ESSA Pharma Inc. 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 30th Annual Prostate Cancer Foundation Scientific Retreat, taking place October 26-28, 2023, in Carlsbad, 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. Poster presentation details: Title: Phase 1/2 Trial of Oral EPI-7386 (masofaniten) in Combination with Enzalutamide (Enz) Compared with Enz Alone in Subjects with Metastatic Castration-Resistant Prostate Cancer (mCRPC): Current Phase 1 (P1) results; Presenting Author: Andrew Laccetti, MD, MS, Memorial Sloan Kettering Cancer Center; Date and time: October 26, 2023; 7:30-10:30 p.m. PT.

Data summary: This Phase 1/2 multicenter, open-label clinical trial is enrolling 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 are from the first four cohorts of patients 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 doses tested through 21 cycles of dosing in some patients. Most frequent adverse events were Grade 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. 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). In the first three cohorts, 90% of patients (9 of 10) achieved PSA50 and PSA90, 80% of patients (8 of 10) achieved PSA90 in less than 90 days, and 70% of patients (7 of 10) achieved PSA <0.2mg/mL.

Across all dose cohorts including patients in the recently enrolled cohort four, 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 56% of patients (9 of 16) achieved PSA <0.2mg/mL. The randomized Phase 2 dose expansion portion of the study is currently enrolling. 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.