Abstract 2580: Single-chain antibody based immunotoxins targeting Her2/neu: design optimization and impact of affinity on antitumor effects

Recombinant immunotoxins, consisting of single-chain antibodies (scFv) genetically fused to polypeptide toxins represent potentially effective candidates for cancer therapeutics. Antibody affinity, internalization rates and intracellular fate of molecular constructs play critical roles in the eventual cytotoxic activities of these constructs although these events related to the design characteristics of fusion constructs have not been comprehensively explored. We evaluated the relationship between affinity of various anti-Her2 scFv fused to recombinant gelonin (rGel) and its effect on cytotoxic activity. A series of rGel-based immunotoxins were created from the human anti-Her2/neu scFv C6.5 and various affinity mutants (designated ML3.9, MH3B1 and B1D3-in increasing affinity order). The affinities of the scFv ranged from 10−8 to 10−11 M and the rGel fusion toxins retained the affinities of the scFv from which they were derived. Against BT474M1 human breast tumor cells, the immunotoxins demonstrated IC50 values of 18.9, 10.9, 3.9 and 1.2nM for the C6.5, ML3.9, MH3B1 and B1D3 based immunotoxins respectively. Targeting indices were highest for the B1D3/rGel construct with the highest affinity. The internalization rate of the fusion constructs into target cells directly correlated with the cytotoxic effects. Competition studies showed that addition of soluble Her2/neu extracellular domain (ECD) as the competitive factor significantly reduced the cytotoxic effects of the highest-affinity fusion construct (B1D3/rGel) while the lower-affinity constructs showed lesser impact of Her2/neu ECD addition on the cytotoxic effect. Animal model efficacy studies against several Her2/neu overexpressing tumors are ongoing but appear to suggest that the construct containing the highest affinity antibody was comparatively less effective in vivo than constructs containing antibodies of intermediate affinity. These studies suggest that human MH3B1/rGel fusion construct with an apparent affinity of 10−10 M displayed an overall optimal cell-killing activity to several Her2/neu positive cancer cells especially in the presence of free Her2/neu ECD. These findings demonstrate the necessity to consider antibody affinity in design considerations for targeted therapeutics. In addition, the potential impact of soluble target or truncated receptor molecules in the environment need to be considered when evaluating the efficacy of targeted therapeutics. Research Conducted, in part, by the Clayton Foundation for Research.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 2580.

Abstract 2437: Engineered human monoclonal antibodies targeting the Müllerian inhibiting substance type II receptor for ovarian cancer therapy

Currently available approaches for ovarian cancer therapy have limited success in the setting of advanced disease. Antibody-based therapeutics offer new and potentially effective strategies. Müllerian inhibiting substance (MIS, or anti-Müllerian hormone, AMH) is known for its role in embryonic development, triggering regression of the Müllerian ducts in mammalian males (in females Müllerian ducts are the source for uteri, Fallopian tubes and upper vagina). MIS signals via a heterodimeric receptor, consisting of type I (MISIR) and type II (MISIIR) receptors, where MISIIR mediates the ligand specificity and MISIR triggers the downstream signaling cascade. Approximately 64% of epithelial ovarian cancers express MISIIR with the most common expression in serous and mucinous tumors (Bakkum-Gamez et. al., Gynecol. Oncol. 2008;108(1):141). MIS has been reported to inhibit the growth of tumor xenografts in vivo (Stephen et al., Clin. Cancer Res. 2002;8(8):2640) and to trigger apoptosis of primary human ovarian cancer cells in vitro (Masaikos et al., Clin. Cancer Res. 1999;5(11):3488). This, along with the highly restricted expression pattern of MISIIR in normal tissues, suggests that MISIIR is a promising target for ovarian cancer therapy. A number of single-chain Fv molecules (scFv) and their bivalent derivatives (scFv: Fc constructs) targeting MISIIR were developed in our lab by means of screening of two large naïve human scFv phage display libraries. However, to date, the potential clinical utility of these anti-MISIIR scFv molecules has been limited due to their low binding affinity.

Here, we describe the screening of a third human scFv phage display library against a fusion protein representing the extracellular domain of MISIIR with the Fc domain of human IgG1 fused to its C-terminus. This screening led to selection of six unique scFv clones that specifically target MISIIR on the surface of cells with varying efficiency. One of these scFv clones, GS45, has demonstrated excellent targeting of MISIIR by flow cytometry, ELISA and surface plasmon resonance analysis. We are currently cloning GS45 into a full-length IgG molecule in order to improve its potential clinical utility.

Antibodies (or antibody-based constructs, such as scFv or scFv: Fc) targeting MISIIR could be used as the basis of a variety of therapeutic strategies. They can be used on their own to induce agonistic signaling or direct antibody-dependent cell-mediated cytotoxicity (ADCC). Alternatively, MISIIR-targeting antibodies can be conjugated to cytotoxic drugs, radiolabels or imaging agents. The anti-MISIIR scFv GS45, described here, is currently being developed for ADCC and immunodrug conjugate applications. We believe it can be the basis of novel therapeutic strategies for ovarian cancer treatment.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 2437.

