Study of therapeutic vaccine shows promise for metastatic colorectal cancer

A therapeutic cancer vaccine has shown effectiveness when given alongside chemotherapy to patients with metastatic colorectal cancer in a phase II trial, according to researchers at Oxford BioMedica (UK) Ltd.

The study found that six of the 17 metastatic colorectal cancer patients in the study showed tumor shrinkage, classified as complete or partial responses following independent expert review.

The study, reported in the August 1 issue of Clinical Cancer Research, a journal of the American Association for Cancer Research, was designed to demonstrate the safety and immunogenicity of the vaccine, called modified vaccinia Ankara-encoding 5T4 (TroVax), when used alongside standard chemotherapy. The research was funded by Oxford BioMedica which is developing the vaccine in partnership with Sanofi-Aventis.

Unlike preventative vaccines, such as the human papillomavirus vaccine to prevent cervical cancer, TroVax is a therapeutic vaccine, designed to stimulate the immune systems of patients who already have cancer. The vaccine consists of an attenuated (non-disease causing) version of the vaccinia virus modified to deliver the gene for 5T4, a protein found in many tumors.

“The idea is that the modified virus enters cells, produces the tumor protein and stimulates the immune system,” said lead study author Richard Harrop, Ph.D., vice president of clinical immunology at Oxford BioMedica. “To give a vaccine alongside chemotherapy might seem counterintuitive, since chemotherapy can weaken the immune system, but our study shows that TroVax could be complementary to standard chemotherapy, enhancing the immune response to tumors.”

The target of this immuno-therapy approach is a tumor antigen called 5T4, a protein embedded within the membrane of cancer cells. The protein is rarely found in normal tissues, but is produced at high levels by a wide range of human cancers, including colorectal, renal, gastric and ovarian. The production of 5T4 has been associated with cancer metastasis and poor prognosis for patients.

“Typically, the immune system doesn’t pay attention to this molecule, so by producing 5T4 artificially in combination with the ‘danger signals’ associated with a viral infection, we are demanding that the immune system take notice,” Harrop said. “TroVax causes cells at the

injection site to produce 5T4 in a way which agitates the immune system into producing antibodies and killer T cells. It is hoped that these two components of the immune system then migrate to tumors and kill them without harming any normal tissues.”

“In essence, it’s like turning up your stereo in the hopes that it will attract the police to your neighbor’s rowdy party,” Harrop said.

Harrop and his colleagues administered the vaccine to 17 patients with metastatic colorectal cancer just before, during and after the patients were treated with the standard chemotherapy regimen FOLFOX which consists of the agents: 5-fluorouracil, folinic acid and oxaliplatin.

Through the course of the study, the researchers monitored the patients for an immune response to 5T4. Eleven of the 17 patients who received the complete course of vaccinations (six injections) mounted strong immune responses to the 5T4 tumor protein. Of these 11 patients, six exhibited significant shrinkage of their tumors and one patient no longer had any detectable tumors. Researchers noted no complications stemming from TroVax vaccination or any other evidence that would call into question the safety of the vaccine.

While the study was not designed to prove that patients survived longer than would normally be expected, the researchers noted that, on average, the overall median survival was 68 weeks in all 17 vaccinated patients and 118 weeks in the 11 patients who received all six vaccinations.

According to Harrop, the researchers are currently testing the vaccine in a phase III trial in renal cancer patients in the U.S. and Europe and Sanofi Aventis is planning a phase III study in colorectal cancer.