Abstract 5425: Novel molecular subtypes identified in prostate cancer: Results from the SPARTAN study

Background: Results from SPARTAN, a phase III placebo (PBO)-controlled study in patients (pts) with nonmetastatic castration-resistant prostate cancer (nmCRPC), show that apalutamide (APA) plus ongoing androgen deprivation therapy (ADT) significantly improves metastasis-free survival (MFS) compared with PBO + ADT. This analysis investigated the effects of APA in biologically distinct molecular subclasses of prostate cancer defined by gene expression profiles. Methods: Gene expression profiles (DECIPHER® prostate test, San Diego, CA) were generated from 233 archival primary prostate tumors; data were summarized based on 160 predefined gene signatures indicative of clinical prognosis and prostate cancer-related biology. Unsupervised consensus clustering identified sets of co-regulated expression signatures, and associations between signature expression, treatment, and signature-treatment interaction with MFS were evaluated using Kaplan-Meier analysis and Cox proportional hazards models. Results: Four co-regulated expression signature classes, each with distinct biological pathway functions, were identified. Class C1 included prognosis-related (risk) signatures; C2 included steroid homeostasis-related signatures; C3 included hormonal therapy nonresponsive basal and neuroendocrine-like signatures; C4 included immune and stromal signatures. Increased C1 expression was associated with shorter MFS in the PBO group (HR [95% CI], 2.18 [1.11-4.28], p = 0.02), while it was associated with longer MFS in the APA group (interaction HR [95% CI], APA vs PBO, 0.36 [0.14-0.95], p = 0.04). Similarly, increased C2 expression was associated with shorter MFS in the PBO group (HR [95% CI], 1.42 [1.02-1.98], p = 0.04), while it was associated with longer MFS in the APA group (interaction HR [95% CI], APA vs PBO, 0.57 [0.35-0.93], p = 0.02). Although there is no significant interaction effect between signature and treatment in C3, pts with low expression of C3 (adeno-like) showed longer MFS on APA vs PBO (HR [95% CI], 0.23 [0.13-0.40], p Conclusion: Expression signatures were clustered into 4 correlated unique biological subclasses. Clinical benefit of APA + ADT was observed in pts with expression profiles with high risk (C1), C2, C3, or C4 compared with PBO + ADT. Citation Format: Clemente Aguilar, Michael Gormley, Shibu Thomas, Paul N. Mainwaring, David Olmos, Fred Saad, Simon Chowdhury, Elai Davicioni, Yang Liu, Deborah S. Ricci, Angela Lopez-Gitlitz, Margaret K. Yu, Matthew R. Smith, Eric J. Small, Felix Feng. Novel molecular subtypes identified in prostate cancer: Results from the SPARTAN study [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 5425.