Pasritamig (JNJ-78278343) is an investigational immunotherapy being studied for people with advanced prostate cancer, specifically metastatic castration-resistant prostate cancer (mCRPC). This is a stage of prostate cancer where the disease has spread to other parts of the body and no longer responds well to hormone-lowering treatments.
Pasritamig is not yet approved for routine clinical use. At this time, it is available only through clinical trials. It is being studied mainly in patients whose cancer has continued to grow despite standard treatments such as hormone therapy and chemotherapy (Stein et al., 2025).
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How Does Pasritamig Work?
Pasritamig (JNJ-78278343) is a type of immunotherapy, which means it works by helping your own immune system fight cancer. The immune system includes T cells, which are white blood cells that can recognize and destroy abnormal cells. Prostate cancer cells often escape detection by these T cells. Pasritamig works by acting like a bridge:
- One part of the drug attaches to T cells
- The other part attaches to a protein called KLK2, which is found mainly on prostate cancer cells
By bringing T cells directly into contact with cancer cells, Pasritamig helps the immune system recognize and attack the tumor more effectively (National Cancer Institute, 2024).
Why Is KLK2 Important In Prostate Cancer?
KLK2 is a protein that is closely linked to prostate tissue and prostate cancer. It is produced mainly by prostate cells and is regulated by male hormones called androgens.
Because KLK2 is largely limited to prostate tissue, targeting it may help focus the immune response on cancer cells while reducing the risk of damage to healthy organs elsewhere in the body. This targeted approach is important because immune-based treatments can sometimes cause side effects when the immune system becomes overly active (Stein et al., 2025).
Who Is Pasritamig Being Studied For?
So far, Pasritamig (JNJ-78278343) has been studied in patients who:
- Have metastatic prostate cancer
- Have disease that no longer responds to hormone therapy
- Have already received other treatments, including chemotherapy and newer prostate cancer drugs
This means Pasritamig (JNJ-78278343) is currently being explored mainly for patients with limited remaining standard treatment options (Baldini et al., 2025).
What Have Clinical Studies Shown So Far?
Early clinical trials have shown encouraging results, although these studies are still in early phases and involved relatively small numbers of patients.
Among patients who received the dose selected for further study:
- About 4 out of 10 patients experienced a significant drop in PSA levels, a blood test used to monitor prostate cancer
- On average, the cancer remained under control on scans for nearly 8 months, which is promising for patients with advanced disease
Some patients were able to remain on treatment for extended periods, suggesting that Pasritamig (JNJ-78278343) may provide lasting benefit for certain individuals (Johnson & Johnson, 2025).
What Side Effects Have Been Reported?
Like many immunotherapies, Pasritamig (JNJ-78278343) can cause side effects related to immune system activation.
The most closely monitored side effect is cytokine release syndrome (CRS). CRS can cause symptoms such as fever, chills, fatigue, and flu-like symptoms.
In clinical studies so far:
- CRS occurred in fewer than 10% of patients
- All reported cases were mild
- No severe CRS was reported at the recommended dose
Other side effects were generally manageable, and doctors closely monitored patients during treatment, especially during the first doses (Baldini et al., 2025).
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How Is Pasritamig Given?
Pasritamig (JNJ-78278343) is given through a vein (intravenous infusion). To reduce the risk of side effects, treatment starts with small step-up doses, allowing the body to adjust. After this initial period, Pasritamig is given at a full dose approximately once every six weeks. This relatively infrequent dosing schedule may make treatment more convenient compared with therapies that require weekly hospital visits (Stein et al., 2025).
Is Pasritamig Approved?
No. Pasritamig (JNJ-78278343) is still an experimental treatment. It is currently being studied in larger clinical trials, including Phase III studies, to determine whether it improves outcomes such as survival and quality of life. Until these studies are completed and reviewed by regulatory authorities, Pasritamig (JNJ-78278343) remains available only through clinical trials (ClinicalTrials.gov, 2025).
What Should Patients Know About Clinical Trials?
Clinical trials are carefully monitored research studies designed to test new treatments. Patients who participate in trials receive close medical supervision and may gain access to promising new therapies before they are widely available.
If you have advanced prostate cancer and are interested in Pasritamig (JNJ-78278343), your oncology team can help determine whether a clinical trial may be appropriate for you and discuss potential benefits and risks (National Cancer Institute, 2024).
What Comes Next For Pasritamig?
Ongoing Phase III studies will help determine whether Pasritamig (JNJ-78278343) becomes a new treatment option for advanced prostate cancer. Researchers are also exploring how best to combine it with other therapies and which patients are most likely to benefit.
While more research is needed, Pasritamig (JNJ-78278343) represents an important step forward in developing more precise and effective immunotherapies for prostate cancer.