Lessons learned from the metastatic castration-resistant prostate cancer phase 1 trial of EPI-506, a first-generation androgen receptor N-terminal domain inhibitor Ronan Le Moigne1, Han-Jie Zhou1, Jon K. Obst2, C. Adriana Banuelos2, Kunzhong Jian3, David Williams3, Peter Virsik1, Raymond J. Andersen3, Marianne D. Sadar2, Frank Perabo1, Kim N. Chi4 1ESSA Pharmaceuticals Inc., Houston, TX and South San Francisco, CA, USA, 2Department of Genome Sciences Centre, BC Cancer Agency, 675 West 10th Avenue, Vancouver, BC V5Z 1L3, Canada, 3Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, BC V6T 1Z1, Canada, 4Department of Medical Oncology, BC Cancer Agency, 600 West 10th Avenue, Vancouver, V5Z 1L3, Canada INTRODUCTION Aniten compounds bind to the N-terminal domain (NTD) of the an- drogen receptor (AR) to inhibit its transcriptional activity. EPI-506, a triacetate prodrug of EPI-002 (ralaniten), was the first AR NTD inhibi- tor tested in a First-in-Human phase 1 study in patients with meta- static castration-resistant prostate cancer (mCRPC) failing enzalut- amide and/or abiraterone (NCT02606123). The drug was well-toler- ated but required high doses to achieve meaningful exposures. At doses >1280 mg, EPI-506 treatment resulted in PSA declines. Howev- er, these did not achieve 50% and were of short duration, reflecting the low potency and short half-life of EPI-002. To understand EPI-506’s metabolic vulnerabilities, patient plasma samples were an- alyzed to identify metabolites. CONCLUSION • EPI-506 was tested in a phase 1 trial and showed PSA declines, but all declines were less than 50% • The drug was well-tolerated, but exposure was insufficient due to significant metabolism • EPI-002 and EPI-506 exhibited extensive metabolism in both human hepatocytes and clinical samples, but demonstrated dif- ferent metabolic pathways in vitro vs. in patients • Oxidation was the major metabolic pathway seen in the phase 1 clinical samples while O-glucuronidation was the major metabolic pathway seen in in vitro hepatocytes • Patient plasma samples identified 19 metabolites, including the highly abundant oxidation metabolite M19, which was inactive in an AR-dependent reporter assay • More potent and stable molecules have been synthesized to ad- dress EPI-506/002’s metabolic and potency limitations. These next-generation Anitens are currently being prepared for IND filing (see Abstract 220, poster board J21) Results: The major metabolic pathway in human hepatocytes is via direct O-glucuronidation. Direct oxidation is also ob- served with the loss of chlorine followed by cysteine conjugation (exemplified by M4), but is much less common. The major metabolic pathway in human plasma samples is via oxidation (exemplified by M19, M18, etc). Direct O-glucuronidation is also observed (exemplified by M15, M12, etc.) but is less common. Time 2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00 18.00 % 0 100 EPI002_HHP 1: TOF MS ES- Sum 0.0200Da 3.82e6 12.84 7.18 5.92 7.96 14.68 17.29 14540 Time 2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00 18.00 % 0 100 EPI_002_QD_AUC 1: TOF MS ES- Sum 0.0200Da 1.60e6 17.33 12.75 5.49 4.84 9.88 10.84 14.70 13.46 15.43 O O OH OH HO HO M3 m/z 375 O O OH OH HO HO O M6 M/Z 389 HO O OH Cl M11 m/z 495 O O O Cl HO HO Gluc +Gluc M13 m/z 745 O O OH Cl HO HO +Gluc O M16 m/z 583 O O OH Cl HO HO O M18 m/z 407 O O OH Cl O HO M19 m/z 377 O O OH OH O HO M8 m/z 359 O O OR Cl RO RO R=H, EPI-002, m/z 393 R=Ac, EPI-506, m/z 521 +Gluc O OH HO HO +Gluc M2 m/z 477 O O O Cl HO HO M15 m/z 569 Gluc 0 25 50 75 100 125 Concentration of compound [ µM] 1 10 100 0.1 EPI-002: IC50 ≈ 12 µM M18: IC50 > 35 µM M19: IC50 > 35 µM PSA-luciferase activity ± SEM (% androgen-induced) -50 -25 0 25 50 75 100 Max % Decline in PSA 1800 mg (BID) 160 mg 320 mg 640 mg 1800 mg (BID) 3600 mg 2400 mg 1280 mg 640 mg 80 mg 1280 mg 80 mg 3600 mg 80 mg 2400 mg 320 mg 3600 mg 320 mg 640 mg 640 mg 160 mg 640 mg 160 mg 1280 mg 2400 mg 640 mg 640 mg 2400 mg 1280 mg 3600 mg 1280 mg 1800 mg (BID) Patients receiving < 1280 mg Patients receiving > 1280 mg 320 mg 0 6 12 18 24 1 10 100 1000 10000 