Randomized phase II study of BR96-doxorubicin conjugate in patients with metastatic breast cancer

Pharmacokinetic Considerations for Antibody-Drug Conjugates against Cancer

Antibody-drug conjugates (ADCs) are ushering in the next era of targeted therapy against cancer. An ADC for cancer therapy consists of a potent cytotoxic payload that is attached to a tumour-targeted antibody by a chemical linker, usually with an average drug-to-antibody ratio (DAR) of 3.5-4. The theory is to deliver potent cytotoxic payloads directly to tumour cells while sparing healthy cells. However, practical application has proven to be more difficult. At present there are only two ADCs approved for clinical use. Nevertheless, in the last decade there has been an explosion of options for ADC engineering to optimize target selection, Fc receptor interactions, linker, payload and more. Evaluation of these strategies requires an understanding of the mechanistic underpinnings of ADC pharmacokinetics. Development of ADCs for use in cancer further requires an understanding of tumour properties and kinetics within the tumour environment, and how the presence of cancer as a disease will impact distribution and elimination. Key pharmacokinetic considerations for the successful design and clinical application of ADCs in oncology are explored in this review, with a focus on the mechanistic determinants of distribution and elimination.

Pharmacokinetic and Toxicodynamic Characterization of a Novel Doxorubicin Derivative

Doxorubicin (Dox) is an effective anti-cancer medication with poor oral bioavailability and systemic toxicities. DoxQ was developed by conjugating Dox to the lymphatically absorbed antioxidant quercetin to improve Dox’s bioavailability and tolerability. The purpose of this study was to characterize the pharmacokinetics and safety of Dox after intravenous (IV) and oral (PO) administration of DoxQ or Dox (10 mg/kg) and investigate the intestinal lymphatic delivery of Dox after PO DoxQ administration in male Sprague–Dawley rats. Drug concentrations in serum, urine, and lymph were quantified by HPLC with fluorescence detection. DoxQ intact IV showed a 5-fold increase in the area under the curve (AUC)—18.6 ± 1.98 compared to 3.97 ± 0.71 μg * h/mL after Dox—and a significant reduction in the volume of distribution (V_ss): 0.138 ± 0.015 versus 6.35 ± 1.06 L/kg. The fraction excreted unchanged in urine (f_e) of IV DoxQ and Dox was ~5% and ~11%, respectively. Cumulative amounts of Dox in the mesenteric lymph fluid after oral DoxQ were twice as high as Dox in a mesenteric lymph duct cannulation rat model. Oral DoxQ increased AUC of Dox by ~1.5-fold compared to after oral Dox. Concentrations of β-N-Acetylglucosaminidase (NAG) but not cardiac troponin (cTnI) were lower after IV DoxQ than Dox. DoxQ altered the pharmacokinetic disposition of Dox, improved its renal safety and oral bioavailability, and is in part transported through intestinal lymphatics.

Antibodies as stratagems against cancer

Antibodies have been in the frontline of anticancer research during the last few decades, since a number of different ways have been discovered to utilize them as parts or main components of anticancer drugs. Antibodies are used as the only component of some anticancer drugs, but they can also be conjugated with a variety of substances. Antibody engineering methods such as humanization, chimerization and Fc engineering are applied in order to modify their properties according to the requirements of anticancer drug application. Given the continuous advances in biology and informatics, the role of antibodies in anticancer treatment is expected to be prominent.

Protein based therapeutic delivery agents: Contemporary developments and challenges

As unique biopolymers, proteins can be employed for therapeutic delivery. They bear important features such as bioavailability, biocompatibility, and biodegradability with low toxicity serving as a platform for delivery of various small molecule therapeutics, gene therapies, protein biologics and cells. Depending on size and characteristic of the therapeutic, a variety of natural and engineered proteins or peptides have been developed. This, coupled to recent advances in synthetic and chemical biology, has led to the creation of tailor-made protein materials for delivery. This review highlights strategies employing proteins to facilitate the delivery of therapeutic matter, addressing the challenges for small molecule, gene, protein and cell transport.

Engineering a humanized bone organ model in mice to study bone metastases

Current in vivo models for investigating human primary bone tumors and cancer metastasis to the bone rely on the injection of human cancer cells into the mouse skeleton. This approach does not mimic species-specific mechanisms occurring in human diseases and may preclude successful clinical translation. We have developed a protocol to engineer humanized bone within immunodeficient hosts, which can be adapted to study the interactions between human cancer cells and a humanized bone microenvironment in vivo. A researcher trained in the principles of tissue engineering will be able to execute the protocol and yield study results within 4-6 months. Additive biomanufactured scaffolds seeded and cultured with human bone-forming cells are implanted ectopically in combination with osteogenic factors into mice to generate a physiological bone 'organ', which is partially humanized. The model comprises human bone cells and secreted extracellular matrix (ECM); however, other components of the engineered tissue, such as the vasculature, are of murine origin. The model can be further humanized through the engraftment of human hematopoietic stem cells (HSCs) that can lead to human hematopoiesis within the murine host. The humanized organ bone model has been well characterized and validated and allows dissection of some of the mechanisms of the bone metastatic processes in prostate and breast cancer.

