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

Targeted Cancer Therapy-on-A-Chip

Targeted cancer therapy (TCT) is gaining increased interest because it reduces the risks of adverse side effects by specifically treating tumor cells. TCT testing has traditionally been performed using two-dimensional (2D) cell culture and animal studies. Organ-on-a-chip (OoC) platforms have been developed to recapitulate cancer in vitro, as cancer-on-a-chip (CoC), and used for chemotherapeutics development and testing. This review explores the use of CoCs to both develop and test TCTs, with a focus on three main aspects, the use of CoCs to identify target biomarkers for TCT development, the use of CoCs to test free, un-encapsulated TCTs, and the use of CoCs to test encapsulated TCTs. Despite current challenges such as system scaling, and testing externally triggered TCTs, TCToC shows a promising future to serve as a supportive, pre-clinical platform to expedite TCT development and bench-to-bedside translation.

Broadening the Utility of Farnesyltransferase-Catalyzed Protein Labeling Using Norbornene-Tetrazine Click Chemistry

Bioorthogonal chemistry has gained widespread use in the study of many biological systems of interest, including protein prenylation. Prenylation is a post-translational modification, in which one or two 15- or 20-carbon isoprenoid chains are transferred onto cysteine residues near the C-terminus of a target protein. The three main enzymes─protein farnesyltransferase (FTase), geranylgeranyl transferase I (GGTase I), and geranylgeranyl transferase II (GGTase II)─that catalyze this process have been shown to tolerate numerous structural modifications in the isoprenoid substrate. This feature has previously been exploited to transfer an array of farnesyl diphosphate analogues with a range of functionalities, including an alkyne-containing analogue for copper-catalyzed bioconjugation reactions. Reported here is the synthesis of an analogue of the isoprenoid substrate embedded with norbornene functionality (C10NorOPP) that can be used for an array of applications, ranging from metabolic labeling to selective protein modification. The probe was synthesized in seven steps with an overall yield of 7% and underwent an inverse electron demand Diels-Alder (IEDDA) reaction with tetrazine-containing tags, allowing for copper-free labeling of proteins. The use of C10NorOPP for the study of prenylation was explored in the metabolic labeling of prenylated proteins in HeLa, COS-7, and astrocyte cells. Furthermore, in HeLa cells, these modified prenylated proteins were identified and quantified using label-free quantification (LFQ) proteomics with 25 enriched prenylated proteins. Additionally, the unique chemistry of C10NorOPP was utilized for the construction of a multiprotein-polymer conjugate for the targeted labeling of cancer cells. That construct was prepared using a combination of norbornene-tetrazine conjugation and azide-alkyne cycloaddition, highlighting the utility of the additional degree of orthogonality for the facile assembly of new protein conjugates with novel structures and functions.

Click Chemistry-Generated Auristatin F-Linker-Benzylguanine for a SNAP-Tag-Based Recombinant Antibody-Drug Conjugate Demonstrating Selective Cytotoxicity toward EGFR-Overexpressing Tumor Cells

Antibody-drug conjugates (ADCs) are bifunctional molecules combining the targeting potential of monoclonal antibodies with the cancer-killing ability of cytotoxic drugs. This simple yet intelligently designed system directly addresses the lack of specificity encountered with conventional anti-cancer treatment regimes. However, despite their initial success, the generation of clinically sustainable and effective ADCs has been plagued by poor tumor penetration, undefined chemical linkages, unpredictable pharmacokinetic profiles, and heterogeneous mixtures of products. To this end, we generated a SNAP-tag-based fusion protein targeting the epidermal growth factor receptor (EGFR)-a biomarker of aggressive and drug-resistant cancers. Here, we demonstrate the use of a novel click coupling strategy to engineer a benzylguanine (BG)-linker-auristatin F (AuriF) piece that can be covalently tethered to the EGFR-targeting SNAP-tag-based fusion protein in an irreversible 1:1 stoichiometric reaction to form a homogeneous product. Furthermore, using these recombinant ADCs to target EGFR-overexpressing tumor cells, we provide a proof-of-principle for generating biologically active antimitotic therapeutic proteins capable of inducing cell death in a dose-dependent manner, thus alleviating some of the challenges of early ADC development.

