Multiregion expression profiling of prostate cancer from men randomized in the STAMPEDE trial: Stage I results of a multistage biomarker analysis.

Abstract

153 Background: The PAM50 gene expression classifier stratifies localized prostate cancer into luminal A/B and basal subtypes: luminal A have superior prognosis compared to luminal B and basal. Drug response scores built using the NCI-60 cell lines predict luminal subtypes are more taxane sensitive compared to basal; luminal B is hypothesized to be the most proliferative and hormone responsive compared to the basal subtype. The STAMPEDE trial provides a framework to validate prognostic and predictive associations of the PAM50 test in tumors from men randomized to androgen deprivation therapy (ADT) alone or with docetaxel or abiraterone. We here present multi-region whole-transcriptome expression array data at completion of Stage I designed to confirm the feasibility of molecular subtyping STAMPEDE tumor blocks. Methods: In collaboration with Decipher Biosciences we generated clinical-grade whole-transcriptome expression array data (Human Exon 1.0 ST GeneChip) performed to CLIA standards on mRNA (from three 10 micron slides) from cancer-enriched areas and applied the PAM50 classifier. Results: As of January 2019, we retrieved blocks from 2012 men from an ITT population of 3879. For Stage I feasibility assessment, we sectioned diagnostic trans-rectal biopsies of the prostate from 50 randomly selected men treated with ADT. We extracted mRNA from 109 cores; > 92% cores (101) from > 94% patients (47/50) passed quality control. The prevalence of PAM50 subtypes was: 31 basal cases (62%), 18 luminal B (36%) and 1 luminal A (2%). 26 cases (52%) were identified as ETS-related gene (ERG) positive. More than one core was interrogated from 35/50 cases (range: 1-6 cores). ERG positive cores were homogeneous across cores from the same patient but PAM50 subtyping identified intra-patient variability across cores from the same case. Conclusions: We confirm the feasibility of expression profiling STAMPEDE tumor blocks and identify a low prevalence of luminal A subtype. Whereas ERG status is consistent across multiple cores, basal and luminal subtypes co-exist in the same prostate. Multi-region analysis will enable further refinement for individual patient classification.