Development of a facile antibody-drug conjugate platform for increased stability and homogeneity

Despite the advances in the design of antibody-drug conjugates (ADCs), the search is still ongoing for novel approaches that lead to increased stability and homogeneity of the ADCs. We report, for the first time, an ADC platform technology using a platinum(ii)-based linker that can re-bridge the inter-chain cysteines in the antibody, post-reduction. The strong platinum-sulfur interaction improves the stability of the ADC when compared with a standard maleimide-linked ADC thereby reducing the linker-drug exchange with albumin significantly. Moreover, due to the precise conserved locations of cysteines, both homogeneity and site-specificity are simultaneously achieved. Additionally, we demonstrate that our ADCs exhibit increased anticancer efficacy in vitro and in vivo. The Pt-based ADCs can emerge as a simple and exciting proposition to address the limitations of the current ADC linker technologies.

A new construct of antibody-drug conjugates for treatment of B-cell non-Hodgkin's lymphomas

The aim of this study was to develop a new class of antibody-drug conjugates (ADCs) with the potential to not only enhance treatment efficacy but also improve tolerability for patients with B-cell lymphomas. Classic ADCs consist of monoclonal antibodies (mAbs) linked to drugs or toxins. They selectively deliver toxic moieties to tumor cells. As such, they greatly improve the therapeutic index compared to traditional chemotherapeutic agents. However, the therapeutic efficacy and safety of ADCs are dependent on linker stability and payload toxicity. Limited payload number on a single antibody (drug-to-antibody ratio, or DAR) has been driving investigators to use extremely toxic agents; however, even very low off-target binding of these ADCs may kill patients. Herein we report a new design of ADCs that consists of rituximab (RTX) and N-(2-hydroxypropyl)methacrylamide (HPMA) copolymer-epirubicin conjugates. The latter was selectively attached to RTX via reduced disulfide bonds. Such design allows the introduction of a large payload of drug on the antibody without adding attachment sites and without compromising the antigen-targeting ability. The binding of the new conjugate, namely RTX-P-EPI, to Ramos cells (with high CD20 expression) was confirmed. The cytotoxicity of RTX-P-EPI against Raji and Ramos cells was also determined. Interestingly, two-fold inhibition of cell proliferation was observed when using RTX-P-EPI compared with their equivalent physical mixture of RTX and P-EPI. Treatment of male SCID mice bearing subcutaneous Ramos B-cell lymphoma tumors demonstrated that RTX-P-EPI possessed superior efficacy when compared to combination of RTX with chemotherapy EPI (RTX+EPI) and P-EPI (RTX+P-EPI), whereas single RTX and a non-specific conjugate IgG-P-EPI only showed marginal effect. The conjugate RTX-EPI in which EPI was directly attached to RTX demonstrated much less antitumor activity compared with RTX-P-EPI. The results suggest that this new design possesses synergistic potential of immunotherapy combined with established macromolecular therapy; moreover, a conventional chemo-agent could be utilized to generate highly effective ADCs and to achieve lower risk of off-target toxicity.

Antibody-Based Cancer Therapy: Successful Agents and Novel Approaches

Since their discovery, antibodies have been viewed as ideal candidates or "magic bullets" for use in targeted therapy in the fields of cancer, autoimmunity, and chronic inflammatory disorders. A wave of antibody-dedicated research followed, which resulted in the clinical approval of a first generation of monoclonal antibodies for cancer therapy such as rituximab (1997) and cetuximab (2004), and infliximab (2002) for the treatment of autoimmune diseases. More recently, the development of antibodies that prevent checkpoint-mediated inhibition of T cell responses invigorated the field of cancer immunotherapy. Such antibodies induced unprecedented long-term remissions in patients with advanced stage malignancies, most notably melanoma and lung cancer, that do not respond to conventional therapies. In this review, we will recapitulate the development of antibody-based therapy, and detail recent advances and new functions, particularly in the field of cancer immunotherapy. With the advent of recombinant DNA engineering, a number of rationally designed molecular formats of antibodies and antibody-derived agents have become available, and we will discuss various molecular formats including antibodies with improved effector functions, bispecific antibodies, antibody-drug conjugates, antibody-cytokine fusion proteins, and T cells genetically modified with chimeric antigen receptors. With these exciting advances, new antibody-based treatment options will likely enter clinical practice and pave the way toward more successful control of malignant diseases.