New Technologies Bloom Together for Bettering Cancer Drug Conjugates

Drug conjugates, including antibody-drug conjugates, are a step toward realizing Paul Ehrlich's idea from over 100 years ago of a "magic bullet" for cancer treatment. Through balancing selective targeting molecules with highly potent payloads, drug conjugates can target specific tumor microenvironments and kill tumor cells. A drug conjugate consists of three parts: a targeting agent, a linker, and a payload. In some conjugates, monoclonal antibodies act as the targeting agent, but new strategies for targeting include antibody derivatives, peptides, and even small molecules. Linkers are responsible for connecting the payload to the targeting agent. Payloads impact vital cellular processes to kill tumor cells. At present, there are 12 antibody-drug conjugates on the market for different types of cancers. Research on drug conjugates is increasing year by year to solve problems encountered in conjugate design, such as tumor heterogeneity, poor circulation, low drug loading, low tumor uptake, and heterogenous expression of target antigens. This review highlights some important preclinical research on drug conjugates in recent years. We focus on three significant areas: improvement of antibody-drug conjugates, identification of new conjugate targets, and development of new types of drug conjugates, including nanotechnology. We close by highlighting the critical barriers to clinical translation and the open questions going forward. SIGNIFICANCE STATEMENT: The development of anticancer drug conjugates is now focused in three broad areas: improvements to existing antibody drug conjugates, identification of new targets, and development of new conjugate forms. This article focuses on the exciting preclinical studies in these three areas and advances in the technology that improves preclinical development.

Dual intra- and extracellular release of monomethyl auristatin E from a neutrophil elastase-sensitive antibody-drug conjugate

Antibody-drug conjugates (ADCs) are targeted therapies, mainly used in oncology, consisting in a three-component molecular architecture combining a highly potent drug conjugated via a linker onto a monoclonal antibody (mAb), designed for the selective delivery of the drug to the tumor site. The linker is a key component, defining the ADC stability and mechanism of action, and particularly the drug release strategy. In this study, we have developed and synthesized a cleavable linker, which possesses an Asn-Pro-Val (NPV) sequence sensitive to the human neutrophil elastase (HNE), overexpressed in the tumor microenvironment. This linker permitted the site-specific conjugation of the cell-permeable drug, monomethyl auristatin E (MMAE), onto trastuzumab, using a disulfide re-bridging technology. The resulting ADC was then evaluated in vitro. This conjugate demonstrated retained antigen (Ag) binding affinity and exhibited high subnanomolar potency against Ag-positive tumor cells after internalization, suggesting an intracellular mechanism of linker cleavage. While no internalization and cytotoxic activity of this ADC was observed on Ag-negative cells in classical conditions, the supplementation of exogenous HNE permitted to restore a nanomolar activity on these cells, suggesting an extracellular mechanism of drug release in these conditions. This in vitro proof of concept tends to prove that the NPV sequence could allow a dual intra- and extracellular mechanism of drug release. This work represents a first step in the design of original ADCs with a new dual intra- and extracellular drug delivery system and opens the way to further experimentations to evaluate their full potential in vivo.

Biomacromolecule-Functionalized Nanoparticle-Based Conjugates for Potentiation of Anticancer Therapy

Cancer is a rapidly growing life-threatening disease that affected 18.1 million people worldwide in 2018. Various conventional techniques like surgery, radiation, and chemotherapy are considered as a mainstream treatment for patients but show some limitations like cytotoxicity due to off-targeted action, poor intra-tumor localization, development of multi-drug resistance by tumor cells, physical and psychological stresses, etc. Such limitations have motivated the scientists to work towards more patient-centric and precision therapy using advanced drug delivery systems like liposomes, nanoparticles, nanoconjugates, etc. However, these carriers also face limitations like poor biocompatibility, lesser payload capacity, leakage of encapsulated drug, and short-term stability. So, this review article explores the profound insights for the development of biomacromolecule-functionalized nanoconjugates to potentiate the anticancer activity of therapeutic agents for various cancers like lung, colorectal, ovarian, breast and liver cancer. Researchers have shown interest in biofunctionalized nanoconjugates because of advantages like biocompatibility, site-specificity with better localization, higher entrapment with long-term stability and lesser off-target toxicity. The progressive trend of biomacromolecule nanoconjugates will encourage further research for the development of effective transport of drugs, nutraceuticals and phytoconstituents for on-site effect at cancer microenvironment and tumor cells with higher safety profile.

