Prostate cancer diagnostic options: past, present, and future

Prostate cancer (PCa) has one of the highest incidence rates in the developed world, affecting one in seven men.¹ Although it is the second highest cause of cancer-related deaths in the United States, only one in 38 of those affected will die from the disease.² Five-year survival is among the highest for all cancers—almost 100 percent for locally constrained cancer (93 percent of all cases), but dramatically lower following metastasis at only 28 percent.¹ In the U.S., 2.7 million men live with a positive PCa diagnosis.

Introduction of the prostate specific antigen (PSA) test and ultrasound-guided biopsy in the 1990s markedly increased PCa detection, largely due to detection of occult disease.³ The PSA test is still the only routinely used biomarker in PCa diagnosis, staging, and treatment response. The biggest risk factor for PCa is age, with detection rates increasing significantly in men over 50 years of age. In fact, PCa is most commonly diagnosed in men from 65 to 75 years of age.

Approximately 20 percent of PCa-related deaths occur in that age group, with 37 percent in 75-to-84 year olds and 33 percent in men 85 and older. Strikingly, cancerous cells were found in approximately 60 percent of 65-to-75 year olds and in 70 percent of 75-to-85 year olds in autopsy studies following non-related deaths.⁴ In fact, the majority of PCas develop extremely slowly over decades, and most men will die of other causes before their PCa becomes clinical relevant. Unfortunately there is currently no way to differentiate between the indolent form of PCa and the less common aggressive forms, which means that over-diagnosis and treatment of clinically non-relevant disease is a major issue. The search for a good way to differentiate is an ongoing concern for oncologists.

Prostate specific antigen as a biomarker

PSA is a glycoprotein that was discovered and isolated in the late 1970s at the Roswell Park Cancer Institute.⁵ It is produced by normal prostate epithelial cells and is elevated in PCa. However, levels also increase with age and can increase with certain benign conditions–notably benign prostate hyperplasia (BPH) and prostatitis (inflammation of the prostate). Impairment of the tissue barrier around the gland lumen allows PSA to enter the bloodstream. After several years of assay development and clinical validation,^6,7 PSA was accepted as a serum test for monitoring of treatment and disease progression (approved by the FDA in 1986). Major simplification in transrectal ultrasound (TRUS) guided biopsy procedures heralded the adoption of PSA as a screening aid for diagnosis by biopsy of PCa. The FDA approved this new application in 1994, and it gained the support of various professional bodies despite reservations over lack of clinical evidence.^9,10 PSA testing had a significant impact in diagnosing early, organ-confined disease¹¹ despite low sensitivity and specificity.

A threshold of greater than four nanograms per milliliter (ng/mL) was adopted for recommending biopsy. At this threshold, detection rates of 80 percent at around 50 percent specificity (i.e., a 20 percent “miss” rate with 50 percent false positives) are better than digital rectal exam (DRE) alone or transrectal ultrasound, but around two-thirds of men with elevated PSA have negative biopsies (mainly due to BPH). The gold standard needle biopsy has a miss rate of 10 to 20 percent, although multiple biopsies have reduced this rate. Around 15 percent of cancers can be missed using the standard threshold;¹² which has prompted recommendations to lower threshold levels (2.5-3.0ng/mL) to enhance sensitivity, albeit at the expense of specificity.

Enhanced PSA testing

With more than 40 million global PSA tests conducted annually, the high false-positive rate results in a large number of unnecessary biopsy procedures, and a number of improvements have been investigated to reduce this burden. Upper limit PSA reference values are now age-adjusted, increasing the threshold by select amounts for men of every decade between ages 40 and 80. PSA was shown to exist in free and complexed forms with a ratio of free:total form greater than 25 percent indicating a much lower risk of PCa. Conversely a ratio less than 10 percent of free:total forms indicates a much higher risk. The combination of free PSA and total PSA was approved for cancer detection by the FDA in 1998 and is useful in cases with moderately elevated total PSA (4.0-10.0 ng/mL).¹³

