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Experimental Therapeutics |
Molecular Medicine Program [K-W. P., C. J. T., S. J. R.], Medical Oncology [E. G., L. C. H.], and Endocrine Research Unit [K. R. K.], Mayo Clinic, Rochester, Minnesota 55905
| ABSTRACT |
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| INTRODUCTION |
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We have been investigating the use of the vaccine strain of measles virus for tumor therapy. Measles is a negative-strand RNA virus (family Paramyxoviridae) that causes rash, fever, cough, coryza, and/or conjunctivitis in an infected person (11) . In 1954, Enders and Peebles isolated measles virus from a patient, David Edmonston, and passaged the virus in cultured human and monkey kidney cells (11) . Tissue culture passage resulted in an attenuated strain with no pathogenicity (11) . From the original Edmonston B strain, a number of vaccine strains were developed and have reduced the incidence of measles infection worldwide (11) . In addition to being an effective vaccine, we reported recently that the live attenuated MV-Edm3 was potent and selective against two B-cell malignancies, non-Hodgkins lymphoma (12) , and multiple myeloma (13) . Infection of lymphoma or myeloma cells in vitro by MV-Edm led to extensive cell-cell fusion and the formation of multinucleated syncytia that eventually became nonviable. Direct intratumoral or i.v. injection of MV-Edm into mice bearing established xenografts resulted in growth inhibition or regression of the tumors (12 , 13) . These B-cell tumor models were chosen on the basis of the lymphotropism of measles virus. Lymphoid cells are particularly sensitive to MV-Edm infection because the virus binds to a cellular receptor, signaling lymphocyte activation molecule (CDw150), present on activated B and T cells (14 , 15) . Through tissue culture passage, attenuated vaccine strains of measles virus have evolved to also infect and enter cells efficiently via a second receptor, CD46 (16, 17, 18) . Coincidentally, CD46 is overexpressed in human ovarian cancer (19) , making it an attractive target for MV-Edm therapy because the virus should enter these receptor-rich cells efficiently. To facilitate the noninvasive monitoring of viral expression profile in treated patients, we developed a recombinant trackable MV-Edm virus expressing an inert soluble marker peptide, the extracellular domain of human carcinoembryonic antigen, MV-hCEA (20) . We chose hCEA because it has no documented biological activity, the normal serum concentration of hCEA is <5 ng/ml, and 90% of epithelial ovarian cancers do not express hCEA (21, 22, 23) . Supernatant hCEA peptide levels in MV-hCEA infected cells were concordant with the level of virus gene expression. By monitoring serum hCEA levels in treated rodents, we could obtain a kinetic profile of viral gene expression in a noninvasive manner (20) . The virally encoded marker peptide can therefore facilitate dose finding and dose scheduling studies, making it a valuable tool for clinical development of MV-Edm for cancer therapy.
In the current study, we have evaluated MV-hCEA as a potential novel therapeutic agent for i.p. therapy of ovarian cancer. Expression levels of the measles virus receptor, CD46, in a panel of epithelial ovarian cancer cells and nontransformed cells from the mesothelium, ovary surface epithelium, and dermal fibroblasts, were compared by flow cytometry. To determine whether the virus was selectively oncolytic for ovarian tumor cells, the extent of syncytial formation induced by measles virus infection was compared between ovarian tumor cells and nontransformed cells. Antitumor efficacy of the virus was evaluated in an established s.c. SKOV3ip.1 tumor xenograft model and in an advanced i.p. SKOV3ip.1 tumor model in athymic mice. The kinetic profiles of viral gene expression in the MV-hCEA-treated animals were obtained by monitoring serum hCEA levels.
| MATERIALS AND METHODS |
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Viruses and Infection Assays.