Importance and Considerations of Antibody Engineering in Antibody-Drug Conjugates Development from a Clinical Pharmacologist's Perspective

Antibody-drug conjugates (ADCs) appear to be in a developmental boom, with five FDA approvals in the last two years and a projected market value of over $4 billion by 2024. Major advancements in the engineering of these novel cytotoxic drug carriers have provided a few early success stories. Although the use of these immunoconjugate agents are still in their infancy, valuable lessons in the engineering of these agents have been learned from both preclinical and clinical failures. It is essential to appreciate how the various mechanisms used to engineer changes in ADCs can alter the complex pharmacology of these agents and allow the ADCs to navigate the modern-day therapeutic challenges within oncology. This review provides a global overview of ADC characteristics which can be engineered to alter the interaction with the immune system, pharmacokinetic and pharmacodynamic profiles, and therapeutic index of ADCs. In addition, this review will highlight some of the engineering approaches being explored in the creation of the next generation of ADCs.

Antibody-drug conjugates for the treatment of lymphoma: clinical advances and latest progress

Antibody-drug conjugates (ADCs) are a promising class of immunotherapies with the potential to specifically target tumor cells and ameliorate the therapeutic index of cytotoxic drugs. ADCs comprise monoclonal antibodies, cytotoxic payloads with inherent antitumor activity, and specialized linkers connecting the two. In recent years, three ADCs, brentuximab vedotin, polatuzumab vedotin, and loncastuximab tesirine, have been approved and are already establishing their place in lymphoma treatment. As the efficacy and safety of ADCs have moved in synchrony with advances in their design, a plethora of novel ADCs have garnered growing interest as treatments. In this review, we provide an overview of the essential elements of ADC strategies in lymphoma and elucidate the up-to-date progress, current challenges, and novel targets of ADCs in this rapidly evolving field.

Targeting PD-1 in CD8+ T Cells with a Biomimetic Bilirubin-5-fluoro-2-deoxyuridine-Bovine Serum Albumin Nanoconstruct for Effective Chemotherapy against Experimental Lymphoma

We fabricated bilirubin-bovine serum albumin (BR-BSA) nanocomplexes as candidates for the delivery of 5-fluoro-2-deoxyuridine (5FUdr) against experimental murine lymphoma. BR was attached to 5FUdr via acid-labile ester bonds mimicking small-molecule drug conjugates. The construct was self-assembled with BSA through strong noncovalent interactions with high drug occupancy in the core and labeled with folic acid (FA) to target cancer cells. The BR-5FUdr-BSA-FA nanoconstruct exhibits excellent biocompatibility, prevents nephrotoxicity, and is tolerated by red blood cells and mononuclear cells. The construct also showed increased accumulation in lymph nodes and tumor cells. BR-5FUdr-BSA-FA caused prolonged growth inhibition and apoptosis, enhanced mitochondrial reactive oxygen species generation, and minimized the viability of parental and doxorubicin-resistant Dalton's lymphoma cells. Treatment of tumor-bearing mice with BR-5FUdr-BSA-FA significantly increased the life span of the animals, improved their histopathological parameters, and downregulated PD-1 expression, suggesting the potential of the construct for 5FUdr delivery to treat lymphoma.

