Cathepsin B-sensitive dipeptide prodrugs. 2. Models of anticancer drugs paclitaxel (Taxol), mitomycin C and doxorubicin

The cell type dependent sorting of CD9- and CD81 to extracellular vesicles can be exploited to convey tumor sensitive cargo to target cells

Extracellular vesicles (EVs) are lipid membrane-bound particles involved in cell-to-cell communication through a delivery of regulatory molecules essential for physiological processes. Since EVs efficiently vectorize specific cargo molecules, they have been proposed as suitable vehicles for therapeutic agents. Drug loading into EVs can be achieved by active, exogenous strategies or by genetic modifications of vesicle-producing cells. With the aim to produce EVs conveying therapeutic proteins, we genetically engineered and compared HEK293 to tumor cells. Tetraspanin-based RFP fusions were found to be more stable and preferentially sorted into EVs in HEK293. EVs isolated from genetically modified HEK293 cells media were captured by cancer cells, efficiently delivering their cargo. Cathepsin B cleavage site introduced between CD9/CD81 and RFP was recognized by tumor specific proteases allowing the release of the reporter protein. Our results indicate HEK293 cells as a preferential system for the production of EVs and pave the way to the development of nano-platforms for the efficient delivery of therapeutic proteins and prodrugs to tumor cells.

Enzymatically Activatable Polymers for Disease Diagnosis and Treatment

The irregular expression or activity of enzymes in the human body leads to various pathological disorders and can therefore be used as an intrinsic trigger for more precise identification of disease foci and controlled release of diagnostics and therapeutics, leading to improved diagnostic accuracy, sensitivity, and therapeutic efficacy while reducing systemic toxicity. Advanced synthesis strategies enable the preparation of polymers with enzymatically activatable skeletons or side chains, while understanding enzymatically responsive mechanisms promotes rational incorporation of activatable units and predictions of the release profile of diagnostics and therapeutics, ultimately leading to promising applications in disease diagnosis and treatment with superior biocompatibility and efficiency. By overcoming the challenges, new opportunities will emerge to inspire researchers to develop more efficient, safer, and clinically reliable enzymatically activatable polymeric carriers as well as prodrugs.

Antibody drug conjugates beyond cytotoxic payloads

For many years, antibody drug conjugates (ADC) have teased with the promise of targeted payload delivery to diseased cells, embracing the targeting of the antibody to which a cytotoxic payload is conjugated. During the past decade this promise has started to be realised with the approval of more than a dozen ADCs for the treatment of various cancers. Of these ADCs, brentuximab vedotin really laid the foundations of a template for a successful ADC with lysosomal payload release from a cleavable dipeptide linker, measured DAR by conjugation to the Cys-Cys interchain bonds of the antibody and a cytotoxic payload. Using this ADC design model oncology has now expanded their repertoire of payloads to include non-cytotoxic compounds. These new payload classes have their origins in prior medicinal chemistry programmes aiming to design selective oral small molecule drugs. While this may not have been achieved, the resulting compounds provide excellent starting points for ADC programmes with some compounds amenable to immediate linker attachment while for others extensive SAR and structural information offer invaluable design insights. Many of these new oncology payload classes are of interest to other therapeutic areas facilitating rapid access to drug-linkers for exploration as non-oncology ADCs. Other therapeutic areas have also pursued unique payload classes with glucocorticoid receptor modulators (GRM) being the most clinically advanced in immunology. Here, ADC payloads come full circle, as oncology is now investigating GRM payloads for the treatment of cancer. This chapter aims to cover all these new ADC approaches while describing the medicinal chemistry origins of the new non-cytotoxic payloads.

Smart Delivery Systems Responsive to Cathepsin B Activity for Cancer Treatment

Cathepsin B is a lysosomal cysteine protease, contributing to vital cellular homeostatic processes including protein turnover, macroautophagy of damaged organelles, antigen presentation, and in the extracellular space, it takes part in tissue remodeling, prohormone processing, and activation. However, aberrant overexpression of cathepsin B and its enzymatic activity is associated with different pathological conditions, including cancer. Cathepsin B overexpression in tumor tissues makes this enzyme an important target for smart delivery systems, responsive to the activity of this enzyme. The generation of technologies which therapeutic effect is activated as a result of cathepsin B cleavage provides an opportunity for tumor-targeted therapy and controlled drug release. In this review, we summarized different technologies designed to improve current cancer treatments responsive to the activity of this enzyme that were shown to play a key role in disease progression and response to the treatment.

Drug conjugate-based anticancer therapy - Current status and perspectives

Drug conjugates are conjugates comprising a tumor-homing carrier tethered to a cytotoxic agent via a linker that are designed to deliver an ultra-toxic payload directly to the target cancer cells. This strategy has been successfully used to increase the therapeutic efficacy of cytotoxic agents and reduce their toxic side effects. Drug conjugates are being developed worldwide, with the potential to revolutionize current cancer treatment strategies. Antibody-drug conjugates (ADCs) have developed rapidly, and 14 of them have received market approval since the first approval event by the Food and Drug Administration in 2000. However, there are some limitations in the use of antibodies as carriers. Other classes of drug conjugates are emerging, such as targeted drugs conjugated with peptides (peptide-drug conjugates, PDCs) and polymers (polymer-drug conjugates, PolyDCs) with the remaining constructs similar to those of ADCs. These novel drug conjugates are gaining attention because they overcome the limitations of ADCs. This review summarizes the current state and advancements in knowledge regarding the design, constructs, and clinical efficacy of different drug conjugates.

