Antibody-Drug Conjugates Guided Missiles Deployed To Fight Cancer

Antibody-Drug Conjugates

Guided Missiles Deployed To Fight Cancer

They’re here.  Antibody-drug conjugates, target-seeking molecular missiles with lethal payloads, have arrived.

Introduction

The advent of modern-day cancer chemotherapy dates back to the mid-1900s when a chemical warfare agent known as nitrogen mustard was seen to destroy the bone marrow and lymph tissue of exposed individuals. In the following years, nitrogen mustard, along with numerous other alkylating agents took center stage in the treatment of various haematological malignancies including leukaemia, lymphoma, Hodgkin's disease and multiple myeloma. Several other serendipitous observations lead to the development of the first primitive classes of cytotoxins. Despite vast progress in the field of cancer chemotherapy, small-molecule cancer drugs (although highly potent) continue to be plagued with the problems of non-specific toxicity (as a result of targeting all rapidly dividing cells), narrow therapeutic windows and increasing resistance rates. These concerns emphasize the need to move away from conventional cancer treatments and explore new ways to tackle the ever-present disease.

What Are Antibody-Drug Conjugates?

Cancer cells differ from normal cells due to genomic mutations in oncogenes and/or tumour suppressor genes. Once the integrity of the genome is compromised, cells are more likely to develop additional genetic faults, some of which may give rise to tumor-specific antigens (found only on the surface of tumor cells) or tumor-associated antigens (overexpressed on tumor cells, but also present on normal cells). Ongoing research has found that several human cancers express unique tumour-specific or tumour-associated cell surface antigens which are of great value as targets for large molecule, monoclonal antibody (mAb)-based therapy.

The use of antibodies as ‘magic bullets’ to treat disease was first proposed more than 100 years ago by the founder of chemotherapy, Paul Ehrlich]. Due to several challenges in the development of human antibodies, it was only in 1997 that the US FDA (Food and Drug Administration) approved the first anti-cancer antibody, rituximab, for the treatment of B-cell non-Hodgkin’s lymphoma. Early mAbs were based on murine or chimeric antibodies that were modified to target human antigens. As these were non-human antibodies, they evoked a strong immune response that prevented the treatment from being successful. The large size of the mAbs also proved to be problematic as it resulted in reduced tumour penetration and poor therapeutic effect. Since then, several advances in antibody engineering have optimized pharmacokinetics and effector function while reducing immunogenicity. This has resulted in a significant increase in the development of antibody-based drugs against cancer.

mAbs exert their therapeutic effect by binding to tumor specific or tumor-associated cell surface antigens. Once bound, the mAb kills the tumor cell by one or more of the following mechanisms; abrogation of tumor cell signalllng, resulting in apoptosis modulation of T-cell function through antibody-dependent cellular cytotoxicity (ADCC), complement dependent cytotoxicity (CDC) or complement-dependent cell mediated cytotoxicity (CDCC) and exertion of inhibitory effects on tumor vasculature and stroma. Despite these various cell-killing mechanisms, most mAbs display insufficient cytotoxic activity. Current efforts in cancer treatment have therefore focused on combining the selectivity of mAbs with the potency of chemotherapeutic small molecules, giving rise to an entirely new class of anti-cancer drugs known as antibody-drug conjugates (ADCs).

Antibody-drug Conjugates or ADCs are empowered antibodies (mAbs) designed to harness the targeting ability of monoclonal antibodies by linking them to cell-killing agents. An ideal ADC has:

• A highly selective monoclonal antibody (mAb) for a tumor-associated antigen that has restricted or no expression on normal (healthy) cells;
• A potent cytotoxic agent (generally a small molecule drug with a high systemic toxicity) designed to induce target cell death after being internalized in the tumor cell and released;
• A linker that is stable in circulation, but releases the cytotoxic agent in target cells.

Antibody-drug Conjugates

• Covalently linked monoclonal antibodies to small molecule drugs that target the a specific cancer cell reduce systemic toxicity
• Increase the cell-killing potential of monoclonal antibodies (mAbs), and
• Confer higher tumor selectivity. As a result, the tolerability of the drug increases.
• Compared to standard chemotherapeutic drugs or biologics, there is limited systemic exposure

An Innovative Therapeutic Application

Antibody-drug conjugates or ADCs represent an innovative therapeutic application that combines the unique, high specificity, properties and antitumor activity of monoclonal antibodies (mAbs) that are tumor-specific but not sufficiently cytotoxic, with the potent cell killing activity of cytotoxic small molecule drugs that are too toxic to be used on their own. In linking monoclonal antibodies with cytotoxic agents, scientists have been able to optimize the features of both components.

