Janssen Initiates Phase 1b Trial of KLK2/CD3 Bispecific Antibody Pasritamig (JNJ-78278343) in Combination with PSMA ADC ARX517 (JNJ-95298177) for Advanced Prostate Cancer

▎Armstrong

On July 24, 2025, Johnson & Johnson registered a Phase 1b clinical trial on Clinicaltrials.gov for the combination therapy of JNJ-78278343 and JNJ-95298177 in patients with advanced prostate cancer.

This Phase 1b clinical trial plans to enroll 110 patients with advanced prostate cancer. According to the clinical trial registry information, JNJ-78278343 is a KLK2/CD3 bispecific antibody, and JNJ-95298177 is a PSMA ADC new drug (ARX517) developed by Ambrx (which has been acquired by Johnson & Johnson).

At the ASCO meeting in June this year, Johnson & Johnson announced the Phase I clinical data of Pasritamig (JNJ-78278343) for the treatment of trend-resistant prostate cancer. Among 84 evaluable patients, the ORR was 8.3%. In the RP2D efficacy analysis of 33 patients, the mPFS was 7.85 months, and the PSA50 response rate was 42.4%.

ARX517 is a differentiated PSMA ADC with AS269 as the payload, utilizing site-specific conjugation with a DAR value of 2.

ARX517 has previously disclosed Phase I clinical data.

Summary

PSMA radiopharmaceuticals have become a star product in the field of prostate cancer, with various drug formats targeting this site actively advancing, including PSMA ADC, PSMA/CD3 bispecific antibodies, and more. In addition, STEAP1, KLK2, and others are also being actively developed for prostate cancer indications.