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Targeted delivery of anticancer agents using antibodies as vectors


An important step in cancer drug research and development is to generate molecules and agents with better outcomes for inhibition of cancer progression. To date, classic cytotoxic chemotherapy is one of the most frequent methods of treatment. Cytotoxic agents currently used frequently have severe side effects. Thus, clinicians are forced to reduce the recommended dosage, or even avoid the extreme usage of these agents because they are nonselective and are distributed to healthy cells as well as cancer cells. Antibody drug conjugates (ADCs) are a new generation of drugs that consist of three parts: a monoclonal antibody (monoclonal antibodies), a small molecule, and a linker between the two. The use of this structure in research pipelines and recently in clinical practice is likely to achieve more precise and selective distribution and delivery of the cytotoxic agents to cancer cells. This review analyzes the structure of ADCs and the advantages of targeted chemotherapeutics. Possible methods of improvement are also discussed.

The activity of a superabundance of chemical compounds was evaluated against clinically significant pathways in cancer progression and increasing interest focused on novel quinazoline derivatives. Activity of the majority of new synthesized agents is usually evaluated by in vitro assays, using human cancer cell lines. For derivatives of quinazoline, the results exhibited moderate to potent cytotoxic, antiinflammatory and antiallergic activity [14]. Lack of specificity of chemotherapeutic agents towards cancer cells is one of the crucial problems in cancer therapy that results in adverse side effects. One means to counter this, is to selectively deliver the drug to the cancer cell.

Human monoclonal antibody technology might be a successful strategy for the treatment of cancer. The first method for the production of hybridomas included humanized animal models, such as murine myeloma cell lines fused with human B cells. Due to their heterogeneity, these hybrids quickly lost the relevant human chromosome and further attempts at using this technology have failed. Over the last few decades, a new generation of monoclonal antibody-based drugs have been developed using Fc part modifications and phage display methods, as well as Epstein-Bar virus transformation procedures. However, these have several disadvantages including loss of specific B-cells and IgM type antibody responses [5, 6].

A new class of therapeutic compounds, called antibody drug conjugates (ADCs), constitutes an increasingly important step in clinical practice. The method of linking monoclonal antibodies with a small compound was described 100 years ago and is still being studied at present [7, 8]. The ADC method uses monoclonal antibodies generated against a specific epitope, which are then linked with a small molecule such as a cytotoxic drug or usually a toxin [9].

Highlighting the hallmarks of ADCs

The success of this new class of anticancer compounds is based on the formulation and manufacturing processes. Although ADC technology is not a new concept, only two drugs (Gemtuzumab ozogamicin, trade name Mylotarg®, targeting CD33 and conjugated with calicheamicin; Brentuximab vedotin, trade name Adcetris®, targeting CD30 (tumor necrosis factor receptor) and conjugated with Monomethyl auristatin E (MMAE)) have been approved by the Food and Drug Administration (FDA). Many other potential drugs are undergoing assessment for cancer therapy, either by the FDA and/or the European Medicines Agency (EMA). Although the two agencies have the same principles and collaborate, they use different procedures and distinct legislation when making assessments [10]. Both the FDA and EMA provide broadly comparable comprehensive postapproval guidance for the identification, monitoring, and minimization of risk to patient safety with some differences in respective implementation toolkits. There is also an increasing tendency toward collaborative efforts between the two regulatory agencies in their approach to monitoring and minimizing risks for patients (Table 1). One example is that the two ADCs have been engineered by Immunogen; MGN388 can attach to an alpha-v integrin targeting antibody and IMGN901 (lorvotuzumab mertansine) is a CD56-targeted ADC with effectiveness against small cell lung cancer (SCLC), multiple myeloma, and ovarian carcinoma [11, 12]. Recently, a new ADC has been developed. Ado-trastuzumab emtansine (trade name Kadcyla®) consists of the monoclonal antibody trastuzumab (Herceptin®) linked to the cytotoxic agent mertansine (DM1). Trastuzumab alone stops the growth of cancer cells by binding to the HER2/neu receptor, whereas mertansine enters cells and destroys them by binding to tubulin. As the monoclonal antibody targets HER2, which is only overexpressed in cancer cells, the conjugate delivers the toxin specifically to tumor cells. In the EMILIA clinical trial of women with advanced HER2 positive breast cancer who were already resistant to trastuzumab alone, it improved survival by 5.8 months compared to the combination of lapatinib and capecitabine [13].

Table 1 Summary of the licensing status of the five mentioned ADCs (Adcetris®, Bexxar®, Zevalin®, Mylotarg® and Kadcyla®)
Table 2 Landmark studies of five ADCs (Adcetris®, Bexxar®, Zevalin®, Mylotarg® and Kadcyla®). A summary of information

In addition to cytotoxic agents, radioactive substances are used to generate cytotoxic effects by selective and local radiation to cancer cells. Bexxar® (tositumomab) is a monoclonal antibody against the CD20 antigen conjugated with radioactive Iodine 131. Zevalin® (ibritumomab tiuxetan) is the same monoclonal antibody (anti-CD20) conjugated with Yttrium 90. Both complexes are used in refractory forms of non-Hodgkin lymphoma, especially after failure of therapy with rituximab where cancer cells still express CD20 antigen (Table 2).