The key components of antibody-drug conjugates include a monoclonal antibody, a stable linker and a cytotoxic agent to target a variety of cancers. The cytotoxic (anticancer) drug is chemically linked (conjugated using disulfide or non-cleavable thioether linker chemistry) to a monoclonal antibody that recognizes a specific tumor-associated antigen, making the drug combination very specific. In simple terms, antibody-drug conjugates deliver “deactivated” cytotoxins to specific cancer cells. Once in the tumor cell – internalization – the cytotoxin is released after which it regains its full – cancer killing – cytotoxic activity. In turn, this leads to rapid cell death.

While the concept of antibody-drug conjugates is relatively easy to understand and relatively straightforward, the design and synthesis of a fully functional and effective antibody-drug conjugate is remarkably challenging, often requiring specialized development teams.

Although the concept of ADCs is theoretically simple, it is difficult to combine its various components into an optimized and functional therapeutic agent. To date, three ADCs have gained entry into the market, of which only two remain. The first ADC to obtain FDA approval was gemtuzumab ozogamicin (Mylotarg®), marketed by Wyeth (now Pfizer), for the treatment of relapsed acute myeloid leukaemia (AML). In 2010, a decade after its approval, gemtuzumab ozogamicin was withdrawn from the market when a clinical trial showed that it had little benefit over conventional cancer therapies and was associated with serious hepatotoxicity. This could have been due to the fact that the linker technology used was not stable enough to prevent the drug from being released in the bloodstream.

The two ADCs currently in the market are brentuximab vedotin (Adcetris® by Seattle Genetics) and trastuzumab emtansine (TDM1; Kadcyla® by Genentech). The former is the first of the two ADCs to be approved and is currently being used to treat patients with relapsed or refractory Hodgkin’s lymphoma or those with relapsed or refractory systemic anaplastic large cell lymphoma. The more recent approval of T-DM1, in 2014, for use against breast cancer, proved that ADCs were capable of targeting solid tumors in addition to haematological malignancies. Although there are only 2 ADCs currently in the market, there are more than 30 ADCs being developed to target a wide range of blood cancers and solid tumors. In contrast with small molecules that have a limited choice of drug targets, the diverse range of ADC targets results in a robust drug pipeline with minimal overlap between different pharmaceutical companies. With the recent FDA approvals of brentuximab vedotin and T-DM1, there has been an increase in research investigating the use of ADCs in the treatment of cancer.

The Make-up of an ADC

Monoclonal antibodies are attached to biologically active drugs by chemical linkers with labile bonds.  By combining the unique targeting of mAbs with the cancer-killing ability of cytotoxic drugs, antibody-drug conjugates allow sensitive discrimination between healthy and diseased tissue.  Antibody-drug conjugates are part of a specialized and technically challenging type of therapy combining innovations from biotechnology and chemistry to form a new class of highly potent biopharmaceutical drugs.  The unique property of antibody-drug conjugates is that these so-called armed antibodies selectively dispatch highly potent cytotoxic anticancer chemotherapies directly to cancer cells while, at the same time, leaving healthy tissue unaffected.

Availability

With the approvals of brentuximab vedotin (Adcetris®; Seattle Genetics/Millennium Pharmaceuticals) and ado-trastuzumab emtansine (Kadcyla®; Genentech/Roche) and more than 50 ADCs in clinical trial pipelines, ADCs are a new drug class. Owing to improved technology and appropriate targeting, the clinical application of ADCs is accelerating rapidly.

Conventional chemotherapy is designed to eliminate fast -growing tumor cells. It can however, also harm healthy proliferating cells, which causes undesirable side effects. [In contrast, ADCs are designed to increase the efficacy of therapy and reduce systemic toxicity, often seen with small molecule drugs.

Linker technology

Antibody-Drug Conjugates deliver highly potent cytotoxic anticancer agents to cancer cells by joining them to monoclonal antibodies by biodegradable, stable linkers and discriminate between cancer and normal tissue. These linkers are either cleavable or non-cleavable.

