High Mesothelin Expression Research Articles

Anetumab ravtansine inhibits tumor growth and shows additive effect in combination with targeted agents and chemotherapy in mesothelin-expressing human ovarian cancer models.

Despite the recent advances in the treatment of ovarian cancer, it remains an area of high unmet medical need. Epithelial ovarian cancer is associated with high levels of mesothelin expression, and therefore, mesothelin is an attractive candidate target for the treatment of this disease. Herein, we investigated the antitumor efficacy of the mesothelin-targeting antibody-drug conjugate (ADC) anetumab ravtansine as a novel treatment option for ovarian cancer in monotherapy and in combination with the antitumor agents pegylated liposomal doxorubicin (PLD), carboplatin, copanlisib and bevacizumab. Anetumab ravtansine showed potent antitumor activity as a monotherapy in ovarian cancer models with high mesothelin expression. No activity was seen in mesothelin-negative models. The combination of anetumab ravtansine with PLD showed additive anti-proliferative activity in vitro, which translated into improved therapeutic in vivo efficacy in ovarian cancer cell line- and patient-derived xenograft (PDX) models compared to either agents as a monotherapy. The combination of anetumab ravtansine with the PI3Kα/δ inhibitor copanlisib was additive in the OVCAR-3 and OVCAR-8 cell lines in vitro, showing increased apoptosis in response to the combination treatment. In vivo, the combination of anetumab ravtansine with copanlisib resulted in more potent antitumor activity than either of the treatments alone. Likewise, the combination of anetumab ravtansine with carboplatin or bevacizumab showed improved in vivo efficacy in the ST081 and OVCAR-3 models, respectively. All combinations were well-tolerated. Taken together, these data support the development of anetumab ravtansine for ovarian cancer treatment and highlight its suitability for combination therapy with PLD, carboplatin, copanlisib, or bevacizumab.

Abstract LB-229: Mesothelin expression is a predict factor for peritoneal recurrence in gastric cancer

Abstract Background: Mesothelin is overexpressed in many solid tumors. Recent studies showed that mesothelin expression is associated with poor outcome in several malignant tumors and it may play a role of cancer cell proliferation, local invasion and metastasis. Clinical trials of mesothelin-targeted immunotherapies are currently in progress. However, the correlation between mesothelin expression and prognosis of gastric cancer is still conflicting. Patients and Methods: Mesothelin expression in tumour cells were immunohistochemically evaluated in 964 advanced gastric cancer patients, and their associations with clinical outcome were analysed. The prediction model for recurrence was developed by cox proportional hazard models. Results: High mesothelin expression was observed in 49.6% and significantly associated with a high stage (P = 0.022) and the presence of recurrence with peritoneal carcinomatosis (P = 0.002). In univariate survival analysis, high mesothelin expression was significantly associated with shorter recurrent-free survival (RFS) (P=0.003) and overall survival (OS) (P=0.006). Multivariate survival analysis revealed that high mesothelin expression was independently associated with poor RFS and OS (P=0.001; P=0.002, respectively) in addition to stage, lymphovascular invasion and Lauren classification (all, P<0.001). In multivariate analysis for recurrence with peritoneal carcinomatosis, high mesothelin expression was also independent prognostic factors in all advanced gastric cancer patients (P<0.001) and in intestinal (P=0.017) and diffuse/mixed type (P=0.005) subgroups. The overall discrimination ability of prediction model for recurrence, the model with mesothelin was better than model without mesothelin. Conclusions: High mesothelin expression was correlated with poor outcome. And, mesothelin expression, along with Lauren classification was meaningful predict factor for peritoneal recurrence. Citation Format: Su-Jin Shin, Hyunki Kim, Yoon Young Choi, Sejung Park, Chung Mo Nam, Hyun Cheol Chung. Mesothelin expression is a predict factor for peritoneal recurrence in gastric cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr LB-229.

Mismatch repair deficiency in cholangiocarcinoma.