Thrombo-tag, an in vivo formed nanotracer for the detection of thrombi in mice by fast pre-targeted molecular imaging

Radioisotope-labelled nanoparticles permit novel applications in molecular imaging, while recent developments in imaging have enabled direct visualization of biological processes. While this holds true for pathological processes that are stable in time, such as cancer, imaging approaches are limited for phenomena that take place in the range of minutes, such as thrombotic events. Here, we take advantage of bioorthogonal chemistry to demonstrate the concept of nanoparticle-based fast pre-targeted imaging. Using a newly designed nanoparticle that targets platelets we show the applicability of this approach developing thrombo-tag, an in vivo produced nanoparticle that labels thrombi. We show that thrombo-tag allows specific labelling of platelets that accumulate in the injured pulmonary vasculature, or that aggregate in brains of mice suffering thrombotic processes. The fast kinetics and high specificity features of thrombo-tag may critically expand the application of molecular imaging to the most prevalent and debilitating diseases in the clinics.

Recent Advances in Peptide-Based Approaches for Cancer Treatment

BACKGROUND

Peptide-based pharmaceuticals have recently experienced a renaissance due to their ability to fill the gap between the two main classes of available drugs, small molecules and biologics. Peptides combine the high potency and selectivity typical of large proteins with some of the characteristic advantages of small molecules such as synthetic accessibility, stability and the potential of oral bioavailability.

METHODS

In the present manuscript we review the recent literature on selected peptide-based approaches for cancer treatment, emphasizing recent advances, advantages and challenges of each strategy.

RESULTS

One of the applications in which peptide-based approaches have grown rapidly is cancer therapy, with a focus on new and established targets. We describe, with selected examples, some of the novel peptide-based methods for cancer treatment that have been developed in the last few years, ranging from naturally-occurring and modified peptides to peptidedrug conjugates, peptide nanomaterials and peptide-based vaccines.

CONCLUSION

This review brings out the emerging role of peptide-based strategies in oncology research, critically analyzing the advantages and limitations of these approaches and the potential for their development as effective anti-cancer therapies.

Nanoparticle Activation Methods in Cancer Treatment

In this review, we intend to highlight the progress which has been made in recent years around different types of smart activation nanosystems for cancer treatment. Conventional treatment methods, such as chemotherapy or radiotherapy, suffer from a lack of specific targeting and consequent off-target effects. This has led to the development of smart nanosystems which can effect specific regional and temporal activation. In this review, we will discuss the different methodologies which have been designed to permit activation at the tumour site. These can be divided into mechanisms which take advantage of the differences between healthy cells and cancer cells to trigger activation, and those which activate by a mechanism extrinsic to the cell or tumour environment.

Multifunctional Scaffolds for Assembling Cancer-Targeting Immune Stimulators Using Chemoselective Ligations

Chemoselective ligations allow chemical biologists to functionalise proteins and peptides for biomedical applications and to probe biological processes. Coupled with solid phase peptide synthesis, chemoselective ligations enable not only the design of homogeneous proteins and peptides with desired natural and unnatural modifications in site-specific locations but also the design of new peptide and protein topologies. Although several well-established ligations are available, each method has its own advantages and disadvantages and they are seldom used in combination. Here we have applied copper-catalyzed azide-alkyne “click,” oxime, maleimide, and native chemical ligations to develop a modular synthesis of branched peptide and polymer constructs that act as cancer-targeting immune system engagers (ISErs) and functionalised them for detection in biological systems. We also note some potential advantages and pitfalls of these chemoselective ligations to consider when designing orthogonal ligation strategies. The modular synthesis and functionalization of ISErs facilitates optimisation of their activity and mechanism of action as potential cancer immunotherapies.

Elimination of factor VIII-specific B cells by immunotoxins composed of a single factor VIII domain fused to Pseudomonas exotoxin A

