SUMMARY  

• In the phase 3 SPARTAN study, which evaluated the efficacy and safety of ERLEADA compared to placebo in patients with high-risk, non-metastatic castration-resistant prostate cancer (nmCRPC) receiving continuous androgen deprivation therapy (ADT)¹, analyses of multiple androgen receptor (AR) aberrations (AR-V7 expression, AR ligand-binding domain [LBD] mutations, and AR amplification) and non-AR aberrations were performed.²
  • The frequency of AR-V7 expression was higher at end of treatment than at baseline (11% vs 6.3%); this difference was not statistically significant (P=0.174). The frequency of any AR aberration and AR amplification were significantly higher at end of treatment than at baseline (AR aberrations: 25.4% vs 13.6% [P=0.019]; AR amplification: 28.5% vs 11.1% [P=0.013]).^2,3
  • In patients who progressed to metastatic castration-resistant prostate cancer (mCRPC) and received abiraterone acetate plus prednisone or enzalutamide as first subsequent therapy, the median second progression-free survival (PFS2) was shorter in patients who were positive for AR amplification (P=0.0004) and any AR aberration (P=0.024); however, the presence of AR-V7 or AR LBD mutations did not have a statistically significant impact on the median PFS2.²
• In the phase 3 TITAN study, which evaluated the efficacy and safety of ERLEADA compared to placebo in patients with metastatic castration-sensitive prostate cancer (mCSPC) receiving continuous ADT,⁴ analyses of detectable/undetectable circulating-tumor DNA (ctDNA), AR aberrations (AR amplification, AR LBD mutations, and AR-V7), and non-AR aberrations were performed.^5,6
  • In patients with both cfDNA and cfRNA assessed, the frequency of AR-V7 expression was higher at the end of treatment than at baseline in both the ERLEADA (13% vs 20%) and placebo (14% vs 36%) groups. Additionally, the proportion of patients with any AR aberration, including AR-V7, AR LBD mutations, and AR amplification, was not increased in the ERLEADA group vs placebo group at the end of treatment.⁵ Median overall survival (OS) and PFS2 were significantly shorter in patients with AR aberrations at the end of treatment. In patients with positive AR aberrations vs negative, the median OS was 11.1 months vs not reached (HR, 3.67; 95% CI, 1.83-7.36; P=0.0001) and the median PFS2 was 8.8 months vs 21.1 months (HR, 2.92; 95% CI, 1.64-5.19; P=0.0001).^5,6

BACKGROUND

Several potential mechanisms have been proposed to explain resistance to AR-targeted agents, including AR amplification, AR overexpression, AR point mutations, AR splice variants such as AR-V7, and altered steroidogenesis.⁷

CLINICAL DATA

AR Aberrations in the Phase 3 SPARTAN Study

Smith et al (2021)² evaluated AR aberrations at baseline and end of treatment in a subset of patients with nmCRPC who were enrolled in the SPARTAN study. Evaluations included determination of aberrations associated with progression from nonmetastatic to metastatic disease in the ERLEADA group and assessment of aberrations longitudinally from nonmetastatic disease at baseline to metastatic disease at progression.