Advances in linker technology needed to attach monoclonal antibodies to cytotoxic anticancer agents to allow control over drug pharmacokinetics and significantly improve delivery of a cytotoxic agent to cancer cells.

A stable link between the antibody and cytotoxic (anti-cancer) agent is a crucial aspect of an ADC. Linkers are based on chemical motifs including disulfides, hydrazones or peptides (cleavable), or thioethers (noncleavable) and control the distribution and delivery of the cytotoxic agent to the target cell. Cleavable and noncleavable types of linkers have been proven to be safe in preclinical and clinical trials. Brentuximab vedotin includes an enzyme-sensitive cleavable linker that delivers the potent and highly toxic antimicrotubule agent monomethyl auristatin E or MMAE, a synthetic antineoplastic agent, to human specific CD30-positive malignant cells. Because of its high toxicity MMAE, which inhibits cell division by blocking the polymerization of tubulin, cannot be used as a single-agent chemotherapeutic drug. However, the combination of MMAE linked to an anti-CD30 monoclonal antibody (cAC10, a cell membrane protein of the tumor necrosis factoror TNF receptor) proved to be stable in extracellular fluid, cleavable by cathepsin and safe for therapy. Trastuzumab emtansine, the other approved ADC, is a combination of the microtubule-formation inhibitor mertansine (DM-1), a derivative of the Maytansine, and antibody trastuzumab (Herceptin/ Genentech/Roche) attached by a stable, non-cleavable linker.

The availability of better and more stable linkers has changed the function of the chemical bond. The type of linker, cleavable or noncleavable, lends specific properties to the cytotoxic (anti-cancer) drug. For example, a non-cleavable linker keeps the drug within the cell. As a result, the entire antibody, linker and cytotoxic (anti-cancer) agent enter the targeted cancer cell where the antibody is degraded to the level of an amino acid. The resulting complex – amino acid, linker and cytotoxic agent – now becomes the active drug. In contrast, cleavable linkers are catalyzed by enzymes in the cancer cell where it releases the cytotoxic agent. The difference is that the cytotoxic payload delivered via a cleavable linker can escape from the targeted cell and, in a process called “bystander killing,” attack neighboring cancer cells.

Another type of cleavable linker, currently in development, adds an extra molecule between the cytotoxic drug and the cleavage site. This linker technology allows researchers to create ADCs with more flexibility without worrying about changing cleavage kinetics. Researchers are also developing a new method of peptide cleavage based on Edman degradation, a method of sequencing amino acids in a peptide.  Future direction in the development of ADCs also include the development of site-specific conjugation (TDCs) to further improve stability and therapeutic index and α emitting immunoconjugates and antibody-conjugated nanoparticles.

How they work

The antibody-drug conjugate is a three-component system including a potent cytotoxic anticancer agent linked via a biodegradable linker to an antibody. The antibody binds to specific markers (antigens or receptors) at the surface of the cancer cell. The whole antibody-drug conjugate is then internalized within the cancer cell, where the linker is degraded and the active drug released.

The focused delivery of the cytotoxic agent to the tumor cell is designed to maximize the anti-tumor effect of ADCs, while minimizing its normal tissue exposure, potentially leading to an improved therapeutic index.

Mechanism of action

In developing antibody-drug conjugates, an anticancer drug (e.g. a cell toxin or cytotoxin) is coupled to an antibody that specifically targets a certain tumor marker (e.g. a protein that, ideally, is only to be found in or on tumor cells). Antibodies track these proteins down in the body and attach themselves to the surface of cancer cells. The biochemical reaction between the antibody and the target protein (antigen) triggers a signal in the tumor cell, which then absorbs or internalizes the antibody together with the cytotoxin.  After the ADC is internalized, the cytotoxic drug is released and kills the cancer.  Due to this targeting, ideally the drug has lower side effects and gives a wider therapeutic window than other chemotherapeutic agents. 