Essentials There is still a need for novel therapeutic approaches for hemophilia A patients with inhibitors. A factor VIII domain was used as the targeting moiety for elimination of FVIII-specific B cells. The immunodominant C2 domain was fused to exotoxin A from Pseudomonas aeruginosa (hC2-ETA). Murine C2 domain-specific B cells were selectively and efficiently eliminated by hC2-ETA ex vivo. SUMMARY: Background Today, the most serious complication for patients with hemophilia A undergoing factor VIII (FVIII) replacement therapy is the development of neutralizing antibodies (inhibitors). Although inhibitors can be eradicated by application of high doses of FVIII, the immune tolerance induction therapy fails in up to 30% of patients. Hence, there is still an urgent need for novel therapeutic approaches for patients with persisting inhibitors. Objectives In the present study, the potential use of immunotoxins containing exotoxin A (ETA) from Pseudomonas aeruginosa for selective elimination of FVIII-specific B cells was explored. Methods The immunodominant C2 domain of human FVIII was used as a targeting moiety instead of the full-length FVIII protein and the resulting human C2 domain-ETA fusion protein (hC2-ETA) was produced in Escherichia coli. Results Binding studies with monoclonal C2 domain-specific antibodies confirmed the conformational integrity of the C2 domain in hC2-ETA. The functionality of hC2-ETA was tested ex vivo by incubation of splenocytes from inhibitor-positive FVIII knockout mice with hC2-ETA and controls. FVIII-specific memory B cells from splenocytes were differentiated by FVIII stimulation in antibody-secreting cells (ASC) and detected by an enzyme-linked immunospot assay. Although the controls showed no effect, incubation of splenocytes with hC2-ETA reduced the number of C2-specific ASC in a dose-dependent fashion, indicating specific and efficient elimination of C2-specific memory B cells. Conclusions Overall, the results of the study support the fact that FVIII domain immunotoxins might be a potential new tool for the elimination of FVIII-specific B cells in patients with hemophilia A and persisting inhibitors.

Improvement and extension of anti-EGFR targeting in breast cancer therapy by integration with the Avidin-Nucleic-Acid-Nano-Assemblies

Nowadays, personalized cancer therapy relies on small molecules, monoclonal antibodies, or antibody–drug conjugates (ADC). Many nanoparticle (NP)-based drug delivery systems are also actively investigated, but their advantage over ADCs has not been demonstrated yet. Here, using the Avidin-Nucleic-Acid-Nano-Assemblies (ANANAS), a class of polyavidins multifuctionalizable with stoichiometric control, we compare quantitatively anti-EGFR antibody(cetuximab)-targeted NPs to the corresponding ADC. We show that ANANAS tethering of cetuximab promotes a more efficient EGFR-dependent vesicle-mediated internalization. Cetuximab-guided ANANAS carrying doxorubicin are more cytotoxic in vitro and much more potent in vivo than the corresponding ADC, leading to 43% tumor reduction at low drug dosage (0.56 mg/kg). Advantage of cetuximab-guided ANANAS with respect to the ADC goes beyond the increase in drug-to-antibody ratio. Even if further studies are needed, we propose that NP tethering could expand application of the anti-EGFR antibody to a wider number of cancer patients including the KRAS-mutated ones, currently suffering from poor prognosis.

The nature of the linker is known to affect the efficacy of antibody–drug conjugate (ADC). Here the authors show cetuximab-guided Avidin-Nucleic-Acid-Nanoassemblies to be superior to cetuximab-doxorubicin conjugate, and show its efficacy in KRAS mutant breast cancer, allowing for therapeutic expansion of anti-EGFR 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.

Influence of protein properties and protein modification on biodistribution and tumor uptake of anticancer antibodies, antibody derivatives, and non-Ig scaffolds

Newly developed protein drugs that target tumor-associated antigens are often modified in order to increase their therapeutic effect, tumor exposure, and safety profile. During the development of protein drugs, molecular imaging is increasingly used to provide additional information on their in vivo behavior. As a result, there are increasing numbers of studies that demonstrate the effect of protein modification on whole body distribution and tumor uptake of protein drugs. However, much still remains unclear about how to interpret obtained biodistribution data correctly. Consequently, there is a need for more insight in the correct way of interpreting preclinical and clinical imaging data. Summarizing the knowledge gained to date may facilitate this interpretation. This review therefore provides an overview of specific protein properties and modifications that can affect biodistribution and tumor uptake of anticancer antibodies, antibody fragments, and nonimmunoglobulin scaffolds. Protein properties that are discussed in this review are molecular size, target interaction, FcRn binding, and charge. Protein modifications that are discussed are radiolabeling, fluorescent labeling drug conjugation, glycosylation, humanization, albumin binding, and polyethylene glycolation.

