ESSA Pharma Presents Updated Phase 1 Masofaniten Clinical Data At The 2024 ASCO Genitourinary Cancers Symposium

Combination of masofaniten plus enzalutamide continues to be well tolerated with deep and durable reductions in PSA in patients with mCRPC

Phase 2 dose expansion currently underway at the RP2CDs of masofaniten 600 mg BID in combination with enzalutamide 160 mg QD

Across all dosing cohorts, 81% of patients achieved PSA90, 69% of patients achieved PSA90 in less than 90 days, and 63% of patients achieved PSA <0.2ng/mL. While the data are still maturing, median time to PSA progression is currently at 16.6 months.

SOUTH SAN FRANCISCO, California and VANCOUVER, Canada, Jan. 25, 2024 /CNW/ - ESSA Pharma Inc. ("ESSA", or the "Company") (NASDAQ:EPIX), a clinical-stage pharmaceutical company focused on developing novel therapies for the treatment of prostate cancer, today announced the presentation of updated dose escalation data from its Phase 1/2 study evaluating masofaniten (formerly EPI-7386) in combination with enzalutamide at the 2024 ASCO Genitourinary Cancers Symposium, taking place January 25 - 27, 2024, in San Francisco, CA. Masofaniten is a first-in-class N-terminal domain androgen receptor ("AR") inhibitor that suppresses androgen activity through a novel mechanism of action and is being developed for the treatment of prostate cancer. The poster presentation is available on the "Publications" section of the Company's website at www.essapharma.com.

"The maturing data from the Phase 1 dose escalation study evaluating masofaniten in combination with enzalutamide continue to demonstrate that the combination is well tolerated, accompanied by deep and durable reductions in circulating prostate-specific antigen ("PSA") levels. While the data are still immature, the durability of the PSA responses is encouraging with a median time to PSA progression currently at 16.6 months. The Phase 2 dose expansion head-to-head portion of the study is underway and is designed to evaluate the proportion of patients with a PSA decline on combination therapy compared to single agent enzalutamide," said David Parkinson, MD, President and CEO of ESSA. "We look forward to evaluating the potential long-term benefits of masofaniten in patients with metastatic castration-resistant prostate cancer ("mCRPC") and to providing future updates."

Poster presentation details:

Title: Phase 1/2 trial of oral EPI-7386 (masofaniten) in combination with enzalutamide (Enz) compared to Enz alone in patients with metastatic castration-resistant prostate cancer (mCRPC): Phase 1 (P1) results and phase 2 (P2) design.

Presenting Author: Christos Kyriakopoulos, MD, University of Wisconsin-Madison Carbone Cancer Center

Abstract #: 141

Date and time: Thursday, January 25, 2024; 11:30-1:00 p.m. PT

Data summary: This Phase 1/2 multicenter, open-label clinical trial enrolled patients with mCRPC who have received androgen deprivation therapy and who are naïve to second-generation antiandrogens but may have been treated previously with one line of prior chemotherapy in the metastatic hormone-sensitive prostate cancer setting. The data presented today include 18 patients across four cohorts in the Phase 1 dose escalation portion of the study. Masofaniten has no effect on enzalutamide exposure, thus allowing the use of full dose per label (160mg) of enzalutamide in combination. Enzalutamide reduces masofaniten exposure but twice daily dosing of masofaniten appears to mitigate the reduction and maintains clinically relevant drug exposures.

In patients evaluable for safety (n=18), masofaniten combined with enzalutamide, continues to be well-tolerated at the dose levels tested through 25 cycles of dosing in some patients. Most frequent adverse events were Grades 1 and 2, related to either AR inhibition or gastrointestinal tract irritation. In Cohort 4, one patient experienced a Grade 3 rash, which was observed immediately following administration of masofaniten combined with enzalutamide and deemed probably related, resulting in the expansion of the cohort from four to seven patients. No additional dose-limiting toxicities (DLTs) were observed, therefore the maximum tolerated dose (MTD) was not reached. The recommended Phase 2 combination doses (RP2CDs) were identified as masofaniten 600 mg twice daily (BID) in combination with enzalutamide 160 mg once daily (QD).

In the patients evaluable for efficacy (n=16), rapid, deep and durable reductions in PSA were observed, regardless of previous chemotherapy status, including in patients who received lower than the full dose of enzalutamide (120 mg). Across all dose cohorts, 88% of patients (14 of 16) achieved PSA50, 81% of patients (13 of 16) achieved PSA90, 69% of patients (11 of 16) achieved PSA90 in less than 90 days, and 63% of patients (10 of 16) achieved PSA <0.2ng/mL. While the data for disease PSA progression are still maturing with a current median follow up of 11.1 months, the median time to PSA progression is at 16.6 months.

The randomized, open-label, two arm, Phase 2 dose expansion portion of the study is underway and is designed to evaluate the combination of masofaniten and enzalutamide versus single agent enzalutamide in patients with mCRPC naïve to second generation anti-androgens. The study is currently enrolling at approximately 25 sites in the USA, Canada and Australia. Expansion to European sites is in progress.

About Masofaniten

Masofaniten (formerly known as EPI-7386) is a first-in-class investigational, highly selective, oral, small molecule inhibitor of the N-terminal domain ("NTD") of the androgen receptor ("AR"). Masofaniten's unique mechanism of action disrupts the AR signaling pathway, the primary pathway that drives prostate cancer growth, by selectively binding to the NTD, a region of the AR that is not currently targeted by other therapies. Masofaniten is currently being studied in an open-label, randomized Phase 2 clinical trial (NCT05075577) in combination with enzalutamide in patients with metastatic castration-resistant prostate cancer (mCRPC) naïve to second-generation antiandrogens. ESSA is also conducting a Phase 1 monotherapy study (NCT04421222) in patients with mCRPC whose tumors have progressed on standard-of-care therapies. The U.S. Food and Drug Administration has granted Fast Track designation to masofaniten for the treatment of adult male patients with mCRPC resistant to standard-of-care treatment. ESSA retains all rights to masofaniten worldwide.