Anetumab ravtansine: a novel mesothelin-targeting antibody-drug conjugate cures tumors with heterogeneous target expression favored by bystander effect.

Mesothelin is a tumor differentiation antigen frequently overexpressed in tumors such as mesothelioma, ovarian, pancreatic, and lung adenocarcinomas while showing limited expression in nonmalignant tissues. Mesothelin is therefore an attractive target for cancer therapy using antibody-drug conjugates (ADC). This study describes the detailed characterization of anetumab ravtansine, here referred to as BAY 94-9343, a novel ADC consisting of a human anti-mesothelin antibody conjugated to the maytansinoid tubulin inhibitor DM4 via a disulfide-containing linker. Binding properties of the anti-mesothelin antibody were analyzed using surface plasmon resonance, immunohistochemistry, flow cytometry, and fluorescence microscopy. Effects of BAY 94-9343 on cell proliferation were first studied in vitro and subsequently in vivo using subcutaneous, orthotopic, and patient-derived xenograft tumor models. The antibody binds to human mesothelin with high affinity and selectivity, thereby inducing efficient antigen internalization. In vitro, BAY 94-9343 demonstrated potent and selective cytotoxicity of mesothelin-expressing cells with an IC(50) of 0.72 nmol/L, without affecting mesothelin-negative or nonproliferating cells. In vivo, BAY 94-9343 localized specifically to mesothelin-positive tumors and inhibited tumor growth in both subcutaneous and orthotopic xenograft models. In addition, BAY 94-9343 was able to induce a bystander effect on neighboring mesothelin-negative tumor cells. Antitumor efficacy of BAY 94-9343 correlated with the amount of mesothelin expressed and was generally superior to that of standard-of-care regimen resulting in complete tumor eradication in most of the models. BAY 94-9343 is a selective and highly potent ADC, and our data support its development for the treatment of patients with mesothelin-expressing tumors.

Evaluation of human pancreatic RNase as effector molecule in a therapeutic antibody platform.

Human antibody-ribonuclease (RNase) fusion proteins, referred to as immunoRNases, have been proposed as an alternative to heterologous immunotoxins, without their immunogenicity and unspecific toxicity issues. In this study, we investigated if human pancreatic RNase will be suitable as effector component in a therapeutic antibody development platform. We generated several fusion proteins consisting of tumor-specific human immunoglobulins (IgGs) and human pancreatic RNase. Transient mammalian cell production was efficient and IgG-RNases were purified to homogeneity. Antigen binding was comparable to the parental antibodies and RNase catalytic activity was retained even in the presence of 50-fold molar excess of human cytosolic RNase inhibitor (RI). Serum stability, cell binding and internalization of IgG-RNases were comparable to the parental IgGs. Despite these promising properties, none of the IgG-RNases revealed significant inhibition of tumor cell growth in vitro even when targeting different antigens putatively employing different endocytotic pathways. The introduction of different linkers containing endosomal protease cleavage sites into the IgG-RNase did not enhance cytotoxicity. Similarly, RI evasive human pancreatic RNase variants mediated only small inhibiting effects on tumor cell growth at high concentrations, potentially reflecting inefficient cytosolic translocation. Taken together, human pancreatic RNase and variants did not prove to be generally suitable as effector component for a therapeutic antibody drug development platform.

Generation and comparative characterization of glycosylated and aglycosylated human IgG1 antibodies.

Monoclonal antibodies (mAbs) are the fastest growing class of biopharmaceuticals reflecting their diverse applications in research and the clinic. The correct glycosylation of mAbs is required to elicit effector functions such as complement-dependent and antibody-dependent cell-mediated cytotoxicity, although these may be undesirable for the treatment of certain chronic diseases. To gain insight into the properties of glycan-deficient mAbs, we generated and characterized six different aglycosylated human IgG1 mAbs (carrying the N297A mutation) and compared them to their glycosylated counterparts. We found no differences in solubility or heterogeneity, and all mAbs the remained stable in stress tests at 4 and 37 °C. Surface plasmon resonance spectroscopy showed no differences in binding affinity, and the in vivo terminal serum half-life and plasma clearance were similar in rats. However, differential scanning calorimetry revealed that the aglycosylated mAbs contained a less stable C(H)2 domain and they were also significantly more susceptible to pH-induced aggregation. We conclude that aglycosylated mAbs are functionally equivalent to their glycosylated counterparts and could be particularly suitable for certain therapeutic applications, such as the treatment of chronic diseases.

Structure of wild-type Plk-1 kinase domain in complex with a selective DARPin.

