403. Toca Retroviral Replicating Vector in Tumor and Blood from Clinical Trial Subjects with Recurrent High Grade Glioma

Abstract

Toca 511 (vocimagene amiretrorepvec) is a retroviral replicating vector (RRV) based on an amphotropic murine gammaretrovirus, that encodes a cytosine deaminase transgene that allows infected cells to selectively convert the antifungal drug 5-fluorocytosine (5-FC) into the antineoplastic drug 5-fluorouracil (5-FU). Toca 511 can be delivered by multiple routes, selectively infects and spreads in tumor cells and through multiple mechanisms can elicit an anti-tumor immune response. Thus, clinically, treatment with Toca 511 and extended-release 5-FC (Toca FC) is expected to selectively destroy tumor cells within the body, while leaving healthy cells unharmed. Toca 511 and Toca FC have been administered to >120 high grade glioma subjects in three phase I studies (NCT01156584, NCT01470794, NCT01985256) and, based on results from these trials, a phase 2/3 trial (Toca 5) has recently started recruitment (NCT02414165). Although Toca 511 plus Toca FC has been well tolerated, we investigated potential off-target viral infection and integration, and whether such off-target events have potential toxicity; we also investigated the stability profile of the Toca 511 genome after infection of tumor targets and off-target tissue (blood). To date, few subjects (approximately 10-25%) show quantifiable viral signal (LOQ approximately 4000 copies/mL) in blood after initial clearance, following Toca 511 injection into the brain tumor or tumor bed. In all cases, quantifiable virus RNA and DNA signal was cleared from blood. In order to better understand interactions among Toca 511 and subjects’ tumor and blood, we systematically mapped Toca 511 integration sites from subject samples with detectable virus and also sequenced, at high depth, integrated Toca 511 genomes, and in some cases RNA genomes, from these samples. Toca 511 integration profiles display a preference for integration near active transcription start sites, as has been seen generally for gammaretroviruses. There was no evidence for clonal expansion of Toca 511 integrated sites/cells or preferential retrieval of sites nearby oncogenes. Toca 511 sequences display an array of mutations in the tumor and blood, including transitions consistent with cytosine deaminase activity. The data provide a molecular correlate to the clinical safety profile of Toca 511 and Toca FC treatment as well as molecular characterization of RRV after infection, replication and therapeutic use in humans.