Study Design/Methods

• SPARTAN (NCT01946204) was a phase 3, randomized, double-blind, placebo-controlled, multicenter study in patients with high-risk (defined as prostate-specific antigen [PSA] doubling time ≤10 months) nmCRPC receiving either ERLEADA 240 mg orally once daily or placebo. All patients received continuous ADT throughout the study. The primary endpoint was metastasis-free survival (MFS), defined as the time from randomization to the time of first evidence of detection of blinded independent central review-confirmed distant metastasis, defined as new bone or soft-tissue lesions or enlarged lymph nodes above the iliac bifurcation, or death due to any cause, whichever occurred first. Study treatment was continued until protocol-defined progression, adverse events, or withdrawal of consent occurred.¹
• PFS2, an exploratory endpoint, was defined as the time from randomization to investigator-assessed disease progression (PSA progression, detection of metastatic disease on imaging, symptomatic progression, or any combination) during the first subsequent treatment for mCRPC or death from any cause.¹
• Blood samples were collected in a subset of patients who consented to biomarker analysis at baseline and at end of study treatment (defined as the time of first MFS event or treatment discontinuation) and the following were assessed:
  • AR LBD mutations (L702H, W742C, H875Y, F877L, and T878A)
  • Non-AR aberrations (CDK12, RB1, BRCA1, BCA2, TP53 inactivation and MET, MYC, and PIK3CA activation)
• Next-generation sequencing of cell-free DNA (cfDNA) isolated from plasma was used to assess ctDNA and genomic AR- and non-AR aberrations. Whole blood-derived quantitative reverse transcription polymerase chain reaction from total RNA was utilized for AR-V7 identification.
• The biomarker population included patients who had at least 1 biomarker sample collection at end of study treatment and analyzed before the first interim analysis (data cut-off of 19 May 2017).
• Associations between MFS, PFS2, and OS with biomarker status were assessed in univariate analyses at baseline and multivariate analyses of biomarker status were assessed in pooled patients from both groups who had all biomarkers measured.

Results

• Summarized below are results specific to AR aberrations.
• Patient baseline characteristics were generally similar between the SPARTAN biomarker population (n=247) and the overall SPARTAN population (N=1207). Significant differences were observed in median PSA doubling time which was shorter, and the proportion of patients with PSA doubling time ≤6 months, which was larger. Median MFS, PFS2, and OS were also significantly shorter in the biomarker population compared with the overall population.
• At the end of the first interim analysis, 247 patients provided blood samples at end of study treatment (193 plasma and whole blood processed samples were from the same patients) and 149 of these patients also provided samples at baseline (110 plasma and whole blood processed samples were from the same patients). A total of 54.6% (131/240) plasma samples and 63.0% (126/200) of whole blood samples at the end of study treatment had matched baseline samples.
• In the biomarker population, progression of disease was reported in 51% of patients (62/121) in the ERLEADA group and 74% of patients (93/126) in the placebo group.
• ctDNA-positive samples were obtained from 40.6% of patients at baseline and from 57.1% of patients at end of treatment (P=0.003). The frequency of AR-V7 expression was higher at end of treatment than at baseline; however, this difference was not statistically significant (P=0.174).³ Refer to Table: AR Aberrations at Baseline and at End of Treatment in the Phase 3 SPARTAN Study.
• At the end of study treatment, no significant differences were observed in any type of AR aberration between the ERLEADA and the placebo groups.
• No significant associations were observed between AR aberration status at baseline and MFS, PFS2, or OS outcomes.
• In patients who progressed to mCRPC and received abiraterone acetate plus prednisone or enzalutamide as first subsequent therapy, the median PFS2 was shorter in patients who were positive for AR amplification and any AR aberration; however, the presence of AR-V7 or AR LBD mutations did not have a statistically significant impact on the median PFS2. Refer to Table: AR Aberration Status at End of Treatment and Efficacy Outcomes in Patients Treated with Androgen Signaling Inhibitors in the SPARTAN Study.

AR Aberration Status at End of Treatment and Efficacy Outcomes in Patients Treated with Androgen Signaling Inhibitors in the SPARTAN Study^2a

Additional Information

Additional information regarding the SPARTAN study, including the clinical study report, protocol, and statistical analysis plan, can be found at: https://www.accessdata.fda.gov/drugsatfda_docs/nda/2018/Erleada_210951_toc.cfm (scroll to the “Sponsor Clinical Study Reports ARN-509-003 SPARTAN NCT # 01946204” section at the bottom of the web page).

AR Aberrations in the Phase 3 TITAN Study

Chi et al (2019)⁵ and (2020)⁶ evaluated the levels of ctDNA and frequencies of AR aberrations (including AR amplification, AR LBD mutations, and AR-V7 mutations), at baseline and end of treatment with ERLEADA plus ADT and impact on OS and PFS2 in patients with mCSPC who were enrolled in the phase 3 TITAN study. Chi et al (2020) additionally evaluated for the presence of ctDNA and non-AR aberrations.

