A novel fusion toxin derived from an EpCAM-specific designed ankyrin repeat protein has potent antitumor activity

Preclinical evaluation of EpCAM-binding designed ankyrin repeat proteins (DARPins) as targeting moieties for bimodal near-infrared fluorescence and photoacoustic imaging of cancer

PURPOSE

Fluorescence-guided surgery (FGS) can play a key role in improving radical resection rates by assisting surgeons to gain adequate visualization of malignant tissue intraoperatively. Designed ankyrin repeat proteins (DARPins) possess optimal pharmacokinetic and other properties for in vivo imaging. This study aims to evaluate the preclinical potential of epithelial cell adhesion molecule (EpCAM)-binding DARPins as targeting moieties for near-infrared fluorescence (NIRF) and photoacoustic (PA) imaging of cancer.

METHODS

EpCAM-binding DARPins Ac2, Ec4.1, and non-binding control DARPin Off7 were conjugated to IRDye 800CW and their binding efficacy was evaluated on EpCAM-positive HT-29 and EpCAM-negative COLO-320 human colon cancer cell lines. Thereafter, NIRF and PA imaging of all three conjugates were performed in HT-29_luc2 tumor-bearing mice. At 24 h post-injection, tumors and organs were resected and tracer biodistributions were analyzed.

RESULTS

Ac2-800CW and Ec4.1-800CW specifically bound to HT-29 cells, but not to COLO-320 cells. Next, 6 nmol and 24 h were established as the optimal in vivo dose and imaging time point for both DARPin tracers. At 24 h post-injection, mean tumor-to-background ratios of 2.60 ± 0.3 and 3.1 ± 0.3 were observed for Ac2-800CW and Ec4.1-800CW, respectively, allowing clear tumor delineation using the clinical Artemis NIRF imager. Biodistribution analyses in non-neoplastic tissue solely showed high fluorescence signal in the liver and kidney, which reflects the clearance of the DARPin tracers.

CONCLUSION

Our encouraging results show that EpCAM-binding DARPins are a promising class of targeting moieties for pan-carcinoma targeting, providing clear tumor delineation at 24 h post-injection. The work described provides the preclinical foundation for DARPin-based bimodal NIRF/PA imaging of cancer.

What is the role of microbial biotechnology and genetic engineering in medicine?

Microbial products are essential for developing various therapeutic agents, including antibiotics, anticancer drugs, vaccines, and therapeutic enzymes. Genetic engineering techniques, functional genomics, and synthetic biology unlock previously uncharacterized natural products. This review highlights major advances in microbial biotechnology, focusing on gene-based technologies for medical applications.

Epithelial cell adhesion molecule‑targeting designed ankyrin repeat protein‑toxin fusion Ec1‑LoPE exhibits potent cytotoxic action in prostate cancer cells

Targeted anticancer therapeutics offer the advantage of reducing cytotoxic side effects to normal cells by directing the cytotoxic payload selectively to cancer cells. Designed ankyrin repeat proteins (DARPins) are promising non-immunoglobulin-based scaffold proteins for payload delivery to cancer-associated molecular targets. Epithelial cell adhesion molecule (EpCAM) is overexpressed in 40–60% of prostate cancers (PCs) and is associated with metastasis, increased risk of PC recurrence and resistance to treatment. Here, we investigated the use of DARPin Ec1 for targeted delivery of Pseudomonas exotoxin A variant (LoPE) with low immunogenicity and low non-specific toxicity to EpCAM-expressing prostate cancer cells. Ec1-LoPE fusion protein was radiolabeled with tricarbonyl technetium-99m and its binding specificity, binding kinetics, cellular processing, internalization and cytotoxicity were evaluated in PC-3 and DU145 cell lines. Ec1-LoPE showed EpCAM-specific binding to EpCAM-expressing prostate cancer cells. Rapid internalization mediated potent cytotoxic effect with picomolar IC₅₀ values in both studied cell lines. Taken together, these data support further evaluation of Ec1-LoPE in a therapeutic setting in a prostate cancer model in vivo.

Bivalent EGFR-Targeting DARPin-MMAE Conjugates

Epidermal growth factor receptor (EGFR) is a validated tumor marker overexpressed in various cancers such as squamous cell carcinoma (SSC) of the head and neck and gliomas. We constructed protein-drug conjugates based on the anti-EGFR Designed Ankyrin Repeat Protein (DARPin) E01, and compared the bivalent DARPin dimer (DD1) and a DARPin-Fc (DFc) to the monomeric DARPin (DM) and the antibody derived scFv425-Fc (scFvFc) in cell culture and a mouse model. The modular conjugation system, which was successfully applied for the preparation of protein-drug and -dye conjugates, uses bio-orthogonal protein-aldehyde generation by the formylglycine-generating enzyme (FGE). The generated carbonyl moiety is addressed by a bifunctional linker with a pyrazolone for a tandem Knoevenagel reaction and an azide for strain-promoted azide-alkyne cycloaddition (SPAAC). The latter reaction with a PEGylated linker containing a dibenzocyclooctyne (DBCO) for SPAAC and monomethyl auristatin E (MMAE) as the toxin provided the stable conjugates DD1-MMAE (drug-antibody ratio, DAR = 2.0) and DFc-MMAE (DAR = 4.0) with sub-nanomolar cytotoxicity against the human squamous carcinoma derived A431 cells. In vivo imaging of Alexa Fluor 647-dye conjugates in A431-xenografted mice bearing subcutaneous tumors as the SCC model revealed unspecific binding of bivalent DARPins to the ubiquitously expressed EGFR. Tumor-targeting was verified 6 h post-injection solely for DD1 and scFvFc. The total of four administrations of 6.5 mg/kg DD1-MMAE or DFc-MMAE twice weekly did not cause any sequela in mice. MMAE conjugates showed no significant anti-tumor efficacy in vivo, but a trend towards increased necrotic areas (p = 0.2213) was observed for the DD1-MMAE (n = 5).

Emerging new therapeutic antibody derivatives for cancer treatment

Monoclonal antibodies constitute a promising class of targeted anticancer agents that enhance natural immune system functions to suppress cancer cell activity and eliminate cancer cells. The successful application of IgG monoclonal antibodies has inspired the development of various types of therapeutic antibodies, such as antibody fragments, bispecific antibodies, and antibody derivatives (e.g., antibody-drug conjugates and immunocytokines). The miniaturization and multifunctionalization of antibodies are flexible and viable strategies for diagnosing or treating malignant tumors in a complex tumor environment. In this review, we summarize antibodies of various molecular types, antibody applications in cancer therapy, and details of clinical study advances. We also discuss the rationale and mechanism of action of various antibody formats, including antibody-drug conjugates, antibody-oligonucleotide conjugates, bispecific/multispecific antibodies, immunocytokines, antibody fragments, and scaffold proteins. With advances in modern biotechnology, well-designed novel antibodies are finally paving the way for successful treatments of various cancers, including precise tumor immunotherapy, in the clinic.

