Multivalent scaffold proteins as superagonists of TRAIL receptor 2-induced apoptosis

A DNA robotic switch with regulated autonomous display of cytotoxic ligand nanopatterns

The clustering of death receptors (DRs) at the membrane leads to apoptosis. With the goal of treating tumours, multivalent molecular tools that initiate this mechanism have been developed. However, DRs are also ubiquitously expressed in healthy tissue. Here we present a stimuli-responsive robotic switch nanodevice that can autonomously and selectively turn on the display of cytotoxic ligand patterns in tumour microenvironments. We demonstrate a switchable DNA origami that normally hides six ligands but displays them as a hexagonal pattern 10 nm in diameter once under higher acidity. This can effectively cluster DRs and trigger apoptosis of human breast cancer cells at pH 6.5 while remaining inert at pH 7.4. When administered to mice bearing human breast cancer xenografts, this nanodevice decreased tumour growth by up to 70%. The data demonstrate the feasibility and opportunities for developing ligand pattern switches as a path for targeted treatment.

Design and Synthesis of DNA Origami Nanostructures to Control TNF Receptor Activation

Clustering of type II tumor necrosis factor (TNF) receptors (TNFRs) is essential for their activation, yet currently available drugs fail to activate signaling. Some strategies aim to cluster TNFR by using multivalent streptavidin or scaffolds based on dextran or graphene. However, these strategies do not allow for control of the valency or spatial organization of the ligands, and consequently control of the TNFR activation is not optimal. DNA origami nanostructures allow nanometer-precise control of the spatial organization of molecules and complexes, with defined spacing, number and valency. Here, we demonstrate the design and characterization of a DNA origami nanostructure that can be decorated with engineered single-chain TNF-related apoptosis-inducing ligand (SC-TRAIL) complexes, which show increased cell killing compared to SC-TRAIL alone on Jurkat cells. The information in this chapter can be used as a basis to decorate DNA origami nanostructures with various proteins, complexes, or other biomolecules.

Redirecting the specificity of tripartite motif containing-21 scaffolds using a novel discovery and design approach

Hijacking the ubiquitin proteasome system to elicit targeted protein degradation (TPD) has emerged as a promising therapeutic strategy to target and destroy intracellular proteins at the post-translational level. Small molecule-based TPD approaches, such as proteolysis-targeting chimeras (PROTACs) and molecular glues, have shown potential, with several agents currently in clinical trials. Biological PROTACs (bioPROTACs), which are engineered fusion proteins comprised of a target-binding domain and an E3 ubiquitin ligase, have emerged as a complementary approach for TPD. Here, we describe a new method for the evolution and design of bioPROTACs. Specifically, engineered binding scaffolds based on the third fibronectin type III domain of human tenascin-C (Tn3) were installed into the E3 ligase tripartite motif containing-21 (TRIM21) to redirect its degradation specificity. This was achieved via selection of naïve yeast-displayed Tn3 libraries against two different oncogenic proteins associated with B-cell lymphomas, mucosa-associated lymphoid tissue lymphoma translocation protein 1 (MALT1) and embryonic ectoderm development protein (EED), and replacing the native substrate-binding domain of TRIM21 with our evolved Tn3 domains. The resulting TRIM21-Tn3 fusion proteins retained the binding properties of the Tn3 as well as the E3 ligase activity of TRIM21. Moreover, we demonstrated that TRIM21-Tn3 fusion proteins efficiently degraded their respective target proteins through the ubiquitin proteasome system in cellular models. We explored the effects of binding domain avidity and E3 ligase utilization to gain insight into the requirements for effective bioPROTAC design. Overall, this study presents a versatile engineering approach that could be used to design and engineer TRIM21-based bioPROTACs against therapeutic targets.

A simeprevir-inducible molecular switch for the control of cell and gene therapies

Chemical inducer of dimerization (CID) modules can be used effectively as molecular switches to control biological processes, and thus there is significant interest within the synthetic biology community in identifying novel CID systems. To date, CID modules have been used primarily in engineering cells for in vitro applications. To broaden their utility to the clinical setting, including the potential to control cell and gene therapies, the identification of novel CID modules should consider factors such as the safety and pharmacokinetic profile of the small molecule inducer, and the orthogonality and immunogenicity of the protein components. Here we describe a CID module based on the orally available, approved, small molecule simeprevir and its target, the NS3/4A protease from hepatitis C virus. We demonstrate the utility of this CID module as a molecular switch to control biological processes such as gene expression and apoptosis in vitro, and show that the CID system can be used to rapidly induce apoptosis in tumor cells in a xenograft mouse model, leading to complete tumor regression.

Targeting TRAIL Death Receptors in Triple-Negative Breast Cancers: Challenges and Strategies for Cancer Therapy

The tumor necrosis factor (TNF) superfamily member TNF-related apoptosis-inducing ligand (TRAIL) induces apoptosis in cancer cells via death receptor (DR) activation with little toxicity to normal cells or tissues. The selectivity for activating apoptosis in cancer cells confers an ideal therapeutic characteristic to TRAIL, which has led to the development and clinical testing of many DR agonists. However, TRAIL/DR targeting therapies have been widely ineffective in clinical trials of various malignancies for reasons that remain poorly understood. Triple negative breast cancer (TNBC) has the worst prognosis among breast cancers. Targeting the TRAIL DR pathway has shown notable efficacy in a subset of TNBC in preclinical models but again has not shown appreciable activity in clinical trials. In this review, we will discuss the signaling components and mechanisms governing TRAIL pathway activation and clinical trial findings discussed with a focus on TNBC. Challenges and potential solutions for using DR agonists in the clinic are also discussed, including consideration of the pharmacokinetic and pharmacodynamic properties of DR agonists, patient selection by predictive biomarkers, and potential combination therapies. Moreover, recent findings on the impact of TRAIL treatment on the immune response, as well as novel strategies to address those challenges, are discussed.

Emerging drug targets for triple-negative breast cancer: A guided tour of the preclinical landscape

INTRODUCTION

Triple-negative breast cancer (TNBC) is the most fatal molecular subtype of breast cancer because of its aggressiveness and resistance to chemotherapy. FDA-approved therapies for TNBC are limited to poly(ADP-ribose) polymerase inhibitors, immune checkpoint inhibitors, and trophoblast cell surface antigen 2-targeted antibody-drug conjugate. Therefore, developing a novel effective targeted therapy for TNBC is an urgent unmet need.