Targeted Delivery of Endosomal Escape Peptides to Enhance Immunotoxin Potency and Anti-cancer Efficacy

This work describes use of anti-carcinoembryonic antigen antibodies (10H6, T84.66) for targeted delivery of an endosomal escape peptide (H6CM18) and gelonin, a type I ribosome inactivating protein. The viability of colorectal cancer cells (LS174T, LoVo) was assessed following treatment with gelonin or gelonin immunotoxins, with or without co-treatment with T84.66-H6CM18. Fluorescent microscopy was used to visualize the escape of immunoconjugates from endosomes of treated cells, and efficacy and toxicity were assessed in vivo in xenograft tumor-bearing mice following single- and multiple-dose regimens. Application of 25 pM T84.66-H6CM18 combined with T84.66-gelonin increased gelonin potency by ~ 1,000-fold and by ~ 6,000-fold in LS174T and LoVo cells. Intravenous 10H6-gelonin at 1.0 mg/kg was well tolerated by LS174T tumor-bearing mice, while 10 and 25 mg/kg doses led to signs of toxicity. Single-dose administration of PBS, gelonin conjugated to T84.66 or 10H6, T84.66-H6CM18, or gelonin immunotoxins co-administered with T84.66-H6CM18 were evaluated. The combinations of T84.66-gelonin + 1.0 mg/kg T84.66-H6CM18 and 10H6-gelonin + 0.1 mg/kg T84.66-H6CM18 led to significant delays in LS174T growth. Use of a multiple-dose regimen allowed further anti-tumor effects, significantly extending median survival time by 33% and by 69%, for mice receiving 1 mg/kg 10H6-gelonin + 0.1 mg/kg T84.66-H6CM18 (p = 0.0072) and 1 mg/kg 10H6-gelonin + 1 mg/kg T84.66-H6CM18 (p = 0.0017). Combined administration of gelonin immunoconjugates with antibody-targeted endosomal escape peptides increased the delivery of gelonin to the cytoplasm of targeted cells, increased gelonin cell killing in vitro by 1,000-6,000 fold, and significantly increased in vivo efficacy.

Linkers: An Assurance for Controlled Delivery of Antibody-Drug Conjugate

As one of the major therapeutic options for cancer treatment, chemotherapy has limited selectivity against cancer cells. Consequently, this therapeutic strategy offers a small therapeutic window with potentially high toxicity and thus limited efficacy of doses that can be tolerated by patients. Antibody-drug conjugates (ADCs) are an emerging class of anti-cancer therapeutic drugs that can deliver highly cytotoxic molecules directly to cancer cells. To date, twelve ADCs have received market approval, with several others in clinical stages. ADCs have become a powerful class of therapeutic agents in oncology and hematology. ADCs consist of recombinant monoclonal antibodies that are covalently bound to cytotoxic chemicals via synthetic linkers. The linker has a key role in ADC outcomes because its characteristics substantially impact the therapeutic index efficacy and pharmacokinetics of these drugs. Stable linkers and ADCs can maintain antibody concentration in blood circulation, and they do not release the cytotoxic drug before it reaches its target, thus resulting in minimum off-target effects. The linkers used in ADC development can be classified as cleavable and non-cleavable. The former, in turn, can be grouped into three types: hydrazone, disulfide, or peptide linkers. In this review, we highlight the various linkers used in ADC development and their design strategy, release mechanisms, and future perspectives.

The Chemistry Behind ADCs

Combining the selective targeting of tumor cells through antigen-directed recognition and potent cell-killing by cytotoxic payloads, antibody-drug conjugates (ADCs) have emerged in recent years as an efficient therapeutic approach for the treatment of various cancers. Besides a number of approved drugs already on the market, there is a formidable follow-up of ADC candidates in clinical development. While selection of the appropriate antibody (A) and drug payload (D) is dictated by the pharmacology of the targeted disease, one has a broader choice of the conjugating linker (C). In the present paper, we review the chemistry of ADCs with a particular emphasis on the medicinal chemistry perspective, focusing on the chemical methods that enable the efficient assembly of the ADC from its three components and the controlled release of the drug payload.

A Complete and Versatile Protocol: Decoration of Cell-Derived Matrices with Mass-Encoded Peptides for Multiplexed Protease Activity Detection

This article provides guidance toward a platform technology for monitoring enzyme activity within the extracellular matrix (ECM) assessed by quantifying reporters secreted into the cell culture supernatant and analyzed by tandem mass spectrometry. The reporters are enzymatically and covalently bound to the ECM by transglutaminases (TG) using the peptide sequence of human insulin-like growth factor I's (IGF-I) D-domain which is known to be bound to the ECM by transglutaminase. The IGF-I D-domain sequence is followed by a peptide sequence cleaved by the intended target protease. This protease-sensitive peptide sequence (PSS) is cleaved off the ECM and can be used to monitor target-enzyme activity by employing a downstream mass tag designed according to isobaric mass encoding strategies, i.e., the combination of isotopically labeled, heavy amino acids. Thereby, cleavage events are linked to the appearance of encoded mass tags, readily allowing multiplexing. This article presents the design and synthesis of these mass reporters. It further aims at detailing the search for peptide sequences responding to target proteases to facilitate future work on enzyme activity measurement for enzymatic activities of hitherto unknown enzymes. In conclusion, the goal of this article is to arm scientists interested in measurements of local enzymatic activities within the ECM with robust protocols and background knowledge.

