Self-hydrolyzing maleimides improve the stability and pharmacological properties of antibody-drug conjugates

The renaissance of chemically generated bispecific antibodies

Bispecific antibodies (bsAbs) target two different epitopes. These are an up-and-coming class of biologics, with two such therapeutics (emicizumab and blinatumomab) FDA approved and on the market, and many more in clinical trials. While the first reported bsAbs were constructed by chemical methods, this approach has fallen out of favour with the advent of modern genetic engineering techniques and, nowadays, the vast majority of bsAbs are produced by protein engineering. However, in recent years, relying on innovations in the fields of bioconjugation and bioorthogonal click chemistry, new chemical methods have appeared that have the potential to be competitive with protein engineering techniques and, indeed, hold some advantages. These approaches offer modularity, reproducibility and batch-to-batch consistency, as well as the integration of handles, whereby additional cargo molecules can be attached easily, e.g. to generate bispecific antibody-drug conjugates. The linker between the antibodies/antibody fragments can also be easily varied, and new formats (types, defined by structural properties or by construction methodology) can be generated rapidly. These attributes offer the potential to revolutionize the field. Here, we review chemical methods for the generation of bsAbs, showing that the newest examples of these techniques are worthy competitors to the industry-standard expression-based strategies.

Homogeneous antibody-drug conjugates: DAR 2 anti-HER2 obtained by conjugation on isolated light chain followed by mAb assembly

Despite advances in medical care, cancer remains a major threat to human health. Antibody-drug conjugates (ADCs) are a promising targeted therapy to overcome adverse side effects to normal tissues. In this field, the current challenge is obtaining homogeneous preparations of conjugates, where a defined number of drugs are conjugated to specific antibody sites. Site-directed cysteine-based conjugation is commonly used to obtain homogeneous ADC, but it is a time-consuming and expensive approach due to the need for extensive antibody engineering to identify the optimal conjugation sites and reduction – oxidation protocols are specific for each antibody. There is thus a need for ADC platforms that offer homogeneity and direct applicability to the already approved antibody therapeutics. Here we describe a novel approach to derive homogeneous ADCs with drug-to-antibody ratio of 2 from any human immunoglobulin 1 (IgG₁), using trastuzumab as a model. The method is based on the production of heavy chains (HC) and light chains (LC) in two recombinant HEK293 independent cultures, so the original amino acid sequence is not altered. Isolated LC was effectively conjugated to a single drug-linker (vcMMAE) construct and mixed to isolated HC dimers, in order to obtain a correctly folded ADC. The relevance of the work was validated in terms of ADC homogeneity (HIC-HPLC, MS), purity (SEC-HPLC), isolated antigen recognition (ELISA) and biological activity (HER2-positive breast cancer cells cytotoxicity assays).

Bridged Cysteine Conjugations

Preparation of antibody-drug conjugates (ADCs) with a highly homogeneous drug loading in general requires site-selective conjugation of a cytotoxic payload. Typically, functionality utilized for attachment of the payload is achieved through engineering of suitable chemical handles or by enzymatic modification of the antibody. Relatively few methods to produce ADCs with homogeneous drug loading via endogenous amino acid conjugation have been developed. Herein we describe a robust method for the conjugation of antibodies using a cysteine rebridging approach to produce ADCs with highly homogeneous drug-to-antibody ratios (DAR) at the native interchain disulfides, called ThioBridge^®. The process described relies upon an elegant cascade of addition-elimination reactions carried out under mild aqueous conditions that can be readily applied to wild-type antibodies without the need for prior modification via recombinant or enzymatic means. Using this method, conversions to a conserved DAR ADC are typically in the range of 70-95% and overall process yields of >70% are readily achieved.

A Facile Procedure for One-Pot Stable Conjugation of Two Proglucagon Cysteine-Containing Peptide Analogs

Optimization of peptides for therapeutic purposes often includes chemical conjugation or modification with substituents that serve to broaden pharmacology or improve pharmacokinetics. We report a convenient and rapid procedure for one-pot, site-specific conjugation of two cysteine-containing peptides that utilizes a bivalent linker comprising maleimide and iodoacetyl functional groups. Following maleimide-mediated peptide conjugation the linker was converted from an unstable thiosuccinimide to a stable thioether bond suitable for biological study by mild aqueous hydrolysis. The procedure is exemplified by peptide-peptide, peptide-small molecule, and peptide-fatty acid conjugations. The method provides a facile approach to search for enhanced biological outcomes through additive and sustained peptide pharmacology unencumbered by the prospect of chemical rearrangement in the course of biological study.

Stimulus-cleavable chemistry in the field of controlled drug delivery

This review comprehensively summarises stimulus-cleavable linkers from various research areas and their cleavage mechanisms, thus provides an insightful guideline to extend their potential applications to controlled drug release from nanomaterials.