AREAS COVERED

In this narrative review, we discuss emerging targets for TNBC treatment discovered in early translational studies. We focus on cancer cell membrane molecules, hyperactive intracellular signaling pathways, and the tumor microenvironment (TME) based on their druggability, therapeutic potency, specificity to TNBC, and application in immunotherapy.

EXPERT OPINION

The significant challenges in the identification and validation of TNBC-associated targets are 1) application of appropriate genetic, molecular, and immunological approaches for modulating the target, 2) establishment of a proper mouse model that accurately represents the human immune TME, 3) TNBC molecular heterogeneity, and 4) failure translation of preclinical findings to clinical practice. To overcome those difficulties, future research needs to apply novel technology, such as single-cell RNA sequencing, thermostable group II intron reverse transcriptase sequencing, and humanized mouse models. Further, combination treatment targeting multiple pathways in both the TNBC tumor and its TME is essential for effective disease control.

Functional Genomic Identification of Predictors of Sensitivity and Mechanisms of Resistance to Multivalent Second-Generation TRAIL-R2 Agonists

Multivalent second-generation TRAIL-R2 agonists are currently in late preclinical development and early clinical trials. Herein, we use a representative second-generation agent, MEDI3039, to address two major clinical challenges facing these agents: lack of predictive biomarkers to enable patient selection and emergence of resistance. Genome-wide CRISPR knockout screens were notable for the lack of resistance mechanisms beyond the canonical TRAIL-R2 pathway (caspase-8, FADD, BID) as well as p53 and BAX in TP53 wild-type models, whereas a CRISPR activatory screen identified cell death inhibitors MCL-1 and BCL-XL as mechanisms to suppress MEDI3039-induced cell death. High-throughput drug screening failed to identify genomic alterations associated with response to MEDI3039; however, transcriptomics analysis revealed striking association between MEDI3039 sensitivity and expression of core components of the extrinsic apoptotic pathway, most notably its main apoptotic effector caspase-8 in solid tumor cell lines. Further analyses of colorectal cell lines and patient-derived xenografts identified caspase-8 expression ratio to its endogenous regulator FLIP(L) as predictive of sensitivity to MEDI3039 in several major solid tumor types and a further subset indicated by caspase-8:MCL-1 ratio. Subsequent MEDI3039 combination screening of TRAIL-R2, caspase-8, FADD, and BID knockout models with 60 compounds with varying mechanisms of action identified two inhibitor of apoptosis proteins (IAP) that exhibited strong synergy with MEDI3039 that could reverse resistance only in BID-deleted models. In summary, we identify the ratios of caspase-8:FLIP(L) and caspase-8:MCL-1 as potential predictive biomarkers for second-generation TRAIL-R2 agonists and loss of key effectors such as FADD and caspase-8 as likely drivers of clinical resistance in solid tumors.

Concentration-Independent Multivalent Targeting of Cancer Cells by Genetically Encoded Core-Crosslinked Elastin/Resilin-like Polypeptide Micelles

Valency is a fundamental principle to control macromolecular interactions and is used to target specific cell types by multivalent ligand-receptor interactions using self-assembled nanoparticle carriers. At the concentrations encountered in solid tumors upon systemic administration, these nanoparticles are, however, likely to show critical micelle concentration (CMC)-dependent disassembly and thus loss of function. To overcome this limitation, core-crosslinkable micelles of genetically encoded resilin-/elastin-like diblock polypeptides were recombinantly synthesized. The amphiphilic constructs were covalently photo-crosslinked through the genetically encoded unnatural amino acid para-azidophenylalanine in their hydrophobic block and they carried different anticancer ligands on their hydrophilic block: the wild-type tenth human fibronectin type III domain, a GRGDSPAS peptide-both targeting α_vβ₃ integrin-and an engineered variant of the third fibronectin type III domain of tenascin C that is a death receptor 5 agonist. Although uncrosslinked micelles lost most of their targeting ability below their CMC, the crosslinked analogues remained active at concentrations up to 1000-fold lower than the CMC, with binding affinities that are comparable to antibodies.

What Therapeutic Regimen Will Be Optimal for Initial Clinical Trials of Pig Organ Transplantation?

We discuss what therapeutic regimen might be acceptable/successful in the first clinical trial of genetically engineered pig kidney or heart transplantation. As regimens based on a calcineurin inhibitor or CTLA4-Ig have proved unsuccessful, the regimen we administer to baboons is based on induction therapy with antithymocyte globulin, an anti-CD20 mAb (Rituximab), and cobra venom factor, with maintenance therapy based on blockade of the CD40/CD154 costimulation pathway (with an anti-CD40 mAb), with rapamycin, and a corticosteroid. An anti-inflammatory agent (etanercept) is administered for the first 2 wk, and adjuvant therapy includes prophylaxis against thrombotic complications, anemia, cytomegalovirus, and pneumocystis. Using this regimen, although antibody-mediated rejection certainly can occur, we have documented no definite evidence of an adaptive immune response to the pig xenograft. This regimen could also form the basis for the first clinical trial, except that cobra venom factor will be replaced by a clinically approved agent, for example, a C1-esterase inhibitor. However, none of the agents that block the CD40/CD154 pathway are yet approved for clinical use, and so this hurdle remains to be overcome. The role of anti-inflammatory agents remains unproven. The major difference between this suggested regimen and those used in allotransplantation is the replacement of a calcineurin inhibitor with a costimulation blockade agent, but this does not appear to increase the complications of the regimen.

Clustering of Death Receptor for Apoptosis Using Nanoscale Patterns of Peptides

The nanoscale spatial organization of transmembrane tumor necrosis factor (TNF) receptors has been implicated in the regulation of cellular fate. Accordingly, molecular tools that can induce specific arrangements of these receptors on cell surfaces would give us an opportunity to study these effects in detail. To achieve this, we introduce DNA origami nanostructures that precisely scaffold the patterning of TNF-related apoptosis-inducing ligand-mimicking peptides at nanoscale level. Stimulating human breast cancer cells with these patterns, we find that around 5 nm is the critical interligand distance of hexagonally patterned peptides to induce death receptor clustering and a resulting apoptosis. We thus offer a strategy to reverse the non-efficacy of current ligand- and antibody-based methods for TNF superfamily activation.