Assessing the use of tumor-specific DARPin-toxin fusion proteins for ex vivo purging of cancer metastases from human ovarian cortex before autotransplantation

OBJECTIVE

To assess the use of tumor-specific designed ankyrin repeat proteins (DARPins) fused to a domain of Pseudomonas aeruginosa exotoxin A for purging of cancer metastases from the ovarian cortex.

DESIGN

Experimental study.

SETTING

University medical center.

PATIENT(S)

Human ovarian cortex.

INTERVENTION(S)

Ovarian cortex harboring artificially induced breast cancer metastases was treated with DARPins targeted to epithelial cell adhesion molecule (EpCAM) and human epidermal growth factor receptor 2 (HER2).

MAIN OUTCOME MEASURE(S)

The presence of any remaining cancer cells after purging was analyzed by (immuno)histochemistry and reverse transcriptase polymerase chain reaction. Effects on the viability of the ovarian cortex were determined by (immuno)histology, a follicular viability assay, and an assay to determine the in vitro growth capacity of small follicles.

RESULT(S)

After purging with EpCAM-targeted DARPin, all EpCAM-positive breast cancer cells were eradicated from the ovarian cortex. Although treatment had no effect on the morphology or viability of small follicles, a sharp decrease in oocyte viability during in vitro growth was observed, presumably due to low-level expression of EpCAM on oocytes. The HER2-targeted DARPins had no detrimental effects on the morphology, viability, or in vitro growth of small follicles. HER2-positive breast cancer foci were fully eliminated from the ovarian cortex, and the reverse transcriptase polymerase chain reaction showed a decrease to basal levels of HER2 mRNA after purging.

CONCLUSION(S)

Purging cancer metastases from ovarian cortex without impairing ovarian tissue integrity is possible by targeting tumor cell surface proteins with exotoxin A-fused DARPins. By adapting the target specificity of the cytotoxic DARPin fusions, it should be possible to eradicate metastases from all types of malignancies.

DARPin_9-29-Targeted Gold Nanorods Selectively Suppress HER2-Positive Tumor Growth in Mice

Simple Summary

Breast cancer is one of the main causes of cancer-related death in women all around the world. The disease becomes largely incurable and lethal after metastasis to distant organs. High level of HER2, a tyrosine kinase receptor, is associated with more aggressive clinical behavior and poor prognosis for breast cancer patients. In this paper, we developed a novel nano-biomaterial for selective photothermal therapy of HER2-positive breast cancers. We demonstrate that bovine serum albumin (BSA)-coated mini gold nanorods (GNRs) chemically conjugated with a HER2-specific designed ankyrin repeat protein, DARPin_9-29, selectively accumulate in HER2-positive xenograft tumors in mice and lead to a strong reduction in the tumor size when being illuminated with near-infrared light.

Abstract

Near-infrared phototherapy has great therapeutic potential for cancer treatment. However, for efficient application, in vivo photothermal agents should demonstrate excellent stability in blood and targeted delivery to pathological tissue. Here, we demonstrated that stable bovine serum albumin-coated gold mini nanorods conjugated to a HER2-specific designed ankyrin repeat protein, DARPin_9-29, selectively accumulate in HER2-positive xenograft tumors in mice and lead to a strong reduction in the tumor size when being illuminated with near-infrared light. The results pave the way for the development of novel DARPin-based targeted photothermal therapy of cancer.

Immunotoxins and nanobody-based immunotoxins: review and update

Immunotoxins (ITs) are protein-based drugs that compose of targeting and cytotoxic moieties. After binding the IT to the specific cell-surface antigen, the IT internalises into the target cell and kills it. Targeting and cytotoxic moieties usually include monoclonal antibodies and protein toxins with bacterial or plant origin, respectively. ITs have been successful in haematologic malignancies treatment. However, ITs penetrate poorly into solid tumours because of their large size. Use of camelid antibody fragments known as nanobodies (Nbs) as a targeting moiety may overcome this problem. Nbs are the smallest fragment of antibodies with excellent tumour tissue penetration. The ability to recognise cryptic (immuno-evasive) target antigens, low immunogenicity, and high-affinity are other fundamental characteristics of Nbs that make them suitable candidates in targeted therapy. Here, we reviewed and discussed the structure and function of ITs, Nbs, and nanobody-based ITs. To gain sound insight into the issue at hand, we focussed on nanobody-based ITs.

Engineering of an EpCAM-targeting cyclic peptide to improve the EpCAM-mediated cellular internalization and tumor accumulation of a peptide-fused antibody

Fusion of a target-specific peptide to a full-length antibody (Ab) can result in a peptide-Ab fusion protein with additional specificity and enhanced activity. We recently developed an intracellular pan-RAS-targeting cytosol-penetrating antibody, RT22-ep59, in which a tumor-specific targeting ability was achieved via the fusion of an epithelial cell adhesion molecule (EpCAM) targeting cyclic peptide (ep133). Here, the aim was to enhance EpCAM-mediated endocytosis and tumor accumulation of the peptide-fused RAS-targeting Ab. Accordingly, we engineered a cyclic peptide (from ep133) that has stronger affinity for EpCAM by using yeast surface display technology and then rationally designed cyclic peptides in the Ab-fused form to enhance colloidal stability. The finally engineered EpCAM-targeting cyclic peptide (ep₆)-fused Ab, ep₆Ras37, has ∼10-fold stronger affinity (K_D ≈ 1.9 nM) for EpCAM than that of RT22-ep59, without deterioration of biophysical properties. Compared with the parental antibody (RT22-ep59), ep₆Ras37 more efficiently reached the cytosol of EpCAM-expressing cells and showed greater preferential tumor homing and accumulation in mice bearing EpCAM-expressing LoVo xenograft tumors. Thus, the high-affinity EpCAM-targeting peptide ensures efficient cellular internalization and better tumor accumulation of the peptide-fused Ab.

