A Retroviral Replicating Vector Encoding Cytosine Deaminase and 5-FC Induces Immune Memory in Metastatic Colorectal Cancer Models

Abstract

Treatment of tumors with Toca 511, a gamma retroviral replicating vector encoding cytosine deaminase, followed by 5-fluorocytosine (5-FC) kills tumors by local production of 5-fluorouracil (5-FU). In brain tumor models, this treatment induces systemic anti-tumor immune responses and long-term immune-mediated survival. Phase 1 Toca 511 and Toca FC (extended-release 5-FC) clinical trials in patients with recurrent high-grade glioma show durable complete responses and promising survival data compared to historic controls. The work described herein served to expand on our earlier findings in two models of metastatic colorectal carcinoma (mCRC). Intravenous (i.v.) delivery of Toca 511 resulted in substantial tumor-selective uptake of vector into metastatic lesions. Subsequent treatment with 5-FC resulted in tumor shrinkage, improved survival, and immune memory against future rechallenge with the same CT26 CRC cell line. Similar results were seen in a brain metastasis model of mCRC. Of note, 5-FC treatment resulted in a significant decrease in myeloid-derived suppressor cells (MDSCs) in mCRC tumors in both the liver and brain. These results support the development of Toca 511 and Toca FC as a novel immunotherapeutic approach for patients with mCRC. A phase 1 study of i.v. Toca 511 and Toca FC in solid tumors, including mCRC, is currently underway (NCT02576665).

Keywords: 5-FU; MDSC; chemoimmunotherapy; durable response; immunotherapy; mCRC; retroviral vector.

Figures

Figure 1

Toca 511 in Combination with 5-FC Was Efficacious in a Multifocal Liver Metastasis Model of mCRC (A) Representative radiance intensities (photon/s/mouse) from the PBS and 5-FC treatment group at various time points throughout the study. After liver metastases were established with Toca 511 pre-transduced CT26-Luc cells delivered intrasplenically, each mouse was imaged by IVIS at indicated time points. 5-FC treatment (500 mg/kg, i.p., BID) was initiated on day 13 for 5 days on and 2 days off for a total of six cycles. The control group received PBS (see also Figure S1). (B) CT26-Luc bioluminescence average signal intensity at indicated time points (n = 9/group). (C) Kaplan-Meier survival analysis. After liver metastases were established with Toca 511 pre-transduced CT26 cells delivered intrasplenically, PBS or 5-FC treatment was started at day 13 post cell inoculation (n = 9/group). A total of six cycles of PBS or 5-FC treatment were administered and survival was examined out to 90 days. The table is the summary of the survival analysis out to 90 days post tumor cell implantations. (D) Cured mice from survival studies (Toca 511/CT26+5-FC; n = 5) were subcutaneously challenged with wild-type CT26 cells on day 0 (day 90 post initial tumor inoculation) at a cell dose of 5 × 105. As a control, CT26 cells were implanted into naive BALB/cJ mice (n = 9) (*p = 0.028; naive versus Toca 511/CT26+5-FC). *Statistical significance was defined as p < 0.05. Error bars represent SEM.

Figure 2

Upon Intrasplenic, Intravenous, or Intraportal Delivery, GFP-Expressing Vector Concentrates in Metastatic Foci in the Liver and Not in Normal Liver Tissue (A) Representative images showing luciferase (CT26-Luc cells) and GFP (Toca GFP vector) signal in livers from mice receiving intrasplenic, intravenous, or intraportal delivery of Toca GFP vector. (B) Flow cytometric analysis of excised metastatic lesions from livers collected 18 days post vector delivery. (C) CT26-Luc tumor-bearing animals received 1 day or 3 days of intravenous delivery of Toca GFP vector. Flow cytometric analysis of excised metastatic lesions from livers collected 14 or 22 days post vector delivery (n = 9/group). Numbers above the columns indicate the average GFP+ cells per group. Error bars represent SEM.

