Murine endoglin-specific single-chain Fv fragments for the analysis of vascular targeting strategies in mice

Targeting the Tumor Microenvironment with Fluorescence-Activatable Bispecific Endoglin/Fibroblast Activation Protein Targeting Liposomes

Liposomes are biocompatible nanocarriers with promising features for targeted delivery of contrast agents and drugs into the tumor microenvironment, for imaging and therapy purposes. Liposome-based simultaneous targeting of tumor associated fibroblast and the vasculature is promising, but the heterogeneity of tumors entails a thorough validation of suitable markers for targeted delivery. Thus, we elucidated the potential of bispecific liposomes targeting the fibroblast activation protein (FAP) on tumor stromal fibroblasts, together with endoglin which is overexpressed on tumor neovascular cells and some neoplastic cells. Fluorescence-quenched liposomes were prepared by hydrating a lipid film with a high concentration of the self-quenching near-infrared fluorescent dye, DY-676-COOH, to enable fluorescence detection exclusively upon liposomal degradation and subsequent activation. A non-quenched green fluorescent phospholipid was embedded in the liposomal surface to fluorescence-track intact liposomes. FAP- and murine endoglin-specific single chain antibody fragments were coupled to the liposomal surface, and the liposomal potentials validated in tumor cells and mice models. The bispecific liposomes revealed strong fluorescence quenching, activatability, and selectivity for target cells and delivered the encapsulated dye selectively into tumor vessels and tumor associated fibroblasts in xenografted mice models and enabled their fluorescence imaging. Furthermore, detection of swollen lymph nodes during intra-operative simulations was possible. Thus, the bispecific liposomes have potentials for targeted delivery into the tumor microenvironment and for image-guided surgery.

Improved monovalent TNF receptor 1-selective inhibitor with novel heterodimerizing Fc

The development of alternative therapeutic strategies to tumor necrosis factor (TNF)-blocking antibodies for the treatment of inflammatory diseases has generated increasing interest. In particular, selective inhibition of TNF receptor 1 (TNFR1) promises a more precise intervention, tackling only the pro-inflammatory responses mediated by TNF while leaving regenerative and pro-survival signals transduced by TNFR2 untouched. We recently generated a monovalent anti-TNFR1 antibody fragment (Fab 13.7) as an efficient inhibitor of TNFR1. To improve the pharmacokinetic properties of Fab 13.7, the variable domains of the heavy and light chains were fused to the N-termini of newly generated heterodimerizing Fc chains. This novel Fc heterodimerization technology, designated "Fc-one/kappa" (Fc1κ) is based on interspersed constant Ig domains substituting the CH3 domains of a γ1 Fc. The interspersed immunoglobulin (Ig) domains originate from the per se heterodimerizing constant CH1 and CLκ domains and contain sequence stretches of an IgG1 CH3 domain, destined to enable interaction with the neonatal Fc receptor, and thus promote extended serum half-life. The resulting monovalent Fv-Fc1κ fusion protein (Atrosimab) retained strong binding to TNFR1 as determined by enzyme-linked immunosorbent assay and quartz crystal microbalance, and potently inhibited TNF-induced activation of TNFR1. Atrosimab lacks agonistic activity for TNFR1 on its own and in the presence of anti-human IgG antibodies and displays clearly improved pharmacokinetic properties.

Endoglin based in vivo near-infrared fluorescence imaging of tumor models in mice using activatable liposomes

BACKGROUND

Endoglin (CD105) is overexpressed on tumor cells and tumor vasculatures, making it a potential target for diagnostic imaging and therapy of different neoplasms. Therefore, studies on nanocarrier systems designed for endoglin-directed diagnostic and drug delivery purposes would expose the feasibility of targeting endoglin with therapeutics.

METHODS

Liposomes carrying high concentrations of a near-infrared fluorescent dye in the aqueous interior were prepared by the lipid film hydration and extrusion procedure, then conjugated to single chain antibody fragments either selective for murine endoglin (termed mEnd-IL) or directed towards human endoglin (termed hEnd-IL). A combination of Dynamic Light Scattering, electron microscopy, cell binding and uptake assays, confocal microscopy and in vivo fluorescence imaging of mice bearing xenografted human breast cancer and human fibrosarcoma models were implemented to elucidate the potentials of the liposomes.