269 Background: Cholangiocarcinoma (CC) is a fatal malignancy with an unmet treatment need. With the approval of immunotherapy for solid tumors with mismatch repair (MMR) deficiency, there is a renewed interest in MMR testing. Little is known about the incidence of MMR deficiency in CC or its correlation to survival, immune cell infiltration, PD-L1 and other proteins expressed in CC such as mesothelin. Methods: CC tumors were identified from patients treated at the University of Virginia from 2000-2014. Tissue microarrays (TMAs) were constructed of 3-4 cores from each tumor and were stained by immunohistochemistry for MMR genes (MLH1, PMS2, MSH2, MSH6), mesothelin, PD-L1 and immune cells. TMAs were scanned using the Leica SCN400 and analyzed using the Digital Image Hub software. Stain intensity thresholds for defining positive cells were determined by two users and recorded as an average of all cores from each tumor. Mesothelin and PD-L1 expression were measured as a percentage of positive tumor cells. Correlation with overall survival was assessed using log-rank tests and classification and regression trees, with p values < 0.05 considered significant. Results: Ninety-one tumors were analyzed: 24 intrahepatic, 33 hilar, and 34 distal. MMR deficiency was found in 20 tumors (22%). None of the MMR deficient tumors co-expressed PD-L1 (>1%), which was found in 15% of the remaining tumors. T cell infiltration (CD4, CD8 and FoxP3) did not differ between MMR deficient or proficient tumors. Patients with MMR deficiency had a trend towards worse survival compared to those with proficiency (median OS: 19.2 vs. 28.1 months, p = 0.07). MMR deficient tumors showed a lower mesothelin expression compared to MMR proficient tumors, median 8 vs. 129 positive cells per TMA (p = 0.08). Patients with MMR deficiency and low mesothelin expression had a worse outcome compared to patients with MMR proficiency and high mesothelin expression (median OS: 14.5 vs. 30.0 months, p = 0.05). Conclusions: Given the high rate of MMR deficiency, all CC tumors should be tested and may benefit from anti-PD-1 therapy. The poor prognosis of MMR deficient CC may be independent of T-cell infiltration and additional studies are needed to better characterize the genetic and molecular landscape of this subset of tumors.

OA 02.01 Randomized Phase II Study of Anetumab Ravtansine or Vinorelbine in Patients with Metastatic Pleural Mesothelioma

Anetumab ravtansine (BAY 94-9343) is a novel fully human anti-mesothelin IgG1 antibody conjugated to the maytansinoid tubulin inhibitor DM4. We report the results of a randomized phase II trial of anetumab ravtansine compared to vinorelbine in patients with advanced malignant pleural mesothelioma (MPM) who have high mesothelin expression and have progressed on platinum/pemetrexed-based first-line chemotherapy (NCT02610140).

Correlation Between Tumor Mesothelin Expression and Serum Mesothelin in Patients with Epithelial Ovarian Carcinoma: A Potential Noninvasive Biomarker for Mesothelin-targeted Therapy.

The cell surface glycoprotein mesothelin is highly expressed in several malignant diseases. Normal mesothelin expression is limited to mesothelial cells lining the pleura, peritoneum, and pericardium, making it a biomarker and an attractive target for cancer therapy. We investigated tumor mesothelin expression and serum mesothelin levels in patients with epithelial ovarian cancer or borderline tumors. In total, 161 patients selected from a previous prospective study were analyzed fortumor mesothelin expressionusing immunohistochemistry and serum mesothelin expressionusing enzyme-linked immunosorbent assay. Eighty-eight (68.8%) epithelial ovarian cancers and eight (24.2%) borderline tumors showed high mesothelin expression, which was associated with shorter progression-free and overall survival. The tumor mesothelin expression status was moderately correlated with serum mesothelin levels in epithelial ovarian cancer patients. Based on receiver operating characteristic analysis, a serum mesothelin level above 2.20nM predicted high tumor mesothelin expression in epithelial ovarian cancer patients (area under the curve=0.81). In 45 patients with recurrent epithelial ovarian cancer, we observed relatively lower levels of serum mesothelin, compared to the level at the primary diagnosis. We also tracked the change in the serum mesothelin level during the course of second-line chemotherapy and found a discrepancy between the clinical response and the serum mesothelin change in some patients, which suggested tumor heterogeneity among the tumor cells with or without mesothelin expression. Serum mesothelin may be a useful noninvasive biomarker surrogate for tumor mesothelin expression in future clinical trials for mesothelin-targeted therapy.

