2042 Background: G207 is a doubly mutated (deletion of both γ134.5 loci, insertional inactivation of UL39) herpes simplex virus (HSV)-1. Safety and efficacy of intracerebral inoculations of G207 to patients suffering from recurrent malignant gliomas have been demonstrated in previous clinical trials. METHODS: In this phase I clinical trial, a total of 1 x 10(9) plaque forming units (pfu) G207 were administered by five stereotactic injections of 0.2 mL each into regions of recurrent malignant glioma defined by MRI, followed by focal radiation therapy 24 hours post injection. Included patients suffered from inoperable pathologically proven recurrent glioblastoma multiforme (GBM) or anaplastic astrocytoma (AA) which was progressive despite radiotherapy or chemotherapy and failed external beam radiotherapy > 5 Gray prior to study enrolment. RESULTS: 9 patients were treated in this phase I study. 5 patients were suffering from relapsed GBM, 4 patients were suffering from relapsed AA. 1 month after treatment, 3 patients (3xGBM) showed SD, 2 patients (1xGBM, 1xAA) PR, and 4 patients (1xGBM, 3xAA) PD. The 2 patients with initial PR (1xGBM, 1xAA) were re-treated with G207/Irradiation at time point of tumor recurrence, showing PR one month after re-treatment again. Median overall survival time for all 9 patients was 229 days (7.6 months), with one patient still alive at time of abstract submission. In patients suffering from relapsed GBM, mOS was 7.4 months, in patients suffering from relapsed AA, mOS was 9.25 months. 20 serious adverse events occurred in this study, only 3 were possible/probable related to G207/irradiation. Within persistent areas of tumor, HSV staining was present by using a polyclonal antibody for HSV, indicating intratumoral G207 replication (proof of concept). CONCLUSIONS: In 9 patients suffering from relapsed GBM or AA, stereotactic intracerebral G207 inoculation followed by radiation therapy was feasible, safe, and induced clinically relevant responses. G207/Irradiation re-treatment was possible and induced anew clinical responses. Median overall survival is superior to published data in this patient population. Therefore, further clinical development of G207 in GBM is medically reasonable. [Table: see text].