Engineering a novel HSV-1 strain for oncolytic therapy of solid tumors

Oncolytic herpes simplex virus (HSV)-1-derived viruses are being developed for cancer treatment. Here, we describe the isolation of a novel strain of HSV-1 and its engineering to safely harness it as an oncolytic therapeutic. This strain (UT1a) was isolated from a de-identified consented patient biorepository. CRISPR-Cas9-based recombination was utilized to insert bacterial artificial chromosome (BAC) genes into the viral <i>UL39</i> and <i>UL40</i> locus, resulting in the deletion of both large and small subunits of the viral ribonucleotide reductase (RR). Subsequent deletion of viral <i>RL1</i> genes encoding the neurovirulence factor γ34.5 resulted in OncoDelta (OncoD), a virus deleted for <i>UL39</i>, <i>UL40</i>, and both copies of <i>RL1</i>. OncoD retained tumor-cell-specific cytotoxicity and replication; was safe and non-toxic in intracranial injections in naive mice up to doses of 5 × 10⁶, the maximal injectable dose for OncoD; and showed significant anti-tumor immune-activating potential in multiple tumor models. Transcriptome profiling of OncoD showed that it impaired DNA damage repair pathways and hence synergized with radiation to improve therapeutic response <i>in vitro</i> and <i>in vivo</i>.