Antibody-drug conjugates: recent advances in conjugation and linker chemistries

The antibody-drug conjugate (ADC), a humanized or human monoclonal antibody conjugated with highly cytotoxic small molecules (payloads) through chemical linkers, is a novel therapeutic format and has great potential to make a paradigm shift in cancer chemotherapy. This new antibody-based molecular platform enables selective delivery of a potent cytotoxic payload to target cancer cells, resulting in improved efficacy, reduced systemic toxicity, and preferable pharmacokinetics (PK)/pharmacodynamics (PD) and biodistribution compared to traditional chemotherapy. Boosted by the successes of FDA-approved Adcetris^® and Kadcyla^®, this drug class has been rapidly growing along with about 60 ADCs currently in clinical trials. In this article, we briefly review molecular aspects of each component (the antibody, payload, and linker) of ADCs, and then mainly discuss traditional and new technologies of the conjugation and linker chemistries for successful construction of clinically effective ADCs. Current efforts in the conjugation and linker chemistries will provide greater insights into molecular design and strategies for clinically effective ADCs from medicinal chemistry and pharmacology standpoints. The development of site-specific conjugation methodologies for constructing homogeneous ADCs is an especially promising path to improving ADC design, which will open the way for novel cancer therapeutics.

Antibody Drug Conjugates for Cancer Therapy

Antibody drug conjugates (ADCs) constitute a family of cancer therapeutics designed to preferentially direct a cytotoxic drug to cells expressing a cell-surface antigen recognized by an antibody. The antibody and drug are linked through chemistries that enable release of the cytotoxic drug or drug adduct upon internalization and digestion of the ADC by the cell. Over 40 distinct ADCs, targeting an array of antigens and utilizing a variety of drugs and linkers, are undergoing clinical evaluation. This review primarily covers ADCs that have advanced to clinical investigation with a particular emphasis on how the individual targets, linker chemistries, and appended drugs influence their behavior.

Cancer Therapy with Radiolabeled and Drug/Toxin-conjugated Antibodies

Radioimmunotherapy and antibody-directed chemotherapy have emerged as cancer treatment modalities with the regulatory approval of products for non-Hodgkin's lymphoma and acute myeloid leukemia. Antibody-toxin therapy is likewise on the verge of clinical fruition. Accumulating evidence suggests that radioimmunotherapy may have the best impact in minimal-disease and adjuvant settings, especially with radioresistant solid tumors. For the latter, ongoing efforts in ‘pretargeting’ to increase deliverable tumor radiation dose, combination therapies, and locoregional applications are also of importance. Antibody-drug conjugates have the potential to increase the therapeutic index of chemotherapy by minimizing systemic toxicity and improving tumor targeting. The design of optimal drug conjugates in this regard is predicated upon the proper choice of the target antigen, the cleavable-linker, and the drug. In respect of antibody-toxin conjugates, considerable progress has been made in chemical and recombinant immunotoxin designs, and in the advancement of many products to clinical trials. Continued development of antibody-directed therapies should expand the options available for the management of cancer.

Prospects and progress of antibody-drug conjugates in solid tumor therapies

INTRODUCTION

Antibody-drug conjugates (ADCs) for targeted chemotherapy have evolved in the past 2-3 decades to become a validated clinical cancer therapy modality. While considerable strides have been made in treating hematological tumors, challenges remain in the more difficult-to-treat solid cancers.

AREAS COVERED

The current model for a successful ADC uses a highly potent cytotoxic drug as the payload, with stringent linker requirements and limited substitutions. In solid tumor treatment, a number of ADCs have not progressed beyond Phase I clinical trials, indicating a need to optimize additional factors governing translational success. In this regard, insights from mathematical modeling provide a number of pointers relevant to target antigen and antibody selection. Together with the choice of targets, these can be expected to complement the gains made in ADC design towards the generation of better therapeutics.

EXPERT OPINION

While highly potent microtubule inhibitors continue to dominate the current ADC landscape, there are promising data with other drugs, linkers, and targets that suggest a more flexible model for a successful ADC is evolving. Such changes will undoubtedly lead to the consideration of new targets and constructs to overcome some of the unique natural barriers that impede the delivery of cytotoxic agents in solid tumor.

Effects of antibody, drug and linker on the preclinical and clinical toxicities of antibody-drug conjugates

Antibody-drug conjugates (ADCs) represent a new class of cancer therapeutics. Their design involves a tumor-specific antibody, a linker and a cytotoxic payload. They were designed to allow specific targeting of highly potent cytotoxic agents to tumor cells whilst sparing normal cells. Frequent toxicities that may be driven by any of the components of an ADC have been reported. There are currently more than 50 ADCs in active clinical development, and a further ∼20 that have been discontinued. For this review, the reported toxicities of ADCs were analysed, and the mechanisms for their effects are explored in detail. Methods to reduce toxicities, including dosing strategies and drug design, are discussed. The toxicities reported for active and discontinued drugs are important to drive the rational design and improve the therapeutic index of ADCs of the future.