An ideal ADC is one that retains the selectivity and killing capacity of a mAb while still being able to release the cytotoxic drug in quantities large enough to kill tumor cells. Each of the steps involved in the mechanism of action is associated with unique challenges that complicate the design of ADCs.  See Fig. 1 below:

ADCs are administered intravenously in order to prevent the mAb from being destroyed by gastric acids and proteolytic enzymes. The mAb component of the ADC enables it to circulate in the bloodstream until it finds and binds to tumor-specific (or tumor-associated) cell surface antigens present on target cancer cells. In the interest of preventing unwarranted release of the cytotoxin and maximizing drug delivery to cancer cells, an ideal linker would not only have to be stable in the bloodstream but also capable of releasing the active form of the cytotoxic drug when required.

Once the mAb component of the ADC is bound to its target antigen, the ADC–antigen complex should theoretically be internalized via receptor-mediated endocytosis. The internalization process finishes with the formation of a clathrin-coated early endosome containing the ADC–antigen complex. An influx of H + ions into the endosome results in an acidic environment that facilitates the interaction between the mAb component of a fraction of ADCs and human neonatal Fc receptors (FcRns). The bound ADCs are transported outside the cell, where the physiological pH of 7.4 enables the release of the ADC from the FcRn. This mechanism acts as a buffer for preventing the death of healthy cells in the case of ADC mis-delivery. Excessive binding of ADCs to tumor cell FcRns might however restrict the release of the cytotoxic drug and prevent the ADC from taking effect. FcRn expression is primarily within the endosomes of endothelial cells.

ADCs that remain in the endosome without binding to FcRn receptors form the late endosome. These subsequently undergo lysosomal degradation, allowing the release of the cytotoxic drug into the cytoplasm. At this stage, it is crucial to ensure that a sufficient (i.e. threshold) concentration of the drug is present within the cancer cell for its destruction to be guaranteed. This is however complicated in practice by the facts that cell-surface antigens are often quite limited and the process of internalization, rather inefficient. Assuming all the steps involved in the mechanism of ADC action have an efficiency of 50 %, only 1 %–2 % of the administered drug will reach tumor cells. This makes the choice of cytotoxin particularly important, as it is required to be highly efficacious at very low concentrations. Drugs that are usually unsuitable for normal chemotherapy (due to excessive toxicity) are therefore a necessary component of ADCs.

 

Different classes of cytotoxic drugs result in cell death using various mechanisms. The common element between all the classes is interference with critical cell functioning and, as a consequence, either direct killing of the cell or induction of apoptosis. As targeted cancer cells die, there is potential for the active cytotoxic drug to kill neighbouring tumor cells and the supporting stromal tissue. The design of ADCs with respect to the choice of target, mAb, linker and cytotoxin are all very important determinants of whether or not the threshold concentration of the cytotoxic drug is reached within the tumor cell. These factors therefore determine the overall success of an ADC.

 

Target Antigen Selection

Successful development of an ADC is dependent on the selection of an appropriate target antigen to which the mAb component of the ADC can bind. Aside from being tumor-specific or tumor associated, cell surface antigens should also undergo efficient internalization, have high levels of expression and possess high penetrance, a characteristic whereby a large percentage of tumour samples test positive for the presence of the antigen. The target of gemtuzumab ozogamicin, an ADC previously used against AML, was cluster designation 33 (CD33), a transmembrane cell-surface glycoprotein expressed on the surface of mature and immature myeloid cells. CD33 has extremely high penetrance with 90 %– 95 % of all AML patients testing positive for the antigen. With regard to tumor specificity and sensitivity however, CD33 performed rather poorly as it was found to have only low levels of expression on not only on mature and immature myeloid cells but also erythroid cells, megakaryocytes and multipotent progenitor cells. Current ADCs aim to execute their therapeutic action by identifying target antigens that fulfill all four of their requirements.

Types of Tumor-Associated Antigens

Although cell-surface proteins are the most commonly used targets for ADCs, there are various other categories of tumor associated antigens including glycoproteins, proteins of the extracellular matrix and aberrant gangliosides on the tumor cell surface. Apart from antigens found on tumor cells, there is a growing interest in targeting antigens present on tissues that support the growth and spread of tumor cells such as neovasculature or extracellular stromal tissue. This is particularly attractive as these tissues have a stable genome unlike cancer cells and are therefore less likely to undergo somatic mutations, reducing the risk of mutation-mediated drug resistance.

Instead, non-tumor tissue targets differentiate themselves from healthy tissue by being in an undeveloped state as a result of their rapid formation and turnover rate.

