Peripheral blood rt-PCR assays for detection and prognosis of prostate cancer. (2003)

The application of molecular biology in medicine has led to new, potentially powerful modalities for the diagnosis and staging of cancer. For example, reverse transcriptase polymerase chain reaction (RT-PCR) has been investigated as a potentially new diagnostic and staging modality for prostate cancer. The RT-PCR assay is a relatively simple and rapid, yet sensitive and specific technique that amplifies a targeted, tissue-specific messenger ribonucleic acid (mRNA) sequence. RT-PCR makes possible the detection of small quantities of cells that are currently undetectable in peripheral blood or within other body fluids or tissues. Since Moreno and associates1 and Vessella and associates2 pioneered the use of RT-PCR for the detection of circulating occult prostate cells, there has been a rapid increase in the number of reports in the literature about the use of RT-PCR for detecting cells expressing prostate-specific antigen (PSA) and other prostatic markers, such as prostate-specific membrane antigen (PSMA) and human glandular kallikrein (hK2). This new assay technology has the potential to allow development and assessment of new therapeutic strategies and to help monitor disease activity more accurately. In cases of localized prostate cancer, it could help identify patients who harbor occult micrometastases, sparing them the morbidity of a radical prostatectomy or radiation therapy that would be ineffective, and/or help select those patients best suited for clinical trials of neo-adjuvant and/or adjuvant therapy.

However, while foci of metastatic prostate cancer detectable by conventional modalities (eg, histology, bone scan, or computed tomography scan) are almost always associated with biologically significant advanced disease, the presence of prostate cells in the circulation does not necessarily indicate that clinically significant metastasis has occurred. Therefore, RT-PCR technology places the burden squarely on investigators to demonstrate the clinical and biological significance of a positive result. Some studies have demonstrated a significant predictive value in RT-PCR for PSA or PSMA for both the pathologic stage of prostate cancer3–6 and progression-free survival.7–9 However, the majority of studies have failed to demonstrate any clinically significant role for RT-PCR for PSA or PSMA.10–14 The limited follow-up time and formidable technical problems regarding sample processing and handling may underlie these conflicting results. Recently, Klein and associates (article 1) and Slawin and associates (article 2) reported on the absence of any association between RT-PCR for PSA and/or PSMA and clinical and pathologic characteristics and biochemical progression in patients who underwent radical prostatectomy for clinically localized disease and who had long-term prospective follow-up (median of 31 and 53 months, respectively). Interestingly, Slawin and his team did find that RT-PCR detection of the native hK2 protein was associated with features of biologically aggressive prostate cancer and clinical outcome (article 2). These studies further emphasize the importance of targeting prostate-specific markers that are associated with biologically and clinically aggressive disease.

One possible limitation of most studies to date is that they have relied on the simple qualitative detection of PSA-expressing cells or on semiquantitative techniques, even if measures were taken to increase the sensitivity by using nested PCR techniques or radioactive detection methods. Because target amplification by PCR is exponential in nature, these approaches show no linearity of reaction to the amount of prostatic target mRNA applied. Moreover, virtually all quantitative detection methods published thus far have the disadvantage of not being easily adapted for standardized clinical application in many centers. To overcome these limitations, Ylikoski and associates recently developed highly sensitive, quantitative, internally standardized RT-PCR assays for PSA15 and hK2,16 and evaluated their utility for diagnosis and staging of prostate cancer (article 3).

In view of the findings of the reviewed articles, and the literature in general, the consensus is that at the present time RT-PCR technology is not ready for use in the clinical management of patients with prostate cancer. However, this molecular test is still highly promising. Technological advances such as immunomagnetic enrichment and digital microscopy, among others, may improve the assays and lead to a quantitative, reproducible assay that can specifically detect those select prostate cancer cells that are associated with the development of clinically evident distant metastases, thereby making them more useful in the clinical setting.