It is not clear if pre-existing anti-virus immunity or multicycle treatment of oncolytic virus (OV) can affect the treatment outcome. This study is to investigate the impact of pre-existing anti Herpes Simplex Virus type-1 (HSV-1) immunity or multicycle OV treatment on the efficacy of VG161, a novel HSV-1 OV armed with IL12, IL15 with its receptor α unit (IL15RA), and a PD-L1 peptide blocker (PDL1B). VG161 has entered phase I trial in Australia and China.

Seventy Balb/c mice were divided into 2 groups. One group (35 mice) was subcutaneously immunized by mVG161 (a version of VG161 expresses mouse IL12, human IL15 and PDL1B) on day 1 (5x105 PFU/mouse) followed by a boost on day 7. The other group (35 mice) was non-immunized. On day 7 post-boost, the blood was collected from tail to determine anti-HSV antibody. At the same time, all 70 mice were subcutaneously implanted with CT26 mouse colon cancer cells. Once the tumors reached 100 mm3, 20 mice (10 from each group) were treated with intra-tumoral (IT) injection of either mVG161 at a dose of 5×106 PFU/mouse or Vehicle, once a day for 5 days.

Hepatic cancer (HC) PDX tissues were dissected into pieces of 27 mm3 and subcutaneously implanted into humanized mice established by transplantation of 1×104 human CD34 stem cells (HLA matched with PDX). When the tumor reached 100-150 mm3, animals were divided into 4 groups (10 mice/group): vehicle control, positive control (oxaliplatin 5 mg/kg and gemcitabine 100 mg/kg), and 2 groups treated with VG161 at high dose (7.75×106 PFU) or low dose (3.10×105 PFU). The OV treatments were given for 3 cycles with IT injection of VG161 once a day for day 1-5 of each cycle.

Tumor, spleen, and peripheral blood mononuclear cells were collected for further analysis.

All HSV pre-immunized mice but not naïve mice were HSV-1 antibody positive. By day 19 post-treatment, mVG161 significantly inhibited CT26 tumor growth in both HSV-1 pre-immunized and non-immunized groups, while all control mice were euthanized due to tumor size reached 2000 mm3. Starting from day 27, tumors grew back in non-immunized group but not in pre-immunized group, resulting in a significant difference of tumor size between the 2 groups on day 31 to 39 (p<0.01).

As compared to the negative controls, HC PDX tumors in both treatment groups were significantly smaller with percentage of T/C less than 40% on day 72. The median time for 3-fold increase of tumor size is 41.5 days (95%CI: 32-58) in the vehicle control, 74 days (95%CI: 51-undefined) in high dose group, and not reached in low dose group (p<0.01). Repeated treatment (2-3 cycles) increased the anti-cancer efficacy of VG161.

The anti-cancer efficacy of VG161 can be enhanced by pre-immunization of HSV-1 or multicycle administration when the virus is given by IT, suggesting potential clinical benefits of HSV-1 based OV therapy in HSV sero-positive patients and multicycle administration of VG161 for long term maintenance treatment.

Citation Format: Jun Ding, Jason Sun, Yana Murad, Erica Lee, Ismael Samudio, William Jia, Ronghua Zhao. Pre-existing HSV-1 Immunity enhances anti-cancer efficacy of a novel immune stimulating oncolytic virus - VG161 [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1913.