Linkers Having a Crucial Role in Antibody-Drug Conjugates

Antibody-drug conjugates (ADCs) comprised of a desirable monoclonal antibody, an active cytotoxic drug and an appropriate linker are considered to be an innovative therapeutic approach for targeted treatment of various types of tumors and cancers, enhancing the therapeutic parameter of the cytotoxic drug and reducing the possibility of systemic cytotoxicity. An appropriate linker between the antibody and the cytotoxic drug provides a specific bridge, and thus helps the antibody to selectively deliver the cytotoxic drug to tumor cells and accurately releases the cytotoxic drug at tumor sites. In addition to conjugation, the linkers maintain ADCs' stability during the preparation and storage stages of the ADCs and during the systemic circulation period. The design of linkers for ADCs is a challenge in terms of extracellular stability and intracellular release, and intracellular circumstances, such as the acid environment, the reducing environment and cathepsin, are considered as the catalysts to activate the triggers for initiating the cleavage of ADCs. This review discusses the linkers used in the clinical and marketing stages for ADCs and details the fracture modes of the linkers for the further development of ADCs.

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.

Strategies and Advancement in Antibody-Drug Conjugate Optimization for Targeted Cancer Therapeutics

Antibody-drug conjugates utilize the antibody as a delivery vehicle for highly potent cytotoxic molecules with specificity for tumor-associated antigens for cancer therapy. Critical parameters that govern successful antibody-drug conjugate development for clinical use include the selection of the tumor target antigen, the antibody against the target, the cytotoxic molecule, the linker bridging the cytotoxic molecule and the antibody, and the conjugation chemistry used for the attachment of the cytotoxic molecule to the antibody. Advancements in these core antibody-drug conjugate technology are reflected by recent approval of Adectris(®) (anti-CD30-drug conjugate) and Kadcyla(®) (anti-HER2 drug conjugate). The potential approval of an anti-CD22 conjugate and promising new clinical data for anti-CD19 and anti-CD33 conjugates are additional advancements. Enrichment of antibody-drug conjugates with newly developed potent cytotoxic molecules and linkers are also in the pipeline for various tumor targets. However, the complexity of antibody-drug conjugate components, conjugation methods, and off-target toxicities still pose challenges for the strategic design of antibody-drug conjugates to achieve their fullest therapeutic potential. This review will discuss the emergence of clinical antibody-drug conjugates, current trends in optimization strategies, and recent study results for antibody-drug conjugates that have incorporated the latest optimization strategies. Future challenges and perspectives toward making antibody-drug conjugates more amendable for broader disease indications are also discussed.

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.

Doxorubicin loaded polymeric gold nanoparticles targeted to human folate receptor upon laser photothermal therapy potentiates chemotherapy in breast cancer cell lines

The current research focuses on the application of folate conjugated and doxorubicin loaded polymeric gold nanoparticles (GNPs) for the targeted treatment of folate receptor overexpressing breast cancers, augmented by adjunctive laser photothermal therapy. Herein, GNPs surface modified with folate, drug doxorubicin and polyethylene glycol were engineered and were used as vehicles for folate receptor targeted delivery of doxorubicin into cancer cells. Subsequently, the GNPs were photo-excited using laser light for mediating hyperthermia in the cancer cells. In vitro studies were performed to validate the efficacy of the combined modality of folate conjugated and doxorubicin loaded polymeric GNP mediated chemotherapy followed by photothermal therapy in comparison to treatment with free drug; and the combination modality showed better therapeutic efficacy than that of plain doxorubicin treatment in MDA-MB-231 breast cancer cells that express increased levels of surface folate receptors when compared to MCF-7 breast cancer cells that express low levels of folate receptor. The mechanism of cell death was investigated using fluorescent microscopy. Immunoassays showed the up-regulation of the pro-apoptotic protein p53 and down-regulation of the anti-apoptotic protein Bcl-2. Collectively, these results suggest that the folate tagged doxorubicin loaded GNPs are an attractive platform for targeted delivery of doxorubicin and are agents suitable for photothermal cancer therapy.

Synthesis of a novel, sequentially active-targeted drug delivery nanoplatform for breast cancer therapy

Breast cancer is the leading cause of cancer deaths among women. Paclitaxel (PTX), an important breast cancer medicine, exhibits reduced bioavailability and therapeutic index due to high hydrophobicity and indiscriminate cytotoxicity. PTX encapsulation in one-level active targeting overcomes such barriers, but enhances toxicity to normal tissues with cancer-similar expression profiles. This research attempted to overcome this challenge by increasing selectivity of cancer cell targeting while maintaining an ability to overcome traditional pharmacological barriers. Thus, a multi-core, multi-targeting construct for tumor specific delivery of PTX was fabricated with (i) an inner-core prodrug targeting the cancer-overexpressed cathepsin B through a cathepsin B-cleavable tetrapeptide that conjugates PTX to a poly(amidoamine) dendrimer, and (ii) the encapsulation of this prodrug (PGD) in an outer core of a RES-evading, folate receptor (FR)-targeting liposome. Compared to traditional FR-targeting PTX liposomes, this sequentially active-targeted dendrosome demonstrated better prodrug retention, an increased cytotoxicity to cancer cells (latter being true when FR and cathepsin B activities were both at moderate-to-high levels) and higher tumor reduction. This research may eventually evolve a product platform with reduced systemic toxicity inherent with traditional chemotherapy and localized toxicity inherent to single-target nanoplatforms, thereby allowing for better tolerance of higher therapeutic load in advanced disease states.

