Novel assay may help identify prostate Bx candidates

A simple blood test that measures PSA structure rather than concentration may be more accurate than PSA in identifying men who need a prostate biopsy, according to the results of a study presented at the Genitourinary Cancers Symposium in Orlando, FL.

IsoPSA is a novel structure-based protein assay that uses water as an experimental probe for changes in PSA structure. The assay produces a single quantitative parameter, K, describing the overall composition of the complex PSA isoforms, using step partitioning of proteins into two aqueous phases. The ratio of the overall complex PSA concentrations in the two phases differentiates between patients with high-grade disease and those with benign or low-grade disease.

Interim data from the study testing the use of IsoPSA were presented by Eric A. Klein, MD, chairman of Glickman Urological and Kidney Institute at Cleveland Clinic.

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According to Dr. Klein, it is important to distinguish low-grade disease from high-grade disease because patients with low-grade cancers are generally not treated immediately and are instead placed on an active surveillance protocol. In contrast, high-grade cancers are usually treated since they have the potential to metastasize and be lethal.

Dr. KleinIn the past, PSA levels have been used to determine whether a prostate biopsy is needed. However, according to Dr. Klein, PSA is prostate specific but not prostate cancer specific, so its use can be confusing.

“Most men with an ‘elevated PSA’ actually have benign prostatic hyperplasia and not prostate cancer, such that many men are biopsied unnecessarily. Our data suggests that IsoPSA more accurately identifies those men who actually have cancer so that only those with a high suspicion of cancer will be biopsied,” Dr. Klein told Urology Times.

For the study, the authors obtained 217 plasma samples from patients previously selected for prostate biopsy. Patients had blood PSA between 2.0 ng/mL and 44.0 ng/mL. IsoPSA was evaluated within 90 days of collection against 12-core biopsy results classified as Gleason of 7 or greater (n=72) or other (either benign or Gleason=6; n=145). The prevalence of high-grade disease was 33%.

Next: High negative-, positive-predictive values

High negative-, positive-predictive values

The authors examined a simple two-parameter logistic regression model comprising IsoPSA K value and age as input parameters. They found that compared with PSA, IsoPSA had a very high negative predictive value (96% or greater) for patients with low-risk disease and a very high positive predictive value (79% or greater) for patients with high-risk disease.

According to the data, these results could effectively reduce the number of unnecessary biopsies by 47%, while identifying high-risk patients who could benefit from biopsy.

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Although the data are preliminary, the study shows early feasibility for the use of the assay to identify high-grade disease across the broad spectrum of patients with PSA levels greater than 2.0 ng/mL.

According to Dr. Klein, once the assay becomes commercially available-within the next year or so-it will be a very easy add-on and will be widely available.

Dr. Klein is a consultant/adviser for Berg Pharma and serves on the speakers’ bureau for GenomeDx and Genomic Health. Several of his co-authors have a financial or other relationship with pharmaceutical companies.

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