18F-C2Am: a targeted imaging agent for detecting tumor cell death in vivo using positron emission tomography

INTRODUCTION

Trialing novel cancer therapies in the clinic would benefit from imaging agents that can detect early evidence of treatment response. The timing, extent and distribution of cell death in tumors following treatment can give an indication of outcome. We describe here an 18F-labeled derivative of a phosphatidylserine-binding protein, the C2A domain of Synaptotagmin-I (C2Am), for imaging tumor cell death in vivo using PET.

METHODS

A one-pot, two-step automated synthesis of N-(5-[18F]fluoropentyl)maleimide (60 min synthesis time, > 98% radiochemical purity) has been developed, which was used to label the single cysteine residue in C2Am within 30 min at room temperature. Binding of 18F-C2Am to apoptotic and necrotic tumor cells was assessed in vitro, and also in vivo, by dynamic PET and biodistribution measurements in mice bearing human tumor xenografts treated with a TRAILR2 agonist or with conventional chemotherapy. C2Am detection of tumor cell death was validated by correlation of probe binding with histological markers of cell death in tumor sections obtained immediately after imaging.

RESULTS

18F-C2Am showed a favorable biodistribution profile, with predominantly renal clearance and minimal retention in spleen, liver, small intestine, bone and kidney, at 2 h following probe administration. 18F-C2Am generated tumor-to-muscle (T/m) ratios of 6.1 ± 2.1 and 10.7 ± 2.4 within 2 h of probe administration in colorectal and breast tumor models, respectively, following treatment with the TRAILR2 agonist. The levels of cell death (CC3 positivity) following treatment were 12.9-58.8% and 11.3-79.7% in the breast and colorectal xenografts, respectively. Overall, a 20% increase in CC3 positivity generated a one unit increase in the post/pre-treatment tumor contrast. Significant correlations were found between tracer uptake post-treatment, at 2 h post-probe administration, and histological markers of cell death (CC3: Pearson R = 0.733, P = 0.0005; TUNEL: Pearson R = 0.532, P = 0.023).

CONCLUSION

The rapid clearance of 18F-C2Am from the blood pool and low kidney retention allowed the spatial distribution of cell death in a tumor to be imaged during the course of therapy, providing a rapid assessment of tumor treatment response. 18F-C2Am has the potential to be used in the clinic to assess early treatment response in tumors.

RALB GTPase: a critical regulator of DR5 expression and TRAIL sensitivity in KRAS mutant colorectal cancer

RAS mutant (MT) metastatic colorectal cancer (mCRC) is resistant to MEK1/2 inhibition and remains a difficult-to-treat group. Therefore, there is an unmet need for novel treatment options for RASMT mCRC. RALA and RALB GTPases function downstream of RAS and have been found to be key regulators of several cell functions implicated in KRAS-driven tumorigenesis. However, their role as regulators of the apoptotic machinery remains to be elucidated. Here, we found that inhibition of RALB expression, but not RALA, resulted in Caspase-8-dependent cell death in KRASMT CRC cells, which was not further increased following MEK1/2 inhibition. Proteomic analysis and mechanistic studies revealed that RALB depletion induced a marked upregulation of the pro-apoptotic cell surface TRAIL Death Receptor 5 (DR5) (also known as TRAIL-R2), primarily through modulating DR5 protein lysosomal degradation. Moreover, DR5 knockdown or knockout attenuated siRALB-induced apoptosis, confirming the role of the extrinsic apoptotic pathway as a regulator of siRALB-induced cell death. Importantly, TRAIL treatment resulted in the association of RALB with the death-inducing signalling complex (DISC) and targeting RALB using pharmacologic inhibition or RNAi approaches triggered a potent increase in TRAIL-induced cell death in KRASMT CRC cells. Significantly, high RALB mRNA levels were found in the poor prognostic Colorectal Cancer Intrinsic Subtypes (CRIS)-B CRC subgroup. Collectively, this study provides to our knowledge the first evidence for a role for RALB in apoptotic priming and suggests that RALB inhibition may be a promising strategy to improve response to TRAIL treatment in poor prognostic RASMT CRIS-B CRC.

A CD40L-targeting protein reduces autoantibodies and improves disease activity in patients with autoimmunity

The CD40/CD40L axis plays a central role in the generation of humoral immune responses and is an attractive target for treating autoimmune diseases in the clinic. Here, we report the generation and clinical results of a CD40L binding protein, VIB4920, which lacks an Fc domain, therefore avoiding platelet-related safety issues observed with earlier monoclonal antibody therapeutics that targeted CD40L. VIB4920 blocked downstream CD40 signaling events, resulting in inhibition of human B cell activation and plasma cell differentiation, and did not induce platelet aggregation in preclinical studies. In a phase 1 study in healthy volunteers, VIB4920 suppressed antigen-specific IgG in a dose-dependent fashion after priming and boosting with the T-dependent antigen, KLH. Furthermore, VIB4920 significantly reduced circulating Ki67⁺ dividing B cells, class-switched memory B cells, and a plasma cell gene signature after immunization. In a phase 1b proof-of-concept study in patients with rheumatoid arthritis, VIB4920 significantly decreased disease activity, achieving low disease activity or clinical remission in more than 50% of patients in the two higher-dose groups. Dose-dependent decreases in rheumatoid factor autoantibodies and Vectra DA biomarker score provide additional evidence that VIB4920 effectively blocked the CD40/CD40L pathway. VIB4920 demonstrated a good overall safety profile in both clinical studies. Together, these data demonstrate the potential of VIB4920 to significantly affect autoimmune disease and humoral immune activation and to support further evaluation of this molecule in inflammatory conditions.

Development and Differentiation in Monobodies Based on the Fibronectin Type 3 Domain

As a non-antibody scaffold, monobodies based on the fibronectin type III (FN3) domain overcome antibody size and complexity while maintaining analogous binding loops. However, antibodies and their derivatives remain the gold standard for the design of new therapeutics. In response, clinical-stage therapeutic proteins based on the FN3 domain are beginning to use native fibronectin function as a point of differentiation. The small and simple structure of monomeric monobodies confers increased tissue distribution and reduced half-life, whilst the absence of disulphide bonds improves stability in cytosolic environments. Where multi-specificity is challenging with an antibody format that is prone to mis-pairing between chains, multiple FN3 domains in the fibronectin assembly already interact with a large number of molecules. As such, multiple monobodies engineered for interaction with therapeutic targets are being combined in a similar beads-on-a-string assembly which improves both efficacy and pharmacokinetics. Furthermore, full length fibronectin is able to fold into multiple conformations as part of its natural function and a greater understanding of how mechanical forces allow for the transition between states will lead to advanced applications that truly differentiate the FN3 domain as a therapeutic scaffold.