Imaging-Guided Therapy Simultaneously Targeting HER2 and EpCAM with Trastuzumab and EpCAM-Directed Toxin Provides Additive Effect in Ovarian Cancer Model

Efficient treatment of disseminated ovarian cancer (OC) is challenging due to its heterogeneity and chemoresistance. Overexpression of human epidermal growth factor receptor 2 (HER2) and epithelial cell adhesion molecule (EpCAM) in approx. 30% and 70% of ovarian cancers, respectively, allows for co-targeted treatment. The clinical efficacy of the monoclonal antibody trastuzumab in patients with HER2-positive breast, gastric and gastroesophageal cancers makes it readily available as the HER2-targeting component. As the EpCAM-targeting component, we investigated the designed ankyrin repeat protein (DARPin) Ec1 fused to a truncated variant of Pseudomonas exotoxin A with reduced immunogenicity and low general toxicity (LoPE). Ec1-LoPE was radiolabeled, evaluated in ovarian cancer cells in vitro and its biodistribution and tumor-targeting properties were studied in vivo. The therapeutic efficacy of Ec1-LoPE alone and in combination with trastuzumab was studied in mice bearing EpCAM- and HER2-expressing SKOV3 xenografts. SPECT/CT imaging enabled visualization of EpCAM and HER2 expression in the tumors. Co-treatment using Ec1-LoPE and trastuzumab was more effective at reducing tumor growth and prolonged the median survival of mice compared with mice in the control and monotherapy groups. Repeated administration of Ec1-LoPE was well tolerated without signs of hepatic or kidney toxicity. Co-treatment with trastuzumab and Ec1-LoPE might be a potential therapeutic strategy for HER2- and EpCAM-positive OC.

Influence of the Position and Composition of Radiometals and Radioiodine Labels on Imaging of Epcam Expression in Prostate Cancer Model Using the DARPin Ec1

Simple Summary

Metastasis-targeting therapy might improve outcomes in oligometastatic prostate cancer. Epithelial cell adhesion molecule (EpCAM) is overexpressed in 40–60% of prostate cancer cases and might be used as a target for specific delivery of toxins and drugs. Radionuclide molecular imaging could enable non-invasive detection of EpCAM and stratification of patients for targeted therapy. Designed ankyrin repeat proteins (DARPins) are scaffold proteins, which can be selected for specific binding to different targets. The DARPin Ec1 binds strongly to EpCAM. To determine an optimal design of Ec1-based probes, we labeled Ec1 at two different positions with four different nuclides (⁶⁸Ga, ¹¹¹In, ⁵⁷Co and ¹²⁵I) and investigated the impact on Ec1 biodistribution. We found that the C-terminus is the best position for labeling and that ¹¹¹In and ¹²⁵I provide the best imaging contrast. This study might be helpful for scientists developing imaging probes based on scaffold proteins.

Abstract

The epithelial cell adhesion molecule (EpCAM) is intensively overexpressed in 40–60% of prostate cancer (PCa) cases and can be used as a target for the delivery of drugs and toxins. The designed ankyrin repeat protein (DARPin) Ec1 has a high affinity to EpCAM (68 pM) and a small size (18 kDa). Radiolabeled Ec1 might be used as a companion diagnostic for the selection of PCa patients for therapy. The study aimed to investigate the influence of radiolabel position (N- or C-terminal) and composition on the targeting and imaging properties of Ec1. Two variants, having an N- or C-terminal cysteine, were produced, site-specifically conjugated to a DOTA chelator and labeled with cobalt-57, gallium-68 or indium-111. Site-specific radioiodination was performed using ((4-hydroxyphenyl)-ethyl)maleimide (HPEM). Biodistribution of eight radiolabeled Ec1-probes was measured in nude mice bearing PCa DU145 xenografts. In all cases, positioning of a label at the C-terminus provided the best tumor-to-organ ratios. The non-residualizing [¹²⁵I]I-HPEM label provided the highest tumor-to-muscle and tumor-to-bone ratios and is more suitable for EpCAM imaging in early-stage PCa. Among the radiometals, indium-111 provided the highest tumor-to-blood, tumor-to-lung and tumor-to-liver ratios and could be used at late-stage PCa. In conclusion, label position and composition are important for the DARPin Ec1.

A Hybrid In Silico and Tumor-on-a-Chip Approach to Model Targeted Protein Behavior in 3D Microenvironments

Simple Summary

Engineered proteins possess a great therapeutic potential, but the development of such therapies is impeded during preclinical studies by the lack of in vitro models that accurately simulate the human physiology. Animal models, on the other hand, also have difficulties predicting human responses, and are ethically concerning. In this study, we employed a hybrid approach where we combined mathematical modeling with 3D in vitro models that mimic aspects of the tumor microenvironment, in order to simulate the delivery of therapeutic proteins targeting cancer cells and to predict the biological activity. By cross-comparing simulated and experimental data from 3D models, we were able to correctly predict the best dose needed to deliver toxic proteins specifically to tumor cells, while leaving the surrounding non-tumor cells untouched. This study shows the potential of combining computational approaches with novel in vitro models to advance the development of protein therapeutics.

Abstract

To rationally improve targeted drug delivery to tumor cells, new methods combining in silico and physiologically relevant in vitro models are needed. This study combines mathematical modeling with 3D in vitro co-culture models to study the delivery of engineered proteins, called designed ankyrin repeat proteins (DARPins), in biomimetic tumor microenvironments containing fibroblasts and tumor cells overexpressing epithelial cell adhesion molecule (EpCAM) or human epithelial growth factor receptor (HER2). In multicellular tumor spheroids, we observed strong binding-site barriers in combination with low apparent diffusion coefficients of 1 µm²·s⁻¹ and 2 µm² ·s⁻¹ for EpCAM- and HER2-binding DARPin, respectively. Contrasting this, in a tumor-on-a-chip model for investigating delivery in real-time, transport was characterized by hindered diffusion as a consequence of the lower local tumor cell density. Finally, simulations of the diffusion of an EpCAM-targeting DARPin fused to a fragment of Pseudomonas aeruginosa exotoxin A, which specifically kills tumor cells while leaving fibroblasts untouched, correctly predicted the need for concentrations of 10 nM or higher for extensive tumor cell killing on-chip, whereas in 2D models picomolar concentrations were sufficient. These results illustrate the power of combining in vitro models with mathematical modeling to study and predict the protein activity in complex 3D models.

Neovascular Age-Related Macular Degeneration: Therapeutic Management and New-Upcoming Approaches

Age-related macular degeneration (AMD) constitutes a prevalent, chronic, and progressive retinal degenerative disease of the macula that affects elderly people and cause central vision impairment. Despite therapeutic advances in the management of neovascular AMD, none of the currently used treatments cures the disease or reverses its course. Medical treatment of neovascular AMD experienced a significant advance due to the introduction of vascular endothelial growth factor inhibitors (anti-VEGF), which dramatically changed the prognosis of the disease. However, although anti-VEGF therapy has become the standard treatment for neovascular AMD, many patients do not respond adequately to this therapy or experience a slow loss of efficacy of anti-VEGF agents after repeated administration. Additionally, current treatment with intravitreal anti-VEGF agents is associated with a significant treatment burden for patients, caregivers, and physicians. New approaches have been proposed for treating neovascular AMD. Among them, designed ankyrin repeat proteins (DARPins) seem to be as effective as monthly ranibizumab, but with greater durability, which may enhance patient compliance with needed injections.

