Development and independent validation of a predictive gene expression signature based on RB1, PTEN, and TP53 in patients with metastatic hormone-sensitive metastatic prostate cancer (mHSPC)

5083 Background: Alterations on the tumor suppressor genes (TSG) RB1, PTEN and TP53 are associated with treatment resistance and aggressive clinical evolution of prostate cancer patients (pts). Here, we developed and validated a TSG gene expression signature in mHSPC pts. Methods: This is a multicenter retrospective biomarker study in mHSPC pts. Mutations on TSG were assessed by targeted sequencing, mRNA expression by nCounter platform and protein by IHC in FFPE tumor samples. Expression data of the training cohort were used to establish the cut-off applied to the other cohorts. TSGlow was considered when ≥2 out of 3 TSG presented low expression and TSGwt in the remaining cases. CHAARTED trial pts with microarray data (Hamid, Ann Oncol, 2021) were analyzed as an independent validation cohort. TSG signature was correlated with castration resistance-free survival (CRPC-FS) (primary endpoint) and overall survival (OS) by Kaplan Meier and multivariate Cox analysis. Results: 218 pts were included: 125 treated with ADT + docetaxel (ADT+D) and 93 with ADT. 160 pts from the CHAARTED dataset were analyzed (84 with ADT+D and 76 with ADT). ADT+D pts with available TSG gene expression and targeted sequencing were considered as the training cohort (n=54). Pts with low PTEN and RB1 gene expression had lower IHC expression (p<0.01). Low PTEN pts had a higher frequency of PTEN mutations (p<0.05). TSGlow (25.9%) pts had a worse CRPC-FS (14.3 vs 21.7 months (m); HR 2.2 (95% CI, 1.1 – 4.4) p=0.022). In the internal validation cohort of ADT+D-treated pts (n=71), TSGlow (9.9%) correlated with a worse CPRC-FS (11.7 vs 20 m, p=0.027) and OS (27.6 vs 58.1 m, p=0.012). TSGlow was independently associated with CPRC-FS (HR 2.7 (95% CI, 1.2 – 6.1) p=0.02) and OS (HR 3.7 (95% CI, 1.3 – 10.1) p=0.012). In the ADT cohort (TSGlow 21.5%), no differences in either CRPC-FS or OS were observed. When analyzed together the ADT+D and ADT cohorts, we observed that TSGwt pts treated with ADT+D had the largest CRPC-FS (21.6 m; p=0.014) and OS (58.1 m; p=0.029), while TSGlow pts receiving ADT+D had a CRPC-FS and OS similar to ADT-treated pts. Similarly, in the CHAARTED dataset, TSGlow (20.2%) pts treated with ADT+D had a worse CPRC-FS (14.8 vs 29.4 m, HR 2.2 (95% CI, 1.1 – 4.3) p=0.023) and OS (32.9 vs 57.4 m, HR 2.2 (95% CI, 1 – 4.5) p=0.04). TSGlow (35.5%) was not predictive of either CRPC-FS or OS in ADT-treated pts. Analyzing together ADT+D and ADT pts, TSGwt pts treated with ADT+D had the best CRPC-FS and OS (p<0.01). Conclusions: Our results suggest that a TSGlow expression may predict the lack of benefit to taxanes in mHSPC pts, which may be useful to personalize the treatment. [Table: see text]