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Cancer metastasis is the life-threatening aspect of cancer and is usually resistant to standard treatment. We report here a targeted therapy strategy for cancer metastasis using a modified strain of Salmonella typhimurium. The genetically modified strain of S. typhimurium is auxotrophic for the amino acids arginine and leucine. These mutations preclude growth in normal tissue but do not reduce bacterial virulence in tumor cells. The tumor-targeting strain of S. typhimurium, termed A1-R and expressing green fluorescent protein (GFP), was administered to both axillary lymph and popliteal lymph node metastasis of human pancreatic cancer and fibrosarcoma, respectively, as well as lung metastasis of the fibrosarcoma in nude mice. The bacteria were delivered via a lymphatic channel to target the lymph-node metastases and systemically via the tail vein to target the lung metastasis. The cancer cells expressed red fluorescent protein (RFP) in the cytoplasm and GFP in the nucleus linked to histone H2B, enabling color-coded real-time imaging of the bacteria targeting the metastatic tumors. After 7–21 days of treatment, the metastases were eradicated without the need of chemotherapy or any other treatment. No adverse effects were observed. This new strategy demonstrates the clinical potential of targeting and curing cancer metastasis with engineered bacteria without the need of toxic chemotherapy.
It has been known for approximately 60 years that anaerobic bacteria can selectively grow in tumors [Coley, 1906; Malmgren and Flanigan, 1955; Moese and Moese, 1964; Gericke and Engelbart, 1964; Thiele et al., 1964; Kohwi et al., 1978; Brown and Giaccia, 1998; Fox et al., 1996; Lemmon et al, 1997; Sznol et al., 2000; Low et al., 1999; Clairmont et al., 2000; Yazawa et al., 2000; Yazawa et al., 2001; Kimura, et al., 1980]. The conditions that permit anaerobic bacterial growth (i.e., impaired circulation and extensive necrosis) are found in many tumors. Several approaches to developing tumor-therapeutic anaerobic bacteria have been described. Yazawa et al. [2000; 2001] showed that the anaerobic bacterium Bifidobacterium longum could selectively grow in the hypoxic regions of solid tumors. Dang et al.  created a strain of Clostridium novyi depleted of its lethal toxin (C. novyi-NT). C. novyi spores germinated within the avascular regions of tumors in mice and destroyed surrounding viable tumor cells [Dang et al., 2001]. The main efficacy of these anaerobic bacteria was in combination with chemotherapy [Dang et al., 2001]. Eradication of tumors was also achieved in mice by combining C. novyi-NT with radiation [Bettegowda et al., 2003]. It was recently shown that treatment of mice bearing large, established tumors with C. novyi-NT plus a single dose of liposomal doxorubicin could lead to eradication of the tumors. The bacterial factor responsible for the enhanced drug release was identified as a protein termed liposomase [Cheong et al., 2006; Juliano, 2007].
The facultative anaerobe Salmonella typhimurium was first attenuated by purine and other auxotrophic mutations to be used for cancer therapy [Low et al., 1999; Hoiseth and Stocker, 1981; Pawelek et al., 1997]. These bacteria replicated in the tumor to >1,000-fold compared with normal tissues [Low et al., 1999]. Salmonella lipid A was also genetically modified by disrupting the msbB gene to reduce septic shock [Low et al., 1999]. Melanomas in mice treated with the Salmonella msbB mutant were 6% the size of tumors in untreated controls [Low et al., 1999]. However, these Salmonella variants did not cause tumor regression or eradication, only growth inhibition. S. typhimurium with attenuated lipid A has been evaluated in a Phase I clinical trial [Toso, et al., 2002].