Peptide-Drug Conjugates with Different Linkers for Cancer Therapy

Drug conjugates are chemotherapeutic or cytotoxic agents covalently linked to targeting ligands such as an antibody or a peptide via a linker. While antibody-drug conjugates (ADCs) are now clinically established for cancer therapy, peptide-drug conjugates (PDCs) are gaining recognition as a new modality for targeted drug delivery with improved efficacy and reduced side effects for cancer treatment. The linker in a drug conjugate plays a key role in the circulation time of the conjugate and release of the drug for full activity at the target site. Herein, we highlight the main linker chemistries utilized in the design of PDCs and discuss representative examples of PDCs with different linker chemistries with the related outcome in cell and animal studies.

Evaluation of PNU-159682 antibody drug conjugates (ADCs)

PNU-159682 is a highly potent secondary metabolite of nemorubicin belonging to the anthracycline class of natural products. Due to its extremely high potency and only partially understood mechanism of action, it was deemed an interesting starting point for the development of a new suite of linker drugs for antibody drug conjugates (ADCs). Structure activity relationships were explored on the small molecule which led to six linker drugs being developed for conjugation to antibodies. Herein we describe the synthesis of novel PNU-159682 derivatives and the subsequent linker drugs as well as the corresponding biological evaluations of the small molecules and ADCs.

An optimal "Click" formulation strategy for antibody-drug conjugate synthesis

As a versatile reaction for bioconjugation, Cu(I)-catalyzed alkyne-azide cycloaddition (CuAAC) has enormous potential in the synthesis of antibody-drug conjugates (ADCs). In order to optimize CuAAC-based ADC synthesis, we characterized kinetically different formulation processes by mimicking ADC synthesis using small molecules and subsequently revealed unique kinetic behaviors of different combinations of alkyne and azide conditions. Our results indicate that under ADC synthesis conditions, for an alkyne-containing drug, its concentration has minimal impact on the reaction rate when an antibody has a non-metal-chelating azide but is proportional to concentration when an antibody contains a metal-chelating azide; however, for an alkyne-containing antibody, the ADC synthesis rate is proportional to the concentration of a drug with a non-metal-chelating azide but displays almost no dependence on drug concentration with a metal-chelating azide. Based on our results, we designed and tested an optimal "click" formulation strategy that allowed rapid and cost-effective synthesis of a new ADC.

Improving the Stability of Maleimide-Thiol Conjugation for Drug Targeting

Maleimides are essential compounds for drug conjugation reactions via thiols to antibodies, peptides and other targeting units. However, one main drawback is the occurrence of thiol exchange reactions with, for example, glutathione resulting in loss of the targeting ability. A new strategy to overcome such retro-Michael exchange processes of maleimide-thiol conjugates by stabilization of the thiosuccinimide via a transcyclization reaction is presented. This reaction enables the straightforward synthesis of stable maleimide-thiol adducts essential in drug-conjugation applications.

Engineering precision nanoparticles for drug delivery

In recent years, the development of nanoparticles has expanded into a broad range of clinical applications. Nanoparticles have been developed to overcome the limitations of free therapeutics and navigate biological barriers — systemic, microenvironmental and cellular — that are heterogeneous across patient populations and diseases. Overcoming this patient heterogeneity has also been accomplished through precision therapeutics, in which personalized interventions have enhanced therapeutic efficacy. However, nanoparticle development continues to focus on optimizing delivery platforms with a one-size-fits-all solution. As lipid-based, polymeric and inorganic nanoparticles are engineered in increasingly specified ways, they can begin to be optimized for drug delivery in a more personalized manner, entering the era of precision medicine. In this Review, we discuss advanced nanoparticle designs utilized in both non-personalized and precision applications that could be applied to improve precision therapies. We focus on advances in nanoparticle design that overcome heterogeneous barriers to delivery, arguing that intelligent nanoparticle design can improve efficacy in general delivery applications while enabling tailored designs for precision applications, thereby ultimately improving patient outcome overall.

Advances in nanoparticle design could make substantial contributions to personalized and non-personalized medicine. In this Review, Langer, Mitchell, Peppas and colleagues discuss advances in nanoparticle design that overcome heterogeneous barriers to delivery, as well as the challenges in translating these design improvements into personalized medicine approaches.