As a key regulator of mitosis, the Ser/Thr protein polo-like kinase-1 (Plk-1) is a well validated drug target in cancer therapy. In order to enable structure-guided drug design, determination of the crystal structure of the kinase domain of Plk-1 was attempted. Using a multi-parallel cloning and expression approach, a set of length variants were identified which could be expressed in large amounts from insect cells and which could be purified to high purity. However, all attempts to crystallize these constructs failed. Crystals were ultimately obtained by generating designed ankyrin-repeat proteins (DARPins) selective for Plk-1 and using them for cocrystallization. Here, the first crystal structure of the kinase domain of wild-type apo Plk-1, in complex with DARPin 3H10, is presented, underlining the power of selective DARPins as crystallization tools. The structure was refined to 2.3 A resolution and shows the active conformation of Plk-1. It broadens the basis for modelling and cocrystallization studies for drug design. The binding epitope of 3H10 is rich in arginine, glutamine and lysine residues, suggesting that the DARPin enabled crystallization by masking a surface patch which is unfavourable for crystal contact formation. Based on the packing observed in the crystal, a truncated DARPin variant was designed which showed improved binding characteristics.

A miniaturized high-throughput screening assay for fucosyltransferase VII.

Fucosyltransferase VII (FucTVII) is a very promising drug target for treatment of inflammatory skin diseases. Its activity is required for synthesis of the sialyl-Lewis X glycoepitopes on the E- and P-selectin ligands, necessary for lymphocyte migration into the skin. High-throughput screening (HTS) of large chemical libraries has become the main source of novel chemical entities for the pharmaceutical industry. The screening of very large compound collections requires the use of specialized assay techniques that minimize time and costs. We describe the development of a miniaturized scintillation proximity assay for human FucTVII based on a oligosaccharide acceptor substrate that is identical to the glycosylation of the physiological substrate. In addition to assay development, the assay performance in a HTS campaign is shown. We screened 798,131 compounds from the Schering AG HTS library and identified 233 IC50 hits; 229 hits were FucTVII specific in so far as they did not inhibit either alpha-fucosidase or galactosyltransferase. In addition to screening a drug-like small-molecule collection, we worked on rational approaches to develop inhibitors or glycosidic decoys based on oligosaccharide-substrate analogues. The structure-activity relationship observed thereby is very narrow and shows strict requirements that are consistent with the described substrate specificity of FucTVII.

Structural basis for a high affinity inhibitor bound to protein kinase MK2.

The Ser/Thr protein kinase MAPKAP kinase 2 (MK2) plays a crucial role in inflammation. We determined the structure of the kinase domain of MK2 in complex with a low molecular mass inhibitor in two different crystal forms, obtained from soaking and co-crystallization. To our knowledge, these are the first structures of MK2 showing the binding mode of an inhibitor with high binding affinity (IC50 8.5 nM). The two crystal forms revealed conformational flexibility in the binding site and extend the experimental basis for rational drug design. Crystal form-1 contained one MK2 molecule per asymmetric unit. Form-2 contained 12 molecules, which arrange into two different types of MK2 trimers. One of them may serve as a model for an intermediate state during substrate phosphorylation, as each MK2 monomer places its activation segment into the substrate peptide binding groove of the trimer neighbor.

Obligate multivalent recognition of cell surface tomoregulin following selection from a multivalent phage antibody library.

A therapeutic antibody candidate (AT-19) isolated using multivalent phage display binds native tomoregulin (TR) as a mul-timer not as a monomer. This report raises the importance of screening and selecting phage antibodies on native antigen and reemphasizes the possibility that potentially valuable antibodies are discarded when a monomeric phage display system is used for screening. A detailed live cell panning selection and screening method to isolate multivalently active antibodies is described. AT-19 is a fully human antibody recognizing the cell surface protein TR, a proposed prostate cancer target for therapeutic antibody internalization. AT-19 was isolated from a multivalent single-chain variable fragment (scFv) antibody library rescued with hyperphage. The required multivalency for isolation of AT-19 is supported by fluorescence activated cell sorting data demonstrating binding of the multivalent AT-19 phage particles at high phage concentrations and failure of monovalent particles to bind. Pure monomeric scFv AT-19 does not bind native receptor on cells, whereas dimeric scFv or immunoglobulin G binds with nanomolar affinity. The isolation of AT-19 antibody with obligate bivalent binding activity to native TR is attributed to the use of a multivalent display of scFv on phage and the method for selecting and screening by alternate use of 2 recombinant cell lines.

Identifying protein construct variants with increased crystallization propensity--a case study.