Dual Targeting of Cancer Cells with DARPin-Based Toxins for Overcoming Tumor Escape

Simple Summary

Targeted therapy of solid tumors represents a great challenge because of heterogeneity of tumor-associated antigen expression. To overcome this obstacle we propose a dual targeting therapy based on protein preparations capable of recognizing different of tumor-associated antigens on a tumor cell producing a directed cytotoxic effect. The dual specific therapy of breast carcinoma-bearing mice using the designed preparations eliminates both the primary tumor and distant metastases. The mono-targeting therapy aimed at single tumor-associated antigen did not suppress metastases at all. The proposed approach can serve as a potential therapeutic strategy that surpasses mono-specific targeting strategies in the anti-cancer efficacy.

Abstract

We report here a combined anti-cancer therapy directed toward HER2 and EpCAM, common tumor-associated antigens of breast cancer cells. The combined therapeutic effect is achieved owing to two highly toxic proteins—a low immunogenic variant of Pseudomonas aeruginosa exotoxin A and ribonuclease Barnase from Bacillus amyloliquefaciens. The delivery of toxins to cancer cells was carried out by targeting designed ankyrin repeat proteins (DARPins). We have shown that both target agents efficiently accumulate in the tumor. Simultaneous treatment of breast carcinoma-bearing mice with anti-EpCAM fusion toxin based on LoPE and HER2-specific liposomes loaded with Barnase leads to concurrent elimination of primary tumor and metastases. Monotherapy with anti-HER2- or anti-EpCAM-toxins did not produce a comparable effect on metastases. The proposed approach can be considered as a promising strategy for significant improvement of cancer therapy.

Radionuclide Molecular Imaging of EpCAM Expression in Triple-Negative Breast Cancer Using the Scaffold Protein DARPin Ec1

Efficient treatment of disseminated triple-negative breast cancer (TNBC) remains an unmet clinical need. The epithelial cell adhesion molecule (EpCAM) is often overexpressed on the surface of TNBC cells, which makes EpCAM a potential therapeutic target. Radionuclide molecular imaging of EpCAM expression might permit selection of patients for EpCAM-targeting therapies. In this study, we evaluated a scaffold protein, designed ankyrin repeat protein (DARPin) Ec1, for imaging of EpCAM in TNBC. DARPin Ec1 was labeled with a non-residualizing [¹²⁵I]I-para-iodobenzoate (PIB) label and a residualizing [^99mTc]Tc(CO)₃ label. Both imaging probes retained high binding specificity and affinity to EpCAM-expressing MDA-MB-468 TNBC cells after labeling. Internalization studies showed that Ec1 was retained on the surface of MDA-MB-468 cells to a high degree up to 24 h. Biodistribution in Balb/c nu/nu mice bearing MDA-MB-468 xenografts demonstrated specific uptake of both [¹²⁵I]I-PIB-Ec1 and [^99mTc]Tc(CO)₃-Ec1 in TNBC tumors. [¹²⁵I]I-PIB-Ec1 had appreciably lower uptake in normal organs compared with [^99mTc]Tc(CO)₃-Ec1, which resulted in significantly (p < 0.05) higher tumor-to-organ ratios. The biodistribution data were confirmed by micro-Single-Photon Emission Computed Tomography/Computed Tomography (microSPECT/CT) imaging. In conclusion, an indirectly radioiodinated Ec1 is the preferable probe for imaging of EpCAM in TNBC.

Feasibility of Imaging EpCAM Expression in Ovarian Cancer Using Radiolabeled DARPin Ec1

Epithelial cell adhesion molecule (EpCAM) is overexpressed in 55%–75% of ovarian carcinomas (OC). EpCAM might be used as a target for a treatment of disseminated OC. Designed ankyrin repeats protein (DARPin) Ec1 is a small (18 kDa) protein, which binds to EpCAM with subnanomolar affinity. We tested a hypothesis that Ec1 labeled with a non-residualizing label might serve as a companion imaging diagnostic for stratification of patients for EpCAM-targeting therapy. Ec1 was labeled with ¹²⁵I using N-succinimidyl-para-iodobenzoate. Binding affinity, specificity, and cellular processing of [¹²⁵I]I-PIB-Ec1 were evaluated using SKOV-3 and OVCAR-3 ovarian carcinoma cell lines. Biodistribution and tumor-targeting properties of [¹²⁵I]I-PIB-Ec1 were studied in Balb/c nu/nu mice bearing SKOV-3 and OVCAR-3 xenografts. EpCAM-negative Ramos lymphoma xenografts served as specificity control. Binding of [¹²⁵I]I-PIB-Ec1 to ovarian carcinoma cell lines was highly specific and had affinity in picomolar range. Slow internalization of [¹²⁵I]I-PIB-Ec1 by OC cells confirmed utility of non-residualizing label for in vivo imaging. [¹²⁵I]I-PIB-Ec1 provided 6 h after injection tumor-to-blood ratios of 30 ± 11 and 48 ± 12 for OVCAR-3 and SKOV-3 xenografts, respectively, and high contrast to other organs. Tumor targeting was highly specific. Saturation of tumor uptake at a high dose of Ec1 in SKOV-3 model provided a rationale for dose selection in further studies using therapeutic conjugates of Ec1 for targeted therapy. In conclusion, [¹²⁵I]I-PIB-Ec1 is a promising agent for visualizing EpCAM expression in OC.

The Role of Interleukin-37 in the Pathogenesis of Allergic Diseases

Cytokines of the interleukin-1 (IL-1) family play an important role in the realization of the protective functions of innate immunity and are the key mediators involved in the pathogenesis of a wide range of diseases, including various manifestations of allergy. The IL-1 family includes more than 11 members. However, the functions of many of them remain to be elucidated. Recently, new members of the IL-1 family have been discovered. In 2000, several independent research groups reported the discovery of a new interleukin of this family, which was named IL-37, or IL-1F7 (according to the new nomenclature). IL-37 was assigned to the IL-1 family based on its structural similarity with other members of this family. The study of its biological properties showed that its activity changes in inflammatory diseases, such as rheumatoid arthritis, psoriasis, as well as allergic diseases (allergic rhinitis, bronchial asthma, and atopic dermatitis). However, unlike most members of the IL-1 family, IL-37 acts as a negative regulator of inflammation. Activation of IL-37 suppresses inflammation, resulting in the suppression of inflammatory cytokines and chemokines, which in turn prevents infiltration of pro-inflammatory cells, mainly eosinophils and neutrophils. The exact molecular and cellular mechanisms of the anti-inflammatory effect of IL-37 in the development of allergic diseases (AD) have not been fully studied. This review summarizes and analyzes the accumulated experimental data on the role of IL-37 in the pathogenesis of AD, such as allergic rhinitis, bronchial asthma, and atopic dermatitis.