We have previously developed [Zhao et al., 2005] a genetically modified bacterial strain of S. typhimurium, auxotrophic for leucine and arginine, which also expresses green fluorescent protein (GFP), termed S. typhimurium A1. When introduced i.v. or intratumorally, A1 invaded and replicated intracellularly in various cancer cells in vivo as well as in vitro. When A1 was injected intratumorally, the tumor completely regressed by day 20. There were no obvious adverse effects on the host when the bacteria were injected i.v. or intratumorally. The S. typhimurium A1 strain grew throughout the tumor, including viable malignant tissue, but could not sustain growth in normal tissue. This result is in marked contrast to the anaerobic bacteria evaluated previously for cancer therapy that were confined to necrotic areas of the tumor as discussed above. The ability to grow in viable tumor tissue may account, in part, for the unique antitumor efficacy of the A1 strain [Zhao et al., 2005].
The A1-R strain was reisolated from A1-targeted tumor tissue in vivo. The idea was to increase the tumor targeting capability of the bacteria. As a consequence of this selective step, the tumor-cell targeting of the reisolated A1 increased in vivo as well as in vitro. The reisolated A1 bacteria, termed A1-R, administered i.v., resulted in human breast cancer [Zhao et al., 2006] and prostate cancer [Zhao et al., 2007] regression, including tumor eradication in orthotopic nude-mouse models.
Cancer metastasis is the life threatening aspect of cancer. Importantly, metastasis is most often treatment-resistant. In the present study, we demonstrate that S. typhimurium A1-R can directly target and eradicate cancer metastases demonstrating the potential of bacterial therapy to treat otherwise incurable disease.
XPA1 and HT-1080 cells labeled with RFP in the cytoplasm and GFP in the nucleus were grown in 24-well tissue culture plates to a density of 104 cells per well. Bacteria were grown in LB and harvested at late-logarithmic phase, diluted in cell culture medium and added to the tumor cells (1 × 105 colony-forming units (CFU) per well). After 1 h incubation at 37°C, the cells were rinsed and cultured in medium containing gentamycin sulfate (20 μg/ml) to kill external but not internal bacteria. Interaction between bacteria and tumor cells was observed at the indicated time points. An Olympus BH 2-RFCA fluorescence microscope equipped with a mercury 100-W lamp power supply and GFP filter set (Chroma Technology, Brattleboro, VT) was used for fluorescence microscopy.
To obtain metastasis in the axillary lymph node, XPA1-RFP cells were injected into the inguinal lymph node (afferent lymph node to the axillary) in nude mice. Nudemice were anesthetized with a ketamine mixture (10 μLketamine HCL, 7.6 μL xylazine, 2.4 μL acepromazine maleate, and 10 μL H2O) via s.c. injection. A one-cm incision was made in the abdominal skin to expose the inguinal lymph node. The inguinal lymph node was exposed without injuring the lymphatic. The skin was fixed on a flat stand. A total of 10 μL medium containing 5 × 105 cancer cells was injected into the center of the inguinal lymph node. Just after injection, cancer cells were observed trafficking in the efferent lymph duct toward the axillary lymph node. Seven days later (day 0), mice were anesthetized, and the axillary lymph node was observed for metastasis. A two-cm incision was made at the center of the chest wall. The greater pectoral muscle was released from the sternum to expose the axillary lymph node. The connective tissue on the axillary lymph node was separated. The lymph node was imaged for metastasis. Lymph nodes which had less than a one-mm tumor were included in this study. Five mice were treated with A1-R, and 5 were used as controls for each cell line. A1-R bacteria (108 CFU) were injected in the inguinal lymph node. After injection, the axillary lymph node was observed repeatedly at different time points with the Olympus OV100 Small-Animal Imaging System (Olympus Corp., Tokyo, Japan). The size of metastasis (fluorescent area [mm2]) was measured at each imaging time point.