A revised model of TRAIL-R2 DISC assembly explains how FLIP(L) can inhibit or promote apoptosis

The long FLIP splice form FLIP(L) can act as both an inhibitor and promoter of caspase‐8 at death‐inducing signalling complexes (DISCs) formed by death receptors such as TRAIL‐R2 and related intracellular complexes such as the ripoptosome. Herein, we describe a revised DISC assembly model that explains how FLIP(L) can have these opposite effects by defining the stoichiometry (with respect to caspase‐8) at which it converts from being anti‐ to pro‐apoptotic at the DISC. We also show that in the complete absence of FLIP(L), procaspase‐8 activation at the TRAIL‐R2 DISC has significantly slower kinetics, although ultimately the extent of apoptosis is significantly greater. This revised model of DISC assembly also explains why FLIP's recruitment to the TRAIL‐R2 DISC is impaired in the absence of caspase‐8 despite showing that it can interact with the DISC adaptor protein FADD and why the short FLIP splice form FLIP(S) is the more potent inhibitor of DISC‐mediated apoptosis.

Genomically informed small-molecule drugs overcome resistance to a sustained-release formulation of an engineered death receptor agonist in patient-derived tumor models

Extrinsic pathway agonists have failed repeatedly in the clinic for three core reasons: Inefficient ligand-induced receptor multimerization, poor pharmacokinetic properties, and tumor intrinsic resistance. Here, we address these factors by (i) using a highly potent death receptor agonist (DRA), (ii) developing an injectable depot for sustained DRA delivery, and (iii) leveraging a CRISPR-Cas9 knockout screen in DRA-resistant colorectal cancer (CRC) cells to identify functional drivers of resistance. Pharmacological blockade of XIAP and BCL-X_L by targeted small-molecule drugs strongly enhanced the antitumor activity of DRA in CRC cell lines. Recombinant fusion of the DRA to a thermally responsive elastin-like polypeptide (ELP) creates a gel-like depot upon subcutaneous injection that abolishes tumors in DRA-sensitive Colo205 mouse xenografts. Combination of ELP_depot-DRA with BCL-X_L and/or XIAP inhibitors led to tumor growth inhibition and extended survival in DRA-resistant patient-derived xenografts. This strategy provides a precision medicine approach to overcome similar challenges with other protein-based cancer therapies.

Novel Apoptosis-Inducing Agents for the Treatment of Cancer, a New Arsenal in the Toolbox

Evasion from apoptosis is an important hallmark of cancer cells. Alterations of apoptosis pathways are especially critical as they confer resistance to conventional anti-cancer therapeutics, e.g., chemotherapy, radiotherapy, and targeted therapeutics. Thus, successful induction of apoptosis using novel therapeutics may be a key strategy for preventing recurrence and metastasis. Inhibitors of anti-apoptotic molecules and enhancers of pro-apoptotic molecules are being actively developed for hematologic malignancies and solid tumors in particular over the last decade. However, due to the complicated apoptosis process caused by a multifaceted connection with cross-talk pathways, protein–protein interaction, and diverse resistance mechanisms, drug development within the category has been extremely challenging. Careful design and development of clinical trials incorporating predictive biomarkers along with novel apoptosis-inducing agents based on rational combination strategies are needed to ensure the successful development of these molecules. Here, we review the landscape of currently available direct apoptosis-targeting agents in clinical development for cancer treatment and update the related biomarker advancement to detect and validate the efficacy of apoptosis-targeted therapies, along with strategies to combine them with other agents.

Molecular Mode of Action of TRAIL Receptor Agonists-Common Principles and Their Translational Exploitation

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) and its death receptors TRAILR1/death receptor 4 (DR4) and TRAILR2/DR5 trigger cell death in many cancer cells but rarely exert cytotoxic activity on non-transformed cells. Against this background, a variety of recombinant TRAIL variants and anti-TRAIL death receptor antibodies have been developed and tested in preclinical and clinical studies. Despite promising results from mice tumor models, TRAIL death receptor targeting has failed so far in clinical studies to show satisfying anti-tumor efficacy. These disappointing results can largely be explained by two issues: First, tumor cells can acquire TRAIL resistance by several mechanisms defining a need for combination therapies with appropriate sensitizing drugs. Second, there is now growing preclinical evidence that soluble TRAIL variants but also bivalent anti-TRAIL death receptor antibodies typically require oligomerization or plasma membrane anchoring to achieve maximum activity. This review discusses the need for oligomerization and plasma membrane attachment for the activity of TRAIL death receptor agonists in view of what is known about the molecular mechanisms of how TRAIL death receptors trigger intracellular cell death signaling. In particular, it will be highlighted which consequences this has for the development of next generation TRAIL death receptor agonists and their potential clinical application.

Spy&Go purification of SpyTag-proteins using pseudo-SpyCatcher to access an oligomerization toolbox

Peptide tags are a key resource, introducing minimal change while enabling a consistent process to purify diverse proteins. However, peptide tags often provide minimal benefit post-purification. We previously designed SpyTag, forming an irreversible bond with its protein partner SpyCatcher. SpyTag provides an easy route to anchor, bridge or multimerize proteins. Here we establish Spy&Go, enabling protein purification using SpyTag. Through rational engineering we generated SpyDock, which captures SpyTag-fusions and allows efficient elution. Spy&Go enabled sensitive purification of SpyTag-fusions from Escherichia coli, giving superior purity than His-tag/nickel-nitrilotriacetic acid. Spy&Go allowed purification of mammalian-expressed, N-terminal, C-terminal or internal SpyTag. As an oligomerization toolbox, we established a panel of SpyCatcher-linked coiled coils, so SpyTag-fusions can be dimerized, trimerized, tetramerized, pentamerized, hexamerized or heptamerized. Assembling oligomers for Death Receptor 5 stimulation, we probed multivalency effects on cancer cell death. Spy&Go, combined with simple oligomerization, should have broad application for exploring multivalency in signaling.

