Molecular determinants of outcome for metastatic castration-sensitive prostate cancer (mCSPC) with addition of apalutamide (APA) or placebo (PBO) to androgen deprivation therapy (ADT) in TITAN.

Abstract

5535 Background: In TITAN, addition of APA to ADT improved radiographic progression-free survival (rPFS) and overall survival (OS) versus PBO plus ADT in patients (pts) with mCSPC. In this post hoc analysis, we performed transcriptome-wide profiling of tumor samples and assessed association of molecular subtypes with rPFS. Methods: The DECIPHER platform (Decipher Biosciences, Inc.) was used to assess gene expression in archival primary prostate tumors from TITAN. Samples were classified into high versus low to average risk of metastases (DECIPHER genomic classifier [GC] > 0.6 and ≤ 0.6, respectively), basal and luminal A/B (PAM50 classifier), and androgen receptor activity (AR-A) signature high and low. Associations between subtypes with rPFS were assessed with Cox proportional hazards model. Results: The biomarker population included 222 pts (APA, 110; PBO, 112). Benefit in rPFS from APA in the biomarker population (HR [95% CI]; p value; 0.49 [0.31-0.78]; 0.002) resembled that in the overall study population (0.49 [0.40-0.61]; < 0.0001). The majority of TITAN pts had GC high scores (n = 166, 75%). GC high risk subtype in the PBO group had poorer prognosis for rPFS than GC low to average risk subtype (median rPFS 18.2 mos for GC high vs not reached [NR] for GC low to average, 0.28 [0.11-0.69]; 0.006), but there was no difference in prognosis between high and low to average GC risk subtypes in the APA group (GC high NR vs GC low to average NR; 0.81 [0.35-1.89]; 0.625). Pts were further stratified based on basal/luminal and AR-A signatures. Basal (n = 112, 50%) and AR-A low (n = 96, 43%) subtypes, known to be nonresponsive to ADT, both showed significant benefit from APA vs PBO (0.30 [0.16-0.57]; < 0.001 and 0.25 [0.12-0.52]; < 0.001, respectively). The majority of AR-A low subtype (74%, 71/96) overlapped with basal subtype. Further conclusions for risk of rPFS in GC low, luminal, and AR-A high subtypes and OS across all subtypes will be assessed as more events occur. Conclusions: In TITAN, addition of APA to ADT improved rPFS for all subtypes of pts with mCSPC. APA overcame the poor prognosis of GC high risk subtype and prolonged rPFS in ADT-resistant AR-A low and basal molecular subtypes, suggesting APA is beneficial especially for the highest risk molecular subtypes. Clinical trial information: NCT02489318 .