Lessons learned from the metastatic castration-resistant prostate cancer phase 1 trial of EPI-506, a �rst-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

INTRODUCTIONAniten 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 �rst 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, re�ecting 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 insu�cient due to signi�cant 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 identi�ed 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 �ling (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 (exempli�ed by M4), but is much less common. The major metabolic pathway in human plasma samples is via oxidation (exempli�ed by M19, M18, etc). Direct O-glucuronidation is also observed (exempli�ed by M15, M12, etc.) but is less common.

14625

Time2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00 18.00

%

0

100

2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00 18.00

%

0

100EPI002_RHP 1: TOF MS ES-

Sum 0.0200Da3.74e6

12.85

11.085.67 7.99

14.70

13.44 17.2915.44

EPI002_HHP 1: TOF MS ES- Sum 0.0200Da

3.82e612.84

7.18

5.92

7.96

14.68

17.29

14540

Time2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00 18.00

%

0

100EPI_002_QD_AUC 1: TOF MS ES-

Sum 0.0200Da1.60e6

17.33

12.75

5.49

4.849.88

10.84

14.70

13.46

15.43

O OOH

OH

HO

HOM3m/z 375

O OOH

OH

HO

HO OM6M/Z 389

HO OOH

Cl

M11m/z 495

O OO

Cl

HO

HO

Gluc

+Gluc

M13m/z 745

O OOH

Cl

HO

HO

+Gluc

O

M16m/z 583

O OOH

Cl

HO

HO O

M18m/z 407

O OOH

ClO

HO M19m/z 377

O OOH

OHO

HO M8m/z 359

O OOR

Cl

RO

ROR=H, EPI-002, m/z 393R=Ac, EPI-506, m/z 521

+Gluc

O OHHO

HO

+Gluc

M2m/z 477

O OO

Cl

HO

HO

M15m/z 569

Gluc

0

25

50

75

100

125

Concentration of compound [µM]

1 10 1000.1

EPI-002: IC50 ≈ 12 µM

M18: IC50 > 35µMM19: IC50 > 35µM

PSA-

luci

fera

se a

ctiv

ity ±

SEM

(% a

ndro

gen-

indu

ced)

-50

-25

0

25

5075

100

Max

% D

eclin

e in

PSA

1800

mg

(BID

)16

0 m

g32

0 m

g

640

mg

1800

mg

(BID

)36

00 m

g24

00 m

g12

80 m

g64

0 m

g80

mg

1280

mg

80 m

g36

00 m

g80

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 mgPatients receiving > 1280 mg

320

mg

0 6 12 18 241

10

100

1000

10000

100000

EPI-002 PK - day 8

Time (hr)

EPI-0

02 [p

lasm

a] in

ng/

mL

80 mg160 mg320 mg640 mg1280 mg2400 mg3600 mg

EPI-002 cell IC50

1800 mg bid1800 mg bid simulated

EPI-002 (active drug)

EPI-506 (tri-acetate prodrug)

OO

(R)

HO(S)

Cl

OHHO

EPI-002

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 pro�les across EPI-506 dose cohorts. EPI-002 plasma pro�les 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 �rst 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 e�ective 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 �rst-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 insu�cent exposure

A

C

B

Below Quanti�cation Limit (<5.00 ng/mL)ª n=3 for t1/2b n=6 for t1/2c n=4 for t1/2 and Vz/F

EPI-002 is highly metabolized via oxidation and glucuronida-tion in patient samples, suggesting a �rst pass e�ect

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 di�erent Aniten compounds, in the presence of androgens (R1881).

EPI-002 metabolites are inactive on the AR path-way

AndrogenAndrogen

DeprivationCYP17

inhibitors

Antiandrogens

Reduce levelof 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

�

M2M2a

M4

M5a

M6 M8

M11M12

M12a

M15

M15aM17

EPI-002

M18M19

M2

M1

M4/M5

M3

M6

M8

M10/M11

M13

M15

M17

EPI-002

M18

M19

M7

M9 M12M14 M16

Figure 3: EPI-002 metabolite identi�ca-tionExtracted ion chromatograms from metabo-lite pro�ling 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 1EPI-506

80 mg qd

Cohort 2EPI-506

160 mg qd

Cohort 3EPI-506

320 mg qd

Cohort 4EPI-506

640 mg qd

Cohort 5EPI-506

1280 mg qd

Cohort 6EPI-506

2400 mg qd

Cohort 7EPI-506

1800 mg bid

Cohort 8EPI-506

3600 mg qdn 3 4 4a 7b 7c 4 3 2

Cmax

(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)

tmax

(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)

AUCtau

(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)

t1/2

(h)- -

1.6(0.4)

3.2(1.8)

5.4(1.2) - - -

Abstract Number: 257Contact info:rlemoigne@essapharma.com