Study examines vaccine therapy in prostate cancer with rising prostate-specific antigen (PSA) levels

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A vaccine therapy that stimulates the body's own immune defenses can be given safely and earlier in the course of prostate cancer progression, according to investigators from a multi-institutional cooperative group. Their study aims to increase treatment options for patients with prostate cancer who have an early relapse.

As part of a phase II clinical trial, adult patients with advanced prostate cancer (as evidenced by two rising prostate-specific antigen [PSA] values and no visible metastasis) whose cancer is resistant to hormone therapy and had either surgery or radiation were recruited from member institutions in the ECOG-ACRIN Cancer Research Group.

ECOG-ACRIN investigators examined two different experimental treatment options in this work, published in the current online edition of European Urology (2014; doi:10.1016/j.eururo.2014.12.010).

In step one, patients were treated with PROSTVAC-V/TRICOM and PROSTVAC-F/TRICOM. PROSTVAC-V is derived from a vaccinia virus that was used for many years to vaccinate against smallpox. This virus is modified to produce a PSA protein that helps focus the body's immune response to the PSA in the prostate tumor. In addition, it is modified to produce three other proteins that help increase an immune cell's ability to destroy its target (TRICOM).

PROSTVAC-F is made from the fowlpox virus, which is found in birds and not known to cause any human disease. It contains the same genetic material as PROSTAC-V, but is given multiple times to further boost the body's immune system.

Patients in the study were given one cycle of PROSTVAC-V/TRICOM followed by PROSTVAC-F/TRICOM for subsequent cycles in combination with a drug known as GM-CSF. GM-CSF is a protein normally made by the body to increase the amount of certain white blood cells and make them more active. When in drug form, it is used to boost the body's immune system to fight off disease.

After 6 months from first treatment, 25 of 40 eligible patients (63%) were found to have no disease progression and experienced minimal toxicity. Median pretreatment PSA velocity was 0.13 log (PSA)/month as compared to median postregistration (6 months) velocity of 0.09 log (PSA)/month, which represents an improvement in PSA doubling time from 5.3 months to 7.7 months.

The second part of the study included the addition of hormone therapy (androgen ablation) to the PROSTVAC-VF/TRICOM combination. In the 27 patients eligible for this step, 20 patients (74%) experienced a complete response at 7 months.

“Previous studies by the ECOG-ACRIN Cancer Research Group and others have shown it is optimal to explore agents like PROSTVAC that harness the body's own defenses in shutting down cancer. With our current findings demonstrating the safe use of combination vaccine therapy earlier in the course of prostate cancer progression, we are laying the groundwork for future immunotherapy options for this patient population,” said Cancer Institute of New Jersey Director Robert S. DiPaola, MD, who is the lead author on the report.

DiPaola and colleagues noted that while this research is supportive of larger federally funded studies, the study is limited by the small number of patients and absence of a control group.

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