DARPins: Promising Scaffolds for Theranostics

Monoclonal antibodies are the classical basis for targeted therapy, but the development of alternative binding proteins has made it possible to use non-immunoglobulin proteins as targeting modules. The advantages of DARPins, scaffold proteins based on ankyrin repeats, over antibodies are as follows: small size, stability over a wide range of temperatures and pH values, low aggregation tendency, and ease of production in heterologous expression systems. The differences in the structure of the paratope of DARPin and antibodies broaden the spectrum of target molecules, while the ease of creating hybrid fusion proteins allows one to obtain bispecific and multivalent constructs. In this article, we summarize recent data on the development of therapeutic and imaging compounds based on DARPins.

Potent and specific fusion toxins consisting of a HER2‑binding, ABD‑derived affinity protein, fused to truncated versions of Pseudomonas exotoxin A

Fusion toxins consisting of an affinity protein fused to toxic polypeptides derived from Pseudomonas exotoxin A (ETA) are promising agents for targeted cancer therapy. In this study, we examined whether fusion toxins consisting of an albumin binding domain‑derived affinity protein (ADAPT) interacting with human epidermal growth factor receptor 2 (HER2), coupled to the ETA‑derived polypeptides PE38X8 or PE25, with or without an albumin binding domain (ABD) for half‑life extension, can be used for specific killing of HER2‑expressing cells. The fusion toxins could easily be expressed in a soluble form in Escherichia coli and purified to homogeneity. All constructs had strong affinity for HER2 (KD 10 to 26 nM) and no tendency for aggregation could be detected. The fusion toxins including the ABD showed strong interaction with human and mouse serum albumin [equilibrium dissociation constant (KD) 1 to 3 nM and 2 to 10 nM, respectively]. The in vitro investigation of the cytotoxic potential revealed IC50‑values in the picomolar range for cells expressing high levels of HER2. The specificity was also demonstrated, by showing that free HER2 receptors on the target cells are required for fusion toxin activity. In mice, the fusion toxins containing the ABD exhibited an appreciably longer time in circulation. The uptake was highest in liver and kidney. Fusion with PE25 was associated with the highest hepatic uptake. Collectively, the results suggest that fusion toxins consisting of ADAPTs and ETA‑derivatives are promising agents for targeted cancer therapy.

Influence of size and charge of unstructured polypeptides on pharmacokinetics and biodistribution of targeted fusion proteins

Alternative non-IgG binding proteins developed for therapy are small in size and, thus, are rapidly cleared from the circulation by renal filtration. To avoid repeated injection or continuous infusion for the maintenance of therapeutic serum concentrations, extensions of unfolded polypeptides have been developed to prolong serum half-life, but systematic, comparative studies investigating the influence of their size and charge on serum half-life, extravasation, tumor localization and excretion mechanisms have so far been lacking. Here we used a high-affinity Designed Ankyrin Repeat Protein (DARPin) targeting the tumor marker epithelial cell adhesion molecule (EpCAM) in a preclinical tumor xenograft model in mice, and fused it with a series of defined unstructured polypeptides. We used three different sizes of two previously described polypeptides, an uncharged one consisting of only Pro, Ala and Ser (termed PAS) and a charged one consisting of Pro, Ala, Ser, Thr, Gly, Glu (termed XTEN) and performed for the first time a precise comparative localization, distribution and extravasation study. Pharmacokinetic analysis showed a clear linear relationship between hydrodynamic radius and serum half-life across both polypeptides, reaching a half-life of up to 21 h in mice. Tumor uptake was EpCAM-dependent and directly proportional to half-life and size, showing an even tumor penetration for all fusion proteins without unspecific accumulation in non-target tissue. Unexpectedly, charge had no influence on any parameter, neither tumor nor tissue accumulation nor kidney elimination kinetics. Thus, both polypeptide types have a very similar potential for precise half-life modification and tumor targeting.

HER2-Specific Targeted Toxin DARPin-LoPE: Immunogenicity and Antitumor Effect on Intraperitoneal Ovarian Cancer Xenograft Model

High immunogenicity and systemic toxicity are the main obstacles limiting the clinical use of the therapeutic agents based on Pseudomonas aeruginosa exotoxin A. In this work, we studied the immunogenicity, general toxicity and antitumor effect of the targeted toxin DARPin-LoPE composed of HER2-specific DARPin and a low immunogenic exotoxin A fragment lacking immunodominant human B lymphocyte epitopes. The targeted toxin has been shown to effectively inhibit the growth of HER2-positive human ovarian carcinoma xenografts, while exhibiting low non-specific toxicity and side effects, such as vascular leak syndrome and liver tissue degradation, as well as low immunogenicity, as was shown by specific antibody titer. This represents prospects for its use as an agent for targeted therapy of HER2-positive tumors.

Programed Assembly of Nucleoprotein Nanoparticles Using DNA and Zinc Fingers for Targeted Protein Delivery

With a growing number of intracellular drug targets and the high efficacy of protein therapeutics, the targeted delivery of active proteins with negligible toxicity is a challenging issue in the field of precision medicine. Herein, a programed assembly of nucleoprotein nanoparticles (NNPs) using DNA and zinc fingers (ZnFs) for targeted protein delivery is presented. Two types of ZnFs with different sequence specificities are genetically fused to a targeting moiety and a protein cargo, respectively. Double-stranded DNA with multiple ZnF-binding sequences is grafted onto inorganic nanoparticles, followed by conjugation with the ZnF-fused proteins, generating the assembly of NNPs with a uniform size distribution and high stability. The approach enables controlled loading of a protein cargo on the NNPs, offering a high cytosolic delivery efficiency and target specificity. The utility and potential of the assembly as a versatile protein delivery vehicle is demonstrated based on their remarkable antitumor activity and target specificity with negligible toxicity in a xenograft mice model.