Structural biology of antibody recognition of carbohydrate epitopes and potential uses for targeted cancer immunotherapies

Monoclonal antibodies represent the most successful class of biopharmaceuticals for the treatment of cancer. Mechanisms of action of therapeutic antibodies are very diverse and reflect their ability to engage in antibody-dependent effector mechanisms, internalize to deliver cytotoxic payloads, and display direct effects on cells by lysis or by modulating the biological pathways of their target antigens. Importantly, one of the universal changes in cancer is glycosylation and carbohydrate-binding antibodies can be produced to selectively recognize tumor cells over normal tissues. A promising group of cell surface antibody targets consists of carbohydrates presented as glycolipids or glycoproteins. In this review, we outline the basic principles of antibody-based targeting of carbohydrate antigens in cancer. We also present a detailed structural view of antibody recognition and the conformational properties of a series of related tissue-blood group (Lewis) carbohydrates that are being pursued as potential targets of cancer immunotherapy.

Site-specific antibody-drug conjugates: the nexus of bioorthogonal chemistry, protein engineering, and drug development

Antibody–drug conjugates (ADCs) combine the specificity of antibodies with the potency of small molecules to create targeted drugs. Despite the simplicity of this concept, generation of clinically successful ADCs has been very difficult. Over the past several decades, scientists have learned a great deal about the constraints on antibodies, linkers, and drugs as they relate to successful construction of ADCs. Once these components are in hand, most ADCs are prepared by nonspecific modification of antibody lysine or cysteine residues with drug-linker reagents, which results in heterogeneous product mixtures that cannot be further purified. With advances in the fields of bioorthogonal chemistry and protein engineering, there is growing interest in producing ADCs by site-specific conjugation to the antibody, yielding more homogeneous products that have demonstrated benefits over their heterogeneous counterparts in vivo. Here, we chronicle the development of a multitude of site-specific conjugation strategies for assembly of ADCs and provide a comprehensive account of key advances and their roots in the fields of bioorthogonal chemistry and protein engineering.

Microtubule inhibitor-based antibody-drug conjugates for cancer therapy

The specificity of monoclonal antibodies represents a potential therapeutic advantage, but their use as single agents in oncology has proven limited to date. The development of antibody-drug conjugates (ADCs) takes advantage of the specificity of the monoclonal antibody and potent cytotoxic effect of chemotherapy, leading to enhanced cytotoxicity in target cells and limiting toxicity to normal tissue. Microtubules represent a validated oncologic target in a range of tumor types, with a number of anti-microtubule targeting cytotoxic drugs approved for cancer use. The systemic use of potent microtubule-binding agents is limited by their effects in normal cells, which leads to toxicity including myelosuppression and peripheral neuropathy. Linking these agents to monoclonal antibodies may limit toxicity to normal tissues and increase drug concentration in target tissues, also allowing the use of more potent agents which would be too toxic to administer in their unbound form. Two such ADCs have been approved for clinical use and many others are in development. Here we review the characteristics of each of the ADC components that have led to efficacious therapies and discuss some of the tubulin inhibitor-based ADCs in development for cancer therapy.

Using the Lessons Learned From the Clinic to Improve the Preclinical Development of Antibody Drug Conjugates

The treatment options for cancer patients include surgery, chemotherapeutics, radiation therapy, antibody therapy and various combinations of these therapies. The challenge with each therapy is finding the balance between maximizing the anti-tumor efficacy while minimizing the dose limiting toxicities. Antibodies, unlike small molecule chemotherapeutics, selectively bind to cell surface tumor antigens and can be used to deliver radionucleotides or small molecule chemotherapeutic drugs directly to the tumor. Advances in antibody engineering, linker chemistry and the identification of potent cytotoxic drugs led to the recent approval of two antibody drug conjugates to treat breast cancer and lymphoma patients. We will discuss how the observations from the clinical development of antibody drug conjugates can guide the preclinical development of the next generation of antibody drug conjugates.

Site-specific antibody drug conjugates for cancer therapy

Antibody therapeutics have revolutionized the treatment of cancer over the past two decades. Antibodies that specifically bind tumor surface antigens can be effective therapeutics; however, many unmodified antibodies lack therapeutic activity. These antibodies can instead be applied successfully as guided missiles to deliver potent cytotoxic drugs in the form of antibody drug conjugates (ADCs). The success of ADCs is dependent on four factors—target antigen, antibody, linker, and payload. The field has made great progress in these areas, marked by the recent approval by the US Food and Drug Administration of two ADCs, brentuximab vedotin (Adcetris^®) and ado-trastuzumab emtansine (Kadcyla^®). However, the therapeutic window for many ADCs that are currently in pre-clinical or clinical development remains narrow and further improvements may be required to enhance the therapeutic potential of these ADCs. Production of ADCs is an area where improvement is needed because current methods yield heterogeneous mixtures that may include 0–8 drug species per antibody molecule. Site-specific conjugation has been recently shown to eliminate heterogeneity, improve conjugate stability, and increase the therapeutic window. Here, we review and describe various site-specific conjugation strategies that are currently used for the production of ADCs, including use of engineered cysteine residues, unnatural amino acids, and enzymatic conjugation through glycotransferases and transglutaminases. In addition, we also summarize differences among these methods and highlight critical considerations when building next-generation ADC therapeutics.