RESULTS

The mEnd-IL and hEnd-IL were highly selective for the respective murine- and human endoglin expressing cells in vitro and in vivo. Hence, the hEnd-IL bound distinctly to the tumor cells and enabled suitable fluorescence imaging of the tumors, whereas the mEnd-IL bound the tumor vasculature, but also to the liver, kidney and lung vasculature of mice.

CONCLUSIONS

The work highlights key differences between targeting vascular (murine) and neoplastic (human) endoglin in animal studies, and suggests that the hEnd-IL can serve as a delivery system that targets human endoglin overexpressed in pathological conditions.

GENERAL SIGNIFICANCE

The endoglin-targeting liposomes presented herewith represent strategic tools for the future implementation of endoglin-directed neoplastic and anti-angiogenic therapies.

Inhibition of HER3 activation and tumor growth with a human antibody binding to a conserved epitope formed by domain III and IV

Human epidermal growth factor receptor 3 (HER3, also known as ErbB3) has emerged as relevant target for antibody-mediated tumor therapy. Here, we describe a novel human antibody, IgG 3-43, recognizing a unique epitope formed by domain III and parts of domain IV of the extracellular region of HER3, conserved between HER3 and mouse ErbB3. An affinity of 11 nM was determined for the monovalent interaction. In the IgG format, the antibody bound recombinant bivalent HER3 with subnanomolar affinity (K_D = 220 pM) and HER3-expressing tumor cells with EC₅₀ values in the low picomolar range (27 - 83 pM). The antibody competed with binding of heregulin to HER3-expressing cells, efficiently inhibited phosphorylation of HER3 as well as downstream signaling, and induced receptor internalization and degradation. Furthermore, IgG 3-43 inhibited heregulin-dependent proliferation of several HER3-positive cancer cell lines and heregulin-independent colony formation of HER2-overexpressing tumor cell lines. Importantly, inhibition of tumor growth and prolonged survival was demonstrated in a FaDu xenograft tumor model in SCID mice. These findings demonstrate that by binding to the membrane-proximal domains III and IV involved in ligand binding and receptor dimerization, IgG 3-43 efficiently inhibits activation of HER3, thereby blocking tumor cell growth both in vitro and in vivo.

Evaluating combinations of costimulatory antibody-ligand fusion proteins for targeted cancer immunotherapy

Combinatory strategies are becoming of increasing interest in cancer immunotherapy. Costimulation by individual members of the immunoglobulin-like (Ig)- and TNF superfamily have already shown promising antitumor potential, thus prompting the exploration of their synergistic abilities in combinatorial approaches. Here, we pursued a targeted strategy with antibody-fusion proteins composed of a tumor-directed antibody and the extracellular domain of the costimulatory ligand B7.1, 4-1BBL, OX40L, GITRL or LIGHT, respectively. Costimulatory activity was assessed in an experimental setting where initial T cell activation was induced by a bispecific antibody (tumor-related antigen × CD3). Advantage of combined targeted costimulation was shown for either B7.1 or 4-1BBL with OX40L, GITRL, LIGHT and 4-1BBL in terms of T cell proliferation and IFN-γ release. Since encouraging results were obtained by the combination of B7.1 and 4-1BBL, we adapted the model system for a time-shift setting. Here, enhanced proliferation and granzyme B expression as well as reduced PD-1 expression on the T cell population demonstrated the benefit of costimulation-assisted restimulation. Finally, the antitumor potential of this combinatorial setting was confirmed in vivo in a lung metastasis mouse model. Thus, combinatorial approaches with costimulatory antibody-ligand fusion proteins seem a promising strategy to be further investigated for cancer immunotherapy.

