Janux Therapeutics has initiated Phase 1b expansion studies of JANX007, a PSMA-targeting Tumor Activated T Cell Engager (TRACTr), in patients with metastatic castration-resistant prostate cancer (mCRPC). This step follows promising results from the Phase 1a dose-escalation trial and reflects the company’s continued efforts to develop safer, more effective T cell-based therapies for solid tumors.

JANX007 is designed to remain inactive in the bloodstream and activate only within the tumor microenvironment, reducing off-tumor toxicity while maximizing anti-cancer activity. In the Phase 1a portion of the ENGAGER-PSMA-01 trial, 16 mCRPC patients—who had received a median of four prior treatments—were enrolled. As of April 21, 2025, data showed a median radiographic progression-free survival (rPFS) of 7.5 months across all patients, with those receiving the 6mg and 9mg doses achieving a slightly longer median rPFS of 7.9 months. The six-month rPFS rates were 65% for the full cohort and 78% for the higher-dose group. The safety profile remained consistent with prior updates, reinforcing confidence in the tolerability of JANX007.

The first Phase 1b expansion study will focus on taxane-naïve mCRPC patients, testing two dose regimens (0.3/2/6mg and 0.3/2/9mg), delivered either weekly or every two weeks. Additional Phase 1b studies are planned to evaluate JANX007 in combination with androgen receptor inhibitors in taxane-experienced patients, as a monotherapy in patients resistant to PARP inhibitors, and in populations previously treated with next-generation hormone therapies and taxanes. These studies are intended to inform dose selection for future registrational trials.

Later this year, Janux plans to share additional clinical data on JANX007 as well as on JANX008, its EGFR-targeting TRACTr candidate. The company will also host an R&D Day to highlight progress across its pipeline of tumor-activated immunotherapies. Through its TRACTr platform, Janux aims to deliver potent T cell activation where it is needed most—at the tumor site—offering a new path forward for patients with difficult-to-treat solid tumors.

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