Utility of Prostate-Specific Antigen Isoforms and Prostate Health Index in the Diagnosis of Metastatic Prostate Cancer

Introduction

Prostate cancer (PCa), the second most frequently diagnosed cancer accounting for more than 1.4 million new cases in 2020, and the fifth leading cause of mortality worldwide, poses major challenges in prevention and management.^[1,2] Despite advances in diagnosis and treatment, the incidence of PCa has seen a significant increase in recent years and is the third most common cancer among Indian males.^[3,4] According to the report from the National Cancer Registry Program, the cumulative risk of developing PCa is 1 in 125 with a projected incidence of 41,532 cases in 2020.^[3] Changes in lifestyle, increased screening, and increase in life expectancy are the major reasons for the rise in PCa incidence.^[5,6]

In early stages, PCa can run an indolent course, emphasizing the need to dissociate diagnosis from treatment and consider active surveillance (AS) for men with low- or intermediate-risk disease.^[7] Hence, the major challenge in PCa treatment is to detect high-risk individuals who could not be managed with AS but require immediate aggressive treatment within the window of curability.^[8] In this context, measurement of serum total prostate-specific antigen (tPSA) plays a crucial role in the diagnosis of high-risk PCa.^[9,10]

The National Comprehensive Cancer Network (NCCN) defines high-risk PCa as PSA greater than 20 ng/mL or grade group 4 or 5 or clinical T3a.^[11] The different treatment options available for high-risk PCa include radical prostatectomy (RP),^[12] external beam radiotherapy (EBRT) with androgen deprivation therapy (ADT) or EBRT plus brachytherapy and ADT,^[13,14] all of which have their own benefits but may be associated with posttreatment unwarranted effects on the quality of life. Hence, management of high-risk PCa involves making a choice between AS and all the available treatment options, and this is usually a shared decision taken by the clinical team and the patient/patient caregivers or family. Making this decision based on only few variables such as PSA levels, biopsy findings, and clinical staging may prove to be difficult. Hence, there is a need for more multivariable approach using several informative markers. This is the key for the development of newer markers for diagnosis of high-risk metastatic PCa.

In the blood, PSA is predominantly (70–90%) bound to serum protease inhibitors, and around 10 to 30% exist in free state—free PSA (fPSA).^[15] Among the fPSA forms, isoform [-2] proPSA (p2PSA), primarily found in tumor extracts,^[8] plays a key role in early detection and in the prediction of aggressive disease.^[16] Consequently, the prostate health index (PHI) that combines all the three forms of PSA (tPSA, fPSA, and p2PSA) to give a single score using the formula: (p2PSA/fPSA) × √PSA)^[17] has been studied across the globe for its usefulness as a diagnostic and prognostic test of PCa.^[8,17-19] In multiple prospective international trials,^[17] PHI was shown to outperform tPSA and fPSA measurements in early diagnosis of PCa. PHI is also associated with an increase in Gleason score following RP, hence useful as an additional test during AS.^[8,12,20]

Although several studies across different ethnic groups in various countries have unequivocally demonstrated the reliability of PHI for diagnosis of high-risk metastatic PCa, there is paucity of information with reference to the Indian population. The current study was therefore undertaken to investigate the utility of measuring PSA, p2PSA (%p2PSA), and PHI in the diagnosis of metastatic PCa.

Materials and Methods

The present study was performed between March 2016 and May 2019, in a Joint Commission International accredited Hospital in Bengaluru, Karnataka, India, whose laboratory services were accredited by the National Accreditation Board for Testing and Calibration Laboratories (NABL). The study protocol was approved by the Institutional Ethics Committee Clinical Studies (vide letter # 001/02-16 dated February 29, 2016).

Results

Of the 85 patients, 81.2% (n = 69) presented (identified) with evidence of metastasis and the remaining (n = 16) did not show any evidence of metastasis at the time of diagnosis of PCa.

The distribution of the patients in different age groups is given in ►Table 1. The age group of 71 to 80 years had maximum cases of metastasis accounting for 40.5% of the total 69 cases of metastatic PCa. Interestingly, this age group has also recorded maximum cases of nonmetastatic PCa accounting for 50% of this cohort (►Table 1).

The median tPSA (ng/mL), p2PSA (pg/mL), %p2PSA, and PHI values in all the quartiles were significantly higher in the group with evidence of metastasis. The median %fPSA values in all quartiles was significantly lower in the group which showed evidence of metastasis (►Table 2).

The validity indicators, namely, sensitivity, specificity, negative predictive value (NPV), and positive predictive value (PPV) were calculated for the ability of the tests to diagnose metastatic high-risk PCa in comparison to the histopathological and imaging studies which acted as the gold standard based on which the presence of metastasis had been confirmed (►Table 3).

In our study, when we used a cutoff value of 20 ng/mL for tPSA based on the NCCN criterion for diagnosis of high-risk PCa,^[11] tPSA had a sensitivity of 92.7% and specificity 37.5% with NPV of 54.5% and PPV of 86.4% for diagnosis of metastatic PCa.