Isolation of anti-MISIIR scFv molecules from a phage display library by cell sorter biopanning

While cell surface antigens represent the most common targets for antibody-based cancer therapy, isolation of new antibodies specific for these targets from single-chain Fv phage display libraries has been hindered by limitations associated with traditional selection techniques. Solid phase panning is often associated with conformational changes to the target protein due to its immobilization on plastic tubes that can limit the ability of the isolated scFv to bind to conformational epitopes and solution panning methods require the use of secondary tags that often mask desired sequences and create unintended epitopes. Commonly utilized cell-based panning methods typically yield a panel of single-chain Fv (scFv) molecules that are specific for numerous cell surface antigens, often obscuring the desired clones. Here, we describe a novel cell sorter-based system to isolate single-chain Fv molecules specific for defined antigen targets expressed on stably-transformed mammalian cells. We employed these methods to isolate promising scFv clones that bind specifically to the Müllerian inhibiting substance type II receptor, a cell surface ovarian cancer antigen that has proven to be a difficult target for selection strategies.

Development of engineered antibodies specific for the Müllerian inhibiting substance type II receptor: a promising candidate for targeted therapy of ovarian cancer

The Müllerian inhibiting substance type II receptor (MISIIR) is involved in Müllerian duct regression as part of the development of the male reproductive system. In adult females, MISIIR is present on ovarian surface epithelium and is frequently expressed on human epithelial ovarian cancer cells. Müllerian inhibiting substance has been found to be capable of inhibiting the growth of primary human ovarian cancer cells derived from ascites and ovarian cancer cell lines. This suggested to us that MISIIR could be an attractive target for antibody-based tumor targeting and growth inhibition strategies. Here, we describe the production of recombinant human MISIIR extracellular domain-human immunoglobulin Fc domain fusion proteins and their use as targets for the selection of MISIIR-specific human single-chain variable fragments (scFv) molecules from a human nonimmune scFv phage display library. The binding kinetics of the resulting anti-MISIIR scFv clones were characterized and two were employed as the basis for the construction of bivalent scFv:Fc antibody-based molecules. Both bound specifically to human ovarian carcinoma cells in flow cytometry assays and cross-reacted with mouse MISIIR. These results indicate that antibody-based constructs may provide a highly specific means of targeting MISIIR on human ovarian carcinoma cells for the purpose of diagnosing and treating this disease.

Isolation of scFvs to in vitro produced extracellular domains of EGFR family members

The members of the epidermal growth factor receptor (EGFR) family are over expressed in a variety of malignancies and are frequently linked to aggressive disease and a poor prognosis. Although clinically effective monoclonal antibodies (MAbs) have been developed to target HER2 and EGFR, the remaining two family members, HER3 and HER4, have not been the subject of significant efforts. In this paper, we have taken the initial steps required to generate antibodies with potential clinically utility that target the members of the EGFR family. The genes for the extracellular domains (ECDs) of all four members of the EGFR family were cloned and used to stably transfect 293 (HEK) cells. Milligram quantities of each ECD were produced and characterized. The HER3, HER4, and EGFR ECDs were then employed as targets for the selection of antibodies from naïve human scFv (single-chain Fv) phage display libraries. Six unique scFv clones were isolated that bound specifically to HER3, 13 unique clones were isolated with specificity for HER4 and 52 unique anti-EGFR clones were isolated. These scFvs provide a valuable and potentially clinically relevant panel of agents to target the members of the EGFR family.

A Single Treatment of Yttrium-90-labeled CHX-A″–C6.5 Diabody Inhibits the Growth of Established Human Tumor Xenografts in Immunodeficient Mice

Antitumor diabody molecules are noncovalent single-chain Fv dimers that recapitulate the divalent binding properties of native IgG antibodies. Diabodies are capable of substantial accumulation in tumor xenografts expressing relevant antigens in immunodeficient mouse models. With a Mr of approximately 55,000, diabodies are rapidly cleared from the circulation, resulting in tumor-to-blood ratios that significantly exceed those achieved early after the administration of monoclonal antibodies. We have evaluated the therapeutic potential of the beta-emitting isotope yttrium-90 (t1/2, 64 hours) conjugated to the C6.5K-A diabody that specifically targets the HER2/neu human tumor-associated antigen. We have found that a single intravenous dose of 150 microCi (200 microg) 90Y-CHX-A"-C6.5K-A diabody substantially inhibits the growth rates of established MDA-361/DYT2 human breast tumor xenografts in athymic nude mice. In contrast, 300 microCi (300 microg) 90Y-CHX-A"-C6.5K-A diabody resulted in only a minor delay in the growth of SK-OV-3 human ovarian cancer xenografts. The maximum tolerated dose was also dependent on the tumor xenograft model used. These studies indicate that genetically engineered antitumor diabody molecules can be used as effective vehicles for radioimmunotherapy.