Selective staining and eradication of cancer cells by protein-carrying DARPin-functionalized liposomes

Since their discovery, liposomes have been widely employed in biomedical research. These nano-size spherical vesicles consisting one or few phospholipid bilayers surrounding an aqueous core are capable of carrying a wide variety of bioactive compounds, including drugs, peptides, nucleic acids, proteins and others. Despite considerable success achieved in synthesis of liposome constructs containing bioactive compounds, preparation of ligand-targeted liposomes comprising large quantities of encapsulated proteins that are capable of affecting pathological cells still remains a big challenge. Here we described a novel method for preparation of small (80-90 nm in diameter) unilamellar liposomes containing very large quantities (thousands of protein molecules per liposome) of heme-containing cytochrome c, highly fluorescent mCherry and highly toxic PE40 (Pseudomonas aeruginosa Exotoxin A domain). Efficient encapsulation of the proteins was achieved through electrostatic interaction between positively charged proteins (at pH lower than pI) and negatively charged liposome membrane. The proteoliposomes containing large quantities of mCherry or PE40 and functionalized with designed ankyrin repeat protein (DARPin)_9-29, which targets human epidermal growth factor receptor 2 (HER2) were shown to specifically stain and kill in sub-nanomolar concentrations HER2-positive cells, overexpressing HER2, respectively. Specific staining and eradication of the receptor-positive cells demonstrated here makes the DARPin-functionalized liposomes carrying large quantities of fluorescent and/or toxic proteins a promising candidate for tumor detection and therapy.

Extrahepatic Targeting of Oligonucleotides with Receptor-Binding Non-Immunoglobulin Scaffold Proteins

Although recent clinical successes of antisense, splice-switching, and siRNA oligonucleotides have established the therapeutic utility of this novel class of medicines, the efficient systemic application for non-liver targets remains elusive. Exploitation of active receptor-mediated targeting followed by efficient and productive cellular uptake is required for enabling the therapy of extrahepatic diseases on the expressional level. Evasion of liver accumulation and organ-specific targeting and also efficient cytosolic delivery after endosomal internalization are currently insufficiently solved issues. Lipid and polymer-based nanoparticles can be engineered for efficient cellular uptake and enhancement of endosomal escape, but are characterized by preferential liver accumulation based on biodistribution largely determined by particle size and biophysical properties. Oligonucleotide bioconjugates with receptor-binding ligands have been evolved for highly efficient targeting, but frequently result in a large extent of endosomal entrapment and consequently a lack of sufficient cytosolic concentrations. Non-immunoglobulin protein-based receptor recognition affords high cell-type selectivity and is promising for achieving nonhepatic oligonucleotide targeting. The use of such novel protein scaffolds, including designed ankyrin repeat proteins (DARPins), for oligonucleotide delivery is attractive for achieving effective tissue targeting. Issues for further development and optimization to advance approaches for extrahepatic oligonucleotide delivery by nanoparticles or bioconjugates are discussed.

Adnectin-drug conjugates for Glypican-3-specific delivery of a cytotoxic payload to tumors

Tumor-specific delivery of cytotoxic agents remains a challenge in cancer therapy. Antibody-drug conjugates (ADC) deliver their payloads to tumor cells that overexpress specific tumor-associated antigens-but the multi-day half-life of ADC leads to high exposure even of normal, antigen-free, tissues and thus contributes to dose-limiting toxicity. Here, we present Adnectin-drug conjugates, an alternative platform for tumor-specific delivery of cytotoxic payloads. Due to their small size (10 kDa), renal filtration eliminates Adnectins from the bloodstream within minutes to hours, ensuring low exposure to normal tissues. We used an engineered cysteine to conjugate an Adnectin that binds Glypican-3, a membrane protein overexpressed in hepatocellular carcinoma, to a cytotoxic derivative of tubulysin, with the drug-to-Adnectin ratio of 1. We demonstrate specific, nanomolar binding of this Adnectin-drug conjugate to human and murine Glypican-3; its high thermostability; its localization to target-expressing tumor cells in vitro and in vivo, its fast clearance from normal tissues and its efficacy against Glypican-3-positive mouse xenograft models.

Advances in the Application of Designed Ankyrin Repeat Proteins (DARPins) as Research Tools and Protein Therapeutics

Nonimmunoglobulin scaffolds have been developed to overcome the limitations of monoclonal antibodies with regard to stability and size. Of these scaffolds, the class of designed ankyrin repeat proteins (DARPins) has advanced the most in biochemical and biomedical applications. This review focuses on the recent progress in DARPin technology, highlighting the scaffold's potential and possibilities.

Synthesis, Characterization, and Selective Delivery of DARPin-Gold Nanoparticle Conjugates to Cancer Cells

We demonstrate that the designed ankyrin repeat protein (DARPin)_9-29, which specifically targets human epidermal growth factor receptor 2 (HER 2), binds tightly to gold nanoparticles (GNPs). Binding of the protein strongly increases the colloidal stability of the particles. The results of experimental analysis and molecular dynamics simulations show that approximately 35 DARPin_9-29 molecules are bound to the surface of a 5 nm GNP and that the binding does not involve the receptor-binding domain of the protein. The confocal fluorescent microscopy studies show that the DARPin-coated GNP conjugate specifically interacts with the surface of human cancer cells overexpressing epidermal growth factor receptor 2 (HER2) and enters the cells by endocytosis. The high stability under physiological conditions and high affinity to the receptors overexpressed by cancer cells make conjugates of plasmonic gold nanostructures with DARPin molecules promising candidates for cancer therapy.

Designed Ankyrin Repeat Proteins as Her2 Targeting Domains in Chimeric Antigen Receptor-Engineered T Cells

Chimeric antigen receptor (CAR) engineering is a branch of cancer immunotherapy that equips immune cells to target tumor antigens expressed on the cell surface using antibody-derived single-chain variable fragments (scFvs). However, other antibody mimetics, such as designed ankyrin repeat proteins (DARPins), can also serve as antigen-binding domains in CARs. This study shows that CAR-engineered T (CAR-T) cells utilizing Her2-targeting DARPins G3 and 929 can target human epidermal growth factor receptor 2 (Her2)-overexpressing cancer cells as effectively as CAR-T cells with the scFv 4D5 in vitro, and G3 CAR-T cells can slow or eliminate tumor growth in vivo as effectively as 4D5 CAR-T cells. Some DARPins may offer an attractive alternative to scFv usage in CARs, as they are smaller, thermodynamically stable, poorly immunogenic, and can be generated with different binding properties from DARPin libraries.