Development of Novel Antibody-Camptothecin Conjugates

We have developed a highly active and well-tolerated camptothecin (CPT) drug-linker designed for antibody-mediated drug delivery in which the lead molecule consists of a 7-aminomethyl-10,11-methylenedioxy CPT (CPT1) derivative payload attached to a novel hydrophilic protease-cleavable valine-lysine-glycine tripeptide linker. A defined polyethylene glycol stretcher was included to improve the properties of the drug-linker, facilitating high antibody-drug conjugate (ADC) drug loading, while reducing the propensity for aggregation. A CPT1 ADC with 8 drug-linkers/mAb displayed a pharmacokinetic profile coincident with parental unconjugated antibody and had high serum stability. The ADCs were broadly active against cancer cells in vitro and in mouse xenograft models, giving tumor regressions and complete responses at low (≤3 mg/kg, single administration) doses. Pronounced activities were obtained in both solid and hematologic tumor models and in models of bystander killing activity and multidrug resistance. Payload release studies demonstrated that two CPTs, CPT1 and the corresponding glycine analog (CPT2), were released from a cAC10 ADC by tumor cells. An ADC containing this drug-linker was well tolerated in rats at 60 mg/kg, given weekly four times. Thus, ADCs comprised of this valine-lysine-glycine linker with CPT drug payloads have promise in targeted drug delivery.

Antibody-drug conjugates as targeted therapies: Are we there yet? A critical review of the current clinical landscape

Antibody-drug conjugates (ADCs) are targeted therapies with the expectation of broadened therapeutic window due to tumor-specific drug delivery. Recent approvals, including ADCs with a novel payload class, topoisomerase-1 inhibitors, generated renewed excitement in the field. We provide a critical review of approved and late-stage molecules, discuss strategies in solid tumors and ADCs outside oncology. Our pharmacokinetics-based assessment of targeting suggests that ADCs, especially in solid tumors, rely on additional mechanisms for efficacy including slow-release of the payload to the circulation at potentially efficacious levels. Further adjustments in the technology are needed to fulfill the promise of true targeted drug delivery.

Cisplatin Prodrug-Loaded Nanoparticles Based on Physalis Mottle Virus for Cancer Therapy

Nanoparticle-based prodrugs offer an effective strategy to improve the safety and delivery of small-molecule therapeutics while reducing the risk of drug resistance. Here, we conjugated a maleimide-functionalized cisplatin prodrug containing Pt(IV) to the internal and/or external surface of virus-like particles (VLPs) derived from Physalis mottle virus (PhMV) to develop a pH-sensitive drug delivery system. The internally loaded and PEGylated VLPs (Pt-PhMVCy5.5-PEG) were taken up efficiently by cancer cells where they released platinum, presumably as a reduced, DNA-reactive Pt(II) complex, rapidly under acidic conditions in vitro (>80% in 30 h). The efficacy of the VLP-based drug delivery system was demonstrated against a panel of cancer cell lines, including cell lines resistant to platinum therapy. Furthermore, Pt-PhMVCy5.5-PEG successfully inhibited the growth of xenograft MDA-MB-231 breast tumors in vivo and significantly prolonged the survival of mice compared to free cisplatin and cisplatin-maleimide. Pt-PhMVCy5.5-PEG therefore appears promising as a prodrug to overcome the limitations of conventional platinum-based drugs for cancer therapy.

On-Demand Detachment of Succinimides on Cysteine to Facilitate (Semi)Synthesis of Challenging Proteins

The maleimide group is a widely used reagent for bioconjugation of peptides, proteins, and oligonucleotides employing Michael addition and Diels–Alder cycloaddition reactions. However, the utility of this functionality in chemical synthesis of peptides and proteins remains unexplored. We report, for the first time that Pd^II complexes can mediate the efficient removal of various succinimide derivatives in aqueous conditions. Succinimide removal by Pd^II was applied for the synthesis of two ubiquitin activity-based probes (Ub-ABPs) employing solid phase chemical ligation (SPCL). SPCL was achieved through a sequential three segment ligation on a polymer support via a maleimide anchor. The obtained probes successfully formed the expected covalent complexes with deubiquitinating enzymes (DUBs) USP2 and USP7, highlighting the use of our new method for efficient preparation of unique synthetic proteins. Importantly, we demonstrate the advantages of our newly developed method for the protection and deprotection of native cysteine with a succinimide group in a peptide fragment derived from thioredoxin-1 (Trx-1) obtained via intein based expression to enable ligation/desulfurization and subsequent disulfide bond formation in a one-pot process.

Controlled coupling of an ultrapotent auristatin warhead to cetuximab yields a next-generation antibody-drug conjugate for EGFR-targeted therapy of KRAS mutant pancreatic cancer

BACKGROUND

Antibody-drug conjugate (ADC) construction poses numerous challenges that limit clinical progress. In particular, common bioconjugation methods afford minimal control over the site of drug coupling to antibodies. Here, such difficulties are overcome through re-bridging of the inter-chain disulfides of cetuximab (CTX) with auristatin-bearing pyridazinediones, to yield a highly refined anti-epidermal growth factor receptor (EGFR) ADC.