MEDI3039, a novel highly potent tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) receptor 2 agonist, causes regression of orthotopic tumors and inhibits outgrowth of metastatic triple-negative breast cancer

Background

TNF-related apoptosis-inducing ligand (TRAIL) receptor agonists are attractive anti-tumor agents because of their capability to induce apoptosis in cancer cells by activating death receptors (DR) 4 and 5 with little toxicity against normal cells. Despite an attractive mechanism of action, previous clinical efforts to use TRAIL receptor agonists have been unsuccessful. In this study, we examined MEDI3039, a highly potent multivalent DR5 agonist, in breast cancer cell lines and in vivo models.

Methods

As in vitro model systems, we used 19 breast cancer cell lines that are categorized into four subtypes: ER+, HER2 amplified, basal A (triple-negative breast cancer) TNBC, and basal B TNBC. Cell viability was analyzed by MTS and RealTime live/dead assays. As in vivo model systems, MDA-MB231T orthotopic primary tumor growth in the mammary fat pad (MFP) and two experimental lung metastasis models were used. The effect of MEDI3039 on MFP tumors was assessed with immunohistochemical analysis. Lung metastases were analyzed with Bouin’s and H&E staining.

Results

MEDI3039 killed multiple breast cancer cell lines, but the sensitivity varied among different subtypes. Sensitivity was basal B TNBC >> basal A TNBC > HER2 amplified > ER+ (average IC₅₀ = 1.4, 203, 314, 403 pM, respectively). While the pattern of relative sensitivity was similar to GST-TRAIL in most cell lines, MEDI3039 was at least two orders of magnitude more potent compared with GST-TRAIL. In the MFP model, weekly treatment with 0.1 or 0.3 mg/kg MEDI3039 for 5 weeks inhibited tumor growth by 99.05% or 100% (median), respectively, compared with the control group, and extended animal survival (p = 0.08 or p = 0.0032 at 0.1 or 0.3 mg/kg, respectively). MEDI3039-induced caspase activation was confirmed in tumors grown in MFP (p < 0.05). In an experimental pulmonary metastasis model, MEDI3039 significantly suppressed outgrowth of surface (p < 0.0001) and microscopic metastases (p < 0.05). In an established lung metastasis model, MEDI3039 significantly inhibited growth of metastases (p < 0.01 in surface [> 4 mm], p < 0.01 in tumor percentage) and extended animal survival (p < 0.0001).

Conclusion

MEDI3039 is a potent DR5 agonist in breast cancer cells in vitro and in vivo and has potential as a cancer drug in breast cancer patients, especially those with basal B TNBC.

Electronic supplementary material

The online version of this article (10.1186/s13058-019-1116-1) contains supplementary material, which is available to authorized users.

Targeting scFv-Fc-scTRAIL fusion proteins to tumor cells

Fusion proteins combining hexavalent TRAIL with antibody fragments allow for a targeted delivery and efficient apoptosis induction in tumor cells. Here, we analyzed scFv-Fc-scTRAIL molecules directed against EGFR, HER2, HER3, and EpCAM as well as an untargeted Fc-scTRAIL fusion protein for their potentials to induce cell death both in vitro and in a xenograft tumor model in vivo. The scFv-Fc-scTRAIL fusion protein directed against EGFR as well as the fusion protein directed against EpCAM showed targeting effects on the two tested colorectal carcinoma cell lines Colo205 and HCT116, while a fusion protein targeting HER3 was more effective than untargeted Fc-scTRAIL only on Colo205 cells. Interestingly, another anti-HER3 scFv-Fc-scTRAIL fusion protein exhibiting approximately 10-fold weaker antigen binding as well as the HER2-directed molecule were unable to increase cytotoxicity compared to Fc-scTRAIL. A comparison of EC50 values of cell death induction and antigen binding supports the assumption that high affinity antigen binding is one of the requirements for in vitro targeting effects. Furthermore, a minimal number of expressed target antigens might be required for increased cytotoxicity of targeted compared to non-targeted molecules. In a Colo205 s.c. xenograft tumor model, strongest antitumor activity was observed for the anti-HER3 scFv-Fc-scTRAIL fusion protein based on antibody 3-43, with complete tumor remissions after six twice-weekly injections. Surprisingly, a similar in vivo activity was also observed for untargeted Fc-scTRAIL in this tumor model, indicating that additional factors contribute to the potent efficacy of targeted as well as untargeted hexavalent Fc-scTRAIL fusion proteins in vivo.

Optoacoustic Detection of Early Therapy-Induced Tumor Cell Death Using a Targeted Imaging Agent

Purpose: The development of new treatments and their deployment in the clinic may be assisted by imaging methods that allow an early assessment of treatment response in individual patients. The C2A domain of Synaptotagmin-I (C2Am), which binds to the phosphatidylserine (PS) exposed by apoptotic and necrotic cells, has been developed as an imaging probe for detecting cell death. Multispectral optoacoustic tomography (MSOT) is a real-time and clinically applicable imaging modality that was used here with a near infrared (NIR) fluorophore-labeled C2Am to image tumor cell death in mice treated with a TNF-related apoptosis-inducing ligand receptor 2 (TRAILR2) agonist and with 5-fluorouracil (5-FU).Experimental Design: C2Am was labeled with a NIR fluorophore and injected intravenously into mice bearing human colorectal TRAIL-sensitive Colo205 and TRAIL-resistant HT-29 xenografts that had been treated with a potent agonist of TRAILR2 and in Colo205 tumors treated with 5-FU.Results: Three-dimensional (3D) MSOT images of probe distribution showed development of tumor contrast within 3 hours of probe administration and a signal-to-background ratio in regions containing dead cells of >10 after 24 hours. A site-directed mutant of C2Am that is inactive in PS binding showed negligible binding. Tumor retention of the active probe was strongly correlated (R2 = 0.97, P value < 0.01) with a marker of apoptotic cell death measured in histologic sections obtained post mortem.Conclusions: The rapid development of relatively high levels of contrast suggests that NIR fluorophore-labeled C2Am could be a useful optoacoustic imaging probe for detecting early therapy-induced tumor cell death in the clinic. Clin Cancer Res; 23(22); 6893-903. ©2017 AACR.