Specific gene delivery to liver sinusoidal and artery endothelial cells

Different types of endothelial cells (EC) fulfill distinct tasks depending on their microenvironment. ECs are therefore difficult to genetically manipulate ex vivo for functional studies or gene therapy. We assessed lentiviral vectors (LVs) targeted to the EC surface marker CD105 for in vivo gene delivery. The mouse CD105-specific vector, mCD105-LV, transduced only CD105-positive cells in primary liver cell cultures. Upon systemic injection, strong reporter gene expression was detected in liver where mCD105-LV specifically transduced liver sinusoidal ECs (LSECs) but not Kupffer cells, which were mainly transduced by nontargeted LVs. Tumor ECs were specifically targeted upon intratumoral vector injection. Delivery of the erythropoietin gene with mCD105-LV resulted in substantially increased erythropoietin and hematocrit levels. The human CD105-specific vector (huCD105-LV) transduced exclusively human LSECs in mice transplanted with human liver ECs. Interestingly, when applied at higher dose and in absence of target cells in the liver, huCD105-LV transduced ECs of a human artery transplanted into the descending mouse aorta. The data demonstrate for the first time targeted gene delivery to specialized ECs upon systemic vector administration. This strategy offers novel options to better understand the physiological functions of ECs and to treat genetic diseases such as those affecting blood factors.

The IgM CH2 domain as covalently linked homodimerization module for the generation of fusion proteins with dual specificity

Dimeric assembly of antibody fragments and other therapeutic molecules can result in increased binding and improved bioactivity. Here, we investigated the use of the IgM heavy chain domain 2 (MHD2) as covalently linked homodimerization module. Fusion of single-chain fragment variable (scFv) molecules directed against epidermal growth factor receptor (EGFR) and human epidermal growth factor receptor 2 to the N- and/or C-terminus of the MHD2, respectively, resulted in molecules with single or dual specificity for tumor cells. Bispecific tetravalent molecules were further generated by fusing a bispecific single-chain diabody directed against EGFR and epithelial cell adhesion molecule to the N-terminus of the MHD2. By combining an anti-EGFR scFv with a single-chain derivative of tumor necrosis factor, a tetravalent bifunctional fusion protein was produced. This fusion protein exhibited improved TNF activity, also mimicking the membrane-bound form of TNF, as shown by the activation of TNFR2-mediated cell killing. Furthermore, the scFv moiety allowed for an antigen-dependent delivery of TNF to EGFR-positive cells and an improved stimulatory TNF action on these cells. Thus, we established the MHD2 as a versatile module for the generation of bispecific and bifunctional fusion proteins.

Positron emission tomography imaging of CD105 expression during tumor angiogenesis

PURPOSE

Overexpression of CD105 (endoglin) correlates with poor prognosis in many solid tumor types. Tumor microvessel density (MVD) assessed by CD105 staining is the current gold standard for evaluating tumor angiogenesis in the clinic. The goal of this study was to develop a positron emission tomography (PET) tracer for imaging CD105 expression.

METHODS

TRC105, a chimeric anti-CD105 monoclonal antibody, was conjugated to 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) and labeled with (64)Cu. FACS analysis and microscopy studies were performed to compare the CD105 binding affinity of TRC105 and DOTA-TRC105. PET imaging, biodistribution, blocking, and ex vivo histology studies were performed on 4T1 murine breast tumor-bearing mice to evaluate the ability of (64)Cu-DOTA-TRC105 to target tumor angiogenesis. Another chimeric antibody, cetuximab, was used as an isotype-matched control.

RESULTS

FACS analysis of human umbilical vein endothelial cells (HUVECs) revealed no difference in CD105 binding affinity between TRC105 and DOTA-TRC105, which was further validated by fluorescence microscopy. (64)Cu labeling was achieved with high yield and specific activity. Serial PET imaging revealed that the 4T1 tumor uptake of the tracer was 8.0 ± 0.5, 10.4 ± 2.8, and 9.7 ± 1.8%ID/g at 4, 24, and 48 h post-injection, respectively (n = 3), higher than most organs at late time points which provided excellent tumor contrast. Biodistribution data as measured by gamma counting were consistent with the PET findings. Blocking experiments, control studies with (64)Cu-DOTA-cetuximab, as well as ex vivo histology all confirmed the in vivo target specificity of (64)Cu-DOTA-TRC105.

CONCLUSION

This is the first successful PET imaging study of CD105 expression. Fast, prominent, persistent, and CD105-specific uptake of the tracer in the 4T1 tumor was observed. Further studies are warranted and currently underway.