Antibody-Drug Conjugates: The New Frontier of Chemotherapy

In recent years, antibody-drug conjugates (ADCs) have become promising antitumor agents to be used as one of the tools in personalized cancer medicine. ADCs are comprised of a drug with cytotoxic activity cross-linked to a monoclonal antibody, targeting antigens expressed at higher levels on tumor cells than on normal cells. By providing a selective targeting mechanism for cytotoxic drugs, ADCs improve the therapeutic index in clinical practice. In this review, the chemistry of ADC linker conjugation together with strategies adopted to improve antibody tolerability (by reducing antigenicity) are examined, with particular attention to ADCs approved by the regulatory agencies (the U.S. Food and Drug Administration (FDA) and the European Medicines Agency (EMA)) for treating cancer patients. Recent developments in engineering Immunoglobulin (Ig) genes and antibody humanization have greatly reduced some of the problems of the first generation of ADCs, beset by problems, such as random coupling of the payload and immunogenicity of the antibody. ADC development and clinical use is a fast, evolving area, and will likely prove an important modality for the treatment of cancer in the near future.

Protease-activated prodrugs: strategies, challenges, and future directions

Proteases play critical roles in virtually all biological processes, including proliferation, cell death and survival, protein turnover, and migration. However, when dysregulated, these enzymes contribute to the progression of multiple diseases, with cancer, neurodegenerative disorders, inflammation, and blood disorders being the most prominent examples. For a long time, disease-associated proteases have been used for the activation of various prodrugs due to their well-characterized catalytic activity and ability to selectively cleave only those substrates that strictly correspond with their active site architecture. To date, versatile peptide sequences that are cleaved by proteases in a site-specific manner have been utilized as bioactive linkers for the targeted delivery of multiple types of cargo, including fluorescent dyes, photosensitizers, cytotoxic drugs, antibiotics, and pro-antibodies. This platform is highly adaptive, as multiple protease-labile conjugates have already been developed, some of which are currently in clinical use for cancer treatment. In this review, recent advancements in the development of novel protease-cleavable linkers for selective drug delivery are described. Moreover, the current limitations regarding the selectivity of linkers are discussed, and the future perspectives that rely on the application of unnatural amino acids for the development of highly selective peptide linkers are also presented.

Sulfatase-cleavable linkers for antibody-drug conjugates

Antibody-drug conjugates (ADCs) are a class of targeted drug delivery agents combining the cell-selectivity of monoclonal antibodies (mAbs) and the cytotoxicity of small molecules. These two components are joined by a covalent linker, whose nature is critical to the efficacy and safety of the ADC. Enzyme-cleavable dipeptidic linkers have emerged as a particularly effective ADC linker type due to their ability to selectively release the payload in the lysosomes of target cells. However, these linkers have a number of drawbacks, including instability in rodent plasma and their inherently high hydrophobicity. Here we show that arylsulfate-containing ADC linkers are cleaved by lysosomal sulfatase enzymes to tracelessly release their payload, while circumventing the instability problems associated with dipeptide-linkers. When incorporated with trastuzumab and the highly potent monomethyl auristatin E (MMAE) payload, the arylsulfate-containing ADC 2 and ADC 3 were more cytotoxic than the non-cleavable ADC 4 against HER2-positive cells, while maintaining selectivity over HER2-negative cells. We propose that the stability, solubility and synthetic tractability of our arylsulfate linkers make them an attractive new motif for cleavable ADC linkers, with clear benefits over the widely used dipeptidic linkers.

Novel PIKK inhibitor antibody-drug conjugates: Synthesis and anti-tumor activity

Novel neolymphostin-based antibody-drug conjugate (ADC) precursors were synthesized either through amide couplings between both cleavable and non-cleavable linkers and neolymphostin derivatives, or through Cu(I)-catalyzed acetylene-azide click cycloadditon between non-cleavable linkers and neolymphostin acetal derivatives. These precursors were site-specifically conjugated to cysteine mutant trastuzumab-A114C to provide neolymphostin-based ADCs. Preliminary in vitro data indicated that the corresponding ADCs were active against HER2-expressing tumor cell lines, thus providing a proof-of-concept for using neolymphostin as ADC-based anticancer agents.

Cleavable linkers in antibody–drug conjugates

This tutorial review summarises the advances in the field of cleavable linker technologies for antibody–drug conjugates (ADCs).

Novel Antibody Drug Conjugates Targeting Tumor-Associated Receptor Tyrosine Kinase ROR2 by Functional Screening of Fully Human Antibody Libraries Using Transpo-mAb Display on Progenitor B Cells

Receptor tyrosine kinase-like orphan receptor 2 (ROR2) has been identified as a highly relevant tumor-associated antigen in a variety of cancer indications of high unmet medical need, including renal cell carcinoma and osteosarcoma, making it an attractive target for targeted cancer therapy. Here, we describe the de novo discovery of fully human ROR2-specific antibodies and potent antibody drug conjugates (ADCs) derived thereof by combining antibody discovery from immune libraries of human immunoglobulin transgenic animals using the Transpo-mAb mammalian cell-based IgG display platform with functional screening for internalizing antibodies using a secondary ADC assay. The discovery strategy entailed immunization of transgenic mice with the cancer antigen ROR2, harboring transgenic IgH and IgL chain gene loci with limited number of fully human V, D, and J gene segments. This was followed by recovering antibody repertoires from the immunized animals, expressing and screening them as full-length human IgG libraries by transposon-mediated display in progenitor B lymphocytes ("Transpo-mAb Display") for ROR2 binding. Individual cellular "Transpo-mAb" clones isolated by single cell sorting and capable of expressing membrane-bound as well as secreted human IgG were directly screened during antibody discovery, not only for high affinity binding to human ROR2, but also functionally as ADCs using a cytotoxicity assay with a secondary anti-human IgG-toxin-conjugate. Using this strategy, we identified and validated 12 fully human, monoclonal anti-human ROR2 antibodies with nanomolar affinities that are highly potent as ADCs and could be promising candidates for the therapy of human cancer. The screening for functional and internalizing antibodies during the early phase of antibody discovery demonstrates the utility of the mammalian cell-based Transpo-mAb Display platform to select for functional binders and as a powerful tool to improve the efficiency for the development of therapeutically relevant ADCs.