Antibodies and Derivatives Targeting DR4 and DR5 for Cancer Therapy

Developing therapeutics that induce apoptosis in cancer cells has become an increasingly attractive approach for the past 30 years. The discovery of tumor necrosis factor (TNF) superfamily members and more specifically TNF-related apoptosis-inducing ligand (TRAIL), the only cytokine of the family capable of eradicating selectively cancer cells, led to the development of numerous TRAIL derivatives targeting death receptor 4 (DR4) and death receptor 5 (DR5) for cancer therapy. With a few exceptions, preliminary attempts to use recombinant TRAIL, agonistic antibodies, or derivatives to target TRAIL agonist receptors in the clinic have been fairly disappointing. Nonetheless, a tremendous effort, worldwide, is being put into the development of novel strategic options to target TRAIL receptors. Antibodies and derivatives allow for the design of novel and efficient agonists. We summarize and discuss here the advantages and drawbacks of the soar of TRAIL therapeutics, from the first developments to the next generation of agonistic products, with a particular insight on new concepts.

Superior Properties of Fc-comprising scTRAIL Fusion Proteins

The TNF-related apoptosis-inducing ligand (TRAIL) has been considered as a promising molecule for cancer treatment. However, clinical studies with soluble TRAIL failed to show therapeutic activity, which resulted in subsequent development of more potent TRAIL-based therapeutics. In this study, we applied defined oligomerization and tumor targeting as strategies to further improve the activity of a single-chain version of TRAIL (scTRAIL). We compared three different formats of EGF receptor (EGFR)-targeting dimeric scTRAIL fusion proteins [Diabody (Db)-scTRAIL, scFv-IgE heavy chain domain 2 (EHD2)-scTRAIL, scFv-Fc-scTRAIL] as well as two nontargeted dimeric scTRAIL molecules (EHD2-scTRAIL, Fc-scTRAIL) to reveal the influence of targeting and protein format on antitumor activity. All EGFR-targeted dimeric scTRAIL molecules showed similar binding properties and comparable cell death induction in vitro, exceeding the activity of the respective nontargeted dimeric format and monomeric scTRAIL. Superior properties were observed for the Fc fusion proteins with respect to production and in vivo half-life. In vivo studies using a Colo205 xenograft model revealed potent antitumor activity of all EGFR-targeting formats and Fc-scTRAIL and furthermore highlighted the higher efficacy of fusion proteins comprising an Fc part. Despite enhanced in vitro cell death induction of targeted scTRAIL molecules, however, comparable antitumor activities were found for the EGFR-targeting scFv-Fc-scTRAIL and the nontargeting Fc-scTRAIL in vivoMol Cancer Ther; 16(12); 2792-802. ©2017 AACR.

Exploring the TRAILs less travelled: TRAIL in cancer biology and therapy

The discovery that the tumour necrosis factor-related apoptosis-inducing ligand (TRAIL) can induce apoptosis of cancer cells without causing toxicity in mice has led to the in-depth study of pro-apoptotic TRAIL receptor (TRAIL-R) signalling and the development of biotherapeutic drug candidates that activate TRAIL-Rs. The outcome of clinical trials with these TRAIL-R agonists has, however, been disappointing so far. Recent evidence indicates that many cancers, in addition to being TRAIL resistant, use the endogenous TRAIL-TRAIL-R system to their own advantage. However, novel insight on two fronts - how resistance of cancer cells to TRAIL-based pro-apoptotic therapies might be overcome, and how the pro-tumorigenic effects of endogenous TRAIL might be countered - gives reasonable hope that the TRAIL system can be harnessed to treat cancer. In this Review we assess the status quo of our understanding of the biology of the TRAIL-TRAIL-R system - as well as the gaps therein - and discuss the opportunities and challenges in effectively targeting this pathway.

Using natural products to promote caspase-8-dependent cancer cell death

The selective killing of cancer cells without toxicity to normal nontransformed cells is an idealized goal of cancer therapy. Thus, there has been much interest in tumor necrosis factor-related apoptosis-inducing ligand (TRAIL), a protein that appears to selectively kill cancer cells. TRAIL has been reported to trigger apoptosis and under some circumstances, an alternate death signaling pathway termed necroptosis. The relative importance of necroptosis for cell death induction in vivo is under intensive investigation. Nonetheless, many cancer cells (particularly those freshly isolated from cancer patients) are highly resistant to TRAIL-mediated cell death. Therefore, there is an underlying interest in identifying agents that can be combined with TRAIL to improve its efficacy. There are numerous reports in which combination of TRAIL with standard antineoplastic drugs has resulted in enhanced cancer cell death in vitro. However, many of these chemotherapeutic drugs are nonspecific and associated with adverse effects, which raise serious concerns for cancer therapy in patients. By contrast, natural products have been shown to be safer and efficacious alternatives. Recently, a number of studies have suggested that certain natural products when combined with TRAIL can enhance cancer cell death. In this review, we highlight molecular pathways that might be targeted by various natural products to promote cell death, and focus on our recent work with withanolides as TRAIL sensitizers. Finally, we will suggest synergistic approaches for combining active withanolides with various forms of immunotherapy to promote cancer cell death and an effective antitumor immune response.

Multivalent Conjugates of Sonic Hedgehog Accelerate Diabetic Wound Healing

Despite their preclinical promise, few recombinant growth factors have been fully developed into effective therapies, in part, due to the short interval of therapeutic activity after administration. To address this problem, we developed nanoscale polymer conjugates for multivalent presentation of therapeutic proteins that enhance the activation of targeted cellular responses. As an example of this technology, we conjugated multiple Sonic hedgehog (Shh) proteins onto individual hyaluronic acid biopolymers to generate multivalent protein clusters at defined ratios (i.e., valencies) that yield enhanced Shh pathway activation at equivalent concentrations relative to unconjugated Shh. In this study, we investigated whether these multivalent conjugates (mvShh) could be used to improve the therapeutic function of Shh. We found that a single treatment with mvShh significantly accelerated the closure of full-thickness wounds in diabetic (db/db) mice compared to either an equivalent dose of unconjugated Shh or the vehicle control. Furthermore, we identified specific indicators of wound healing in fibroblasts and endothelial cells (i.e., transcriptional activation and cell migration) that were activated by mvShh in vitro and at concentrations approximately an order of magnitude lower than the unconjugated Shh. Taken together, our findings suggest that mvShh conjugates exhibit greater potency to activate the Shh pathway, and this multivalency advantage improves its therapeutic effect to accelerate wound closure in a diabetic animal model. Our strategy of multivalent protein presentation using nanoscale polymer conjugates has the potential to make a significant impact on the development of protein-based therapies by improving their in vivo performance.