METHODS

In vitro and in vivo assessment of ADC activity was performed in KRAS mutant pancreatic cancer (PaCa) models with known resistance to CTX therapy. Computational modelling was employed for quantitative prediction of tumour response to various ADC dosing regimens.

RESULTS

Site-selective coupling of an auristatin to CTX yielded an ADC with an average drug:antibody ratio (DAR) of 3.9, which elicited concentration- and EGFR-dependent cytotoxicity at sub-nanomolar potency in vitro. In human xenografts, the ADC inhibited tumour growth and prolonged survival, with no overt signs of toxicity. Key insights into factors governing ADC efficacy were obtained through a robust mathematical framework, including target-mediated dispositional effects relating to antigen density on tumour cells.

CONCLUSIONS

Together, our findings offer renewed hope for CTX in PaCa therapy, demonstrating that it may be reformatted as a next-generation ADC and combined with a predictive modelling tool to guide successful translation.

Chemical-Switching Strategy for Synthesis and Controlled Release of Norcantharimides from a Biomass-Derived Chemical

Catalytic strategies were developed to synthesize and release chemicals for applications in fine chemicals, such as drugs and polymers, from a biomass-derived chemical, 5-hydroxymethyl furfural (HMF). The combination of the diene and aldehyde functionalities in HMF enabled catalytic production of acetalized HMF derivatives with diol or epoxy reactants to allow reversible synthesis of norcantharimide derivatives upon Diels-Alder reaction with maleimides. Reverse-conversion of the acetal group to an aldehyde yielded mismatches of the molecular orbitals in norcantharimides to trigger retro Diels-Alder reaction at ambient temperatures and released reactants from the coupled molecules under acidic conditions. These strategies provide for the facile synthesis and controlled release of high-value chemicals.

Genetic Code Expansion Enables Site-Specific PEGylation of a Human Growth Hormone Receptor Antagonist through Click Chemistry

Regulation of human growth hormone (GH) signaling has important applications in the remediation of several diseases including acromegaly and cancer. Growth hormone receptor (GHR) antagonists currently provide the most effective means for suppression of GH signaling. However, these small 22 kDa recombinantly engineered GH analogues exhibit short plasma circulation times. To improve clinical viability, between four and six molecules of 5 kDa poly(ethylene glycol) (PEG) are nonspecifically conjugated to the nine amines of the GHR antagonist designated as B2036 in the FDA-approved therapeutic pegvisomant. PEGylation increases the molecular weight of B2036 and considerably extends its circulation time, but also dramatically reduces its bioactivity, contributing to high dosing requirements and increased cost. As an alternative to nonspecific PEGylation, we report the use of genetic code expansion technology to site-specifically incorporate the unnatural amino acid propargyl tyrosine (pglY) into B2036 with the goal of producing site-specific protein-polymer conjugates. Substitution of tyrosine 35 with pglY yielded a B2036 variant containing an alkyne functional group without compromising bioactivity, as verified by a cellular assay. Subsequent conjugation of 5, 10, and 20 kDa azide-containing PEGs via the copper-catalyzed click reaction yielded high purity, site-specific conjugates with >89% conjugation efficiencies. Site-specific attachment of PEG to B2036 is associated with substantially improved in vitro bioactivity values compared to pegvisomant, with an inverse relationship between polymer size and activity observed. Notably, the B2036-20 kDa PEG conjugate has a molecular weight comparable to pegvisomant, while exhibiting a 12.5 fold improvement in half-maximal inhibitory concentration in GHR-expressing Ba/F3 cells (103.3 nM vs 1289 nM). We expect that this straightforward route to achieve site-specific GHR antagonists will be useful for GH signal regulation.

LILRB4-targeting Antibody-Drug Conjugates for the Treatment of Acute Myeloid Leukemia

