0.67) for total PSA (p < 0.001). For high-grade cancer versus low-grade cancer/benign histology, the AUC was 0.81 (95% CI 0.74–0.86) for IsoPSA versus 0.69 (95% CI 0.61–0.75) for total PSA (p < 0.005)”. Using the pre-selected cutoff to recommend a biopsy, they reported a 48% reduction in false-positive biopsies with IsoPSA. Utilizing a cutoff selected to identify men at low risk of high-risk disease, they reported a 45% reduction in the false-positive rate. They acknowledged that the test was not yet validated. The authors conclude that the IsoPSA outperformed PSA in predicting prostate cancer risk.
The authors acknowledge that clinical and demographic parameters including age, race, and prostate volume could impact the clinical performance of the IsoPSA test was not evaluated in this study which focused on the test’s clinical performance. They also acknowledge weaknesses in the study, including lack of central or standardized pathology review of the biopsies, a lack of distinction between primary and repeat biopsies, and variability in the use of MRI for decisions on the need for in the technique used in biopsies. Additional weakness includes the study design was a prospective trial without controls. This was not designed or powered for superiority, although the authors suggest the test outperformed standard PSA test. Age, race, prostate volume, presence of benign prostatic hypertrophy (BPH) were not evaluated but could impact results. Various pathological labs and methods for biopsy may have some variability that is was not controlled, introducing a risk of bias.
Stovsky et al. 20195 was an industry-sponsored multi-centered prospective validation study. Two hundred seventy-one men scheduled for prostate biopsy due to rising PSA or suspicious DRE over a ten-month period from seven academic and community centers were included. Biopsies were performed under ultrasound or MRI guidance. Exclusion criteria included: serum PSA less than 2 ng/ml; recent (less than 72 hours) prostate manipulation, including digital rectal examination; recent (less than two weeks) urinary tract infection and/or prostatitis; recent (less than 30 days) prostate surgery, urinary catheterization, prostate infarction or endoscopic evaluation; and any other urinary tract malignancy. Three hundred five samples were collected with thirty-four exclusions, including nine due to prolonged storage, forwarded canceled biopsies and fifteen due to PSA <2 (exclusion), and six unrelated, leaving a final cohort of 271.
The primary endpoint was the clinical performance of the test in discriminating power of IsoPSA for the presence of high-grade prostate cancer versus low-grade cancer or benign pathology. Using the data from Klein et al.4 a cutoff value of 17% was selected as the predicted probability of high-grade prostate cancer. The authors concluded using a cutoff of 17% yielded 93% negative predictive value. They calculate that this would reduce unnecessary prostate biopsies by 43%. Using modeling, they estimate that out of 1000 subjects; the test would miss twenty-two high-grade cancers, of which seven would have been Gleason sum 4 + 3 or higher. A sub-analysis of the African American participants showed similar results to the original combined cohort.
Additional analysis on the biopsy technique segregated biopsies done by ultrasound compared to MRI guidance. They observed significant improvement in the IsoPSA AUC when the biopsies were performed under MRI guidance. The number of biopsies done under MRI guidance increased from 4%4 to 41% in this study.
Strengths of the study include using a new patient cohort, the addition of sub-analysis for African Americans and based on biopsy type, test run in the same lab, and pre-specified fixed outcome measurements defined by the previously published study.
Weaknesses of this study include study design as a prospective trial without a control group. While the authors state the test had superior performance to PSA, the trial did not compare the performance of IsoPSA to PSA directly, which would require an appropriately designed and powered superiority study or non-inferiority study to demonstrate at least equally accurate. While the study addressed the test's clinical performance (validity), it did not address the clinical utility. The lack of standardized biopsy technique and lack of central pathology review introduces a risk of bias. Age, prostate volume, presence of BPH were not evaluated but could impact results. The authors acknowledge a significant increase in MRI guidance for the biopsies, which is more accurate than the ultrasound guidance and cannot exclude selection bias.
Cost analysis studies were excluded from evidence review.6
National Comprehensive Cancer Network (NCCN) guidelines recommend considering the use of biomarker tests for men with persistent elevation in PSA between 3 to 10 ng/ml using a validated test in peer-reviewed, multi-site studies using an independent cohort of patients.2,7 At this time, they include Prostate Health Index (PHI), 4Kscore, ExoDx Prostate Test (EPI), PCA3, Confirm MDx, and Select MDx in the guidelines. IsoPSA was not mentioned in these guidelines as of 7/14/2021. The NCCN states the following concerns for additional biomarker tests: lack of validation of the models/algorithms, unclear behavior in other screened populations, lack of clarity regarding the incremental value and cost-effectiveness of these assays. They also bring up concern for gaps in current knowledge regarding upgrading of grade for Group 1 cancer at biopsy at the time of pathological assessment and lack of long-term follow-up of the cohorts to determine whether missed prostate cancers were ultimately detected. The panel concluded they could not recommend their routine use at this time.7
U.S. Preventive Service Task Force Guidelines updated in 2018 recommend PSA screening with a “C” recommendation and did not mention adjunctive screening test.3