Recombinant MV-Edm encoding the soluble extracellular domain of human CEA (MV-hCEA) was generated and propagated on Vero cells as described previously (18)
. The titers of viral stocks were determined by 50% end point dilution assays (TCID50) on Vero cells (27)
. For virus infection assays, 2 x 105 cells were incubated with MV-hCEA at a MOI of 0.01 or 0.1 in 1.0 ml of Opti-MEM (Life Technologies, Inc.) for 2 h at 37°C. At the end of the incubation period, the virus was removed, and the cells were maintained in standard medium. At 48 h after infection, the cells were photographed, harvested, and frozen/thawed twice, and virus titer was determined by TCID50 assay on Vero cells. hCEA levels in the culture supernatants were analyzed using the Bayer Centaur Immunoassay System. For cell viability assays, 7 x 103 cells/50 µl were plated into 96-well plates and infected with MV-hCEA at different MOIs. At 6 days after infection, the cell viability was determined by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay (Promega Corp., Madison, WI) that measures the formation of insoluble formazan by mitochondria in viable cells.
Flow Cytometry.
Cells were harvested, washed twice in cold 2% BSA-PBS, and incubated with mouse antihuman CD46 antibody (PharMingen, San Diego, CA) for 1 h on ice, washed twice, and then incubated with FITC-conjugated antimouse antibody (Sigma Chemical Co., St. Louis, MO) for 1 h on ice. Samples were washed twice and run on a Becton-Dickinson FACScan Plus cytometer and analyzed using the CellQuest software (Becton-Dickinson, San Jose, CA).
In Vivo Experiments.
All procedures involving animals were approved by and performed according to guidelines of the Institutional Animal Care and Use Committee of Mayo Foundation. Female athymic mice (56 weeks of age; Taconic Laboratory, Germantown, NY) were maintained in the barrier facilities of Mayo Clinic. Mice were implanted s.c. in the right flank with 5 x 106 SKOV3ip.1 cells/100 µl PBS or i.p. with 5 x 106 SKOV3ip.1 cells/250 µl PBS. When the tumors reached 0.5 cm in diameter (11 days after implantation) in the s.c. model, MV-hCEA was injected directly into the xenografts with a 28-gauge needle. Mice in the therapy group (n = 10) received a total of five doses of active MV-hCEA (107 pfu/100 µl/dose), delivered three times in the first week and twice thereafter. Control mice (n = 10) were treated with an equivalent dose of virus inactivated by UV irradiation. For the i.p. model, mice received injections i.p. with MV-hCEA (107 pfu/250 µl/dose) 10 days after implantation of cells. The mice received a total of 16 doses delivered over a 6-week period (total dose, 1.6 x 108 pfu). Mice in the control group were given UV-inactivated virus. Animals were routinely bled to obtain serum hCEA levels. Animals were euthanized when tumor diameter reached 1 cm (s.c. model) or if mice lost 10% of body weight or developed ascites (i.p. model).
In Situ Hybridization for MV-Edm Nucleocapsid (N) mRNA.
SKOV3ip.1 tumors were harvested and fixed in 10% formalin. Paraffin-embedded tissue sections (5 µm) were deparaffinized and were probed for the presence of MV N-specific mRNA using digoxigenin-labeled nucleocapsid RNA of negative polarity as described previously (13)
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Rescue of Virus from Residual Tumors.
Tumors were harvested and disaggregated under sterile conditions. The suspension was passed through a 40-µm sieve to obtain single cells. Cells were then counted and plated onto tissue culture plates. Virus titer, hCEA level, and presence of syncytia in these cultures were determined.
Statistical Methods.