Acute myeloid leukemia (AML) is the most common and aggressive blood cancer in adults. In particular, significant unmet medical needs exist for effective treatment strategies for acute myelomonocytic leukemia (M4) and acute monocytic leukemia (M5) AML subtypes. Antibody-drug conjugates (ADC) are a promising drug class for AML therapy, as demonstrated by the FDA-approved anti-CD33 ADC, gemtuzumab ozogamicin (Mylotarg). However, CD33 is expressed in normal hematopoietic stem cells, highlighting the critical need to identify AML-specific targets to minimize the risk of potential adverse effects. We have demonstrated that the leukocyte immunoglobulin-like receptor subfamily B4 (LILRB4) is expressed at significantly higher levels on monocytic M4 and M5 AML cells than on normal counterparts. Here, we test whether LILRB4 is a promising ADC target to kill monocytic AML cells while sparing healthy counterparts. To this end, we generated ADCs from a humanized anti-LILRB4 mAb and the antimitotic payload, monomethyl auristatin F. The conjugates constructed were characterized and evaluated for LILRB4-specific cell killing potency, toxicity to progenitor cells, pharmacokinetics, and therapeutic efficacy. Our ADC linker technology platform efficiently generated homogeneous anti-LILRB4 ADCs with defined drug-to-antibody ratios. The homogeneous anti-LILRB4 ADCs demonstrated the capacity for LILRB4-mediated internalization, suitable physicochemical properties, and high cell killing potency against LILRB4-positive AML cells. Importantly, our data indicate that these ADCs spare normal progenitor cells. One of our homogeneous conjugates exerted a remarkable therapeutic effect and no significant toxicity in a xenograft mouse model of disseminated human AML. Our findings highlight the clinical potential of anti-LILRB4 ADCs in monocytic AML therapy.

Refined construction of antibody-targeted nanoparticles leads to superior antigen binding and enhanced delivery of an entrapped payload to pancreatic cancer cells

Antibody-targeted nanoparticles have shown exceptional promise as delivery vehicles for anticancer drugs, although manufacturability challenges have hampered clinical progress. These include the potential for uncontrolled and random antibody conjugation, resulting in masked or inactive paratopes and unwanted Fc domain interactions. To circumvent these issues, we show that the interchain disulfide of cetuximab F(ab) may be selectively re-bridged with a strained alkyne handle, to permit 'click' coupling to azide-capped nanoparticles in a highly uniform and oriented manner. When compared to conventional carbodiimide chemistry, this conjugation approach leads to the generation of nanoparticles with a higher surface loading of cetuximab F(ab) and with markedly improved ability to bind to the target epidermal growth factor receptor. Moreover, we show that entrapment of a camptothecin payload within these nanoparticles can enhance drug targeting to antigen-expressing pancreatic cancer cells, resulting in superior cytotoxicity versus the conventional nanoformulation. Collectively, this work highlights the critical need to develop refined methods for the construction of targeted nanoparticles that will accelerate their clinical translation through improved performance and manufacturability.

Morphogenesis Guided by 3D Patterning of Growth Factors in Biological Matrices

Three-dimensional (3D) control over the placement of bioactive cues is fundamental to understand cell guidance and develop engineered tissues. Two-photon patterning (2PP) provides such placement at micro- to millimeter scale, but nonspecific interactions between proteins and functionalized extracellular matrices (ECMs) restrict its use. Here, a 2PP system based on nonfouling hydrophilic photocages and Sortase A (SA)-based enzymatic coupling is presented, which offers unprecedented orthogonality and signal-to-noise ratio in both inert hydrogels and complex mammalian matrices. Improved photocaged peptide synthesis and protein functionalization protocols with broad applicability are introduced. Importantly, the method enables 2PP in a single step in the presence of fragile biomolecules and cells, and is compatible with time-controlled growth factor presentation. As a corollary, the guidance of axons through 3D-patterned nerve growth factor (NGF) within brain-mimetic ECMs is demonstrated. The approach allows for the interrogation of the role of complex signaling molecules in 3D matrices, thus helping to better understand biological guidance in tissue development and regeneration.

Genetically Encoded Stealth Nanoparticles of a Zwitterionic Polypeptide-Paclitaxel Conjugate Have a Wider Therapeutic Window than Abraxane in Multiple Tumor Models

Small-molecule therapeutics demonstrate suboptimal pharmacokinetics and bioavailability due to their hydrophobicity and size. One way to overcome these limitations-and improve their efficacy-is to use "stealth" macromolecular carriers that evade uptake by the reticuloendothelial system. Although unstructured polypeptides are of increasing interest as macromolecular drug carriers, current recombinant polypeptides in the clinical pipeline typically lack stealth properties. We address this challenge by developing new unstructured polypeptides, called zwitterionic polypeptides (ZIPPs), that exhibit "stealth" behavior in vivo. We show that conjugating paclitaxel to a ZIPP imparts amphiphilicity to the polypeptide chain that is sufficient to drive its self-assembly into micelles. This in turn increases the half-life of paclitaxel by 17-fold compared to free paclitaxel, and by 1.6-fold compared to the nonstealth control, i.e., ELP-paclitaxel. Treatment of mice bearing highly aggressive prostate or colon cancer with a single dose of ZIPP-paclitaxel nanoparticles leads to near-complete eradication of the tumor, and these nanoparticles have a wider therapeutic window than Abraxane, an FDA-approved taxane nanoformulation.