100000 EPI-002 PK - day 8 Time (hr) EPI-002 [plasma] in ng/mL 80 mg 160 mg 320 mg 640 mg 1280 mg 2400 mg 3600 mg EPI-002 cell IC50 1800 mg bid 1800 mg bid simulated EPI-002 (active drug) EPI-506 (tri-acetate prodrug) O O (R) HO (S) Cl OH HO Figure 2: EPI-506/002 clinical activity and day 8 PK (A) Maximal PSA change at any time from start of multi-dose period. PSA declines (ranging from 8-37%) have been observed, especially in higher dose cohorts (≥ 1,280 mg). (B) Mean steady-state EPI-002 plasma concen- tration-vs-time profiles across EPI-506 dose cohorts. EPI-002 plasma profiles from day 8 multiple-dose samples indicate that EPI-002 plasma drug levels above cellular IC50 were not reached for long-periods. The EPI-506 prodrug was not detected in plasma at any measured time point across dose groups. (C) EPI-002 steady-state plasma pharmacokinetics following 8 days of EPI-506 administration (mean ± SD). N-Terminal Domain (NTD) DNA Binding Domain (DBD) Ligand Binding Domain (LBD) A B Figure 1: EPI-506/002 is a first in class NTD inhibitor of the androgen receptor (A) Structure of the androgen receptor (AR) and mechanisms of inhibitions. The AR is organized in 3 distinct domains: the LBD, involved in binding with androgens, the DBD, and the NTD, which orchestrate the transactivation of the receptor. Inhibiting the NTD with Anitens allows the inactivation of the AR dow- stream of the LBD, and is an effective strategy to counteract the resistance mechanism involving the LBD (mutations, splice vari- ants). (B) EPI-506, a rapidly cleaved triacetate prodrug of EPI-002, was a first-in-class oral small molecule from the Aniten family of compounds. A C B EPI-506/002 showed minor PSA declines in patients with mCRPC, due to insufficent exposure A C B Below Quantification Limit (<5.00 ng/mL) ª n=3 for t1/2 b n=6 for t1/2 c n=4 for t1/2 and Vz/F EPI-002 is highly metabolized via oxidation and glucuronida- tion in patient samples, suggesting a first pass effect Figure 4: EPI-002 metabolites are inactive in LNCaP PSA-Luciferase cell line (A) A dose-dependent decrease in AR pathway activity is demonstrated by incubating LNCaP cells transiently-expressing a PSA-driven luciferase construct with different Aniten compounds, in the presence of androgens (R1881). EPI-002 metabolites are inactive on the AR path- way Androgen Androgen Deprivation CYP17 inhibitors Anti androgens Reduce level of androgen Inhibit synthesis of androgen Block androgen binding to LBD Current Hormonal Therapies Resistance to current hormonal therapy occurs predominantly in the LBD and can cause the AR to remain active M2 M2a M4 M5a M6 M8 M11 M12 M12a M15 M15a M17 EPI-002 M18 M19 M2 M1 M4/M5 M3 M6 M8 M10/M11 M13 M15 M17 EPI-002 M18 M19 M7 M9 M12 M14 M16 Figure 3: EPI-002 metabolite identifica- tion Extracted ion chromatograms from metabo- lite profiling of EPI-002 incubated in human hepatocytes incubation (A) and human plasma samples (B). Samples were analyzed using UPLC/MS method. (A) Samples of EPI-002 were incubated in human hepato- cytes for 4 hours. (B) Three plasma samples from patients (one 80 and two 3,600 mg doses), were pooled across timepoints and metabolites were analyzed. (C) Proposed major metabolic scheme of main metabo- lites of EPI-002. A portion of the data presented in the poster was funded by US National Cancer In- stitute (R01 CA105304) awarded to Marianne Sadar. PK parameter Cohort 1 EPI-506 80 mg qd Cohort 2 EPI-506 160 mg qd Cohort 3 EPI-506 320 mg qd Cohort 4 EPI-506 640 mg qd Cohort 5 EPI-506 1280 mg qd Cohort 6 EPI-506 2400 mg qd Cohort 7 EPI-506 1800 mg bid Cohort 8 EPI-506 3600 mg qd n 3 4 4 a 7 b 7 c 4 3 2 C max (ng/mL) 26.3 (9.31) 134 (95.2) 345 (437) 768 (391) 1,528 (806) 2,888 (1,257) 3,430 (118) 8,397 (1,396) t max (h) 1.00 (0.98-4.00) 0.72 (0.52-1.00) 0.99 (0.50-1.97) 1.93 (0.50-4.00) 1.92 (1.00-4.05) 1.98 (1.90-4.08) 2.00 (1.95-2.00) 2.5 (1.00-4.00) AUC tau (ng.h/mL) 94.3 (8.65) 257 (83.7) 680 (692) 2,439 (678) 5,858 (1,602) 13,545 (5,331) 14,443 (2,429) 42,988 (24,841) t 1/2 (h) - - 1.6 (0.4) 3.2 (1.8) 5.4 (1.2) - - - Abstract Number: 257 Contact info: [email protected]