For PHI, when 55 was considered as cutoff based on previous studies,^[24] PHI had a sensitivity of 98.5% and specificity of 43.7%, while NPV was 87.5% and PPV was 88.3% for diagnosis of metastatic PCa. The area under the receiver operating curve (AUC) was 0.892 with p-value less than 0.001 (►Fig. 1). The Youden index J was 0.69 when the criterion of PHI greater than 90.16 was used with a sensitivity of 87.06 and specificity of 82.2 with 95% confidence interval (CI) of 78.0 to 93.4 and 78.1 to 85.8, respectively.

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Fig. 1

Area under the receiver operating curve (AUC) for prostate health index (PHI).

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When we used a threshold value of 1.66 for %p2PSA,^[25] it had a sensitivity of 94.2% and a specificity of 62.5% with NPV of 71.4% and PPV of 91.5%. The AUC for %p2PSA is shown in ►Fig. 2. The AUC was 0.828 with a p-value of less than 0.001. The Youden index was found to be 0.60 when the criterion of %p2PSA greater than 2.56 was used with a sensitivity of 77.65 and specificity of 82.68 with 95% CI of 67.3 to 86.0 and 78.7 to 86.2, respectively.

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Fig. 2

Area under the receiver operating curve (AUC) for percentage of isoform [-2] proPSA (%p2PSA).

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Discussion

The purpose of this study was to investigate the utility of estimating PSA isoforms and PHI in the diagnosis of metastatic high-risk PCa, in a cohort of Indian population. Majority of the patients in our study showed evidence of metastasis and the medical records of these patients revealed that the sites of metastasis include regional lymph nodes, bone, distant lymph nodes, lungs, and liver in descending order of frequency. Most of these patients belonged to the age group 71 to 80 years similar to findings documented in the literature.^[2]

All the biomarkers evaluated in this study were significantly elevated in the group which showed evidence of metastasis. Previous studies^[26,27] have shown that among the PSA isoforms, the production of p2PSA is selectively increased in cancer and is significantly associated with high-grade PCa at RP, and that higher %p2PSA may be regarded as a diagnostic marker for clinically significant PCa.^[28] In our study, %p2PSA was significantly increased in patients who displayed evidence of metastasis when compared with patients without metastasis. Moreover, %p2PSA with a cutoff of 1.66 turned out to be a better marker both in terms of sensitivity and specificity for metastatic PCa when compared with tPSA and PHI (►Table 3). In a previous meta-analysis study,^[26] which showed that %p2PSA along with PHI could detect more aggressive PCa at the initial prostate biopsy, the AUC for %p2PSA was 0.54. We speculate that a value of 0.82 for the AUC of %p2PSA obtained in the present study could be because majority of the patients presented with metastasis and for the same reason, the %p2PSA turned out to be a better biomarker in terms of specificity of metastatic PCa than tPSA and PHI.

The PHI developed by Beckman Coulter, Inc. is a mathematical formula and uses three biomarkers, p2PSA, fPSA, and tPSA to give an index number. Although the PHI test has regulatory approval in more than 50 countries worldwide, its use in India is limited and used sparingly for either screening or in predicting the aggressiveness of PCa.^[27] Discrepancies exist between clinical cancer staging and pathological staging.^[29] There are few studies which have used PHI in this clinical context, but some studies have shown that among patients with biochemical recurrence, p2PSA and PHI were significantly higher in men with metastatic PCa compared with those without clinical metastasis.^[17,29] In our study also, PHI was significantly elevated in patients presenting with metastasis compared with those without any evidence of metastasis.

There has been no consensus on the most appropriate cutoff value for the %p2PSA and PHI in cancer detection or for predicting clinically significant PCa due to the use of different study designs.^[30] So, when we used a threshold of 30 for PHI as recommended for Asian population,^[31] the specificity of PHI in our study decreased to 25%, with 98% sensitivity and hence according to our study results, a cutoff value of 55 for PHI as recommended worldwide would be better suited for Indian population. When compared with tPSA, PHI was more specific at this cutoff value and may be useful as an adjunct marker for risk stratification at initial diagnosis and in predicting the possibility of metastasis.

This study, which is one of its kind performed among the Indian population adds to the information that is available on %p2PSA and PHI besides underscoring the potential utility of these novel biomarkers in understanding the impact of age, ethnicity, and extent of disease. However, this study is relatively small sized, and there are limitations to generalize the findings to the entire population. Future studies are necessary to further evaluate %p2PSA and PHI for their biological reference ranges and validate their role with appropriate cutoff values in the management of more advanced disease.

Conclusion

Since many treatment options are available including AS for the management of PCa especially when there is a possibility of metastasis, using a multimodal approach to take treatment decisions will be useful rather than based on only pathological and clinical staging. Using many noninvasive or minimally invasive tests such as %p2PSA and PHI as a part of the standard armamentarium will give additional information that will help in the decision-making process for treatment selection for PCa.