Advances in Antibody–Drug Conjugate Design: Current Clinical Landscape and Future Innovations

The targeted delivery of potent cytotoxic molecules into cancer cells is considered a promising anticancer strategy. The design of clinically effective antibody–drug conjugates (ADCs), in which biologically active drugs are coupled through chemical linkers to monoclonal antibodies, has presented challenges for pharmaceutical researchers. After 30 years of intensive research and development activities, only seven ADCs have been approved for clinical use; two have received fast-track designation and two breakthrough therapy designation from the Food and Drug Administration. There is continued interest in the field, as documented by the growing number of candidates in clinical development. This review aims to summarize the most recent innovations that have been applied to the design of ADCs undergoing early- and late-stage clinical trials. Discovery and rational optimization of new payloads, chemical linkers, and antibody formats have improved the therapeutic index of next-generation ADCs, ultimately resulting in improved clinical benefit for the patients.

Bispecific Antibodies and Antibody-Drug Conjugates for Cancer Therapy: Technological Considerations

The ability of monoclonal antibodies to specifically bind a target antigen and neutralize or stimulate its activity is the basis for the rapid growth and development of the therapeutic antibody field. In recent years, traditional immunoglobulin antibodies have been further engineered for better efficacy and safety, and technological developments in the field enabled the design and production of engineered antibodies capable of mediating therapeutic functions hitherto unattainable by conventional antibody formats. Representative of this newer generation of therapeutic antibody formats are bispecific antibodies and antibody–drug conjugates, each with several approved drugs and dozens more in the clinical development phase. In this review, the technological principles and challenges of bispecific antibodies and antibody–drug conjugates are discussed, with emphasis on clinically validated formats but also including recent developments in the fields, many of which are expected to significantly augment the current therapeutic arsenal against cancer and other diseases with unmet medical needs.

Stable Pyrrole-Linked Bioconjugates through Tetrazine-Triggered Azanorbornadiene Fragmentation

An azanorbornadiene bromovinyl sulfone reagent for cysteine-selective bioconjugation has been developed. Subsequent reaction with dipyridyl tetrazine leads to bond cleavage and formation of a pyrrole-linked conjugate. The latter involves ligation of the tetrazine to the azanorbornadiene-tagged protein through inverse electron demand Diels-Alder cycloaddition with subsequent double retro-Diels-Alder reactions to form a stable pyrrole linkage. The sequence of site-selective bioconjugation followed by bioorthogonal bond cleavage was efficiently employed for the labelling of three different proteins. This method benefits from easy preparation of these reagents, selectivity for cysteine, and stability after reaction with a commercial tetrazine, which has potential for the routine preparation of protein conjugates for chemical biology studies.

Synthetic Liposomal Mimics of Biological Viruses for the Study of Immune Responses to Infection and Vaccination

Human viruses possess very complex supramolecular structures. Both icosahedral and enveloped viruses typically display an array of viral-encoded protein antigens at varied spatial densities on the viral particle surface. The viral nucleic acid genome, on the other hand, is encapsulated inside the viral particle. Although both the surface antigen and the interior nucleic acids could independently produce immunological responses, how B cells integrate these two types of signals and respond to a typical virus particle to initiate activation is not well understood at a molecular level. The study of these fundamental biological processes would benefit from the development of viral structural mimics that are well constructed to incorporate both quantitative and qualitative viral features for presentation to B cells. These novel tools would enable researchers to systematically dissect the underlying processes. Here we report the development of such particulate antigens based on liposomes engineered to display a model protein antigen, hen egg lysozyme (HEL). We developed methods to overexpress and purify various affinity mutants of HEL from E. coli. We conjugated the purified recombinant HEL proteins onto the surface of a virion-sized liposome in an orientation-specific manner at defined spatial densities and also encapsulated nucleic acid molecules into the interior of the liposome. Both the chemical conjugation of the HEL antigen on liposome surfaces and the encapsulation of nucleic acids were stable under physiologically relevant conditions. These liposomes elicited antigen-specific B-cell responses in vitro, which validate these supramolecular structures as a novel and effective approach to mimic and systematically isolate the role of essential viral features in directing the B-cell response to particulate antigens.

N-Hydroxysuccinimide-Modified Ethynylphosphonamidates Enable the Synthesis of Configurationally Defined Protein Conjugates

Herein, the application of N-hydroxysuccinimide-modified phosphonamidate building blocks for the incorporation of cysteine-selective ethynylphosphonamidates into lysine residues of proteins, followed by thiol addition with small molecules and proteins, is reported. It is demonstrated that the building blocks significantly lower undesired homo-crosslinking side products that can occur with commonly applied succinimidyl 4-(N-maleimidomethyl)cyclohexane-1-carboxylate (SMCC) under physiological pH. The previously demonstrated stability of the phosphonamidate moiety additionally solves the problem of premature maleimide hydrolysis, which can hamper the efficiency of subsequent thiol addition. Furthermore, a method to separate the phosphonamidate enantiomers to be able to synthesize protein conjugates in a defined configuration has been developed. Finally, the building blocks are applied to the construction of functional antibody-drug conjugates, analogously to FDA-approved, SMCC-linked Kadcyla, and to the synthesis of a functional antibody-protein conjugate.