The stereodynamics of macrocyclic succinimide-thioethers

Maleimide-thiol coupling is a popular bioconjugation strategy, but little is known about the stereoselectivity and the stereodynamics of the succinimide thioether formed in a biopolymer environment. We used thiol 1,4-addition for the macrocyclisation of 5 designed pentapeptides with the ringsize of hexapeptides because they incorporate the succinimide thioether (4-8). Both succinimide diastereomers are observed in the constrained macrocyclic rings in each case. In spite of the low diastereoselectivity of the macrocyclisation reaction, there is a significant influence of the amino acid environment on the epimerization rate of the succinimide. Its half life can be as short as several hours at room temperature when Gly is the amino acid following the succinimide (peptide 8). On the contrary, no epimerization is detectable even after several weeks in the case of d-Phe C-terminal to the succinimide in peptide 4. Already the small selection of examples shows how big the differences in epimerization rates can be and that the local environment has a significant influence. The variation of amino acids in the vicinity of the ligation site points the way towards the synthesis of bioconjugates which are obtained as stable and separable diastereomers.

Site-Specifically Labeled Antibody-Drug Conjugate for Simultaneous Therapy and ImmunoPET

The conjugation of antibodies with cytotoxic drugs can alter their in vivo pharmacokinetics. As a result, the careful assessment of the in vivo behavior, and specifically the tumor-targeting properties, of antibody-drug conjugates represents a crucial step in their development. In order to facilitate this process, we have created a methodology that facilitates the dual labeling of an antibody with both a toxin and a radionuclide for positron emission tomography (PET). To minimize the impact of these modifications, this chemoenzymatic approach leverages strain-promoted azide-alkyne click chemistry to graft both cargoes to the heavy chain glycans of the immuoglobulin's F_c domain. As a proof-of-concept, a HER2-targeting trastuzumab immunoconjugate was created bearing both a monomethyl auristatin E (MMAE) toxin as well as the long-lived positron-emitting radiometal ⁸⁹Zr ( t_1/2 ≈ 3.3 days). Both the tumor targeting and therapeutic efficacy of the ⁸⁹Zr-trastuzumab-MMAE immunoconjugate were validated in vivo using a murine model of HER2-expressing breast cancer. The site-specifically dual-labeled construct enabled the clear visualization of tumor tissue via PET imaging, producing tumoral uptake of ∼70%ID/g. Furthermore, a longitudinal therapy study revealed that the immunoconjugate exerts significant antitumor activity, leading to a >90% reduction in tumor volume over the course of 20 days.

89Zr-Immuno-Positron Emission Tomography in Oncology: State-of-the-Art 89Zr Radiochemistry

Immuno-positron emission tomography (immunoPET) with ⁸⁹Zr-labeled antibodies has shown great potential in cancer imaging. It can provide important information about the pharmacokinetics and tumor-targeting properties of monoclonal antibodies and may help in anticipating on toxicity. Furthermore, it allows accurate dose planning for individualized radioimmunotherapy and may aid in patient selection and early-response monitoring for targeted therapies. The most commonly used chelator for ⁸⁹Zr is desferrioxamine (DFO). Preclinical studies have shown that DFO is not an ideal chelator because the ⁸⁹Zr–DFO complex is partly unstable in vivo, which results in the release of ⁸⁹Zr from the chelator and the subsequent accumulation of ⁸⁹Zr in bone. This bone accumulation interferes with accurate interpretation and quantification of bone uptake on PET images. Therefore, there is a need for novel chelators that allow more stable complexation of ⁸⁹Zr. In this Review, we will describe the most recent developments in ⁸⁹Zr radiochemistry, including novel chelators and site-specific conjugation methods.

