Reduced Shedding of Surface Mesothelin Improves Efficacy of Mesothelin-Targeting Recombinant Immunotoxins

Abstract

Mesothelin (MSLN) is a differentiation antigen that is highly expressed in many epithelial cancers. MSLN is an important therapeutic target due to its high expression in cancers and limited expression in normal human tissues. Although it has been assumed that shed antigen is a barrier to immunotoxin action, a modeling study predicted that shed MSLN may enhance the action of MSLN-targeting recombinant immunotoxins such as SS1P and similar therapeutics by facilitating their redistribution within tumors. We aimed to determine whether shed MSLN enhances or reduces the antitumor effect of MSLNtargeting immunotoxins SS1P and RG7787. We engineered a cell line, A431/G9 (TACE mutant) that expresses a mutant form of MSLN in which the TNF-converting enzyme protease site is replaced with GGGS. We compared the response of the TACEmutant cells with immunotoxins SS1P and RG7787 with that of the parental A431/H9 cell line. We show that TACE-mutant cells shed 80% less MSLN than A431/H9 cells, that TACE-mutant cells show a 2-to 3-fold increase in MSLN-targeted immunotoxin uptake, and that they are about 5-fold more sensitive to SS1P killing in cell culture. Tumors with reduced shedding respond significantly better to treatment with SS1P and RG7787. Our data show that MSLN shedding is an impediment to the antitumor activity of SS1P and RG7787. Approaches that decrease MSLN shedding could enhance the efficacy of immunotoxins and immunoconjugates targeting MSLN-expressing tumors. Mol Cancer Ther; 15(7); 1648-55. Ó2016 AACR.

A new anti-mesothelin antibody targets selectively the membrane-associated form

“…[30][31][32][33][34] The presence of SMRP can also alter the binding capacity of mesothelin-targeting therapeutic agents. 22,23 On the basis of these data, we used the serum of 21 patients with metastatic colon and pancreatic cancers in competition assays with selected Fabs. Serum concentration of mesothelin from the 21 patients and from 2 healthy volunteers (A and B) was first determined by ELISA assay (Fig.…”

“…22 A recent study demonstrated that reduced shedding of surface mesothelin as a result of treatment with paclitaxel (Taxol) improves the efficacy of mesothelin-targeting recombinant immunotoxins. 23 Similarly, mutation of the protein responsible for mesothelin cleavage such that it is not shed results in improved efficacy of the immunotoxin SS1P. 23 Here, we propose a new approach to overcome the effect of shed (soluble) mesothelin on antibody-based therapies.…”

“…23 Similarly, mutation of the protein responsible for mesothelin cleavage such that it is not shed results in improved efficacy of the immunotoxin SS1P. 23 Here, we propose a new approach to overcome the effect of shed (soluble) mesothelin on antibody-based therapies. Our goal was to identify an antibody that exclusively targets the whole cell membrane-associated form of mesothelin without interfering with the soluble form of the protein.…”

A re-engineered immunotoxin shows promising preclinical activity in ovarian cancer

“…Also for other solid tumor types synergistic effects of combined treatment with an immunotoxin and a taxane have been reported 31 – 33 . Several different mechanisms of interaction between these two types of agents have been demonstrated: (i) Taxol treatment can reduce the extent of shedding of mesothelin 34 (the same might not apply for other surface targets of immunotoxins 35 ). (ii) Tumor uptake in-vivo is usually a rate-limiting step for any immunotoxin therapy due to the short circulation half-life of immunotoxins and can be enhanced by taxol treatment 36 .…”

Critical Issues in the Development of Immunotoxins for Anticancer Therapy

“…112 Reduced shedding of surface mesothelin has improved the efficacy of antimesothelin scFv PE38 ITs, such as SSIP and RG7787, in mouse models. 113 Furthermore, antigen shedding may induce immunecomplex formation between a soluble antigen and the IT in blood vessels, thus causing significant type III hypersensitivity. 114 On the other hand, the effect of antigen shedding depends on the number of antigen molecules on the cell surface and on the endocytosis rate.…”