Trastuzumab-monomethyl auristatin E conjugate exhibits potent cytotoxic activity in vitro against HER2-positive human breast cancer

Targeted therapy using specific monoclonal antibodies (mAbs) conjugated to chemotherapeutic agents or toxins has become one of the top priorities in cancer therapy. Antibody-drug conjugates (ADCs) are emerging as a promising strategy for cancer-targeted therapy. In this study, trastuzumab, a humanized monoclonal anti-HER2 antibody, was reduced by dithiothreitol and conjugated to the microtubule-disrupting agent monomethyl auristatin E (MMAE) through a valine-citrulline peptide linker (trastuzumab-MC-Val-Cit-PABC-MMAE [trastuzumab-vcMMAE]). After conjugation, ADCs were characterized by using UV-vis, sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE), and flow cytometry. The antitumor activity of the ADC was evaluated in breast cancer cells in vitro. In addition, ADCs were further characterized using purification by the protein A chromatography, followed by assessment using apoptosis and MTT (3-(4,5-dimethylthiazol-2-yl)-2, 5-diphenyltetrazolium bromide) assays. Hydrophobic interaction chromatography was used to determine drug-to-antibody ratio species of ADCs produced. Our finding showed that approximately 5.12 drug molecules were conjugated to each mAb. H2L2, H2L, HL, H2, H, and L forms of ADCs were detected in nonreducing SDS-PAGE. The binding of trastuzumab-vcMMAE to HER2-positive cells was comparable with that of the parental mAb. The MTT assay showed that our ADCs induced significant cell death in HER2-positive cells, but not in HER2-negative cells. The ADCs produced was a mixture of species, unconjugated trastuzumab (14.147%), as well as trastuzumab conjugated with two (44.868%), four (16.886%), six (13.238%), and eight (10.861%) molecules of MMAE. These results indicated that MMAE-conjugated trastuzumab significantly increases the cytotoxic activity of trastuzumab, demonstrating high affinity, specificity, and antitumor activity in vitro. Trastuzumab-vcMMAE is an effective and selective agent for the treatment of HER2-positive breast tumors.

Experimental evidence of good efficacy and reduced toxicity with peptide-doxorubicin to treat gastric cancer

BACKGROUND

To compare the efficacy and toxicity of peptide-doxorubicin (PDOX) and doxorubicin (DOX) on nude mice models of human gastric cancer.

RESULTS

Both PDOX and DOX could significantly inhibit tumor growth compared with Control (P < 0.05) in both subcutaneous and orthotopic models. Animal survival was much better in PDOX group than DOX group. In peripheral blood test, PDOX group had significantly higher levels of platelets than the Control (P < 0.05), and lymphocyte lower than Control (P < 0.05). There were no significant differences on liver, kidney and cardiac function parameters among three groups (P > 0.05). Immunohistochemistry showed that treatment groups had much higher Tunel than Control (P < 0.05), and PDOX had significantly lower Ki-67 than doxorubicin and Control group (P < 0.01). Western blotting showed that PDOX caused much higher expressions of P53, P21, Aparf-1, pro- and cleaved-caspase 3, compared with DOX.

CONCLUSION

Compared with DOX, PDOX has increased effects but much decreased toxicity in treating animal model of gastric cancer.

MATERIALS AND METHODS

Animals in subcutaneous model were randomized into Control, doxorubicin, PDOX-L, PDOX-M, and PDOX-H groups. Animals in surgical orthotopic implantation model were randomized into Control, doxorubicin and, peptide-doxorubicin groups. The animals were treated, monitored and examined following a set protocol.

Molecular design and synthesis of self-assembling camptothecin drug amphiphiles

The conjugation of small molecular hydrophobic anticancer drugs onto a short peptide with overall hydrophilicity to create self-assembling drug amphiphiles offers a new prodrug strategy, producing well-defined, discrete nanostructures with a high and quantitative drug loading. Here we show the detailed synthesis procedure and how the molecular structure can influence the synthesis of the self-assembling prodrugs and the physicochemical properties of their assemblies. A series of camptothecin-based drug amphiphiles were synthesized via combined solid- and solution-phase synthetic techniques, and the physicochemical properties of their self-assembled nanostructures were probed using a number of imaging and spectroscopic techniques. We found that the number of incorporated drug molecules strongly influences the rate at which the drug amphiphiles are formed, exerting a steric hindrance toward any additional drugs to be conjugated and necessitating extended reaction time. The choice of peptide sequence was found to affect the solubility of the conjugates and, by extension, the critical aggregation concentration and contour length of the filamentous nanostructures formed. In the design of self-assembling drug amphiphiles, the number of conjugated drug molecules and the choice of peptide sequence have significant effects on the nanostructures formed. These observations may allow the fine-tuning of the physicochemical properties for specific drug delivery applications, ie systemic vs local delivery.

Targeted delivery for regenerative medicines: an untapped opportunity for drug conjugates

Regenerative approaches are promising avenues to effectively cure diseases rather than merely treating symptoms, but are associated with concerns around proliferation in other organs. Given that targeted delivery holds the promise of delivering a drug precisely to its desired site of action, usually with the prospect of increasing the therapeutic index, it can be considered as an essential enabler of regenerative medicines. Although significant progress has been made predominantly in oncology for the delivery of cytotoxic drugs using antibody-drug conjugates (ADCs), the physiological conditions and safety requirements for regenerative medicines are very different. Drug conjugates need to be approached differently and, we herein suggest using a broader range of homing modalities and a specific framework to develop safe linkers.

