Six of nine breast cancer patients treated with an experimental gene therapy from Ziopharm Oncology(ZIOP) - Get Report experienced a potentially dangerous immune system-related toxicity known as cytokine release syndrome, the company said Monday.

Ziopharm said the incidence of cytokine release syndrome "was higher than expected" but the small, phase II study will continue.

The company's gene therapy, known as Ad-RTS-hIL-12, is injected directly into a patient's tumor and is designed to produce interleukin-12, a powerful protein known to stimulate T cells in the immune system to kill cancer cells.

Cytokine release syndrome (CRS) occurs when T cells become overstimulated, causing a serious immune system reaction akin to a severe allergic reaction. Severe CRS can be fatal and even moderate cases need to be closely monitored.

On Monday, Ziopharm reported interim results from a phase I/II study of Ad-RTS-hIL-12 in patients with advanced breast cancer. The study is intended to enroll 40 breast cancer patients still responsive to chemotherapy. Upon entering the study, the women take a break from chemotherapy and are injected with Ziopharm's Ad-RTS-hIL-12 gene therapy.

Among the first nine patients enrolled, four patients experienced grade 2 CRS, usually defined as a patient with low blood pressure and low oxygen levels requiring supportive care and organ monitoring.

Two additional breast cancer patients treated with Ziopharm's gene therapy experience grade 1 CRS, which involves a fever and other constitutional symptoms.

While CRS can be fatal in severe cases, the side effect is tolerated and generally acceptable for when T cell therapies wipe out cancer cells completely, as has been reported by Kite Pharma, Juno Therapeutics and Novartis.

Ziopharm's IL-12 gene therapy has not been similarly effective. In Monday's update, one of the nine breast cancer patients entered the study with stable disease and reported a partial tumor response 12 weeks after treatment with Ziopharm's gene therapy. The response disappeared 6 weeks later.

One patient entered the study stable and remains stable at week 12. A third patient entered the study with a partial response but has now seen her tumor start to grow again at week 12. A fourth patient entered the study with stable disease but her tumor started to grow again at week 12.

Ziopharm described these results as "promising," in a statement.

In July, a brain cancer patient died from bleeding in the skull following treatment with Ziopharm's Ad-RTS-hIL-12. The company later attributed the patient's death to his cancer and not its gene therapy. A separate study of Ad-RTS-hIL-12 in skin cancer patients showed no anti-tumor activity.

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