Antibody-drug conjugates—a new wave of cancer drugs

Antibody-drug conjugates (ADCs) consist of cytotoxic drugs covalently linked to monoclonal antibodies directed to antigens differentially overexpressed in tumor cells. These loaded antibodies are expected to selectively deliver lethal cargoes to tumor cells and provide sustained clinical benefit to pre-selected cancer patients while, at the same time, minimizing systemic toxicity. Although on-target adverse events are not completely avoided and the true efficacy of these innovative agents still requires further clarification, proof-of-concept has already been achieved in clinical settings with immunoconjugates containing calicheamicin, auristatin or maytansine-based cytotoxic payloads. In this present article we review the characteristics of the preceding cytotoxic platforms and their chemical conjugation approaches.

"Extremely minimally invasive": recent advances in nanotechnology research and future applications in neurosurgery

The term "nanotechnology" refers to the development of materials and devices that have been designed with specific properties at the nanometer scale (10(-9) m), usually being less than 100 nm in size. Recent advances in nanotechnology have promised to enable visualization and intervention at the subcellular level, and its incorporation to future medical therapeutics is expected to bring new avenues for molecular imaging, targeted drug delivery, and personalized interventions. Although the central nervous system presents unique challenges to the implementation of new therapeutic strategies involving nanotechnology (such as the heterogeneous molecular environment of different CNS regions, the existence of multiple processing centers with different cytoarchitecture, and the presence of the blood-brain barrier), numerous studies have demonstrated that the incorporation of nanotechnology resources into the armamentarium of neurosurgery may lead to breakthrough advances in the near future. In this article, the authors present a critical review on the current 'state-of-the-art' of basic research in nanotechnology with special attention to those issues which present the greatest potential to generate major therapeutic progresses in the neurosurgical field, including nanoelectromechanical systems, nano-scaffolds for neural regeneration, sutureless anastomosis, molecular imaging, targeted drug delivery, and theranostic strategies.

Mechanistic considerations for the use of monoclonal antibodies for cancer therapy

Since the approval of rituximab in 1997, monoclonal antibodies (mAbs) have become an increasingly important component of therapeutic regimens in oncology. The success of mAbs as a therapeutic class is a result of great strides that have been made in molecular biology and in biotechnology over the past several decades. Currently, there are 14 approved mAb products for oncology indications, and there are ten additional mAbs in late stages of clinical trials. Compared to traditional chemotherapeutic agents, mAbs have several advantages, including a long circulating half-life and high target specificity. Antibodies can serve as cytotoxic agents when administered alone, exerting a pharmacologic effect through several mechanisms involving the antigen binding (Fab) and/or Fc domains of the molecule, and mAbs may also be utilized as drug carriers, targeting a toxic payload to cancer cells. The extremely high affinity of mAbs for their targets, which is desirable with respect to pharmacodynamics (i.e., contributing to the high therapeutic selectivity of mAb), often leads to complex, non-linear, target-mediated pharmacokinetics. In this report, we summarize the pharmacokinetic and pharmacodynamics of mAbs that have been approved and of mAbs that are near approval for oncology indications, with particular focus on the molecular and cellular mechanisms responsible for their disposition and efficacy.

Targeted delivery of doxorubicin-loaded poly (ε-caprolactone)-b-poly (N-vinylpyrrolidone) micelles enhances antitumor effect in lymphoma

Background

The present study was motivated by the need to design a safe nano-carrier for the delivery of doxorubicin which could be tolerant to normal cells. PCL₆₃-b-PNVP₉₀ was loaded with doxorubicin (6 mg/ml), and with 49.8% drug loading efficiency; it offers a unique platform providing selective immune responses against lymphoma.

Methods

In this study, we have used micelles of amphiphilic PCL₆₃-b-PNVP₉₀ block copolymer as nano-carrier for controlled release of doxorubicin (DOX). DOX is physically entrapped and stabilized in the hydrophobic cores of the micelles and biological roles of these micelles were evaluated in lymphoma.