ADCs that bind to neovasculature destroy the tumor blood supply and cause tumor cell death via nutrient deprivation and hypoxia. Potential targets in tumor vasculature include vascular endothelial growth factor (VEGF) and its receptors integrin and endoglin. ADCs that target and destroy extracellular stromal tissue cause tumor cell death by reducing the concentration of growth factors produced by stroma. Similar to tumor vasculature, these growth factors are critical in promoting tumor cell survival. Examples of anti-tumour stromal targets include fibroblast activation protein (FAP) and protein tyrosine kinase 7 (Ptk 7), a pseudokinase enzyme commonly found on many cancer and stromal cells. Since all tumours depend on angiogenesis and stromal factors for their survival and growth, ADCs that target such tissues have a wider efficacy.

Choosing Monoclonal Antibodies

mAbs allow ADCs to have high target-specificity, target-affinity and prolonged drug exposure at the tumor site. Based on these features, antibody selection should ideally ensure minimal cross reactivity with healthy tissues, sub-nanomolar affinity to the target antigen and a long pharmacokinetic half-life combined with minimal immunogenicity. Over time, these features result in the accumulation of the ADC at the tumor site and allow it to have an increased therapeutic effect. In addition, it is beneficial for the mAb to possess intrinsic anti-tumor activity resulting from either direct modulation of the biological activity of the target antigen (e.g., anti-HER2 mAbs) [80] and/or via immune effector functions such as ADCC, CDC and CDCC. When constructing the ideal ADC, it is important to maximally preserve the favorable properties of the mAb. With regard to tumor specificity and target affinity, it is important to choose a mAb that does not lose these features through non-specific binding to normal cells. Apart from being toxic to healthy tissue, an antibody lacking tumor specificity may be eliminated from the circulation due to immunogenicity, leading to sub-optimal target exposure and decreased therapeutic effect. The complementarity-determining regions of an antibody (i.e. the antigen-binding sites) should have extremely high (preferably sub-nanomolar) target affinity in order to guarantee efficient internalization.

A major benefit of using mAbs and mAb-based drugs, such as ADCs, is their favorable pharmacokinetics with respect to distribution, metabolism and elimination in comparison with small molecule cancer therapies. Once mAbs are administered into the bloodstream, they can distribute into cancer tissue either via extravasation through pores in the endothelium or via pinocytosis through endothelial cells following a diffusion gradient [89]. The distribution of the ADC (and hence the cytotoxic drug) into tumor tissues is limited by the size of the antibody, which represents more than 90 % of the mass of an ADC.

Specificity and efficacy

The efficacy of antibody-targeted chemotherapy greatly depends on the specific binding of the targeting antibody-drug conjugate to the specific tumor antigen and on the internalization of the antigen-antibody complex to ensure focused delivery of the conjugated cytotoxic agent inside tumor cells.

The focused delivery of the cytotoxic agent to tumor cells maximizes the antitumor effect. It also minimizes normal tissue exposure that results in an improved therapeutic index and less damage to the surrounding, healthy tissue.

Antibody affinity

The monoclonal antibody affinity, the strength with which an antibody binds to an epitope (antigenic determinant), to the selected antigen is an important factor in ADCs. So-called high-affinity antibodies may have a greater uptake, increasing the therapeutic advantage.  However, while the uptake by the ‘outer layer’ of antigen-positive cells may be higher, the penetration and distribution throughout a ‘bulky tumors’ may be reduced.

Conjugation Chemistry

In recent years, a great deal of research has been conducted in order to develop novel conjugation techniques for use in future ADCs. Traditionally, cytotoxic drugs have undergone chemical conjugation to mAbs, whereby reactive portions of native amino acids are made to interact and bind a specific part of the linker molecule. Examples of reactive groups include the epsilonamino end of lysine residues (used in the conjugation of T-DM1) and the thiol side chains present in the partially reduced form of cysteine residues (used in brentuximab vedotin). As this technique relies on native amino acids, conjugation of the drug is limited by the peptide sequence of the antibody, which therefore restricts control over the number and position of attached cytotoxic drugs. The resulting heterogenous ADC mixture is a major drawback to the chemical conjugation technique as it impacts the toxicity, stability and potency of the ADC. Heterogeneity, with respect to the number of cytotoxin molecules attached per antibody (Figure 5), results in only a small portion of the prepared ADC solution being therapeutically active. This is because a subset of ADCs will contain too few cytotoxin molecules to retain their cell-killing capacity whereas others will have too many to maintain their stability in the bloodstream. Furthermore, inactive ADCs directly decrease ADC potency by binding the limited number of target antigens available on tumor cells and blocking the binding of therapeutically-active ADCs.

