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Prolaris: prognostically accurate but of limited utility?

Posted Oct 13 2010 12:00am

According to a media release from  Myriad Genetics , the company’s investigational genetic test (Prolaris™) can accurately predict prostate cancer-specific mortality at 10 years in men known to have clinically localized prostate cancer. This claim  is being made on the basis of data presented earlier this week at the European Society for Medical Oncology (ESMO) annual meeting in Milan, Italy.

Cuzick and his colleagues used the Prolaris genetic test to predict prostate cancer-specific mortality in a cohort of 337 patients who were diagnosed with clinically localized prostate cancer between 1990 and 1996. All diagnoses were made on the basis of pathological examination of “chips” of prostate tissue removed during transurethral resection of the prostate (TURP). These men were all managed “conservatively,” implying that they received no treatment until they required hormone therapy for symptomatic or evident metastatic prostate cancer. Since they were followed over time, their actual prostate cancer-specific mortality is known, allowing confirmation of the predictions of the Prolaris test. Unfortunately, the actual abstract of the presentation by Cuzick is only available to attendees of the ESMO meeting, so we have not been able to see this abstract for ourselves.

The Prolaris test is based on a panel of 46 genes, most of which are involved in cell cycle progression and cell growth. By measuring the capacity of prostate cancer tissue’s ability to divide and grow, the test can be used to assess whether a patient has a relatively indolent or a relatively aggressive form of prostate cancer using standard prostate tumor tissue of the type available to pathologists after TURP or after radical prostatectomy.

Data from the study by Cuzick et al. (“Prognostic value of a cell cycle expression profile score among men with conservatively treated localized prostate cancer”) showed that:

  • Prolaris was capable of accurate prediction of prostate cancer-specific mortality with very high statistical significance (P = 1 x 10-21).
  • 25 percent of patients in this cohort had a “favorable” Prolaris score and  a 98.5 percent probability of surviving their cancer over a 10-year period.
  • 25 percent of patients in this cohort had an “unfavorable” Prolaris score and a 42.4 percent probability of surviving their cancer over a 10-year period.
  • Prolaris was superior to the Gleason score, baseline PSA levels, and other prognostic factors in predicting prostate cancer-specific mortality.

In theory, these data suggest that the Prolaris test should be capable of discriminating between good and poor candidates for active surveillance. However, there may be a fly in the ointment. As yet we are not aware of any data showing that the Prolaris test can make accurate predictions based on tissue from an initial prostate biopsy, but such biopsy-derived tissue is what is available to the pathologist in the vast majority of cases prior to a decision whether to treat or to monitor individual patients. If the Prolaris test can only be used with accuracy on tissue available after a TURP or a radical prostatectomy, this will severely curtail the potential utility of this test.

The “New” Prostate Cancer InfoLink commented on the Prolaris test when it was first introduced , immediately prior to the annual Genitourinary Cancers symposium (in San Francisco earlier this year). At present we continue to take the position that validation of the utility of the Prolaris test will require a clear demonstration that the test can accurately predict prostate cancer mortality based on prostate biopsy tissue. If the test can be used to do that, then its value may well be considerable (although whether the insurance industry will be willing to pay the current asking price is still open to question).

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