Novel Phosphate Modified Cathepsin B Linkers: Improving Aqueous Solubility and Enhancing Payload Scope of ADCs

In an effort to examine the utility of antibody-drug conjugates (ADCs) beyond oncology indications, a novel phosphate bridged Cathepsin B sensitive linker was developed to enable the targeted delivery of glucocorticoids. Phosphate bridging of the Cathepsin B sensitive linkers allows for payload attachment at an aliphatic alcohol. As small molecule drug-linkers, these aqueous soluble phosphate containing drug-linkers were found to have robust plasma stability coupled with rapid release of payload in a lysosomal environment. Site-specific ADCs were successfully made between these drug-linkers and an antibody against human CD70, a receptor specifically expressed in immune cells but also found aberrantly expressed in multiple human carcinomas. These ADCs demonstrated in vitro targeted delivery of glucocorticoids to a representative cell line as measured by changes in glucocorticoid receptor (GR) mediated gene mRNA levels. This novel linker expands the scope of potential ADC payloads by allowing an aliphatic alcohol to be a stable, yet cleavable attachment site. This phosphate linker may have broad utility for internalizing ADCs as well as other targeted delivery platforms.

Prodrug Strategies for Paclitaxel

Paclitaxel is an anti-tumor agent with remarkable anti-tumor activity and wide clinical uses. However, it is also faced with various challenges especially for its poor water solubility and low selectivity for the target. To overcome these disadvantages of paclitaxel, approaches using small molecule modifications and macromolecule modifications have been developed by many research groups from all over the world. In this review, we discuss the different strategies especially prodrug strategies that are currently used to make paclitaxel more effective.

Synthesis and Preclinical Evaluation of a Highly Improved Anticancer Prodrug Activated by Histone Deacetylases and Cathepsin L

Lack of absolute selectivity against cancer cells is a major limitation for current cancer therapies. In the previous study, we developed a prodrug strategy for selective cancer therapy using a masked cytotoxic agent puromycin [Boc-Lys(Ac)-Puromycin], which can be sequentially activated by histone deacetylases (HDACs) and cathepsin L (CTSL) to kill cancer cells expressing high levels of both enzymes. Despite the promise as a selective cancer therapy, its requirement of relatively high dosage could be a potential issue in the clinical setting. To address this issue, we aimed to further improve the overall efficacy of our prodrug strategy. Since the proteolytic cleavage by CTSL is the rate-limiting step for the drug activation, we sought to improve the substrate structure for CTSL activity by modifying the α-amino protecting group of lysine. Here we show that protection with Fmoc [Fmoc-Lys(Ac)-Puromycin] exhibits a marked improvement in overall anticancer efficacy compared to the original Boc-Lys(Ac)-Puromycin and this is mainly due to the highly efficient cellular uptake besides its improved substrate structure. Furthermore, to address a concern that the improved drug efficacy might direct high toxicity to the normal cells, we confirmed that Fmoc-Lys(Ac)-Puromycin still retains excellent cancer selectivity in vitro and no obvious systemic off-target toxicity in vivo. Thus our preclinical evaluation data presented here demonstrate that the Fmoc-Lys(Ac)-Puromycin exhibits substantially improved anticancer efficacy, further supporting our approach for the selective cancer therapy.

Discovery of Pyrophosphate Diesters as Tunable, Soluble, and Bioorthogonal Linkers for Site-Specific Antibody-Drug Conjugates

As part of an effort to examine the utility of antibody-drug conjugates (ADCs) beyond oncology indications, a novel pyrophosphate ester linker was discovered to enable the targeted delivery of glucocorticoids. As small molecules, these highly soluble phosphate ester drug linkers were found to have ideal orthogonal properties: robust plasma stability coupled with rapid release of payload in a lysosomal environment. Building upon these findings, site-specific ADCs were made between this drug linker combination and an antibody against human CD70, a receptor specifically expressed in immune cells but also found aberrantly expressed in multiple human carcinomas. Full characterization of these ADCs enabled procession to in vitro proof of concept, wherein ADCs 1-22 and 1-37 were demonstrated to afford potent, targeted delivery of glucocorticoids to a representative cell line, as measured by changes in glucocorticoid receptor-mediated gene mRNA levels. These activities were found to be antibody-, linker-, and payload-dependent. Preliminary mechanistic studies support the notion that lysosomal trafficking and enzymatic linker cleavage are required for activity and that the utility for the pyrophosphate linker may be general for internalizing ADCs as well as other targeted delivery platforms.

Salvage treatment for relapsed/refractory Hodgkin lymphoma: role of allografting, brentuximab vedotin and newer agents

INTRODUCTION

Second-line, salvage chemotherapy followed by high-dose chemotherapy and autologous stem cell transplantation (AUTO-SCT) is the standard of care for patients with relapsed/refractory (R/R) Hodgkin lymphoma (HL). Approximately 50% of patients relapse after AUTO-SCT and their prognosis is generally poor. Brentuximab Vedotin (BV) has demonstrated efficacy in this setting and allogeneic (ALLO)-SCT represents an option with curative potential in this subgroup of patients.

AREAS COVERED

A systematic review has been conducted to explore the actual knowledge on ALLO-SCT, BV and newer agents in R/R HL.

