Luminal B subtype as a predictive biomarker of docetaxel benefit for newly diagnosed metastatic hormone sensitive prostate cancer (mHSPC): A correlative study of E3805 CHAARTED.

162 Background: Through gene expression profiling (GEP), the PAM50 classifier demonstrates prognostic value in localized prostate cancer (PCa). Pre-clinical drug response models predict increased taxane sensitivity in luminal subtypes compared to basal subtype. Men with mHSPC and high-risk features have greatest benefit from androgen deprivation therapy (ADT) plus docetaxel (D) vs ADT alone. We therefore sought to test the prognostic and predictive value of PAM50 in pre-ADT specimens from E3805 CHAARTED. Methods: Whole transcriptomic profiling of formalin-fixed, paraffin-embedded primary PCa biopsies from pts enrolled in the E3805 CHAARTED trial of ADT vs ADT+D was performed using the Human Exon 1.0 ST microarray platform (Decipher Biosciences). Normalized gene expression was used to classify subjects as luminal A, luminal B or basal subtype. Multivariable analyses (MVA) adjusted for ECOG status, de novo metastasis vs prior local therapy and volume of disease. The primary endpoint was overall survival (OS). Secondary endpoint was time to castration resistant PCa (TTCRPC). Results: Successful GEP was completed in 160 of 198 pts with available specimens. Eighty (50%), 77 (48%) and 3 (2%) pts were classified as luminal B, basal and luminal A, respectively. High volume disease was similarly present in luminal B (79%) and basal (78%) subtypes. In the ADT arm, luminal B subtype was associated with shorter OS vs basal (HR 1.75, p=0.05); consistent in MVA. Pts with luminal B subtype treated with ADT+D showed significant improvement in TTCRPC and OS (Table). By contrast, basal subtype showed no OS benefit from ADT+D even in pts with high volume disease. Conclusions: We demonstrate that GEP identifies tumor subtypes associated with differential benefit from chemohormonal therapy for mHSPC. Luminal B subtype is associated with poorer OS with ADT alone and benefits from addition of D. Basal subtype shows a lack of OS benefit from upfront ADT+D. We plan to validate these findings in independent trial cohorts.[Table: see text]