High affinity restricts the localization and tumor penetration of single-chain fv antibody molecules

Antitumor monoclonal antibodies must bind to tumor antigens with high affinity to achieve durable tumor retention. This has spurred efforts to generate high affinity antibodies for use in cancer therapy. However, it has been hypothesized that very high affinity interactions between antibodies and tumor antigens may impair efficient tumor penetration of the monoclonal antibodies and thus diminish effective in vivo targeting (K. Fujimori et al., J. Nucl. Med., 31: 1191-1198, 1990). Here we show that intrinsic affinity properties regulate the quantitative delivery of antitumor single-chain Fv (scFv) molecules to solid tumors and the penetration of scFv from the vasculature into tumor masses. In biodistribution studies examining a series of radioiodinated scFv mutants with affinities ranging from 10(-7)-10(-11) M, quantitative tumor retention did not significantly increase with enhancements in affinity beyond 10(-9) M. Similar distribution patterns were observed when the scFv were evaluated in the absence of renal clearance in anephric mice, indicating that the rapid renal clearance of the scFv was not responsible for these observations. IHC and IF evaluations of tumor sections after the i.v. administration of scFv affinity mutants revealed that the lowest affinity molecule exhibited diffuse tumor staining whereas the highest affinity scFv was primarily retained in the perivascular regions of the tumor. These results indicate that antibody-based molecules with extremely high affinity have impaired tumor penetration properties that must be considered in the design of antibody-based cancer therapies.

Increasing the affinity for tumor antigen enhances bispecific antibody cytotoxicity

We tested the hypothesis that bispecific Abs (Bsab) with increased binding affinity for tumor Ags augment retargeted antitumor cytotoxicity. We report that an increase in the affinity of Bsab for the HER2/neu Ag correlates with an increase in the ability of the Bsab to promote retargeted cytotoxicity against HER2/neu-positive cell lines. A series of anti-HER2/neu extracellular domain-directed single-chain Fv fragments (scFv), ranging in affinity for HER2/neu from 10(-7) to 10(-11) M, were fused to the phage display-derived NM3E2 human scFV: NM3E2 associates with the extracellular domain of human FcgammaRIII (CD16). The resulting series of Bsab promoted cytotoxicity of SKOV3 human ovarian carcinoma cells overexpressing HER2/neu by human PBMC preparations containing CD16-positive NK cells. The affinity for HER2/neu clearly influenced the ability of the Bsab to promote cytotoxicity of (51)Cr-labeled SKOV3 cells. Lysis was 6.5% with an anti-HER2/neu K(D) = 1.7 x 10(-7) M, 14.5% with K(D) = 5.7 x 10(-9) M, and 21.3% with K(D) = 1.7 x 10(-10) M at 50:1 E:T ratios. These scFv-based Bsab did not cross-link receptors and induce leukocyte calcium mobilization in the absence of tumor cell engagement. Thus, these novel Bsab structures should not induce the dose-limiting cytokine release syndromes that have been observed in clinical trials with intact IgG BSAB: Additional manipulations in Bsab structure that improve selective tumor retention or facilitate the ability of Bsab to selectively cross-link tumor and effector cells at tumor sites should further improve the utility of this therapeutic strategy.

Delivery of the alpha-emitting radioisotope bismuth-213 to solid tumors via single-chain Fv and diabody molecules

Intravenously administered anti-tumor single-chain Fv (scFv) and diabody molecules exhibit rapid clearance kinetics and accumulation in tumors that express their cognate antigen. In an attempt to fit the rate of isotope decay to the timing of delivery and duration of tumor retention, anti-HER2/neu CHX-A" DTPA-C6.5K-A scFv and diabody conjugates were labeled with the alpha-particle emitter (213)Bi (t(1/2) = 47 min). Radioimmunotherapy studies employing 0.64, 0.35, or 0.15 microCi of (213)Bi-labeled C6.5K-A diabody or 1.1, 0.6, or 0. 3 microCi of (213)Bi-labeled C6.5K-A scFv were performed in nude mice bearing early, established SK-OV-3 tumors. Only the 0.3 microCi dose of (213)Bi-labeled C6.5K-A scFv resulted in both acceptable toxicity and a reduction in tumor growth rate. The specificity of the anti-tumor effects was determined by comparing the efficacy of treatment with 0.3 and 0.15 microCi doses of (213)Bi-labeled C6.5K-A scFv and (213)Bi-labeled NM3E2 (an irrelevant scFv) in nude mice bearing large established tumors. The 0.3 microCi dose of (213)Bi on both the C6.5K-A and NM3E2 scFvs resulted in similar anti-tumor effects (p = 0.46) indicating that antigen-specific targeting was not a factor. This suggests that the physical half-life of (213)Bi may be too brief to be effectively paired with systemically-administered diabody or scFv molecules.