SC27.03 Transforming KRAS into a Clinically Relevant Biomarker

SC27.03 Transforming KRAS into a Clinically Relevant Biomarker

Abstract A90: Mesothelin confers tumor cell vulnerabilities for gefitinib treatment

Abstract Combination therapy for pancreatic adenocarcinoma (PDAC) has recently had two successful stage III trials with FOLFIRINOX (combination folinic acid, fluorouracil, irinotecan, and oxalplatin) and combination gemcitabine and paclitaxel. Because of the heterogeneity of PDAC, the benefits of combination therapy are immediately apparent since it targets multiple facets of cancer simultaneously. However, FOLFIRINOX and gemcitabine + paclitaxel are highly toxic and affect all dividing cells, making apparent the need for better understanding of the cell profiles in PDAC. The epidermal growth factor receptor (EGFR) is involved in the pathogenesis and progression of many cancers, with EGFR or its ligands often overexpressed or mutated. Recently we have found that EGFR expression is correlated to mesothelin (MSLN) expression in PDAC. MSLN, a cell surface glycoprotein, has been shown by us and other labs to be overexpressed in 80 to 90% of all PDAC cases. To examine the relationship between MSLN and EGFR, we modulated MSLN expression by generating a MSLN overexpressing cell line from MIA-PaCa2 cells, and a MSLN knockdown cell line from CAPAN-2 cells. The change in EGFR signaling mimicked that of MSLN, with MSLN overexpression significantly increasing EGFR expression and MSLN knockdown significantly decreasing EGFR expression. Furthermore, high MSLN (and therefore high EGFR expression) showed increased sensitivity to gefitinib, an EGFR ATP binding site inhibitor, at concentrations as low as 100 nM, while MSLN low expressing cells did not exhibit any effect from gefitinib at concentrations below 10 µM. Cell Cycle analysis demonstrated that gefitinib increased the number of cells in G1 phase after 24 hours in MSLN high expressing cells, but had no effect on MSLN low expressing cells. In addition, western blot analysis of EGFR downstream pathways showed an increase in phosphorylated phospholipase C-Υ (PLC-Υ) in MSLN high expressing cells, which could be mitigated by treatment with gefitinib. Finally, combination therapy with gemcitabine and 100 nM gefitinib showed an additive effect in MSLN high expressing cells, while in MSLN low expressing cells combination gemcitabine and gefitinib had no additional effect over gemcitabine alone. This study demonstrates the correlation between MSLN expression and EGFR expression and indicates that gefitinib could be a potential addition to combination therapy for PDAC in patients with high MSLN expression. Citation Format: Ethan Poteet, Zhengdong Liang, Youwen Wang, Changyi Chen, Qizhi Yao.{Authors}. Mesothelin confers tumor cell vulnerabilities for gefitinib treatment. [abstract]. In: Proceedings of the AACR Special Conference on Pancreatic Cancer: Advances in Science and Clinical Care; 2016 May 12-15; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2016;76(24 Suppl):Abstract nr A90.

Discovery and Validation of Cell-Surface Protein Mesothelin (MSLN) As a Novel Therapeutic Target in AML: Results from the COG/NCI Target AML Initiative