EXPERT OPINION

The introduction of BV in clinical practice has significantly improved the management of post-AUTO-SCT relapses and the drug can induce durable remissions in a subset of R/R HL. Allografting select patients has been used to improve clinical outcomes and recent case series have begun to explore BV as a potential 'bridge' to allo-SCT, even though the optimal timing of ALLO-SCT after BV response remains undetermined. However, reduced tumor burden at the time of ALLO-SCT is a key factor to decrease relapse risk. Based on the unique composition of the tumor, more recently new agents such as PD-1 inhibitors have been developed. The potential role of PD-1 inhibitors with ALLO-SCT remains to be explored.

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.

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

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

Targeted Delivery of LXR Agonist Using a Site-Specific Antibody-Drug Conjugate

Liver X receptor (LXR) agonists have been explored as potential treatments for atherosclerosis and other diseases based on their ability to induce reverse cholesterol transport and suppress inflammation. However, this therapeutic potential has been hindered by on-target adverse effects in the liver mediated by excessive lipogenesis. Herein, we report a novel site-specific antibody-drug conjugate (ADC) that selectively delivers a LXR agonist to monocytes/macrophages while sparing hepatocytes. The unnatural amino acid para-acetylphenylalanine (pAcF) was site-specifically incorporated into anti-CD11a IgG, which binds the α-chain component of the lymphocyte function-associated antigen 1 (LFA-1) expressed on nearly all monocytes and macrophages. An aminooxy-modified LXR agonist was conjugated to anti-CD11a IgG through a stable, cathepsin B cleavable oxime linkage to afford a chemically defined ADC. The anti-CD11a IgG-LXR agonist ADC induced LXR activation specifically in human THP-1 monocyte/macrophage cells in vitro (EC50-27 nM), but had no significant effect in hepatocytes, indicating that payload delivery is CD11a-mediated. Moreover, the ADC exhibited higher-fold activation compared to a conventional synthetic LXR agonist T0901317 (Tularik) (3-fold). This novel ADC represents a fundamentally different strategy that uses tissue targeting to overcome the limitations of LXR agonists for potential use in treating atherosclerosis.

Antibody-Drug Conjugates: Design, Formulation and Physicochemical Stability

The convergence of advanced understanding of biology with chemistry has led to a resurgence in the development of antibody-drug conjugates (ADCs), especially with two recent product approvals. Design and development of ADCs requires the synergistic combination of the monoclonal antibody, the linker and the payload. Advances in antibody science has enabled identification and generation of high affinity, highly selective, humanized or human antibodies for a given target. Novel linker technologies have been synthesized and highly potent cytotoxic drug payloads have been created. As the first generation of ADCs utilizing lysine and cysteine chemistries moves through the clinic and into commercialization, second generation ADCs involving site specific conjugation technologies are being evaluated and tested. The latter aim to be better characterized and controlled, with wider therapeutic indices as well as improved pharmacokinetic-pharmacodynamic (PK-PD) profiles. ADCs offer some interesting physicochemical properties, due to conjugation itself, and to the (often) hydrophobic payloads that must be considered during their CMC development. New analytical methodologies are required for the ADCs, supplementing those used for the antibody itself. Regulatory filings will be a combination of small molecule and biologics. The regulators have put forth some broad principles but this landscape is still evolving.

Drug delivery and release systems for targeted tumor therapy

Most toxic agents currently used for chemotherapy show a narrow therapeutic window, because of their inability to distinguish between healthy and cancer cells. Targeted drug delivery offers the possibility to overcome this issue by selectively addressing structures on the surface of cancer cells, therefore reducing undesired side effects. In this broad field, peptide-drug conjugates linked by intracellular cleavable structures have evolved as highly promising agents. They can specifically deliver toxophores to tumor cells by targeting distinct receptors overexpressed in cancer. In this review, we focus on these compounds and describe important factors to develop a highly efficient peptide-drug conjugate. The necessary properties of tumor-targeting peptides are described, and the different options for cleavable linkers used to connect toxic agents and peptides are discussed, and synthetic considerations for the introduction of these structures are reported. Furthermore, recent examples and current developments of peptide-drug conjugates are critically evaluated with a special focus on the applied linker structures and their future use in cancer therapy.

Synthesis, identification and in vivo studies of tumor-targeting agent peptide doxorubicin (PDOX) to treat peritoneal carcinomatosis of gastric cancer with similar efficacy but reduced toxicity

Background

This work aimed to synthesize a cathepsin B (CTSB)-cleavable tumor-targeting prodrug peptide doxorubicin (PDOX) and study the in vivo efficacy and toxicities on an animal model of gastric peritoneal carcinomatosis (PC).

Methods

PDOX was synthesized using doxorubicin (DOX) attaching to a CTSB-cleavable dipeptide Ac-Phe-Lys and a para-amino-benzyloxycarbonyl (PABC) spacer. PC model was established by injecting VX2 tumor cells into the gastric sub-mucosa of 40 rabbits, which then were randomized into 4 groups: the Control (n = 10) without treatment, the HIPEC (n = 10) receiving cytoreductive surgery (CRS) plus hyperthermic intraperitoneal chemotherapy (HIPEC), the PDOX (n = 10) and the DOX (n = 10) receiving systemic chemotherapy with PDOX 50.0 mg/kg or DOX 5.0 mg/kg, respectively, after CRS + HIPEC.

Results

The median overall survivals (OS) were 23.0 d (95% CI: 19.9 d - 26.1 d) in the Control, 41.0 d (36.9 d - 45.1 d) in the HIPEC, 65.0 d (44.1 d - 71.9 d) in the PDOX, and 58.0 d (39.6 d - 54.4 d) in the DOX. Compared with the Control, the OS was extended by 70% in the HIPEC (p < 0.001) and further extended by 40% in the DOX (p = 0.029) and by 58% in the PDOX (p = 0.021), and the PC severity was decreased in the HIPEC and further decreased in the PDOX and DOX. Animals receiving DOX treatment showed hematological toxicities with marked reduction of white blood cells and platelets, as well as cardiac toxicities with significant increases in creatine kinase mb isoenzyme, evident myocardium coagulation necrosis, significant nuclear degeneration, peri-nucleus mitochondria deletion, mitochondria-pyknosis, and abnormal intercalated discs. But these toxicities were not evident in the PDOX.

