The serum biomarker most widely used to test for prostate cancer initially and to monitor patients with the disease is prostate-specific antigen (PSA). This article discusses the limitations of this approach and introduces a more effective method for prostate cancer testing.
by Dr B. Cook
The dilemma of routine prostate cancer testing and the arguments for and against national screening programmes have been highlighted by the recent findings of the European Randomised Study of Screening for Prostate Cancer (ERSPC) [1]. The study reported that prostate-specific antigen (PSA) screening reduced mortality from prostate cancer by 20 percent, a figure that rises to 30 percent once the data are adjusted for contamination and non-attendance [2].
Over-diagnosis is often said to be an inevitable effect of cancer screening procedures. However, it is a statistically significant factor in the case of prostate cancer because of problems arising from the lack of specificity in differentiating cancer from benign disease. Separate ERSPC findings confirm that approximately 30 percent of detected cancers actually display non-aggressive features and are ‘indolent’ or slow growing. Such patients will therefore die from other causes rather than prostate cancer [3]. Treatment for the disease may have a profound effect on the quality of life of patients and their families. The present challenge is to find a way of discriminating between cancer and benign disease at an early stage in order to avoid unnecessary treatment.
Worldwide, prostate cancer is now the second leading cause of cancer death in men and its incidence increases with age. The risk rises sharply as men reach the age of 50; two out of three cancers found in men over 65 are prostate cancer. The highest levels are currently found in the United States – perhaps because more ad hoc PSA testing is carried out. However, it is the most common form of malignancy in Europeans; the incidence over the last 20 years has increased faster more for prostate cancer than any other cancer. According to the World Health Organisation, about 190,000 new casesare diagnosed each year.
PSA testing and its limitations
The serum biomarker most widely used to test for prostate cancer initially and to monitor patients with the disease is prostate-specific antigen (PSA). However, serum PSA is often elevated in non-cancerous conditions such as benign prostatic hyperplasia (BPH) and prostatitis, so the test lacks specificity, especially at low concentrations [4]. Low specificity means that large numbers of patients who do not have prostate cancer currently require a biopsy to confirm or rule out cancer. Yet only 20 to 30 percent of men with serum PSA levels from 2 to 4 ng/mL and 30 to 45 percent with serum PSA levels from 4 to 10 ng/mL have a diagnosis of prostate cancer confirmed when they undergo a prostate needle biopsy [5]. This means over two thirds have to undergo an invasive test unnecessarily. This is not only stressful but a waste of valuable resources. Up to half of men diagnosed with prostate cancer after PSA testing and biopsy have indolent disease, where treatment is unnecessary and any they do receive may be detrimental to their quality of life.
With liited ability to discriminate between cancer and benign disease, screening for prostate cancer with PSA remains contentious. While it can help detect some life-threatening tumours, the argument against it is the risk of patients suffering unnecessary and potentially harmful tests and treatment.
Background to ERSPC Study
The ongoing ERSPC study is the world’s largest study into prostate cancer screening, involving a total of 162,000 men in seven countries. They were initially screened for total PSA and followed up over 12 years. Total PSA was measured using the Beckman Coulter Hybritech PSA assay. From 1994 to 2000, the Hybritech Tandem E assay was used. Beckman Coulter migrated the Hybritech assay from the manual Tandem assay to the automated Access immunoassay system, (which provides equivalent results). After 2000, the Beckman Coulter Access assay was used for ERSPC, with the original Hybritech calibration applied to prevent analytical bias [1].
proPSA isoforms
Beckman Coulter has been actively collaborating with some of the world’s leading prostate cancer researchers to help find more specific biomarkers. It has been noted from past research how use of the more specific percentage of free PSA (%fPSA) can offer significantly improved cancer detection rates in the 4 to 10 ng/mL range [6].
Free PSA is made up of several distinct molecular forms. These are enzymatically inactive forms of PSA. Their presence appears to be diagnostically significant, indicating an association with either BPH or prostate cancer [7]. For example:
• proPSA is elevated in cancer tissue [8] and serum [9] in cancer.
• benign PSA (BPSA) and intact PSA (iPSA) are more associated with BPH [10].
Several forms of proPSA have been identified according to the length of the remaining pro-leader peptide. The seven amino acid pro-leader peptide form, [-7] proPSA, is cleaved by human kallikrein 2 and trypsin to yield active PSA. Truncated forms of this peptide (containing leader sequences of five, four or two amino acids) are also present and can be measured using specific immunoassays.
Beckman Coulter has identified several isoforms of free PSA in the blood of prostate cancer patients. When tested using a unique monoclonal antibody (MAb), an increase in individual isoforms – in particular [-2]proPSA – was associated with prostate cancer [9, 10]. The use of this antibody demonstrated greater immunohistochemical (IHC) staining in prostate tumour tissue than in benign prostate tissue [11].
p2PSA - a promising marker
This increased specificity of the [-2]proPSA isoform, measured with the Access Hybritech p2PSA assay*, demonstrates its potential role in the detection of prostate cancer when assessing elevated levels of PSA [12]. Several retrospective studies have shown that ratios of total proPSA and [-2]proPSA to fPSA (%proPSA and %[-2]proPSA, respectively) were more cancer-specific than combinations of total PSA and fPSA [12].
[-2]proPSA levels have been correlated with clinically significant cancer, including the more advanced pathologic stage, higher tumour volume, and higher tumour grade. [-2]proPSA has also been shown to improve discrimination where there is particular diagnostic uncertainty, such as patients with %fPSA > 25 and total PSA levels of 2 to 4 ng/mL [9]. All these studies used archived samples.