In efforts to discover genes uniquely expressed in childhood AML, we performed transcriptomesequencing (RNA-Seq) in pediatric AML and contrasted the expression signature to that in normal marrow hematopoiesis. This effort led to the discovery of over 200 genes that lack expression in normal hematopoietic cells, but are variably expressed in pediatric AML cells. Mesothelin(MSLN) was discovered to be one of the most highly expressed genes in a subset of childhood AML cases (p<10-15).Mesothelin is a cell-surface protein that is expressed onmesothelial cells ofserosal lining. MSLN is over-expressed on a variety of solid tumors, including lung, pancreatic, and ovarian cancers, and is associated with increased malignant transformation, cellular proliferation, and tumor aggressiveness. Given its cell surface expression, MSLN has emerged as an attractive target for immunotherapeutic interventions in solid tumors in adults.In this study, RNA obtained from diagnostic bone marrow specimens from childhood AML (N=434) and normal marrow (N=20) was subjected to wholetranscriptomesequencing and MSLN expression was quantified and normalized and reported as reads perkilobaseof exon per million reads mapped (RPKM). Similar data was obtained from adult TCGA AML database. Quantitative RT-PCR (qRT-PCR) and multidimensional flowcytometry(MDF) was used for confirmation of the transcript and cell surface protein expression. TARGET AML methylation data was used for correlation with transcript expression.Of the 434 specimens analyzed, MSLN mRNA expression was variably expressed (RPKM range 0-618.8), with expression detected in 119 patients (27%). Confirmatory studies by qRT-PCR on specimens with and without MSLN expression (N=137) showed correlation between RNA-Seqand PCR data. Cell surface MSLN expression was assessed by MDF using a PE-conjugated MSLN antibody (PE-mesoAb) and verified expression of MSLN protein on the leukemic cell surface in every case with MSLN transcript expression (Figure 1A). Evaluation of CD34+/CD38- hematopoietic progenitor cells by PE-mesoAbdemonstrated lack of MSLN expression by MDF. Evaluation of matched diagnostic and relapse specimens from MSLN-expressing patients (n=27) confirmed that MSLN expression was largely stable (R2=0.87), thus substantiating its expression in the major AML clone.Comparison of MSLN expression in pediatric vs. adult AML demonstrated a higher prevalence in pediatric AML (TARGET: 27% vs. TCGA: 11%)(Figure 1B). Evaluation of the clinical and biologic features in MSLN expressing (MSLN+) and non-MSLN expressing (MSLN-) pediatric patients revealed that MSLN expression was rarely observed in patients with normal karyotype (p<0.001) or with the most common somatic mutations of FLT3/ITD, NPM1, CEBPA (p<0.001 in all cases). However, MSLN expression was significantly higher in patients with inv(16), t(8;21) and MLL translocations (p<0.001, p<0.001, and p=0.02 respectively; Figure 1C). Given that a majority of patients with core binding factor (CBF) AML were MSLN+, we evaluated the clinical implications of MSLN expression in this favorable risk cohort. Among CBF patients, MLSN+ patients (n=95) had a relapse risk of 51% vs. 32% in the MSLN- (n=62) cohort (p=0.03; Figure 1D), with a corresponding disease free survival of 46% vs. 64% respectively (p=0.03).We further inquired about the mechanism by which MSLN expression might be regulated. Whole genome sequencing data failed to identify any genomic alterations in MSLN that could result in high expression. Therefore, we interrogated the possibility of epigenetic regulation of MSLN expression. Integration of the expression and methylation profiling cases with matching RNA-Seqand methylation data (N=246) demonstrated thathypomethylationof the MSLN promoter significantly correlated with high MSLN expression, implicating epigenetic regulation in the expression of MSLN in AML.Mesothelin, a therapeutic target in solid tumors, is highly expressed in biologically distinct subsets of childhood AML. High expression on leukemic blasts and lack of expression in normal hematopoiesis makes this antigen an ideal target for therapeutic intervention in AML. As a cell surface protein, this antigen avails itself for immune targeting by antibody drug conjugates, CAR-T cells, and T cell receptor mediated targeting. [Display omitted] DisclosuresLoken:Hematologics: Employment, Equity Ownership. Pardo:Hematologics, Inc: Employment.