Conclusions

PDOX is a newly synthesized tumor-targeting prodrug of DOX. Compared with DOX, PDOX has similar efficacy but reduced hematological and cardiac toxicities in treating rabbit model of gastric PC.

Targeted therapy for Hodgkin lymphoma and systemic anaplastic large cell lymphoma: focus on brentuximab vedotin

Despite the relative success of chemotherapy for Hodgkin lymphoma (HL) and systemic anaplastic large cell lymphoma (ALCL), novel therapeutic agents are needed for refractory or relapsed patients. Targeted immunotherapy has emerged as a novel treatment option for these patients. Although unconjugated anti-cluster of differentiation (CD)30 antibodies showed minimal antitumor activity in early clinical trials, development of antibody–drug conjugates (ADCs) appears promising. Brentuximab vedotin is an ADC composed of an anti-CD30 antibody linked to a potent microtubule-disrupting agent monomethyl auristatin E (MMAE). It has the ability to target CD30-positive tumor cells and, once bound to CD30, brentuximab vedotin is internalized and MMAE is released to induce cell cycle arrest and apoptosis. In two Phase II trials, objective response was reported in 75% and 86% of patients with refractory or relapsed HL and systemic ALCL, respectively, with an acceptable toxicity profile. Based on these studies, the US Food and Drug Administration (FDA) granted accelerated approval of brentuximab vedotin in August 2011 for the treatment of refractory and relapsed HL and ALCL. We review the key characteristics of brentuximab vedotin, clinical data supporting its therapeutic efficacy, and current ongoing trials to explore its utility in other CD30-positive malignancies.

Targeting therapy of hepatocellular carcinoma with doxorubicin prodrug PDOX increases anti-metastatic effect and reduces toxicity: a preclinical study

BACKGROUND

This study was to investigate the effects and safety of cathepsin B-cleavable doxorubicin (DOX)-prodrug (PDOX) for targeting therapy of metastatic human hepatocellular carcinoma (HCC) using DOX as a positive control drug.

METHODS

The orthotopic nude mice model of highly metastatic HCC was established and the animals were randomized and treated with PDOX, DOX and saline, respectively. Hematology, biochemistry and tumor markers were studied. At autopsy, liver tumor weight and size, ascites, abdominal lymph nodes metastases, experimental peritoneal carcinomatosis index (ePCI), and tumor-host body weight ratio were investigated. Immunohistochemical studies and western blotting were done to investigate key molecules involved in the mechanism of action.

RESULTS

Compared with Control, both PDOX and DOX could similarly and significantly reduce liver tumor weight and tumor volume by over 40%, ePCI values, retroperitoneal lymph node metastases and lung metastases and serum AFP levels (P < 0.05). The PDOX group had significantly higher WBC than the DOX group (P < 0.05), and higher PLT than Control (P < 0.05). Serum BUN and Cr levels were lower in the PDOX group than DOX and Control groups (P < 0.05). Compared with Control, DOX increased CK and CK-MB; while PDOX decreased CK compared with DOX (P < 0.05). Multiple spotty degenerative changes of the myocardium were observed in DOX-treated mice, but not in the Control and PDOX groups. PDOX could significantly reduce the Ki-67 positive rate of tumor cells, compared with DOX and Control groups. PDOX produced the effects at least via the ERK pathway.

CONCLUSION

Compared with DOX, PDOX may have better anti-metastatic efficacy and reduced side effects especially cardio-toxicities in this HCC model.

Antibody-drug conjugates for the treatment of cancer

With over 20 antibody-drug conjugates in clinical trials as well as a recently FDA-approved drug, it is clear that this is becoming an important and viable approach for selectively delivering highly cytotoxic agents to tumor cells while sparing normal tissue. This review discusses the critical aspects for this approach with an emphasis on the properties of the linker between the antibody and the cytotoxic payload that are required for an effective antibody-drug conjugate. Different linkers are illustrated with attention focused on (i) the specifics of attachment to the antibody, (ii) the polarity of the linker, (iii) the trigger on the linker that initiates cleavage from the drug, and (iv) the self-immolative spacer that liberates the active payload. Future directions in the field are proposed.

Linker technologies for antibody-drug conjugates

Antibody-drug conjugates (ADCs), which combine the specificity, favorable pharmacokinetics, and biodistribution of a monoclonal antibody (mAb) with the cytotoxic potency of a drug, are promising new therapies for cancer. Along with the development of monoclonal antibodies (mAbs) and cytotoxic drugs, the design of the linker is of essential importance, because it impacts the efficacy and tolerability of ADCs. The linker needs to provide sufficient stability during systemic circulation but allow for the rapid and efficient release of the cytotoxic drug in an active form inside the tumor cells. This review provides an overview of linker technologies currently used for ADCs and advances that have resulted in linkers with improved properties. Also provided is a brief summary of some considerations for the conjugation of antibody and drug linker such as drug-to-antibody ratio and site of conjugation.

