Johnson & Johnson reported first Phase 1 results for pasritamig plus JNJ-9401 in advanced prostate cancer: 11 of 15 patients at the recommended regimen had PSA falls of at least 50 percent, but the readout is small and preliminary, with a Phase 3 trial already under way.
Johnson & Johnson has reported deep falls in a blood marker of prostate cancer from an early trial of a two-antibody treatment, with nearly three quarters of patients at the chosen dose seeing their levels at least halve. The combination pairs pasritamig, which directs T cells at a protein called KLK2, with JNJ-9401, also listed in the conference programme as JNJ-87189401, which targets a second protein, PSMA, to give the immune system two complementary signals against the tumour. The results, from 15 patients, were presented at the Society for Immunotherapy of Cancer annual meeting in Phoenix.
At the recommended regimen, 11 of the 15 patients, 73 percent, had prostate-specific antigen, or PSA, fall by at least half, and seven of them, 47 percent, saw falls of at least 90 percent. Four patients reached undetectable PSA, below 0.2 nanograms per millilitre, a level the presenting researcher described as very uncommon in men as heavily pretreated as these. All seven patients with falls of 90 percent or more stayed on treatment for over six months without their disease progressing. The patients had metastatic castration-resistant prostate cancer that had already been through a median of four previous lines of systemic therapy.
The safety picture in this early readout was also encouraging. No patient had a treatment-related adverse event of grade 3 or higher, and there were no dose-limiting toxicities or treatment-related events leading to stopping treatment or death. Three patients, 20 percent, developed cytokine release syndrome, an inflammatory reaction that can follow T-cell activation, but all cases were mild grade 1 fever that needed no treatment with tocilizumab. No cases of a related brain toxicity known as ICANS were reported.
Fifteen patients is a small base for percentages, and the company says so itself: the release warns that the findings rest on a small number of patients and that larger studies are needed to establish clinical benefit and safety. The PSA figures describe a blood marker, not survival, and the trial is open-label, so there is no comparison group at this dose. The numbers do check out against the meeting presentation, though: the same 11 of 15 and 7 of 15 figures appear in the conference report, along with the four undetectable PSA results and follow-up showing responses still going at a median of 7.4 months. The regimen details also match, with JNJ-9401 given as a 200 milligram injection under the skin every six weeks and pasritamig as a 300 milligram intravenous dose on the same schedule, after two weekly step-up doses.
Prostate cancer has proved stubborn ground for immunotherapy. Checkpoint inhibitors, which have transformed treatment in several other cancers, have shown minimal benefit here, and men whose disease keeps growing despite hormone-blocking treatment face a median survival of roughly two to three years. The dual-target idea is to attack the tumour on two fronts at once: pasritamig aims T cells at KLK2, a protein found almost only on prostate cells, while JNJ-9401 adds a second, costimulatory signal through PSMA to keep those T cells working. If the approach works in larger trials, it would give doctors an immune treatment for a disease where few exist, delivered every six weeks in an outpatient setting.
The combination is already moving into that larger test. A phase 3 study called KLK2-comPAS is under way to see whether adding JNJ-9401 to pasritamig improves outcomes in late-line metastatic castration-resistant prostate cancer, and pasritamig plus the chemotherapy docetaxel is being tested in a separate phase 3 trial in first-line disease. Laura Graham of the University of Colorado, who presented the data, said the depth of PSA falls, including responses sustained for six months or longer, provided encouraging early evidence that warranted further investigation in a larger study. The phase 1 study itself continues, with 144 patients enrolled across its dose levels as of 15 June 2026.