Platinum Priority – Prostate CancerEditorial by Anders Bjartell on pp. 905–907 of this issueTMPRSS2-ERG Status in Circulating Tumor Cells as a Predictive Biomarker of Sensitivity in Castration-Resistant Prostate Cancer Patients Treated With Abiraterone Acetate
Introduction
Abiraterone acetate (AA) is a 17α-hydroxylase/C17,20-lyase (CYP17) inhibitor that blocks androgen synthesis in the testes and adrenal glands and in prostate cancers. AA has been shown to prolong survival in men with castration-resistant prostate cancer (CRPC) who have progressed after chemotherapy with docetaxel, and AA was recently approved by the US Food and Drug Administration (FDA) for this indication [1]. The results validate earlier profiling studies showing that overexpression of androgen biosynthesis pathway enzymes and increased intratumoral androgen levels were frequent alterations in CRPC. Notable in the trials was the pattern of posttreatment prostate-specific antigen (PSA) changes, which ranged from dramatic declines in some patients to continued elevations in others. This wide variation in PSA-decline rates suggested the presence of predictive molecular markers of sensitivity in the tumor [2], [3], [4], [5], [6], with a number of biomarkers being available for evaluation. Analogous to the sequence of trials used to evaluate new therapeutic approaches is the need for defined metrics to determine which biomarkers warrant testing in large-scale phase 3 trials.
Transmembrane protease, serine 2 (TMPRSS2)–v-ets erythroblastosis virus E26 oncogene homolog (ERG) fusion, first identified in 2005, is created by the translocation of the androgen-driven 5′ TMRPSS2 chromosomal region to the ETS transcription factor family member ERG(21q22.2) [7]. This fusion, present in approximately 50% (range: 30–70%) of newly diagnosed prostate cancers [7], [8], [9], [10], represents >90% of the identified ETS translocations. In experimental models, the TMPRSS2-ERG fusion has shown a limited role in prostate tumorigenesis [9]. In clinical studies, presence of the fusion was associated with low-grade disease [11] but not with higher risk of biochemical recurrence, metastases, or death [9], [10]. A role in androgen-dependent tumor growth has been postulated [12], and a relationship between TMPRSS2-ERG status and degree of PSA decline was shown in chemotherapy-naive CRPC patients treated with AA, suggesting a role as a predictive biomarker of AA sensitivity [13].
For prostate cancer in particular, establishing the clinical significance of a molecular determinant is hindered by the difficulty of obtaining representative tumor samples for analysis in a routine clinical practice setting. Circulating tumor cells (CTCs) isolated from peripheral blood have the potential to fulfill this unmet need. Before clinical utility can be established, it is essential that a robust assay that gives consistent results be available [14]; this process is termed analytic validation and is outlined in the FDA Critical Path [15].
This report describes the development and analytic validation of a sensitive polymerase chain reaction (PCR)–based assay to detect TMPRSS2-ERG fusions in CTC and explores the relationship between fusion detection and clinical outcome of patients treated in phase 2 AA trials at Memorial Sloan-Kettering Cancer Center (MSKCC).
Section snippets
Patients
Samples were obtained from 48 men with progressive metastatic CRPC following docetaxel-based chemotherapy who were treated at MSKCC in two sequential phase 2 trials with AA 1000 mg plus prednisone 10 mg daily and continuously until clinical or radiologic progression [2], [4], according to PSA Working Group 1 criteria [16]. Declines in PSA were illustrated by waterfall plots [17]. Informed consent was obtained prior to the initiation of institutional review board–approved protocols.
Laboratory methods
CTCs were
Standard procedures for specimen collection and handling
Blood samples were collected prior to drug administration, first in CellSave tubes for CTC enumeration (n = 48) and second in an EDTA tube for RNA profiling (n = 41). Samples were delivered and processed in the clinical chemistry laboratory within 4 h.
Assay performance characteristics
The intra-assay coefficient of variation (CV) was 0.34% (n = 1152). Repeat experiments performed in triplicate by different analysts at different time points showed an interassay Pearson correlation coefficient of 0.994, while the interassay CV across a
Discussion
The revolution in cancer genomics is rapidly changing the field of cancer diagnostics and therapeutics, bringing us to an era of personalized medicine in which treatment selection is based on the molecular characteristics of an individual patient's tumor. Achieving this objective in clinical practice requires the identification of predictive biomarkers of sensitivity, the validation of assays to measure the biomarker, and, separately, prospective clinical trials designed to qualify the
Conclusions
A significant proportion of patients have the prostate cancer–specific TMPRSS2-ERG fusion product present in CTCs. These results demonstrate the feasibility of CTCs as an easily obtained tissue for molecular analysis such as the detection of kinase mutations in other tumor types [29], [30]. The results also illustrate the importance of developing standards for biomarker development that include establishing the performance of the assay itself, followed by a prospectively planned sequence of
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