A high-throughput pharmacoviral approach identifies novel oncolytic virus sensitizers

Abstract

Oncolytic viruses (OVs) are promising anticancer agents but like other cancer monotherapies, the genetic heterogeneity of human malignancies can lead to treatment resistance. We used a virus/cell-based assay to screen diverse chemical libraries to identify small molecules that could act in synergy with OVs to destroy tumor cells that resist viral infection. Several molecules were identified that aid in viral oncolysis, enhancing virus replication and spread as much as 1,000-fold in tumor cells. One of these molecules we named virus-sensitizers 1 (VSe1), was found to target tumor innate immune response and could enhance OV efficacy in animal tumor models and within primary human tumor explants while remaining benign to normal tissues. We believe this is the first example of a virus/cell-based "pharmacoviral" screen aimed to identify small molecules that modulate cellular response to virus infection and enhance oncolytic virotherapy.

Figure 1

Identification of novel viral sensitizers by high-throughput screening. (a) Dot plot representation of the high-throughput screening data. The y axis represents the viral sensitization factor where positive values suggest viral sensitization. This factor is defined as the logarithm of the cytotoxicity of compounds in absence of VSV over cytotoxicity of compounds in presence of VSV (refer to Materials and Methods section). The average of assay duplicates is plotted for each compound. The x axis represents each of the 12,280 compounds. Compounds exhibiting viral sensitization factor values >0.3 were considered for further validation (dots within shaded area). (b) The potential viral sensitizers identified were retested in a 96-well plate format for VSVΔ51-enhancing activity on 4T1 cells using 10 µmol/l concentrations of drug and a VSVΔ51 MOI of 0.03. A VSVΔ51 strain expressing RFP was used to visualize virus spread after 24 hours using fluorescence microscopy. SAHA (10 µmol/l) was used as a positive control. (c) Fold change in viral titers from supernatants collected from (b) after 48-hours incubation relative to vehicle-treated control. Arrow points to inset panel showing the molecular structure of VSe1 (3,4-dichloro-5-phenyl-2,5-dihydrofuran-2-one). CTRL, control; MOI, multiplicity of infection; RFP, red fluorescent protein; SAHA, suberoylanilide hydroxamic acid; VSe, virus-sensitizers; VSV, vesicular stomatitis virus.

Figure 2

VSe1 enhances VSVΔ51 spread and leads to synergistic cell killing in resistant cells. (a) VSVΔ51 resistant 4T1 murine breast cancer cells, CT26 murine colon cancer cells, 786-0 human renal cancer, and U251 human glioma cells were challenged with an RFP-expressing VSVΔ51 at an MOI of 0.01 following a 2–4 hours pretreatment with either VSe1 20 µmol/l or control. Normal human GM38 fibroblasts were also tested but challenged with an MOI of 0.03. Fluorescence pictures were taken 40 hours postinfection. (b) VSVΔ51 titers were determined by plaque assay on Vero cells from supernatants collected at 40 hours postinfection (VSVΔ51, MOI of 0.01) of 4T1, CT26, and 786-0 cells treated with either vehicle control, 20 or 40 µmol/l VSe1. Data represent average from three to five independent experiments *P = 0.007, **P = 0.04, ^#P = 0.02, ^##P = 0.01, ^$P = 0.005, ^$$P = 0.009 (ANOVA). Error bars represent the SE. (c) CT26 cells were treated with VSe1 20 µmol/l or vehicle control then challenged with a wild-type VSV (MOI = 0.0003). Viral titers were assessed by plaque assay on Vero cells from supernatants collected at 18, 28, and 36 hours postinfection. (d) 4T1 and CT26 cells were treated with serial dilutions of a fixed ratio combination mixture of VSVΔ51 and VSe1 (500 PFU: 1 µmol/l VSVΔ51:VSe1). Cytotoxicity was assessed using alamar blue reagent after 48 hours. Combination indexes (CI) were calculated according to the method of Chou and Talalay¹² using Calcusyn (see Materials and Methods section). Plots represent the algebraic estimate of the CI in function of the fraction of cells affected (Fa). Error bars indicate the estimate SE. ANOVA, analysis of variance; CTRL, control; MOI, multiplicity of infection; RFP, red fluorescent protein; PFU, plaque-forming units; SAHA, suberoylanilide hydroxamic acid; VSe, virus-sensitizers; VSV, vesicular stomatitis virus.