Cytotoxic drugs

There are thousands of cellular toxins from either natural sources or chemical synthesis, but only a very few are suitable as components for use in an Antibody-drug Conjugate (ADCs).  In the development of early ADCs, researchers used clinically approved chemotherapeutic drugs. One reason is that these agents  were readily available and their toxicological properties were well known.

However, these early ADCs were only moderately potent and generally less cytotoxic for the targeted tumor cells than the corresponding unconjugated agents.

To solve this problem, scientists started to look at compounds found to be too toxic when tested as a stand-alone chemotherapeutic agent. But the number of these high potent toxins, generally 100 to 1,000 times more toxic than traditional anticancer agents, and are stable, is quite limited.

As ADCs are most often prepared in an aqueous solution and administered intravenously, it is important that the cytotoxic drug has prolonged stability in such environments to prevent damage to healthy cells and increase the availability of the drug at the tumor site. Similarly, it is important that the molecular structure of the cytotoxin allows for its conjugation to the linker while avoiding immunogenicity, maintaining the internalization rate of the mAb and promoting its anti-tumor effects.

Typically, the cytotoxins used in ADCs are a 100–1000 times more potent than regular chemotherapeutics and preferably have sub-nanomolar potency. Most classes of cytotoxins act to inhibit cell division and are classified based on their mechanism of action. Since many ADCs utilize cytotoxins that target rapidly dividing cells, there is a decreased risk of unwanted toxicity if the ADC mistakenly delivers the drug to a non-replicating cell. As the cytotoxin is most commonly released in the lysosome following cleavage of the linker molecule, it is important to ensure that the cytotoxin remains stable in low pH environments and has the capacity to move into the cytosolic or nuclear compartments of the cell within which it takes effect. The choice of the specific cytotoxin to be used in an ADC depends on its mechanism of action and the type of cancer.

Among these are highly potent, biologically active anti-microtubule agents, alkylating agents and DNA minor groove binding agents are being used in combination to humanized mAb targeting agents. These drugs are biologically active at the ng/Kg level placing them in the most potent class of advanced cancer drugs.

ADCs in Development

Many lessons have been learnt since the FDA withdrawal of gemtuzumab ozogamicin regarding the design and development of ADC components. Pharmaceutical companies investing in ADC research have made significant advances in linker technology, conjugation chemistry, antibody engineering and the identification of potent cytotoxins, resulting in rapid evolution of the field. This has not only led to the recent approvals of brentuximab vedotin and trastuzumab emtansine but has also driven the clinical development of ADCs. Currently approved ADCs and those in advanced clinical development are listed in the table below:

Antibody-drug Conjugates combine the unique targeting capabilities of monoclonal antibodies (mAbs) with the specific cancer killing ability of cytotoxic drugs. By attaching biologically active chemotherapeutic drugs, radioactive isotopes, cytokines or cytotoxins via chemical linkers with liable bonds to a monoclonal antibody directed to antigens differentially overexpressed in tumor cells, ADCs significantly improve sensitive discrimination between healthy and diseased tissue. Anitibody-drug Conjugates are part of a specialized subset of highly potent active pharmaceutical ingredients (APIs).

Over the past decade pharmaceutical companies have started to commercialize antibody-drug conjugates. Today, about 50 molecules are in clinical development. Of these, approximately 25% are in Phase II or Phase III of development, leading to a rapidly expanding pre-clinical pipeline. With nearly 190+ active clinical trials, antibody-drug conjugates are gaining acceptance across the globe as they offer superior pharmacological efficiency along with minimized side effects.

 

Manufacturing

In order to be effective naked, unconjugated, therapeutic monoclonal antibodies or TMAs used in the treatment of human disease, require significant quantities as well as multiple treatments. Clinical evidence shows that antibody-drug conjugates require a lower doses to be effective.