Expression of mesothelin in thymic carcinoma and its potential therapeutic significance

ObjectivesAdvanced thymic epithelial tumors (TETs) lack adequate treatment options in part due to absence of well characterized tumor-specific antigens. Mesothelin, a cell surface antigen, has been used successfully as a target for tumor-directed therapy. We sought to determine tumor expression and serum levels of mesothelin in patients with TETs. Patients and methodsTissue samples were obtained from 71 patients with histologically confirmed, unresectable advanced TETs and evaluated for mesothelin expression by immunohistochemistry. The evaluation was blinded for clinical data and outcome. Mesothelin expression and its association with clinico-pathological parameters and survival were assessed. ResultsThymic carcinoma, thymoma, and thymic neuroendocrine tumors (NETs) accounted for 34 (48%), 29 (41%), and 8 (11%) cases respectively. Mesothelin expression was seen in a significantly larger proportion of thymic carcinoma (27/34, 79%) than thymoma (3/29, 10%) (P<0.0001) and was absent in thymic NETs. Among thymic carcinomas 13/34 (38%) showed expression in nearly all tumor cells. Immunoreactivity was membranous, strong, and homogenous. Patients with thymic carcinoma and high mesothelin expression (in >50% of tumor cells) had significantly improved overall survival (median not reached, n=19) compared to patients with no or low mesothelin expression (1.60 years; 95% CI: 1.24–4.94 years; n=15; HR=4.46, 95% CI: 1.55–12.80; p=0.0026). ConclusionMesothelin expression is frequently observed in advanced thymic carcinomas, infrequently in thymomas and is absent in thymic NETs. Due to strong, membranous expression mesothelin is a potential therapeutic target in thymic carcinoma.

In vitro and in vivo targeting imaging of pancreatic cancer using a Fe3O4@SiO2 nanoprobe modified with anti-mesothelin antibody.

Pancreatic cancer is a highly malignant disease with a 5-year survival rate <5% mainly due to lack of early diagnosis and effective therapy. In an effort to improve the early diagnostic rate of pancreatic cancer, a nanoprobe Fe3O4@SiO2 modified with anti-mesothelin antibody (A-MFS) was prepared to target cells and tumor tissues highly expressing mesothelin in vitro (human pancreatic cancer cell line SW1990) and in vivo (subcutaneously transplanted tumors) studies. The A-MFS probe was successfully prepared and was spherical and uniform with a hydrodynamic diameter between 110 and 130 nm. Cell Counting Kit-8 testing indicated that A-MFS was nontoxic in vitro and in vivo studies. The in vitro study showed that the A-MFS probe specifically targeted SW1990 cells with high mesothelin expression. The in vivo study was conducted in Siemens 3.0 T magnetic resonance imaging. The average T2-weighted signal values of the xenografts were 966.533±31.56 before injecting A-MFS and 691.133±56.84 before injecting saline solution. After injection of 0.1 mL A-MFS via nude mouse caudal vein for 2.5 hours, the average T2-weighted signal of the xenograft decreased by 342.533±42.6. The signal value decreased by −61.233±33.9 and −58.7±19.4 after injection of the saline and Fe3O4@SiO2. The decrease of tumor signal by A-MFS was much more significant than that by saline and Fe3O4@SiO2 (P<0.05). The results demonstrated the high stability and nontoxicity of A-MFS, which effectively targeted pancreatic cancer in vitro and in vivo. A-MFS is a promising agent for diagnosis of pancreatic cancer.

Mesothelin as a target for chimeric antigen receptor-modified T cells as anticancer therapy.

Mesothelin is a promising target for immune-based therapy, specifically for mesothelioma and pancreatic and ovarian cancers that have high levels of mesothelin expression. Many preclinical and clinical studies that target tumors with high mesothelin expression with antibodies, immunotoxins, antibody-drug conjugates and vaccines have shown the potential of mesothelin as a target. Studies of T cells genetically modified with chimeric antigen receptors (CAR) report significant efficacy in hematologic malignancies, and antimesothelin CAR T cells are currently being investigated in clinical studies. Here we outline the rationale for using mesothelin as a target for immunotherapy, review the clinical and preclinical studies evaluating mesothelin-directed therapies and explore the promise of CAR T cells directed against mesothelin for immunotherapy in the future.