Study Design/Methods

• TITAN (NCT02489318) was a phase 3, randomized, double-blind, placebo-controlled, multicenter study in patients with mCSPC receiving either ERLEADA 240 mg orally once daily or placebo. All patients received ADT via gonadotropin releasing hormone analog or surgical castration. The dual primary endpoints were radiographic progression-free survival (rPFS), defined as time from randomization to first imaging-based documentation of progressive disease (progression of soft tissue lesions measured by computed tomography or magnetic resonance imaging or new bone lesions on bone scan) or death, whichever occurred first, and OS.⁴
• PFS2, an exploratory endpoint, was defined as time from randomization to first occurrence of investigator-assessed disease progression (PSA progression, progression on imaging, or clinical progression) while the patient was receiving first subsequent treatment for prostate cancer, or death from any cause, whichever occurred first.⁴
• Blood samples were collected in a subset of patients who consented to biomarker analysis at baseline and end of treatment (defined as first rPFS event or discontinuation of treatment) and the following was assessed:
  • Detectable ctDNA, AR amplification, AR LBD mutations, and AR-V7 mRNA⁵
  • Detectable/undetectable ctDNA, AR amplification, AR LBD mutations, non-AR aberrations, and AR splice variant (AR-V7)⁶
• Next-generation sequencing was utilized to assess plasma-derived cfDNA for presence of detectable ctDNA, AR amplification, and AR LBD mutations (L702H, W742C, H875Y, F877L, and T878A), and Chi et al (2020) additionally assessed undetectable ctDNA and non-AR aberrations (TP53, RB1, and CDK12 inactivation, and PIK3CA, MYC, and MET activation). Whole blood-derived quantitative reverse transcription polymerase chain reaction from total RNA was utilized for AR-V7 identification.^5,6
• Associations of PFS2 and OS with biomarker status were assessed in univariate analyses and using multivariate analyses in pooled ERLEADA and placebo group patients.⁶

Results

• Summarized below are results specific to AR aberrations.
• Patient baseline characteristics were similar between the TITAN biomarker population (n=265) and the overall TITAN population (N=1052).^5,6
• The detection of AR amplification at baseline was associated with significantly longer duration of prior ADT treatment (AR amplification positive [n=7] vs negative [n=52]: 4.2 months vs 1.6 months; P=0.034).^5,6
• Detection of AR aberrations increased from baseline to end of treatment. At end of treatment, any AR aberration was significantly less frequent in the ERLEADA group compared to the placebo group (48% vs 67%, respectively; P=0.041).^5,6 Refer to Table: AR Aberrations at Baseline and End of Treatment in the Phase 3 TITAN Study and Table: AR Genomic Aberrations at Baseline and End of Treatment in the Phase 3 TITAN Study.
• At end of treatment, the proportion of patients with baseline AR-V7, AR LBD mutations, and AR amplification did not increase in the ERLEADA group vs the placebo group.⁵ Refer to Table: Change in AR Aberrations from Baseline to End of Treatment in the Phase 3 TITAN Study.
• Median OS and PFS2 were significantly shorter in patients with AR aberrations at the end of treatment.^5,6 Refer to Table: OS and PFS2 with AR Aberrations in the Phase 3 TITAN Study.
• In the univariate analysis, any AR aberration at end of treatment was significantly associated with shorter PFS2 (HR, 2.9; P=0.0001) and OS (HR, 3.7; P=0.0001). There was no significant association with PFS2 (HR, 1.9; P=0.066) and OS (HR, 1.9; P=0.088) outcomes in the multivariate analysis.⁶

Literature Search

A literature search of MEDLINE^®, Embase^®, BIOSIS Previews^®, and Derwent Drug File (and/or other resources, including internal/external databases) was conducted on 30 June 2024. Summarized in this response are relevant data limited to the phase 3 SPARTAN and TITAN studies in patients with nmCRPC and mCSPC, respectively.