Results

DOX loaded PCL₆₃-b-PNVP₉₀ block copolymer micelles (DOX-PCL₆₃-b-PNVP₉₀) shows enhanced growth inhibition and cytotoxicity against human (K-562, JE6.1 and Raji) and mice lymphoma cells (Dalton's lymphoma, DL). DOX-PCL₆₃-b-PNVP₉₀ demonstrates higher levels of tumoricidal effect against DOX-resistant tumor cells compared to free DOX. DOX-PCL₆₃-b-PNVP₉₀ demonstrated effective drug loading and a pH-responsive drug release character besides exhibiting sustained drug release performance in in-vitro and intracellular drug release experiments.

Conclusion

Unlike free DOX, DOX-PCL₆₃-b-PNVP₉₀ does not show cytotoxicity against normal cells. DOX-PCL₆₃-b-PNVP₉₀ prolonged the survival of tumor (DL) bearing mice by enhancing the apoptosis of the tumor cells in targeted organs like liver and spleen.

Antibody-drug conjugates: an emerging concept in cancer therapy

Traditional cancer chemotherapy is often accompanied by systemic toxicity to the patient. Monoclonal antibodies against antigens on cancer cells offer an alternative tumor-selective treatment approach. However, most monoclonal antibodies are not sufficiently potent to be therapeutically active on their own. Antibody-drug conjugates (ADCs) use antibodies to deliver a potent cytotoxic compound selectively to tumor cells, thus improving the therapeutic index of chemotherapeutic agents. The recent approval of two ADCs, brentuximab vedotin and ado-trastuzumab emtansine, for cancer treatment has spurred tremendous research interest in this field. This Review touches upon the early efforts in the field, and describes how the lessons learned from the first-generation ADCs have led to improvements in every aspect of this technology, i.e., the antibody, the cytotoxic compound, and the linker connecting them, leading to the current successes. The design of ADCs currently in clinical development, and results from mechanistic studies and preclinical and clinical evaluation are discussed. Emerging technologies that seek to further advance this exciting area of research are also discussed.

Antibody-based therapy in colorectal cancer

Treatment in patients with nonresectable and resectable colorectal cancer at the advanced stage is challenging, therefore intensive strategies such as chemotherapy, signaling inhibitors and monoclonal antibodies (mAbs) to control the disease are required. mAbs are particularly promising tools owing to their target specificities and strong antitumor activities through multiple mechanisms, as shown by rituximab in B-cell non-Hodgkin's lymphoma and trastuzumab in breast cancer. Three mAbs (cetuximab, bevacizumab and panitumumab) have been approved for the treatment of colorectal cancer in the USA and many other mAbs are being tested in clinical trials. The potential of antibody therapy is associated with several mechanisms including interference of vital signaling pathways targeted by the antibody and immune cytotoxicity selectively directed against tumor cells by tumor-bound antibody through the Fc portion of the antibody, such as antibody-dependent cellular cytotoxicity and complement-dependent cytotoxicity. Moreover, recent experimental findings have shown that immune complexes formed by therapeutic mAbs with tumor-released antigens could augment the induction of tumor-specific cytotoxic CD8(+) T cells through activation of APCs. In addition, antibodies targeting immune checkpoints on hematopoietic cells have recently opened a new avenue for the treatment of cancer. In this review, we focus on mAb treatment in colorectal cancer and its immunological aspects.

Antibody-drug conjugates: current status and future directions

Antibody-drug conjugates (ADCs) aim to take advantage of the specificity of monoclonal antibodies (mAbs) to deliver potent cytotoxic drugs selectively to antigen-expressing tumor cells. Despite the simple concept, various parameters must be considered when designing optimal ADCs, such as selection of the appropriate antigen target and conjugation method. Each component of the ADC (the antibody, linker and drug) must also be optimized to fully realize the goal of a targeted therapy with improved efficacy and tolerability. Advancements over the past several decades have led to a new generation of ADCs comprising non-immunogenic mAbs, linkers with balanced stability and highly potent cytotoxic agents. Although challenges remain, recent clinical success has generated intense interest in this therapeutic class.

Trends in cancer-targeted antibody-drug conjugates

Better knowledge of engineered antibodies and tumour biology has led to the development of novel targeted therapies, such as antibody-drug conjugates (ADCs). ADCs combine a monoclonal antibody, directed toward specific antigen highly expressed on the cancer cell, to potent cytotoxic drug through a stable linker. ADCs are designed to bind selectively to cancer cells and to deliver cytotoxic drugs into the cancer cell, which may preserve normal cells. ADCs should be stable and non-toxic in circulation. Upon binding to antigen, ADCs are internalized by different processes, followed by the intracellular release of an active form of the cytotoxic drug, which in turn kills the cancer cell. This technology has the potential to further improve the anticancer activity while limiting toxicity. First results from ongoing clinical trials are encouraging. Favourable pharmacokinetic profile was observed showing good stability in circulation. Clinical studies demonstrated that ADCs provide clinical efficacy with an acceptable safety profile. Objective responses and clinical benefits were demonstrated with the investigated ADCs. Major toxicities frequently associated to chemotherapy were barely or not reported with ADCs. Taken together, ADCs may become the new wave of anticancer drugs in the future.