Multifunctional αvβ6 Integrin-Specific Peptide-Pt(IV) Conjugates for Cancer Cell Targeting

Increasing the specificity of cancer therapy, and thereby decreasing damage to normal cells, requires targeting to cancer-cell specific features. The α_vβ₆ integrin is a receptor involved in cell adhesion and is frequently up-regulated in cancer cells compared to normal cells. We have selected a peptide ligand reported to bind specifically to the β₆ integrin and have synthesized a suite of multispecific molecules to explore the potential for targeting of cancer cells. A combination of solid-phase peptide synthesis and chemoselective ligations was used to synthesize multifunctional molecules composed of integrin-targeting peptides, cytotoxic platinum(IV) prodrugs, and fluorescent or affinity probes joined with flexible linkers. The modular synthesis approach facilitates the construction of peptide-drug conjugates with various valencies and properties in a convergent manner. The binding and specificity of the multifunctional peptide conjugates were investigated using a cell line transfected with the β₆ integrin and fluorescence microscopy. This versatile and highly controlled approach to synthesizing labeled peptide-drug conjugates has the potential to target potent cytotoxic drugs specifically to cancer cells, reducing the doses required for effective treatment.

Strategies and challenges for the next generation of antibody-drug conjugates

Antibody-drug conjugates (ADCs) are one of the fastest growing classes of oncology therapeutics. After half a century of research, the approvals of brentuximab vedotin (in 2011) and trastuzumab emtansine (in 2013) have paved the way for ongoing clinical trials that are evaluating more than 60 further ADC candidates. The limited success of first-generation ADCs (developed in the early 2000s) informed strategies to bring second-generation ADCs to the market, which have higher levels of cytotoxic drug conjugation, lower levels of naked antibodies and more-stable linkers between the drug and the antibody. Furthermore, lessons learned during the past decade are now being used in the development of third-generation ADCs. In this Review, we discuss strategies to select the best target antigens as well as suitable cytotoxic drugs; the design of optimized linkers; the discovery of bioorthogonal conjugation chemistries; and toxicity issues. The selection and engineering of antibodies for site-specific drug conjugation, which will result in higher homogeneity and increased stability, as well as the quest for new conjugation chemistries and mechanisms of action, are priorities in ADC research.

Design, synthesis and evaluation of anti-CD123 antibody drug conjugates

Leukemia stem cells (LSCs) account for the development of drug resistance and increased recurrence rate in acute myeloid leukemia (AML) patients. Targeted drug delivery to leukemia stem cells remains a major challenge in AML chemotherapy. Overexpressed interleukin-3 receptor alpha chain, CD123, on the surface of leukemia stem cells was reported to be a potential target in AML treatment. Here, we designed and developed an antibody drug conjugate (CD123-CPT) by integrating anti-CD123 antibody with a chemotherapeutic agent, Camptothecin (CPT), via a disulfide linker. The linker is biodegradable in the presence of Glutathione (GSH, an endogenous component in cells), which leads to release of CPT. Anti-CD123 antibody conjugates showed significant higher cellular uptake in CD123-overexpressed tumor cells. More importantly, CD123-CPT demonstrated potent inhibitory effects on CD123-overexpressed tumor cells. Consequently, these results provide a promising targeted chemotherapeutical strategy for AML treatment.

Site-Directed Conjugation of Antibodies to Apoferritin Nanocarrier for Targeted Drug Delivery to Prostate Cancer Cells

Herein, we describe a novel approach for targeting of ubiquitous protein apoferritin (APO)-encapsulating doxorubicin (DOX) to prostate cancer using antibodies against prostate-specific membrane antigen (PSMA). The conjugation of anti-PSMA antibodies and APO was carried out using HWRGWVC heptapeptide, providing their site-directed orientation. The prostate-cancer-targeted and nontargeted nanocarriers were tested using LNCaP and HUVEC cell lines. A total of 90% of LNCaP cells died after treatment with DOX (0.25 μM) or DOX in nontargeted and prostate-cancer-targeted APO, proving that the encapsulated DOX toxicity for LNCaP cells remained the same. Free DOX showed higher toxicity for nonmalignant cells, whereas the toxicity was lower after treatment with the same dosage of APO-encapsulated DOX (APODOX) and even more in prostate-cancer-targeted APODOX. Hemolytic assay revealed exceptional hemocompatibility of the entire nanocarrier. The APO encapsulation mechanism ensures applicability using a wide variety of chemotherapeutic drugs, and the presented surface modification enables targeting to various tumors.