Titratable Avidity Reduction Enhances Affinity Discrimination in Mammalian Cellular Selections of Yeast-Displayed Ligands

Yeast surface display selections against mammalian cell monolayers have proven effective in isolating proteins with novel binding activity. Recent advances in this technique allow for the recovery of clones with even micromolar binding affinities. However, no efficient method has been shown for affinity-based selection in this context. This study demonstrates the effectiveness of titratable avidity reduction using dithiothreitol to achieve this goal. A series of epidermal growth factor receptor binding fibronectin domains with a range of affinities are used to quantitatively identify the number of ligands per yeast cell that yield the strongest selectivity between strong, moderate, and weak affinities. Notably, reduction of ligand display to 3,000-6,000 ligands per yeast cell of a 2 nM binder yields 16-fold better selectivity than that to a 17 nM binder. These lessons are applied to affinity maturation of an EpCAM-binding fibronectin population, yielding an enriched pool of ligands with significantly stronger affinity than that of an analogous pool sorted by standard cellular selection methods. Collectively, this study offers a facile approach for affinity selection of yeast-displayed ligands against full-length cellular targets and demonstrates the effectiveness of this method by generating EpCAM-binding ligands that are promising for further applications.

Recombinant targeted toxin based on HER2-specific DARPin possesses a strong selective cytotoxic effect in vitro and a potent antitumor activity in vivo

DARPins fused with other proteins are promising non-immunoglobulin scaffolds for specific binding to target cells. In this study HER2-specific DARPin (DARPin_9-29) was used as a tumor-targeting moiety for the delivery of a cytotoxic agent - the fragment of Pseudomonas aeruginosa exotoxin A. It was determined that DARPin-PE40 possesses a considerable cytotoxic activity and induces apoptosis in HER2-positive cells. Cytotoxic effect of DARPin-PE40 strongly correlates with the HER2 expression level. The effect of intravenous administration of DARPin-PE40 was tested in the xenograft model of breast cancer. It was shown that treatment of animals with DARPin-PE40 caused strong and prolonged suppression of xenograft tumor growth.

Expansion of bioorthogonal chemistries towards site-specific polymer–protein conjugation

Bioorthogonal chemistries have been used to achieve polymer-protein conjugation with the retained critical properties.

Targeting CD44, ABCG2 and CD133 markers using aptamers: in silico analysis of CD133 extracellular domain 2 and its aptamer

Truncated CSC marker aptamers penetrate tumor spheres and inhibits cell proliferation; a bioinformatics approach to decipher their structural interactions.

A Fluorescent Imaging Probe Based on a Macrocyclic Scaffold That Binds to Cellular EpCAM

Here, we report a fluorescent probe based on a macrocyclic peptide scaffold that specifically stains EpCAM-expressing MCF7 cells. The 14-mer macrocyclic peptide binding to the extracellular domain of EpCAM with a dissociation constant in the low nM range (1.7 nM) was discovered using the random non-standard peptide-integrated discovery system. Notably, this probe containing a fluorescence tag is less than 3000 Da in total and able to visualize nearly every live cell under high cell-density conditions, which was not achieved by the conventional mAb staining method. This suggests that the molecular probe based on the compact macrocyclic scaffold has great potentials as an imaging tool for the EpCAM biomarker as well as a delivery vehicle for drug conjugates.

Designed ankyrin repeat proteins (DARPins): binding proteins for research, diagnostics, and therapy

Designed ankyrin repeat proteins (DARPins) can recognize targets with specificities and affinities that equal or surpass those of antibodies, but because of their robustness and extreme stability, they allow a multitude of more advanced formats and applications. This review highlights recent advances in DARPin design, illustrates their properties, and gives some examples of their use. In research, they have been established as intracellular, real-time sensors of protein conformations and as crystallization chaperones. For future therapies, DARPins have been developed by advanced, structure-based protein engineering to selectively induce apoptosis in tumors by uncoupling surface receptors from their signaling cascades. They have also been used successfully for retargeting viruses. In ongoing clinical trials, DARPins have shown good safety and efficacy in macular degeneration diseases. These developments all ultimately exploit the high stability, solubility, and aggregation resistance of these molecules, permitting a wide range of conjugates and fusions to be produced and purified.

Target-specific cytotoxic effects on HER2-expressing cells by the tripartite fusion toxin ZHER2:2891-ABD-PE38X8, including a targeting affibody molecule and a half-life extension domain

Development of cancer treatment regimens including immunotoxins is partly hampered by their immunogenicity. Recently, deimmunized versions of toxins have been described, potentially being better suited for translation to the clinic. In this study, a recombinant tripartite fusion toxin consisting of a deimmunized version of exotoxin A from Pseudomonas aeruginosa (PE38) genetically fused to an affibody molecule specifically interacting with the human epidermal growth factor receptor 2 (HER2), and also an albumin binding domain (ABD) for half-life extension, has been produced and characterized in terms of functionality of the three moieties. Biosensor based assays showed that the fusion toxin was able to interact with human and mouse serum albumin, but not with bovine serum albumin and that it interacted with HER2 (KD=5 nM). Interestingly, a complex of the fusion toxin and human serum albumin also interacted with HER2 but with a somewhat weaker affinity (KD=12 nM). The IC50-values of the fusion toxin ranged from 6 to 300 pM on SKOV-3, SKBR-3 and A549 cells and was lower for cells with higher surface densities of HER2. The fusion toxin was found specific for HER2 as shown by blocking available HER2 receptors with free affibody molecule before subjecting the cells to the toxin. Analysis of contact time showed that 10 min was sufficient to kill 50% of the cells. In conclusion, all three regions of the fusion toxin were found to be functional.

Antibody-Drug Conjugates for Tumor Targeting-Novel Conjugation Chemistries and the Promise of non-IgG Binding Proteins

Antibody-drug conjugates (ADCs) have emerged as a promising class of anticancer agents, combining the specificity of antibodies for tumor targeting and the destructive potential of highly potent drugs as payload. An essential component of these immunoconjugates is a bifunctional linker capable of reacting with the antibody and the payload to assemble a functional entity. Linker design is fundamental, as it must provide high stability in the circulation to prevent premature drug release, but be capable of releasing the active drug inside the target cell upon receptor-mediated endocytosis. Although ADCs have demonstrated an increased therapeutic window, compared to conventional chemotherapy in recent clinical trials, therapeutic success rates are still far from optimal. To explore other regimes of half-life variation and drug conjugation stoichiometries, it is necessary to investigate additional binding proteins which offer access to a wide range of formats, all with molecularly defined drug conjugation. Here, we delineate recent progress with site-specific and biorthogonal conjugation chemistries, and discuss alternative, biophysically more stable protein scaffolds like Designed Ankyrin Repeat Proteins (DARPins), which may provide such additional engineering opportunities for drug conjugates with improved pharmacological performance.