Maleimide-based metal-free ligation with dienes: a comparative study

Maleimide-based Diels–Alder strategies for bioconjugation are compared in terms of dienes accessibility and stability, reactions rates, as well as products isolation and stability.

An Overview of the Current ADC Discovery Landscape

The prototypical ADC mechanism involving antigen-mediated uptake and lysosomal release is both elegantly simple and scientifically compelling. However, recent clinical-stage failures have prompted a reevaluation of this delivery paradigm and have resulted in an array of new technologies that have the potential to improve the safety and efficacy of up and coming programs. These innovations can generally be categorized into seven areas that will be elaborated on in this chapter: (1) Exploiting new payload mechanisms; (2) Increasing the drug-antibody ratio (DAR); (3) Increasing the antibody penetration; (4) Overcoming ADC resistance mechanisms; (5) Increasing the efficiency of ADC uptake and processing; (6) Mitigating off-target payload exposure; and (7) Employment of noncytotoxic payloads. It is our belief that these seven areas capture the current "landscape" of innovations that are taking place in the design of next-generation ADCs. Together, these advancements are reshaping the ADC field and providing a path forward in the face of the recent clinical setbacks.

Assessing localized conformational stability of antibody-drug conjugate by protein conformation assay

Antibody-drug conjugates (ADCs) are a class of attractive therapeutic agents to fight cancer with conjugation of potent chemical agents on target-selective antibodies. The conceptually elegant approach has encountered mounting practical challenges in combining the mAb and potent drug while maintaining the conformational and physiochemical stability of the bioconjugates. The attachment of hydrophobic drug-linker with antibody could potentially alter the antibody conformational scaffold, locally or globally. Here we propose to use a protein conformation assay (PCA) to measure the higher-order structure of antibodies upon drug-linker conjugation. The PCA analysis provides insights into the formation of partially unfolded ADCs, which may correlate with protein stability and aggregation propensity. To further elucidate the cause of the unfolding events, in-depth peptide mapping combined with the PCA conformational footprints were performed on a commercial ADC trastuzumab emtansine in this study. The locally altered conformational hot-spots observed in PCA matched with conjugation sites with high occupancy rate identified in peptide mapping. In summary, by combining PCA and in-depth peptide mapping, a snapshot of ADC structural conformation and stability profile could be obtained and provide a swift and convenient measurement of the 'fitness' of ADC to facilitate payload selection, conjugation process development and early predictive developability assessment.

Targeting Triple Negative Breast Cancer Cells with Novel Cytotoxic Peptide-Doxorubicin Conjugates

In this study, we have designed and synthesized two novel peptide-drug conjugates (PDCs) where the drug, doxorubicin (Dox), is linked to the peptide via a succinimidyl thioether bond or a hydrazone linker. A highly specific and proteolytically stable breast cancer cell targeting peptide (WxEAAYQrFL) is conjugated to Dox to synthesize peptide-Dox thioether (1) or hydrazone (2) conjugate. The evaluation of the stability in water, media, and human serum showed that the conjugate 1 with the succinimidyl thioether linkage is more stable compared to the acid-sensitive hydrazone containing conjugate 2. The cytotoxicity studies showed that the two PDCs were as toxic as free Dox toward the triple negative breast cancer (TNBC) cells and were 7-30 times less toxic (IC₅₀ 1.2-4.7 μM for TNBC cells versus 15-39 μM for noncancerous cells) toward the noncancerous breast cells compared to the free doxorubicin (IC₅₀ 0.35-1.5 μM for TNBC cells versus 0.24 μM for noncancerous cells). The results from the comparative study of the two PDCs suggest that both may have translational potential for TNBC treatment.

Computationally designed antibody-drug conjugates self-assembled via affinity ligands

Antibody-drug conjugates (ADCs) combine the high specificity of antibodies with cytotoxic payloads. However, the present strategies for the synthesis of ADCs either yield unstable or heterogeneous products or involve complex processes. Here, we report a computational approach that leverages molecular docking and molecular dynamics simulations to design ADCs that self-assemble through the non-covalent binding of the antibody to a payload that we designed to act as an affinity ligand for specific conserved amino acid residues in the antibody. This method does not require modifications to the antibody structure and yields homogenous ADCs that form in less than 8 min. We show that two conjugates, which consist of hydrophilic and hydrophobic payloads conjugated to two different antibodies, retain the structure and binding properties of the antibody and its biological specificity, are stable in plasma and improve anti-tumour efficacy in mice with non-small cell lung tumour xenografts. The relative simplicity of the approach may facilitate the production of ADCs for the targeted delivery of cytotoxic payloads.