Trends in translational medicine and drug targeting and delivery: new insights on an old concept-targeted drug delivery with antibody-drug conjugates for cancers

According to the JPS Drug Delivery Clinical Trials Database (jpharmscidatabase.org), 37,738, 14,104, and 8060 clinical trials are registered to evaluate (1) drug delivery technology, (2) biomolecule platform, and (3) drug metabolism and pharmacokinetic (PK)-pharmacodynamic (PD) interactions. These numbers represent a 19%-29% increase since 2012. Within biomolecules in clinical testing, antibodies constitute the majority and approximately 6% carry drug conjugates. Paul Ehrlich introduced the antibody-drug conjugate or "magic bullet" concept about a century ago. A monoclonal antibody (mAb)-drug conjugate Mylotarg was licensed for treating cancer in 2000 and exhibits significant liver toxicity and immune hypersensitivity. Plasma drug instability and a bacterial-derived drug may be partly to blame. Progress in antibody-drug conjugation chemistry, understanding how biologic systems respond to antibody-drug conjugates, and unwavering efforts of scientists have enabled successful development of highly potent and effective second-generation antibody-drug conjugates. With the approval of Adcetris for lymphoma in 2011 and Kadcyla in 2013, about a twofold to fourfold gain in cancer response rate is attributed to drug conjugates. With a demonstrated higher safety profile, many more antibody-drug conjugates are in development. The clinical success of Adcetris and Kadcyla has raised hope that antibody-guided "drug bullets" may be truly "magical" in leading to a cure for cancer.

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.

Therapeutic targeting of Lewis(y) and Lewis(b) with a novel monoclonal antibody 692/29

Background

Several monoclonal antibodies (mAbs) recognising Lewis^y, such as BR96, have reached the clinic but have failed to show good anti-tumour responses with an acceptable level of toxicity. No Lewis^b mAbs have been trialled in patients. In this study we compare the specificity of three mAbs; BR96 (Lewis^y), 2-25 LE (Lewis^b) and 692/29 that recognises a unique facet of both Lewis^y and Lewis^b. We then assessed the in vivo therapeutic effect of 692/29 using xenograft models.

Methodology/Principal Findings

Using a glycan array, each mAb was shown to display a different binding pattern with only 692/29 binding to both Lewis^y and Lewis^b. 692/29 was able to kill tumour cells over-expressing Lewis^y/b directly, as well as by antibody and complement mediated cytotoxicity (ADCC/CDC), but failed to kill cells expressing low levels of these haptens. In contrast, BR96, directly killed cells expressing either high or low levels of Lewis^y perhaps explaining its toxicity in patients. 2-25 LE failed to cause any direct killing but did mediate ADCC/CDC. Both 692/29 and BR96 bound to >80% of a panel of over 400 colorectal tumours whereas 2-25 LE showed lower reactivity (52%). 692/29 demonstrated more restricted normal tissue reactivity than both BR96 and 2-25 LE. 692/29 anti-Lewis^y/b mAb also showed good in vivo killing in xenograft models.

Conclusions/Significance

MAbs targeting both Lewis^y and Lewis^b may have a therapeutic advantage over mAbs targeting just one hapten. 692/29 has a more restricted normal tissue distribution and a higher antigen threshold for killing which should reduce its toxicity compared to a Lewis^y specific mAb. 692/29 has an ability to directly kill tumours whereas the anti-Lewis^b mAb does not. This suggests that Lewis^y but not Lewis^b are functional glycans. 692/29 showed good anti-tumour responses in vivo and is a strong therapeutic candidate.

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.

Immunoconjugates and long circulating systems: origins, current state of the art and future directions

Significant progress has been made recently in the area of immunoconjugated drugs and drug delivery systems (DDS). The immuno-modification of either the drug or DDS has proven to be a very promising approach that has significantly improved the targeted accumulation in pathological sites while decreasing its undesirable side effects in healthy tissues. The arrangement for both prolonged life in the circulation and specific target recognition represents another potent strategy in the development of immuno-targeted systems. The longevity of immuno-targeted DDS such as immunoliposomes and immunomicelles improves their targetability even in the presence of the additional passive accumulation in areas with a compromised vasculature. The added use of the immuno-targeted systems takes advantage of the specific microenvironment of pathological sites including lowered pH, increased temperature, and variation in the enzymatic activity. "Smart" stimulus-responsive systems combine different valuable functionalities including PEG-protection, targeting antibody, cell-penetration, and stimulus-sensitive functions. In this review we examined the evolution, current status and future directions in the area of therapeutical immunoconjugates and long-circulating immuno-targeted DDS.

Antibody-drug conjugate payloads

Toxin payloads, or drugs, are the crucial components of therapeutic antibody-drug conjugates (ADCs). This review will give an introduction on the requirements that make a toxic compound suitable to be used in an antitumoral ADC and will summarize the structural and mechanistic features of four drug families that yielded promising results in preclinical and clinical studies.