Beyond peptides and mAbs--current status and future perspectives for biotherapeutics with novel constructs

Biotherapeutics are attractive anti-cancer agents due to their high specificity and limited toxicity compared to conventional small molecules. Antibodies are widely used in cancer therapy, either directly or conjugated to a cytotoxic payload. Peptide therapies, though not as prevalent, have been utilized in hormonal therapy and imaging. The limitations associated with unmodified forms of both types of biotherapeutics have led to the design and development of novel structures, which incorporate key features and structures that have improved the molecules' abilities to bind to tumor targets, avoid degradation, and exhibit favorable pharmacokinetics. In this review, we highlight the current status of monoclonal antibodies and peptides, and provide a perspective on the future of biotherapeutics using novel constructs.

Efficient cell-specific uptake of binding proteins into the cytoplasm through engineered modular transport systems

Through advances in protein scaffold engineering and selection technologies, highly specific binding proteins, which fold under reducing conditions, can be generated against virtually all targets. Despite tremendous therapeutic opportunities, intracellular applications are hindered by difficulties associated with achieving cytosolic delivery, compounded by even correctly measuring it. Here, we addressed cytosolic delivery systematically through the development of a biotin ligase-based assay that objectively quantifies cytosolic delivery in a generic fashion. We developed modular transport systems that consist of a designed ankyrin repeat protein (DARPin) for receptor targeting and a different DARPin for intracellular recognition and a bacterial toxin-derived component for cytosolic translocation. We show that both anthrax pores and the translocation domain of Pseudomonas exotoxin A (ETA) efficiently deliver DARPins into the cytosol. We found that the cargo must not exceed a threshold thermodynamic stability for anthrax pores, which can be addressed by engineering, while the ETA pathway does not appear to have this restriction.

Artificial affinity proteins as ligands of immunoglobulins

A number of natural proteins are known to have affinity and specificity for immunoglobulins. Some of them are widely used as reagents for detection or capture applications, such as Protein G and Protein A. However, these natural proteins have a defined spectrum of recognition that may not fit specific needs. With the development of combinatorial protein engineering and selection techniques, it has become possible to design artificial affinity proteins with the desired properties. These proteins, termed alternative scaffold proteins, are most often chosen for their stability, ease of engineering and cost-efficient recombinant production in bacteria. In this review, we focus on alternative scaffold proteins for which immunoglobulin binders have been identified and characterized.

Novel prodrug-like fusion toxin with protease-sensitive bioorthogonal PEGylation for tumor targeting

Highly potent biotoxins like Pseudomonas exotoxin A (ETA) are attractive payloads for tumor targeting. However, despite replacement of the natural cell-binding domain of ETA by tumor-selective antibodies or alternative binding proteins like designed ankyrin repeat proteins (DARPins) the therapeutic window of such fusion toxins is still limited by target-independent cellular uptake, resulting in toxicity in normal tissues. Furthermore, the strong immunogenicity of the bacterial toxin precludes repeated administration in most patients. Site-specific modification to convert ETA into a prodrug-like toxin which is reactivated specifically in the tumor, and at the same time has a longer circulation half-life and is less immunogenic, is therefore appealing. To engineer a prodrug-like fusion toxin consisting of the anti-EpCAM DARPin Ec1 and a domain I-deleted variant of ETA (ETA″), we used strain-promoted azide alkyne cycloaddition for bioorthogonal conjugation of linear or branched polyethylene glycol (PEG) polymers at defined positions within the toxin moiety. Reversibility of the shielding was provided by a designed peptide linker containing the cleavage site for the rhinovirus 3C model protease. We identified two distinct sites, one within the catalytic domain and one close to the C-terminal KDEL sequence of Ec1-ETA″, simultaneous PEGylation of which resulted in up to 1000-fold lower cytotoxicity in EpCAM-positive tumor cells. Importantly, the potency of the fusion toxin was fully restored by proteolytic unveiling. Upon systemic administration in mice, PEGylated Ec1-ETA″ was much better tolerated than Ec1-ETA″; it showed a longer circulation half-life and an almost 10-fold increased area under the curve (AUC). Our strategy of engineering prodrug-like fusion toxins by bioorthogonal veiling opens new possibilities for targeting tumors with more specificity and efficacy.

Engineered proteins with desired specificity: DARPins, other alternative scaffolds and bispecific IgGs

Specific binding proteins have become essential for diagnostic and therapeutic applications, and traditionally these have been antibodies. Nowadays an increasing number of alternative scaffolds have joined these ranks. These additional folds have raised a lot of interest and expectations within the last decade. It appears that they have come of age and caught up with antibodies in many fields of applications. The last years have seen an exploration of possibilities in research, diagnostics and therapy. Some scaffolds have received further improvements broadening their fields of application, while others have started to occupy their respective niche. Protein engineering, the prerequisite for the advent of all alternative scaffolds, remains the driving force in this process, for both non-immunoglobulins and immunoglobulins alike.

Increasing the antitumor effect of an EpCAM-targeting fusion toxin by facile click PEGylation

Fusion toxins used for cancer-related therapy have demonstrated short circulation half-lives, which impairs tumor localization and, hence, efficacy. Here, we demonstrate that the pharmacokinetics of a fusion toxin composed of a designed ankyrin repeat protein (DARPin) and domain I-truncated Pseudomonas Exotoxin A (PE40/ETA″) can be significantly improved by facile bioorthogonal conjugation with a polyethylene glycol (PEG) polymer at a unique position. Fusion of the anti-EpCAM DARPin Ec1 to ETA″ and expression in methionine-auxotrophic E. coli enabled introduction of the nonnatural amino acid azidohomoalanine (Aha) at position 1 for strain-promoted click PEGylation. PEGylated Ec1-ETA″ was characterized by detailed biochemical analysis, and its potential for tumor targeting was assessed using carcinoma cell lines of various histotypes in vitro, and subcutaneous and orthotopic tumor xenografts in vivo. The mild click reaction resulted in a well-defined mono-PEGylated product, which could be readily purified to homogeneity. Despite an increased hydrodynamic radius resulting from the polymer, the fusion toxin demonstrated high EpCAM-binding activity and retained cytotoxicity in the femtomolar range. Pharmacologic analysis in mice unveiled an almost 6-fold increase in the elimination half-life (14 vs. 82 minutes) and a more than 7-fold increase in the area under the curve (AUC) compared with non-PEGylated Ec1-ETA″, which directly translated in increased and longer-lasting effects on established tumor xenografts. Our data underline the great potential of combining the inherent advantages of the DARPin format with bioorthogonal click chemistry to overcome the limitations of engineering fusion toxins with enhanced efficacy for cancer-related therapy.