Atara Biotherapeutics Inc. said Tab-cel, its investigational T cell immunotherapy, showed durable effects in patients infected with a certain tumor virus after undergoing transplant procedures in two mid-stage trials.

The phase 2 trials evaluated Tab-cel, or tabelecleucel, in patients with Epstein-Barr virus associated post-transplant lymphoproliferative disorder, or EBV-PTLD, after undergoing either a solid organ transplant or an allogeneic hematopoietic cell transplant, or HCT. These patients had failed initial treatment with Rituxan, or rituximab, Roche Holding AG's blood cancer drug.

Epstein-Barr virus is a common virus that causes infectious mononucleosis and is associated with certain cancers such as Burkitt lymphoma, immunoblastic lymphoma and nasopharyngeal carcinoma. Lymphoproliferative disorders refer to several conditions in which infection-fighting cells of the immune system, called lymphocytes, are produced in excessive quantities.

For the 35 patients with EBV-PTLD following a hematopoietic cell transplant who were treated with Tab-cel, 68% were alive for one year while 55% survived for three years. The average period of time that the patients were alive was not reached after 23.3 months of follow-up.

The expected median survival for patients with EBV-PTLD following HCT who have failed rituximab first-line therapy is 16 to 56 days.

Meanwhile, in the 14 patients with EBV-PTLD after a solid organ transplant, 64% survived for a year while 43% were alive for three years. The average survival period for the group was 21.3 months, versus the expected 12- to 13-month median survival in patients with EBV-PTLD following organ transplant who fail to achieve a complete response to initial treatment with rituximab.

Tab-cel reduced the disease in 69% of patients who had received a hematopoietic cell transplant and in 50% of patients who had gotten a solid organ transplant.

The South San Francisco, Calif.-based company said results from the first phase 3 study for Tab-cel and the submission of an EU conditional marketing authorization application are expected in the first half of 2019.