RG7386, a Novel Tetravalent FAP-DR5 Antibody, Effectively Triggers FAP-Dependent, Avidity-Driven DR5 Hyperclustering and Tumor Cell Apoptosis

Dysregulated cellular apoptosis and resistance to cell death are hallmarks of neoplastic initiation and disease progression. Therefore, the development of agents that overcome apoptosis dysregulation in tumor cells is an attractive therapeutic approach. Activation of the extrinsic apoptotic pathway is strongly dependent on death receptor (DR) hyperclustering on the cell surface. However, strategies to activate DR5 or DR4 through agonistic antibodies have had only limited clinical success. To pursue an alternative approach for tumor-targeted induction of apoptosis, we engineered a bispecific antibody (BsAb), which simultaneously targets fibroblast-activation protein (FAP) on cancer-associated fibroblasts in tumor stroma and DR5 on tumor cells. We hypothesized that bivalent binding to both FAP and DR5 leads to avidity-driven hyperclustering of DR5 and subsequently strong induction of apoptosis in tumor cells but not in normal cells. Here, we show that RG7386, an optimized FAP-DR5 BsAb, triggers potent tumor cell apoptosis in vitro and in vivo in preclinical tumor models with FAP-positive stroma. RG7386 antitumor efficacy was strictly FAP dependent, was independent of FcR cross-linking, and was superior to conventional DR5 antibodies. In combination with irinotecan or doxorubicin, FAP-DR5 treatment resulted in substantial tumor regression in patient-derived xenograft models. FAP-DR5 also demonstrated single-agent activity against FAP-expressing malignant cells, due to cross-binding of FAP and DR5 across tumor cells. Taken together, these data demonstrate that RG7386, a novel and potent antitumor agent in both mono- and combination therapies, overcomes limitations of previous DR5 antibodies and represents a promising approach to conquer tumor-associated resistance to apoptosis. Mol Cancer Ther; 15(5); 946-57. ©2016 AACR.

Challenges and opportunities for non-antibody scaffold drugs

The first candidates from the promising class of small non-antibody protein scaffolds are now moving into clinical development and practice. Challenges remain, and scaffolds will need to be further tailored toward applications where they provide real advantages over established therapeutics to succeed in a rapidly evolving drug development landscape.

DED or alive: assembly and regulation of the death effector domain complexes

Death effector domains (DEDs) are protein–protein interaction domains initially identified in proteins such as FADD, FLIP and caspase-8 involved in regulating apoptosis. Subsequently, these proteins have been shown to have important roles in regulating other forms of cell death, including necroptosis, and in regulating other important cellular processes, including autophagy and inflammation. Moreover, these proteins also have prominent roles in innate and adaptive immunity and during embryonic development. In this article, we review the various roles of DED-containing proteins and discuss recent developments in our understanding of DED complex formation and regulation. We also briefly discuss opportunities to therapeutically target DED complex formation in diseases such as cancer.

Genetic and Pharmacological Screens Converge in Identifying FLIP, BCL2, and IAP Proteins as Key Regulators of Sensitivity to the TRAIL-Inducing Anticancer Agent ONC201/TIC10

ONC201/TIC10 is a small-molecule inducer of the TRAIL gene under current investigation as a novel anticancer agent. In this study, we identify critical molecular determinants of ONC201 sensitivity offering potential utility as pharmacodynamic or predictive response markers. By screening a library of kinase siRNAs in combination with a subcytotoxic dose of ONC201, we identified several kinases that ablated tumor cell sensitivity, including the MAPK pathway-inducer KSR1. Unexpectedly, KSR1 silencing did not affect MAPK signaling in the presence or absence of ONC201, but instead reduced expression of the antiapoptotic proteins FLIP, Mcl-1, Bcl-2, cIAP1, cIAP2, and survivin. In parallel to this work, we also conducted a synergy screen in which ONC201 was combined with approved small-molecule anticancer drugs. In multiple cancer cell populations, ONC201 synergized with diverse drug classes, including the multikinase inhibitor sorafenib. Notably, combining ONC201 and sorafenib led to synergistic induction of TRAIL and its receptor DR5 along with a potent induction of cell death. In a mouse xenograft model of hepatocellular carcinoma, we demonstrated that ONC201 and sorafenib cooperatively and safely triggered tumor regressions. Overall, our results established a set of determinants for ONC201 sensitivity that may predict therapeutic response, particularly in settings of sorafenib cotreatment to enhance anticancer responses.

Thioether analogues of disulfide-bridged cyclic peptides targeting death receptor 5: conformational analysis, dimerisation and consequences for receptor activation

Cyclic peptides containing redox-stable thioether bridges might provide a useful alternative to disulfide-bridged bioactive peptides. We report the effect of replacing the disulfide bridge with a lanthionine linkage in a 16-mer cyclic peptide that binds to death receptor 5 (DR5, TRAIL-R2). Upon covalent oligomerisation, the disulfide-bridged peptide has previously shown similar behaviour to that of TNF-related apoptosis inducing ligand (TRAIL), by selectively triggering the DR5 cell death pathway. The structural and biological properties of the DR5-binding peptide and its desulfurised analogue were compared. Surface plasmon resonance (SPR) data suggest that these peptides bind DR5 with comparable affinities. The same holds true for dimeric versions of these peptides: the thioether is able to induce DR5-mediated apoptosis of BJAB lymphoma and tumorigenic BJELR cells, albeit to a slightly lower extent compared to its disulfide homologue. NMR analysis revealed subtle variation in the conformations of the two peptides and suggests that the thioether peptide is slightly less folded than its disulfide homologue. These observations could account for the different capability of the two dimers to cluster DR5 receptors on the cell surface and to trigger apoptosis. Nevertheless, our results suggest that the thioether peptide is a potential candidate for evaluation in animal models.

Apo2L/TRAIL and the death receptor 5 agonist antibody AMG 655 cooperate to promote receptor clustering and antitumor activity

Death receptor agonist therapies have exhibited limited clinical benefit to date. Investigations into why Apo2L/TRAIL and AMG 655 preclinical data were not predictive of clinical response revealed that coadministration of Apo2L/TRAIL with AMG 655 leads to increased antitumor activity in vitro and in vivo. The combination of Apo2L/TRAIL and AMG 655 results in enhanced signaling and can sensitize Apo2L/TRAIL-resistant cells. Structure determination of the Apo2L/TRAIL-DR5-AMG 655 ternary complex illustrates how higher order clustering of DR5 is achieved when both agents are combined. Enhanced agonism generated by combining Apo2L/TRAIL and AMG 655 provides insight into the limited efficacy observed in previous clinical trials and suggests testable hypotheses to reconsider death receptor agonism as a therapeutic strategy.

