Intra-individual dose escalation of abiraterone (ABI) according to its plasma exposure in patients (pts) with progressive metastatic castration-resistant prostate cancer (mRCPC): Results of the OPTIMABI trial

159 Background: Abiraterone acetate (ABI) improves survival in mCRPC. In a previous observational study, worse PSA response and shorter progression-free survival (PFS) were associated with mean ABI plasma through concentration (ABI Cmin) < 8.5 ng/mL within the first three months (ms) of treatment (Carton E et al. Eur J Cancer 2017). The OPTIMABI study, a multicenter non-randomized study (NCT 03458247), aimed to investigate whether a dose escalation of ABI improve PFS in underexposed progressive mCRPC pts. Methods: In step 1, pts with docetaxel-naive progressive mCRPC received ABI at the standard dose of 1000 mg/d + Prednisone 10 mg/d. Mean ABI Cmin was calculated from three monthly dosages. Pts experiencing progression within the first 6 ms and for whom mean ABI Cmin was < 8.5 ng/mL were included in step 2 and received 1000 mg twice daily (2000 mg/d) of ABI. ABI Cmin was assessed every month. The primary endpoint was the non-progression rate at 12 weeks in step 2. Identification of risk factors of 6 ms-PFS was done using Cox regression analysis. Results: From 06/2018 to 09/2021, 81 of 93 pts (median age: 72 yrs) included in step 1 were evaluable for statistical analysis. The median ABI Cmin was 10.0 (IQR: 4.5-15. 0) ng/mL over the first three ms. Inter-individual variability was 59%. The 6 ms-PFS was 68% (IC 95%: 56% - 80%) with no statistical difference between pts with mean ABI Cmin ≥ or < 8.5 ng/mL (log-rank test, p= 0.24). In multivariate analysis, liver metastasis (Hazard ratio, HR = 5.95, CI95% = 1.22–28.92; p=0.027) and creatinine clearance < 60 mL/min (HR = 3.51, CI95% = 1.12–10.99; p=0.031) were independently associated with 6 ms-PFS. Among 21 pts (25.9%), with mean ABI Cmin < 8.5 ng/mL, 8 pts (38.1%) experienced tumor progression within the first 6 ms; four of them could be included in step 2. None of them fulfilled the primary endpoint despite a significant increase in plasma ABI Cmin (p