Cathepsin B-cleavable doxorubicin prodrugs for targeted cancer therapy (Review)

Doxorubicin (DOX) is one of the most effective cytotoxic anticancer drugs used for the treatment of hematological malignancies, as well as a broad range of solid tumors. However, the clinical applications of this drug have long been limited due to its severe dose-dependent toxicities. Therefore, DOX derivatives and analogs have been developed to address this issue. A type of DOX prodrug, cleaved by cathepsin B (Cat B), which is highly upregulated in malignant tumors and premalignant lesions, has been developed to achieve a higher DOX concentration in tumor tissue and a lower concentration in normal tissue, so as to enhance the efficacy and reduce toxicity to normal cells. In this review, we focused on Cat B-cleavable DOX prodrugs and discussed the efficacy of these prodrugs, demonstrated by preclinical and clinical developments.

Enzyme responsive materials: design strategies and future developments

Enzyme responsive materials (ERMs) are a class of stimuli responsive materials with broad application potential in biological settings. This review highlights current and potential future design strategies for ERMs and provides an overview of the present state of the art in the area.

The discovery and development of brentuximab vedotin for use in relapsed Hodgkin lymphoma and systemic anaplastic large cell lymphoma

Progress has been made recently in developing antibody-drug conjugates (ADCs) that can selectively deliver cancer drugs to tumor cells. In principle, the idea is simple: by attaching drugs to tumor-seeking antibodies, target cells will be killed and nontarget cells will be spared. In practice, many parameters needed to be addressed to develop safe and effective ADCs, including the expression profiles of tumor versus normal tissues, the potency of the drug, the linker attaching the drug and placement of the drug on the antibody, and the pharmacokinetic and stability profiles of the resulting ADC. All these issues had been taken into account in developing brentuximab vedotin (Adcetris), an ADC that recently received accelerated approval by the US Food and Drug Administration for the treatment of relapsed Hodgkin lymphoma and systemic anaplastic large cell lymphoma (ALCL). Research is under way to extend the applications of brentuximab vedotin and to advance the field by developing other ADCs with new linker and conjugation strategies.

Development of self-immolative dendrimers for drug delivery and sensing

Traditional dendrimers possess unique cascade-branched structural properties that allow for multivalent modifications with drug cargos, targeting/delivery agents and imaging tools. In addition to multivalency, the dendrimer's macromolecular size also brings about the enhanced permeability and retention (EPR) effect, which makes it an attracting agent for drug delivery and biosensing. Similar to other macromolecules, therapeutic application of dendrimers in the human body faces practical challenges such as target specificity and toxicity. The latter represents a substantial issue due to the dendrimer's unnatural chemical structure and relatively large size, which prohibit its in vivo degradation and excretion from the body. To date, a class of self-immolative dendrimers has been developed to overcome these obstacles, which takes advantage of its unique structural backbone to allow for cascade decompositions upon a simple triggering event. The specific drug release can be achieved through a careful design of the trigger, and as a result of the fragmentation, the generated small molecules are either biodegradable or easily excreted from the body. Though still at a preliminary stage, the development of this novel approach represents an important direction in nanoparticle-mediated drug delivery and sensor design, thereby opening up an insightful frontier of dendrimer based applications.

Cathepsin B cleavable novel prodrug Ac-Phe-Lys-PABC-ADM enhances efficacy at reduced toxicity in treating gastric cancer peritoneal carcinomatosis: an experimental study

BACKGROUND

Doxorubicin (Adriamycin) is effective in gastric cancer treatment, but with severe dose-dependent toxicities. A novel prodrug of doxorubicin (Ac-Phe-Lys-PABC-ADM) is designed to deliver free doxorubicin relying on cathepsin B and reduce side effects. The authors examined the antitumor effect and toxicities of Ac-Phe-Lys-PABC-ADM against gastric cancer peritoneal carcinomatosis.

METHODS

SGC-7901 gastric cancer cell line was used for the study. The in vitro study investigated the effects of doxorubicin and Ac-Phe-Lys-PABC-ADM on cell growth dynamics and cell cycle. The in vivo study investigated the efficacy and toxicity of Ac-Phe-Lys-PABC-ADM on a nude mice model of peritoneal carcinomatosis, with doxorubicin as positive control.

RESULTS

In the in vitro study, Ac-Phe-Lys-PABC-ADM had a lower dose-dependent inhibitory effect on SGC-7901 cells. In the in vivo study of control, doxorubicin, and Ac-Phe-Lys-PABC-ADM groups, the median experimental peritoneal carcinomatosis indexes were 6, 1.5, and 1, respectively (P = .004); the body weights were 24.32 ± 1.40 g, 18.40 ± 2.97 g, and 23.61 ± 0.80 g, respectively (P = .000). Biochemical studies showed that Ac-Phe-Lys-PABC-ADM had significantly lower toxicities on the bone marrow, liver, kidney, and particularly heart. Histopathological studies of the control, doxorubicin, and Ac-Phe-Lys-PABC-ADM groups found significant myocardium toxicities in 3, 7, and 4 animals, respectively.

CONCLUSIONS

Ac-Phe-Lys-PABC-ADM could be an effective molecular targeting drug to treat gastric cancer peritoneal carcinomatosis with enhanced efficacy and reduced toxicity.

Prodrug-based intracellular delivery of anticancer agents

There are numerous anticancer agents based on a prodrug approach. However, no attempt has been made to review the ample available literature with a specific focus on the altered cell uptake pathways enabled by the conjugation and on the intracellular drug-release mechanisms. This article focuses on the cellular interactions of a broad selection of parenterally administered anticancer prodrugs based on synthetic polymers, proteins or lipids. The report also aims to highlight the prodrug design issues, which are key points to obtain an efficient intracellular drug delivery. The chemical basis of these molecular concepts is put into perspective with the uptake and intracellular activation mechanisms, the in vitro and in vivo proofs of concepts and the clinical results. Several active targeting strategies and stimuli-responsive architectures are discussed throughout the article.