Background: Combination of CDKi with endocrine therapy is a key treatment for mHRBC due to survival benefit and favorable safety profile. However, progressive disease inevitably develops and outcomes after CDKi discontinuation (dc) are not well-described. Within our institution, we previously reported clinical characteristics and outcomes for a cohort of 140 mHRBC patients who received CDKi therapy. Median progression-free survival (PFS) and overall survival (OS) post-CDKi dc were 7.0 and 15.4 months, respectively. However, 29% experienced rapid progression or death within 4 months following CDKi dc. Molecular predictors of rapid progression after CDKi are unknown and may help define therapies to improve outcomes. In this study, we sought to identify molecular predictors for rapid disease progression after CDKi dc in mHRBC. Methods: We identified within our cohort 34 patients with mHRBC who progressed on CDKi with next-generation sequencing (NGS) performed on pre-CDKi tissue samples. PFS and OS, measured from CDKi dc, were analyzed with the Kaplan-Meier estimator and log-rank test. Rapid progression was analyzed with logistic regression and Fisher’s exact test to evaluate association between pre-CDKi tumor mutation and rapid progression post-CDKi. Results: NGS of pre-CDKi tumor biopsies found 12 genes (FGF3, FGF4, FGFR, PIK3CA, PTEN, AKT, RB1, CDKN2A, MYC, CCND1, ESR1, TP53) that were altered in ≥3 of the 34 patients. The six patients (18%) with a PTEN mutation (mut) had a median PFS of 3 months and median OS of 4 mo. In comparison, median PFS and OS of PTEN wild-type (wt) patients were 7 mo. (log-rank p=0.008) and 21 mo. (log-rank p<0.001), respectively. Moreover, those with PTENmut tumors were more likely to experience rapid progression compared to PTENwt (odds ratio = 7.0, 95% CI: 1.1 – 60.5, p=0.048). Notably, in the 10 rapid progression patients with pre-CDKi NGS results, alterations to PI3K pathway constituents were prevalent: PTENmut (40%), FGFRmut (50%), AKTmut (20%) and PIK3CAmut (40%). Conclusions: PI3K pathway alterations are prevalent in mHRBC patients who develop rapid progression post-CDKi dc, with PTENmut being the most significant predictor. These hypothesis-generating findings provide the basis for ongoing investigations to find clinical and molecular biomarkers that can help improve outcomes for mHRBC at risk of rapid progression post-CDKi therapy.