Stabilization of the third fibronectin type III domain of human tenascin-C through minimal mutation and rational design

Non-antibody scaffolds are increasingly used to generate novel binding proteins for both research and therapeutic applications. Our group has developed the tenth fibronectin type III domain of human tenascin-C (TNfn3) as one such scaffold. As a scaffold, TNfn3 must tolerate extensive mutation to introduce novel binding sites. However, TNfn3's marginal stability (T(m) ∼ 59°C, ΔG(unfolding) = 5.7 kcal/mol) stands as a potential obstacle to this process. To address this issue, we sought to engineer highly stable TNfn3 variants. We used two parallel strategies. Using insights gained from structural analysis of other FN3 family members, we (1) rationally designed stabilizing point mutations or (2) introduced novel stabilizing disulfide bonds. Both strategies yielded highly stable TNfn3 variants with T(m) values as high as 83°C and ΔG(unfolding) values as high as 9.4 kcal/mol. Notably, only three or four mutations were required to achieve this level of stability with either approach. These results validate our rational design strategies and illustrate that substantial stability increases can be achieved with minimal mutation. One TNfn3 variant reported here has now been successfully used as a scaffold to develop two promising therapeutic molecules. We anticipate that other variants described will exhibit similar utility.

Death receptor agonist therapies for cancer, which is the right TRAIL?

The activation of cell-surface death receptors represents an attractive therapeutic strategy to promote apoptosis of tumor cells. Several investigational therapeutics that target this extrinsic pathway, including recombinant human Apo2L/TRAIL and monoclonal agonist antibodies directed against death receptors-4 (DR4) or -5 (DR5), have been evaluated in the clinic. Although Phase 1/1b studies provided encouraging preliminary results, findings from randomized Phase 2 studies failed to demonstrate significant clinical benefit. This has raised multiple questions as to why pre-clinical data were not predictive of clinical response. Results from clinical studies and insight into why current agents have failed to yield robust responses are discussed. In addition, new strategies for the development of next generation death receptor agonists are reviewed.

Enhanced catalysis of oxime-based bioconjugations by substituted anilines

The conjugation of biomolecules by chemoselective oxime ligation is of great interest for the site-specific modification of proteins, peptides, nucleic acids, and carbohydrates. These conjugations proceed optimally at a reaction pH of 4-5, but some biomolecules are not soluble or stable under these conditions. Aniline can be used as a nucleophilic catalyst to enhance the rate of oxime formation, but even in its presence, the reaction rate at neutral pH can be slower than desired, particularly at low reagent concentrations and/or temperature. Recently, alternative catalysts with improved properties were reported, including anthranilic acid derivatives for small molecule ligations, as well as m-phenylenediamine at high concentrations for protein conjugations. Here, we report that p-substituted anilines containing an electron-donating ring substituent are superior catalysts of oxime-based conjugations at pH 7. One such catalyst, p-phenylenediamine, was studied in greater detail. This catalyst was highly effective at neutral pH, even at the low concentration of 2 mM. In a model oxime ligation using aminooxy-functionalized PEG, catalysis at pH 7 resulted in a 120-fold faster rate of protein PEGylation as compared to an uncatalyzed reaction, and 19-fold faster than the equivalent aniline-catalyzed reaction. p-Phenylenediamine (10 mM) was also an effective catalyst under acidic conditions and was more efficient than aniline throughout the pH range 4-7. This catalyst allows efficient oxime bioconjugations to proceed under mild conditions and low micromolar concentrations, as demonstrated by the PEGylation of a small protein.

Sorafenib sensitizes solid tumors to Apo2L/TRAIL and Apo2L/TRAIL receptor agonist antibodies by the Jak2-Stat3-Mcl1 axis

Background

Approximately half of tumor cell lines are resistant to the tumor-selective apoptotic effects of tumor necrosis factor-related apoptosis-inducing ligand (Apo22L/TRAIL). Previously, we showed that combining Apo2L/TRAIL with sorafenib, a multikinase inhibitor, results in dramatic efficacy in Apo2L/TRAIL-resistant tumor xenografts via inhibition of Mcl-1. Soluble Apo2L/TRAIL is capable of binding to several surface receptors, including the pro-apoptotic death receptors, DR4 and DR5, and decoy receptors, DcR1 and DcR2. Monoclonal antibodies targeting either of these death receptors are being investigated as antitumor agents in clinical trials. We hypothesized that sorafenib and Apo2L/TRAIL or Apo2L/TRAIL death receptor agonist (TRA) antibodies against DR4 (mapatumumab) and DR5 (lexatumumab) will overcome resistance to Apo2L/TRAIL-mediated apoptosis and as increase antitumor efficacy in Apo2L/TRAIL-sensitive solid tumors.

Methodology/Principal Findings

We found that Apo2L/TRAIL or TRA antibodies combined with sorafenib synergistically reduce cell growth and increase cell death across a panel of solid tumor cell lines in vitro. This panel included human breast, prostate, colon, liver and thyroid cancers. The cooperativity of these combinations was also observed invivo, as measured by tumor volume and TUNEL staining as a measure of apoptosis. We found that sorafenib inhibits Jak/Stat3 signaling and downregulates their target genes, including cyclin D1, cyclin D2 and Mcl-1, in a dose-dependent manner.

Conclusions/Significance

The combination of sorafenib with Apo2L/TRAIL or Apo2L/TRAIL receptor agonist antibodies sensitizes Apo2L/TRAIL-resistant cells and increases the sensitivity of Apo2L/TRAIL-sensitive cells. Our findings demonstrate the involvement of the Jak2-Stat3-Mcl1 axis in response to sorafenib treatment, which may play a key role in sorafenib-mediated sensitization to Apo2L/TRAIL.

Fibronectin type III domains engineered to bind CD40L: cloning, expression, purification, crystallization and preliminary X-ray diffraction analysis of two complexes

Tn3 proteins are a novel class of binding molecules based on the third fibronectin type III domain of human tenascin C. Target-specific Tn3 proteins are selected from combinatorial libraries in which three surface-exposed loops have been diversified. Here, the cocrystallization of two different Tn3 proteins in complex with CD40L, a therapeutic target for immunological disease, is reported. These crystal structures are the first to be reported of Tn3 proteins and will help to reveal how these engineered molecules achieve specific recognition of a cognate target.