Subtiligase-Catalyzed Peptide Ligation

Subtiligase-catalyzed peptide ligation is a powerful approach for site-specific protein bioconjugation, synthesis and semisynthesis of proteins and peptides, and chemoproteomic analysis of cellular N termini. Here, we provide a comprehensive review of the subtiligase technology, including its development, applications, and impacts on protein science. We highlight key advantages and limitations of the tool and compare it to other peptide ligase enzymes. Finally, we provide a perspective on future applications and challenges and how they may be addressed.

Identification and Validation of a Novel Biologics Target in Triple Negative Breast Cancer

The goal of this study was to identify a novel target for antibody-drug conjugate (ADC) development in triple negative breast cancer (TNBC), which has limited treatment options, using gene expression datasets and in vitro siRNA/CRISPR and in vivo functional assays. We analyzed 4467 breast cancers and identified GABRP as top expressed gene in TNBC with low expression in most normal tissues. GABRP protein was localized to cell membrane with broad range of receptors/cell (815–53,714) and expressed by nearly half of breast cancers tissues. GABRP gene knockdown inhibited TNBC cell growth and colony formation in vitro and growth of MDA-MB-468 xenografts in nude mice. Commercially available anti-GABRP antibody (5–100 μg/ml) or de novo generated Fabs (20 μg/ml) inhibited TNBC cell growth in vitro. The same antibody conjugated to mertansine (DM1) also showed significant anticancer activity at nanomolar concentrations. Our results indicate that GABRP is a potential novel therapeutic target for ADC development.

Precision Chemoradiotherapy for HER2 Tumors Using Antibody Conjugates of an Auristatin Derivative with Reduced Cell Permeability

The most successful therapeutic strategies for locally advanced cancers continue to combine decades-old classical radiosensitizing chemotherapies with radiotherapy. Molecular targeted radiosensitizers offer the potential to improve the therapeutic ratio by increasing tumor-specific kill while minimizing drug delivery and toxicity to surrounding normal tissue. Auristatins are a potent class of anti-tubulins that sensitize cells to ionizing radiation damage and are chemically amenable to antibody conjugation. To achieve tumor-selective radiosensitization, we synthesized and tested anti-HER2 antibody-drug conjugates of two auristatin derivatives with ionizing radiation. Monomethyl auristatin E (MMAE) and monomethyl auristatin F (MMAF) were attached to the anti-HER2 antibodies trastuzumab and pertuzumab through a cleavable linker. While MMAE is cell permeable, MMAF has limited cell permeability as free drug resulting in diminished cytotoxicity and radiosensitization. However, when attached to trastuzumab or pertuzumab, MMAF was as efficacious as MMAE in blocking HER2-expressing tumor cells in G₂-M. Moreover, MMAF anti-HER2 conjugates selectively killed and radiosensitized HER2-rich tumor cells. Importantly, when conjugated to targeting antibody, MMAF had the advantage of decreased bystander and off-target effects compared with MMAE. In murine xenograft models, MMAF anti-HER2 antibody conjugates had less drug accumulated in the normal tissue surrounding tumors compared with MMAE. Therapeutically, systemically injected MMAF anti-HER2 conjugates combined with focal ionizing radiation increased tumor control and improved survival of mice with HER2-rich tumor xenografts. In summary, our results demonstrate the potential of cell-impermeable radiosensitizing warheads to improve the therapeutic ratio of radiotherapy by leveraging antibody-drug conjugate technology.

First platinum(II)-based metal-organic linker technology (Lx®) for a plug-and-play development of antibody-drug conjugates (ADCs)

Introduction: Compared to the antibody and drug components of an ADC, the linker part has been somewhat neglected. However, its importance for the reduction of failures in ADC approvals is increasingly recognized. Next of being a stable glue between drug and antibody, an ideal linker should improve the manufacturability and widen the therapeutic window of ADCs. Areas covered: The biopharmaceutical company LinXis started an ADC development program in which platinum(II) is the key element of the first metal-organic linker. The cationic complex [ethylenediamineplatinum(II)]²⁺, herein called 'Lx®', is used successfully for conjugation of drugs to antibodies. Expert opinion: Based on lessons learned from ADC development, Lx linker technology fulfills most of the desirable linker characteristics. Lx allows large-scale cost-effective manufacturing of ADCs via a straightforward two-step 'plug-and-play' process. First clinical candidate trastuzumab-Lx-auristatin F shows favorable preclinical safety as well as outstanding in vivo tumor targeting performance and therapeutic efficacy.