The statistical significance of difference between the survival of mice treated with MV-hCEA or UV-inactivated virus was compared using the log-rank test in the JMP program. P
0.05 indicates that the survival of the groups is significantly different.
| RESULTS |
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To compare and contrast the cytopathic killing of ovarian tumor cells and nontransformed cells by MV-hCEA, we infected (MOI 0.01) a panel of three ovarian tumor cell lines and three nontransformed cell cultures (normal dermal fibroblasts, nontransformed OSE cells, and mesothelial cells) and compared the extent of syncytial formation (Fig. 3)
. Large multinucleated (>40 nuclei) syncytia were seen in SKOV3ip.1, OV202, and OV207 ovarian tumor cultures by 48 h after infection. These syncytia eventually became nonviable and floated off from the tissue culture plates. In contrast, no such syncytia nor cytotoxic killing were seen in the normal dermal fibroblasts, nontransformed OSE cells, or mesothelial cells (Fig. 3)
. To assess the cell killing of MV-hCEA, tumor cell lines and normal untransformed cells were plated in 96-well plates and infected with the virus at different doses, and cell viability was determined at 6 days after infection. As shown in Fig. 4
, tumor cells were susceptible to MV-hCEA killing, and <20% of cells were viable at MOI 10.0. In contrast, 8099% of normal cells were still viable at that dose of virus.
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Serum hCEA Profiles Helped Elucidate the Kinetics of Virotherapy in Treated Mice.
Mice from the treatment and control groups were bled regularly to determine serum hCEA levels during the course of treatment. Rodents are not susceptible to measles virus infection because murine cells lack the appropriate receptor for virus entry. Thus, serum from tumor-free mice injected with MV-hCEA or tumor-bearing mice treated with UV-inactivated virus tested negative for hCEA (data not shown). As expected, serum from all tumor-bearing mice that received the active virus became positive for hCEA (Fig. 6)
. Fig. 6A
shows the hCEA profiles of MV-hCEA-treated mice bearing s.c. tumors. In the 8 mice that responded to therapy with complete tumor regression, the corresponding hCEA levels peaked between 10 and 300 ng/ml and subsequently decreased to baseline levels. In contrast, serum hCEA did not return to baseline levels in the two mice with palpable tumors after MV-hCEA therapy, indicating persistence of virus in these nonresponsive tumors.
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| DISCUSSION |
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It is important to emphasize that it is the tissue culture-adapted attenuated strain of measles virus, not wild-type measles virus, that is being investigated for antitumor activity. Wild-type measles virus is a serious human pathogen and is responsible for a million deaths worldwide each year (11) . Most importantly, the tropism of wild-type MV is restricted to lymphoid cells (via signaling lymphocyte activation molecule entry) and produces cytopathic effects only in these cells (28) . In contrast, the attenuated MV strains attach and enter cells efficiently through CD46, a ubiquitous regulator of complement activation universally present on all human cells. Ovarian cancer is therefore a particularly attractive target for MV-Edm therapy because ovarian tumor cells, in contrast to nontransformed cells from mesothelium and ovarian surface epithelium, overexpress the CD46 receptor (19) . Thus, MV-hCEA is naturally "targeted" to the tumor cells, producing a markedly enhanced cytopathic effect in the ovarian cancer cells versus the nontransformed cells. A single infected tumor cell expressing the viral hemagglutinin and fusion glycoproteins becomes a focus for cell-to-cell fusion with neighboring cells, forming a multinucleated syncytium that eventually becomes nonviable and dies by apoptosis (29) .4 The biochemical mechanisms underlying the remarkable selective oncolytic activity of MV-Edm for tumor cells have not yet been elucidated. However, we are currently testing the hypothesis that CD46 receptor overexpression is a major contributing factor, promoting the process of virus-mediated cell-to-cell fusion. With a high level of CD46 receptors on neighboring noninfected tumor cells, binding and fusion with virus-infected cells expressing the measles envelope glycoproteins might be facilitated, amplifying the spread of the virus and cytopathic effects within the culture. In contrast to the MV receptor, the adenovirus receptor, (CAR) is very weakly expressed in ovarian tumor cells (30) , which are therefore naturally resistant to infection by oncolytic adenoviruses. To overcome this limitation, significant progress was made in retargeting adenoviruses for entry into cancer cells by adding targeting moieties, such as TAG-72, RGD, and anti-erbB2 single-chain antibody, on the surfaces of the viruses to enhance entry into the ovarian cancer cells (30, 31, 32) . We have demonstrated recently that it is possible to retarget entry of MV-Edm via display of targeting ligands such as epidermal growth factor, insulin-like growth factor, and single-chain antibodies, and this could offer an additional level of specificity for MV-Edm infection of tumor cells (33 , 34) .