A Novel Recombinant Anti-CD22 Immunokinase Delivers Proapoptotic Activity of Death-Associated Protein Kinase (DAPK) and Mediates Cytotoxicity in Neoplastic B Cells

The serine/threonine death-associated protein kinases (DAPK) provide pro-death signals in response to (oncogenic) cellular stresses. Lost DAPK expression due to (epi)genetic silencing is found in a broad spectrum of cancers. Within B-cell lymphomas, deficiency of the prototypic family member DAPK1 represents a predisposing or early tumorigenic lesion and high-frequency promoter methylation marks more aggressive diseases. On the basis of protein studies and meta-analyzed gene expression profiling data, we show here that within the low-level context of B-lymphocytic DAPK, particularly CLL cells have lost DAPK1 expression. To target this potential vulnerability, we conceptualized B-cell-specific cytotoxic reconstitution of the DAPK1 tumor suppressor in the format of an immunokinase. After rounds of selections for its most potent cytolytic moiety and optimal ligand part, a DK1KD-SGIII fusion protein containing a constitutive DAPK1 mutant, DK1KD, linked to the scFv SGIII against the B-cell-exclusive endocytic glyco-receptor CD22 was created. Its high purity and large-scale recombinant production provided a stable, selectively binding, and efficiently internalizing construct with preserved robust catalytic activity. DK1KD-SGIII specifically and efficiently killed CD22-positive cells of lymphoma lines and primary CLL samples, sparing healthy donor- or CLL patient-derived non-B cells. The mode of cell death was predominantly PARP-mediated and caspase-dependent conventional apoptosis as well as triggering of an autophagic program. The notoriously high apoptotic threshold of CLL could be overcome by DK1KD-SGIII in vitro also in cases with poor prognostic features, such as therapy resistance. The manufacturing feasibility of the novel CD22-targeting DAPK immunokinase and its selective antileukemic efficiency encourage intensified studies towards specific clinical application. Mol Cancer Ther; 15(5); 971-84. ©2016 AACR.

Cutting-edge mass spectrometry methods for the multi-level structural characterization of antibody-drug conjugates

Antibody drug conjugates (ADCs) are highly cytotoxic drugs covalently attached via conditionally stable linkers to monoclonal antibodies (mAbs) and are among the most promising next-generation empowered biologics for cancer treatment. ADCs are more complex than naked mAbs, as the heterogeneity of the conjugates adds to the inherent microvariability of the biomolecules. The development and optimization of ADCs rely on improving their analytical and bioanalytical characterization by assessing several critical quality attributes, namely the distribution and position of the drug, the amount of naked antibody, the average drug to antibody ratio, and the residual drug-linker and related product proportions. Here brentuximab vedotin (Adcetris) and trastuzumab emtansine (Kadcyla), the first and gold-standard hinge-cysteine and lysine drug conjugates, respectively, were chosen to develop new mass spectrometry (MS) methods and to improve multiple-level structural assessment protocols.

Bioconjugation – using selective chemistry to enhance the properties of proteins and peptides as therapeutics and carriers

Both peptide and protein therapeutics are becoming increasingly important for treating a wide range of diseases. Functionalisation of theseviasite-selective chemical modification leads to enhancement of their therapeutic properties.

Field Guide to Challenges and Opportunities in Antibody-Drug Conjugates for Chemists

Antibody-drug conjugates have attracted a great amount of attention as a therapeutic strategy for diseases where targeting specific tissues and cells are critical components, such as in cancer therapy. Although promising, the number of approved ADC drugs is relatively limited. This emanates from the challenges associated with generating the conjugates and the complexities associated with the stability requirements for these conjugates during circulation and after reaching the target. Here, we provide a comprehensive overview of the design challenges facing the ADC field. These challenges also provide several unique research and development opportunities, which are also highlighted throughout the review.

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.