Mesothelin as a potential therapeutic target in human cholangiocarcinoma

Killing of resistant cancer cells with low Bak by a combination of an antimesothelin immunotoxin and a TRAIL Receptor 2 agonist antibody

PURPOSE

Many solid tumors express cell surface mesothelin making them attractive targets for antibody-based therapies of cancer. SS1P [antimesothelin(Fv)PE38] is a recombinant immunotoxin (RIT) that has potent cytotoxic activity on several cancer cell lines and clinical activity in mesothelioma patients. Pancreatic cancers express mesothelin and are known to be resistant to most chemotherapeutic agents. The goal of this study is to treat pancreatic cancer with RIT by targeting mesothelin.

EXPERIMENTAL DESIGN

We measured the cytotoxic activity of an antimesothelin immunotoxin on pancreatic cancer cells. We also measured the levels of several pro- and antiapoptotic proteins, as well as the ability of TNF-related apoptosis-inducing ligand (TRAIL) or the anti-TRAIL receptor 2 agonist antibody (HGS-ETR2) to kill pancreatic cells, and the cytotoxic activity of the two agents together in cell culture and against tumors in mice.

RESULTS

In two pancreatic cancer cell lines, immunotoxin treatment inhibited protein synthesis but did not produce significant cell death. The resistant lines had low levels of the proapoptotic protein Bak. Increasing Bak expression enhanced the sensitivity to immunotoxins, whereas Bak knockdown diminished it. We also found that combining immunotoxin with TRAIL or HGS-ETR2 caused synergistic cell death, and together triggered caspase-8 recruitment and activation, Bid cleavage and Bax activation. Combining SS1P with HGS-ETR2 also acted synergistically to decrease tumor burden in a mouse model.

CONCLUSION

Our data show that low Bak can cause cancer cells to be resistant to immunotoxin treatment and that combining immunotoxin with TRAIL or a TRAIL agonist antibody can overcome resistance.

HN125: A Novel Immunoadhesin Targeting MUC16 with Potential for Cancer Therapy

Background: The mucin MUC16 expresses the repeating peptide epitope CA125 that has been known for decades to be a well-validated cancer marker that is overexpressed on the cell surface of ovarian cancers and other malignant tumors. In spite of recent efforts to make mouse monoclonal antibodies to MUC16 to treat ovarian cancer, a human monoclonal antibody against this mucin has not been described. MUC16 interacts with mesothelin, a protein that mediates heterotypic cancer cell adhesion, indicating that MUC16 and mesothelin play an important role in the peritoneal implantation and metastasis of ovarian tumors. Therefore, a suitable candidate for therapeutic targeting of MUC16 would functionally block the interaction of MUC16 and mesothelin.

Methodology/Principal Findings: Here we report the generation of a novel immunoadhesin, HN125, against MUC16 that consists of a functional MUC16 binding domain of mesothelin (IAB) and the Fc portion of a human antibody IgG1. The yield for purified HN125 proteins is over 100 µg/mL of HEK-293 culture supernatant. We show that HN125 has high and specific affinity for MUC16-expressing cancer cells by flow cytometry and immunohistochemistry. HN125 has the ability to disrupt the heterotypic cancer cell adhesion mediated by the MUC16-mesothelin interaction. Moreover, it elicits strong antibody-dependent cell mediated cytotoxicity against MUC16-positive cancer cells in vitro.

Conclusion/Significance: This report describes a novel human immunotherapeutic agent highly specific for MUC16 with potential for treating ovarian cancer and other MUC16-expressing tumors. Because of its lower immunogenicity in patients, a fully human protein is the most desirable format for clinical applications. We believe that the methods developed here may apply to the generation of other tumor-targeting immunoadhesins when it is difficult to obtain a human monoclonal antibody to a given antigen for clinical applications. The resultant immunoadhesins can have advantages usually found in monoclonal antibodies such as ease of purification, high binding affinity and effector functions.

In vivo imaging of human malignant mesothelioma grown orthotopically in the peritoneal cavity of nude mice

Malignant mesothelioma (MM) causes significant morbidity and mortality in patients. With increasing efforts devoted to developing therapeutics targeting mesothelioma, a xenograft mouse model with in vivo tumor imaging is especially desired for evaluating anti-tumor therapies. In the present study, we fluorescently labeled the NCI-H226 human mesothelioma cell line by a lentiviral vector harboring a luciferase-GFP (Luc/GFP) fusion gene driven by the RNA polymerase II promoter. After single-cell cloning by flow cytometry, a clone (named LMB-H226-GL) that stably expresses high levels of Luc/GFP was obtained. The in vivo tumorigenicity of Luc/GFP-labeled LMB-H226-GL was determined by using intraperitoneal injections of the cells in nude mice. LMB-H226-GL was found to be able to consistently form solid tumors in the peritoneum of mice. Tumor growth and aggressive progression could be quantitated via in vivo bioluminescence imaging. The model exhibited the pathological hallmarks consistent with the clinical progression of MM in terms of tumor growth and spread inside the peritoneal cavity. To evaluate the in vivo efficacy of drugs targeting mesothelioma, we treated mice with SS1P, a recombinant immunotoxin currently evaluated in Phase II clinical trials for treatment of mesothelioma. All the tumor-bearing mice had a significant response to SS1P treatment. Our results showed that this is a well-suited model for mesothelioma, and may be useful for evaluating other novel agents for mesothelioma treatment in vivo.