Ensem Therapeutics has dosed the first patient in a phase 1/2 clinical trial of ETX-636, a first-in-class allosteric pan-mutant-selective PI3Ka inhibitor and degrader, marking a major milestone for the biotech’s precision oncology pipeline. The investigational therapy targets activating PI3Ka mutations, which are found in up to 40% of hormone receptor-positive (HR+)/HER2-negative advanced breast cancers and are implicated in a variety of solid tumors.

ETX-636 is designed to selectively inhibit and degrade mutant PI3Ka proteins, sparing the wildtype form, which has been a limitation of earlier generation PI3Ka inhibitors due to associated toxicities like hyperglycemia. The molecule binds to a unique allosteric site on the PI3Ka catalytic subunit (p110a), allowing for highly specific targeting of hotspot kinase and helical domain mutations. This dual mechanism of inhibition and proteasomal degradation drives potent and sustained tumor pathway suppression.

The first patient was enrolled at START San Antonio under the supervision of Dr. Amita Patnaik. The trial, cleared by the US FDA in April 2025, will assess safety, pharmacokinetics, pharmacodynamics, and early antitumor activity. ETX-636 will be tested both as a monotherapy and in combination with fulvestrant, a standard-of-care estrogen receptor degrader.

“Dosing our first patient in this first-in-human trial is a significant achievement,” said Ron Peck, Chief Medical Officer at Ensem. “ETX-636 offers the potential for a safer, more effective treatment option by targeting only mutant PI3Ka.”

CEO Shengfang Jin added that ETX-636 is the second novel oral therapy from Ensem to enter clinical development, with more pipeline INDs expected in 2026.

Preclinical data demonstrated that ETX-636 delivers deep tumor regression without disrupting glucose metabolism, setting it apart from earlier therapies in the PI3Ka space.