Ambrx Biopharma provides update on APEX-01

Ambrx Biopharma announced a clinical update regarding its proprietary anti-PSMA antibody drug conjugate, ADC, ARX517 in the ongoing APEX-01 Phase 1 / 2 dose escalation and dose expansion clinical trial in metastatic castration-resistant prostate cancer. Following the completion of the 21-day DLT observation period from APEX-01 dose escalation Cohort 9, no DLTs or SAEs were observed. Two of the three patients in Cohort 9 had a rapid PSA reduction of 91% and 33% at the first PSA assessment at three weeks post-treatment and following only one dose of ARX517. A PSA decrease was not reported in the third patient because the patient had non-PSA secreting mCRPC. On Monday November 27, the safety monitoring committee voted to dose escalate to 4.5 mg/kg as well as expand patients at the 3.4 mg/kg dose. All expansion cohorts, Cohort 4, Cohort 6, and Cohort 8 are now fully enrolled. “We continue to observe no DLTs or SAEs at the highest ARX517 dose tested. We believe this is a direct result of the stability of conjugation supported by PK data presented at ESMO last month. While we are opportunely positioned to explore higher doses, we have already observed significant PSA50 decline and long duration on treatment at 2.0 mg/kg,” said Sandra Aung, Ph.D., Chief Clinical Officer of Ambrx. “The high interest in our PSMA-targeting ADC and pace of enrollment in the APEX-01 study has been outstanding and based on this we are anticipating to reach a recommended phase 2 dose by early next year.”