Rinzimetostat and Darolutamide: Himalayas-1 Phase 3 Clinical Trial

Himalayas‑1 is a newly initiated global Phase 3 registrational trial testing rinzimetostat (ORIC-944) in combination with darolutamide in men with metastatic castration‑resistant prostate cancer (mCRPC) who have previously been treated with abiraterone. The trial is designed as a pivotal study intended to support approval if the results are positive, with about 600 patients planned across more than 250 sites in over 20 countries, randomized 1:1 to rinzimetostat 400 mg once daily plus darolutamide versus physician’s choice of another androgen receptor inhibitor or chemotherapy.

Rinzimetostat blocks PRC2 by targeting the EED subunit. PRC2 is the complex, and EZH2 is one of its main components. Together, they help cancer cells silence genes that would normally limit growth. In prostate cancer, this pathway is linked to resistance to hormone therapy and more aggressive disease, which is why combining this drug with an androgen receptor blocker makes sense. Darolutamide is a potent androgen receptor antagonist with a distinct binding conformation and relatively low CNS penetration, which makes it a suitable backbone drug for long‑term combination therapy targeting AR signaling.

The 400 mg once‑daily dose of rinzimetostat used in Himalayas‑1 was selected based on a Phase 1b dose‑optimization trial in mCRPC, where rinzimetostat given at 400 mg or 600 mg once daily with darolutamide 600 mg twice daily produced encouraging PSA and radiographic outcomes with a manageable safety profile. In the Phase 1b study, rinzimetostat (ORIC-944) combined with apalutamide or darolutamide showed broad and deep PSA responses in men with mCRPC. The regimen produced a 59% PSA50 response rate, including 47% confirmed responses plus one additional response pending confirmation, and a 24% confirmed PSA90 response rate. Responses were durable, with several patients remaining on treatment for nearly a year or longer, while adverse events were mostly low‑grade gastrointestinal symptoms and fatigue with few dose reductions. These findings, together with preclinical evidence that EED‑directed PRC2 inhibition can suppress growth and induce apoptosis in prostate adenocarcinoma and neuroendocrine prostate cancer models, underpinned the choice of rinzimetostat plus darolutamide as the Himalayas‑1 regimen.

The primary endpoint of Himalayas‑1 is radiographic progression‑free survival, with overall survival as the key secondary endpoint and PSA response, objective response, and patient‑reported outcomes as additional secondary measures. If the trial demonstrates a clear improvement in progression‑free survival and a favorable overall survival signal versus physician’s choice of AR inhibition or chemotherapy, rinzimetostat plus darolutamide could emerge as a new oral, biomarker‑agnostic option in the post‑abiraterone mCRPC setting, directly targeting both androgen receptor signaling and PRC2‑mediated epigenetic resistance mechanisms.

Note: Although the trial has been initiated operationally, patient enrollment has not yet begun.

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