Multiple isoforms of free PSA have been identified, most recently using mass spectometry.¹⁴ proPSA is elevated in cancer but not benign disease whereas BPSA, discovered later, is associated with benign disease. A third form, inPSA, is inversely correlated with proPSA and is elevated in the absence of cancer. Clinical utility of these isoforms remains to be proven.¹⁵ PSA density (adjusted for prostate volume determined by ultrasound) can improve test accuracy at intermediate PSA levels,¹⁶ and PSA velocity, change in PSA level over time, has also been examined to differentiate tumor from benign disease.¹⁷ The National Comprehensive Cancer Network (NCCN) and American Urology Association (AUA) recommend that biopsies be considered by men with PSA velocities greater than 0.35ng/mL per year, but recent data suggests further unnecessary biopsies would offset benefits.¹⁸

Controversy over PSA screening

Despite improving early diagnosis, the low specificity of PSA for PCa versus benign disease coupled with the inability to stratify aggressive and non-aggressive forms of the disease result in a high false positive rate and over-diagnosis of clinically non-relevant disease. This results in patient anxiety and morbidity due to unnecessary biopsies and over-treatment for low-risk cancer. Potential long-term side effects include impotence, incontinence, and gastrointestinal tract damage. Recent data from the prospective Prostate, Lung, Colorectal, and Ovarian (PLCO)¹⁹ trial in the U.S. and the European Randomized Study of Screening for Prostate Cancer (ERSPC)²⁰ both showed increased cancer detection with screening. However, no cancer-specific mortality benefit related to PSA screening (at 13 years follow-up) was seen in the U.S. trial, and only a marginal benefit was seen (at 11 years follow-up) in the EU trial.²¹ These results prompted the American Cancer Society, which does not advocate PSA screening, to include warnings on the limitations of PSA screening with a recommendation that patients receive information on the uncertainties and risks as well as potential benefits.²² Despite this, PSA testing in the U.S. remains popular, whereas in Europe the lack of clinical evidence and risk from potential harms argue against routine screening.²³

Secondary screening biomarkers

Additional biomarkers are being evaluated as secondary screening tools to reduce unnecessary biopsies. Recent data from a cohort of nearly 2,000 men with PSA greater than 2.5ng/mL showed that PCa Gene 3 (PCA3) testing of prostate cells released into urine after manual prostate massage could reduce false positives by 35.4 percent with a marginal increase in false negatives (5.6 percent). 24 IL-8, TNF-[alpha] and particularly sTNFR1 have also been identified as potential markers for biopsy negative individuals with high PSA levels. Further prospective trials will be required for validation.²⁵

Future perspectives

The emerging field of epigenetics has opened up new approaches to diagnosing disease. Put simply, epigenetics is the study of non-gene sequence-based changes in phenotype and gene expression. Examples include DNA methylation mediated gene silencing, post-translational modifications of histone proteins for dynamic chromatin structural regulation, and miRNA mediated transcriptional silencing of complementary mRNA.²⁶ Importantly, epigenetic changes can precede tumorogenesis, potentially allowing early diagnosis,²⁷ and tumor-specific mutation profiles have been identified in biopsy material. One of the first clinical applications is the assessment of DNA methylation status of GSTP1, APC, and RASSF1 genes to confirm negative prostate biopsy results (90 percent negative predictive value).²⁸ Methylated GSTP1 has also been detected in the plasma of PCa patients with a sensitivity of 80 percent and a specificity of 82 percent, highlighting the potential for a future blood test rather than a biopsy.²⁹ MicroRNA profiling using PCR arrays identified miR-205 and miR-214 as being down regulated in the urine of PCa patients (n=40) compared to healthy individuals (89 percent sensitivity, 80 percent specificity).³⁰ 

Global levels of epigenetic modifications (as opposed to gene-specific alterations) can also provide diagnostic information.³¹ When cells die, the nuclear material fragments and nucleosomes—147 base pair sequences of DNA wrapped around four pairs of histone proteins—are released into the bloodstream.³² These nucleosomes can be quantified and epigenetic information contained within them can be profiled and correlated with specific diseases. The term histo-oncoproteins has been proposed for histone modifications linked to cancer.³³ For example, methylation of lysine residues at position 27 of histone 3 in cell free nucleosomes has been shown to be reduced significantly in patients with metastatic disease.³⁴ Immunoassays for detection of various nucleosome-associated epigenetic features are being evaluated in large-scale clinical trials for diagnosis of various types of cancer and have shown potential to differentiate cancer from benign disease.³⁵

Whatever the future holds for prostate cancer diagnosis, it will require extensive clinical validation of robust biomarkers that can clearly identify aggressive forms of prostate cancer. Given the potential demand for screening, affordability and access will also be keys to consider.

References

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