To obtain spontaneous lymph node metastasis, 5 × 106 HT-1080-GFP-RFP cells in 20 μl of Matrigel basement membrane matrix (BD Bioscience, San Jose, CA) were injected into the foot pad in nude mice. The presence of popliteal lymph node metastasis was determined by fluorescence imaging every week after tumor injection. Mice were given the ketamine mixture for anesthesia and laid out in the prone position. The entire limb was observed with the OV100 imaging system without any traumatic procedures. Once metastasis was confirmed in the popliteal region, bacteria therapy was started targeting the metastasis (day 0). A1-R bacteria (108 CFU) were injected subcutaneously in the foot pad. The size of metastasis and primary tumor, and body weight was measured every week. Six mice were treated with A1-R, and 6 were used as controls. Another two mice were used for imaging immediately and at day-7 after bacteria injection. The experiment was terminated when the control primary tumor invaded the popliteal region or the mouse died. The experimental data are expressed as the mean ± SE. Statistical analysis was done using the two-tailed Student’s t-test.
Four week old, female nude mice were used. To obtain lung metastasis, HT-1080-GFP-RFP cells (1 × 106 in 100μl PBS) were injected into the tail vein of the nude mice (day 0). On day 4 and day 11, 5 × 107 CFU bacteria per mouse were injected into the tail vein. Five mice were treated with bacteria and five mice were used as untreated control. On day16, all animals were sacrificed and the lungs were imaged to determine the efficacy of bacteria therapy for lung metastasis. To observe the lung metastasis at lower magnification, an RFP filter was used (excitation 545 nm, emission 570–625 nm).
All animal studies were conducted in accordance with the principles and procedures outlined in the National Institutes of Health Guide for the Care and Use of Laboratory Animals under assurance no. A3873-1.
The total number of metastases on the surface of the lung was counted using the Olympus OV100 Small Animal Imaging System (Olympus Corporation, Tokyo, Japan). The pictures were taken in same setting (exposure time 1000 msec, 0.56×lens). The number of metastases in each group was expressed as mean ± SE. Statistical analysis was done using the two-tailed Student’s t-test.
We first tested the ability of S. typhimurium A1-R to kill human pancreatic cancer and fibrosarcoma cells in vitro. A1-R expressing GFP was observed to invade and replicate intracellularly in the XPA1 human pancreatic cancer and HT1080 human fibrosarcoma cell lines expressing GFP in the nucleus and RFP in the cytoplasm. Intracellular bacterial infection led to cell fragmentation and rapid cell death (Fig. 1).
A new experimental model of lymph node metastasis was developed for this study. To obtain an experimental metastasis in the axillary lymph node, XPA1-RFP human pancreatic cancer cells were injected into the inguinal lymph node in the nude mice (Fig. 2A). Just after injection, cancer cells were imaged trafficking in the efferent lymph duct to the axillary lymph node [Hayashi et al., 2007]. Metastasis in the axillary lymph node was subsequently formed. A1-R bacteria were then injected into the inguinal lymph node to target the axillary lymph node metastasis. Just after bacterial injection, a large amount of bacteria were visualized around the axillary lymph node metastasis (Fig. 2B and Supplementary Movie-1). By day 7, all lymph node metastases had been eradicated in contrast to growing metastases in the control group (Figs. 2C and and3).3). There were very few bacteria in the lymph node by day 7, and no bacteria were detected after day 10. This route of administration was therefore able to deliver sufficient bacteria to eradicate the lymph node metastasis after which the bacteria became undetectable.
The average tumor size (fluorescent area) in the axillary lymph nodes on day 0 was 0.4 ± 0.19 mm2 in the treatment group and 0.46 ± 0.08 in the untreated group, respectively. On day 7, it was 0 mm2 in the treatment group and 0.98 ± 0.17 in the untreated group.
We then tested bacterial therapy strategy for spontaneous lymph node metastasis from a tumor growing in the footpad. At first, only A1-R bacteria were injected in the foot pad in nude mice in order to determine any adverse effects. No infection, skin necrosis, or body weight loss or fatality was detected (data not shown). Then HT1080-GFP-RFP human fibrosarcoma cells were injected into the footpad of additional nude mice. Before treatment, the presence of popliteal lymph node metastasis was determined by weekly imaging. Once the metastasis was detected, A1-R bacteria were injected subcutaneously in the footpad.