Combining the Antimesothelin Immunotoxin SS1P With the BH3-mimetic ABT-737 Induces Cell Death in SS1P-resistant Pancreatic Cancer Cells

SS1P is an antimesothelin recombinant immunotoxin (RIT). Pancreatic ductal adenocarcinoma (PDAC) cell lines are resistant to SS1P, despite high mesothelin expression. The aim of this study is to examine whether combining SS1P and BH3-mimetic ABT-737 induces cell death in a panel of PDAC cell lines. ABT-737 binds and neutralizes several antiapoptotic BCL2 family proteins, but has a low affinity for the short-lived MCL1 and BCL2A1. SS1P inhibits protein synthesis, which has shown to downregulate MCL1. PDAC cell lines KLM-1, BxPc-3, and Panc 3.014 were resistant to SS1P or ABT-737 alone. Combining both compounds led to a significant increase in cell death. After 48 hours of treatment, cell death was observed in 92% of KLM-1, 55% of BxPc-3, and 23% of Panc 3.014 cells. Panc 3.014 had the highest number of mesothelin-binding sites (92×10(3)), followed by KLM-1 (58×10(3)) and BxPc-3 (3×10(3)). ABT-737 had no effect on SS1P internalization, but enhanced SS1P-induced protein synthesis inhibition significantly in KLM-1, to a lesser extent in BxPc-3, and very little in Panc 3.014. SS1P alone or in combination with ABT-737 downregulated MCL1 in KLM-1 and BxPc-3, but not in Panc 3.014. Similar observations were made for BCL2A1, which had the highest levels in Panc 3.014. Compared with KLM-1, Panc 3.014, and BxPc-3 also had lower proapoptotic BAK and a trend toward higher MCL1. Proapoptotic BAX was similar in KLM-1 and BxPc-3, but lower in Panc 3.014. In conclusion, combining SS1P with ABT-737 overcomes SS1P-resistance in PDAC, although to a variable extent. The efficacy of the combination is mainly associated with the RIT-associated inhibition of protein synthesis and the ability to downregulate MCL1 and BCL2A1, while levels of other key apoptotic proteins may also be important. Our data support the combination of an RIT and a BH3-mimetic, and identify factors that potentially limit the efficacy of such therapeutic approach.

Mesothelin Overexpression Promotes Mesothelioma Cell Invasion and MMP-9 Secretion in an Orthotopic Mouse Model and in Epithelioid Pleural Mesothelioma Patients

Mesothelin (MSLN) is a tumor-associated antigen, being investigated as a biomarker and therapeutic target in malignant pleural mesothelioma (MPM). The biologic function of MSLN overexpression in MPM is unknown. We hypothesized that MSLN may promote tumor invasion in MPM, a tumor characterized primarily by regional aggressiveness and rare distant metastases. Human and murine MPM cells with MSLN forced expression and short hairpin RNA knockdown were examined for proliferation, invasion, and matrix metalloproteinase (MMP) secretion. The influence of MSLN overexpression on MPM cell invasion was assessed in an orthotopic mouse model and in patient samples. MSLN expression promotes MPM cell invasion and MMP secretion in both human and murine MPM cells. In an orthotopic MPM mouse model characterized by our laboratory, MPM cells with MSLN overexpression preferentially localized to the tumor invading edge, colocalized with MMP-9 expression, and promoted decreased survival without an increase in tumor burden progression. In a tissue microarray from epithelioid MPM patients (n = 139, 729 cores), MSLN overexpression correlated with higher MMP-9 expression at individual core level. Among stage III MPM patients (n = 72), high MSLN expression was observed in 26% of T2 tumors and 51% of T3 tumors. Our data provide evidence elucidating a biologic role for MSLN as a factor promoting tumor invasion and MMP-9 expression in MSLN expressing MPM. As regional invasion is the characteristic feature in MSLN expressing solid cancers (MPM, pancreas, and ovarian), our observations add rationale to studies investigating MSLN as a therapeutic target.

