# Liquid Biopsy Biomarkers in Metastatic Castration-Resistant Prostate Cancer Treated with Second-Generation Antiandrogens: Ready for Clinical Practice? A Systematic Review

## TL;DR

This review examines liquid biopsy biomarkers in prostate cancer patients treated with antiandrogens, finding that only a few markers have consistent evidence for clinical use.

## Contribution

The study systematically identifies the most promising and well-characterized liquid biopsy biomarkers in metastatic castration-resistant prostate cancer.

## Key findings

- Androgen receptor overexpression and splice variant 7 are among the best-supported biomarkers.
- PTEN, RB1, and TP53 tumor suppressor gene alterations show some consistency.
- Most biomarkers lack sufficient study or show inconsistent results.

## Abstract

Liquid biopsy techniques (circulating and exosomal nucleic acids and circulating tumor cells) represent a convenient method to study the molecular landscape of metastatic castration-resistant prostate cancer. Our systematic review attempted to identify the most promising and best-characterized liquid biopsy biomarkers. We concluded that, despite the large number of studies, relatively few liquid biopsy markers have been studied in sufficient depth and with sufficiently consistent results: nonspecific markers, such as circulating tumor DNA fraction, circulating tumor cell count, as well as androgen receptor splice variant 7 and androgen receptor overexpression and amplification, and potentially also three tumor suppressor genes, namely, PTEN, RB1, and TP53. This highlights the need for more in-depth studies of potentially promising biomarkers, ideally in large and diverse populations.

Background: Second-generation androgen receptor signaling inhibitors are one of the main treatment options in metastatic castration-resistant prostate cancer (mCRPC). Nonetheless, a considerable proportion show limited response to treatment, which indicates the need for convenient, easily accessible predictor biomarkers, a role suited for liquid biopsy. Methods: We conducted a PRISMA-compliant systematic review of four databases (Embase, Medline, Scopus, Web of Science) to identify all studies (observational studies and clinical trials) investigating cell-free DNA, circulating tumor cells, exosomes, and circulating RNAs as prognostic markers in metastatic castration-resistant patients starting androgen receptor signaling inhibitors. We excluded studies that evaluated combination therapies, rare histological subtypes or included nonmetastatic or castrate-sensitive disease. We also evaluated whether published papers followed reporting guidelines (REMARK, STROBE, or CONSORT for abstracts). Results: We identified a total of 123 reports, from which we identified only a few well-studied and consistent biomarkers: androgen receptor overexpression/copy number gain and splice variant 7, as well as disease burden markers (circulating tumor DNA fraction and circulating tumor cell concentration). Alterations or copy number loss in tumor suppressors PTEN, RB1, and TP53 were second in terms of quantity and consistency of evidence. However, a large majority of identified biomarkers were relatively understudied or inconsistent. We identified two potential vulnerabilities: inconsistent adherence to reporting guidelines and the under-inclusion of patients of non-Western European ancestry. Conclusions: A large number of biomarkers were linked to worse outcomes in prostate cancer; nonetheless, in most cases, the evidence is limited or inconsistent, or even contradictory. The main exceptions pertain to androgen receptor signaling and disease burden, and, to a smaller extent, to certain tumor suppressor genes. Further studies are needed to confirm their clinical utility, using clear and consistent methodologies and including patients from currently understudied populations.