ESSA Pharma (EPIX) shares were down more than 65% premarket Friday after the company said it decided to terminate the phase 2 trial evaluating the combination of masofaniten and enzalutamide in patients with metastatic castration-resistant prostate cancer compared with enzalutamide alone.

The company said in a Thursday statement that the trial is unlikely to achieve the primary endpoint as interim data showed that "there was no clear efficacy benefit seen with the combination of masofaniten plus enzalutamide compared to enzalutamide single agent."

The study's primary endpoint is the proportion of patients reaching a PSA90 response and ESSA Pharma said trial data showed a "much higher" rate of PSA90 response in patients treated with enzalutamide monotherapy.

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