Heterogeneity of canonical prostate cancer markers across lesions in metastatic castration-resistant prostate cancer.

219 Background: Metastatic castration-resistant prostate cancers (mCRPC) show a diversity of clinical responses and phenotypes, suggesting molecular heterogeneity. Understanding molecular phenotypes is critical for patient stratification and selection of therapeutic targets. Treatment with androgen receptor (AR) pathway inhibitors are thought to drive AR-negative disease and the emergence of neuroendocrine lineages. There are several emerging biomarkers for mCRPC, including novel targets, such as PSMA and STEAP1, and markers of neuroendocrine and aggressive disease, such as INSM1, DLL3, and loss of p53, RB1 and PTEN. Our understanding of the co-expression of such biomarkers across multiple metastatic sites is currently limited. This study aimed to determine biomarker expression in multiple anatomical sites within individual mCRPC patients. Methods: Through the rapid-autopsy CASCADE program at the Peter MacCallum Cancer Centre, tissue was obtained from 5-14 tumor sites from 12 individual mCRPC patients yielding samples from a total of 110 tumor sites (8 primary and 102 metastatic). Ten patients were diagnosed with prostate adenocarcinoma (89 sites), and 2 patients with neuroendocrine histopathology in all sites (21 sites). Tissue was stained for AR, PSMA, STEAP1, INSM1, DLL3 p53, RB1 and PTEN by immunohistochemistry (IHC). Serial sections were reviewed and scored by a pathologist. IHC results were corroborated with genetic information derived from DNA sequencing. Results: In the adenocarcinoma cohort, on average 77% of sites of individual patients had AR expression, suggesting intra-patient heterogeneity. Co-expression of AR and PSMA was common (55/89 sites), but 6/10 adenocarcinoma patients had 1 to 6 PSMA + AR - lesions. Co-expression of PSMA and STEAP1 was also common in adenocarcinoma patients (60/89 sites), although 8/10 patients had 1 to 2 PSMA - STEAP1 + lesions, and 7 patients had 1 to 3 PSMA + STEAP1 - lesions. Expression of INSM1 and DLL3 was mostly seen in the 2 patients (20/21 sites) with neuroendocrine prostate cancer. In contrast, only 2 INSM1 + DLL3 + tumor sites in liver and 1 INSM1 - DLL3 + prostate cancer sample were detected in the adenocarcinoma cohort. Of the 110 sites, p53 loss was observed in 9, RB1 loss in 30 and PTEN loss in 25 sites. Of these, 8/9, 14/30 and 8/25 occurred in INSM1 + DLL3 + sites, suggesting that loss of these markers does not always correlate with INSM1 and DLL3 expression. Conclusions: While AR, PSMA and STEAP1 expression are frequently detected in a single site of disease, this co-expression is not consistent across all sites. In this dataset, INSM1 and DLL3 expression was almost exclusively confined to neuroendocrine prostate cancer. Loss of RB1 and PTEN often occurs without INSM1 and DLL3 expression. Understanding differential biomarker expression in mCRPC may guide future targeted treatments and warrants further research.