Both naked antibodies as well as antibody-drug conjugates are approved for manufacture in mammalian expression and bioreactor systems, which require expensive upstream and downstream processes. In addition, the production of antibody-drug conjugates involves additional, specialized, technologies – including chemical conjugation steps – complicating production streams. These factors result in the outsourcing of 70%-80% of all ADC manufacturing to a limited number of specialized contract manufacturers or CMOs capable of manufacturing monoclonal antibodies, linkers and cytotoxins. In addition, even fewer CMOs have the ability to provide specific conjugation services required to manufacture the actual ADC.

Global Antibody-Drug Market 

The global next-generation antibody therapeutics market was worth $ 1,328.3 million in 2014, and it is expected to grow at a CAGR of 44% during 2015-2020. Next-generation antibody therapeutics market is escalating with high growth rate due to growing prevalence of chronic diseases. Some of the major reasons leading to increase in number of chronic disease cases include unhealthy lifestyle, poor diet, and addictions, such as smoking, and others. In addition, various pharmaceutical and biotechnology companies are collaborating with other companies and organizations for R&D of next-generation antibody therapeutics which further drives the market of next-generation antibody therapeutics.

 

The next-generation antibody therapeutics industry has made huge growth in the recent years. The growing healthcare expenditure has increased the overall research and development investment by various organizations. This leads to the growth of global next-generation antibody therapeutics market. The Autoimmune/Inflammatory market is growing at an average annual growth rate of over 46.4%. The Fc Engineered Antibodies market on the other hand grew at a CAGR of 56.8% during 2015-2020.

The restraints associated with next-generation antibody therapeutics market include high cost, stringent regulatory requirements and time consuming approval process of drugs.  A significant capital investment is required for the research and manufacturing of next-generation therapeutics. The manufacturing process of next-generation antibody therapeutics is very complex, as it requires specialized equipment and careful handling techniques. Next-generation antibody therapeutics is a biologic product, and thus requires specific downstream process for their production. This leads to the high cost of manufacturing next-generation antibody therapeutics.

Different countries have distinct regulatory requirements for launching a product in the market. Strict regulatory requirements and extended approval time hinders the market for next-generation antibody therapeutics.

The key companies operating in the global next-generation antibody therapeutics market include F. Hoffmann-La Roche Ltd, Seattle Genetics Inc., Pfizer Inc., ImmunoGen Inc., Amgen Inc., Biogen, Kyowa Hakko Kirin Co. Ltd., Xencor Inc., Bristol-Myers Squibb Company, AstraZeneca PLC., Dyax Corp., Takeda Pharmaceuticals Company Limited and Bayer AG.

Conclusion

The major challenges associated with the development of ADCs arise from factors that interfere with ADC efficacy and/or those that result in ADC-mediated non-target cell toxicity. All three components of an ADC contribute to these challenges and need to be optimized to create a successful conjugate. Once the therapeutic effect of an ADC has been maximized, it is also desirable to prolong these effects by avoiding resistance mechanisms that decrease the duration of ADC efficacy. All targeted cancer therapies (including ADCs) are prone to resistance mechanisms that alter the function of the target antigen and render the treatment redundant. The future of ADCs will mainly depend on our ability to tackle these challenges.

Despite complexities in designing ADCs, the promise of this therapeutic class has generated intense interest for decades. A robust clinical pipeline and the recent FDA approvals of Adcetris1 and Kadcyla1 suggest that the potential benefit of ADCs may finally be realized. Evolving clinical data will continue to drive technological advancements in the field. Current methods for preclinical lead selection typically rely on systematic in vitro evaluation of a matrix of various mAbs, linkers and cytotoxic payloads. Whether in vitro models are sufficient to predict response remains to be seen; until further understanding of ADCs is realized, early in vivo studies might be crucial. Progress in site-specific conjugation modalities, optimization of linkers with balanced stability and identification of novel, potent cytotoxic agents should pave the way for greater insight into the contribution of these various factors to ADC efficacy, PK and safety. Challenges in target tumor selection will be addressed as the roles of antigen expression, heterogeneity and internalization rate are further elucidated. Guiding principles for the selection of an ideal antibody Fc format are, as of yet, lacking and prompt validation of current assumptions regarding antibody-dependent properties, such as specificity and immune effector functions. Ongoing efforts to address these issues will continue to broaden the impact of ADCs as targeted therapeutics for the treatment of cancer and potentially other diseases.