Levels of proPSA, alongside DNA content, can also point to patients who should be monitored. In a group of 71 men who underwent annual surveillance biopsies, the ratio of serum [-2]proPSA /fPSA was found to be significantly higher in those who had a poor biopsy result (Gleason score ≥7). The [-5,7]proPSA staining was greater in the benign area adjacent to the biopsy in these 39 men. DNA content – excess of optical density (OD) in the benign-adjacent area – and the standard deviation of optical density in the cancer area, were also significant predictors of an unfavourable biopsy [13]. These studies used archived samples.
The challenge has been to find ways of improving the specificity of biomarker assessment when clinicians are faced with elevated PSA levels. With this in mind, Beckman Coulter has developed a mathematical formula called Prostate Health Index (phi), which combines readings of total Access Hybritech PSA, fPSA and p2PSA. It has been validated in men with PSA levels from 2 to 10 ng/mL and is available in Europe, beginning this month (November 2009).
The new p2PSA test is easily performed, along with traditional PSA and free PSA tests, using Beckman Coulter‘s family of Access immunoassay systems. When the Prostate Health Index is installed, the analyser automatically computes the Index using the PSA, free PSA and p2PSA test results. The phi result gives a good indication of the probability of prostate cancer, providing a significant increase in specificity over total or free PSA alone [Table 1].What patients do not always appreciate is that many prostate cancers (a significant proportion of which are high grade) are found in men with PSA levels considered to be in the “normal” range of 2 to 4 ng/mL [Table 2], [14]. In fact, biopsy-detected prostate cancer is not a rare occurrence among men with these PSA levels. A test with a high specificity for prostate cancer, even in subjcts who fall within the range of 2 to 4 ng/mL, has the potential to identify early-stage cancer in these men.
Effectiveness of p2PSA and Prostate Health Index (phi)
The first prospective study looking at the effectiveness of p2PSA and phi for prostate cancer screening in men is currently ‘in press’. It has examined the relationship between p2PSA and prostate cancer detection in 2034 men undergoing prostate screening. The study assessed the accuracy of total PSA, the ratio of free PSA (fPSA) to total PSA (%fPSA), the ratio of p2PSA to fPSA (%p2PSA) and the phi formula in predicting prostate cancer among men from the study with PSA levels of 2.5-10 ng/mL and non-suspicious digital rectal examination (DRE) who underwent prostate biopsy. Results will be generated using the Access Hybritech PSA, free PSA and p2PSA assays.It is anticipated that the study will help confirm the promise of p2PSA in improving discrimination between prostate cancer and benign disease even amongst subjects with similar total PSA levels. Maintaining the clinical sensitivity for prostate cancer at 88.5 percent, the study will show whether p2PSA offers improvements in specificity, positive predictive value and negative predictive value using p2PSA as a percentage of free PSA. The overall performance characteristics of the Prostate Health Index formula using results from the three markers is also being assessed.
Role for biomarkers in reducing overdiagnosis
Prostate cancer is a major cause of mortality for men and the risk of it increases with age. While PSA screening has been shown to reduce deaths from prostate cancer by up to 31 percent [2], the test is also associated with overdiagnosis and potential overtreatment. An elevated PSA level is an indicator for further investigation but it does not discriminate between benign disease and cancer. Patients may still require an invasive biopsy for a more definitive diagnosis that carries with it the risk of side effects. To justify the cost of screening, research has been taking place to identify newer, more specific markers.
It is well established that the use of free PSA (%fPSA), rather than total serum PSA, significantly improves cancer detection rates in the 4 to 10 ng/mL range [6]. Measurement of levels of proPSA isoforms, which are specifically associated with prostate cancer – and in particular serum p2PSA – are expected to improve detection accuracy even further because this marker is better able to discriminate between cancer and benign disease [12].When combined with Access Hybritech PSA and free PSA in the new phi index, p2PSA promises to provide far greater accuracy in assessing the need for a prostate biopsy, particularly at low levels of total PSA (in the 2-10 ng/mL range) – by increasing the clinical specificity for prostate cancer. This has the potential to reduce the number of unnecessary biopsies. This, in turn, should increase confidence in PSA testing.
*In development for U.S. market, pending FDA approval
References
1. Schröder FH et al. New Engl J Med 2009; 360:1320-1328
2. Roobol MJ et al. Eur Urol 2009; 56: 584-591.
3. Draisma G et al. J Natl Cancer Inst 2003;95:868-878
4. Sokoll LJ et al. J of Urology 2008;180:539-546.
5. Roehl KA, Antenor JA, Catalona WJ. J Urol 2002; 168: 922.
6. Catalona WJ et al. JAMA 1998; 279: 1542,
7. Mikolajczyk SD et al. Urology 2002: 59: 797.
8. Mikolajczyk SD et al. Cancer Res 2000; 60: 756.
9. Mikolajczyk SD et al. Cancer Res 2001; 61: 6958.
10. Khan MA et al. Urology 2004; 64: 1160.
11. Chan TY et al. Urology 2003; 62: 177
12. Catalona WJ et al. J Urol 2003; 170: 2181.
13. Poster presentation, American Urology Association: annual meeting, Chicago, May, 2009.
14. Thompson IM et al. J of National Cancer Institute 2006 Apr 19;98 (8):529-534.
The author
Dr Bernard Cook
Group Manager Scientific and
Professional Relations
Immunodiagnostics
Beckman Coulter, Inc.
Fullerton, CA, USA