Abstract 2726: Efficacy and candidate biomarker evaluation for the anti-mesothelin antibody drug conjugate (ADC) BAY 94-9343, mesothelin-ADC in mesothelin-positive preclinical xenograft models

Abstract BAY 94-9343 is a novel antibody drug conjugate consisting of a fully human monoclonal antibody directed against mesothelin conjugated with the maytansinoid derivative DM4 by a linker containing a disulfide bridge (SPDB) and currently in Phase I clinical testing. Mesothelin is expressed in a range of tumor types, such as mesothelioma, ovarian cancer, pancreatic cancer and lung cancer. Membrane-anchored forms may play a role in cellular adhesion and metastasis. Here we report on the correlation of tumor mesothelin antigen expression levels, as determined by a newly developed mesothelin IHC staining protocol, and the efficacy of BAY 94-9343 in 13 preclinical tumor models. Further, mesothelin levels in xenograft tumors were compared to those in clinical samples of human tumors. Efficacy of BAY 94-9343 in tumor models with high mesothelin expression levels were generally higher compared to models with low mesothelin expression. OVCAR6677, OVCAR6668 and MIAPaCa-meso models characterized by high expression of mesothelin showed 90-100 % tumor growth inhibition, when treated with 2.5 mg/kg BAY 94-9343. PAXF736 and PAXF1881, tumor models with no mesothelin expression, did not show tumor growth inhibition at 2.5 mg/kg mesothelin-ADC, but responded to higher doses. Analysis of mesothelin expression by IHC using human tumor samples (mesothelioma, ovarian and pancreatic cancer) demonstrated that tumor cell mesothelin expression varied both within and between tumor types. IHC scoring revealed that 92% of mesothelioma tumors, 80% of ovarian tumors, and 63% of pancreatic cancer tumors assayed were mesothelin positive. Mesothelin expression was stable when comparing primary ovarian cancers with relapsing tumor tissue from the same patient. Maximum mesothelin expression in ovarian cancer is at least 400,000 antigenic sites per cell determined by Quantibrite technology. In preclinical models analysis of serum mesothelin extracellular domain (ECD) by ELISA revealed an initial rise in serum mesothelin ECD as early as 1 days after 10 mg/kg BAY 94-9343 followed by a decrease to below-baseline levels. Increased levels of the apoptosis marker CK18 were observed following BAY 94-9343 administration. Changes in serum mesothelin ECD levels as well as CK18 will be explored as an early pharmacodynamic (PD) response markers in clinical trials for BAY 94-9343. In summary, these results demonstrate that targeted delivery of DM4 resulted in higher efficacy of BAY 94-9343 in tumor models with high mesothelin expression. Responsive xenograft models were representative of clinical samples with respect to mesothelin expression scored by IHC. Quantification of mesothelin by IHC is a candidate stratification biomarker for the antibody drug conjugate BAY 94-9343 currently in Phase I testing. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 2726. doi:1538-7445.AM2012-2726

Mesothelin as a Potential Therapeutic Target in Human Cholangiocarcinoma

Hepatocellular carcinoma (HCC) and cholangiocarcinoma (CCA) are the two most common primary liver cancers, yet there have been no significant advances in effective therapeutics. Mesothelin has been reported as a new therapeutic target in various types of cancer. Here, we investigated the expression of mesothelin in liver cancer and its potential role as a novel therapeutic target for immunotherapy. HCC and CCA specimens were examined by immunohistochemistry for mesothelin expression. Protein expression was assessed by immunoblotting and flow cytometry. The SS1P immunotoxin targeting mesothelin was evaluated in the well-established CCA cell lines HuCCT1, HuH-28, KMBC, KMCH, Mz-ChA-1 and OZ. We showed strong immunochemical mesothelin staining in 33% of the surgically resected CCA specimens and 3 of 6 CCA cell lines (OZ, KMBC and KMCH). No mesothelin staining was found in HCC or normal liver tissue. Mesothelin was primarily localized to the cellular plasma membrane and the mature form (molecular weight, ~40 kDa) was expressed at a high level in CCA tissues. Moreover, 22% of CCA specimens had a high mesothelin expression level which was comparable to the CCA cell line models. Interestingly, SS1P showed very high and specific growth inhibition when added to mesothelin-expressing CCA cells with IC(50) values ranging from 0.5 to 11 ng/mL. Mesothelin is overexpressed in one-third of CCA tissues. SS1P targeting mesothelin reveals a remarkable single agent activity against CCA in vitro. These findings indicate a potential for SS1P in the immunotherapeutic treatment of CCA.