This study describes an efficient multiparallel automated workflow of cloning, expression, purification, and crystallization of a large set of construct variants for isolated protein domains aimed at structure determination by X-ray crystallography. This methodology is applied to MAPKAP kinase 2, a key enzyme in the inflammation pathway and thus an attractive drug target. The study reveals a distinct subset of truncation variants with improved crystallization properties. These constructs distinguish themselves by increased solubility and stability during a parallel automated multistep purification process including removal of the recombinant tag. High-throughput protein melting point analysis characterizes this subset of constructs as particularly thermostable. Both parallel purification screening and melting point determination clearly identify residue 364 as the optimal C terminus for the kinase domain. Moreover, all three constructs that ultimately crystallized feature this C terminus. At the N terminus, only three amino acids differentiate a noncrystallizing from a crystallizing construct. This study addresses the very common issues associated with difficult to crystallize proteins, those of solubility and stability, and the crucial importance of particular residues in the formation of crystal contacts. A methodology is suggested that includes biophysical measurements to efficiently identify and produce construct variants of isolated protein domains which exhibit higher crystallization propensity.

Imaging of tumor angiogenesis using 99mTc-labeled human recombinant anti-ED-B fibronectin antibody fragments.

UNLABELLED: The aim of this study was to target the angiogenesis-associated extracellular matrix protein ED-B fibronectin for molecular imaging of solid tumors. Recombinant and chemically modified derivatives of the single-chain antibody fragment (scFv) L19, capable of being labeled with 99mTc, were synthesized and radiolabeled. The resulting compounds 99mTc-AP39, 99mTc-L19-His, and 99mTc-L19-Hi20 were assessed for their imaging properties in vivo. METHODS: L19 was genetically modified by inserting either the (Gly)3-Cys-Ala (AP39) or a (His)6 tag (L19-His) sequence at the C-terminal end. Chemical modifications were performed by conjugating the bifunctional chelator Hi20 (L19-Hi20) at epsilon-Lys-NH2 residues of the molecule to allow for a direct chelator-based labeling with 99mTc. Tumor-targeting, pharmacokinetic, and scintigraphic imaging properties of the radiolabeled scFvs were evaluated in nude mice bearing murine F9 teratocarcinoma. RESULTS: 99mTc labeling of the L19 derivatives yielded radiochemically pure proteins maintaining high immunoreactivity to ED-B fibronectin, as measured by affinity chromatography. Size-exclusion high-performance liquid chromatographic analysis of labeled L19 derivatives demonstrated either dimeric species (L19-His) or a mixture of predominantly associative dimeric and monomeric species (AP39, L19-Hi20). 99mTc-AP39 showed the most favorable biodistribution and imaging properties with high and fast tumor uptake (8.3 percentage injected dose per gram at 3 h after injection), rapid blood clearance and renal excretion, leading to high signal-to-noise ratios (tumor-to-blood ratio of 6.4 at 3 h after injection), and excellent planar scintigraphy in vivo. CONCLUSION: ED-B fibronectin can be efficiently targeted by 99mTc-AP39 and scintigraphically visualized in tumor-bearing mice, providing a potentially useful clinical tool for imaging of angiogenesis-associated ED-B fibronectin-expressing human tumors.

Targeting tomoregulin for radioimmunotherapy of prostate cancer.

Radiotherapy is an effective approach for the treatment of local prostate cancer. However, once prostate cancer metastasizes, radiotherapy cannot be used due to the distribution of multiple metastases to lymph nodes and bones. In contrast, radioimmunotherapy should still be efficacious in metastatic prostate cancer as radioisotopes are brought to tumor cells by targeting antibodies. Here we identify and validate a prostate-expressed molecule, tomoregulin, as a target for radioimmunotherapy of prostate cancer. Tomoregulin is a transmembrane protein selectively expressed in the brain, prostate, and prostate cancer, but not expressed in other normal tissues. Immunohistochemical studies of tomoregulin protein in clinical samples show its location in the luminal epithelium of normal prostate, benign prostatic hyperplasia, and prostatic intraepithelial neoplasia. More importantly, the tomoregulin protein is expressed in primary prostate tumors and in their lymph node and bone metastases. The nature of tomoregulin as a transmembrane protein and its tissue-specific expression make tomoregulin an attractive target for radioimmunotherapy, in which tomoregulin-specific antibodies will deliver a radioisotope to prostate tumor cells and metastases. Indeed, biodistribution studies using a prostate tumor xenograft model showed that the (111)In-labeled anti-tomoregulin antibody 2H8 specifically recognizes tomoregulin protein in vivo, leading to a strong tumor-specific accumulation of the antibody. In efficacy studies, a single i.p. dose of 150 microCi (163 microg) (90)Y-labeled 2H8 substantially inhibits the growth rate of established LNCaP human prostate tumor xenograft in nude mice but produces no overt toxicity despite cross-reactivity of 2H8 with mouse tomoregulin. Our data clearly validate tomoregulin as a target for radioimmunotherapy of prostate cancer.