Figure 3

Intravenous Delivery of Toca 511, Followed by Treatment with 5-FC, Was Efficacious against Colorectal Liver Metastasis (A) Kaplan-Meier survival analysis. Mice inoculated with CT26-Luc cells intrasplenically were then administered Toca 511 i.v. for 5 consecutive days starting on day 3 post cell inoculation. PBS or 5-FC treatment started at day 12 post cell inoculation. Six cycles of PBS or 5-FC treatment were administered, and survival was examined out to 90 days. The table is the summary of the survival analysis out to 90 days post tumor cell implantations. Toca 511 and 5-FC treated animals lived significantly longer compared to Toca 511 and PBS control animals (p = 0.037). (B) Radiance intensities (photon/s/mouse) from representative animals at indicated time points. 5-FC treatment (500 mg/kg, i.p., BID) started on day 13 for 5 days on and 2 days off for 6 cycles. Control group received PBS (see also Figure S2). (C) Cured mice from survival studies (Toca 511+5-FC; n = 5) were subcutaneously challenged with wild-type CT26 cells on day 0 (day 90 post initial tumor inoculation) at a cell dose of 5 × 105. As a control, CT26 cells were implanted into naive BALB/cJ mice (n = 9) (*p = 0.04; naive versus Toca 511+5-FC). *Statistical significance was defined as p < 0.05. (D) WBCs, LYMs, NEUs, and PLTs were determined in the PBS and 5-FC treatment groups. Dotted line represents LLN for age-matched mice. Age-matched mouse LLN = WBC, 6 × 109 cells/L; LYM, 3.4 × 109 cells/L; PLT, 200 × 109 cells/L; and NEU, 3.4 × 109 cells/L. Error bars represent SEM.

Figure 4

Toca 511 and 5-FC Resulted in Concentrated 5-FU within Liver Metastases in a Murine Model of mCRC LC/MS measurement of 5-FC and 5-FU in plasma and tumor from mice with Toca 511 pre-transduced CT26-Luc liver metastases treated for 3.5 days with 5-FC (500 mg/kg, i.p., BID). Numbers above columns indicate average values for each group. Error bars represent SEM.

Figure 5

Systemic 5-FU Treatment Was Not Efficacious in a Mouse Model of Colorectal Liver Metastasis (A) Kaplan-Meier analysis showing the survival of animals with liver metastasis treated with 5-FC (500 mg/kg, i.p., BID) or 5-FU (20 mg/kg, i.p., SID) for 5 consecutive days, followed by 2 days without drug for a total of 4 cycles. Animals received no vector in this experiment. 5-FC, without the presence of vector, has no effect on survival. (B) Hematologic analysis, including values for WBCs, LYMs, and NEUs from animals treated with 5-FC or 5-FU. Animals had established CT26 liver metastases but were not given Toca 511 vector. Error bars represent SEM.

Figure 6

Myeloid-Derived Suppressor Cells Decreased with Toca 511 and 5-FC Treatment in a Liver Metastasis Model of mCRC Liver metastases treated with 5-FC visually show a decrease in MDSC populations. Mice were administered three cycles of 5-FC (500 mg/kg, i.p., BID) or PBS. (A) Immunofluorescence color staining of CD11b (myeloid cells), Gr-1 (monocytes and neutrophils), Ly6C (monocytes), and Gr-1/Ly6C (MDSCs) confirms decrease of expression with treatment of 5-FC. (B) Quantitative analysis of percentage area (n = 6) of CD11b, Gr-1, and Ly6C show decrease of MDSCs with treatment of 5-FC compared with PBS. *Statistical significance was defined as p 

Figure 7

Toca 511 and 5-FC Prolonged Survival and Promoted Long-Term Anti-tumor Immunity in a Brain Metastasis Model of mCRC (A) Kaplan-Meier analysis. BALB/cJ mice received Toca 511 pre-transduced CT26 cells intracranially and were subsequently treated with cycles of 5-FC (500 mg/kg, i.p., BID) (n = 12) or PBS (n = 5). Treatments started at day 10 post cell inoculation and were for 7 days on and 7 days off for four cycles. Survival was examined out to 150 days. (B) Long-term survivors (n = 5) were rechallenged subcutaneously at day 65 post intracranial tumor implant with parental CT26 cells, and tumor growth was monitored over time. As a control, CT26 cells were implanted into naive, age-matched, BALB/cJ mice (n = 5) (*p = 0.0001; naive versus Toca 511+5-FC). (C) Recipient athymic animals received splenocytes from either cured or naive animals and were inoculated subcutaneously with parental CT26 cells and tumor growth was monitored over time (n = 6/group) (*p = 0.04). *Statistical significance was defined as p 

Figure 8

Myeloid-Derived Suppressor Cells Decreased with Toca 511 and 5-FC Treatment in a Brain Metastasis Model of mCRC BALB/c mice received Toca 511 pre-transduced CT26 cells intracranially and were treated with 5-FC (500 mg/kg, i.p., BID) or PBS. Treatments started at day 10 post cell inoculation and were 7 days on and 7 days off for one cycle. (A and B) The tumor (A) (*p + Gr-1+ Ly6C+ cells in the tumor (A) and spleen (B). Error bars represent SEM.