Monoclonal antibody selection is the first crucial step in ADC design. It is highly important for the antigen to be expressed at high levels in the tissue of preference to minimize off-target toxicity, as well as to maximize the ADC binding percentage to the cancer cell locus [14]. Once the monoclonal antibody has been selected, it has to be linked covalently with a compound with a known effect against cancer cells. The majority of agents used in this procedure are small molecules, radioisotopes, proteins, or bacterially derived toxins with anticancer effects [15]. The bioconjugation process includes amine coupling of lysine amino acid residues (typically through amine-reactive succinimidyl esters), sulfhydryl coupling of cysteine residues (via a sulfhydryl-reactive maleimide), and photochemically initiated free radical reactions, which have broader reactivity [16]. Furthermore, the development of a linker that couples the selected cytotoxic agent to the monoclonal antibodies is one of the most fundamental steps in ADC generation. The molecule has to be sufficiently stable to allow the antibody to carry the toxic material to the cell of interest [17, 18]. Linkers are categorized as cleavable and non-cleavable. A non-cleavable linker keeps the drug within the cell. As a result, the entire antibody, linker and cytotoxic (anticancer) agent enter the targeted cancer cell where the antibody is degraded to its constituent amino acids. The resulting complex—amino acid, linker, and cytotoxic agent—now acts as the active drug. In contrast, cleavable linkers are catalyzed by enzymes in the cancer cell, where they release the cytotoxic agent [19, 20] (Fig. 1).

Fig. 1

Antibody drug conjugates mechanism of action

According to recent studies, ADCs need to be analyzed microscopically as well as macroscopically because they have complex molecular structures. The drug to antibody ratio may vary, and thus, methods such as mass spectrometry and ligand to epitope interactions should be used [21].

ADCs technology advantages

Currently, the majority of ADCs are under development or in clinical trials and are designed for oncological and hematological indications. This is primarily driven by the availability of monoclonal antibodies targeting various types of cancer antigens. The first crucial advantage of the usage of these heterogeneous mixtures is that potent and highly toxic agents acting against tumors can be delivered into the cells. Monomethyl auristatin E (MMAE), a synthetic antineoplastic agent, acts as an antimicrotubule agent to human specific CD30-positive malignant cells. Recent studies demonstrated encouraging results when using MMAE in an ADC design with tumor initiating cells [20].

The clinical potential of ADCs has been greatly enhanced by improved choices of specific targets conjugated to highly potent drugs with improved stability. This has greatly expanded our knowledge of ADC cell biology, pharmacokinetics, and pharmacodynamics [21]. The engineering of fully human monoclonal antibodies that have specific targets allow the mixture to target the cell directly, excluding almost all nonspecific binding. Thus, when the specificity of a monoclonal antibody for a target antigen is combined with the delivery of a highly potent cytotoxic drug, clinicians can achieve lower doses of the drug, and as a result, fewer side effects might occur in patients.


As hundreds of approaches and strategies have been developed to deplete cancer cells patients, a more effective model has to be developed. ADC technology includes the following key points: (a) well-characterized antigen target in malignant cells, (b) well-designed, fully human monoclonal antibodies, (c) an already proved cytotoxic–cytostatic agent and finally (d) a linker without adverse effects. Combining all the above, the results seem to be promising. Although some drug developers are also looking to expand the application of ADCs beyond oncology and hematology to other important disease areas, these ADCs are only in the initial drug discovery or preclinical stages of development. An upcoming, first-in-class antibody-drug conjugate for the potential treatment of CD25-expressing lymphomas and leukemias may be the HuMax-TAC-ADC. ADC Therapeutics Sarl and Danish Genmab A/S agreed to develop a new ADC combining HuMax®-TAC antibody and a PBD-based warhead and linker technology. The companies have conducted in vitro and in vivo studies since 2012 to investigate different warhead and linker combinations with HuMax-TAC, and are now starting pre-IND preclinical development. In addition, a new collaboration agreement has also been announced for an antibody against PSMA-positive prostate cancers. In conclusion, SGN-75, which targets CD70; SGN-LIV1A, which targets LIV-1; and ASG-22ME, which targets Nectin-4 are some examples of upcoming ADCs that are either in phase I of clinical trials or are planned to start clinical trials this year.


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Correspondence to Ioannis Papasotiriou MD, PhD.

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Toloudi, M., Papasotiriou, I. Targeted delivery of anticancer agents using antibodies as vectors. memo 6, 262–266 (2013).

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  • Immunoconjugate
  • Cytotoxin
  • Heterogeneous mixture
  • Linker conjunction
  • Targeted drug delivery