Figure 3

VSe1 inhibits IFN-α induced transcription and its antiviral effects. (a) 293T cells were co-transfected with an ISRE-luciferase reporter and β-galactosidase (control). Six hours post-transfection, cells were treated with indicated concentrations of VSe1 or vehicle. Twenty hours after receiving VSe1, media was replaced and cells were treated with IFN-α. The following day, cells were lysed and measured for luciferase activity. β-Galactosidase activity was also measured and used for data normalization. *P = 0.03, **P = 0.005, ***P = 0.03, ^#P = 0.003, ^##P = 0.006, ^###P = 0.002. (b) Human U251 glioma cells were cotreated with 200 U/ml Intron A and VSe1 (or vehicle) then challenged with GFP-expressing VSVΔ51 at an MOI of 0.01. Supernatants were collected 40 hours later and titered by plaque assay on Vero cells.*P = 6.4 × 10⁻³, **P = 1.6 × 10⁻⁴, ***P = 6.8 × 10⁻⁵ (ANOVA) error bars represent SE, n = 3. ANOVA, analysis of variance; IFN, interferon; ISRE, IFN-responsive promoter element; MOI, multiplicity of infection; PFU, plaque-forming units; RFP, red fluorescent protein; VSe, virus-sensitizers; VSV, vesicular stomatitis virus.

Figure 4

VSe1 represses VSVΔ51-induced genes. CT26 cells were pretreated with either SAHA 5 µmol/l, VSe1 20 µmol/l, or vehicle for 4 hours then challenged with VSVΔ51 at an MOI of 0.03 (or mock treated). Twenty-four hours postinfection, cells were harvested and RNA was extracted. RNA was subsequently processed for hybridization on Affymetrix Mouse Gene 1.0 ST arrays. Expression of genes was normalized to values obtained for vehicle-treated, mock-infected control. In (a,b) points along the x axis represent each gene increased by over twofold by VSVΔ51 infection and are indicated by filled circles. (a) Fold change in gene expression of genes induced by VSVΔ51 in presence of VSe1 20 µmol/l are indicated by open circles. (b) Fold change in gene expression of genes induced by VSVΔ51 in presence of SAHA 5 µmol/l are indicated by open circles. Note that in order to maintain comparable y axes between (a) and (b), one gene (RSad2, gene rank #6, Supplementary Table S1) that was induced nearly 40-fold by SAHA/VSVΔ51 is not represented. MOI, multiplicity of infection; SAHA, suberoylanilide hydroxamic acid; VSe, virus-sensitizers; VSV, vesicular stomatitis virus.

Figure 5

VSe1 exhibits VSVΔ51-sensitizing activity in immunocompetent mice and in human clinical samples. (a) 3 × 10⁵ VSVΔ51-resistant CT26 cells were implanted subcutaneously (s.c) in syngeneic Balb/c mice 11 days before first treatment. On day 11 VSe1 (or vehicle) was administered intraperitoneally (i.p) at 0.4 mg/mouse. Four hours later, 1 × 10⁸ VSVΔ51 (or PBS) was administered intratumorally (i.t). Two more doses of VSe1 were administered on days 13 and 15. Mouse tumor volume was measured using caliper and average tumor volumes relative to day 11 are shown. Error bars represent SE. *P < 0.005, **P < 0.05, ***P < 0.1 (ANOVA). N = 5 mice per group. (b) False-color (LUT) fluorescence microscopy images of representative human colon tumor slices infected with 1 × 10⁷ PFU of GFP-expressing VSVΔ51 (or PBS, left panel) 24 hours post-treatment with either vehicle (middle panel) or 40 µmol/l VSe1 (right panel). Green color is indicative of intense GFP fluorescence and virus replication whereas blue color represents low level background tissue autofluorescence. Pictures were taken after 72-hours incubation. (c) Human tumor or normal tissue slices were treated as in (b) with either 20 or 40 µmol/l VSe1. Seventy-two hours later tissue samples were collected and homogenized for subsequent tittering on Vero cells by plaque assay. ANOVA, analysis of variance; CTRL, control; GFP, green fluorescent protein; PBS, phosphate-buffered saline; PFU, plaque-forming units; VSe, virus-sensitizers; VSV, vesicular stomatitis virus.