ATROSAB, a humanized antagonistic anti-tumor necrosis factor receptor one-specific antibody

Tumor necrosis factor (TNF) signals through two membrane receptors, TNFR1 and TNFR2, and TNFR1 is known to be the major pathogenic mediator of chronic and acute inflammatory diseases. Present clinical intervention is based on neutralization of the ligand TNF. Selective inhibition of TNF receptor 1 (TNFR1) provides an alternative opportunity to neutralize the pro-inflammatory activity of TNF while maintaining the advantageous immunological responses mediated by TNFR2, including immune regulation, tissue homeostasis and neuroprotection. We recently humanized a mouse anti-human TNFR1 monoclonal antibody exhibiting TNFR1-neutralizing activity. This humanized antibody has been converted into an IgG1 molecule (ATROSAB) containing a modified Fc region previously demonstrated to have greatly reduced effector functions. Purified ATROSAB, produced in CHO cells, showed strong binding to human and rhesus TNFR1-Fc fusion protein and mouse embryonic fibroblasts transfected with a recombinant TNFR1 fusion protein with an affinity identical to the parental mouse antibody H398. Using chimeric human/mouse TNFR1 molecules, the epitope of ATROSAB was mapped to the N-terminal region (amino acid residues 1-70) comprising the first cysteine-rich domain (CRD1) and the A1 sub-domain of CRD2. In vitro, ATROSAB inhibited typical TNF-mediated responses like apoptosis induction and activation of NFκB-dependent gene expression such as IL-6 and IL-8 production. These findings open the way to further analyze the therapeutic activity of ATROSAB in relevant disease models in non-human primates.

The effects of affinity and valency of an albumin-binding domain (ABD) on the half-life of a single-chain diabody-ABD fusion protein

Fusion of small recombinant antibody fragments to an albumin-binding domain (ABD) from streptococcal protein G strongly extends their plasma half-life. This ABD binds with nanomolar affinity to human (HSA) and mouse serum albumin (MSA). It was speculated that an increase in albumin-binding affinity should lead to a further increase in half-life. In the present study, we analyzed the effects of affinity and valency of the ABD on the pharmacokinetic properties of a bispecific single-chain diabody (scDb), applied previously to investigate various half-life extension strategies. The scDb is directed against carcinoembryonic antigen (CEA) and CD3 capable of mediating T cell retargeting to tumor cells. Two scDb derivatives with increased (scDb-ABD-H) and decreased (scDb-ABD-L) affinity as well as an scDb molecule fused to two ABD (scDb-ABD(2)) were generated and produced in mammalian cells. The altered binding of these constructs to HSA and MSA was confirmed by ELISA and quartz crystal microbalance measurements. All constructs bound efficiently to CEA and CD3-positive cells and were able to activate T cells in a target cell-dependent manner, although T cell activation was reduced in the presence of serum albumin. All three derivatives showed a strongly increased half-life in mice as compared with scDb. Compared with the wild-type scDb-ABD, the half-life of scDb-ABD-H exhibited a prolonged half-life and scDb-ABD-L a reduced half-life, while the half-life scDb-ABD(2) was almost identical to that of scDb-ABD. However, these changes were only moderate, indicating that the half-life-extending property of the ABD in mice is only weakly influenced by affinity for serum albumin or valency of albumin binding.

Targeted delivery of SiRNA to CD33-positive tumor cells with liposomal carrier systems

SiRNA molecules represent promising therapeutic molecules, e.g. for cancer therapy. However, efficient delivery into tumor cells remains a major obstacle for treatment. Here, we describe a liposomal siRNA carrier system for targeted delivery of siRNA to CD33-positive acute myeloid leukemia cells. The siRNA is directed against the t(8;21) translocation resulting in the AML1/MTG8 fusion protein. The siRNA was encapsulated in free or polyethylene imine (PEI)-complexed form into PEGylated liposomes endowed subsequently with an anti-CD33 single-chain Fv fragment (scFv) for targeted delivery. The resulting siRNA-loaded immunoliposomes (IL) and immunolipoplexes (ILP) showed specific binding and internalization by CD33-expressing myeloid leukemia cell lines (SKNO-1, Kasumi-1). Targeted delivery of AML1/MTG8 siRNA, but not of mismatch control siRNA, reduced AML1/MTG8 mRNA and protein levels and decreased leukemic clonogenicity, a hallmark of leukemic self-renewal. Although this study revealed that further modifications are necessary to increase efficacy of siRNA delivery and silencing, we were able to establish a targeted liposomal siRNA delivery system combining recombinant antibody fragments for targeted delivery with tumor cell-specific siRNA molecules as therapeutic agents.