Bacteria are small particles and when injected subcutaneously, the lymph system immediately collects them from the site of injection. The lymph system is well known as a drainage route for bacterial infection. One mouse was used to observe the injected bacteria trafficking in the lymphatic channel. The popliteal region was exposed just after bacteria injection and a large amount of GFP bacteria targeting the popliteal lymph node metastasis was observed by fluorescence imaging (Fig. 4A, Supplementary Movie-2). Dual-color labeling of the cancer cells distinguished them from the GFP bacteria. After treatment, the popliteal lymph node was observed every week by fluorescence imaging. One mouse was used to image the bacteria by exposing the popliteal lymph node on day 7. All lymph node metastases shrank and 5 out of 6 were eradicated within 7 to 21 days after treatment in contrast to growing metastases in the control group (Fig. 4B and C).
To obtain lung metastasis, dual-colored HT-1080 cells were injected into tail vein of the nude mice (day 0). On day 4 and day 11, bacteria were injected into the tail vein. On day 16, all animals were sacrificed and the lungs were imaged to determine the effect of bacteria therapy on lung metastasis. To observe the lung metastasis at lower magnification, an RFP filter was used (excitation 545 nm, emission 570–625 nm). In the bacterial-treatment group, only a few cancer cells were observed in contrast to multiple metastases in the control (untreated) group (Fig. 5A and B). The number of metastases on the surface of the lung was significantly lower in the treatment group than in the control group (P<0.005) (Fig. 5C).
The strategy employed in our studies for bacterial therapy of cancer has significant advantages over that using obligate anaerobic bacteria such as Clostridium. Anaerobic bacteria only grow in the necrotic regions of tumors and therefore require additional cytotoxic chemotherapy in order to kill viable tumor tissue and effect eradication [Dang et al., 2001]. The facultative anaerobic S. typhimurium, in contrast, grows in viable as well as necrotic regions of tumors and therefore can eradicate tumors without additional chemotherapy. Thus, our method of bacterial treatment can target spontaneous metastasis and eradicate them without toxic chemotherapy, a major advance over other approaches to bacterial therapy which require combination with chemotherapy in order to effect eradication [Cheong et al., 2006; Juliano, 2007]. For lymph node metastasis, we used a lymphatic targeting approach that was highly effective. The lung metastases in our model were eradicated by systemic injection. However, no adverse effects were observed.
The lymphatic system is a critical route for both bacteria drainage and cancer spread. In humans, subcutaneous injection of tumor-targeting bacteria around the primary tumor should be an effective way to target lymphatic metastasis. Alternatively, the tumor-targeting bacteria can be injected directly in the lymph duct similar to lymphangiography. The bacteria could then reach regional lymph nodes and potentially eradicate the metastasis. The adverse effects should be much less than systemic administration. Intralymphatic administration of bacteria in humans should require only low doses of bacteria.
GFP-labeled S. typhimurium A1-R are seen trafficking in a lymphatic duct to the axillary lymph node metastasis of the XPA1-RFP pancreatic cancer cell line. The bacteria were injected in the inguinal lymph node. Real time images were obtained with the Olympus OV100 Small Animal Imaging System. The lymph duct was exposed with a skin-flap.
GFP-labeled S. typhimurium A1-R are seen trafficking in a lymphatic duct to the popliteal lymph node metastasis of the HT1080-GFP-RFP human fibrosarcoma cell line. The bacteria were injected in the foot pad. Real time images were obtained with the Olympus OV100 Small Animal Imaging System. The lymph duct was exposed with a skin-flap.
This study was supported in part by National Institutes of Health grant CA119841 and U.S. Army Medical Research and Materiel Command Department of Defense Prostate Cancer Research Program contract award W81XWH-06-1-0117.