Randomized phase 2 therapeutic equivalence study of abiraterone acetate fine particle formulation vs. originator abiraterone acetate in patients with metastatic castration-resistant prostate cancer: The STAAR study

Urol Oncol. 2018 Feb;36(2):81.e9-81.e16. doi: 10.1016/j.urolonc.2017.10.018. Epub 2017 Nov 14.


Background: This multicenter, randomized, open-label, active-controlled study evaluated therapeutic equivalence, steady-state pharmacokinetics, and safety of a novel abiraterone acetate fine particle formulation (AAFP) 500mg plus methylprednisolone vs. the originator AA (OAA) 1000mg plus prednisone in men with metastatic castrate-resistant prostate cancer (mCRPC). The primary endpoint was a comparison of average of serum testosterone levels on treatment days 9 and 10 between groups.

Methods: Men with progressive mCRPC, receiving gonadotropin-releasing hormone agonist or antagonist therapy, and with a serum testosterone level of <50ng/dl were randomized 1:1 to either AAFP 500mg daily plus 4mg methylprednisolone orally twice daily (BID), or OAA 1000mg daily plus 5mg prednisone BID for 84 days. Serum testosterone, serum prostate-specific antigen (PSA), steady-state (trough) abiraterone pharmacokinetics, and safety were evaluated.

Results: Fifty-three patients were enrolled (n = 24, AAFP; n = 29, OAA). Mean age was 75.1 years and 54.7% had Gleason>7. Over 90% of patients in each group achieved absolute testosterone levels of ≤1ng/dl during the study. The averaged absolute testosterone levels ≤0.1ng/dl were achieved in 25% of AAFP-treated patients and 17% of OAA-treated patients. A PSA-50 response was observed in>65% of patients in both groups on days 28, 56, and 84 (P = NS, all timepoints). Days 9 and 10 averaged rounded-up least squares (LS) mean (SE) serum testosterone levels were comparable (1.05ng/dl [0.04], AAFP; 1.02ng/dl [0.03], OAA; P = 0.4703 for LS mean difference). The geometric mean ratio between groups was 1.021 (90% CI: 0.965-1.081); the 90% CI fell within 80.0% to 125.0% equivalence limits. The LS mean differences in abiraterone trough plasma concentrations were not statistically significant at any visit. Adverse event frequency was comparable between arms (75.0%, AAFP; 82.8%, OAA). Musculoskeletal events were more common among OAA-treated patients (37.9% vs. 12.5%).

Conclusion: Therapeutic equivalence between AAFP 500mg daily and OAA 1000mg daily based on serum testosterone levels was confirmed in mCRPC patients. Both agents led to similar PSA-50 response rates. Abiraterone trough levels were similar between treatments. No new safety concerns were observed.

Keywords: Abiraterone; CYP17 inhibitor; Metastatic castrate-resistant prostate cancer; Prostate-specific antigen; Testosterone.

Publication types

  • Clinical Trial, Phase II
  • Equivalence Trial
  • Multicenter Study
  • Randomized Controlled Trial
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Abiraterone Acetate / administration & dosage
  • Abiraterone Acetate / adverse effects
  • Abiraterone Acetate / pharmacokinetics
  • Aged
  • Aged, 80 and over
  • Antineoplastic Combined Chemotherapy Protocols / adverse effects
  • Antineoplastic Combined Chemotherapy Protocols / pharmacokinetics
  • Antineoplastic Combined Chemotherapy Protocols / therapeutic use*
  • Gastrointestinal Diseases / chemically induced
  • Humans
  • Male
  • Methylprednisolone / administration & dosage
  • Methylprednisolone / adverse effects
  • Methylprednisolone / pharmacokinetics
  • Middle Aged
  • Neoplasm Metastasis
  • Particle Size
  • Prednisone / administration & dosage
  • Prednisone / adverse effects
  • Prednisone / pharmacokinetics
  • Prostatic Neoplasms, Castration-Resistant / drug therapy*
  • Prostatic Neoplasms, Castration-Resistant / pathology
  • Therapeutic Equivalency


  • Abiraterone Acetate
  • Prednisone
  • Methylprednisolone