Serum hCEA levels were persistently elevated in MV-hCEA-treated mice bearing i.p. ovarian cancer cells, although the mice appeared healthy and were free of ascites. Postmortem done on the animals revealed residual tumors, some with necrotic centers, containing foci of MV-hCEA-infected cells demonstrated by in situ hybridization for MV-nucleocapsid mRNA. These sites of viral RNA synthesis appeared as scattered foci in the tumor sections. The explanted tumor cells still expressed CD46 receptors and were able to fuse when plated in tissue culture plates, indicating the presence of virus. Indeed, replication-competent viruses that expressed hCEA were rescued from these tumor cell isolates. Harrison et al. (35) also recovered high levels of titratable adenoviruses from persistent tumors after oncolytic virotherapy in a lung cancer therapy model and attributed the failure of the virus to totally eradicate the tumor because of limitation in viral spread by tumor matrix in vivo. Clearly, in these tumors, there existed a dynamic equilibrium between formation of new tumor cells and death of infected tumor cells. Viable tumor cells continue to divide and contribute to the tumor mass but also provide a substrate for virus replication. Death of virally infected tumor cells keeps progression of the disease in check. This model of a dynamic equilibrium will explain the apparent "cured" status of the mice with persistently high serum hCEA levels. Using mathematical modeling, Wodarz (36) predicted this state of equilibrium of virus persistence in tumors, ongoing death of infected tumor cells, and growth of uninfected tumor cells. According to this mathematical model, the therapeutic outcome will depend heavily on the degree of cytotoxicity of the virus and replication rate of the virus and that of the tumor cells. Obviously, with an intact immune system, the dynamics of virus persistence, infection, and death of infected tumor cells can be altered by cytotoxic T-cell responses and humoral antibodies. It is very likely that in these immunocompetent individuals, virally infected cells/tumor will be recognized by the immune system and will be cleared. Moreover, additional cytotoxic genes can be inserted into the viral genome to enhance/modulate its cytotoxicity to achieve maximal tumor cell killing at the optimal time (36) .
Most ovarian cancer patients will have been vaccinated against measles virus and therefore have anti-measles antibodies that might compromise the therapeutic efficacy of the virus. However, neutralizing antibodies would be a more serious concern for systemic therapy than they will be for i.p. therapy of ovarian cancer. Previous studies have shown that the presence of antibodies to replication-competent viruses does not negate the efficacy of therapy or prevent virus replication in the tumor sites (37, 38, 39) . Indeed, it has been shown that, despite high levels of circulating antibodies in the serum of mice that received multiple injections of adenovirus, the titer of neutralizing antibodies in the peritoneal cavity was undetectable (40) . Also, if neutralizing antibodies are present in ascites, the fluid can be easily aspirated from the peritoneal cavity before therapy. Thus, i.p. therapy is an attractive route for delivery of the virus to maximize virus/tumor cell interaction and infection.
With respect to safety, reversion of attenuated vaccine strains of MV-Edm leading to reemergence of a pathogenic "wild-type" MV has not been observed despite many years of human exposure with live measles vaccine. However, should a pathogenic variant emerge, it is worth noting that measles is a self-limiting disease, and most people recover completely. In addition, caregivers would be expected to have antibodies against the virus, and this would limit person-to-person transmission of the disease.
