Antitumour efficacy of the nitroreductase-armed oncolytic adenovirus ONYX-411NTR in combination with dinitrobenzamide mustard prodrugs in preclinical models

Abstract

Oncolytic viruses that selectively replicate in and lyse cancer cells are a promising approach for the treatment of tumours that are resistant to conventional therapies. Clinical experience has shown that oncolytic viruses are safe and well tolerated but possess modest single agent activity. One approach to improve the efficacy of oncolytic viruses is to utilise their tumour tropism to deliver genes encoding enzymes able to activate prodrugs. ONYX-411 is an oncolytic adenovirus that replicates in cells that carry dysfunctions in the retinoblastoma (pRb) pathway, a common hallmark of cancer. ONYX-411 was ‘armed’ by inserting the Escherichia coli nfsB nitroreductase (NTR) gene into the E3B region of the viral genome under the control of the endogenous E3 viral transcriptional machinery. NTR is an oxygen-insensitive nitroreductase that is capable of activating dinitrobenzamide mustard (DNBM) prodrugs to cytotoxic metabolites. The main objective of this thesis was to determine the extent and mechanism of the therapeutic interaction between ONYX-411NTR and DNBM prodrugs. A fluorogenic probe was developed to monitor NTR activity non-invasively and revealed robust, replication dependent NTR activity in ONYX-411NTR-infected neoplastic but not primary human cell lines. In vitro exposure of ONYX-411NTR-infected cells to therapeutically relevant concentrations of the DNBM prodrugs (SN 27686 or PR-104A) did not inhibit virus replication. Tumour growth delay studies of systemic ONYX-411NTR followed by prodrug demonstrated different outcomes in three models (H1299, C33A, 22Rv1). To establish predictable viral infection of tumours a pre-infection model was developed using HCT 116 xenografts. This methodology demonstrated that prodrug administration (SN 28343 or PR-104) provided significant inhibition of tumour growth without suppression of ONYX-411NTR replication. Follow-on studies using intravenous virus administration confirmed titre amplification with time (24-fold between day 3 and 13 post administration; P < 0.001) and a marked survival gain for the virus/prodrug combinations. Neither the prodrugs nor ONYX-411NTR were active as single agents. The improvement in efficacy for the combination of ONYX-411NTR and prodrug was conditional on NTR-dependent prodrug activation resulting in improved virus distribution within the tumour. PR-104 is currently in clinical development making the combination of ONYX-411NTR with PR-104 a promising strategy for cancer selective therapy.