SS1P was developed at the National Cancer Institute and is designed to target proteins that are over-expressed in mesothelioma tumors. This is one of a few promising immunotherapy drugs emerging for mesothelioma.

Making Immunotherapy More Effective

This trial is testing an experimental immunotherapy drug (SS1P) in combination with chemotherapy drugs, pentostatin and cyclophosphamide. The purpose of the chemotherapy is to make the new immunotherapy more effective for the patients. This trial is for patients with either peritoneal or pleural mesothelioma.

Medications in this Trial

  • Pentostatin
    Pentostatin – This is an antibiotic chemotherapy drug that is most commonly used to treat leukemia. It stops the growth of cancerous cells and can suppress the immune system.
  • Cyclophosphamide
    Cyclophosphamide – This chemotherapy drug is also used to treat leukemia and is known as an alkylating agent. Alkylating agents poison cells rather than inhibiting growth. It is also effective as an immunosuppressive agent.
  • SS1P
    SS1P – This immunotherapy agent is known as an immunotoxin. It is a genetically engineered antibody (a key component of the immune system) loaded with a chemical that is toxic to mesothelioma cells.

The trial with SS1P is a good opportunity for patients who haven’t responded to chemotherapy and those who aren’t eligible for surgical treatment. This trial is designed for patients who have expended all other options. This is a non-randomized study, so every participant is treated with the experimental drug.

Get Help Enrolling in This Trial

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Mesothelioma clinical trials offer a variety of treatments that aren’t available through standard treatments. This is a notable benefit for mesothelioma patients since treatment options become more limited as the disease spreads.

Patients may choose to participate in this trial because:
  • SS1P can inhibit tumor growth in patients for whom chemotherapy isn’t effective.
  • This trial is geared toward patients with limited options or a late stage diagnosis.
  • Patients receive care from one of the most prestigious cancer centers in the world.

How Do Immunotoxins Like SS1P Work?

SS1P was developed at the National Cancer Institute by Dr. Ira Pastan. The drug is designed to target a protein that is commonly found on the surface of mesothelioma cells. This protein is called mesothelin.

Immunotoxins like SS1P are able to target mesothelin because they are actually antibodies loaded with toxic material. Antibodies are proteins essential to the function of a healthy immune system. Antibodies sound the alarm for the immune system to destroy harmful intruders by combining with them. However, immunotoxins only need to combine with mesothelioma cells to begin delivering the toxic material.

The antibody in SS1P is one that specifically combines with mesothelin. Once it combines with the protein, it delivers the toxin to the mesothelioma cell to which it is attached on the surface.

Patients who participated in the Phase I of this study experienced a major regression in their disease.

Suppressing the Immune System

Immunotoxins have been successful in certain types of cancer, especially leukemia. Part of the reason they have been successful is because leukemia is a disease that severely weakens the immune system.

Mesothelioma patients have normal immune systems, complete with strong and abundant immune cells. This is why suppressing the immune system of the mesothelioma patients is a pivotal part of the SS1P clinical trial. This needs to be done so that the immune cells can’t attack the immunotoxin before it reaches the mesothelioma cells.

In patients with mesothelioma, the immune system sees SS1P as an intruder and starts building defenses against it after the first round of treatment. Any following rounds of treatment with SS1P become increasingly ineffective as a result. The way to combat this natural immune reaction is to suppress the immune system with chemotherapy. This is vital as it then allows the immunotoxin to reach its target.

The immunosuppressive properties of pentostatin and cyclophosphamide may be the answer to SS1P’s effectiveness. The most common side effect as noted by the American Cancer Society in both of these chemotherapy agents is a weakened immune system.

Who Is This Trial For?

Like most immunotherapy trials, this trial is for late stage mesothelioma patients who have already received some form of treatment. This trial is geared toward patients without many treatment options remaining.

General eligibility criteria includes:

  • Epithelioid or biphasic cell type
  • Patient has been treated with standard chemotherapy regimen
  • Prognosis greater than 3 months
  • Must not be eligible for surgery
  • Must have been 4 weeks since last treatment


There is an estimated enrollment of 47 participants for this study. This clinical trial is being conducted solely at the National Cancer Institute at the National Institutes of Health. Dr. Hassan, the principal investigator, is an internationally renowned mesothelioma specialist who conducts many clinical trials.

  • National Institutes of Health Clinical Center

    National Institutes of Health Clinical Center

    Principal Investigator: Dr. Raffit Hassan
    Dr. Hassan is one of the leaders in immunotherapy research for mesothelioma. He made an encouraging breakthrough with the first phase of the SS1P trial, with 30 percent of the enrolled patients experiencing tumor shrinkage and increased survival time. “In addition,” said Dr. Hassan, “two patients who had previously progressed on chemotherapy had a tumor response when treated with chemotherapy following SS1P.”

Getting Enrolled

This clinical trial is best for patients who have received standard treatment for their disease and are looking for more options. Patients who aren’t responding to chemotherapy may especially benefit. If you are considering participating in this study, we can get you connected. Get connected to this clinical trial today.