In conclusion, we believe that MV-hCEA warrants further investigation for i.p. therapy of ovarian cancer. It is potent and selective for tumor cells, and the profile of viral gene expression can be followed through the marker peptide. i.p. delivery circumvents the potential problem that circulating neutralizing antibodies might compromise the therapeutic efficacy of the virus. The virally encoded soluble marker peptide provides important feedback on viral expression kinetics that will facilitate safer dose escalation studies in humans. hCEA is an inert self-peptide and thus nonimmunogenic. It is expressed in <10% of ovarian tumors, and by monitoring the hCEA profile, we can safely move up the MV-hCEA dose range, identifying threshold doses giving no viral gene expression, self-limited expression, or excessive expression. Hence, we intend to test this trackable attenuated measles virus in a Phase I dose escalation clinical trial for advanced stage/recurrent ovarian cancer, and we expect to gain useful insight into the kinetics of virotherapy by monitoring the profile of hCEA expression.
| ACKNOWLEDGMENTS |
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| FOOTNOTES |
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1 This work was supported by grants from the Mayo Foundation, Harold Siebens Foundation, George Eisenberg Foundation, Fraternal Order of Eagles, and the Mayo Clinic Cancer Center Ovarian Cancer Research Group. ![]()
2 To whom requests for reprints should be addressed, at Molecular Medicine Program, Guggenheim 18, Mayo Foundation, Rochester, MN 55905. Phone: (507) 284-8357; Fax: (507) 284-8388; E-mail: peng.kah{at}mayo.edu ![]()
3 The abbreviations used are: MV-Edm, Edmonston B strain of measles virus; hCEA, human carcinoembryonic antigen; NHDF, normal human dermal fibroblast; OSE, ovarian surface epithelial; FBS, fetal bovine serum; MOI, multiplicity of infection; pfu, plaque-forming unit(s). ![]()
4 K-W. Peng, C. TenEyck, S. J. Russell, unpublished results. ![]()
Received 1/22/02. Accepted 6/13/02.
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A. Gauvrit, S. Brandler, C. Sapede-Peroz, N. Boisgerault, F. Tangy, and M. Gregoire Measles Virus Induces Oncolysis of Mesothelioma Cells and Allows Dendritic Cells to Cross-Prime Tumor-Specific CD8 Response Cancer Res., June 15, 2008; 68(12): 4882 - 4892. [Abstract] [Full Text] [PDF] |
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G. D. Aletti, M. M. Gallenberg, W. A. Cliby, A. Jatoi, and L. C. Hartmann Current Management Strategies for Ovarian Cancer Mayo Clin. Proc., June 1, 2007; 82(6): 751 - 770. [Abstract] [Full Text] [PDF] |
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D. M. Duelli, S. Hearn, M. P. Myers, and Y. Lazebnik A primate virus generates transformed human cells by fusion J. Cell Biol., November 7, 2005; 171(3): 493 - 503. [Abstract] [Full Text] [PDF] |
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S. J. Russell Measles Virus: A Potent, Versatile, Retargetable Oncolytic Platform Am. Assoc. Cancer Res. Educ. Book, April 1, 2005; 2005(1): 389 - 394. [Full Text] [PDF] |
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S. Vongpunsawad, N. Oezgun, W. Braun, and R. Cattaneo Selectively Receptor-Blind Measles Viruses: Identification of Residues Necessary for SLAM- or CD46-Induced Fusion and Their Localization on a New Hemagglutinin Structural Model J. Virol., January 1, 2004; 78(1): 302 - 313. [Abstract] [Full Text] [PDF] |
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D. Grote, R. Cattaneo, and A. K. Fielding Neutrophils Contribute to the Measles Virus-induced Antitumor Effect: Enhancement by Granulocyte Macrophage Colony-stimulating Factor Expression Cancer Res., October 1, 2003; 63(19): 6463 - 6468. [Abstract] [Full Text] [PDF] |
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K.-W. Peng, K. A. Donovan, U. Schneider, R. Cattaneo, J. A. Lust, and S. J. Russell Oncolytic measles viruses displaying a single-chain antibody against CD38, a myeloma cell marker Blood, April 1, 2003; 101(7): 2557 - 2562. [Abstract] [Full Text] [PDF] |
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